Competition between birds and mammals: A comparison of giving-up densities between crested larks and gerbils
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Evolutionary Ecology 1997, 11, 757±771 Competition between birds and mammals: A comparison of giving-up densities between crested larks and gerbils JOEL S. BROWN,* BURT P. KOTLER and WILLIAM A. MITCHELLz Department of Biological Sciences, University of Illinois at Chicago, 845 W. Taylor St., Chicago, IL 60607, USA and Mitrani Center for Desert Ecology, Blaustein Institute for Desert Research, Ben-Gurion University, Sede Boqer Campus, 84 993 Israel Summary We combined the concept of mechanisms of co-existence with the approach of giving-up densities to study inter-taxon competition between seed-eating birds and mammals. We measured feeding behaviour in food patches to de®ne and study the guild of seed-eating vertebrates occupying sandy habitats at Bir Asluj, Negev Desert, Israel. Despite a large number of putatively granivorous rodents and birds at the site, two gerbil species (Allenby's gerbil, Gerbillus allenbyi, and the greater Egyptian gerbil, G. pyramidum) dominated noc- turnal foraging, and a single bird species (crested lark, Galerida cristata) contributed all of the daytime foraging. We used giving-up densities to quantify foraging behaviour and foraging eciencies. A low giving- up density demonstrates the ability of a forager to pro®tably harvest food at low abundances and to pro®tably utilize the foraging opportunities left behind by the less ecient forager. Gerbils had lower giving-up densities in the bush than open microhabitat, and lower giving-up densities in the semi-stabilized than stabilized sand habitats. Crested larks showed the opposite: lower giving-up densities in the open than bush, and on the stabilized than semi-stabilized sand habitats. Despite these patterns, gerbils had substantially lower giving-up densities than crested larks in both microhabitats, all sand habitats, and during each month. Several mech- anisms may permit the crested lark to co-exist with the gerbils. Larks may be cream skimmers on the high spatial and temporal variability in seed abundances. Larks may rely on insects, fruit or smaller seeds. Or, larks may rely on adjacent rocky habitats. Keywords: foraging theory; Galerida cristata; Gerbillus allenbyi; Gerbillus pyramidum; giving-up density; granivory; habitat selection; inter-taxon competition; mechanism of co-existence; Negev Desert; patch use; predation risk; sand dunes Introduction MacArthur and Pianka (1966) envisioned foraging theory as a conceptual tool for understanding population and community level phenomena. The expectation that organisms should feed sensibly greatly restricts the set of observable behaviours (prey are unlikely to exhibit behaviours of nuz- zling up to their predators) and greatly increases the likelihood of successfully using feeding behaviours to investigate population interactions (Rosenzweig, 1981), population dynamics (Fryxell and Lundberg, 1994) and communities (Kotler and Brown, 1988; Werner, 1992). Com- bining optimal patch use behaviour from foraging theory (MacArthur and Pianka, 1966; Charnov, 1976) with mechanisms of co-existence from resource theory (Tilman, 1982) generates the giving- * Address all correspondence to J.S. Brown, Department of Biological Sciences, University of Illinois at Chicago, 845 W. Taylor St., Chicago, IL 60607, USA. z Present address: Department of Zoology, University of Wisconsin, Madison, WI 53706, USA. 0269-7653 Ó 1997 Chapman & Hall
758 Brown et al. up density (GUD) approach to studying feeding behaviours, predation risk, habitat selection and species co-existence (Brown, 1988, 1992). Conceptually, a forager should abandon a depletable food patch when the bene®t derived from its present harvest rate no longer exceeds the sum of metabolic, predation and missed opportunity costs of foraging. Empirically, measuring GUDs (the amount of food remaining in a patch when abandoned by a forager) from experimental food patches measures the forager's costs and bene®ts of feeding. At the community level, the GUD, as the density of food to which a forager can pro®tably exploit the patch, provides one measure of competitive ability (Tilman, 1982). When species co-existence involves the ability of feeding ani- mals to thoroughly exploit the resources of particular environments, the predictions of the asso- ciated mechanisms of co-existence can often be couched in terms of GUDs and other measures of activity and feeding behaviour. The giving-up density approach has been applied to examine the mechanisms of co-existence in desert