Behavioral Ecology Vol. 13 No. 4: 497-502
© 2002 International Society for Behavioral Ecology
Measuring the benefit of habitat selection
a Department of Life Sciences, Ben Gurion University, Beer Sheva, Israel b Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721-0088, USA
Address correspondence to Z. Abramsky. E-mail: zvika{at}bgumail.bgu.ac.il .
Received 25 January 2000; revised 1 August 2001; accepted 10 October 2001.
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ABSTRACT |
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We used a behavioral bioassay to estimate the advantages that two species of gerbils (Gerbillus allenbyi and G. pyramidum) experienced by preferring a semistabilized dune habitat over a stabilized sand habitat. We used the magnitude of foraging effort by the gerbils to signal the difference between the two habitats. When they were foraging as much in stabilized sand as in semistabilized dune, we inferred that these habitats were providing equivalent rewards. We performed a series of experiments in two 1-ha field enclosures, each containing similar proportions of stabilized sand and semistabilized dune. Each enclosure contained a population of only one of the species. By varying the amount of seeds added (either 0.5, 1, 2, or 3 g of seeds in 18 seed trays) to each habitat and monitoring the behavior of the gerbils, we were able to fit a curve that reflected the change in habitat preference as a function of seed addition rate. We were also able to show how much seed addition had to be added to bring the two habitats into equal use. Each species required only 13 g/ha/night to entirely offset the advantage of the semistabilized dune.
Key words: behavioral bioassay, Gerbillus, gerbils, habitat selection, foraging activity.
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INTRODUCTION |
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Most animals exhibit a certain degree of habitat preference. Few organisms are generalists that can live equally well in different habitats. How large are the advantages that accrue from habitat preference? How many resources need to be added to a given area to change habitat preference? In this article we report the results of experiments to answer these questions for a pair of gerbil species in two habitat types.
An established body of behavioral ecology theory predicts how the
well-adapted forager should distribute its foraging among habitats (e.g.,
Morris, 1987
;
Rosenzweig, 1981). Models of
density-dependent habitat selection for a population of one species are most
relevant to the present work (Fretwell,
1972; Fretwell and Lucas,
1970; Morris, 1988; Rosenzweig
and Abramsky, 1985). These models show that foraging decisions
yield information on how an animal views its environment (e.g.,
Stephens and Krebs, 1986).
Thus, the gerbils can signal the difference between habitats by their foraging
behavior. When they forage as much in stabilized sand as in semistabilized
dune, one may assume that these habitats are providing equivalent rewards.
During the last 20 years, we have studied the ecology of the two most
common gerbils of Israeli desert sands (see
Rosenzweig and Abramsky, 1997,
for a review). Our work on habitat selection in gerbils forms the basis for
the present study. We have established that, in a system containing two sandy
habitat types (semistabilized dune and stabilized sand), both gerbil species
prefer the same habitat—the semistabilized dune—and that their
habitat preference depends on their intra- and interspecific densities.
Because the present study deals only with monospecific populations of the two
species, we will mention only the intraspecific preferences. For Gerbillus
allenbyi, the stabilized sand is a secondary habitat type. As its
population increases, this gerbil spends a greater proportion of its foraging
time in this secondary habitat (Abramsky et
al., 1990). For G. pyramidum, the stabilized sand is only
a tertiary habitat type. Nevertheless, as its population increases, this
gerbil also spends a greater proportion of its foraging time in this tertiary
habitat (Abramsky et al., 1990;
Rosenzweig and Abramsky,
1986).
Seeds play a major role in the ecology of desert regions. They constitute
dormant, resistant life-history stages that maintain large populations of
annual plants for a long time (Brown and
Davidson, 1977). Seeds, with their high-energy content, are the
primary food resources (Bar et al.,
1984) for which many desert rodents compete
(Brown and Heske, 1990). Seeds
are the main diet item of gerbils during the summer months, when most of our
experiments were conducted (Bar et al.,
1984). Thus, we decided to use commercial millet seeds to alter
the foraging rewards of a habitat.
