The ecology of selected grasshopper species along an elevational gradient
by David Harrison Wachter
A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in
Entomology
Montana State University
© Copyright by David Harrison Wachter (1995)
Abstract:
The extent to which grasshopper communities along an elevational gradient change in southwest
Montana was studied. In addition to this, four species were chosen to examine changes in phenology
along the gradient. This thesis examines the hypothesis that species assemblages change with elevation
and that intraspecific variation in phenology occurs with changes in elevation.
I used detrended correspondence analysis to evaluate changes in grasshopper and plant species across
eleven sampling sites of varying elevation and aspect. Weekly sampling of each site allowed for the
plotting of phenologies for Melanoplus sanguinipes, Melanoplus bivittatus, Chorthippus curtipennis,
and Melanoplus oregonensis.
The results showed non-random distribution of grasshopper species along the gradient. Grasshopper
species distribution was significantly correlated to elevation, precipitation, proportion of grasses and
forbs, and the total number of plant species. Phenological studies showed thatnymphal emergence was
later at sites greater in elevation. Adults from higher elevation sites appeared at collection dates nearly
equal to those from lower elevation sites. THE ECOLOGY OF SELECTED GRASSHOPPER SPECIES
ALONG AN ELEVATIONAL GRADIENT
by
David Harrison Wachter
A thesis submitted in partial fulfillment
of the requirements for the degree
of
Master of Science
in
Entomology
MONTANA STATE UNIVERSITY
Bozeman, Montana
December 1995
Uii+*
APPROVAL
of a thesis submitted by
David Harrison Wachter
This thesis has been read by each member of the thesis committee and has been
found to be satisfactory regarding content, English usage, format, citations, bibliographic
style, and consistency, and is ready for submission to the College of Graduate Studies.
Approved for the Major Department
Date
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Approved for the College of Graduate Studies
LL
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STATEMENT OF PERMISSION TO USE
In presenting this thesis in partial fulfillment of the requirements for a master’s
degree at Montana State University, I agree that the Library shall make it available to
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If I have indicated my intention to copyright this thesis by including a copyright
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from or reproduction of this thesis in whole or parts may be granted only by the copyright
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Signature
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ACKNOWLEDGEMENTS
I express my sincere thanks to.my advisor, Kevin O’Neill, for his support,
guidance, and patience throughout this project. Thanks also to Bill Kemp, Matt Lavin, and
Bob Nowierski for their support and insightful reviews of the thesis.
I wish to acknowledge the Center for Mountain Studies at Montana State University
for their support and the classes they provided through which I gained valuable insight into
the uniqueness of mountain environments. I also wish to acknowledge the U.S. Forest
Service for allowing me to maintain weather stations on land in the Gallatin National
Forest.
My sincere thanks go to all Entomology faculty and staff for all their help. Lastly,
thanks to my family for their never ending love and support.
V
TABLE OF CONTENTS.
Page
APPROVAL ......................................................................................................................
ii
STATEMENT OF PERMISSION TO USE ............................................................... iii
ACKNOW LEDGEM ENTS
...........................................
TABLE OF CONTENTS .........
iv
v
LIST OF TABLES ....................................................
vii
LIST OF FIGURES ...........................................................................
vii
A BSTRACT
.......................
ix
1. INTRODUCTION ........................................................................................................
I
Importance of grasshopper studies ................................................................... I
Climate of southwestern Montana ...........................................
2
Vegetation of southwestern Montana ........................................
3
Grasshoppers in steppe and mountain environments ......................................
4
Distribution o f Acrididae ................................................................................
5
Phenology o f Acrididae .............. — ........ ....................... ................................ 8
Objectives of distribution and phenology studies .......................................... 10
2.
MATERIALS AND METHODS ................................
11
Study Sites ........................................................................................................
11
Sam pling Techniques ............
12
Grasshoppers
Plants
.............................................................
.........................................................................................................
12
13
Temperature Data and Phenology ................................................................... 13
3.
Identification of Acrididae ..................
15
Data A nalysis ......................................................................
16
RESULTS
........................................................ '........................................................
17
vi
TABLE OF CONTENTS (cont.)
Species Distribution ...........................................................................................
Grasshoppers
17
......................................................................................... 17
Grasshopper DCA Results ......................................
22
Plants
25
........................................................................................
Plant DCA Results ................................................................................ 29
Grasshopper phenology ..........
4.
31
Melanoplus sahguinipes ...................
31
Melanoplus bivittatus .....................................................................
33
Chorthippus curtipennis ........
36
Melanoplus oregonensis .........................................................................
36
DISCUSSIO N ...........................................................................
41
Summary of patterns of grasshopper and plant distribution.............................41
Environmental correlates of grasshopper distribution......................................42
Summary o f patterns of grasshopper development ......................................
47
Environmental correlates of grasshopper developm ent.....................................48
BIBLIOGRAPHY
............................................................................................................
52
vii
LIST OF TABLES
Table
Page
1. Grasshopper species collected ................................................................... 18
2. Julian dates for the first collection of second instars ................................. 23
3. Correlation computed for comparisons between DCA axes scores and
environmental variables ..............
26
4. Plant species collected and used for DCA analysis........ .........................
28
A.
viii
LIST OF HGURES
Figure
Page
1. Number of species as a function of e lev a tio n ..........................................
19
2. Number of individuals collected from each sampling s it e ..........................20
3. Detrended correspondence analysis of 11 grasshopper sampling sites across
4 vegetation zones .............. ............. ................. ............................. 24
4. Detrended correspondence analysis of 25 grasshopper species for 11
sampling sites across 4 vegetation zones .................;......................... 27
5. Detrended correspondence analysis of 11 plant communities across
4 vegetation zones ..............................................................................
30
6. M. sanguinipes development as a function o f Julian d a te .......................... 32
7. Mean instar of M. sanguinipes plotted against accumulated degree days......34
8. Plots of Melanoplus bivittatus development presented as mean instar as a
function o f Julian date .......................................................................
35
9. Plots of C. curtipennis development presented as mean instar as a
function of Julian date ..............................
37
10. Plots of M. oregonensis development presented as mean instar as a
function
of Julian date ................................
38
11. Mean instar of M. oregohensis plotted against accumulated degree-days.... 40
ABSTRACT
The extent to which grasshopper communities along an elevational gradient change
in southwest Montana was studied. In addition to this, four species were chosen to
examine changes in phenology along the gradient. This thesis examines the hypothesis that
species assemblages change with elevation and that intraspecific variation in phenology
occurs with changes in elevation.
I used detrended correspondence analysis to evaluate changes in grasshopper and
plant species across eleven sampling sites of varying elevation and aspect. Weekly
sampling of each site allowed for the plotting of phenologies for Melanoplus sanguinipes,
Melanoplus bivittatus, Chorthippus curtipennis, and Melanoplus oregonensis.
The results showed non-random distribution of grasshopper species along the
gradient. Grasshopper species distribution was significantly correlated to elevation,
precipitation, proportion of grasses and forbs, and the total number of plant species.
Phenological studies showed thatnymphal emergence was later at sites greater in elevation.
Adults from higher elevation sites appeared at collection dates nearly equal to those from
lower elevation sites.
I
INTRODUCTION
Importance of Grasshopper Studies
Southwest Montana is composed of regions of steppe and mountain ranges
(Price, 1981). Grasslands within these regions are distributed within a complex
topography. This regional variation creates environmental gradients that affect
plant and animal assemblages at the landscape and community scale (Senft et al.
1987). These gradients also provide ecologists with opportunities to study the
spatial and temporal aspects of insect diversity and abundance.
In the temperate grasslands of North America, grasshoppers (Orthoptera:
Acndidae) are among the most abundant and economically important grazing
herbivores (Otte 1981). Their abundance and food preferences occasionally
make them strong competitors with humans because they consume food crops
and livestock forage. For this reason, they are a relatively well-studied group
(Alexander and Hilliard 1969, Evans 1988, Joem 1979, 1982, Kemp et al.
