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Geographic Variation in
Plant Species Richness:
Lessons from the Sonoran Desert, U.S.A. and
Mexico, and Northern Territory, Australia
Tony L. Burgess 1 , Julio L. Betancourt2 , and John R. Busby3
Abstract .. -Proposed plans for computerized networks of ecological
data will involve costly digitizing of existing biological inventories, such
as herbaria and museum collections. A potential use of such data is to
evaluate and monitor regional biodiversity. We examined variation in the
number of vascular plant species per 10 latitude x 10 longitude in digitized
inventories for the Sonoran Desert, U.S.A. and Mexico, and Northern
Territory, Australia. In both data bases, sampling intensity, rather than
environmental or biological factors, explains most of the geographic
variation in species richness. The peril to management is that an
apparent stability or even an increase in perceived species richness can
be created by manipulating the effort given to sampling. Attempts to
monitor species richness at landscape to subregional scales will be
affected by the quality of the appropriate data bases, and the ability to
remedy or correct for sampling bias.
toring and predicting how regional diversity
might be affected by land use and management .
A common concern about even the best of biological inventories is the degree to which
geographic variation in species richness results
from unevenness of effort across a region (Connor
and Simberloff 1976; Miller and Wiegert 1989) or
sampling intensity-i.e., the accumulation of new
species as the time spent collecting increases (Soberon and Llorente 1993). Issues such as sample
size and sampling effort are dealt with routinely
at the plot scale (e.g., Magurran 1988). However,
these biases are seldom considered in analyses of
species richness at landscape to continental scales,
except by paleobiologists sensitive to the fragmentary nature of the fossil record (Raup 1976;
Koch 1987). Ecologists working at these larger
scales often rely on multiple sources of information collected for various purposes and subject to
different biases.
Here, we evaluate the effect of sampling effort
on plant species richness at the one degree scale in
the Sonoran Desert, USA and Mexico, and Northern Territory, Australia. These are two of the
better digitized inventories of plant distribution
in the subtropics. We use this simple exercise to
INTRODUCTION
.
Temporal and spatial variation in diversity are
central themes in theoretical and applied ecology.
For example, forecasts for future extinctions may
be based on the mathematical relation between
numbers of species and geographic area, with
habitat loss used as a predictor for species loss.
Unfortunately, precise estimates of geographic
variation in species richness (the simplest measure of diversity), much less the relative
abundances of species, are unavailable for most
organisms, except for well-known guilds that contain a limited number of species (e.g., oaks in
northern Mexico). Even in areas such as the southwestern United States, long considered a haven
for naturalists, biological inventories are woefully
inadequate for evaluating species-area relations;
for relating geographic differences in species richness to historical or physical factors (e.g. climate,
soils, or topographic heterogeneity); or for moni1Desert laboratory, University of Arizona, Tucson.
20esert Laboratory, U.S. Geological Survey, Tucson.
3EnvironmentBI Resources Information Network (ERIN), Australian National Parks & Wildlife Service, Canberra.
84
illustrate how uneven sampling efforts contaminate available data bases and bias our perceptions
of regional diversity. These sampling problems
should be considered in plans to develop regional
and national networks for ecological data.
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METHODS
We chose to examine species richness for vascular plants in two relatively well-known
semi-arid regions, the Sonoran Desert, U.S. and
Mexico, and Northern Territory, Australia. Both
areas offered large digitized data bases, each consisting of 80,000-100,000 observations for
3,000-4,000 species distributed across 100-130 one
degree cells. Observations consist of either a
voucher specimen in a herbarium (Northern Territory), or both voucher specimens and field
sightings by a select group of botanists (Sonoran
Desert). We compiled the number of species (species richness) and the number of observations
(sampling intensity) in each 1 latitude x 1 longitude cell. This is a convenient sampling scale to
study the influence of climate, landscape attributes and historical processes on regional diversity
(Rickelefs 1987).
The Sonoran Desert analysis is based on an
electronic data base of vascular plant distribution
(95,000 observations) compiled from 30 years of
field logs and supplemented by voucher specimens deposited in regional herbaria (Hastings et
al. 1972; Turner et al. in press). We limited our
analysis to the 100 grid cells that had more than 15
observations in and around the Sonoran Desert
(fig. 1).