How much seed needs to be added to bring the two habitats to a state of equal reward? The amount of seed required to achieve habitat equality provides a convenient measure of the advantage of the semistabilized dune habitat. The energy the seed contains is a measure of the difference between the habitats to the fitness of the gerbils. We performed a series of experiments in a set of large field enclosures. To each enclosure, we added seeds to one of the two habitats. By varying the seed addition rate and monitoring the behavioral response of the gerbils, we were able to fit a curve that not only reflected the change in habitat preference as a function of seed addition rate but also showed how much seed was needed to bring the two habitats into equal use. It turned out that merely 13 g/ha/night was required to entirely offset the advantage of the semistabilized dune.
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METHODS |
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Study site
Holot Mashabim Nature Reserve (31°01' N, 34°45' E) is situated in the Halutza region 50 km south of Beer Sheva, Israel. Average annual precipitation is 108 mm. Rainfall comes in winter, and dew forms on approximately 250 nights per year.
Sandy areas at the study site comprise two habitat types based on mobility
of the sand and on the dominant perennial plant species
(Danin, 1978). The perennial
shrub Artemesia monosperma and dead remnants of the grass
Stipagrostis scoparia dominate dunes in the process of being
stabilized (semistabilized dunes). In this habitat type, perennial vegetation
cover is relatively sparse, open patches of sand are relatively common, and
some portions of the dunes are still mobile. A monosperma and another
shrub, Retama raetam, dominate long-stabilized dunes (stabilized
sand). Here, shrub cover is relatively dense, open patches are smaller, soil
crust is common, and none of the sand is mobile.
Enclosures
We conducted the experiment in two 1-ha enclosures measuring 100 m x
100 m. We made the fencing of 6-mm mesh hardware cloth, buried 40 cm below the
soil surface and extending 60 cm above the ground. Atop both sides of the
fence, we put a 15-cm wide ribbon of aluminum flashing to prevent rodents from
climbing over. Each 1-ha enclosure consisted of similar proportions of
semistabilized sand and stabilized sand habitats. In both 1-ha enclosures the
semistabilized sand consisted of 40% of continuous area. In addition to the
two enclosed plots, we also sampled the gerbils in two 1-ha-unfenced plots.
During the time of these experiments, densities in these two control plots
were 15 G. allenbyi and 3 G. pyramidum/ha.
Gerbils
For each experiment, we first removed all gerbils from the two enclosures
during 3-5 days. G. allenbyi (average mass 24 g) and G.
pyramidum (average mass 40 g) were specifically marked by clipping the
hind outside right toe (G. allenbyi) or outside left toe (G.
pyramidum). Toe clipping was conducted to be able to distinguish
species-specific tracks in the sand. Individuals were kept in the laboratory
for a maximum of 1 week.
Earlier studies (e.g., Abramsky et al.,
1990,
1991,
1994) showed that each of the
two gerbil species forage in the two 1-ha enclosures as if the enclosures are
identical. On 1 July, we simultaneously introduced 17 individuals of G.
allenbyi to one enclosure (enclosure 1) and 5 individuals of G.
pyramidum to the other enclosure (enclosure 2). We set the densities of
both species to be slightly higher then natural densities at the time of the
experiments. We did not change these densities during the course of the
experiments.
Before we began the experiments, we allowed the gerbils more than 1 month
to habituate to the enclosures. To avoid fence effects
(Krebs et al., 1969), no
gerbil was kept enclosed for more than 2.5 months. After the experiments,
portals in the fences were opened, allowing both species free access to both
enclosures and to the outside.
Protocol
In the two enclosures, we first measured the nocturnal activity of each
gerbil species during 1-5 August. These data represented the gerbils' activity
in the absence of seed augmentation. Then we conducted two experimental tests,
one 6-15 August and the second 16 August-4 September. In these two
experimental tests, seeds were added to either one of the habitats. We
estimated activity of the gerbils on all nights, using their tracks left in
the sand (see "Sand tracking and habitat preference," below).
