Effects of Mule Deer and Bison on
Regeneration of Island Scrub Oak on Santa
Catalina Island, California1
Thad A. Manuwal2 and Rick A. Sweitzer2
Abstract
Islands commonly harbor unique species that are particularly susceptible to damage by
introduced organisms. Historically, no large ungulates existed on Santa Catalina Island, but
several were introduced, including mule deer (Odocoileus hemionus) and bison (Bos bison). A
recently completed study suggested bison may alter the structure of island scrub oak (Quercus
pacifica) woodlands but no data exists for assessing impacts of mule deer to endemic trees
and shrubs on the island. We are investigating this and other aspects of deer ecology on Santa
Catalina Island during a study from January 2005 to August 2007. Our research was designed
to estimate habitat use and diets, measure browse use of trees and shrubs, characterize current
regeneration of scrub oak woodlands, and identify factors impinging on oak seedling survival,
using data from multiple plots in which 1,600 total seedlings were planted in 2005. This
report focuses on scrub oak woodland regeneration and seedling mortality. Our results suggest
scrub oak seedling numbers are recovering in some areas of the island, likely related to recent
removal of feral pigs and feral goats. Relatively large seedlings remain uncommon,
potentially due to browsing by abundant mule deer. Nevertheless, physical damage to
seedlings by bison appears more important for reduced seedling survival than browsing by
mule deer.
Keywords: Bison, California, feral pigs, feral goats, Quercus pacifica, regeneration
Introduction
The introduction and spread of nonnative species is an important conservation
problem in general but especially on islands (Cree and others 1995, Savidge 1987),
which typically have higher proportions of endemic species than mainland
ecosystems and are more prone to invasion (Lodge 1993, Simberloff 1994). Large
ungulates introduced to islands can be highly irruptive (Andersen and Linnell 2000)
where they may cause important browse damage to endemic trees and shrubs that
lack adequate chemical and structural defenses (Bowen and Van Vuren 1997,
Husheer and others 2003).
Santa Catalina Island (hereafter Catalina Island), part of California’s Channel
Islands (a small group of islands off the coast of Southern California), has a long
history of introduced ungulates, including mule deer (Odocoileus hemionus). Catalina
Island was occupied by feral goats (Capra hircus) before mule deer were introduced
in the early 1930s (Coblentz 1977), whereas bison (Bos bison) and feral pigs (Sus
scrofa) were introduced around the same time as mule deer (Sweitzer and others
2005). By the 1980s, conservationists were more aware of the myriad ecological
problems with introduced species. In the 1990s, the Santa Catalina Island
1
An abbreviated version of this paper was presented at the Sixth California Oak Symposium: Today’s
Challenges, Tomorrow’s Opportunities, October 9-12, 2006, Rohnert Park, California.
2
Graduate student and Associate Professor, respectively, Department of Biology, University of North
Dakota, Grand Forks, ND 58202. e-mail: thad.manuwal@und.nodak.edu;
richard_sweitzer@und.nodak.edu.
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GENERAL TECHNICAL REPORT PSW-GTR-217
Conservancy (hereafter Conservancy), a non-profit organization that owns and
manages 88 percent of Catalina Island, implemented an active restoration program to
eradicate feral goats and feral pigs (Schuyler and others 2002). By early 2005, feral
goats were eradicated and nearly all feral pigs had been removed (Schuyler and
others 2002, Conservancy records). Recently completed research on the bison
population provided useful information on their ecological effects (Constible and
others 2005, Sweitzer and others 2003), thereby supporting a controversial decision
by the Conservancy to reduce but not entirely remove the bison herd. Mule deer
remain widespread and abundant at an unknown level, however, and very little is
known of their foraging behaviors and population ecology useful for management.
