Competition and the Structure of Granivore

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Competition and the Structure
of Granivore Communities
Diane W. Davidson, James H. Brown, and Richard S. Inouye
Historically, studies of competitive in­
t e r a c t i o n s in e c o s y s t e m s h a v e b e e n
based on one of two approaches. A com­
parative approach has dominated inves­
tigations of competition between taxo n o m i c a l l y r e l a t e d s p e c i e s that s h a r e
similar morphological,
physiological,
and behavioral traits. Notwithstanding
their general likenesses, these species of­
ten differ from each other in one or a few
characteristics that reduce overlap in the
use of resources. Ecologists have inter­
preted the differences in the body sizes,
feeding structures, microhabitat special­
izations, and foraging behaviors of re­
lated and coexisting species as evidence
for t h e i m p o r t a n c e o f c o m p e t i t i o n in
structuring natural communities. The
theoretical underpinnings o f this com­
parative approach to the study of com­
petition predict "limiting similarities** in
characteristics relevant to allocation of
scarce resources (May and MacArthur
1972, and reviewed in Schoener 1974).
A second approach to studying com­
petition employs experimental manipula­
tions of resources and/or competitive re­
gimes. Because experimental methods
do not depend on similarities of form and
function between presumed competitors,
this approach is more general and has
proven especially useful in analyzing in­
teractions between phylogenetically un­
related species. Such interactions are
particularly common in space-limited
systems; for example, organisms as dis­
tant taxonomically as plants and sessile
a n i m a l s c o m p e t e for s p a c e o n r o c k y
ocean shores (Lubchenko and Menge
1978). Like competition for space, com­
petition for food resources is not limited
to closely related species. As investiga­
tions have expanded to include distantly
r e l a t e d t a x a , t h e i n a d e q u a c i e s o f the
Davidson is an assistant professor of biology, Uni­
versity of Utah, Salt Lake City, UT 84112; Brown is
a professor and Inouye is a graduate student, both in
the Department of Ecology and Evolutionary Biol­
ogy, University of Arizona, Tucson, AZ 85721. ©
1980 American Institute of Biological Sciences. All
rights reserved.
April 1980
comparative approach have b e c o m e in­
creasingly apparent, and experimenta­
tion has gradually assumed a more prom­
i n e n t role in c o m m u n i t y - e c o l o g i c a l
research.
Here w e trace the development of our
investigations of granivory in desert eco­
systems, illustrating the synthesis of the
comparative and experimental approach­
es and noting the essential contributions
of both. In the process, w e also call at­
tention to several major difficulties inher­
ent to experimentation on this scale and
describe some relatively unconventional
experiments designed to circumvent
some of these problems.
DESERT
GRANIVORES
A N D THEIR
RESOURCES
Associations of seed consumers in arid
habitats of the southwestern United
States have proven very useful systems
for analyzing competitive interactions in
e c o s y s t e m s . Local granivore assem­
blages frequently contain representa­
tives of several kinds of specialized seedeaters, including birds, rodents, ants,
and other insects. Our studies have em­
phasized ants and rodents, t w o groups of
seed harvesters that are abundant and
permanent residents of local habitats and
have relatively generalized diets. As
many as 14 c o m m o n species of seed-eat­
ing ants and rodents may coexist and
share seed resources in local habitats
(see Figure 1).
Seeds of both annual and perennial
p l a n t s are c o n s u m e d b y d e s e r t granivores, but those of annuals predominate
in the diets of ants and rodents. The
seeds of annuals may regularly consti­
tute more than 95% of the total standing
crop of seeds by number and more than
80% by b i o m a s s ( N e l s o n a n d C h e w
1977). Rapid, facultative development of
annuals in response to available moisture
effectively couples production of green
vegetation and seeds to the infrequent
and unpredictable precipitation that lim­
its productivity in arid regions. Geo­
graphic gradients in the magnitude and
predictability of precipitation are reflect­
ed in patterns of seed production. East­
ward and northward from the Mojave
Desert of southern California, both the
mean annual precipitation and the mean
amount of predictable precipitation in­
crease, and there are corresponding in­
creases in the productivity and standing
crop biomass of both vegetation and
seeds.