1989,1990, Uvarov, 1931). However, researchers have only recently addressed
the factors that influence grasshopper distribution (Evans 1987, Kemp et al,
1990a,b). In this thesis, I present the results from studies of grasshopper
distribution along an environmental gradient from the steppe regions in the
Gallatin Valley of southwest Montana to alpine regions of surrounding mountain
ranges.
2
Climate of Southwestern Montana
The climate of southwestern Montana is influenced by latitude,
continentality, and altitudinal variation. The climate is characterized by low
annual precipitation, large quantities of solar radiation, and wide daily and
annual temperature fluctuations. The steppe habitat of this region (1400-1600 m
altitude) is characterized by hot dry summers and cold winters with precipitation
amounts increasing with higher elevations. Precipitation in the region typically
ranges from 20 to 35 cm annually depending on the local elevation.
The mountain ranges surrounding the Gallatin Valley of southwest Montana
begin at elevations around 1800 m and rise to heights from 2800 to 3500 m.
Mean annual precipitation in the mountains varies from 25 cm at lower
elevations to 50 cm at higher elevations. Most of the precipitation comes in the
form of snow. Tree line in the mountains begins around 2700 to 2800 m
depending on the local conditions. Conditions, which create such variable
climates, include elevation, slope angle, and aspect. For example,
mountainsides facing southwest tend to be the warmest and driest slopes because
earlier evaporation of dew allows the sun to more rapidly heat the soil surface;
in contrast north-facing slopes tend to be wetter and cooler (Shreve 1924, Greg
1963). At any time, there can be great variation in weather conditions over
short distances.
3
Vegetation of Southwest Montana
Grasslands composed mainly of native vegetation were chosen for this study.
Sampling sites were chosen from four regional or landscape vegetation
categories: steppe, piedmont, montane, and alpine (Price, 1981). The piedmont
or foothill zone generally represents an integration of steppe and montane
vegetation, and is an area where precipitation tends to increase most abruptly.
The steppe zone is found in the central portion of the Gallatin Valley, a
sinking block between faults from which rise the Bridger and the Gallatin
mountain ranges to the east and south, respectively. The elevation of the valley
varies from 1200 m to 1600 m. In the western portion of the valley, grassland
sites are relatively hot and dry with some vegetation characteristics similar to
those found in the Great Basin desert (Mueggler and Stewart 1983).
As the valley floor rises to the east into the foothills of the Bridger
Mountains and Gallatin Range (1400 m to 1800 m), there is a corresponding
increase in mean annual precipitation from 35 to 50 cm. The steppe/montane
interface or foothill zone occurs at an elevation between 1500 m and 1700 m.
The montane region is characterized by cool, moist coniferous forest.
Montane grasslands occur at elevations between 1800 m to 2700 m with
vegetation varying dramatically within this area due to local differences in
conditions governed by slope angle, aspect, soil type, and elevation.
The alpine regions in the Bridger and Gallatin ranges are found at elevations
4
between 2700m and 3100m. In this region, we see a shift from the coniferous
forest to alpine tundra. The growing season is extremely short and plants
possess adaptations to survive the harsh climate. These adaptations include a
bunched and shortened morphology and increased plant hair to increase
insulation.
Many of these sites, including the montane and alpine sites, have been grazed
by both domestic and wild animals, but their precise grazing history is unknown.
During the course of the study, grazing was observed on the steppe sites, one
foothills site, and one montane site.
Grasshoppers in Steppe and Mountain Environments
In mountainous regions, the effect of altitude on the evolution of insect
morphology is similar to that of latitude. A number of insect traits vary with
increasing altitude (Mani 1968, Alexander and Hilliard 1968, Somme 1982).
First, the reduction in body size with increasing altitude is one of the most
striking forms of variation within and between species (Mani 1968). Second,
insects tend to exhibit increased aptery with increasing altitude. Uvarov (1977)
suggested that wing reduction in alpine grasshoppers was an important adaptation
to the alpine environment arising from the selection pressure of the climate.
Uvarov felt that given the short growing season, it is more important to put
more of the energy stores toward reproductive structures rather than wings.
5
Third, some insect species exhibit increased melanism with increased altitude.
Kingsolver (1983, 1985b) documented the importance of melanism in pierine
butterflies at high elevations and its relation to thermoregulation.
Fourth, insects exhibit increased cold-hardiness with increasing elevation
(e.g. Salt 1961, Duman et al. 1982, Somme 1982). Physiological cold-hardiness
seems to come from one of two methods (e.g. Salt 1961, Duman et al. 1982,
Somme 1982). Freezing-tolerant species are able to survive the formation of ice
crystals in their tissue. In contrast, freezing-susceptible insects rely on
supercooling to survive. In addition to these processes, insects inhabiting higher
elevations can perform their daily activities at temperatures lower than those of
their lowland counterparts (Gillis and Smeigh 1985).
Lastly, due to cooler temperatures, the rate of development may be
retarded at higher altitudes. This may result in a need for two or more years to
complete the life cycle. It may also result in the compression of the length of
each instar or a reduced number of instars. For example, White (1978) found
that three species of grasshoppers in the Craigiebum Range of New Zealand had
two-year life cycles.
Distribution of Acrididae
Approximately 200 species of Acrididae have been identified in the
grasslands of the western United States, (Pfadt, 1968). Kemp et al. (1990a)
6
identified more than forty species inhabiting steppe and foothill regions in the
Gallatin Valley of Montana. Unfortunately, the economic impact of most of
these species is unknown, because they have not been studied in enough detail to
understand their true status as pests. However, two grasshopper species
frequently considered as pests of Montana agriculture include Melanoplus
sanguinipes (Fabricius) and Aulocara ellioti Thomas.
Alexander and Hilliard (1969), working in the Colorado Rocky Mountains,
cataloged 94 species of Orthoptera, including 73 species of Acrididae along an
altitudinal gradient from 1530 m to 4265 m. The gradient was divided into 1000
ft bands. Each band was placed in one of nine vegetation zones, based on
vegetation patterns and dominant plant species. (The results showed a clear
segregation of particular grasshopper species into distinct zones along the
gradient and a reduction in species numbers with increasing elevation. There
was also evidence that some species showed compressed life cycles during
development from nymph to adult and an expanded duration of the egg phase.
This study was descriptive in nature and did not quantitatively examine how
factors other than vegetative zones influenced distribution or phenology.
Scoggen and Brusven (1973), in a study to detect grasshopper species
association with plant communities, used several biotic and abiotic factors in a
cluster analysis. Their study included an altitudinal gradient of 2000 m. Their
findings showed that, out of 10 plant communities, five vegetation communities
7
were significant indicators of certain assemblages of grasshopper species.
However, their analysis was not based on quantitative data using grasshopper
abundance. Rather, they used the arbitrary terms "accidental","occasional", or
"common".
Evans (1987) examined how fire, topography, and vegetation influenced
grasshopper communities on tall grass prairie. The scale of study was defined
and smaller than most previous work (Alexander and Hilliard 1968; Scoggen and
Brusven 1973). Using detrended correspondence analysis (DCA), Evans showed
that grasshopper distribution was most strongly related to the frequency of bum
and the soil type. He suggested that this was probably due to the effect of these
factors on the local plant communities. However, Evans noted that his study
could not differentiate whether grasshopper assemblages were being influenced
by plant species or by the physical structure of the environment.
Work by Kemp et al. (1990a) revealed non-random distributions of
grasshopper species existed along a rangeland environmental gradient. This
gradient consisted of changes in elevation, moisture, and plant communities.
The vegetation communities analyzed were chosen based on the concept of
habitat type (Mueggler and Stewart 1980). These gradients appeared to play a
role in structuring grasshopper community composition. Grasshopper
community composition was found to vary both between habitat type patches and
within habitat type patches.
Significant associations were found between the
8
number of species as represented by subfamily within a given habitat type,
suggesting that, given information on patch resources, one could generally
predict species distribution. Yet it remains unknown how dynamic these
associations are in space and time.