In the first experimental test, we improved the stabilized sand habitat. We added either 0.5, 1, 2, or 3 g of millet seeds to 18 seed trays located in stabilized sand. Each seed addition rate was conducted twice (except for 0.5 g, which was conducted three times) in the stabilized sand of each enclosure. The order of the seed addition rates was randomly selected to be 3, 2, 1, 1, 0.5, 3, 2, 0.5, and 0.5 g seeds/tray.
During the second experimental test, we improved the semistabilized dune habitat. In each enclosure, we moved the 18 seed trays from stabilized sand to semistabilized dune. Then we repeated the protocol of the first experiment, except that the order of seed addition rates was 1, 3, 1, 2, 0.5, 0.5, 2, and 3 g seeds/tray.
On each experimental day we added the seeds and smoothed the 40 tracking plots (see below) before sunset. On any single night, we added only one portion of seeds in each enclosure's trays. Tracks left by the gerbils in each tracking plot were read 2 h after sunset.
Sand tracking and habitat preference
We measured foraging activity by counting gerbil tracks left in 40 0.4 m
x 0.4 m sand-tracking plots located at 20 stations. Ten stations were
located in semistabilized sand and 10 in stabilized sand. At each station, one
tracking plot was placed under a shrub and the second in the open.
We smoothed the tracking plots at sunset and read them 2 h later. The score
given to a tracking plot ranged from 1 to 4. It depended on the footprint
coverage: 0 = no tracks; 1 = 
track coverage; 2 = 
track
coverage; 3 = 
track coverage; 4 = full track coverage. We summed the
activity density scores of the 40 tracking plots of each enclosure to produce
a measure of foraging activity density in each plot. The sum of G.
allenbyi scores is AGA, the activity density of G. allenbyi (in
enclosure 1). Similarly, AGP is the activity density of G. pyramidum
(in enclosure 2).
We have been using this sand-tracking method successfully for the last 10
years (Abramsky and Pinshow,
1989; Abramsky et al.,
1990,
1991,
1992,
1994,
1996,
1997,
1998; Kotler et al.,
1991,
1993;
Mitchell et al., 1990;
Rosenzweig et al., 1997).
These studies and another (Ziv et al.,
1993) show that track coverage is highly and significantly
correlated with population size. They also show that our sampling method does
not interfere with natural rodent behavior. Finally, these tracks constitute
activity data that are more sensitive than raw population size in assessing
potential interactions between gerbils. Activity is related to resource
acquisition and therefore reflects competitive pressure and the degree of risk
(Werner, 1991). Through their
activity, animals can adaptively balance trade-offs between food and safety
(Lima and Dill, 1990;
Werner and Anholt, 1996).
Following Abramsky et al.
(1990), we measured each
night's habitat preference as the proportion of the gerbil activity on the
semistabilized sand habitat.
Time frame
Gerbil activity decreases significantly during hours with considerable
moonlight (Kotler, 1984).
Hence, we restricted our experiments to phases of the moon when there was
little or no moonlight. Experiments were conducted during August and
September. Earlier studies (Abramsky et
al., 1990) showed that habitat selection does not change during
this period.
Seed trays
We introduced 18 trays to each enclosure, first to the stabilized sand and
then (on 16 August) to the semistabilized dune habitat. Each seed tray was
circular with a 30 cm diam. The trays contained 2 l of sand. Trays were
regularly spaced in each habitat but were never closer than 1 m to a tracking
plot. The location of a tray was not changed during the experiment.
Before sunset, we introduced the predetermined amount of seeds to each tray and thoroughly mixed the seeds with 2 l of sand. On experimental nights, the gerbils found all trays and removed most of the seeds during the 2 h.
Statistical analysis
We used t test and regression analysis to test for the
significance of our results. We conducted the experiments in an unreplicated
pair of 1-ha enclosures. Each species' experiments were conducted in a 1-ha
enclosure. Because the experiments were not replicated in space, they might be
regarded as pseudoreplication. However, this kind of design is entirely
appropriate when working with systems, such as ours, where treatments are very
costly (Hurlbert, 1984). Also,
during the 2 months of the experiments, we first conducted the treatment in
the stabilized sand and only later conducted it in the semistabilized dune.