THE COMPARATIVE
APPROACH
The distribution and coexistence of
ants and rodents over the southwestern
deserts afforded an excellent opportunity
to gain rapid insight into processes deter­
mining the diversity and structure of
granivore communities. We began by
censusing rodents (Brown 1973, 1975)
and ants (Davidson 1977a, b, Brown and
Davidson 1977) over a gradient of precip­
itation and seed productivity.
Along a transect eastward from south­
ern California into southern Arizona and
southwestern N e w M e x i c o , patterns in
the species diversity of ants and rodents
are strikingly alike. Within each fauna,
species richness increases along the gra­
dient of resource productivity. The most
diverse local assemblages of granivores
occupy the extreme southeastern corner
of Arizona, where rainfall and seed pro­
duction are most bountiful and predict­
able. Estimates of ant and rodent numbers
and biomass also vary directly with pro­
ductivity. Within each of the t w o major
taxa, the number of c o m m o n species that
occur together in a local habitat is signifi­
cantly fewer (by approximately five spe­
cies for both rodents and ants) than the
number of species w h o s e geographic dis­
tributions make them potential colonists
of these sites. These patterns constituted
our first indirect evidence that coexis­
tence of granivores might be limited by
competition for food.
Further indication of the importance
of resource competition emerged from
233
analyses of spatial pattern in granivore
communities over the same environmen­
tal gradient. Ant species characterized
by small foraging territories and limited
worker mobility decrease in abundance
as mean annual precipitation and pro­
ductivity decline (Davidson 1977a). For
these stationary, ground-nesting species,
availability of seed resources depends on
access to space. Colonies with limited
foraging areas are probably unable to ac­
quire sufficient food for maintenance and
reproduction where seed densities are
very low. Within individual localities,
uniform intraspecific spacing of ant colo­
nies (Bernstein 1975, Holldobler 1976)
and of mounded burrow systems of some
rodents (Schroder and Geluso 1975) re­
flect the desirability of excluding conspecifics from the feeding grounds.
Indirect evidence for resource c o m ­
petition also c o m e s from the composi­
tion of local granivore a s s o c i a t i o n s .
Where food resources limit consumer
populations, differential use of resources
should be a requirement for coexistence.
Interspecific morphological and behav­
ioral specializations characterize co-oc­
curring species of both ants and rodents.
The most striking patterns involve inter­
specific differences in body sizes (see
Figure 1). Coexisting associations of har­
vester ants often differ conspicuously in
worker body size, although species of
similar body size can coexist if they dif­
fer in colony foraging m o d e (Davidson
1977a). Experiments and observations
suggest that column-foraging ants have
relatively p o p u l o u s c o l o n i e s that spe­
cialize temporally and spatially on ener­
getically rich c o n c e n t r a t i o n s of s e e d ,
whereas species w h o s e workers forage
solitarily gather primarily dispersed seeds
(Davidson 1977b).
Harvester species that are alike in
both worker body size and colony forag­
ing behavior tend not to coexist locally
within homogeneous habitats, e v e n at lo­
calities that fall within the distributional
ranges of both species. The interspecific
territorial defense that occurs between
s o m e such s p e c i e s pairs (Holldobler
1974, Whitford et al. 1976) suggests that
these species are strong competitors and
serve as ecological replacements for one
another (Davidson 1977a).