Phenology of Acrididae
Knowledge of life cycle variation has both applied and theoretical
ramifications for ecologists and pest managers. Kemp (1991) discussed the
value of understanding phenological patterns for pest control purposes. First,
understanding how environmental gradients affect life history patterns can lead
to more cost efficient and timely pest control. Second, studying life history
variation, particularly within species, may illuminate factors that influence
adaptation and speciation.
Early studies of grasshopper development noted that low temperatures
accelerated the development of eggs (e.g. Uvarov 1931, Moore 1945, Parker
1930, Bodine 1928). These low temperatures were below what was considered
the lower threshold of development (sometimes below 0° c), but above a level
that was injurious. Researchers from this period also noted that these thresholds
seemed to vary with each life stage (Uvarov 1931).
This phenomena suggested
to Uvarov that the use of a mean daily temperature was inappropriate for
analyzing questions regarding development.
9
Few studies have examined intraspecific variation in development at
different altitudes. Handford (1961), found that mean inStars of Camnula
pellucida (Seudder) for a given date were lower at higher elevation sites in the
900 foot elevation gradient study.
Handford also calculated degree-days above
60° F from air temperatures recorded from a nearby airport. He chose 60° F
because the lower threshold for Camnula pellucida (Scudder) was unknown at
the time.
Dingle et al. (1990) studied how the life cycle of M. sanguinipes varied
with altitude in the Sierra Mountains of California. They sampled six
populations from sites at coastal elevations of 90 m to mountain elevations of
2700 m. They computed degree-days from air and soil temperatures using
weather stations on each site, with 15° C the lower developmental threshold
temperature.
Phenologies were calculated as percent instar composition by
date. At higher altitudes, they found later spring emergence and a more rapid
passage through the instars to maturity. They attributed these differences to the
length of the growing seasons based on soil temperatures. They believe that air
temperatures do not produce enough degree-days to complete nymphal
development. If, however, soil temperatures are used there is adequate heat
available for full grasshopper development to occur.
Using laboratory experiments, Dingle’s study also showed all populations
of M. sanguinipes to be univoltine, but populations varied in their intensity of
10
egg diapause and rates of growth. At high altitudes females laid diapausing
eggs. At mid-altitude females laid a mixture diapausing and nondiapausing eggs.
They believed these differences reflected genetic variation among populations.
Objectives of distribution and phenology studies
First, using the methods of Kemp et al. (1990a) as a model, I examined
local variation within mountain ranges by sampling mountain slopes of different
elevation and aspect to determine local grasshopper species variation. My null
hypothesis was that there was no variation in species assemblages between sites
of different elevation and vegetation. Second, I examined how grasshopper
species varied from steppe elevations to the highest elevations of the surrounding
mountain ranges. Third, I examined variation between the Bridger and Gallatin
mountain ranges to see how species varied between mountains.
The objectives of the development study were, first, to examine
intraspecific variation in phenology of grasshopper species from sites of varying
altitude and aspect. My null hypothesis was that there was no intraspecific
variation in grasshopper phenology between patches. Second, I wanted to use
the concept of degree-days to measure intraspecific and interspecific variation in
phenology. Third, I wanted to examine interspecific variation in grasshopper
phenology between species inhabiting local patches or having divergent ranges.
11
MATERIALS AND METHODS
Study Sites
The study area was located in the northern part of Gallatin County,
Montana (longitudes l l l o40’- lll° 4 1 ’ east-west, latitudes
46 °00’-45 °30’north-south).. Sites were chosen from four regional vegetation
zones (Price, 1981): steppe, montane, alpine, and the transition between steppe
and montane often known as the foothill zone. At each site an area of 50 m x
80 m was laid out. Within this area, 9 linear transects were constructed using
flags as markers. I chose two steppe sites from those sampled by Kemp et al.
1990a near Three Forks, Gallatin County, MT. Steppe sites were between 1400
m and 1500 m in elevation and were flat in their aspect. Two foothill sites
located on the western front of the Bridger and Gallatin ranges were sampled.
The first, located at the entrance of North Cottonwood Creek in the Bridger
range at an elevation of 11550 m (longitude 111°- east-west, latitude TM0OT
north-south). The second western foothill site was located at the entrance of
Hyalite Canyon near the Gallatin range at an elevation of approximately 1850m
(longitude I l l 0OI ’east-west, latitude 45°33’ north-south). A third site, located
on the eastern slopes of the Bridgers, at approximately 1950 m in the Brackett
Creek drainage was sampled (longitude 110°52’ east-west, latitude
45 °50’north-south). The montane and alpine sites were selected from the
12
Bridger Mountain Range and the Gallatin Mountain Range. In the Bridger range
two sites were selected for weekly sampling. On the west side of the range at
an elevation of 2270 m a west-facing meadow was chosen located between
Corbly Gulch and Limestone Canyon (Longitude 111°00’ east-west, latitude
45°52’ north-south. On the east side of the range, located near Fairy Lake a
mountain meadow that was relatively flat in aspect and approximately 2350 m in
elevation was selected (Longitude 111°02’ east-west, latitude 45°55’ east-west).
An east-facing montane site located in the Gallatin range was sampled. It is
located in the Lick Creek drainage at an elevation of 2275 m (longitude
111 °0Least-west, latitude 45°33’ north-south). Two Bridger alpine sites were
selected. The west-facing site is located on a western slope directly below
Sacagawea Peak at an elevation of 2950 m (longitude 111°02’ east-west, latitude
45°53’ north-south). The east-facing Bridger alpine site is located on the
north-east side of Hardscrabble Peak at approximately 2850 m (longitude
I l l 0OV east-west, latitude 45°55’ north-south). In the Gallatin range, an
east-facing alpine site was chosen on the slopes of Mt. Blackmore, at an
elevation of 3050 m (longitude 111°00’,latitude 45°32’).
Sampling Techniques
Grasshoppers
Sampling methodology consisted of two hundred sweeps each traversing
13
an arc of 180° with a 40 cm diameter sweep net, along permanent linear
transects. These 200 sweeps were divided into 50 sweeps along four transects
randomly chosen each sampling period. In 1991, the sampling period was
between 0900 h and 1600 h. In 1992, the sampling period was shortened
compressed to three hours from 1200 h to 1500 h. The period was shortened in
an attempt to increase relative abundance, particularly in the mountains.
Samples were collected from each site every 7-10 days. In 1991, the seasonal
sampling period ran from 15 June to 25 August. In 1992, the sampling period
ran from 5 May to 25 August. Samples were placed on ice until they could be
delivered to a laboratory freezer.
Plants
Vegetation was sampled from mid-June through August of 1992. Twenty
to thirty quadrants (20 cm x 50 cm) were sampled for cover (Daubenmire 1959)
and species type along a transect within the area of the grasshopper sampling
site. In addition to percent canopy coverage for each plant species, litter, and
bare ground coverage were estimated. The variation in the number of quadrants
sampled was due to traveling and weather constraints.
Temperature Data and Phenology
Temperature data was collected from stations located at six of the eleven
sites (see Table I for specific sites). The foothill station was located on the
14
I
western front of the Bridger range. The other stations that I installed consisted
of two on the west side of the Bridgers and the two on the east side of the
Gallatins located in the Hyalite region. Stations were erected in the central
portion of each site. A Omnidata two channel Li-cor device was used to take
hourly air (5cm) and subsurface (-5mm) temperatures.
Temperature data was reduced to daily maximums and minimums. From
this data, degree days were calculated from 5 May to dates on which only adults
were collected in samples. Degree days were calculated only for
sanguinipes
and Mi oregonensis. This choice was due to their abundance across sites and
thermoregulatory studies, that I carried out. The base temperature chosen for
calculating degree days for M=. sanguinipes was 17 0C (Kemp and Dennis 1991).
The base temperature chosen for Melanoplus oregonensis Thomas was 5 0C.
There is no available research data on M. oregonensis developmental thresholds.
I chose 5°C for two reasons, first, from thermoregulatory studies of Moregonensis. I found that alpine populations were able to walk and jump at
temperatures in this range. Second, I found nymphs in the alpine regions at
temperatures below 10°C.