Catalina Island contains 6 island endemic and 15 Channel Islands endemic plant
species that may be in danger from impacts of introduced ungulates. Recent
observations of island scrub oak (Quercus pacifica), a Channel Islands endemic, have
indicated low regeneration rates and browse damage caused by mule deer. Due to the
ecological importance of oak species (Pavlik and others 1995) we designed a study to
investigate the impacts of introduced ungulates on oak seedling recruitment.
Specifically, our objectives in this study were to use seedling belt transects to
characterize the current regeneration potential of island scrub oak woodlands, and
experimentally assess factors contributing to island scrub oak seedling mortality. We
hypothesize that mule deer browsing is limiting regeneration of island scrub oak and
browsing activities will be an important factor of island scrub oak seedling mortality.
In addition, various environmental factors (vegetative height and protective cover)
should positively influence seedling survival by protecting the seedling from
ungulate disturbance.
Methods
Study Area
Catalina Island is a 194-km2 island located 40 km south of coastal Los Angeles, in
Los Angeles County. Elevation on the island ranges from sea level to 640 m, with a
topography dominated by a northwest-southeast mountain range containing a series
of lateral canyons (Schuyler and others 2002). The climate is Mediterranean with
relatively mild temperatures throughout the year and a long-term mean annual
precipitation of 290 mm, mostly occurring between November and April (Schoenherr
and others 1999). There are four common habitat types on the island: (1) coastal sage
scrub, characterized by coastal sage (Artemisia californica) and prickly pear cactus
(Opuntia littoralis); (2) grassland, dominated by exotic annual grasses and forbs,
such as wild oats (Avena fatua) and storksbill (Erodium spp.), interspersed with
native bunch grasses (Nasella spp.); (3) island chaparral, represented by evergreen
and drought-resistant shrubs and low trees such as island scrub oak and lemonade
berry (Rhus integrifolia); and (4) riparian habitats limited to a few permanent or
ephemeral streams in relatively deep canyons and marshy wetland areas that are
represented by cottonwood (Populus trichocarpa), willow (Salix spp.) various sedges
and rushes, and mule fat (Baccharis pilularis) (Knapp 2002).
Three cross-island fences were established on Catalina Island in the 1990s to
facilitate the eradication of feral pigs and goats (Schuyler and others 2002),
effectively dividing the island into four zones (fig. 1). These two feral ungulates were
then eradicated in each zone sequentially (zone one– 1998, zone two – 2000, zone
three – 2003, zone four – all goats and most pigs by January 2005). Because feral
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Effects of Mule Deer and Bison on Regeneration of Island Scrub Oak on Santa Catalina Island,
California
pigs are known to negatively impact tree seedling survival by rooting (Sweitzer and
Van Vuren 2002) and feral goats seriously damage trees and shrubs by browsing
(Coblentz 1977), we anticipated potential differences among zones one, two, and
three for the number and size of scrub oak seedlings. Further, because bison on
Catalina Island rarely ventured across the isthmus into zone one historically, and
were prevented from crossing into zone one by fence after the early 1990s (Sweitzer
and others 2005), any possible bison-related impacts to tree seedlings would be
focused in zones two and three.
Scrub Oak Regeneration
Feral ungulates may impact tree seedling survival in general and scrub oak
regeneration on Catalina Island in particular by rooting (feral pigs), browsing (mule
deer, feral goats), and trampling or wallowing (bison). Tree seedling “belt transects”
can be used to assess the impacts of foraging ungulates on seedling survival and
forest regeneration (Bruinderink and Hazebroek 1996, Sweitzer and Van Vuren
Figure 1—Map of Catalina Island illustrating locations of three cross-island fences,
established in the 1990s, which partitioned the island into four feral animal removal
zones. Inset in the upper right shows the location of Catalina Island in relation to
mainland southern California and several nearby Channel Islands.
