Effects of predatory risk and resource renewal on the timing of foraging activity in a gerbil community

The foraging decisions of animals are often influenced by risk of predation and by the renewal of resources. For example, seed-eating gerbils on sand dunes in the Negev Desert of Israel prefer to forage in the bush microhabitat and during darker hours due to risk of predation. Also, daily renewal of seed resource patches and timing of nightly foraging activity in a depleting environment play important roles in species coexistence. We examined how these factors influence the timing of gerbil foraging by quantifying foraging activity in seed resource patches that we experimentally renewed hourly during the night. As in previous work, gerbils showed strong preference for the safe bush microhabitat and foraged less in response to high levels of illumination from natural moon light and from artificial sources. We demonstrate here for the first time that gerbils also responded to temporal and spatial heterogeneity in predatory risk through their timing of activity over the course of each night. Typically, gerbils concentrated their activity early in the night, but this changed with moon phase and in response to added illumination. These results can be understood in terms of the nature of patch exploitation by gerbils and the role played by the marginal value of energy in determining the cost of predation. They further show the dynamic nature of gerbil foraging decisions, with animals altering foraging efforts in response to time, microhabital, moon phase, illumination, and resource availability.     

Competition between birds and mammals: A comparison of giving-up densities between crested larks and gerbils

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 define 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 (Allenbys gerbil, Gerbillus allenbyi, and the greater Egyptian gerbil, G. pyramidum) dominated nocturnal 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 efficiencies. A low giving-up density demonstrates the ability of a forager to profitably harvest food at low abundances and to profitably utilize the foraging opportunities left behind by the less efficient 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 mechanisms 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.     

Foraging and community consequences of seed size for coexisting Negev Desert granivores

We examined the effects of seed size on patch use and diet selection for three co-existing Negev Desert granivores: Allenby's gerbil ( Gerbillus allenbyi ), greater Egyptian sand gerbil ( Gerbillus pyramidum ), and crested lark ( Galerida cristata ). We manipulated size and spatial distribution of seeds in experimental food patches and quantified foraging behavior by measuring giving-up densities (GUDs: the amount of food remaining in a resource patch following exploitation by a forager). In one experiment, we presented small (<1.4 mm in diameter cracked wheat), medium (2.0–3.3 mm), and large (>3.4 mm) seeds in separate trays; in a second, we presented small and medium seeds separately and mixed together. Gerbils had a higher handling time efficiency on smaller seeds, but a much higher encounter probability on larger seeds (20 times higher on large than medium seeds, and 2–5 times higher on medium than small seeds). This led gerbils to have significantly lower GUDs on larger seeds than smaller seeds and to harvest a higher proportion of the larger seeds. When presented with rich and poor patches, G. allenbyi tended to equalize GUDs in both patches, indicating a quitting harvest rate rule for patch exploitation. In contrast, larks appeared to use a fixed time rule for patch exploitation. For larks, seed size did not influence encounter probabilities, and they showed no seed-size selectivity. Still, larks had higher handling efficiencies on smaller than larger seeds, and consequently had a significantly lower GUD on small than medium seeds. Despite large differences between the gerbils and larks in their foraging, our results do not support species coexistence via seed-size partitioning: the larks had much higher GUDs than the gerbils on all seed sizes. Nonetheless, seed size, seed abundance, seed distribution and the animal's patch use behavior all played major roles in determining gerbils' and larks' diet selectivities and GUDs.     

Influence of temporal fluctuations in seed abundance on the foraging behaviour of harvester ants (Pogonomyrmex spp.) in the central Monte desert,Argentina

The simultaneous study of the temporal dynamics of foraging behaviour, diet and seed abundance is essential to assess the way in which resources affect the behaviour and ecology of harvester ants. Here, we evaluate how fluctuations in grass seed abundance during three consecutive growing seasons influenced the foraging behaviour and diet of the harvester ants Pogonomyrmex rastratus, P. mendozanus and P. inermis in the central Monte desert, Argentina. Seed abundance of the most consumed grasses varied greatly through ant activity season, and ants altered their foraging behaviour in response to those changes. Foragers spent more time travelling and searching for food, and their foraging trips took longer during the low seed availability season. Foraging distance was very similar among species and, contrary to our expectations, did not vary between seasons. Foraging success of P. rastratus and P. inermis increased during the high availability season. This matched the seasonal pattern of foraging activity, suggesting that colonies may detect seed abundance and regulate their foraging effort with the rate of forager success. Although grass seeds were the main item in the diet of the three species, P. mendozanus, and to a lesser extent P. rastratus, turned more generalist when grass seeds were scarce. In contrast, P. inermis showed a very narrow diet breadth, only harvesting grass seeds in both seasons. Our results indicate the relevance of seed availability on foraging behaviour of harvester ants, which should be taken into account when predicting and evaluating the effect of ants on seed resources as well as numerical responses of harvester ant populations to the temporal and spatial variations in grass seed abundance.     