The importance of worker body size to
coexistence of harvester ants is perhaps
best exemplified by the pattern of size
specialization in relationship to faunistic
diversity (Davidson 1978a). Dominating
the depauperate communities of the rela­
tively arid and unproductive Mojave and
C o l o r a d o d e s e r t s is Veromessor
per-
234
Fig. 1 . Seed-eating ants and rodents inhabiting the Chiricahua Bajada study site near
Portal, Arizona. Within each group note the obvious body size differences among coexist­
ing species. From left to right, the ants are (top row): Aphaenogaster (Novomessor) cocfcerelli (I = individual forager); Pogonomyrmex rugosus (C = column forager); P. desertorum ( I ) ; a n d ( b o t t o m r o w ) : Pheidole desertorum
(I, worker and soldier castes);
Solenopsis xyloni (C, minor media, and major); Ph. xerophila (C, worker and soldier) and
Ph. sitarches (C, worker and soldier). Rodents in the leflhand column include only spe­
cialized seed-eaters. From top to bottom, these are Perognathus flavus, P. penicillatus,
Dipodomys merriami, D. ordi, and D. spectabilis. In the righthand column are included two
additional species whose more generalized diets also contain many seeds. From top to
bottom, these are Perognathus flavus, Reithrodontomys
megalotus, P. penicillatus,
Peromyscus maniculatus, Dipodomys merriami, D. ordi, and D. spectabilis.
gandei, whose colonies display a contin­
uous polymorphism in worker body size.
Where this species is the only c o m m o n
harvester ant, the polymorphism is most
extreme, and workers range from 3.5 to
8.5 mm in body length. The pattern of
w o r k e r - s i z e p o l y m o r p h i s m in V. pergandei responds in a remarkedly precise
way to changes in the competitive envi­
ronment (Davidson 1978a). In more pro­
ductive habitats at the eastern edge of its
distribution, this ant occurs with larger
and smaller ant species, and its colonies
have relatively monomorphic workers of
an intermediate body size. In the still
more diverse and species-rich Chihuahuan Desert of southeastern Arizona, V.
pergandei
is replaced by more special­
ized monomorphic species.
Patterns in the organization of commu-
Bioscience Vol. 30 No. 4
nities of harvester ants suggest that dif­
ferences in worker body sizes function­
ally differentiate the ecological roles of
coexisting species. T w o kinds of empiri­
cal data support this interpretation (Da­
v i d s o n 1977a, 1978a). In e x p e r i m e n t s
where ants were permitted to select from
four sizes of quality-standardized seeds,
worker size was strongly and positively
correlated with an index of seed size
both in interspecific comparisons and
within colonies of V. pergandei.
Similar
relationships hold for ants collecting na­
tive seeds.
Mechanistic upper limits and energetic
lower limits appear to determine the siz­
es of seeds that can be exploited profit­
ably by worker ants of a given body size
(Davidson 1978b). H o w e v e r , body size is
a complex character and may influence
ecological function in many w a y s . Re­
cent multivariate analyses relating har­
vester ant diets to the pattern of resource
a v a i l a b i l i t y s u g g e s t ( t h o u g h d o not
prove) that large body size also enhances
both worker mobility and the capacity
for ants to feed selectively on rare and
patchily distributed but energetically su­
perior resources (Davidson, unpublished
analysis).
Similarly, body size patterns provide
circumstantial evidence for competitive
organization of rodent communities. In
sand dune habitats spanning a latitudinal
gradient of increasing precipitation, pro­
ductivity, and species richness, the body
sizes of coexisting rodents differ more
than would be anticipated on the basis of
random co-occurrence of those species
w h o s e geographic ranges allow them
potential a c c e s s to these sites (Brown
1973). Structurally and functionally con­
vergent rodent communities characterize
geographically distinct deserts of similar
productivity (Brown 1975). Different
species with similar body size and locomotory specialization represent eco­
logical counterparts inhabiting geograph­
ically disjunct deserts.