Our experience in working in this system for the last 13 years shows that
habitat selection does not change much during the summer months
(Abramsky et al., 1990,
Ziv et al., 1993). We assume
that this was also the case during the present study.
Although there was no spatial replication, there was replication of seed addition rates at different times. Finally, the reader will see that the results were quite similar for the two species. This suggests that our results did not depend very much (if at all) on spatial differences between the hectares.
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RESULTS |
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Confirmation of previous results (no seed augmentation)
Both gerbil species preferred the semistabilized dune habitat. The proportion of the tracks of G. allenbyi in the semistabilized habitat was 0.65 ± 0.04 (mean ± SD, n = 5). The proportion of the tracks of G. pyramidum in the semistabilized habitat was 0.85 ± 0.09. G. pyramidum's preference for the semistabilized sand was significantly higher (paired t = 6.37; p =.001, n = 5) than that of G. allenbyi. Similar results were reported by Abramsky et al. (1990
).
Current results
In response to seed addition to the stabilized sand habitat, each of the
two species increased its total activity. The increases were similar and
significant (Figure 1A).
However, in neither species' case did total activity respond to seed addition
in the semistabilized sand habitat (Figure
1B).
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) and G. pyramidum
(x) as a function of seed addition rates. (A) Seed addition to the
stabilized sand (n = 14). Total activity increased significantly in
these experiments. (B) Seed addition to the semistabilized dune (n =
13). In these experiments, there was no significant increase in total
activity.Underlying the patterns of Figure 1 lies a simple difference. When we added seeds to the stabilized sand, the gerbils did not significantly shift their activity away from the semistabilized dune habitat. Instead, they maintained it there while significantly increasing their activity on the stabilized sand (Figure 2A). But when we added seeds to the semistabilized dune, both gerbil species shifted their activity from stabilized sand to semistabilized dune (Figure 2B). Only their preference changed significantly.
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In Figure 2A, we also see the neutralization of the semistabilized habitat preference. When seeds were not added to the stabilized sand, this habitat was almost avoided. But with an addition of only 0.5 g/tray/night, the stable sand seemed just about as acceptable as the semistabilized. The polynomial regressions in the figure cross at about 0.7 g/tray/night, but such a precise value is more than our few experiments can offer. Nevertheless, the quantitative similarity of the results for the two species is apparent.
In Figure 3, we plotted the preferences of the two gerbil species as they responded to the seed additions in the two habitats. This affords us an alternative view of the data plotted in Figure 2. Seed addition to the semistabilized sand habitat significantly intensified the gerbils' preference for this habitat. On the other hand, adding seeds to the stabilized sand habitat reversed the habitat preference of both species. At an addition rate between 0.5 g/tray and 1 g/tray, the habitat preference was neutralized. At 2 g (or above), both G. pyramidum and G. allenbyi preferred foraging in the stabilized sand.
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Neither species used the bush microhabitat significantly more than the open. Proportional use of bush was 0.53 ± 0.07 (n = 22) for G. allenbyi and 0.51 ± 0.07 (n = 22) for G. pyramidum. The difference between the species is not significant (paired t test = 1.14, p =.27, n = 22). The proportional use of bush did not change as a function of seed addition to either species or habitat type (largest r =.31, p =.28, n = 14).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Organisms are exposed to different types of biotic interactions—predation, competition, and mutualism. Habitat selection is a reflection of the strength of these intra- and interspecific interactions plus the adaptation of a species to them. Animals choose habitats because they provide better cover or more food, or because they provide escape from some interspecific competition. In the present work, we eliminated the interspecific competition factor by using pure populations of each species, but lack of any interspecific competitors did not eliminate preference for the semistabilized habitat. Thus, it is clear that preference for the semistabilized habitat must involve something besides escape from some interspecific competition.
What is the advantage of the semistabilized habitat? Our experiments showed
that food supply must be involved. We did nothing to alter the cover afforded
by the two habitats. We just added seeds. If the semistabilized habitat were
better only because it provided better cover, then our seed additions would
not have altered gerbil foraging preferences at all. However, as we did
nothing to alter cover, we also cannot eliminate it as a factor in the
superiority of the semistabilized habitat. Gerbils might well prefer the
semistabilized sand habitat because it provides a combination of more food and
better cover. In fact, we suspect that it is the combination of the two
factors that make the semistabilized sand a better habitat
(Abramsky et al., 1990,
Ziv et al., 1995).