The goal of the Sonoran Desert inventory was
to extend the earlier biogeographic work of
Shreve and Wiggins (1964) in studying the relationships between climate and plant distributions.
J.R. Hastings, R.M. Turner, R. Warren and others
who compiled this Atlas traveled along roads in
Baja California and Sonora, Mexico, making lists
of perennial plants at 5-mile intervals, with supplementary observations as needed. The
distributional data were augmented by searching
collections in regional herbaria (Desert Botanical
Garden, San Diego Museum of Natural History,
Arizona State University, University of CaliforniaBerkeley, University of Texas, and University of
Arizona) and copying specimen label data for particular species of interest, so that their total ranges
could be represented. Additional data for California and Arizona were taken from site lists
gathered from a variety of sources, including the
0
SONORAN DESERT BOUNDARY
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NUMBER OF SPECI ES
,....
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Figure 1.-Map of Sonoran Desert, with number of species per one
degree cell.
Bureau of Land Management, The Nature Conservancy, doctoral dissertations, and helpful field
ecologists.
Various biases have influenced the Sonoran
Desert data. The focus was on perennial plants
and there are few observations of annuals and
short-lived perennials. A general lack of interest
in grass identification led to significant underrepresentation of grasses. Observations were
concentrated near roads, and many disjunct populations in rugged terrains probably were
overlooked. This bias has made it especially difficult to accurately represent the upper elevational
limits for most species. More importantly, the Atlas focused on desert elevations and ignores plant
distributions in the so-called "sky islands" of
southern Arizona and northern Mexico.
The central Australian data consist of an
electronic data base for 78,000 voucher specimens
in the Northern Territory Herbarium, Darwin,
which has been digitized into the Environmental
Resources
Information
Network
(ERIN),
Australian National Parks and Wildlife Service.
ERIN was established in 1989 to provide
geographically-related environmental information
of an extent, quality and availability required for
planning and decision making (Slater 1992;
85
Tindale-Biscoe 1992). We selected Northern
Territory because it is subtropical region of
comparable size (129 one degree cells) to the
Sonoran Desert with an available digitized
inventory of vascular plants.
To assess geographic sampling bias, we constructed one degree gridded maps of the Sonoran
Desert and Northern Territory showing variations
in the number of species and number of observations (figs. 1-4). In addition, we composed scatter
plots of number of species versus number of observations per one degree cell in each of the two
regions (figs. 5-6). Because most of the statistical
assumptions would have been violated, we did
not attempt to fit curves to these data to predict
the sample size needed to sample species richness
in the "average" grid cell. Although we do not
present them here, species accumulation curves
for individual cells might have been used to predict the total number of species expected in a
given cell (Soberon and Llorente 1993).
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RESULTS
Figure 2 shows that sampling was clearly not
consistent throughout the Sonoran Desert region.
The Sonora coast and the Vizcaino region of Baja
California are better represented than the Lower
Colorado Valley of southeastern California and
the northeastern edge of the Sonoran Desert in
Central Arizona. The maps of the number of plant
species (fig. 1) and observations in each grid cell
(fig. 2) indicate that the pattern of species richness
is strongly associated with the number of observations and with latitude. A plot of the number of
species versus the number of observations
showed that up to a certain threshold (ca. 1900
observations), the number of species observed in
the Sonoran Desert is closely related to the
number of observations (fig. 5). The large scatter
in number of species in grid cells on the higher
end of the curve suggests that, in the Sonoran Desert, 2000 observations may be an appropriate
sample size to estimate species richness. At this
sample size, there may be real rather than perceived differences in the number of species
between grid cells.
The maps of the Northern Territory (fig. 3 and
4) reflect relatively heavy sampling in the vicinities of Darwin and Alice Springs, with low
numbers of observations thoughout the remainder of the Territory. This confirms spatial analysis
of the Census of Australian Vascular Plants, which
showed a strong tendency for plant species rich-
t""
141
Figure 2.-Map of Sonoran Desert, with number of observations per
one degree cell.
ness to reflect the population density of plant collectors (Bullen 1991). Figure 6 shows an even
closer relationship between number of species
and number of observations in Northern Territory
than in the Sonoran Desert, even in grid cells with
more than 2000 observations. There are at least
two explanations: (1) sampling intensity affects inventories from voucher specimens in herbaria
more than databases that include additional
sources of information; (2) because of less topographic heterogeneity, there are a few abundant
species and many rare ones in Northern Territory,
requiring greater sampling than in the Sonoran
Desert, where many species occur in common
abundance.
