Functional extinction of a desert rodent: implications for seed fate and vegetation dynamics

Population declines of once‐abundant species have often preceded understanding of their roles within ecosystems. Consequently, important drivers of environmental change may remain undiagnosed because we simply do not know how species that are now rare or extinct shaped ecosystems in the past. Australia's desert rodents are thought to have little numerical impact on seed fate and vegetation recruitment when compared with ants or with desert rodents on other continents. However most research on granivory by Australian desert rodents has occurred in areas where rodents were rare or functionally extinct. Here we ask if the paradigm that rodents are relatively un‐important granivores in Australian deserts is an artefact of their historical decline. In the Strzelecki Desert, the endangered rodent, Notomys fuscus is rare where introduced mesopredators are abundant but common where dingoes (an apex predator) suppress mesopredator populations. We used foraging trays to compare rates of seed removal for a common shrub (Dodonaea viscosa angustissima hopbush) between areas where N. fuscus, hopbush shrubs and their seedlings were rare and common and found that seed removal was consistently higher where rodents were common and hopbush rare. By excluding ants and rodents from foraging trays we show that ants removed more seeds than rodents where rodents were rare but rodents removed far more seeds than ants where rodents were common. By manipulating rodents’ access to the soil seed‐bank we show that hopbush seeds persisted in greater numbers where rodents were excluded than where they had access. Our results support the hypothesis that granivory by rodents may once have been a far more important process influencing the fate of seeds and shaping plant communities in arid Australia and suggest that dingo extirpation has cascading effects on shrub seeds. Our study highlights that functional extinction of rodents may be an under‐appreciated driver of vegetation change.     

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.     

Harvester ants modify seed rain using nest vegetation and granivory

1. In annual plant communities, the seed bank determines each generation's potential emergence and any factor that influences it could alter community structure. Harvester ants are common seed predators that may cause seed sources and seed rain composition to vary. 2. In semi‐arid shrubland of the Negev desert of Israel, we hypothesised that the positive effects of dense vegetation on nests of the harvester ant Messor ebeninus Sant. would outweigh reduction by granivory. 3. To test whether nests were seed sources, we removed vegetation on 10 of 20 sampled nests prior to seeding. We tested the interaction between seed sources and granivory by locating seed traps at three locations: nests, foraging trails, and unvisited areas. Trapped seeds were recorded during spring and summer dispersal. Plant recruitment from the altered seed bank was recorded the following spring. 4. During April, seeds were more abundant near nests than the other locations and at uncut compared with cut nests. Foraging trails had fewer species and higher equitability compared with nests or unvisited areas. May and June to August had highest species richness at nests, but richness of cut and uncut nests were similar. Ordination of plant recruitment showed slightly lower vegetation community variation by diminished species in the cut seed bank. 5. While intact nest vegetation initially had high abundance of locally deposited seeds, it apparently blocked wind‐dispersed seeds later in the summer. Conversely, granivory reduced seed species richness only on trails and decreased dominance structure by preferential removal of some species.     

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.     

Mechanisms affecting the fate of Prosopis flexuosa (Fabaceae,Mimosoideae) seeds during early secondary dispersal in the Monte Desert,Argentina

Abstract The fate of seeds during secondary dispersal is largely unknown for most species in most ecosystems. This paper deals with sources of seed output of Prosopis flexuosa D.C. (Fabaceae, Mimosoideae) from the surface soil seed‐bank. Prosopis flexuosa is the main tree species in the central Monte Desert, Argentina. In spite of occasional high fruit production, P. flexuosa seeds are not usually found in the soil, suggesting that this species does not form a persistent soil seed‐bank. The magnitude of removal by animals and germination of P. flexuosa seeds was experimentally analysed during the first stage of secondary dispersal (early autumn). The proportion of seeds removed by granivores was assessed by offering different types of diaspores: free seeds, seeds inside intact endocarps, pod segments consisting of 2–3 seeds, and seeds from faeces of one herbivorous hystricognath rodent, the mara (Dolichotis patagonum). The proportion of seeds lost through germination was measured for seeds inside intact endocarps, seeds inside artificially broken endocarps, and free seeds. Removal by ants and mammals is the main factor limiting the formation of a persistent soil seed‐bank of P. flexuosa: >90% of the offered seeds were removed within 24 h of exposure to granivores in three of four treatments. Seeds from the faeces of maras, on the other hand, were less vulnerable to granivory than were other types of diaspores. These results suggest that herbivory might be an indirect mechanism promoting seed longevity in the soil (and likely germination) by discouraging granivore attack. On the other hand, germination did not seem to have an important postdispersal impact on the persistence of P. flexuosa seeds in the soil. Both direct and indirect interactions between vertebrate herbivores and plants may foster P. flexuosa's seed germination in some South American arid zones.     