Degree days were calculated beginning with the installation date of the
temperature recording devices. For the 1991 field season, installation occurred
after hatch particularly at the lower elevation sites. In 1992, all stations were
installed prior to hatch (i.e. approximately Julian date 125) (Kemp and Dennis
15
1991).
Four species were chosen for phonological analysis. These choices
reflected three factors: I) their collection from early nymph stages through
adulthood across two or more vegetation zones or at sites of different aspect, 2)
their occurrence in numbers high enough to analyze, and 3) my ability to make
accurate identifications of nymphs.
Identification of Acrididae
Most of the Acrididae found from these sites are well-documented species,
particularly those in the subfamilies Gomphocerinae, Acridinae, and Oedipodinae
(Otte 1981). Adults from the steppe and foothill regions were identified using
keys from Pfadt (1989), Capinera and Sechrist (1982), Hebard (1928), and
Brooks (1958). Nymphs from these regions were identified using Brusven
(1972), Scoggan and Brusven (1972), and Hebard (1928). Mountain species,
except for certain species of Melanoplus adults and nymphs, could often be
identified using the above keys. The subfamily Melanoplinae is a poorly
understood group taxonomically (Otte 1993). Few taxonomic keys even refer to
certain alpine Melanoplus. in particular M. oregonensis and Melanoplus
montanus (Thomas). To aid in the identification of these, Brooks(1913) and
Alexander (1948) were used extensively. Information on the nymphs of these
problematic species is even more difficult to find. Typically, adult male
16
characteristics, are well enough developed by the fourth instar to allow
identification. Identification of the early instars was possible by comparing them
with later instars and adults and male morphology was distinguishable often by
the fourth instar.
Data Analysis
Given that species along the gradient dropped out of samples with
changes in elevation and aspect, I used detrended correspondence analysis
(DCA) (ter Braak 1988).
DCA is an ordination technique that can summarize
count data and arrange it into two-dimensional space. Within these dimensions
most of the variation among sites will be accounted for. Increasing distance
between samples along the axis shows a decrease in similarity. The axis scores
were correlated to site variables using Spearmann rank correlation (rs).
17
RESULTS
Species Distribution
Grasshoppers
I collected twenty-five species of Acrididae from the eleven sites during
two field seasons (Table I). The number of species tended to decrease with
increasing elevation (rs =-0.72, P = 0.012)(Fig. I). The total number of
individuals collected also decreased with increasing altitude (rs =-0.68,P
=0.021, Fig. 2). The montane elevation sites (2250 - 2350 m), showed a slight
increase in species numbers and individuals, before decreasing again at the
alpine elevation sites.
I found an uneven distribution of grasshopper subfamilies along the
environmental gradient (Table I). The most obvious trend was the increase in
the percent of Melanoplinae (all Melanoplus spp.) with an increase in altitude:
steppe 31%, foothills 56%, montane 64%, and alpine 88% (chi-square 2x4
contingency table analysis, x2 = 6.99, P = 0.07). In fact, only I of 102
grasshoppers collected on the alpine sites was not a Melanoplus.
Melanoplus comprised a minority of species only on the steppe sites.
However, even here they made up the majority of grasshoppers collected,
because of the great abundance of Melanoplus infantilis Scudder and M.
sanpuinipes (74% at the 1400 m site and 69% at the 1500 m site). Other species
Table I. Grasshopper (Orthoptera: Acrididae) Species Collected. Number of Individuals Collected by Elevation(meters) and Aspect. Superscript key:
I - Only nymphs collected, 2 - Only adults collected, 3 - Both nymphs and adults collected. AsterisksQ indicates sites with temperature recording
stations.
Alpine
Montane
Foothills
Steppe
2850
2950* 3050*
2350*
1950
2250*
2275*
1400* 1500* 1800*. 1850
W
NE
E
SW
E
SE
SW
E
W
Flat
Flat
Oedipodlnae
Arphia pseudoneatoma1
Arphia conspersa2
Camnula pellucida2
Spharagemon collare1
Trachyrhachys kiowa3
T r i m e r o t r o p i s suffusa'
Xanthippus corallipes3
Subtotal
-
—
-
—
—
I
2
2
I
4
2
19
30
—
-
-
—
—
-
—
—
—
-
—
—
7
—
2
4
z.
2
3
6
—
—
—
-
24
>7
A
?
Gomphocerinae &
Acridinae
Aeropedellus clavatus3
Ageneotettix deorum3
Amphitornus coloradus3
Aulocara eliotti3
Chorthippus curtipennis3
Psoleossa delicatula3
Subtotal
Helanoplinae
Helanoplus alpinus2
Helanoplus bruneri3
Helanoplus bivittatus3
Helanoplus confusus2
Helanoplus dawsonii3
Helanoplus fasciatus3
Helanoplus huroni2
Helanoplus infantalis3
Helanoplus montanus2
Helanoplus oregonensis3
Helanoplus packardii3
Helanoplus sanguinipes3
Subtotal
11
98
20
111
5
—
5
25
54
44
48
2
-240
I
38
82
649
770
14
185
_
-
—
260
46
168
474
-
15
92
7
90
—
-
12
—
i
50
266
—
—
-
—
—
—
—
-
-
-
-
5
—
—
I
5
-
—
—
9
9
I
5
—
—
—
-
—
—
—
—
12-
W
„
_
.
—
—
-
—
-
—
—
—
—
—
—
—
—
—
2
2
17
25
70
103
—
—
—
17
25
—
70
103
-
-
-
3
—
-
-
21
—
I
I
I
“
I
—
—
2
5
—
—
—
—
—
—
—
—
—
—
20
-
9
3
14
12
—
7
24
11
—
—
42
13
11
—
—
-
—
—
—
5
8
2
75
I
44
-
89
—
88
25
—
—
—
3
—
—
188
.I
218
—
20
9
6
?
I
87
2
22
91
I S
26
*
12
11
cn
10
CD
U
CD
9
Ql
if)
O
8
7
6
CD
5
E
4
Z5
Z
3
2
__ I
1000
1500
2000
2500
Elevation (m )
Figure I.
Num be r of S p e c i e s a s a function of elevation.
3000
3500
\
C/)
O
3
"O
>
't5
C
~
I 100
1050
1000
950
900
850
800
750
700
650
600
r
-
-
« ISS:
S HS :
c
^
z
350 300 250 200
-
150 100
50 -
13
14
15
16
17
18
O
O
19
20
21
22
23
24
25
26
Elevation x 100 (m )
Figure 2.
Number of Individuals collected from each
sampling site as represented by elevation.
27
28
29
30
31
21
replaced these two as the most abundant Melanoplus in other zones. Melanoplus
bivitattus (Say) (35%) and Melanoplus dawsonii (Scudder) (28%) were most
abundant in the middle of the gradient (foothills and montane). Melanoplus
oregonensis predominated at highest altitudes (montane and alpine), where it
made up 91% of the grasshoppers collected.
The fact that nymphs of Melanoplus alpinus Scudder, Melanoplus bruneri
Scudder, M. bivittatus. and M. oregonensis were collected on the montane sites
suggests that adults of these species oviposit in this zone. Only M. oregonensis
was collected throughout its life cycle in the alpine zone.
The greatest diversity of non-melanoplines also occurred on the steppe
sites. I found adults of only one gomphocerine on the foothill and montane
sites. Although low in abundance at all sites, Camnula pellucida was the only
oedipodine collected in all four zones. In contrast, others occurred only on the
steppe sites and Trimerotropis suffusa was collected only in the montane zone.
No species of Gomphoceiinae were widely distributed across the gradient. The
most commonly collected species, Chorthippus curtipennis (Harris) was found at
five sites in two zones.
Comparisons between the two mountain ranges showed montane species
numbers ranging between 6 and 10. The west-facing montane site in the
Bridgers (2250 m) contained seven species. The east-facing montane Bridger
sites contained six species, while the east-facing montane Gallatin site produced
22
10 species. The Bridger alpine site (east-facing) contained only one species (M.
oregonensis), while the east-facing Gallatin alpine site yielded four species.