DISCUSSION
~Iost ecologists have an intuitive grasp of diversity patterns that is actually supported by
gross comparison of comprehensive regional inventories. For instance, few of us would object to
Rzedowski's (1993) rough sketch of floristic richness in Mexico, which contrasts the impoverished
Yucatan' Platform with the rich band across the
Sierra Madre del Sur (Oaxaca) and Sierra Madre
86
de Chiapas. But geographic variations in floristic
richness may not always be this apparent, even at
this gross scale. For example, one might assume
that strong Mexican influence on both the Sonoran Desert and sky islands would contribute to
greater species richness in Arizona than in New
Mexico. In reality, there is very little difference
between the two floras. Arizona has 3,370 species
spread across 294,000 km2 (0.0115 species/km2)
(Kearney and Peebles 1951); New Mexico has
3,728 species spread across 314,260 km2 (0.0119
species/km2) (Martin and Huizona, high floen the
Sonoran oands is offset by low species richness on
the Colorado Plateau. New Mexico includes three
relatively rich floristic provinces with a number of
endemics: the northern Chihuahuan Desert, the
Southern Rockies, and the southern High Plains.
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Figure 4.-Map of Northern Territory, Australia, with number
of observations per one degree cell.
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As the scale of analysis is narrowed to a one
degree cell, or approximately 1/30 of either Arizona or New Mexico, there is an overlap in size
with natural landscapes (e.g., one of the sky islands), as well as typical land management units
(e.g., a district within a particular National Forest
or a BLM management area). At this scale, we can
no longer rely on comprehensive floras, but must
depend on inventories that, if available, are of
variable quality. One degree is also a geographical
scale where local and regional processes interact
to determine species richness (Ricklefs; 1987). The
analysis of patterns in these kinds of data bases
promises many insights into the relative importance of historical and environmental factors in
producing biodiversity, but the validity of conclusions will depend on how well the effects of
-
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Figure 3.-Map of Northern Territory, Australia, with number
of plant species per one degree cell.
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Figure 5.-Scatter plot showing the number of species as an
increasing functions of the number of observation~ In the
Sonoran Desert data base (y = 66.397 + 0.109x, r2 = 0.70).
Figure 6.-Scatter plot showing the number of species as an
increasing functions of the number of observations in the
Northern Territory data base (y 105.21 + 0.195x, ,2 0.85).
sampling bias can be estimated. Obviously, any
attempt to monitor species richness at the landscape scale will be affected by both the
availability and quality of the appropriate data
bases.
There appear to be four important corollaries
from our findings:
1" Our understanding of biotic diversity is obscured by inadequate sampling. This hinders the prediction of consequences
deriving from proposed management actions.
2. Not only does inadequate sampling underestimate biodiversity, but an apparent stability
or even an increase in perceived species
richness also can be created by manipulating the effort given to sampling.
3. Monitoring and evaluating diversity in species-rich, complex landscapes such exist in
the southwestern U.S. and northern Mexico
will require heroic efforts that will be hard
to fund on a long-term basis. Add to this
the political complexity of an international
boundary and several federal and state
agencies, and the task at hand seems even
more daunting. Even so, assembling and
maintaining such data sets will be critical to
the success of biodiversity and global
change research as well as to effective management.
It is unconventional to offer methodological
hindsight in a scientific paper, but in the context
of this conference we believe that sharing our experience in the assembling a data base for the
Sonoran Desert may help others avoid similar pitfalls. To effectively manage and maintain
biodiversity, we need a much better understanding of how the existing pattern of diversity
emerges from the behaviors of populations" With
an understanding of dynamics in patterned landscapes, the possible consequences of proposed
management actions will become clearer. Learning how to collect and analyze information most
effectively is a critical skill for organizations
charged with management responsibilities.
