Generalist Predator Populations Research Articles

Diet Specialization in an Insular Population of Coastal Peregrine Falcons

Individual diet specialization is known to occur in populations of generalist predators, where specific individuals develop specialist feeding strategies. Diet specialization has been reported in many raptor species and it may be an important driver of intraspecific population structure. Here, we quantify the diet of Peregrine Falcons Falco peregrinus from five breeding territories on an island determined from prey remains collected over four breeding seasons. Three prey species accounted for 69.8% of total prey frequency, with Manx Shearwater Puffinus puffinus being the primary prey accounting for 47.3% by frequency and 40.8% by biomass. European Herring Gull Larus argentatus was the second most important prey species by frequency (13.8%) and biomass (29.8%) followed by Feral Pigeon Columba livia (frequency: 8.7%, biomass: 7.0%). Prey frequency on specific prey groups varied substantially between breeding pairs and months. Two pairs specialized on Manx Shearwater, one pair specialized on Herring Gull and Manx Shearwater, with the remaining two pairs having a relatively generalist diet of Manx Shearwaters, Feral Pigeon and small passerines. Prey frequency on Manx Shearwaters increased throughout the breeding season with a peak in total diet frequency of 63.8% in July, with a concurrent decrease in Herring Gull prey frequency. Higher percentage of Manx Shearwater in the diet explained 87% of the variation in a narrower dietary breadth for the Peregrine pairs. Our results suggest individual diet specialization may be one of multiple factors important for understanding population density in insular raptor populations.

Intercropping Okra and Castor Bean Reduces Recruitment of Oriental Fruit Moth, Grapholita molesta (Lepidoptera: Tortricidae) in a Pear Orchard.

Intercrops can lower pest densities by increasing plant diversity, altering chemical communication in the arthropod community, and integrating well with other IPM tactics. We used two years of field observations and Y-tube olfactometer assays to explore the effects of intercropping a pear orchard with okra and castor bean on the cosmopolitan fruit-boring pest Grapholita molesta (Lepidoptera: Tortricidae). Intercropping okra reduced G. molesta trap catches in the pear orchard in both years, and intercropping with castor bean reduced them in the second year. Hydrocarbons, phenols, and ketones predominated in the GC-MS assay of okra volatiles, whereas castor bean volatiles were rich in aldehydes, ketones, and esters. Five of the commercially available volatiles released by these plants exhibited repellency to G. molesta in olfactometer trials, especially cinnamaldehyde, dibutyl phthalate, and thymol; the former compound also exhibited attraction to the egg parasitoid Trichogamma dendrolimi (Hymenoptera: Trichogrammatidae). In addition to their repellent properties, okra and castor bean may enhance integrated control of G. molesta in orchards by hosting prey that support populations of generalist predators that either provide biological pest control services within the orchard ecosystem or generate non-consumptive effects that contribute to pest deterence. Among the plant volatiles evaluated, cinnamaldehyde has the best potential for deployment in orchards to repel G. molesta without disrupting augmentative releases of T. dendrolimi.

Impacts of river fragmentation on limiting individual dietary specialization of Amazonian predatory fish.

Individual dietary specialization is one of the factors that promotes variation in resource use at the individual level. Here we used stable isotope analysis of multiple tissues with different turnover rates to examine the degree of individual specialization in two sub-populations of the predator Cichla temensis inhabiting both fragmented and undammed rivers within the Uatumã River basin of the Amazon. Our results showed that the undammed river provides better conditions to promote individual dietary specialization than the fragmented river. This study contributes to the understanding of how specific life history characteristics of populations of generalist predators are impacted by fragmentation within megadiverse environments such as the Amazon basin.

Earthworm abundance and availability does not influence the reproductive decisions of black‐tailed godwits in an agricultural grassland