Melanoplus oregonensis also predominated at this site. Table 2 shows the first
collection dates of second instars (1992 data only) from sites of different aspects.
Chorthippus curtipennis. found predominantly in the montane zone, was
collected 9 days earlier (ID 181) at the east-facing Gallatin site and the
west-facing Bridger site than the east-facing Bridger site (JD190). This Bridger
site is approximately 100 meters higher than the other 2 sites. A similar
situation is seen for M. bivittatus in the montane zone. All collection dates were
within a four day range. M. oregonensis showed a greater range of variation
within vegetation zones. At montane elevations, the Bridget west-facing site
was the earliest (ID 145) while the Gallatin east-facing site was the latest
(JD158). In the alpine region, the Gallatin east-facing site was the earliest
(JD180), while the Bridget east-facing site was much later (JD210). The
Bridger east-facing site had large amounts of snow until early July.
Grasshopper DCA Results
The DCA on grasshopper study sites, showed that a high proportion of
variation in the samples is accounted for by the first DCA axis (Fig. 3).
Ninety-four percent of the variance in species data was accounted for in axis I.
Spearmann rank correlations showed that axis I scores were significantly
23
Table 2. Julian Dates for the first collection of 2nd instars from sites o f varying
aspect within a given vegetation zone. These dates are for 1992 data only. Dates
with (G) indicate a Gallatin Range site, all others are Bridger Range sites.
Vegetation Zone
C. c u r t i p e n n i s
M . bivittatus
M. o r e g o n e n s i s
Foothill
East West
Montane
East West
160
152
East
Alpine
West
190
181(G )
18 1
166
1 6 9 (G)
168
150
1 5 8 (G)
145
210
1 8 0 (G)
2.5
M%Sbm
2.0
2850 m
1.5
Axis
C\2
1600 m
2275 m
2250 m
1500 m
1.0
1800 m
0.5
2
1850 m
1950
0.0
—0.5 -------------- '---------------- L
-I
0
1
12350
m
j_________ i__________ i__________ i__________ i
2
3
4
5
6
Axis I
Fig. 3.
D e n t r e n d e d C o r r e s p o n d e n c e Analysis of 11 g r a s s h o p p e r sa m p li n g
s i t e s a c r o s s 4 v e g e t a t i o n zones.
a n d 0.19, r e s p e c t i v e l y .
E ig e n v a lu e s for x a n d y axes a r e 0.94
25
correlated with elevation, precipitation, proportion of grasses and forbs, and
total number of plant species (Table 3).
Axis 2 accounted for nineteen percent of the variance in the data (Fig. 3).
Axis 2 correlations were significantly correlated (P<0.05) with elevation,
precipitation, and total number of plant species.
Figure 4 shows the DCA site scores for the grasshopper species. The
species are distributed along the plot in a manner similar to the study sites.
Species that tend to be found in steppe regions are found toward the left portion
, of the plot, while foothill, montane, and alpine species are increasingly to the
right of the plot.
Plant Distribution
Table 4 lists the plant species used for DCA including their percent
canopy coverage by site. My studies of steppe vegetation showed that these
grassland sites were dominated by Stipa comata Trin. & Rupr. and Bouteloua
gracilis (H.B.K.) Lag. exSteud. These plant communities occur in the most arid
of the study sites, with annual precipitation of 20-35 cm. Other grasses such as
Agropyron spicatum (Pursh) Scribn., Agropyron smithii Rydb., Carex filifolia
Nutt., Koeleria cristata (L.) Pers., and Poa sanbergii Vasey were found in lesser
densities. Forb species associated with this grassland type include; Astragalus
spp.. Aster spp.. Liatris punctata (Hook.), Phlox hoodii Richards., and
26
T a b le 3 . C o r r e l a t i o n s co m p u te d f o r c o m p a r is o n s b e tw e e n DCA
a x e s s c o r e s and e n v ir o n m e n t a l v a r i a b l e s
C o r r e la tio n s
r/
P
DCA: G r a s s h o p p e r C o m m u n itie s A x is I s c o r e s
A x is 2 s c o r s e
DCA:
v s . e le v a tio n
v s . p r e c ip ita tio n
v s. asp ect
v s . p r o p o r tio n o f
g ra sses
v s . p r o p o r tio n o f
fo r b s
v s . T o ta l # o f
p la n t s p e c ie s
0.82
0.88
0.70
-0.76
0.002
0.000
0.163
0.006
-0.73
0.002
0.61
0.001
v s . e le v a tio n
v s . p r e c ip ita tio n
v s . asp ect
v s . T o ta l # o f
p la n t s p e c ie s
0.62
0.63
0.26
-0.47
0.038
0.035
0.432
0.021
v s . e le v a tio n
v s . p r e c ip ita tio n
vs. asp ect
v s . p r o p o r tio n o f
g ra sses
v s . p r o p o r tio n o f
fo r b s
vs. to ta l # of
p la n t s p e c ie s
0.01
0.01
0.25
-0.39
0.987
0.973
0.480
0.265
-0.37
0.289
0.62
0.060
v s . e le v a tio n
v s . p r e c ip ita tio n
vs. asp ect
vs. to ta l # of
p la n t s p e c ie s
0.73
0.71
0.67
-0.18
0.016
0.022
0.031
0.618
P l a n t C o m m u n itie s -
A x is I s c o r e s
A x is 2 s c o r e s
4
mm
2
mo
me
oec
CV
I
mp
CA)
X
igs
ms
op
mdmbi
mb
O
CC
-I
mf
ts
-2
I
-3
-2
-I
J___________ I
0
I
2
3
_L
4
to
mh
J __
5
j
6
Axis I
Fig. 4. Detrended C orrespondence Analysis of 25 grasshopper sp ecies for 11 sam pling sites
across 4 vegetation zones. Eigenvalues for x and y are 0.94 and 0.19, respectively. Key to
grasshopper species: K C - A r p h ia c o n s p e r s a , A D - A g e n e o te ttix d e o r u m , A e C - A e r o p e d e llu s c la v a tu s ,
A m C - A m p h ito m u s c o lo r a d u s , A P - A r p h ia p s e u d o n e to m e , C C -C h o r th ip p u s c u r tip e n n is , C P C a m n u la p e llu c id a , M A -M e la n o p L u s a lp in u s , M B -M e la n o p lu s b r u n e r i, M B i-M e la n o p lu s b iv itta tu s ,
M C -M e L a n o p lu s c o n f u s u s , M D -M e L a n o p lu s d a w s o n ii, MF—M e la n o p lu s f a s c i a t u s , M H - M e la n o p lu s
h u ro n i, M m - M e la n o p lu s m o n ta n u s , M o -M e la n o p lu s o r e g o n e n is is , M p - M e la n o p lu s p a c k a r d ii, M s M e la n o p lu s s a n g u in ip e s , P d - P s o l e o s s a d e lic u tu la , S c - S p h a r a g e m o n c o lla r e , T k - T r a c h y r h a c y s k io w a ,
T s - T r i m e r o t r o p i s s u f f u s a , X c - X a n t h i p p u s c o r a llip e s .
Table 4.
Plant species used for DCA, represented by percentage of canopy coverage.
S te p p e
1400
1500
F la t
F la t
F o o th ills
1800
1850
W
SW
2250
SW
M o n ta n e
2275
E
2350
SE
2850
NE
—
—
—
—
—
—
—
—
—
—
ii
-
8
—
—
11
4
—
3
5
2
6
I
5
G rasses
Stlpa comata
Bouteloua gracilis
Agropyron splcatum
Agropyron smithii
Carex filifolia
Koeleria cristata
Poa sanbergii
Poa pratensis
Festuca idahoensis
P o a spp.
Phleum alpinum
Danthonia intermedia
C a r e x spp.
Deschampsia caespitosa
21
12
4
I
5
4
—
-
21
6
18
—
19
10
I
—
—
—
-
4
13
3
—
I
I
4
29
3
5
2
I
2
—
3
I
7
26
I
4
I
2
I
—
2
—
16
I
2
4
—
10
3
2
2
—
5
7
A lp in e
2950
W
3050
E
—
—
—
—
—
—
14
3
5
2
11
15
11
4
3
I
—
5
13
22
to
OO
Forbs
A s t r a g a l u s spp.