rodents (Brown, 1989; Kotler et al., 1993a,b; Brown et al., 1994a; Hughes et al., 1994; Bouskila, 1995), squirrels (Smith, 1991; Jedlicka, 1993) and bark-feeding insectivorous birds (Morgan, 1994). In these examples, as in most studies of species co-existence, members of a feeding guild (Root, 1967) are close taxonomic relatives. This eases the task of studying the mechanisms that promote or inhibit co-existence. Close taxonomic relatives share most traits in common and dier in just a few salient features such as body size (in birds and mammals) or bill size (in birds). Close taxonomic relatives are generally amenable to the same sets of experimental procedures. And, close taxonomic relatives generally respond to experimentation on similar temporal and spatial scales. In reality, feeding guilds contain more distantly related taxa (e.g. Paine, 1966; Dayton, 1971; Colwell and Fuentes, 1975; Brown and Davidson, 1977; Lubchenco and Menges, 1978; Brown et al., 1979, 1981, 1986; Turkington and Harper, 1979; Inouye, 1981; Morse, 1981; Davidson et al., 1984; Thompson et al., 1991). For example, desert granivores (Brown et al., 1979, 1986) include mostly birds, rodents and ants. Any of these taxa can consume nearly all of the annual productivity of seeds (see Price and Jenkins, 1986, and references therein). In an eort to deduce mechanisms of species co-existence, we applied the giving-up density approach to measure habitat selection and foraging eciencies of seed-eating birds and mammals inhabiting sand dunes of the Negev Desert, Israel. We de®ned guild composition as those species feeding from our food patches. Despite a high diversity of putatively granivorous birds in the vicinity (see Table 1), only crested larks (Galerida cristata) fed from our food patches. From among ®ve rodent species, two gerbils, Allenby's gerbil (Gerbillus allenbyi) and the greater Egyptian gerbil (G. pyramidum), were responsible for most (> 95%) of the rodent GUDs in our patches (Brown et al., 1994a). Competition between the diurnal larks and the nocturnal gerbils occurs only through the joint exploitation of seeds. Co-existence between gerbils and larks can occur if: (1) each species possesses a habitat in time or space in which it has the lowest GUD; (2) each species possesses a class of foods on which it has the lowest GUD; or (3) the species with the higher GUD has a compensatory competitive advantage in terms of priority at food patches (Kotler et al., 1993b), lower travel costs (Brown, 1986, 1989) or higher foraging eciency at high resource abundances (Stewart and Levin, 1973). We tested from among the ®rst category of mechanisms of co-existence. Failure to support any from this category will indicate a need for the second or third categories to understand co- existence of gerbils with larks. On the sand dunes at our study site, potentially relevant habitats in time and space include season (habitat selection in time), the areas near and away from perennial shrub cover (bush and open microhabitats), and the mosaic of habitats distinguished by dierent degrees of sand stabi- lization due to algal soil crusts and vegetation. If larks and gerbils each possess seasons, micro-
Competition between birds and mammals 759
Table 1. The granivorous and partially granivorous birds and mammals in the habitats of Bir Asluj
Species Habitat
Birds
Chukar (Alectoris chukar) rock and loess
Sand partridge (Ammoperdix heyi) rock
Quail (Coturnix coturnix) sand and loess b
Black-bellied sandgrouse (Pterocles orientalis) rock and loess
Rock dove (Columba livia) rock and loess
Turtle dove (Streptopelia turtur) rock and loess a
Palm dove (Streptopelia senegalensis) rock and loess
Desert lark (Ammomanes deserti) rock
Short-toed lark (Caladrella brachydactyla) loess a
Lesser short-toed lark (Calandrella rufescens) rock b
Crested lark (Galerida cristata) sand and rock
Skylark (Alauda arvensis) loess b
Corn bunting (Emberiza calandra) loess b
Cretzschmar's bunting (Emberiza caesia) rock and loess b
b
Ortolan (Embiriza hortulana) sand, rock and loess
Green®nch (Carduelis chloris) sand and loess
Trumpeter ®nch (Rhodepechys githaginea) rock and loess
House sparrow (Passer domesticus) rock and loess
Rodents
Allenby's gerbil (Gerbillus allenbyi) stabilized sand
Greater Egyptian gerbil (Gerbillus pyramidum) semi-stabilized sand
Pygmy gerbil (Gerbillus henleyi) sand and loess
Wagner's gerbil (Gerbillus dasyurus) rock and loess
Common jerboa (Jaculus jaculus) sand and loess
Buxton's jird (Meriones sacrementi) stabilized sand
Gentle jird (Meriones crasus) loess and rock
House mouse (Mus musculus) loess
a
Present in spring, summer and autumn.