Relationships between seed banks and spatial heterogeneity of North American alvar vegetation

Abstract. This is the first quantitative study of seed bank characteristics in North American alvar habitats. We assessed seed bank density, species richness, and species composition in 75 plots distributed among five alvar sites in Bruce Peninsula National Park, Ontario, Canada, each of which displayed areas of high and low vegetation cover within the alvar and a fully forested perimeter area. Forested habitats immediately adjacent to alvar patches contained minimal seed banks for species restricted to the alvar patches. Open alvars contained less than 1% seeds from woody forest species. This suggests that forest is not invading adjacent alvar habitat via seeds and that adjacent forest does not contain a reservoir of alvar seeds. When compared to areas on the alvar with high vascular plant cover, areas with low cover contained a slightly smaller viable seed bank, but seed banks from high and low vegetation cover plots had similar species composition and species richness. High vegetation cover plots had slightly higher mean and maximum soil depths compared with low cover plots, but no differences in other physical and chemical parameters. Thus, spatial heterogeneity in plant cover is associated only weakly with heterogeneity in below‐ground factors. Despite the availability of seed and soil resources, vegetation dynamics are constrained in areas with low plant cover, and thus alvar community development seems to respond non‐linearly to resource availability.     

Does food‐searching ability determine habitat selection? Foraging in sand of three species of gerbilline rodents

The distribution of three gerbil species in the Negev Desert, Israel differs in relation to sandy habitats. Gerbillus gerbillus occurs in sand only, Gerbillus dasyurus lives in all habitat types except sand, Gerbillus henleyi occurs in sand at high density periods only. We hypothesized that the reason for this distribution pattern is differential ability of species to forage in sand. We tested the ability of gerbils to search for seeds in sand layers of different depths and predicted that a) G. gerbillus will have equal search success at different sand depths, b) G. henleyi (which is twice smaller than G. gerbillus and G. dasyurus) will find seeds in thin layers better than in deep layers, and c) G. dasyurus will successfully find seeds in the thinnest layers only. We predicted also d) that G. gerbillus relies on olfaction for seed location, so its foraging success will be higher in searching for whole seeds than for seed kernels, whereas this will not be the case for G. henleyi and G. dasyurus. We examined the responses of the three species to odor of whole seeds vs seed kernels in Y-maze. In regards to seed depth, the search success of G. gerbillus was significantly higher when the whole seeds rather than kernels were offered in all treatments except the control, but sand depth did not influence the search success. The search success of G. dasyurus and G. henleyi did not depend on the type of seeds offered. The search success of G. dasyurus was lower in experimental (1, 3, 5 and 8 cm sand depth) than in control (1 mm sand depth) treatments, but did not differ among most experimental treatments. The search success of the smallest G. henleyi depended on sand depth for both whole seeds and kernels. Gerbillus gerbillus and G. dasyurus did not lose body masses in any treatment, whereas body mass changes of G. henleyi were influenced by the depth of sand in which gerbils foraged. No species demonstrated differences in response to whole seeds vs kernels in Y-maze tests.     