A s t e r spp.
Liatris punctata
Lupinus sericeus
Phlox hoodii
Achillea millefolium
Artemisia ludoviciana
Balsamorhiza sagitatta
Guara coccinea
Geranium viscosissimum
Silene acaulis
Gentian parryi
Potentilla gracilis
T r i f o l i u m spp.
Helianthus rigidus
7
5
5
3
4
I
7
3
—
—
—
—
11
—
-
—
—
10
20
12
6
8
I
32
I
21
8
8
21
I
I
3
8
I
3
2
15
2
8
I
3
5
10
12
12
2
13
2
-
-
8
12
7
-
-
4
3
14
—
2
2
19
3
16
—
—
—
16
—
I7
—
—
—
—
—
-
10
I
—
—
—
—
-
I
13
5
—
17
3
3
—
—
—
—
-
2
3
—
—
I
2
-
—
—
—
5
2
7
6
—
7
I
4
2
—
29
Sphaeralcea coccinea (Pursh) Rydb.
In the foothill zone, the dominant grasses were Festuca idahoensis
(Elmer.) and Agropyron spicatum Rydb. (Mueggler and Stewart 1974). Other
important grasses in this community include Danthonia intermedia (Vasey),
Koeleria cristata (L.) Pers., Poa pratensis L., and Stipa comata. There is also a
greater representation of forbs, the dominant species being Achillea millefolium
L., Artemisia ludoviciana Nutt., Balsamorhiza sagitatta (Pursh) Nutt., Guara
coccinea Pursh., Liatris punctata Hook., and Lupinus sericeus Kell.
The vegetation found at the montane sites included Festuca idahoensis in
association with the grasses Ai spicatum. and Deschampsia caespitosa (L.)
Beauv, and the forbs Achillea millefolium. Lupinus sericeus Kell., Geranium
viscosissimum F. & M., Danthonia spp., and Poa spp.
Alpine vegetation typically consisted of Festuca idahoensis in association
with D .caespitosa. Silene acaulis L., and Gentian parryi. Other grass species
present included Phleum alpinum L., and Carex spp. Forbs included Potentilla
diversifola L. and Trifolium spp.
Plant DCA Results
Plant species scores, using DCA, for axis I accounted for seventy-three
percent of the data variance (Figure 5). Spearmann rank correlations showed
that axis I scores were not significantly correlated to any environmental
3 .0
r2350 m
2.5
2 .0
C\2
Axis
W
<
X
C
1.5
2850 m
• r—
1500 m
2250 m
1 .0
2275 m
1600 m
.
1850 m
1950 m
g
0.5
0 .0
—
1800 m
l
-0 .5
1
0
l
l
1
l
2
l
3
l
4
l
5
6
Axis I
Fig. 5.
D e t r e n d e d C o r r e s p o n d e n c e Analysis of 11 p l a n t c o m m u n i t i e s
a c r o s s 4 v e g e t a t i o n z o n es.
0.44, r e s p e c t iv e ly .
E ig e n v a lu e s for x a n d y axes a r e 0.73 a n d
31
variables (P<0.05) (Table 3). Axis 2 scores were significantly correlated with
elevation, precipitation, and aspect.
Grasshopper Phenology
I chose C. curtipennis. M. sanguinipes. M. bivittatus. and M. oregonensis
for phonological analysis. Their selection was based on three criteria. First,
their distribution needed was broad enough to allow for comparisons between
vegetation zones or multiple sites within a vegetation zone. Second, they were
abundant enough to study. Third, I was able to identify the species, particularly
when studying early instars. The determination that a species completed its
development at a site was based on the collection of that species throughout its
instar development period and as an adult. The seasonal phenologies were
computed from weekly sweep net samples and the mean instar was calculated for
a given collection date. It is not known if these populations are univoltine.
Melanoplus sanguinipes
Melanoplus sanguinipes was found to complete its development at the two
steppe sites and one of the foothill zone sites located on the western side of the
Bridger range (Figure 6). In 1991, this grasshopper was only collected from the
two steppe sites. I found no apparent differences in development between the
two rangeland sites in either year. In 1992, M. sanguinipes was collected from
the 1850 m foothill site as well. Nymphs appeared at the foothill site
7
1991
MEAN INSTAR
6
O
5
4
3
O = I 400m
□ = 1500m
2
O
I
I
225
0
150
200
175
1992
MEAN INSTAR
w
to
O =1 400m
JULIAN
F ig u re 6.
□
= I 500m
A
= I 800m
DATE
P l o t s of M. s a n g u in ip e s d e v e l o p m e n t as m e a n i n s t a r a g a i n s t Julian d a te .
33
approximately 15 days later than the rangeland sites, but adults appeared
approximately on the same dates for all sites. Spring emergence was
approximately 10 days earlier in 1992 for all sites.
Adult M- sanguinpes not only appeared later in 1991 than 1992 (Fig. 6), but
required a greater number of degree days to complete development (Fig. 7).
This may reflect the colder and wetter spring of 1991. Comparisons between
steppe and foothill sites for 1992 only (no nymphs of M- sanguinipes were
collected from the foothill site in 1991) show that M- sanguinipes at the foothill
site, completed development in fewer accumulated degree-days then on the
steppe sites.
Melanoplus bivittatus
M- bivittatus was found throughout its life cycle at all foothill and montane
sites. Figure 8 shows development for all sites. In 1991, M- bivittatus was
only collected in sweep net samples from one Bridger Mountains site, although I
observed the grasshopper at all Bridger mountain sites collected in 1991. In the
Bridger mountains in 1992, M- bivittatus was first collected at 1800 m on Julian
date 143. At the elevations 2250m and 2350 m, it was first collected on date
157. This time lag between elevations was observed throughout its development
too. A similar initial time lag was observed in the Gallatin Mountains between
sites 1850 m and 2275 m, but the differences disappeared later in the season.
□
6
I
INSTAR
□
O
V
□
"1>
ii ^
□
□
MEAN
-r<>
v
•
= Steppe (1991)
v = Steppe (1992)
□
= FoothiIIsO 992)
i_____________I_____________I_____________I_____________I____________ I____________ I____________ l _
0
100
200
300
400
500
600
700
_I
800
ACCUMULATED DEGREE DAYS
Fig. 7. Mean i n s t a r of M. s a n g u in ip e s p l o t t e d a g a i n s t a c c u m u l a t e d
d e g r e e - d a y s ( b a s e 17°C) fo r two p o p u l a t i o n s o v e r two field seaso n s.
Gallatin M o u n ta in s
B r i d g e r M o u n t a in s
7
1800m
V
= 2275m
1991
In s ta r
5
Mean
6
3
4
V
2
1
I -
0
0 -
7
INSTAR
5
MEAN
6
3
O
— 1800m
V
□
= 2250m
= 2350m
j
x
150
O
V
=
175
200
225
250
1850m
w
Ul
= 2275m
4
2
I
0
Julian
Da t e
Julian
Date
F i g u r e 8. P l o t s of M. b i v i t ta t u s d e v e l o p m e n t p r e s e n t e d as m e a n i n s t a r as a
as a f u n c t i o n of Ju lian d a te .
36
Chorthippus curtipennis
C. curtipennis was collected from all montane sites sampled in both
mountain ranges. In 1991, only two montane sites were sampled (Fig. 9), the
sites were similar in elevation, but the Bridger site was west-facing and the
Gallatin site was east-facing. Nymphal development was similar at the two sites
throughout the season with adults appearing between dates 220 and 230. In
1992, a lower montane site was sampled in the Bridger range (1950 m). This
site when compared to the 2250 m site shows that nymph development began
earlier and instars progressed faster than the higher site. However, adults from
both sites appeared at the same time. Comparisons of development between
years shows that nymphs appeared much earlier in 1992 than in 1991.