b
Present in winter and during spring and autumn migration.
habitats or sand stabilization habitats within which they have the lowest GUDs, then the patch use
behaviour of the birds and rodents across the relevant habitats promotes co-existence. Here, we
compared the GUDs of gerbils and crested larks to examine how season, bush and open micro-
habitats, and habitats varying in the degree of sand stabilization in¯uence GUDs.
Interpreting the giving-up density
Charnov's marginal value theorem can be generalized to show that a forager should leave a
depletable food patch when (Brown, 1988):
H C P MOC 1
where H, C, P and MOC are the quitting harvest rate, metabolic cost, predation cost and missed
opportunity cost of foraging, respectively. In general, H and C can be thought of as having units of
energy per unit time (e.g. J min)1). P can be converted into units of energy per unit time by760 Brown et al. multiplying the predation risk (probability of death per unit time) by the marginal rate of sub- stitution of energy for safety. The marginal rate of substitution considers at what rate the animal can sacri®ce energy for safety and still maintain the same level of ®tness (Caraco, 1979; Brown, 1988). The cost of predation includes such terms as predation risk, marginal value of energy and the animal's state (Brown, 1992). The missed opportunity cost gives the marginal value of time in units of energy per unit time. As such, it measures the quality of the environment following exploitation by the feeding animals. In an in®nitely repeating environment (one that does not deplete following some ®xed time period), MOC is positive, negative or zero depending upon whether the forager's expected per capita growth rate is positive, negative or zero, respectively. If the environment becomes depleted during the course of a day, night or other relevant time period, then the foragers should cease feeding and retire to a burrow, nest or den as a way of conserving energy and remaining safe. In this case, MOC becomes the negative of the cost of resting in one's refuge (Brown, 1992; Brown et al., 1994b). A number of studies have indicated that the patch use behaviour of feeding animals conforms to Equation (1). In appropriate experimental food patches, there is a monotonic relationship between the forager's quitting harvest rate and its GUD (Kotler and Brown, 1990; Brown et al., 1994b; Smith, 1995). As predicted, GUDs increase with metabolic costs (Kotler et al., 1993a), predation (Brown et al., 1988; Kotler et al., 1988, 1991) and missed opportunity costs of foraging (Brown et al., 1992b). Giving-up densities in depletable food patches have been used to investigate animals' diets (Brown and Morgan, 1995), patch use strategies (Valone and Brown, 1989), predation risk (Kotler et al., 1991) and habitat preferences (Brown and Alkon, 1992; Bowers et al., 1993; Hughes and Ward, 1993; Hughes et al., 1995). As an approach to studying community level processes, GUDs measure foraging eciencies (Tilman, 1982; Vance, 1985; Brown, 1988). The species with the lower GUD has an advantage over other species in that it depresses resource abundances below the other species' subsistence levels. The only exceptions occur when environments are unusually rich or poor. In a rich environment, a high GUD may manifest the forager's high state, or low marginal value of energy (both of which raise the predation cost). Such an environment is not sustainable, in that either the forager's population size will grow accordingly, or the environment's richness will deplete. Similarly, the low GUD of animals in a poor environment may re¯ect the desperation of an animal's low state and high marginal value of energy (both lower the cost of predation). This `Stalingrad' eect (manifest in the behaviour of starving German soldiers in the winter of 1942 prior to their capitulation to the encircling Russians; Craig, 1982) is also not stable, in that the forager's population size will decline accordingly, or the environment's quality will recover. In summary, for the purposes of this study: · Giving-up densities in experimental food patches provide a surrogate for the forager's quitting harvest rate. · Giving-up densities provide an estimate of foraging costs. · The species with the lower GUD is the more ecient forager. Methods Study site We established two grids of 16 stations each on sand dunes at Bir Asluj, Holot Mashabim Nature Reserve, northwestern Negev Desert, Israel. The grids were 4 ´ 4 arrays with 40 m intervals between stations. The site oers a hierarchy of habitats. At the largest scale, distinct boundaries separate extensive and continuous tracts of sandy and rocky habitats. Each of our grids lay entirely