Harvest rates and foraging strategies in Negev Desert gerbils

We examined the foraging strategy and quantified the foragingtraits of two nocturnal rodent species, Allenby's gerbil (Gerbillusallenbyi) and the greater Egyptian sand gerbil (Gerbillus pyramidum).In the laboratory, both species used two distinct foragingstrategies: either they immediately consumed seeds found ina patch (seed tray); or they collected and delivered the seedsto their nest box for later consumption. Moreover, we founda transition in foraging strategy among individual G. allenbyi under laboratory conditions; they all began by consuming theseeds on the tray and, after 7 days on average, switched tothe collecting strategy. By contrast, in the field both speciesused only one foraging strategy; they collected and deliveredthe seeds to their burrow or to surface caches for later consumption.Furthermore, G. allenbyi and G. pyramidum collected seeds atsignificantly higher rates in the field than in the laboratorybecause the seed encounter rates for both species were higherin the field. This suggests that in natural conditions, probablyinvolving predation risk and competitive pressure, gerbilsmust respond in two ways: (1) they must choose a foraging strategythat reduces predation risk by minimizing time spent feedingoutside their burrows; and (2) they must forage more efficiently.In the field, seed handling time of the larger species, G. pyramidum, was shorter than that of the smaller one, G. allenbyi.This difference may give G. pyramidum an advantage when resourcelevels are high and when most of a forager's time is spent handling seeds rather than searching for more seeds. Additionally,our field study showed that the seed encounter rate of G. allenbyiwas higher than that of G. pyramidum. This difference may giveG. allenbyi an advantage when resource levels are low and whensearching occupies most of the forager's time. The differentadvantages that each species has over the other, under differentconditions, may well be factors promoting their coexistenceover a wide range of resource densities.     

Giving-up densities of foraging gerbils: the effect of interspecific competition on patch use

We studied the effect of a dominant species, Gerbillus pyramidum (Egyptian sand gerbil), on the patch use of its subordinate competitor, G. andersoni allenbyi (Allenby's gerbil), to better understand interspecific competition between the two species. We used manipulated resource patches (seed trays) covered with cages with two adjustable species-specific gates (either opened or closed to the bigger-dominant species, but always opened to the subordinate one). We recorded species tracks around and on the seed trays and giving-up densities (GUDs) of seeds in the trays after each night of foraging. G. a. allenbyi depleted seed patches to a lower level whenever G. pyramidum was given the opportunity to forage on the seed trays (i.e., present on the grid). This result held regardless of whether G. pyramidum was actually present at a particular station. We suggest that competition from G. pyramidum occurs both directly by interference, in which G. a. allenbyi is forced to be active in the late part of the night, and indirectly by exploitation via resource depletion by G. pyramidum in the early part of the night. The results suggest that interspecific competition from G. pyramidum reduces seed availability and the richness of the environment for G. a. allenbyi enough to affect the marginal value of energy for G. a. allenbyi individuals and cause them to experience lower costs of predation and manifest lower GUDs.     

The effects of gerbil foraging on the natural seedbank and consequences on the annual plant community

We explicitly tested the following predictions using Gerbillus allenbyi and G. pyramidum foraging on artificial arrays of natural seedbank on a semi-stabilized sand dune: 1. that the gerbils will forage using a quitting harvest rate rule, 2. that larger seeds are preferred due to higher encounter rates, and 3. that there are community level consequences for the annual plants as a direct result of foraging by the desert rodents. Natural seedbank, separated into two size classes, was placed in seed trays in the field at four different densities (1/16×, 1/4×, 1×, and 2×normal). Following exposure to granivory, the remaining seeds were weighed and germinated to test for community level effects. Only the 1× and 2×normal density plots were heavily foraged, and at both seed sizes, which suggests that the gerbils employed a quitting harvest rate rule. In support of our second prediction, the two species of gerbils tended to consume more of the larger seeds, particularly at higher densities. The mean number and total number of plant species that germinated in plots exposed to granivores was not significantly different from unexposed samples. At the community level however, there was no net association of germinated plant species in the four treatment groups exposed to granivores, but a significant net positive association in the unexposed control seedbank. Gerbil foraging on annual plant seedbank may thus subtly change the entire community structure (from positive to neutral), although not necessarily shift the species distributions significantly. Our results corroborate other studies involving artificial food types such as millet and suggest that foraging decisions may affect the plant community.     

Harvester ant response to spatial and temporal heterogeneity in seed availability: pattern in the process of granivory

The influence of temporal and spatial heterogeneity in seed availability on the foraging behaviour of the harvester ant Messor arenarius was studied in an arid shrubland in the Negev Desert, Israel. The study investigated the implications of behavioural responses to heterogeneity in seed availability for the seed predation process and the potential for feedback effects on vegetation. Vegetation and seed rain were monitored across two landscape patch types (shrub patches and inter-shrub patches) in 1997. Shrub patches were shown to have higher plant and seed-rain density than inter-shrub patches. Patch use and seed selection by M. arenarius foragers were monitored through the spring, summer and autumn of 1997. After a pulse of seed production in the spring, the ants exhibited very narrow diet breadth, specialising on a single annual grass species, Stipa capensis. At this time, ants were foraging and collecting seeds mainly from inter-shrub patches. In the summer, diet breadth broadened and use of shrub patches increased, although the rate of seed collection per unit area was approximately equal in the two patch types. The increase in the use of shrub patches was due to colony-level selection of foraging areas with relatively high shrub cover and an increase in the use of shrub patches by individual foragers. In the autumn, a pulse of seed production by the shrub species Atractylis serratuloides and Noaea mucronata led to a reduction in diet breadth as foragers specialised on these species. During this period, foragers exhibited a large increase in the proportion of time spent in shrub patches and in the proportion of food items collected from shrub patches. The seasonal patterns in foraging behaviour showed linked changes in seed selection and patch use resulting in important differences in the seed predation process between the two landscape patch types. For much of the study period, there was higher seed predation pressure on the inter-shrub patches, which were of relatively low productivity compared with the shrub patches. This suggests that the seed predation process may help maintain the spatial heterogeneity in the density of ephemeral plants in the landscape.     