Abstract Maintaining the biodiversity of agricultural ecosystems has become a global imperative. Across Europe, species that occupy agricultural grasslands, such as black‐tailed godwits (Limosa limosa limosa), have undergone steep population declines. In this context, there is a significant need to both determine the root causes of these declines and identify actions that will promote biodiversity while supporting the livelihoods of farmers. Food availability, and specifically earthworm abundance (Lumbricidae), during the pre‐breeding period has often been suggested as a potential driver of godwit population declines. Previous studies have recommended increasing the application of nitrogen to agricultural grasslands to enhance earthworm populations and aid agricultural production. Here we test whether food availability during the pre‐breeding period affects when and where godwits breed. Using large‐scale surveys of food availability, a long‐term mark‐recapture study, focal observations of foraging female godwits, and tracking devices that monitored godwit movements, we found little evidence of a relationship between earthworm abundance and the timing of godwit reproductive efforts or the density of breeding godwits. Furthermore, we found that the soils of intensively managed agricultural grasslands may frequently be too dry for godwits to forage for those earthworms that are present. The increased application of nitrogen to agricultural grasslands will therefore likely have no positive effect on godwit populations. Instead, management efforts should focus on increasing the botanical diversity of agricultural grasslands, facilitating conditions that prevent hardening soils, and reducing the populations of generalist predators.

Seasonal dynamics of prey utilization and individual specialization in a generalist spider in a pear orchard

Populations of generalist predators are often comprised of a mixture of specialist and generalist individuals and this intraspecific variation in prey utilization may affect pest suppression. However, little is known about seasonal changes in prey utilization by generalist predators, especially during winter, when some generalist predators exert strong biocontrol effects. We used prey preference experiments on Philodromus cespitum (Araneae: Philodromidae) and molecular gut-content analyses of Philodromus spp. (dominated 85% by P. cespitum) to study the dynamics of spider specialization in a pear orchard during the non-growing season. In the laboratory, P. cespitum preferred springtails (Collembola) over the pear psylla, Cacopsylla pyri, an important pest of pear. The presence of springtails in guts of Philodromus spp. did not affect detection of psylla by molecular gut-content analysis in specimens collected from the pear orchard. The philodromid population comprised of a mixture of specialists and generalists in winter, but was composed mainly of generalists in spring. Greater variation in prey utilization during winter most likely reflects a patchy distribution of prey and philodromids switching to a less active hunting mode. Seasonal specialization of individual generalist predators could be the result of their ability to change hunting mode in response to changing environmental conditions and prey distribution. Our study demonstrates that alternative (collembolan) prey are unlikely to affect philodromid predation on psyllids and suggests that changes occur in the spatial structure of predator-pest interactions from winter to spring.

Reducing anthropogenic subsidies can curb density of overabundant predators in protected areas

Protected areas safeguard biodiversity and provide opportunities for human recreation. However, abundant anthropogenic food subsidies associated with human activities in protected areas can lead to high densities of generalist predators, posing a threat to rare species at broad spatial scales. Reducing anthropogenic subsidies could curb populations of overabundant predators, yet the effectiveness of this strategy is unclear. We characterized changes in the foraging ecology, body condition, and demography of a generalist predator, the Steller's jay, three years after implementation of a multi-faceted management program to reduce anthropogenic subsidies in a protected area in California. Stable isotope analysis revealed that the proportional contribution of anthropogenic foods to jay diets declined from 88% to 47% in response to management. Overlap between jay home ranges decreased after management began, while home range size, body condition, and individual fecundity remained stable. Adult density in subsidized areas decreased markedly from 4.33 (SE: ±0.91) to 0.65 (±0.20) jays/ha after the initiation of management, whereas density in unsubsidized areas that were not expected to be affected by management remained stable (0.70 ± 0.22 pre-management, 0.58 ± 0.38 post-management). Thus, the response of jays to management was density-dependent such that reduced densities facilitated the maintenance of individual body condition and fecundity. Importantly, though, jay population size and collective reproductive output declined substantially. Our study provides evidence that limiting anthropogenic subsidies can successfully reduce generalist predator populations and be part of a strategy to increase compatibility of species protection and human recreation within protected areas.

Effect of Herbage Density, Height and Age on Nutrient and Invertebrate Generalist Predator Abundance in Permanent and Temporary Pastures