Foraging behavior and morphology: seed selection in the harvester ant genus, Pogonomyrmex

Optimally foraging animals can be behaviorally or morphologically adapted to reduce the energetic and time costs of foraging. We studied the foraging behavior and morphology of three seed harvester ant species, Pogonomyrmex barbatus, P. desertorum, and P. occidentalis, to determine the importance of behavioral strategies and morphological features associated with load carriage in reducing the costs of foraging. We found that none of five morphological features we measured had a significant impact on seed selection. Also, body size did not influence running speed, an important variable in time costs of foraging. Temperature had the largest effect on running speed in these species. Our results show that these species have foraging strategies which minimize the time costs of traveling with seeds. We also describe a pattern where the running speed in individual-foraging species is less affected by increasing seed size than in trunk-trail foragers, when temperature and body mass are held constant. These results support previous work which showed that time costs are most important in seed selection for Pogonomyrmex, and suggest that central place foraging theory may need to accommodate variation in foraging strategy to more accurately predict optimal seed size selection in harvester ants. Received: 16 June 1997 / Accepted: 15 December 1997     

Diet of two sympatric Pheidole spp. ants in the central Monte desert: implications for seed–granivore interactions

Ants of the genus Pheidole are important seed consumers in several desert ecosystems. In South American deserts, although several Pheidole spp. have been characterized as seed harvesters, studies on their diet and ecological role are still missing. Pheidole spininodis (Mayr) and Pheidole bergi (Mayr) are capable of removing seeds in the central Monte desert. The aim of this study was to quantify and compare the diet of these species and to interpret the results in the context of seed–granivore interactions. Diet was estimated during mid-summer by collecting items brought back to the nest by foragers in ten colonies per species. While P. spininodis was mainly granivorous, P. bergi was mainly insectivorous. However, they both collected ~40% of other types of items. Among seeds, the diet of P. spininodis included mostly grass seeds, whereas the diet of P. bergi was mainly made up of shrub and tree seeds, usually retrieved cooperatively. This behavior allowed P. bergi to carry larger seeds, resulting in diet partitioning in terms of seed size. However, diet of P. spininodis is very similar to that of three sympatric Pogonomyrmex species. Thus, specialized harvester ants remove large quantities of grass seeds in the central Monte desert during the summer, potentially affecting their abundance in the soil seed bank. P. bergi directs its feeding pressure to shrub and tree seeds, and although seeds constitute ~10% of its diet, its high colony density and high activity levels, added to the lower proportion of large seeds in the soil seed bank, indicate that their importance as seed consumers cannot be ruled out.     

Effect of three mound-building ant species on the formation of soil seed bank in mountain grassland

The effect of three ant species (Lasius flavus, Formica spp., Tetramorium caespitum) on soil seed bank formation was studied in temperate mountain grassland. Seed removal experiments, analysis of soil seed content and seed survival experiments were carried out to evaluate the influence of ground ants on the seed fate. In the seed removal experiment seeds of 16 species, including 5 species with elaiosome-bearing seeds (myrmecochores), were exposed and their removal followed for 39 h. On average, ants removed 63.8% of myrmecochorous seeds and 10.9% of seeds without adaptation to ant dispersal. Analysis of soil seed content revealed that myrmecochores, in spite of expectations that they would accumulate in nests of seed dispersing ants, were most abundant in the soil of control plots. Evidence on seed relocation to the ant nests was obtained from a comparison of mounds of seed dispersing and seed non-dispersing ant species, as more seeds were found in the mounds of Formica spp. and Tetramorium caespitum (seed dispersers) in comparison with the mounds of Lasius favus (non-disperser).The soil seed bank of the compared microhabitats (control plots and mounds of 3 ant species) differed in their species composition, seed abundance and vertical distribution. The most distinct qualitative differences were between seed flora of control plots and mounds of Tetramorium caespitum. Control plots had approximately 30,000 propagules per m², which was double the number of seeds found in the ant mounds. In control plots, abundance and diversity of seeds steeply declined with depth; this trend was not observed in the mounds probably due to bioturbation. In the seed survival experiment, more seeds (2 out of 3 species) survived in control plots, which may also contribute to the higher seed abundance in this microhabitat.This study showed that seed relocation by ants does not contribute significantly to seed bank build-up at this study site. Ants may, however, increase the regeneration success of myrmecochores, mainly by dispersal for distance and placement in a larger spectrum of microsites, in contrast to species not adapted for myrmecochory.     