A field assessment of optimal foraging in ants: trail patterns and seed retrieval by the European harvester ant Messor barbarus

Summary: The ant Messor barbarus is a major seed predator on annual grasslands of the Mediterranean area. This paper is an attempt to relate the foraging ecology of this species to resource availability and to address several predictions of optimal foraging theory under natural conditions of seed harvesting.¶Spatial patterns of foraging trails tended to maximise acquisition of food resources, as trails led the ants to areas where seeds were more abundant locally. Moreover, harvesting activity concentrated on highly frequented trails, on which seeds were brought into the nest in larger numbers and more efficiently, at a higher mean rate per worker.¶The predictions of optimal foraging theory that ants should be more selective in both more resource-rich and more distant patches were tested in the native seed background. We confirm that selectivity of ants is positively related to trail length and thus to distance from the nest of foraged seeds. Conversely, we fail to find a consistent relationship between selectivity and density or species diversity of seed patches. We discuss how selectivity assessed at the colony level may depend on factors other than hitherto reported behavioural changes in seed choice by individual foragers.     

Interspecific variation in extraction of buried seeds within an assemblage of sparrows

Most North American sparrows forage almost exclusively on herbaceous seeds during the winter months. Limited availability of surface seeds forces some birds to scratch for seeds buried beneath soil, snow, or litter. Artificial seed trays were used to test the ability of five different sparrow species to extract seeds buried at different depths in soil. The results suggest three functional groups based on relative scratching ability. Strong scratchers, which included eastern towhee and song and white-throated sparrows, met or exceeded their energetic requirements when foraging on seeds buried at all depths (down to a maximum depth of 1.50–2.25 cm). A weak scratching species, Savannah sparrow, scratched with the same frequency as the strong scratchers, but experienced negative energy budgets when forced to forage on sub-surface seeds. Finally, a non-scratching species, field sparrow, failed to extract any buried seeds. Level of scratching ability may influence foraging efficiency at low resource densities. As a result, interspecific differences in scratching ability may contribute to habitat selection. Strong scratchers may be adapted to foraging near woody vegetation where intense resource competition and abundant litter limit the availability of surface seeds. Weak scratchers, on the other hand, may be forced to feed away from areas where surface seeds are limited. Since woody vegetation serves as a primary source of protective cover in early successional habitats, a tradeoff between foraging efficiency and the risk of predation may promote the local coexistence of species that differ in relative scratching ability and adaptations to evading predators.     

Foraging behaviour of harvesting ants determines seed removal and dispersal

Harvesting ants can affect the regeneration of plants through at least two different processes: seed removal and seed dispersal. We analyse the role of different foraging strategies of ants on patterns of seed removal and dispersal by three Messor species with considerable differences in their foraging systems. Messor capitatus workers rarely leave the nest in well-formed columns, while the other two species form foraging trails, with M. bouvieri forming temporary trails and M. barbarus foraging on a stable system of permanent foraging trails. Overall seed intake of M. capitatus colonies is considerably less than that of the two group-foraging species. There are also differences in the size of seeds collected: M. barbarus and M. capitatus harvest similar amounts of large and small seeds, while M. bouvieri harvests small seeds more intensely than large ones, due to the smaller size of the worker caste. The three Messor species differ in the percent of seed dropping of the different seed type and in the seed dispersal distance. Moreover, M. bouvieri and M. capitatus redistributed dropped seeds preferentially in bare soil and low sparse vegetation habitats, while M. barbarus redistributed seeds mainly in the high vegetation habitat. These results show that the foraging systems of these harvesting ants determine different patterns of seed removal and dispersal and, thus, affect the abundance and redistribution of seeds in the area.     