Because rodents are much larger in
relationship to their resources than are
ants, simple differences in the sizes of
seeds foraged are less likely to provide a
f u n c t i o n a l b a s i s for b o d y s i z e dis­
placement. For seed types differing in
size and dispersion, the foraging effi­
ciencies of rodents are apt to be deter­
mined by a number of morphological and
behavioral characteristics that correlate
with body size. For example, the greater
energetic expenditures of larger bipedal
rodents such as kangaroo rats (genus
Dipodomys)
may constrain them to for­
age on large seeds or clumps of smaller
April 1980
seeds, whereas smaller scansorial ro­
dents can profitably use small scattered
seeds (Brown 1975, Brown and Lieberman 1 9 7 3 , H u t t o 1978, P r i c e 1978a,
Reichman and Oberstein 1977).
Tendencies for ecological separation
in foraging microhabitats are correlated
with locomotor specializations in desert
rodents. While quadrupedal pocket mice
(genus Perognathus)
tend to feed pre­
dominantly in or under vegetation, bi­
pedal kangaroo rats (genus
Dipodomys)
and kangaroo mice (Microdipodops)
are
w e l l a d a p t e d t o l o c a t e s e e d s in the
open. Their saltatorial locomotion per­
mits rapid coverage of the territory sepa­
rating resource patches and may also fa­
cilitate escape from predators (Brown
1975, Brown and Lieberman 1973, Lemen and Rosenzweig 1978, Price 1978b,
Rosenzweig 1973, Wondolleck 1978).
That such microhabitat separation
m a y b e m a i n t a i n e d in part b y inter­
specific competition is evidenced by
shifts in r e s o u r c e u t i l i z a t i o n a c c o m ­
panying changes in the competitive envi­
ronment. On isolated sand dune habitats
in the eastern Great Basin, where Dipo­
domys ordi occurs with a few other ro­
dent species, this species forages in a
wider range of microhabitats, broadens
the seed diet, and attains greater popu­
lation densities (Brown 1973) than where
it coexists with diverse species of ro­
dents on less isolated dunes.
1
Thus, studies based on the compara­
tive approach have revealed remarkably
similar patterns of community diversity
and structure in t w o major taxa of generalist granivores. We can infer that com­
petition has played a prominent role in
the assembly of local communities of
both harvester ants and seed-eating ro­
dents. Although circumstantial, the evi­
dence for competition is strong because
it has been derived from several inde­
pendent lines of argument. Patterns of
e c o l o g i c a l separation within the two
g r o u p s h a v e p r o b a b l y a r i s e n b y dis­
tinctive routes that reflect basic biologi­
cal differences between the t w o taxa.
Despite such differences, the form of re­
source allocation is similar within ant
and rodent communities, probably be­
cause of the similarities in the availabili­
ty of seed resources to both groups.
Extensive biological differences be­
tween ants and rodents do not exclude
the potential for competitive interactions
between them, but they do preclude our
'Unpublished data from J. H. Brown, coauthor, and
Eric Larsen, Department of Ecology and Evolution­
ary Biology, University of Arizona, Tucson.
use of the comparative approach to seek
indirect evidence of such competition.
Broad overlap in the sizes (Brown and
Davidson 1977) and species (Table 1) of
seeds used by the two taxa suggests the
potential for competition. But though
dietary overlap is a necessary condition
for c o m p e t i t i o n , it is not a sufficient
criterion.
Tentative biogeographic evidence also
hints at the possibility of competition
between harvester ants and seed-eating
rodents ( B r o w n and D a v i d s o n 1977).
Whereas both faunas b e c o m e increas­
ingly abundant and species-rich along a
longitudinal gradient of enhanced precip­
itation and production, harvester ants
decline in diversity and biomass over a
similar but latitudinal gradient. Cooler
mean temperatures at high latitudes in
the Great Basin Desert may confront
ectothermic ants with abbreviated forag­
ing seasons and actually reduce the level
of seed productivity effectively harvestable by them. Rodents appear to com­
pensate for the missing ants by becoming
e v e n more populous and diverse (Brown
and Davidson 1977). But the possibility
of niche expansion and density compen­
sation in the rodent fauna must be viewed
cautiously, since translation of precipita­
tion to productivity need not be identical
along the two environmental gradients
where the seasonality of precipitation is
notably different. Cross-community com­
parisons fail to provide really convincing
data on the extent and consequences of
competition between ants and rodents.