Competition between birds and mammals 761 within the sandy habitat, with distances of greater than 100 m between grid margins and the closest rocky habitat. The sandy habitat oers a mosaic of sand stabilization types based on soil crusts and organic matter. Zvika Abramsky, as an expert and independent observer, scored the sand habitat at each station on a scale of 1±4 based on the degree of sand stabilization within a 5 m radius of the station, where 1 sand completely stabilized by algal soil crust (7 stations), 2 sand partially stabilized by algal soil crust (10 stations), 3 mostly shifting sand with broken patches of soil crust (7 stations), 4 shifting sand bare of any soil crust (8 stations). Substrate habitat scores of 1 and 2 correspond to stabilized sand dune, and habitat scores of 3 and 4 correspond to semi-stabilized sand dune. At the smallest scale, each station oered bush and open microhabitats: bush next to the canopy of a perennial shrub, open about 2 m from the nearest shrub. The dominant perennial plants are Artemisia monosperma and Retama raetam (Abramsky et al., 1985). Seed-eating rodents and birds The ecologies of Allenby's gerbil and the greater Egyptian gerbil at Bir Asluj are well known (Abramsky et al., 1985, 1990, 1991; Abramsky and Pinshow, 1989; Mitchell et al., 1990; Kotler et al., 1993a,b; Ziv et al., 1993). Allenby's gerbil (25 g) occupies all sand habitats found at the site, whereas the greater Egyptian gerbil (39 g) occupies primarily the semi-stabilized sand (sand hab- itats 3 and 4). Analysis of the rodent data from this project (Brown et al., 1994a) revealed that both gerbils had lower GUDs on the semi-stabilized than on the stabilized sand habitats, and in the bush than in open microhabitat. In all sand habitats and in both microhabitats, Allenby's gerbil had a lower GUD than the greater Egyptian gerbil. Other rodents found on the sand dunes were in- consequential either because of their low abundances (pygmy gerbil, G. henleyi, and Buxton's jird, Meriones sacramenti), or because of low foraging eciencies that usually precluded them as the ®nal forager in the food patches (common jerboa, Jaculus jaculus). Allenby's gerbil and the greater Egyptian gerbil co-exist via daily renewal of resource patches and a trade-o between foraging eciency at high versus low resource abundance (Kotler et al., 1993b). The greater Egyptian gerbil arrives at resource patches earlier than the smaller All- enby's gerbil, leaves earlier and, on average, forages on richer patches. Allenby's gerbil forages later in the evening, but depletes resource patches to a lower GUD than the greater Egyptian gerbil. Bir Asluj oers a diverse community of birds as potential foragers in our food patches. All of the species listed in Table 1 were seen on, over or within 500 m of our grids. Throughout the 15 months of the study, however, crested larks were the only birds known to forage from our food patches, based on visual spot checks of birds in the food patches and from footprints in the patches' sand. Of the birds at Bir Asluj, crested larks are distinct in having a long hindtoe, followed by a similarly long claw. Crested larks range over most of Europe, North and East Africa, the Middle East and the steppes of Central Asia. Their diet includes insects and fruits, but mostly seeds. In comparison to other granivorous birds of the Negev, the crested lark (39 g) uses habitats opportunistically, digs vigorously with its beak to break soil crusts, and biases its diet towards larger seeds (Shkedy, 1990, 1992; Shkedy and Safriel, 1991). At Bir Asluj, the crested larks moved in pairs or singly during the breeding season (February± March). For the rest of the year, crested larks could still be found singly or in pairs, but more frequently they associated loosely in ¯ocks numbering 4±10 individuals. On a grid, one or two of these ¯ocks would move from station to station when exploiting the food patches. Frequently, the ¯ock would spread out so that individuals simultaneously exploited food patches at two or three stations.