Temporally fluctuating prey and coexistence among unequal conspecific interferers

Coexistence among unequal conspecific interferers should be unlikely to persist if stronger interferers always experience a relative fitness increment from their higher foraging rates. In this study, we suggest that decreased relative costs to weaker interferers with increasing temporal fluctuations in prey availability may be a mechanism enhancing coexistence of unequal conspecific interferers. Previous work on fluctuation and coexistence has dealt with oscillations over a time-scale measured in generations of competitor species and their resources, while our work shows that fluctuations in prey availability facilitate coexistence of different phenotypic strategies within species and generations, and over short time-scales. With increasing amplitude of temporal fluctuation about an average prey density, cumulative intakes for differently strong interferers are affected differently. Because of the prey-dependent effect of interference, high amplitudes of fluctuation allow for relatively lower foraging-rate costs in weaker interferers, which decreases the difference in foraging success between strong and weaker interferers. This decreased difference in foraging success could thus significantly relax the conditions allowing for unequal interferer coexistence.     

Ant foraging strategies vary along a natural resource gradient

Food selection by foragers is sensitive to the availability of resources, which may vary along geographical gradients. Hence, selectivity of food types by foragers is expected to track these resource gradients. Here we addressed this hypothesis and asked if foraging decisions of seed-eating ants differ along a geographic gradient of habitat productivity. The study was carried out for two years at five sites along a natural climatic gradient, ranging from arid to Mediterranean, where plant productivity varies six-fold across a short geographic distance of 250 km. We found that in ant colonies of the genus Messor, collective foraging decisions differed along the gradient. Specifically, at the high-productivity sites, a stronger association was found between plant seed availability and selectivity, suggesting that colonies respond more accurately to within-patch variation in food amounts. In contrast, colonies in low-productivity sites foraged in patches with higher concentration of seeds, suggesting that they respond more accurately to among-patch variation in food amounts. Moreover, at the high-productivity sites, colonies were more discriminating in their choice of food and preferred bigger seeds, while in the low-productivity sites, where smaller seeds were relatively more abundant, food collection depended mostly on seed availability. An experiment with artificial seed patches performed along the same climatic gradient, revealed no difference in food selectivity across sites when food type and availability were similar, and a general preference for bigger over medium-sized seeds. Overall, our findings suggest that resource availability is an important factor explaining food choice along a climatic gradient and imply that in low-productivity regions small-seeded species incur high predation pressure, whereas in high-productivity regions, large-seeded species suffer higher predation. This could have important consequences for plant species composition, particularly at the face of climate change, which could dramatically alter the foraging decisions of granivores.     

Invasive plant species alters consumer behavior by providing refuge from predation

Understanding the effects of invasive plants on native consumers is important because consumer-mediated indirect effects have the potential to alter the dynamics of coexistence in native communities. Invasive plants may promote changes in consumer pressure due to changes in protective cover (i.e., the architectural complexity of the invaded habitat) and in food availability (i.e., subsidies of fruits and seeds). No experimental studies have evaluated the relative interplay of these two effects. In a factorial experiment, we manipulated cover and food provided by the invasive shrub Amur honeysuckle (Lonicera maackii) to evaluate whether this plant alters the foraging activity of native mammals. Using tracking plates to quantify mammalian foraging activity, we found that removal of honeysuckle cover, rather than changes in the fruit resources it provides, reduced the activity of important seed consumers, mice in the genus Peromyscus. Two mesopredators, Procyon lotor and Didelphis virginiana, were also affected. Moreover, we found rodents used L. maackii for cover only on cloudless nights, indicating that the effect of honeysuckle was weather-dependent. Our work provides experimental evidence that this invasive plant species changes habitat characteristics, and in so doing alters the behavior of small- and medium-sized mammals. Changes in seed predator behavior may lead to cascading effects on the seeds that mice consume.     

Indirect interactions among tropical tree species through shared rodent seed predators: a novel mechanism of tree species coexistence

The coexistence of numerous tree species in tropical forests is commonly explained by negative dependence of recruitment on the conspecific seed and tree density due to specialist natural enemies that attack seeds and seedlings (‘Janzen–Connell’ effects). Less known is whether guilds of shared seed predators can induce a negative dependence of recruitment on the density of different species of the same plant functional group. We studied 54 plots in tropical forest on Barro Colorado Island, Panama, with contrasting mature tree densities of three coexisting large seeded tree species with shared seed predators. Levels of seed predation were far better explained by incorporating seed densities of all three focal species than by conspecific seed density alone. Both positive and negative density dependencies were observed for different species combinations. Thus, indirect interactions via shared seed predators can either promote or reduce the coexistence of different plant functional groups in tropical forest.     