The aim of this research was to assess differences in the quantity and quality of herbage and invertebrate generalist predator abundance among permanent and temporary pastures. Two permanent pastures and four temporary ley pastures (either one year or two years since being sown) were monitored weekly for 10 weeks in the spring. Permanent pastures included a diverse range of native UK grass species, and temporary ley pastures were predominantly perennial ryegrass (Lolium perenne) with or without white clover (Trifolium repens). Weekly measurements of herbage height (in centimeters), herbage cover (fresh and dry matter in kg per hectare) and herbage density (fresh and dry matter in kg per hectare per centimeter) were obtained for each field, along with lycosid spider and carabid beetle abundance. Weekly pasture samples were used to obtain nutrient concentrations of dry matter, crude protein, neutral detergent fibre (NDF), acid detergent fibre (ADF), ash, oil, sugars, digestible organic matter in the dry matter (DOMD) and metabolisable energy (ME) in the herbage as a measure of forage quality for grazing or harvesting. A linear mixed model was used to assess the effect of sward age, herbage density and height on herbage production, nutrient concentrations and invertebrate abundance. Although this study showed that permanent pastures were associated with lower nutrient concentrations of crude protein, ash, oil and ME compared to younger and predominantly perennial ryegrass pastures, the older pastures were associated with higher carabid numbers. Furthermore, permanent pastures had a higher density of dry matter herbage compared to younger pastures, and more dense and taller swards were associated with higher lycosid numbers. The study suggests that within pastures of 3 to 20 cm height, increasing the height and density of swards increases both ME and oil concentrations in herbage, therefore enhancing forage nutrient quality. Older and more permanent pastures can be beneficial for plant and invertebrate generalist predator populations, and still provide a useful source of nutrients for forage production.

The potential of highly nutritious frozen stages of Tyrophagus putrescentiae as a supplemental food source for the predatory mite Amblyseius swirskii

ABSTRACTAstigmatid mites have potential as supplementary prey items to support generalist predator populations in crops. However, applying living prey mites has some disadvantages; if not predated they have the potential to cause crop damage and allergies. In this study, we evaluated various diets based on the astigmatid mite Tyrophagous putrescentiae (Schrank) as a supplemental food source for the predatory mite Amblyseius swirskii Ahias-Henriot. Eggs and larvae of T. putrescentiae were reared on a diet of dog food (rich in proteins and fat) or bran (rich in carbohydrate); they were offered either frozen or alive, and either with or without cattail pollen (Typha angustifolia L.). Oviposition rate of A. swirskii fed with frozen mite larvae reared on dog food was similar to the rate observed when they were fed with cattail pollen or living prey mites, but developmental time of A. swirskii was longer on this frozen diet than on a diet of living prey mites or pollen. Both living and frozen prey mites were, in contrast with cattail pollen, not suitable for oviposition by western flower thrips, Frankliniella occidentalis Pergande. In a greenhouse study, the use of frozen prey mite stages as supplemental food on chrysanthemum plants allowed populations of A. swirskii to establish, but not increase; in contrast, provision of living prey mites and pollen increased A. swirskii populations on plants. Hence, our study shows that living prey mites, but not frozen prey mites, had the greatest potential as a supplemental food source for A. swirskii.

Spatio-temporal trends in the predation of large gulls by peregrine falcons (Falco peregrinus) in an insular breeding population

Abstract Individual diet specialization occurs in many populations of generalist predators, with specific individuals developing specialist strategies in their feeding behaviour. Intraspecific resource partitioning is hypothesised to be common amongst species in higher trophic levels where competition for resources is intense, and a key driver in breeding success and community structure. Though well-studied in other predators, there is sparse data on ecological specialization in raptors, which are important drivers of community and trophic structure. In this study, the breeding season diet of an insular population of peregrine falcons (Falco peregrinus) was determined from indirect analysis of prey remains collected over three years. An unexpected result was the high proportion of large gulls (Laridae), of the genus Larus, in the diet of two breeding pairs of peregrines. Large gulls made up 18.44% by frequency of total prey recorded and 30.81% by biomass. Herring gulls (Larus argentatus) were the most common large gull prey, with immatures most frequent (67.95%) compared to adults (19.23%). Overall, most gulls predated were immatures (80.77%). Frequency of predation varied between breeding pairs and months, but was consistent over the three years. Most gulls were taken in April (37.17%), followed by May (19.23%), with a smaller peak of immature herring gulls taken in August and September. The pattern of regular predation by peregrines on large gulls is a new observation with important implications for understanding individual diet specialization in raptors, and its effect on bird populations and community structure.