Complex foraging ecology of the red harvester ant and its effect on the soil seed bank

Granivory is an important interaction in the arid and semi-arid zones of the world, since seeds form an abundant and nutritious resource in these areas. While species of the genus Pogonomyrmex have been studied in detail as seed predators, their impact on seed abundance in the soil has not yet been explored in sufficient depth. We studied the impact of the harvesting activities of the ant Pogonomyrmex barbatus on seed abundance in the soil of the Zapotitlán valley, Mexico. We found that P. barbatus activity significantly impacts the abundance of seeds in the soil, which is lower in the sites where P. barbatus forages than it is in sites with no recorded foraging. We also found that P. barbatus distributes intact seeds of three tree species, two of which are nurse plants, and could consequently be promoting the establishment of these species. Using tools derived from graph theory, we observed that the ant-seed interactions exhibit a nested pattern; where more depredated seed species seem to be the more spatially abundant in the environment. This study illustrates the complex foraging ecology of the harvester ant P. barbatus and elucidates its effect on the soil seed bank in a semi-arid environment.     

The effects of harvester ant (Messor ebeninus Forel) nests on vegetation and soil properties in a desert dwarf shrub community in north-eastern Arabia

In desert ecosystems, harvester ants have been shown to be important granivores and seed dispersers. Because many desert plants exist as seeds in the ground for long periods, harvester ants may greatly influence plant population dynamics. In this study, we examine the effects of harvester ant nests on vegetation and soil properties in a desert ecosystem dominated by the dwarf shrub Rhanterium epapposum in Kuwait, north-eastern Arabia. Soil properties were greatly modified in the circular refuse zone around the nests of the harvester ants, with elevated levels of nutrients (except nitrogen) and organic matter. Plant species richness and productivity were also significantly enhanced in the refuse zone. The vast majority of species inhabiting this zone were desert annuals, but there is no evidence to suggest that the species involved have any distinct association with ant nests. Harvester ants therefore contribute substantially to small-scale spatial heterogeneity in this aridland plant community. It is also suggested that the significance of ant nests in providing favourable sites for plant growth is enhanced in drier years, when many species in off-nest sites may die prematurely due to drought. Furthermore, because ants occasionally abandon their nests and create new colonies, the addition of nutrient-rich patches to the landscape over time could represent an important mechanism for maintaining fertility of desert soils, possibly with long-term implications for plant biodiversity. Harvester ants can therefore be regarded as a key ecosystem engineer in this relatively undisturbed desert ecosystem of Kuwait.     

Influence of seed size on feeding preferences and diet composition of three sympatric harvester ants in the central Monte Desert,Argentina

Selective seed consumption by harvester ants may affect seed abundance and composition and, ultimately, plant communities. We evaluated the influence of seed size on preferences and diet of Pogonomyrmex mendozanus, P. rastratus, and P. inermis in the central Monte Desert, Argentina. In choice experiments with Pappophorum spp. seeds of different sizes, P. mendozanus and P. rastratus preferred large seeds, maximizing energy reward. P. inermis showed a less-marked preference for large seeds, which was probably due to morphological constraints imposed by its small body size. Under natural conditions, none of the three species selected larger Pappophorum spp. seeds probably because of high travel and handling costs. Seeds of intermediate size predominated in the diet of the three species but a slight size match was detected as P. mendozanus carried larger seeds than P. rastratus and this than P. inermis, matching body-size differences. Thus, ants probably maximize energy reward but face morphological restrictions and higher costs when carrying and holding large seeds. While seeds of intermediate size are the most vulnerable ones to ant predation, small seeds are favored, as they are abundant in the soil seed bank and lowly predated.     