762 Brown et al. Experimental protocol From November 1986 until January 1988, we completed nine rounds of data collection (see Brown et al., 1994a). A round consisted of live-trapping small mammals, measuring rodent activity in tracking stations, and measuring the GUDs of rodents and crested larks from food patches. Food patches consisted of aluminium seed trays (45 ´ 60 ´ 2.5 cm deep) ®lled with 3 g of millet seed mixed into 5 litres of sifted sand. Each station of each grid received a pair of food patches. One tray of a pair was placed in the open and the other in the bush microhabitat. Data collection consisted of identifying the forager species based on footprints in the tray's sand and in the surrounding sand, sifting the sand from the tray to recover the remaining seeds, and recharging the tray's sand with 3 g of millet. The GUD of the tray was credited to the one or several species whose tracks were visible in the tray. If a species' tracks were found adjacent to, but not in, a foraged tray, then we assumed that it foraged in the tray, but that individuals of other species came later to forage from the tray and covered the sand with their own tracks. Seeds recovered from trays were cleaned of debris and weighed to measure the forager's GUD (Brown, 1988). Data were collected at dawn and dusk. Seed trays set up at dawn and collected at dusk measured the foraging behaviour of crested larks, and those set up at dusk and collected at dawn measured the foraging of the two gerbil species. For each round, we collected GUD data for 6±7 mornings and afternoons. Days of data collection were consecutive, weather and logistics per- mitting. Results To test for the eects of season, microhabitat and sand habitat on the GUDs of larks and gerbils, we used a partially hierarchical ANOVA (Brownlee, 1965), with taxon (gerbil or lark), month (the nine rounds of data collection), microhabitat (bush and open), sand habitat (1±4; stabilized to semi-stabilized sand dunes, respectively) and station (32 stations) as independent variables. Taxon, month and microhabitat were fully-crossed factors, and station was a factor nested within sand habitat. As the dependent variable, we used the mean GUD at a station across the 6±8 days of a round. We calculated separate means for the gerbils (morning data collection) and the larks (afternoon data collection). This analysis gave us 1152 data points (2 taxa ´ 9 rounds ´ 2 mi- crohabitats ´ 32 stations). By using the mean GUDs across days, we simultaneously lose information, streamline the statistical analysis, and avoid possible non-independences associated with repeated measures of the same tray. We lose two types of information. The ®rst is the eect of day within a round of data collection; day is a variable nested within round. Including day as a variable when analysing data for gerbils or larks separately may be useful. However, while it is possible to pair lark and gerbil data by station, it is arbitrary to do so for day because the taxa are temporally separated. Does one pair the previous night with the subsequent day, or vice-versa? The second loss of information occurs because the mean GUDs do not discriminate between the two gerbil species. We did not discriminate because we were interested in comparing the mammalian granivores with the avian granivores in their eciencies at depressing seed abundances. Elsewhere, we report on the results from comparing the foraging behaviour and GUDs of Allenby's gerbil, the greater Egyptian gerbil and the common jerboa (Brown et al., 1994a). The largest eect in the ANOVA (Table 2) showed that crested larks had a signi®cantly higher GUD than the gerbils (2.149 vs 0.593 g). The higher GUD of larks occurred across all months, microhabitats and sand habitats, despite the signi®cant interaction eects of taxon with month, microhabitat and sand habitat. The three-way interactions were not signi®cant. The main eects of
Competition between birds and mammals 763 Table 2. The ANOVA showing the eects of species (lark vs gerbil), microhabitat (bush vs open), sand habitat (four grades of stabilization from stabilized to semi-stabilized) and month on giving-up densities. Station is a variable nested within sand habitat. The model yields r2 0:89 Factor d.f. MS F Species 1 683.11 7101.90 *** Microhabitat 1 1.318 13.71 *** Sand habitat 3 0.375 1.87 Month 8 13.45 139.80 *** Species ´ microhabitat 1 6.91 71.80 *** Species ´ sand habitat 3 2.133 22.20 *** Species ´ month 8 2.757 28.70 *** Species ´ microhabitat ´ sand habitat 3 0.040 0.41 Species ´ microhabitat ´ month 8 0.120 1.25 Species ´ sand habitat ´ month 24 0.071 0.74 Station (nested within sand habitat) 28 0.201 2.14 * Error 1063 0.094 * P < 0:05; *** P < 0:001. month and microhabitat were signi®cant, but are not particularly interesting in light of the strong interaction eects with taxon. Because of the strong interaction eect of taxon with month, we performed an