Environmental correlates of breeding abundance and population change of Eurasian Curlew Numenius arquata in Britain

ABSTRACTCapsule: Across Britain, breeding Eurasian Curlew Numenius arquata are less numerous and have shown greater population declines in areas with more arable farming, woodland cover and higher generalist predator abundance.Aims: We present the first national-scale analysis of the potential drivers of Curlew population change in Britain, which is needed to guide conservation action for this globally near-threatened, declining species.Methods: Breeding Bird Survey data and environmental predictors were used to model variation in Curlew abundance in 1995–99 and 2007–11, and population change between these periods.Results: Arable farming and woodland cover were negatively associated with Curlew abundance and population declines. Curlew abundance was positively associated with extent of protected area coverage and gamebird numbers. Abundance and population change were positively associated with cooler temperatures and higher summer rainfall, but negatively associated with numbers of generalist predators.Conclusions: We found support for the negative effects of intensive agriculture, forestry, increases in generalist predator populations and climate warming on Curlew abundance and population change. Effective site protection and measures to reduce generalist predator abundance may be important conservation measures, together with improving breeding habitat quality in the wider countryside.

The contribution of a beneficial insectary plant Scaevola aemula to survival and long-term establishment of flightless Harmonia axyridis in greenhouses

The fairy fan flower, Scaevola aemula R. Br., is a primary candidate insectary plant for maintaining populations of generalist predators. We conducted release experiments in greenhouses of cultivated eggplants to evaluate the effects of intercropping S. aemula on the establishment of flightless Harmonia axyridis Pallas. Compared with a monoculture of eggplant, all release experiments showed that flightless H. axyridis remained in greater numbers in plots with S. aemula planted alongside eggplant. In the release experiment of flightless H. axyridis larvae, the incidence of aphids in the plot with transplanted S. aemula was suppressed compared with that in the release plot without transplanted S. aemula. In a laboratory experiment, the longevity of flightless H. axyridis adults on blossom stems of S. aemula was greater than when open flowers and buds were removed, suggesting that the insects fed on floral resources such as the pollen of S. aemula. Our findings showed that the floral resources of S. aemula can enhance aphid suppression by improving the establishment of flightless H. axyridis.

Drivers of intrapopulation variation in resource use in a generalist predator, the macaroni penguin

Intrapopulation variation in resource use occurs in many populations of generalist predators with important community and evolutionary implications. One of the hypothesised mechanisms for such widespread variation is ecological opportunity, i.e. resource availability determined by intrinsic constraints and extrinsic conditions. In this study, we combined tracking data and stable isotope analysis to examine how breeding constraints and prey conditions influenced intrapopulation variation in resource use in a generalist predator, the macaroni penguin Eudyptes chrysolophus. Isotopic variation was also examined as a function of breeding success, individual traits and individual specialisation. Variation in isotope ratios was greatest across multiple tissue types when birds were able to undertake mid-range foraging trips (i.e. during incubation and pre-moult). This variation was highly consistent between years that spanned a 3-fold difference in local prey Euphausia superba density, and was also highly consistent at the individual level between 2 years that had similar prey densities. Furthermore, by comparing our results with previous work on the same population, it appeared that a decrease in local prey availability can also increase intrapopulation variation in resource use during periods with more restricted foraging ranges (i.e. during brood-guard and creche). This study highlights the importance of considering ecological interactions that operate on multiple spatio-temporal scales when examining the drivers of resource use in populations of generalist predators

Biological control in winter: novel evidence for the importance of generalist predators

Summary The role of generalist predators in pest control has been neglected because generalists are not able to track pest populations. Generalist predators are suggested to be important in spring before specialist predators become active. Here, we show that some generalist predators are important even during winter, when the majority of arthropod pests and their enemies are dormant. We quantified the role of winter‐active generalist predators on the suppression of pear psylla during winter using a discrete nonlinear model of an intraguild predation system. To parameterize our model, we conducted a series of experiments on (i) functional responses, (ii) prey preferences and (iii) ontogenetic development and made observations on the population densities of spiders and potential prey. We ran the model for different winter scenarios, that is, for very cold and very warm winters. Synthesis and applications. We found that winter‐active predators considerably suppressed the pear psylla population. Predators exerted a stronger effect in a warmer winter than in a colder one. Orchard growers thus should avoid use of non‐selective pesticides during this period and instead aim to support the community of generalist predators. Our results suggest more attention should be given to encouraging generalist predator populations in other systems, even in periods when crops are not producing.