Individual foraging components of harvester ants: movement patterns and seed patch fidelity

Summary We investigated individual foraging components of the western harvester ant,Pogonomyrmex occidentalis, in the native seed background of a shrub-steppe environment. Our study identified factors affecting foraging movements and seed selection by individual ants. Some assumptions and predictions of central-place foraging theory and a correlated random walk were evaluated for individual foragers. Results showed that ant size was only weakly correlated with the seed sizes harvested; seed size was a more important constraint than a predictor of seed selection. Individual ants spent more time in localized search behavior than traveling between search areas and nests.P. occidentalis foragers encountered seeds randomly with respect to time, and handled a mean of 1.7 seeds/trip. A correlation of increased search effort with greater travel distances was consistent with central-place foraging theory but, contrary to it, search and travel effort were not associated with energetic reward.Individual ants exhibited fidelity in both search site and native seed species. Spatial analyses of foraging movements showed a highly oriented travel path while running, and an area-restricted path while searching. Searching ants moved in a manner consistent with a correlated random walk. The deterministic component of patch fidelity and the stochastic component of search may override energetic foraging decisions in individualP. occidentalis ants.     

Differential weed seed removal in dryland cereals

Weeds persist in rain-fed cereal fields in NE Spain, despite intense herbicide use and high seed removal rates by granivorous harvester ants. Herbicide resistance is involved, but certain weed species also appear to escape seed removal by granivores. To identify the mechanisms involved, we measured seed removal rates (three fields in 2010) and the timing of seed shed (one field in 2009 and three fields in 2010) and used an existing model, which integrates short-term rates of seed shed, burial and removal, to estimate long-term seed removal rates. Averaged over years, fields and weed species, the long-term seed removal rate was estimated at 72?% (range 46?C100?%). Fifteen to 25?% of the seeds of Bromus diandrus avoided removal by being less attractive (low removal rates), and another 0?C29?% escaped through crop harvest, which made seeds inaccessible to granivores. Similarly, 20?C32?% of the Papaver rhoeas seeds escaped through crop harvest, while another 13?C17?% escaped by burial into the soil (small seed size). Other species, such as Galium spurium or Diplotaxis erucoides, had no means of avoiding seed removal by harvester ants. In particular, the more troublesome weeds, such as B. diandrus, P. rhoeas and L.?rigidum, combined herbicide resistance or tolerance with avoidance mechanisms against granivory.     

Foraging energetics of the ant,Paraponera clavata

The energy currencies used by foraging animals are expected to relate to the energy costs and benefits of resource collection. However, actual costs of foraging are rarely measured. We measured the ratio of energetic benefit relative to cost (B/C) during foraging for the giant tropical ant, Paraponera clavata. The B/C ratio was 3.9 for nectar-foragers and 67 for insect prey foragers. In contrast, the B/C ratio during foraging for seed harvester ants (Pogonomyrmex occidentalis) is over 1000, demonstrating that the B/C ratio can vary widely among ants. The B/C ratio was 300 times lower for nectar-foraging Paraponera than for the seed-harvesting Pogonomyrmex because of: (1) a 5-fold lower energetic benefit per trip, (2) a 10-fold greater cost due to longer foraging distances, and (3) a 6-fold greater energy cost per meter due to larger body size. For Paraponera daily colonial energy intake rates are similar to expeditures and may limit colony growth and reproduction. In contrast, for Pogonomyrmex energy intake rates are an order of magnitude higher than estimated costs, suggesting that Pogonomyrmex colonies are unlikely to be limited by short-term energy intake. We suggest that variation in individual B/C ratios may explain why the foraging behavior of Paraponera but not Pogonomyrmex appears sensitive to foraging costs.     

Effects of seed predation by ants on Mediterranean grassland related to seed size

Questions: 1. Do harvester ants (Messor barbarus) promote seed mortality in Mediterranean grassland?; 2. Is this effect greater in large‐seeded species? Location: Central Spain. Methods: We established an ant‐exclusion experiment of five circular (1.5 m diameter) plots from where ants were excluded during one year, along with ten control plots. We recorded the seed bank of all species in the plots both before and after the treatment. The effect of seed length and weight was analysed after transforming data into phylogenetically independent contrasts, and alternatively by dividing the species data set into morphological groups. Results: Longer and heavier seeded species significantly increased in the seed banks under the exclusion treatment, although ants did not significantly modify overall seed densities. Conclusions Although the ants do not collect large numbers of seeds, they differentially affect the composition of the seed banks by selecting the longest or heaviest seeds, or both. The persistence of this short‐term effect in the seed bank may result, over a number of years, in the system evolving towards a predominance of small‐seeded annuals, congruent with the species composition actually observed in Mediterranean grasslands.     