Seasonal dietary niche contraction in coexisting Neotropical frugivorous bats (Stenodermatinae)

Tropical dry forests are characterized by punctuated seasonal precipitation patterns that drive primary production and the availability of fruits, seeds, flowers, and insects throughout the year. In environments in which the quantity and quality of food resources varies seasonally, consumers should adjust their foraging behavior to maximize energy intake while minimizing overlap with competitors during periods of low food availability. Here, we investigated how the diets of frugivorous bats in tropical dry forests of NW Mexico varied in response to seasonal availability and how this affected dietary overlap of morphologically similar species. We performed stable isotope analyses to understand temporal and interspecific patterns of overall isotopic niche breadth, trophic position, and niche overlap in the diet of six frugivorous species of closely related New World leaf-nosed bats (family Phyllostomidae, subfamily Stenodermatinae). We estimated seasonal changes in resource abundance in two complementary ways: (a) vegetative phenology based on long-term remote sensing data and (b) observational data on food availability from previously published insect and plant fruiting surveys. In all species, there was a consistent pattern of reduced isotopic niche breadth during periods of low food availability. However, patterns of niche overlap varied between morphologically similar species. Overall, results from our study and others suggest that seasonal food availability likely determines overall dietary niche breadth in Phyllostomidae and that despite morphological specialization, it is likely that other mechanisms, such as opportunistic foraging and spatiotemporal niche segregation, may play a role in maintaining coexistence rather than simply dietary displacement.     

Spatial variation in seed limitation of plant species richness and population sizes in floodplain tallgrass prairie

The relative importance of seed availability versus biotic interactions that affect early life stages in limiting plant population sizes and determining composition of plant communities is a central debate in plant ecology. We conducted a seed addition experiment in restored tallgrass prairie in central Kansas to determine (1) whether addition of seed of 18 native forb species produced persistent (three growing seasons) increases in the species' population sizes and plant species richness, (2) what properties of recipient communities best explained spatial variation in added species' establishment, and (3) whether seed size explained interspecific patterns in establishment success. Adding seed led to persistent increases in the number of added species present and in plant species richness at one of three sites. Increased species richness at the one site where community composition was structured by seed availability largely resulted from greater densities of four species. Seed size did not predict species' establishment success. Pre-existing plant species richness was correlated with added species' establishment success, but the direction of the relationship (positive vs. negative) varied among sites. Living aboveground plant biomass in experimental plots in the year of seed addition was negatively correlated with the number of added species established three years later. Our results provide further evidence for large spatial variation in seed limitation of plant community composition. Surprisingly, mean light availability and heterogeneity in light, both important parameters in conceptual models of grassland plant coexistence, did not predict the response of the recipient plant community to seed addition as well as pre-existing plant species richness and living aboveground biomass.     

Effect of harvester ants of the genus Pogonomyrmex on the soil seed bank around their nests in the central Monte desert,Argentina

1. The abundance and composition of soil seed banks is a key determinant of plant community structure. Harvester ants can remove huge quantities of preferred seeds close to the nest affecting composition and spatial distribution of plants. 2. In the central Monte desert (Argentina) ants of the genus Pogonomyrmex have high seed removal rates, especially of the five main grasses. The aim of this study was to establish if their foraging activity affects spatial patterns of the soil seed bank around their nests. Our hypotheses were: (1) removal by ants decreases seed abundance of preferred species in the soil; and (2) the effect varies in time. 3. Soil seed abundance was assessed at different distances from Pogonomyrmex nests in the litter and in bare soil at the beginning, the middle, and the end of the season (late spring‐early autumn). 4. A lower seed abundance of preferred species was observed close to the nest in the litter at the end of the season. Non‐preferred species showed no distance gradient. 5. The lower foraging activity and seed consumption at the beginning of the season could explain the temporal variation of the spatial effect. This was only observed in the litter, probably because of the higher removal frequency in this substrate. 6. Colonies of Pogonomyrmex spp. could enhance the heterogeneity of soil seed banks in the central Monte desert from the summer to the beginning of the autumn. Implications for vegetation dynamics depend on the degree to which seed density limits perennial grasses recruitment after ant activity season.