Networks of prey specialization in an Arctic monomorphic seabird

Generalist predator populations are sometimes made up of individuals that specialize on particular prey items. To examine specialization in thick‐billed murres Uria lomvia during self‐feeding we obtained stomach contents and muscle stable isotope values for 213 birds feeding close to five colonies in the Canadian Arctic. Adults were less specialized during self‐feeding than during chick‐provisioning. Nonetheless, particular specialists clustered together within the foraging network. While sexes showed similar levels of specialization, individuals of the same sex clustered together within the foraging network. The significant degree of clustering regardless of sex showed that individuals specializing on one prey item tend to also specialize on another, although network topology varied from colony to colony. Adult muscle stable isotope values correlated with the stable isotope values of the prey found in stomachs, at least at the one colony with relevant prey data, suggesting that specializations are maintained over time. Degree of specialization increased with niche width across the five colonies, but similarity in gastro‐intestinal and bill morphology was independent of dietary similarity. Thus, although individual specialization is thought to play a key role in sympatric speciation through trophic specialization, we found no support for an association between morphology and foraging patterns in our species. We conclude that self‐feeding murres show clustered dietary specialization, and that specialization is highest where diet is most diverse.

Vacant land conversion to community gardens: influences on generalist arthropod predators and biocontrol services in urban greenspaces

Due to economic decline and the recent rise in home foreclosure, many U.S. cities are faced with managing large acreages of vacant land. Interest in local food production on this land has the potential to dramatically reshape the composition of greenspace found within urban landscapes. This study examined how the conversion of vacant land to urban gardens and farms influenced arthropod generalist predator populations and their ability to support biocontrol services. We found that the abundance of Coccinellidae and Syrphidae, and the activity density of Carabidae, Formicidae, and Lycosidae were equivalent among vacant lot and urban garden sites. Dolichopodidae abundance and the activity density of Linyphiidae and Opiliones were reduced in urban gardens whereas Anthocoridae abundance and the activity density of Staphylinidae were greater within urban gardens relative to vacant lots. The biocontrol service supplied by generalist predators was measured using sentinel eggs (Helicoverpa zea (Noctuiidae)) and pupae (Sarcophaga bullata (Sarcophagidae) and Musca domestica (Muscidae)). We found no difference in the biocontrol of H. zea eggs or M. domestica pupae among the focal greenspaces. We found no difference in the removal of S. bullata pupae by predators in June 2010, whereas in August 2010 a greater proportion of pupae were removed in urban garden sites relative to vacant lots. Based on these findings, we discuss the potential of the urban landscape to support generalist predators and sustainable urban gardening and farming.

Predation, individual variability and vertebrate population dynamics

Both predation and individual variation in life history traits influence population dynamics. Recent results from laboratory predator-prey systems suggest that differences between individuals can also influence predator-prey dynamics when different genotypes experience different predation-associated mortalities. Despite the growing number of studies in this field, there is no synthesis identifying the overall importance of the interactions between predation and individual heterogeneity and their role in shaping the dynamics of free-ranging populations of vertebrates. We aim to fill this gap with a review that examines how individual variability in prey susceptibility, in predation costs, in predator selectivity, and in predatory performance, might influence prey population dynamics. Based on this review, it is clear that (1) predation risk and costs experienced by free-ranging prey are associated with their phenotypic attributes, (2) many generalist predator populations consist of individual specialists with part of the specialization associated with their phenotypes, and (3) a complete understanding of the population dynamic consequences of predation may require information on individual variability in prey selection and prey vulnerability. Altogether, this work (1) highlights the importance of maintaining long-term, detailed studies of individuals of both predators and prey in contrasting ecological conditions, and (2) advocates for a better use of available information to account for interactive effects between predators and their prey when modelling prey population dynamics.

Absence of effects of predator control on nesting success of Northern Lapwings Vanellus vanellus: implications for conservation

The control of generalist predator populations is increasingly adopted as a management tool to combat declines in ground-nesting bird populations. However, compensatory predation by uncontrolled species frequently occurs, so determining the relative impacts of different predatory species, and hence the relative benefits of their control, can be difficult. Islands, with their reduced faunas, provide natural experimental units for investigating specific predator–prey interactions in detail. We studied Northern Lapwing Vanellus vanellus breeding success on an island where feral Ferrets Mustela furo and Hooded Crows Corvus cornix were subjected to trial control regimes over 2 years. In both years, Lapwing hatching success was >80%, with neither Ferret nor Crow control selected as important predictors. Fledging rates in both years were 0.7 young per pair, despite highly effective predator removal, although Crow control potentially resulted in compensatory predation by Common Ravens C. corax. Neither mustelid nor corvid control produced significant immediate benefits for Lapwings. This suggests that mesopredator release of mustelids in mainland situations is unlikely to be a consistent threat to Lapwing, and provides further evidence that declines in this species are unlikely to be tackled successfully through predator management alone.