EXPERIMENTAL
APPROACHES
An early experiment was designed as a
simple and direct test of the hypothesis
that desert ants and rodents compete for
limiting seed resources. T o confirm this
hypothesis, it was necessary to demon­
strate that each major taxon grows more
abundant when the other group is re­
moved. Choosing a broad area of homo­
geneous Sonoran Desert scrub habitat,
we established two replicates each of four
experimental treatments (Brown and Da­
vidson 1977). From two circular plots 18
m in radius, w e excluded rodents alone
with hardwarecloth fencing, partially
buried to discourage tunneling from out­
side. By poisoning, w e eliminated ants
from two similar but unfenced plots. We
removed both ants and rodents from a
third set of exclosures, and left one final
pair of plots unmanipulated as controls.
After only two years, we observed re­
ciprocal density compensations in the
235
TABLE 1.
Items composing > 5 % of the diets of ant and rodent granivores at the Chiricahua Bajada study site.*
Rodentsf
Seeds
Amaranthus
palmeri
Astragalus sp.
Boerhaavia
coulteri
Bouteloua sp.
Chorizanthe sp.
Cryptantha sp.
Descurainia
pinnata
Ephedra trifurca
Eriogonum
abertianum
Eriogonum
trichopes
Euphorbia
micromera
Euphorbia
serpyllifolia
Kallstroemia
grandiflora
Larrea tridentata
Lepidium
medium
Panicum
arizonicum
Prosopis juliflora
Tidestromia
lanuginosa
Unidentified grass
Unidentified seed X
1
2
Antsf
3
4
5
6
7
8
+
9
10
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
*Based on seeds robbed from laden ants or found in rodent cheek pouches (or D.s. mounds) during spring, fall, and summer over the years 1 9 7 2 - 7 3 for rodents and
1 9 7 4 - 7 8 for ants. Because ant and rodent diets were sampled under different resource regimes, the overlap in 2 5 % of resource categories used by the two groups
probably represents an underestimate of actual overlap.
f K E Y : Rodents: 1 = Dipodomys spectabilis, 2 = Dipodomys merriami, 3 = Perognathus pennicillatus, 4 = Perognathus flavus. Ants: 5 = Aphaenogaster
cockereHi,
6 = Pogonomyrmex rugosus, 7 = Pogonomyrmex desertorum, 8 = Pheidole desertorum, 9 = Pheidole xerophila, 10 = Pheidole sitarches.
two faunas. In comparision with control had qualitatively different effects on the species may indirectly benefit ants by fa­
plots, ant colonies reached 7 1 % higher plant community (Inouye et al. 1980). cilitating growth and seed production by
densities on plots from which rodents The predominant ants on the plots were small-seeded plants. The reciprocal den­
had been excluded. Rodent populations small column-foraging species of Phei­ sity compensations suggest that, at least
increased 20% numerically and 29% in dole that specialized on the tiny but nu­ in the short term, competition for shared
biomass in the absence of ants. Simulta­ merically dominant seeds of Filago cali- resources is the dominant interaction.
neous monitoring of seed levels on one fornicus (Family Compositae). Predation E v e n if resources do not interact, the po­
of the r e p l i c a t e d s e t s of t r e a t m e n t s by the ant community as a whole may tential importance of indirect pathways
(Reichman 1980, Brown et al. 1979) sug­ commonly fall most heavily on abundant can complicate studies of competition in
gested that these population responses seed types (Davidson 1980), promoting c o m m u n i t i e s o f t h r e e or m o r e c o m ­
were mediated through exploitative re­ diversity within the surviving seed pool.