Melanoplus oregonensis
M- oregonensis completed its life cycle at all six montane or alpine sites.
In 1991, the difference between montane and alpine hatch dates was
approximately one week in both the Bridger and Gallatin mountainss (Fig. 10).
In 1992, there were three to five week differences between the montane and the
alpine hatch. There were no differences in the duration of instars between
montane and alpine elevations.
In the Bridgers, M- oregonensis on the montane sites appeared to reach
G alla tin M ountain s
Bridger M ountains
L,
CO
-t-J
W
6
5
a
4
C
cd
3
CU
V
2275 m
= 2250 m
Y
2
V
1
0
__ I___________ |
150
175
200
1950 m
2250 m
2350 m
225
250
= 2275 m
150
Julian
Date
1992
175
200
Julian
225
250
Date
F i g u r e 9. P l o t s of C h o r th ip p u s c u r tip e n n is d e v e l o p m e n t p r e s e n t e d a s m e a n i n s t a r
as a f u n c t i o n of Julian d a t e .
Gallatin Mountains
Bridger Mountains
7
7
Mean Instar
6
5
4
3
3
^
2
V
2
O =2250 m
V =2950 m
"V
I
__I___________ I
0
150
175
O = 2275 m
V = 30 5 0 m
I
---- 1--------- 1 0
225
200
250
Mean Instar
O
i
^
J u l i a n Date
O
□
V
A
=2250
=2350
=2930
=2850
m
m
m
m
T
V
V
O = 2275 m
V = 3050 m
J u lia n Date
F i g u r e 10. D e v e l o p m e n t o f M. o r e g o n e n s i s p r e s e n t e d a s m e a n i n s t a r byJu lian date.
39
equivalent stages of development with fewer accumulated degree-days than on
the alpine sites. (Fig. 11). However, the alpine populations reached the adult
stage about the same date as the montane sites in 1991, at both sites,
development took longer than in 1992. This may reflect the colder and wetter
spring of 1991.
Gallatin Mountains
Mean I n s t a r
Bridger Mountains
O =2250 m
V = 2950 ci
800
1000
O =2275 m
V =3050 m
1000
1200
Mean I n s t a r
V
V
1992
200
400
600
800
O =2250 m
O =2275 m
V = 2930 m
V =3050 m
1000 1200 1400
A c c u m u l a t e d Degree Days
200 400 600 800 I 0001 2001 4001 6001 800
A c c u m u l a t e d Degr ee Days
F ig u re 11. D e v e l o p m e n t of M. o r e g o n e n s is p r e s e n t e d as m e a n i n s t a r by a c c u m u l a t e d
d e g r e e —days.
41
DISCUSSION
Summary of Patterns of Grasshopper and Plant Distribution
My surveys revealed variation in grasshopper community composition across
four vegetation/elevation zones. DCA analysis on the grasshopper species
showed different species and subfamily representation within each vegetation
zone. Clustering of species occurred according to their representation in a given
vegetation zone. Several clusters are apparent: I) species found only at the
steppe sites including the Oedipodines Spharagemon collare (Scudder),
Trachyrhachys Mowa Thomas, Xanthippus corallipes Haldeman, and the
Gomphocerines Ageneotettix deorum (Scudder), Amphitomus coloradus
(Thomas), A. ellioti Thomas, Psoloessa delicutula Scudder, and the
Melanoplines Melanoplus infantalis Scudder and Melanoplus packardii Scudder
2) steppe/foothill species including Aeropedellus clavatus (Thomas) and M.
sanguinipes 3) foothill/montane species including C. curtipennis. M. bruneri.
M. bivittatus. and M. dawsonii 4) montane species including T. suffusa, M.
alpinus Scudder, Melanoplus fasciatus (F. Walker), and Melanoplus huronii
Blatchley 4) montane/alpine species including M. montanus and M.
oregonenis. Lastly, both M. oregonensis and C. pellucida Scudder ranged from
the foothills to the alpine region. C. pellucida was collected in small numbers (5
or less) from several sites within the three zones, while M. oregonensis was
42
collected in relatively large numbers from montane and alpine sites whereas in
the foothill region smaller numbers of individuals were collected only as adults.
These clusters support the results shown in Table I of increasing percentage of
melanoplinae presence with elevation. The greatest diversity of nonmelanoplinaes occurred in the steppe zone and C. curtipennis and M. bivitattus
were restricted to the upper foothill and montane zone.
The DCA on study sites showed non-random distribution patterns. For
each vegetation zone, except the montane, study sites clustered according to
their elevation, precipitation, and the number of plant species. Within zones, in
spite of variation in the plant communities, I observed similar species at each
study site.
Study sites within both mountain ranges showed more similarities than
differences. In the montane zone, there was an increase in plant richness
relative to other zones, a comparable number of species relative to the lower
zones, and population sizes that were comparable to lower vegetation zones. In
the alpine region, there seemed to be decreases in plant richness, species
numbers, and number of individuals. Comparisons between the two mountain
ranges failed to show distinct differences.
Environmental Correlates of Grasshopper Distribution
Grasshopper communities can be evaluated on many spatial scales. This
43
study has shown that the elevation, precipitation, and changes in plant species at
a landscape scale are correlated to changes in grasshopper communities. Several
studies have found positive correlations between vegetation patterns and
grasshopper communities (Scoggen and Brusven 1973, Otte 1976, Evans 1987,
Kemp 1990a). These studies are providing indirect evidence for the driving
force behind community structure. Joem (1986) summarized possible ecological
mechanisms responsible for grasshopper community structure. These
mechanisms included interspecific competition, species-specific host plant use,
and species-specific microhabitat use. Kemp (1992) discusses possible
facultative associations of grasshopper species based on available resources.
Grasshopper feeding patterns can help explain the patterns I observed along this
environmental gradient, regardless of how ephemeral they are. Melanoplines,
which are primarily forb-feeders with a wide diet breath, showed an increasing
presence with elevation. This coincides with an increase in forbs and species
richness from the steppe region to the montane.
Lawton et al. (1987) suggested that species richness declines with
elevation due to diminishing resources and increasingly unfavorable
environments. In reality, this picture is more complex than Lawton portrays. It
is true that the growing season shortens at higher elevations. However, as seen
in my study, precipitation and plant species richness often increase at montane
elevations (Price 1981). The most likely factors limiting habitat use are the
44
available heat energy, the genetics, and the thermoregulatory abilities of a
species.
Montane sites supported larger population sizes than did the foothill
sites. There also exists the occurrence of a slight mid-elevational rise in species
numbers in the montane zone due to the presence of C. curtipennis and M.
oregonensis (Table I.). There has been debate over mid-elevational peaks in
species richness (Janzen 1973, Lawton et al. 1987, McCoy 1990). When
comparing montane versus foothill sites (Table I), there is an increase in number
of species and in population sizes. This suggests an increase in habitat
complexity allowing for more species and larger population sizes. Furthermore,
I collected the following species in montane habitats, including C. pellucida. A.
clavatus. Melanoplus confusus (Scudder), Melanoplus fasciatus (F. Walker),
Melanoplus dodgei (Thomas), and M. montanus as nymphs from montane areas
not included in this study. This suggests that species numbers do not necessarily
decrease with elevation. Montane communities of grasshoppers vary in species
numbers and population size between patches.
McCoy (1990) suggested that latitude, sampling regime, and disturbance
produce different patterns of species richness along an elevational gradient.
McCoy claims that short-term sampling produces mid-elevational peaks due to a
more rapid species turnover rate at lower elevations. While he does not define
in precise terms the differences between short-term and long-term, it is apparent
from his work that short-term sampling is not continuous throughout the growing
45
season. I believe that my sampling would not be considered short-term within
each growing season and, therefore, I would not be missing species due to
turnover. Given the number of seasons sampled, I might be missing species
shifts due to bi-voltinism or transitions in and out of a given patch in different
years.
Another factor which merits discussion is sampling technique. For
example, given the low height of alpine vegetation, sweep net sampling may not
be the best method for assessing species types and numbers. Anecdotically, my
alpine sweep net counts never seemed to correspond to the number of
grasshoppers I observed.