array of post-hoc tests (Bonferroni adjustments to experimentwise error rate) (Fig. 1). These tests compared the GUDs of the gerbils with those of the larks on a month by month basis (nine comparisons). Among gerbils and larks, respectively, we made all of the pairwise comparisons of months (an additional 72 comparisons). During each month, gerbils had a signi®cantly lower GUD than crested larks (P < 0:001 for all comparisons). For gerbils, months fell into two categories. Feb- ruary and April had signi®cantly higher GUDs than the other 7 months (P < 0:001 for the 14 comparisons, and P > 0:5 for the comparison of February with April). For crested larks, months fell into four categories. February, April and June had similar GUDs (P > 0:4 for the three comparisons) that were signi®cantly higher than the other months (P < 0:001 for the 18 com- parisons). November 1987 had the lowest GUD followed by January 1987 (November 1987 < January 1987, P < 0:001). Both of these months had signi®cantly lower GUDs than other months. A Spearman rank correlation revealed a positive, but non-signi®cant, relationship between the GUDs of gerbils and larks R 0:55; P > 0:1. Most striking were the high GUDs of both gerbils and larks during February and April. June was the most divergent month because larks had a relatively high GUD and gerbils had a relatively low GUD (Fig. 1). But, monthly variation within each taxon paled in comparison to dierences between taxa. We analysed the interaction eect of sand habitat with taxon in a similar fashion to that of month (Fig. 2). We compared the GUDs of gerbils and larks for each of the sand habitats (four comparisons) and then compared among sand habitats separately for gerbils and crested larks (an additional 12 comparisons). In all sand habitats, gerbils had signi®cantly lower GUDs than crested larks (P < 0:001, Bonferroni-adjusted post-hoc test). For gerbils, the habitats fell into two cate- gories. The least stabilized sand habitat (#4) had a signi®cantly lower GUD than the other three habitats (4 and 1, P < 0:001; 4 and 2, P < 0:05; 4 and 3, P < 0:01), which did not dier signi®cantly from each other. For crested larks, habitats also fell into two categories. In contrast to the gerbils, larks had a signi®cantly lower GUD in sand habitat (#1) than in the other three (1 and 2,
Figure 1. The monthly giving-up densities of the gerbil species and the crested lark. The species' bars have been superimposed, not stacked. Hence, the top of each species' bar indicates its GUD. During each period, the crested lark had a signi®cantly higher giving-up density than the gerbils. The error bars indicate one standard error around the mean. Figure 2. The eect of sand habitat on the giving-up densities of the gerbil species and the crested lark. The habitat scores of 1±4 represent a continuum from stabilized to semi-stabilized sand habitats. The crested lark had a signi®cantly lower giving-up density in the most stabilized sand habitat. The gerbils showed the opposite, and had a signi®cantly lower giving-up density in the least stabilized sand habitat.
Competition between birds and mammals 765 Figure 3. The eect of the bush and open microhabitat on the giving-up densities of the gerbil species and the crested lark. The gerbils had a signi®cantly lower giving-up density in the bush than open microhabitat, and vice versa for the crested lark. The error bars indicate one standard error around the mean. P < 0:001; 1 and 3, P < 0:01; 1 and 4, P < 0:001). A perfect negative rank-correlation existed between the GUDs of gerbils and larks across sand habitats. For the interaction eect between taxon and microhabitat, we used a Bonferroni-adjusted post- hoc test to compare gerbils and larks in each microhabitat (two comparisons) and to compare bush and open microhabitats separately for each taxon (two additional comparisons) (Fig. 3). Gerbils had a signi®cantly lower GUD in the bush than in the open microhabitat P < 0:01, while larks had a signi®cantly lower GUD in the open than in the bush microhabitat P < 0:001. In both microhabitats, gerbils had signi®cantly lower GUDs than larks (P < 0:001 for both comparisons). Discussion On the sand dunes at Bir Asluj, the two gerbil species (Allenby's gerbil and the greater Egyptian gerbil) were more ecient granivores (as measured by giving-up densities) than the crested lark during every month, on every sand habitat, and in both the bush and open microhabitat. Several related issues merit discussion: 1. Is the lower eciency of larks more apparent than real? 2. What mechanisms of co-existence might promote the presence of larks in the community? 3. What is the role of predation risk in shaping the foraging behaviour of larks and gerbils? 4. How does the foraging behaviour of gerbils and larks ®t into the larger desert landscape of sandy, rocky and loessal habitats? 5. Do birds and mammals restrict each other's distribution and diversity?