petitors (Levine 1976). Circumstantial
source competition. Seed abundance, as
In contrast, the rodents (principally evidence (Davidson 1980) indicates that
measured either by numbers or biomass, Dipodomys
rugosus, SL large and ag­
merriami, Perognathus
am- Pogonomyrmex
was reduced to approximately the same plus, and P. penicillatus)
foraged selec­ gressive harvester ant, may indirectly fa­
level on plots where either ants or ro­ tively on large-seeded species such as cilitate coexisting ant species by dif­
dents or both were present, while seeds Er odium cicutarium,
E. texanum,
and ferentially interfering with foraging and
accumulated to approximately four times Lotus humistratus,
an in­
that tend to dominate nest-founding by P. desertorum,
this level on the plot from which both annual plant biomass. With their greater termediate-sized ant w h o s e resource re­
faunas had been removed.
s t a r t i n g r e s e r v e s , t h e s e s p e c i e s m a y quirements strongly overlap those of both
Recent analyses of plant responses on g r o w r a p i d l y t o o u t c o m p e t e s m a l l e r larger and smaller species.
these experimental plots (Inouye et al. seeded plants. On plots containing ro­
To decipher further the mechanisms of
1980) h a v e d o c u m e n t e d c o m p e t i t i o n dents, differential harvesting of Erodium interaction among granivores and their
within the producer trophic level as well, and Lotus species permitted Euphorbia resources, we have repeated and ex­
and underscore the potential complexity polycarpa
to reach significantly higher panded our experiments in southeastern
of interactions in this sytem. Density-de­ densities. Presumably, the smaller seeds Arizona on the eastern bajada of the
pendent germination and/or mortality of this species (approximately .18 mg in Chiricahua M o u n t a i n s . H e r e , an e x ­
among annual plants is suggested by the contrast to 1.5 mg for the larger-seeded ceptionally rich association of granivores
fact that, while seed densities quadru­ species) were less attractive to rodent includes s e v e n c o m m o n harvester ants
pled in the absence of ant and rodent pre- granivores.
and five-to-seven relatively abundant
dation, plant densities merely doubled
Competition among producers greatly species of seed-eating rodents (see Fig­
(Brown et al. 1979). Watering and thin­ enhances the potential complexity of in­ ure 1). In so diverse an assemblage of
ning experiments undertaken on the same t e r a c t i o n s b e t w e e n a n t s and r o d e n t s . species, the number of potential direct
plots r e v e a l e d significant c o m p e t i t i v e Not only may these groups compete for and indirect interactions is very great,
inhibition of germination, growth rates, shared resources, but they may interact and firm hypotheses about particular in­
biomass, and fecundity in annual plants indirectly through their use of non-over­ terspecific interactions are difficult to
(Inouye et al. 1980).
lapping but competing resources. For ex­ pose and test.
Seed predation by ants and rodents ample, rodent predation on large-seeded
Our strategy has been instead to ma-
236
Bioscience Vol. 30 No. 4
nipulate broad parameters which prelim­
inary observations and experiments sug­
gest may have demonstrable effects on
community organization. In a number of
treatments, w e are supplementing natu­
ral resources with milo s e e d s , applied ei­
ther continuously (V12 of the total annual
subsidy supplied each month) or in a
single annual pulse coincident with sum­
mer seed set by ephemeral plants. Early
observations indicate interspecific dif­
ferences within each of the two major
taxa in the proficiencies with which spe­
cies use aggregated and dispersed re­
sources, and preliminary experiments
(Brown et al. 1975) suggest that ants and
rodents differ functionally with respect
to this same trait. Greater mobility and a
capacity to gather large quantities of
seed into external, fur-lined cheek
pouches enable heteromyid rodents to be
far more successful than ants at deplet­
ing dense concentrations of experimen­
tally supplied seed. Rodents tend to ig­
nore low-density seed subsidies, perhaps
because the higher metabolic costs of
endothermy prevent them from foraging
profitably at low seed densities. Ants are
apparently equally adept at using lowand high-density seed supplements. Both
enhanced searching efficiencies and re­
duced foraging costs, associated with eusociality and ectothermy, respectively,
probably contribute to the ability of ants
to f e e d e c o n o m i c a l l y f r o m d i s p e r s e d
seed distributions.