We measured the energetic value of habitat preference by measuring the
response of the gerbils to controlled amounts of food addition. The gerbils'
response is behavioral—they increased their activity in the habitats
with the seed addition. By so doing, they demonstrated the cost associated
with the process of habitat selection. In fact, one may conclude from their
responses that the difference between the two habitats was equivalent to about
1g/tray (18 g of seeds/ha.). Their responses parallel those made in response
to competition (Abramsky et al.,
2000) and to risk of predation
(Abramsky et al., 2002).
Others have studied adaptive foraging strategies by monitoring behavioral
decisions (e.g., Abrahams and Dill,
1989; Brown, 1988;
Holbrook and Schmitt, 1988;
Kotler and Blaustein, 1995;
Nonacs and Dill, 1990;
Todd and Cowie, 1990). These
studies measured the cost of predation risk, whereas ours measured the value
of habitat selection.
By adding seeds to the system, we might be changing the energy status of
individuals. However, energy status adds to the residual variance and is only
one of many sources of variation (such as travel cost). We cannot entirely
eliminate them because we conduct the experiments in the field. Nevertheless,
so far, we have been able to tease signal from noise, even when the signal was
quite weak (e.g., Abramsky et al.,
1990,
1991,
1994,
1997,
1998; Rosenzweig and Abramsky,
1985,
1986;
1997;
Rosenzweig et al., 1997).
In addition, we were adding relatively small amounts of seeds that hardly exceeded the gerbils' daily energetic requirements. Enclosure 1 contained 17 G allenbyi individuals, and enclosure 2 contained 5 G. pyramidum, each consuming about 3-4 g of seeds/day, respectively (Abramsky et al., personal observations). Hence, the entire population consumed 57-68 g/ha/day in enclosure 1 and 15-20 g/ha/day in enclosure 2. Thus, the amounts we added were mostly consumed within the same night we added them. Moreover, between August and September, we allowed a period of 12 days without any seed addition. Therefore, whatever seed reservoirs the gerbils did accumulate during August would have disappeared during the 12-day break between the 2 months of experimental tests, when seeds were not added.
We added less than 0.5 kg of seeds/ha during August and September. Data on
seed production and seed in the soil bank is missing, but one may obtain some
estimates from data obtained in other deserts. The amount we added is much
less than estimates of seed production or seed in the soil banks in deserts
(80-1480 kg/ha; Brown et al.,
1979) or the seeds found in the seed bank (380 kg/ha) or in the
seed rain (167 kg/ha) of the Mojave Desert
(Price and Joyner, 1997). It
is also less than the 192 kg/ha/year added by Thompson et al. (1991) while
working with granivores in the Chihuahua Desert. And it is much smaller than
the amounts reported for deserts by Childs and Goodall
(1973), Nelson and Chew
(1977), Parmenter and MacMahon
(1983), and Price and Reichman
(1987).
Our results indicate that the value of the better habitat is quite modest,
only about 0.7 g/tray/night. How does this amount compare to other results for
the same gerbil species? Using the same gerbil system and similar experimental
protocols, we have measured the energetic costs, in grams of millet seed
addition, of interspecific competition (from G. pyramidum on G.
allenbyi) and the risk of predation by trained barn owls. The cost for
interspecific competition is about 4 g/tray/night
(Abramsky et al., 2000), while
that for risk of predation is about 6-12 g/tray/night (Abramsky et al., 2001).
Thus, the value of habitat preference seems to be much lower than the cost of
interspecific competition and risk of predation. Viewed as a set, these
behavioral bioassays are getting close to reducing the various dynamic
influences on fitness to the common currency of energy.
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ACKNOWLEDGEMENTS |
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The research was supported by BSF grant number 97-000008. We thank Lora Herztog and Moshe Elbaz for assisting in the fieldwork.
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