2002), and we therefore enumerated and measured sizes of naturally growing tree
seedlings along multiple 4-m X 40-m belt transects in three different “zones” on
Catalina Island, demarcated by cross-island fences. To begin to understand the
potential impacts of mule deer and feral pigs and goats on tree seedling survival, we
used the Santa Catalina Island geographic information system (GIS) to randomly
identify starting positions for 5, 12, and 13 seedling belt transects in large blocks of
scrub oak woodlands in zone one, zone two, and zone three, respectively. Fewer
transects were done in zone one because the overall area of scrub oak woodland is
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GENERAL TECHNICAL REPORT PSW-GTR-217
less in this region of Catalina Island than in zones two and three (2839, 5192, and
6509 ha in zone one, two, and three; respectively).
Belt transects were assessed during July and August 2005. From starting
positions for each belt transect, we randomly selected an azimuth from 0 to 359° N
and laid out a 40-m line transect. All tree seedlings encountered within 2 m of either
side of the tape were identified and measured from ground level to the terminal bud.
A moveable 2x4 m rope frame was used to facilitate sampling seedlings along the 40
m transect line.
A one-way Analysis of Variance (ANOVA) was used to compare the mean
number of oak seedlings among the three zones (data on mean seedling
number/transect were transformed by natural log prior to analysis; Zar 1999). In a
second analysis of data from seedling transects, we calculated an index to
regeneration potential as the ratio of large scrub oak seedlings (>200 mm) to total
scrub oak seedlings for each belt transect (Sweitzer and Van Vuren 2002). A low
regeneration index suggests that relatively few tree seedlings survived to a relatively
large size in which they may be less prone to current and future mortality caused by
mule deer or bison activity. Regeneration index data were arcsine transformed to
meet assumptions of normality and evaluated by ANOVA.
Oak Seedling Mortality Factors
From January to March 2005, we planted 1,600 (1- to 3-year-old) greenhouse-grown
island scrub oak seedlings in sixteen 30-m by 30-m experimental seedling plots in
island scrub oak chaparral habitats (100 oak seedlings/plot). Prior to out-planting, all
seedlings were measured for total height and ranked by health status. Data on
seedling sizes were compiled and used to partition seedlings into four different size
classes (size class one <100mm, size class two- 101 to 200mm, size class three- 201
to 300mm, size class four >300mm). The distribution of tree seedlings among the
size classes constrained our design such that each seedling plot could include 33
seedlings within size class one, 34 seedlings within size class two, 20 seedlings
within size class three, and 13 seedlings within size class four. We used a
randomization procedure to identify and select 100 individual seedlings among the
four size classes for each of the 16 different experimental seedling plots.
The Santa Catalina Island Conservancy’s GIS database was used to identify
seedling plot locations, based on the goal of planting a minimum of three seedling
plots within each of the five focus areas (we planted an extra plot in the Twin Rocks
focus area, fig. 2). A 100-m by 100-m grid was placed over island scrub oak habitats
that fell within a 400-m buffer strip around drivable roads on the island (fig. 2),
rendering plot locations reasonably accessible for hand-carrying seedlings and
planting equipment to the area. We utilized a randomization method to select grid
cells for planting; the first three random grids in each focus area that encompassed a
30-m by 30-m area suitable for planting were used for seedling plots. Grid areas were
considered unsuitable for planting when soils were very rocky, overly steep, or not
scrub oak woodland.
Each 30-m by 30-m seedling plot (hereafter seedling plot) was subdivided into
100 3-m by 3-m cells for planting. Individual tree seedlings were planted in the
approximate center of each plot cell unless trees, rocks or other obstacles prevented
planting access to that area of the cell with a two person gas-powered soil auger. The
soil auger was fitted with a 20-cm diameter bit for excavating a 40- to 45-cm planting
532
Effects of Mule Deer and Bison on Regeneration of Island Scrub Oak on Santa Catalina Island,
California
hole. Augered planting holes were cleared of loose soil whereupon seedlings were
carefully extracted from their nursery tubes with root balls intact and placed into the
hole with stems at approximate ground level. Excavated soil was used to backfill
around each seedling and tamped firmly into place using a hand trowel.