766 Brown et al. Foraging eciencies of gerbils versus larks Within our food patches, gerbils harvested millet seeds to a much lower level than larks. We feel that this supports the conclusion that gerbils also deplete native seedbanks to a lower density than larks. Two concerns must be addressed. First, the experimental food patches might give an ad- vantage to gerbils that is not relevant under natural conditions. If larks cannot exploit the full depth of 2.5 cm of sand, then maybe larks are more ecient foragers on surface seeds while gerbils can access deeper recesses of the patch. This is unlikely. Larks swung their beaks from side to side to dig thoroughly through the sand and, based on their lowest GUDs, larks are capable of depleting the seeds to less than one-third of their average GUDs. The high GUDs of the larks in our trays stem from an unwillingness to forage longer, rather than the inability to access all of the seeds in the patch. Secondly, larks may overtake gerbils in foraging eciency as seed size declines. Millet is a relatively large seed (c. 6 mg). However, it has recently been demonstrated that the foraging eciency of the crested lark changes little with seed size, although that of the gerbil species drops sharply on smaller seeds (c. 1.5 mg; J. Garb, B.P. Kotler and J.S. Brown, unpublished results). Regardless, the gerbils had lower GUDs than larks on all seed sizes examined (1.5±6 mg). The mechanism of co-existence between larks and gerbils When a lark leaves a food patch, the patch remains a valuable feeding opportunity for a gerbil. In contrast, when a gerbil leaves a food patch, the patch must experience considerable resource renewal before providing a pro®table foraging opportunity for a crested lark. The conditions for gerbils to persist in the presence of crested larks are obviously met, but less clear is how crested larks co-exist with gerbils. Larks forage during the hours of resource renewal by wind action (mornings and afternoons; Kotler et al., 1993b). In this way, a mechanism of co-existence would involve temporal and spatial variability in seed abundances, and a trade-o among the taxa in eciency (higher in gerbils) and speed/mobility (greater in larks) (Brown, 1986, 1989). Larks and gerbils would function as `cream skimmers' and `crumb pickers', respectively. Alternatively, mechanisms such as diet selection may explain co-existence. Gerbils may be more ecient foragers on seeds, whereas crested larks may be more ecient foragers on insects (J.S. Brown and B.P. Kotler, unpublished results). Habitat selection at a larger scale may provide yet another mecha- nism of co-existence. While individual gerbils are restricted to the sand dunes, the mobility of larks and the landscape of Bir Asluj permits individual larks to exploit rocky, loessal and sandy habitats. Larks may require the other habitats to prevent competitive exclusion. The role of predation risk for crested larks and gerbils Crested larks had lower GUDs in the open than bush microhabitat, and lower GUDs on the most stabilized sand habitat. Because the same ¯ocks of larks had access to food patches in both microhabitats and all sand habitats, the dierences in GUDs should re¯ect predation costs rather than metabolic or missed opportunity costs of foraging. At Bir Asluj, predators of crested larks on the sand dunes include the great gray shrike (Lanius excubitor), hen harrier (Circus cyaneus), pallid harrier (C. macrourus) and monitor lizard (Varanus griseus). Lima (1992, 1993), in his review of escape tactics of birds, found that several granivorous sparrows and larks of arid and semi-arid habitats should favour the open rather than the bush microhabitat. Similarly, for the crested lark, the bush microhabitat may oer higher predation risk by constraining escape ¯ights and facili- tating ambushes by monitor lizards and perching shrikes.