The demonstrated importance of size
differentiation in coexisting granivores
s u g g e s t s the p o s s i b i l i t y o f p e r t u r b i n g
community organization by altering the
spectrum of seed sizes. Among the newly
established continuous seed addition
plots, treatments are differentiated with
respect to the particle sizes (small, large,
and heterogeneous) of experimentally
supplied milo.
In addition to repeating our whole
faunal removal experiments, w e have es­
tablished replicated plots from which in­
dividual dominant ant or rodent species
have been eliminated. These latter ex­
periments should contribute to our un­
derstanding of e c o s y s t e m structure. Re­
cently, ecologists have hypothesized that
high diversity may be maintained in com­
plex e c o s y s t e m s if interactions center in
small associations of closely interacting
species that interact relatively weakly
with other such groups. Provided such
s u b s t r u c t u r e e x i s t s in o u r s y s t e m of
granivores and resources, we might an­
ticipate a priori that organization should
follow taxonomic lines. Single-species
removal experiments should lend insight
April 1980
to the problem of whether or not inter­
actions are significantly stronger within
than between major taxa.
Biogeographic comparisons of granivore communities hint at the possible
role that "guild structure" may play in
the maintenance of diversity. Over a lati­
tudinal transect where precipitation in­
creases but temperatures change very
little, greater diversity is achieved by al­
most parallel increases in species rich­
ness in ant and rodent faunas, rather
than by adding members of only one tax­
onomic association or the other (Brown
and Davidson 1977). A b o v e a certain
threshold of seed productivity, diversity
is enhanced by addition of a third (avian)
guild of granivores. The finding that die­
tary similarity among coexisting rodent
species e x c e e d s that between cooccurring rodents and ants (Davidson, unpub­
lished data) also lends support to the sug­
g e s t i o n of t a x o n o m i c a l l y based guild
structure.
T w o years following initiation of these
experiments, our results are still prelimi­
nary; only qualitative statements are jus­
tified at this time. First, granivore popu­
lations have responded more gradually
to our perturbations than in past experi­
ments. At this time, w e do not know
w h e t h e r the s y s t e m ' s r e s i s t a n c e to
change represents enhanced stability as­
sociated with a more complex and/or
c o e v o l v e d interaction structure or sim­
ply the obviation of competitive stress
by the recent bounty of natural seed pro­
duction. Competition need act only in­
termittently to be instrumental in regu­
lating c o m m u n i t y
structure,
and
prolonged maintenance of experimental
treatments may be necessary to demon­
strate its effects.
Considerable time may also be re­
quired before we can distinguish tempo­
rary from equilibrial responses to our
manipulations. The most immediate re­
sponses to seed additions and species re­
movals may occur in species that are op­
portunists rather than in the specialized
species that eventually will prove to be
the superior competitors for available re­
sources. Opportunism is promoted by
the ability to colonize through immigra­
tion and/or reproduction. Where kanga­
roo rat species (genus Dipodomys)
have
been excluded from experimental plots,
pocket mice (genus Perognathus)
have
quickly immigrated to take advantage of
surplus resources. Although harvester
ants (Pogonomyrmex
rugosus) have been
observed to capitalize on Erodium seeds
accumulating on rodent-free plots, this
expansion in resource use has not yet
b e e n translated through reproduction
into higher colony densities.
On Chiricahua Bajada plots where
ant densities have risen, Pheidole
desertorum has contributed most significantly
to the increase. This true opportunist can
colonize by overground colony move­
ment and grow from small single-colony
units to fill expansive nests with many
disjunct units. N e w l y established queens
of the species are similarly well-suited
for taking advantage of underutilized for­
aging territory; their first worker broods
are larger than those of coexisting ant
species, and worker development is sig­
nificantly more rapid.