Figure 2—Locations of sixteen experimental seedling plots (black
diamonds) established in five study areas (black ovals) of Catalina
Island, California. Shaded areas represent island scrub oak woodland
habitats.
Seedlings at two seedling plots were “watered in” with 7.5-9.5 l of water per seedling
slowly drained into the soil from plastic water containers placed on the edges of the
refilled holes. Watering in was not done at the other 14 seedling plots because soils
were very moist from winter rains that occurred before and during the planting
period.
We visited and evaluated all seedling plots approximately every six weeks from
March to August in 2005, and from January to July in 2006, for a total of eight
monitoring periods. During each plot visit and for each individual seedling, we
recorded data on seedling status (alive/dead), general health (poor, good, or very
good), seedling height (tallest apical bud standing in a natural position ± 2 mm),
evidence of deer browse of seedlings (any branchlets that were browsed were hand
clipped perpendicular to axial growth to preclude recounting on subsequent visits),
direct (hoof print on seedling, bison wallow) and indirect ungulate activity within 1m radius of each seedling, surrounding cover type (beneath tree canopy, open, within
cover such as woody debris or prickly pear cactus), and vegetative cover (height of
herbaceous vegetation immediately surrounding the seedling).
We used a logistic regression model to investigate factors contributing to
survival and/or mortality of island scrub oak seedlings in the seedling plots. Predictor
variables included in the model were browse disturbance [BR_DIST (expressed as
533
GENERAL TECHNICAL REPORT PSW-GTR-217
the number of times an individual seedling was browsed/the number of times that
seedling was recorded as alive)], ungulate disturbance [UNG_DIST (total number of
times an individual seedling was disturbed by ungulate activity with the exception of
browse/ total number of times seedling was alive)], other disturbances
[OTHER_DIST (number of times an individual seedling was disturbed by rodents,
insects or other/ number of times seedling was alive)], indirect disturbances
[IND_DIST(number of times ungulate activity was observed within 1-m radius of
seedling/ total number of times seedling was alive)], cover [COVER (whether or not
seedling was positioned in protective cover or not)], canopy position [CNPY
(whether or not seedling was positioned under canopy or not)], and vegetative cover
[VEG_COV (expressed as the mean of vegetation height minus seedling height over
all monitoring periods)]. All predictor variables that were ratios were transformed
using the arcsine transformation prior to analyses (Zar 1999). A backward selection
method was used to reduce the model, in which the least significant terms (highest Pvalues) were removed sequentially until all terms remaining in the model were
significant (α < 0.05). Logistic regression analyses were performed using statistical
software R (R Development Core Team 2005).
Results
Scrub Oak Regeneration
Scrub oak seedlings were significantly more numerous for belt transects conducted in
zone one of Catalina Island compared to zones two and three (table 1; F2,26 = 4.24, P
= 0.025). Based on analyses of the ratios of large seedlings to total seedlings,
however, there were proportionally similar numbers of large seedlings among the
three zones (fig. 3; F2,26 = 1.08, P = 0.35).
Oak Seedling Mortality Factors
Seventy-six percent of the 1,600 island scrub oak seedlings we planted in the
experimental seedling plots remained alive as of July 2006, and most (70 percent) are
in good to excellent condition. On average, 4.9% ± SE 0.57 of the 1,600 planted
seedlings experienced some type of disturbance between monitoring periods. Deer
browsing was by far the most common type of disturbance observed; an average of
3.7% ± SE 0.3 of the 1,600 planted seedlings had evidence of browsing during each
monitoring period (fig. 4). In general, the large majority of disturbance to planted oak
seedlings was by nonnative ungulates (fig. 4).
534
Effects of Mule Deer and Bison on Regeneration of Island Scrub Oak on Santa Catalina Island,
California
Table 1—Summary data for island scrub oak seedlings detected along seedling belt transects
for different areas of Catalina Island, California. Data are from July and August 2005.