Temporal patterns of seed use and availability in a guild of desert ants

ABSTRACT.

• 1 Temporal patterns of seed use were studied from late winter to autumn in three species of seed-harvesting ants in the Sonoran Desert. Measures of effective foraging activity, dietary niche breadth and dietary niche overlaps were obtained each month and were tested for correlation with estimates of the available seed resource.
• 2 Seeds were the only numerically important type of food in the diets of all species.
• 3 The ants partitioned the resource according to both seed species and seed size, although there was considerable overlap.
• 4 Pheidole xerophila had the smallest forager body size and is a specialist on small seeds because it harvested them in greater proportion than their rank in the soils and expanded its diet to larger seeds only when the abundance of small seeds declined.
• 5 When the abundance of the small seeds of Bouteloua barbata decreased, the middle-sized ant, Veromessor pergandei, showed a decrease in foraging activity, increase in niche breadth, and a decrease in overlap with P.xerophila.
• 6 Seed size preferences of V.pergandei did not vary seasonally, except that during the month of highest seed abundance, V.pergandei showed no size preference.
• 7 Pogonomyrmex rugosus was the largest ant; it preferred larger seeds and was inactive when small seeds were most abundant. Seasonal foraging activity and niche parameters were random in relation to seed abundance.
• 8 We suggest that nocturnal foraging by P.rugosus during the summer months was a response to interference with diurnal foraging by either predation frorn horned lizards or competition from V.pergandei.
• 9 Seasonal abundance of small seeds explains most of the seasonal foraging patterns of P.xerophila and V.pergandei. The summertime abundance of larger seeds during years of adequate precipitation may account for the seasonal activity patterns of P.rugosus.

     

Impact of Formica exsecta Nyl. on seed bank and vegetation patterns in a subalpine grassland ecosystem

The mound building ant Formica exsecta Nyl. is widely distributed in grassland ecosystems of the Central European Alps. We studied the impact of these ants on seed bank and vegetation patterns in a 11 ha subalpine grassland, where we counted over 700 active ant mounds. The mounds showed a distinct spatial distribution with most of them being located in tall‐grass, which was rarely visited by ungulates (red deer; Cervus elaphus L.). Heavily grazed short‐grass, in contrast, seemed to be completely avoided by ants as only few mounds were found in this vegetation type. The species composition of the ant mound and grassland seed banks was quite similar, i.e. from 15 common plant species 12 were found in both seed bank types. We found the same proportions of myrmecochorous seeds in ant mound and grassland soil samples. In contrast, the number of seeds was 15 times higher in mound compared with the grassland soil samples. Also, the vegetation growing on ant mounds significantly differed from the vegetation outside the mounds: graminoids dominated on ant mounds, herbaceous and myrmecochorous species in the grassland vegetation. We found significant continuous changes in vegetation composition on gradients from the ant mound centre to 1 m away from the mound edge. Overall, F. exsecta was found to have a considerable impact on seed bank and vegetation patterns in the grassland ecosystem studied. These insects not only altered grassland characteristics in the close surrounding of their mounds, but also seem to affect the entire ecosystem including, for example, the spatial use of the grassland by red deer.     

Positive interactions between desert granivores: localized facilitation of harvester ants by kangaroo rats