Finally, short-term and long-term re­
sponses may differ for yet another rea­
son. Responses involving very indirect
routes of interaction may be expressed
much later than those mediated through
relatively short and direct pathways.
CONCLUSIONS
T w o fundamentally different method­
o l o g i e s h a v e m a d e u n i q u e and o f t e n
complementary contributions to our un­
derstanding of granivory in desert eco­
systems. Biogeographic comparisons of
species numbers and characteristics in
independently assembled communities
quickly provided an overview of the eco­
logical processes underlying community
organization. While lending useful per­
spective on interactions between closely
related groups of c o m p e t i t o r s , crosscommunity comparisons have proven
less fruitful in identifying patterns of as­
sociation between taxonomically distant
competitors. Experiments provide an al­
ternative approach that we have em­
ployed successfully to demonstrate com­
petitive interactions between ants and
r o d e n t s as w e l l a s a m o n g primary
producers.
At least two factors complicate the de­
s i g n i n g of a d d i t i o n a l e x p e r i m e n t s to
p h r a s e and t e s t e x p l i c i t h y p o t h e s e s
about interactions among these distantly
related taxa. It is unclear which of the
many differences between ant and rodent
granivores favor their coexistence in des­
ert e c o s y s t e m s . Functional differenti­
ation of the two groups may well depend
on numerous minor distinctions in the
use of seed resources. Secondly, poten­
tial interactions among granivores through
distinct but competing resources, as well
as through those that are shared, not
only complicate hypothesis-formulation
but may prolong the time period over
which the system responds to a given
perturbation.
237
Our ongoing experiments are uncon­
ventional in the sense that their immedi­
ate goal is description rather than hypoth­
esis testing. B y manipulating resource
parameters whose natural variation figures
p r o m i n e n t l y in r e s o u r c e s u b d i v i s i o n
among closely related competitors, w e
hope to alter community structure to a
degree that will enable us to recognize
major patterns in the organization of
consumer-resource interactions.
Brown, J. H. 1973. Species diversity of seedeating desert rodents in sand dune habitats.
Ecology 54: 775-787.
1975. Geographical ecology of desert
rodents. Pages 315-341 in M. L. Cody and
J. M. Diamond, eds. Ecology and Evolution
of Communities. Harvard University Press,
Cambridge.
Brown, J. H. and D. W. Davidson. 1977.
Competition between seed-eating rodents
and ants in desert ecosystems. Science 196:
880-882.
Brown, J. H., D. W. Davidson, and O. J.
Reichman. 1979. An experimental study of
competition between seed-eating desert ro­
ACKNOWLEDGMENTS
dents and ants. Am. Zool. 19: 1129-1143.
We thank the many colleagues, s t u ­ • Brown, J. H., J. J. Grover, D. W. Davidson,
and G. A. Lieberman. 1975. A preliminary
dents, and assistants w h o have contrib­
study of seed predation in desert and mon­
uted in major ways to our research on
tane habitats. Ecology 56: 987-992.
g r a n i v o r e s . S p e c i a l t h a n k s g o to A .
Brown, J. H., and G. A. Lieberman. 1973.
Brown, G. Byers, J. Munger, and K.
Resource utilization and coexistence of
Zemanek. D. Alstad provided helpful
seed-eating rodents in sand dune habitats.
criticism of an early draft of the manu­
Ecology 54: 788-797.
script. The investigations summarized
Davidson, D. W. 1977a. Species diversity and
here have been supported by the Desert
community organization in desert seed-eat­
Biome, US/IBP, and by grants from the
ing ants. Ecology 58: 711-724.
National Science Foundation.
1977b. Foraging ecology and commu­
nity organization in desert seed-eating ants.
Ecology 58: 725-737.
1978a. Size variability in the worker
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