Competition between birds and mammals 767 The GUDs of the lark suggest that predation risk increases as the degree of sand stabilization declines. The soil crust surrounding the experimental food patches (the patches themselves were identical) may be important to crested larks walking on the ground. Soil crusts may provide better traction and support for initiating and directing escape ¯ights or for running across the surface. The role of other habitats in the interaction between gerbils and larks The primary interaction and co-existence of seed-eating birds and mammals may actually occur at the larger scale of rocky and sandy habitats. Rodents may predominate on the sand by virtue of high foraging eciencies and may be less abundant on rocky habitats by virtue of high predation costs (e.g. escape substrate; Brown et al., 1992) and competition from birds. Allenby's gerbils had much higher GUDs on sandy habitats than on rocky or loessal habitats. And, their GUDs in- creased steadily the farther into the rocky habitat the gerbils ventured from the sand (Brown et al., 1992). Interestingly, the GUD of crested larks on the sand (about 2 g) was similar to that of gerbils in the same seed trays placed in the rocky habitat. The diverse community of granivorous birds (Table 1) may rely on and occupy the rocky and loessal habitats. In this way, each major habitat (sand and rock) may possess a taxon-speci®c guild of vertebrate granivores. But, the ghost of competition past that drives species to occupy dierent habitats (Rosenzweig, 1981; Brown and Rosenzweig, 1986) may not be complete. Crested larks join the gerbil species on the sand; and some rodents (Wagner's gerbil, G. dasyurus, and common spiny mice, Acomys caharhinus) join diverse species of birds on the various grades of rocky habitats. Do birds and mammals restrict each other's distribution and diversity? With a few exceptions (e.g. Thompson et al., 1991), studies of community organization in mam- mals or birds have not considered the in¯uence or role of the other taxon. Examples of potential resource competition between birds and mammals include fox squirrels and blue jays consuming acorns, nutcrackers and red squirrels consuming pine nuts, frugivorous birds and primates in tropical forests (Howe, 1980, 1990), sugar gliders and honeyeaters or lorikeets consuming nectar, shrews and thrushes consuming invertebrates among leaf litter, and mustelids and owls consuming voles. In these examples, the similarities in diets between the mammal and bird species often contrast with dierences in foraging tactic and foraging scale. Super®cially, mammals often appear to forage more intensively while birds forage more extensively. These features of tactic and scale that promote the co-existence of birds and mammals may partially explain the absence of joint community studies of birds and mammals; the scale and tools for studying one taxon often fail to yield meaningful data for another taxon. In addition, if mammals and birds do competitively exclude each other, then feeding guilds with high diversities of mammals may tend to be depauperate in birds and vice versa. On the other hand, there may be situations where birds and rodents, through their foraging behaviour, create opportunities for each other. Thompson et al. (1991) suggest that granivorous birds and rodents may facilitate each other through an indirect mutualism. By preferentially consuming large seeds, rodents promote plants producing the small seeds favoured by some birds. In general, the mechanisms of co-existence (diet choice, habitat selection, variance partitioning) reported for mammal communities (e.g. Rosenzweig, 1966; Terborgh, 1983; Kreuger, 1986; Kotler and Brown, 1988; Morris et al., 1989; McNaughton, 1993) are the same as those reported for bird communities (e.g. MacArthur, 1958; Cody, 1974, 1985; Dunning, 1986; Terborgh et al., 1990; Benkman, 1991; Sohonen et al., 1993). It is likely that these mechanisms also explain co-existence, and limits to co-existence, between birds and mammals.
768 Brown et al.
Acknowledgements
We thank Charles Eesley, M.F. Eyphat, Matthew Goldowitz, Oren Hasson, Reuven Yosef, Berry
Pinshow, Jean Powlesland, Aziz Subach and Laurie Zaarur for assistance with ®eldwork and seed
sorting. We thank Zvika Abramsky, John Fryxell, Douglas Morris, Michael Rosenzweig and
James Thorson for stimulating and valuable comments. B.P.K is a Bat-Sheva de Rothschild
Fellow. This work was supported by United States-Israel Binational Science Foundation Grant
No. 86-00087 (to B.P.K., Z. Abramsky and M. Rosenzweig) and Grant No. 93-00236 (to B.P.K.
and J.S.B.). The Jacob Blaustein International Center for Desert Studies provided ®nancial as-
sistance for J.S.B. and W.A.M. This is publication #225 of the Mitrani Center for Desert Ecology.
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