Facilitation, when one species enhances the environment or performance of another species, can be highly localized in space. While facilitation in plant communities has been intensely studied, the role of facilitation in shaping animal communities is less well understood. In the Chihuahuan Desert, both kangaroo rats and harvester ants depend on the abundant seeds of annual plants. Kangaroo rats, however, are hypothesized to facilitate harvester ants through soil disturbance and selective seed predation rather than competing with them. I used a spatially explicit approach to examine whether a positive or negative interaction exists between banner-tailed kangaroo rat (Dipodomys spectabilis) mounds and rough harvester ant (Pogonomyrmex rugosus) colonies. The presence of a scale-dependent interaction between mounds and colonies was tested by comparing fitted spatial point process models with and without interspecific effects. Also, the effect of proximity to a mound on colony mortality and spatial patterns of surviving colonies was examined. The spatial pattern of kangaroo rat mounds and harvester ant colonies was consistent with a positive interspecific interaction at small scales (<10 m). Mortality risk of vulnerable, recently founded harvester ant colonies was lower when located close to a kangaroo rat mound and proximity to a mound partly predicted the spatial pattern of surviving colonies. My findings support localized facilitation of harvester ants by kangaroo rats, likely mediated through ecosystem engineering and foraging effects on plant cover and composition. The scale-dependent effect of kangaroo rats on abiotic and biotic factors appears to result in greater founding and survivorship of young colonies near mounds. These results suggest that soil disturbance and foraging by rodents can have subtle impacts on the distribution and demography of other species.     

Effect of seed predation on seed bank size and seedling recruitment of bush lupine (Lupinus arboreus)

Whether seed consumers affect plant establishment is an important unresolved question in plant population biology. Seed consumption is ubiquitous; at issue is whether seedling recruitment is limited by safe-sites or seeds. If most seeds inhabit sites unsuitable for germination, post-dispersal seed consumption primarily removes seeds that would otherwise never contribute to the population and granivory has minimal impacts on plant abundance. Alternatively, if most seeds ultimately germinate before they lose viability, there is greater potential for seed consumption to affect plant recruitment. Of the many studies on seed consumption, few ask how seed loss affects seedling recruitment for species with long-lived seed banks. We examined post-dispersal seed predation and seedling emergence in bush lupine (Lupinus arboreus), a woody leguminous shrub of coastal grasslands and dunes in California. We followed the fate of seeds in paired experimental seed plots that were either protected or exposed to rodent granivores in grassland and dune habitats. Significantly more seeds were removed by rodents in dunes than grasslands. In dunes, where rodent granivory was greatest (65% and 86% of seeds removed from plots by rodents in two successive years), there is a sparse seed bank (6.6 seeds m⁻²), and granivory significantly reduced seedling emergence (in the same two years, 18% and 19.4% fewer seedlings emerged from exposed versus protected plots), suggesting seed rather than safe-site limited seedling recruitment. In contrast, rodents removed an average of 6% and 56% of seeds from grassland plots during the same two years, and the grassland seed bank is 43-fold that of the dunes (288 seeds m⁻²). Even high seed consumption in the second year of the study only marginally influenced recruitment because seeds that escaped predation remained dormant. Burial of seeds in both habitats significantly reduced the percentage of seeds removed by rodents. Results suggest that granivores exert strong but habitat-dependent effects on lupine seed survival and seedling emergence. Received: 24 October 1996 / Accepted: 4 February 1997     

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

Harvester ants usually go through temporal fluctuations in environmental seed abundance and composition which could influence their behaviour and ecology. The aim of this study was to evaluate how these fluctuations influence the diet of Pogonomyrmex rastratus, P. pronotalis and P. inermis (Hymenoptera, Formicidae) in the central Monte desert during three consecutive growing seasons. Although seeds were the main item in the diet, these ants turned more generalist when seed abundance of the most consumed species (grasses Aristida spp., Trichloris crinita, Pappophorum spp., Digitaria californica and Stipa ichu) was low. Accordingly, diversity of items in the diet decreased with seed abundance in a logarithmical fashion, showing higher foraging efficiency for seeds at higher seed abundance. Seed diversity, however, was not related to seed abundance as ants always included several species in their diet, with alternating prevalence. The proportion of the most consumed species increased logarithmically in the diet of P. rastratus and P. pronotalis along with their abundance in the environment probably as a consequence of diet switching (from forb and shrub seeds to grass seeds) and by an increase in foraging efficiency at higher seed densities. In contrast, foraging activity of P. inermis was very low at low seed abundance and its diet included only the five grasses. Among the most consumed species, proportion in the diet was not associated with relative abundance in the environment. Aristida spp., Pappophorum spp. and D. californica were overall highly selected. However, the flexibility in the diet of P. pronotalis and P. rastratus and the low foraging activity of P. inermis during periods of low resource abundance could attenuate potential top‐down effects in the central Monte desert. This study shows that bottom‐up effects are important in ant‐seed interactions and should be considered when predicting and evaluating ants' effects on seed resources.