Inventories for Baja California and Sonora
were started before reliable maps were available.
Our location data for these areas were necessarily
limited to a precision of 0.1 degree of latitude and
longitude. Locality data on herbarium specimens
was often imprecise, hence much time was spent
resolving uncertain location descriptions, and trying to estimate elevation for geographic locations.
This level of resolution and uncertainty has constrained the utility of the Sonoran Desert database
for future research. The production of detailed
topographic maps for Mexico and the decreasing
cost of global position systems promise much
greater precision in location coordinates at a reduced cost per location, There is no longer any
excuse for professional collectors in the region to
have sloppy locality data"
Our understanding of basic autecology would
have been greatly expanded had we developed
and used a consistent, digitizable system to describe the landscape context where observations
=
4. As with many good investments, the payback
from thorough inventories will be slow at
first, and the full profit can only be realized
with a sustained commitment to providing
fundamental information,
88
=
and they now face the formidable task of building
data bases and information systems at a time of
dwindling resources.
Research scientists tend to focus on general
phenomena, and hence they are often less concerned with tracking local details and history.
Research institutes and laboratories may not be
the best nexuses for ecosystem monitoring. Local
naturalists tend to focus on details and natural
history, and may not perceive trends or patterns at
other scales. They may be unwilling or unable to
undertake analysis and evaluation of complex
data. Managers need resolution of local details
and history in a context of general phenomena
and large-scale trends. Hence the organizational
framework for monitoring and evaluation must
connect scientists, local naturalists and ecosystem
managers in a way that empowers and inspires
each group.
It is important for the credibility of management that managers are not solely responsible for
evaluating effects of their management. If an organization is to continue receiving a mandate for
ecosystem management, managers and evaluators
should be at least partly independent. Present
trends for downsizing indicate that our land and
resource management agencies will lack sufficient
resources for monitoring and analyzing biotic diversity at the levels desired. The mandate for
ecosystem management may impose an unsustainable burden on present institutional resources.
Of governmental bodies in the United States, the
newly created National Biological Survey seems
to be an appropriate context to build the appropriate environmental data sets and information
systems. However, an example of what could be
accolnplished in a relatively short time frame is
the five-year (1980-1994) effort to develop the Environmental Resource Information Network
(ERIN) in Australia.
were made. An early example is the British Ecological Survey habitat system designed for
punch-cards (Elton 1966). Descriptive coding systems for the following information would greatly
augment the utility of field observations:
1. Radiant loading, as estimated by slope and aspect.
2. Rainfall runoff or run-on, estimated by topographic position.
3, Vegetation structure. The strictly structural
categories used by Mueller-Dombois & Ellenberg (1974) would be adequate.
4. Associated plant species. If listing immediate
neighbors is too time-consuming, at least include a biogeographic context, perhaps using the biotic provinces system of Brown
and Lowe (1980; see also Brown 1982).
5. Geomorphic context. The categories developed by Peterson (1981) for the Basin and
Range physiographic province are appropriate for the southwestern U.S. and Mexico.
6. Lithological and pedological context. These
need not be excessively detailed. Simple
categories of limestone, granitic, etc. for
rock types and alluvium, calcrete, aeolian
sand, etc. for soil origin would serve.
7. Substrate surface characteristics, estimated by
texture categories that extends from boulder to clay size classes. Most of the critical
properties of soil for vegetation dynamics
in arid and semiarid systems can be discerned from the top 40 cm of the profile.
8. For species of interest, estimates of the population status in terms of local abundance
would be an important addition to a biodiversity database. This could take the form
of size distributions within a site, which
would be time-consuming to record, or simple densities estimated to the nearest power
of two (McAuliffe 1990). The presence and
abundance of seedlings should be estimated to evaluate whether or not populations are self-sustaining across the species'
range.
Finally, proposed plans to create clearinghouses for ecological data (Raven and Wilson
1992; Craft 1994; National Research Council 1993;
Stone 1994) will be compromised by several disturbing trends. Funding for museum, herbaria
and other "collecting" institutions is on the downswing, as is expertise in systematics and
identification (Systematics Agenda 2000, 1994).
Historically, federal agencies have not pooled resources to archive and digitize ecological data,
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