McCoy’s reference to human disturbance may have some relevance to
grasshopper populations in southwest Montana. Mid-elevation peaks could be
caused by lower elevations being disturbed through agricultural practices and
development. This would be fairly easy to test given the lack of reduction I
observed in plant species with increasing elevation. If plant species stayed
constant across sites of different elevation, then one could test for other effects
such as disturbance. If disturbance does play a role in shifting species richness
it could explain why we see certain species such as M. bivittatus inhabiting
cropland borders and weedy areas yet find no significant populations of these
species in steppe patches. Disturbance might also explain the narrow bands of
distribution of certain species (e.g. M. dawsonii and C. curtipennis).
46
Alexander and Hilliard (1968) did not find mid-elevational peaks in their
study of grasshopper distributions in the Colorado Rockies. Their data showed a
steady decline in species number which they attributed to a shortening of the
growing season with increasing altitude. A noticeable difference between our
studies was their findings of much wider distributions for several species along
the gradient. Based on their work, I expected to find certain species to occur
across the entire gradient (e.g. A. clavatus. X. corallipes. and Mi sanguinipes).
While M. sanguinipes and C. pellucida were collected across all zones, they
were only found as adults above the foothill zone. In fact, I found no single
species residing across the gradient. Species that occur in Colorado as high
mountain species such as M. dawsonii were found in the foothill zone and into
the lower montane zone. Melanoplus sanguinipes which from Alexander and
Hilliard’s work occurred across their gradient as a resident species was relatively
restricted when considering the regional scale. At smaller scales, certain species
occurred in large numbers at several sites within certain zones (e.g. M.
sanguinipes. M. oregonensis. and C. curtipennis). Species that were
wide-ranging in Colorado, were found to be more restricted in their distribution.
These differences suggest that regional environmental factors between the
southern Rockies and the Northern Rockies tend to lower and narrow the
distribution of certain grasshopper species.
47
Summary of Patterns of Grasshopper Development
The four species chosen for the development study: M. sanguinipes. M.
bivittatus. C. curtipennis. and M. oregonensis were concentrated in different
vegetation zones along the environmental gradient. M. sanguinipes primarily
inhabited the steppe study sites. Nymphs from the steppe sites began to emerge
in May and develop over the course of the summer, with adults appearing from
mid-July on. There was no strong divergence between the two steppe sites. In
1992, M. sanguinipes was collected from the 1850 m foothill site. Development
at this site was compressed compared to the steppe sites with adults appearing at
approximately the same dates for all sites (Fig. 6).
Melanoplus bivittatus. an inhabitant of foothill and montane sites, showed
some divergence in development, particularly in the Bridger mountains (Fig. 8).
Nymphs followed a pattern of later emergence dates with increasing elevation.
Foothill nymphs appeared first, then lower montane nymphs and finally upper
montane nymphs. This pattern continued for their entire development cycle.
Chorthippus curtipennis appears to be primarily a montane species. In
1991, nymphs appeared later in the growing season (Fig. 9). In 1992, nymphs
appeared up to two weeks earlier yet finished development at approximately the
same time as the 1991 populations. Chorthippus curtipennis was collected at
multiple sites only in the Bridger mountains during 1992. Nymphs appeared
earliest at the 1950 m site. Montane nymphs were collected approximately I
48
week later. The appearance of adults was not significantly later in time at
higher elevations.
Melanoplus oregonensis was collected primarily from montane and alpine
sites. Again as with previous species, nymphs first collection date is later with
increasing elevation. Using 1992 to illustrate differences in development, there
was strong divergence of first collection dates between the montane and alpine
sites, particularly in the Bridger mountains. This variance decreased as the
populations moved to adulthood suggesting that the alpine populations were
moving through their development more quickly. Bridger populations utilized
equivalent amounts of heat for both field seasons (Fig. 11). In the Gallatin
mountains, the patterns are not as clear. In 1991, due to missing most of the
first 2 instars there is not a complete picture for the 2275 m site. The alpine
population on Mt. Blackmore used nearly 600 degree-days from instar 3.5 to
adulthood in 1991. In 1992, the Gallatin montane population used 1500
degree-days to complete their development. The alpine population again used
nearly 600 degree-days for their entire development. This coincides with the
length of winter conditions that persisted on Mt. Blackmore well into June of
1992.
Environmental Correlates of Grasshopper Development
The phenologies of the 4 species studied reflect differences in local
49
ambient conditions and length of the growing season. Populations appeared to
be univoltine, but showed differences in the rate of development. All species
demonstrated later development times and a compression of life cycles with
increasing elevation. For three species, M. bivittatus. C. curtipennis. and M.
oregonensis. slope aspect most likely affected development by influencing local
temperature regimes and vegetation patterns. Most notably was the east-facing
Bridger mountains alpine site where M. oregonensis was two weeks behind the
,
other two alpine sites (Table 3).
Species appear to be more restricted than the available habitat and
temperature regimes suggest. DCA scores did not correlate with air or ground
temperatures (Table 2). This is a surprising result since we often consider
temperatures to be a gauge of the heat energy in a patch and thus a major
influence on life in the patch. From my temperature data, mountain air
temperatures provide more available heat than ground temperatures during the
spring and early summer. This is the opposite of the lower elevation sites where
ground temperatures track air temperatures more evenly throughout the growing
season. As long as snow is covering the ground it would seem that ground level
heat is of little value to nymphs. Temperature profiles from sites in each zone
show growing seasons start later with increasing elevation, air temperatures are
similar to lower elevation sites and can be greater at montane elevations in July
and August, and ground temperatures reflect the snow melt patterns but catch up
50
to lower elevation sites. Dingle et al (1990), believed that soil temperatures
predict growing season length for M. sanguinipes along an altitudinal gradient.
More research is needed to explore this question. Having seen first instar
nymphs at alpine sites covered with large patches of snow and wet soil, I
question whether nymphs are using the substrate as their primary source of heat.
If hatching occurs before soils warm, nymphs may be using other microhabitats
that provide warmer air temperatures such as exposed rocks.
The phenologies for the melanoplines, to some extent, illustrate the
community organization at local patches. Joem (1986) discusses several
standard models of community organization including independent assortment,
interspecific competition, and compensatory mortality. My results show that
nymphal development overlaps between the melanoplines studied. In the
foothills, M. sanguinipes and M. bivittatus nymphs development times
overlapped. In the montane region, M. bivittatus and M. oregonensis
development overlapped.
Population sizes for these two species contrasted.
At the 2250 m montane site, M. bivittatus numbers were low relative to M.
oregonensis. while at the 2275 m montane site M. bivittatus numbers were
larger while M. oregonensis numbers went down. It is not possible for me to
make any conclusions about their food base, because I do not know what types
of plants each of these species was utilizing. Mortality could be a factor
influencing population sizes.
M. bivittatus tends to be the larger of the two
51
species and may be more visible to predators. Lastly, one or the other of these
species may be using a particular part of the vegetation structure that may affect
its capture during sweeping. The only non-melanopline, C. curtipennis.
appeared later in the growing season than the Melanoplines. It is a grass feeder
and may be timing its life cycle to the grasses. Also the fact that C. curtipennis
has four instars may allow it to be more flexible in areas with shortened growing
seasons.
These results present grasshopper community organization and phenology
for two growing seasons. During this time, shifts in species presence/absence
(ie. M- bivittatusl were observed at individual sites and may reflect the
ephemeral nature to certain grasshopper populations. Grasshoppers in mountain
environments are faced with conditions that in some cases reflect the limits of
their geographic range, particularly in the alpine regions. Conditions such as
frost after hatch, large variance in temperature, and short growing seasons are
all thought to increase mortality at lower elevations. In the mountains, these
conditions are typical throughout the summer months. It appears that the
summer season in the mountains of this region are long enough to support one
generation per year and that the compression of the life cycle enables
grasshoppers to populate the montane and alpine zones. Long-term studies are
required to assess the viability of these populations over time.
52
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