Mean number
Seedling density
Total oak
Number
Total area
oak seedlings
Zone
2
(seedlings/m2)
seedlings
transects sampled (m )
(SE)
1
5
800
2312
462 (217.1)
2.89
2
11
1760
458
42 (21.3)
0.26
3
13
2080
665
51 (20.3)
0.32
Figure 3—Comparison of mean regeneration ratios (± 1 SE) for island scrub oak
seedlings based on seedling belt transects in different areas of Catalina Island in
2005.
Logistic regression analyses of data on scrub oak seedlings after eight monitoring
visits (17 to 18 months after seedlings were planted) identified ungulate disturbance,
protective cover, overhead canopy, and vegetative cover as important predictors of
survival (table 3). Our final reduced model was:
g ( x ) = 2.25 − 0.87 (UNG _ DIST ) − 1.48( IND _ DIST ) − 0.006 (VEG _ COV )
Seedlings that experienced ungulate disturbance were located in areas with
repeated ungulate activity, and those within herbaceous vegetation taller than the
seedling were less likely to survive than seedlings experiencing no ungulate
disturbance, those that were not in areas of ungulate use, and those that were taller
than the surrounding herbaceous layer (table 3). Seedlings that had some evidence of
browsing by mule deer did not experience lower probabilities of survival than nonbrowsed seedlings (table 2).
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GENERAL TECHNICAL REPORT PSW-GTR-217
Figure 4—Identified sources of disturbance (browsing, trampling, chewing, etc.)
for scrub oak seedlings planted among 16 experimental seedling plots on
Catalina Island, California.
Table 2—Summary of coefficient estimates and their associated statistics for the full logistic
regression model with response variable - oak seedling survival. See Methods for description
of variables.
Variable
CONSTANT
BR_DIST
UNG_DIST
OTHER_DIST
IND_DIST
COVER
CNPY
VEG_COV
536
Coefficient
2.267
0.501
-0.810
-0.791
-1.438
0.365
-0.198
-0.005
S.E.
0.160
0.365
0.382
1.085
0.217
0.237
0.141
0.001
z statistic
14.187
1.375
-2.117
-0.730
-6.626
1.540
-1.408
-8.621
p-value
< 0.0001
0.1692
0.0343
0.4655
< 0.0001
0.1236
0.1593
< 0.0001
Effects of Mule Deer and Bison on Regeneration of Island Scrub Oak on Santa Catalina Island,
California
Table 3—Summary of coefficient estimates and their associated statistics for the reduced
logistic regression model with response variable - oak seedling survival. See Methods for
description of variables.
Variable
CONSTANT
UNG_DIST
IND_DIST
VEG_COV
Coefficient
2.246
-0.874
-1.477
-0.006
S.E.
0.115
0.382
0.214
0.001
z statistic
19.605
-2.288
-6.899
-9.170
p-value
< 0.0001
0.0222
< 0.0001
< 0.0001
Discussion
Data from seedling belt transects suggest important variation in numbers of oak
seedlings growing in different areas of Catalina Island. Oak seedlings appeared over
10 times as abundant on the west end of the island (zone one, fig 1) compared to two
regions encompassing the central portion of the island (table 1, fig 1). Feral pigs and
feral goats were eradicated from the zone one area of Catalina Island nearly four
years and more than six years before these ungulates were eradicated from zone two
and zone three, respectively (Schuyler and others 2002). Bison have also been
excluded from zone one historically. Together these preliminary findings suggest
seedling numbers are recovering in zone one by release from rooting and other
activities of feral pigs and goats. The large number of seedlings found in zone one
may also reflect the absence of bison trampling activities in this area. Even though
seedlings were more abundant in zone one, there was no difference in the number of
large seedlings relative to total seedlings in this area compared to zones two and three
(fig. 3). It is possible that the persistent lack of large seedlings in the western region
of Catalina Island and in zones two and three is because of ongoing browsing by
mule deer, which are abundant in all three zones of the island.