Addition of a new resource in agroecosystems: Do cover crops alter the trophic positions of generalist predators?

Generalist predators can play an important role in agroecosystems by controlling herbivores via top-down effects. As cover crops are increasingly used in agroecosystems, the effects of this resource on generalist predator diet need to be evaluated. We studied the effect of adding a cover crop, Brachiaria decumbens, on trophic niches of generalist predators in a banana agroecosystem by analysing stable isotope variation in C and N for Cosmopolites sordidus, a major banana pest, and its potential predators (spiders, ants, centipedes, and earwigs). While addition of the new resource did not change the trophic niche of the banana pest C. sordidus, the trophic position of the generalist predators was changed as indicated by δ 13C signature. Cover crop provided resources that are likely to support a community of insect herbivores, which are alternative preys for generalist predators. The failure of the cover crop to increase δ 15N signature is inconsistent with the hypothesis that the cover crop would increase intraguild predation. By providing alternative preys, the addition of a new resource in agroecosystems has the potential to increase populations of generalist predators and therefore pest control.

Geographic variation in North American gypsy moth cycles: subharmonics, generalist predators, and spatial coupling

Many defoliating forest lepidopterans cause predictable periodic deforestation. Several of these species exhibit geographical variation in both the strength of periodic behavior and the frequency of cycles. The mathematical models used to describe the population dynamics of such species commonly predict that gradual variation in the underlying ecological mechanisms may lead to punctuated (subharmonic) variation in outbreak cycles through period-doubling cascades. Gypsy moth, Lymantria dispar, in its recently established range in the northeastern United States may represent an unusually clear natural manifestation of this phenomenon. In this study we introduce a new statistical spatial-smoothing method for estimating outbreak periodicity from space-time defoliation data collected with spatial error. The method statistically confirms the existence of subharmonic variation in cyclicity among different forest types. Some xeric forest types exhibit a statistical 4-5 year period in outbreak dynamics, some mesic forest types a 9-10 year period, and some intermediate forest types a dominant 9-10 year period with a 4-5 year subdominant superharmonic. We then use a theoretical model involving gypsy moth, pathogens, and predators to investigate the possible role of geographical variation in generalist predator populations as the cause of this variation in dynamics. The model predicts that the period of gypsy moth oscillations should be positively associated with predator carrying capacity and that variation in the carrying capacity provides a parsimonious explanation of previous reports of geographical variation in gypsy moth periodicity. Furthermore, a two-patch spatial extension of the model shows that, in the presence of spatial coupling, subharmonic attractors can coexist whereas nonharmonic attractors (i.e., where the cycle lengths are not integer multiples of one another) cannot.

Cattle grazing, raptor abundance and small mammal communities in Mediterranean grasslands

Abundance and diversity of small mammals are usually affected strongly by grazing either due to decreased food availability or quality, decreased suitability of soil for building burrow systems due to trampling and/or due to increased predation risk in the structurally simpler grazed areas. We estimated the effects of grazing-induced changes in vegetation and soil and of increased predation on small mammals in a Mediterranean grassland landscape. We measured vegetation structure, soil compaction and small mammal abundance and species composition in 22 plots of 8 Sherman live traps each, arranged according to an unbalanced two-way ANOVA design with two grazing levels (grazed areas and cattle exclosures) and two predator abundance levels (increased densities of Eurasian kestrels Falco tinnunculus by means of nest boxes and control). Plots were sampled during 2 consecutive years in early summer and early fall. Exclosure from cattle increased significantly vegetation height and volume and decreased soil compaction. Grazing-induced changes in vegetation height and volume and in soil compaction produced strong effects on small mammal abundance and species richness. Increased kestrel densities did not have significant additive or interactive effects, with the effects of grazing-induced vegetation and soil gradients on abundance or richness of small mammals. Our results suggest that the effects of grazing on small mammal communities in Mediterranean montane grasslands were mainly due to reduced food availability and by negative effects of trampling on the suitability of soils for building burrow systems. Decreased food quality and increased predation in grazed areas seemed to play a minor role, if any. Reductions in stock densities would then favor generalist predator populations in Mediterranean grasslands through the expected positive effects of such reductions on the availability of food and burrows for small mammals.