Mean regeneration ratios in zone two exhibited a high level of variability
compared to regeneration ratios in zones one and three. Although we are currently
unable to offer an explanation for this pattern based on the activities of mule deer,
bison, or recently eradicated feral pigs and goats, we anticipate that an additional
series of seedling transects will aid in verifying that the pattern was not an artifact of
sampling effort.
Direct and indirect disturbance to tree seedlings in the form of trampling and
bison wallowing were more important predictors of island scrub oak seedling
mortality than browsing by mule deer. Several studies have addressed physical
disturbances to oak seedlings (Sweitzer and Van Vuren 2002) but few have
specifically addressed trampling activities by large ungulates. Similar to our results,
Coppedge and Shaw (1997) found that bison on the Tallgrass Prairie Preserve in
Oklahoma significantly impacted saplings and shrubs by horning and rubbing
activities. They further concluded that bison activity could have effectively limited
the historic distribution of woody vegetation in the Great Plains. Although our
seedlings were too small for rubbing or horning, the above studies support our
findings that disturbances by large ungulates can limit recruitment of trees and
shrubs.
When designing the study, we originally anticipated that vegetative cover would
obscure seedlings from foraging mule deer, thus positively influencing seedling
survival. However, the logistic regression model identified relatively tall vegetative
cover as a negative influence on seedling survival. These results are in accordance
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GENERAL TECHNICAL REPORT PSW-GTR-217
with observations in the field that a large proportion of seedlings growing in areas
dominated by tall and dense non-native grasses appeared less vigorous. Although our
study does not address the mechanisms by which this process occurs, it is possible
that herbaceous vegetation [mostly non-native grasses (Avena spp., Brachypodium
distachyon, Bromus spp.) in our study plots] reduces water, soil nutrients, and light
availability for the relatively small scrub oak seedlings. Interactions between oak
seedlings and exotic annual grasses were identified as limiting factors to seedling
survival in other studies. Danielsen and Halvorson (1991) found that valley oak
(Quercus lobata) seedling growth and survivorship were limited by exotic annual
grasses, and Gordon and Rice (2000) determined the importance of soil water
potential competition between blue oak (Quercus douglasii) seedlings and exotic
grasses. Our results suggest that island scrub oak seedlings may experience similar
detrimental effects from nonnative grass species.
Oak seedling survival can vary significantly with differences in annual
precipitation, with low seedling survival in drought years and high seedling survival
in wet years. Catalina Island’s annual precipitation for calendar year 2005 was
527.05 ± SE 23.84 mm, the second highest in recorded history. This is approximately
200 mm above long-term average precipitation and likely contributed to the high
survival of our planted seedlings. During periods of drought or more normal rainfall
conditions, seedling survival could be reduced from the 76 percent we report 1.5
years after planting. In addition, the wet conditions likely increased the amount of
available forage for mule deer, potentially decreasing browsing pressure on
seedlings. Browsing pressure from deer could become a more significant mortality
factor for island scrub oak seedlings in years with limited available forage. Continued
monitoring of these plots will allow a more comprehensive look at inter-annual
precipitation effects on island scrub oak seedling mortality.
The results we have presented from this study are important for identifying the
effects of introduced ungulates on scrub oak woodland habitats on Catalina Island.
The recent removal of feral pigs and feral goats may be contributing to establishment
of many oak seedlings on one part of the island (zone one), but relatively large
seedlings that may eventually recruit to canopy forming trees and thereby contribute
to the maintenance of scrub oak woodland habitat appear limited in number islandwide. Although browsing by mule deer does not appear to kill scrub oak seedlings
outright, it may prevent free-growing seedlings from becoming saplings of sufficient
size/height to escape herbivory by mule deer. Further, data from our experimental
seedling plots have revealed that trampling and wallowing by bison may be at least
partly responsible for the relatively low numbers of oak seedlings detected along
seedling belt transects in zones two and three compared to zone one. Recall that bison
have not historically ranged into the zone one area of Catalina Island during their 80plus year history on the island.
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