Cyclicity and variability in prey dynamics strengthens predator numerical response: the effects of vole fluctuations on white stork productivity

Theory predicts that optimality of life-long investment in reproduction is, among other factors, driven by the variability and predictability of the resources. Similarly, during the breeding season, single resource pulses characterized by short periods and high amplitudes enable strong numerical responses in their consumers. However, it is less well established how spatio-temporal dynamics in resource supplies influence the spatio-temporal variation of consumer reproduction. We used the common vole (Microtus arvalis)—white stork (Ciconia ciconia) resource—consumer model system to test the effect of increased temporal variation and periodicity of vole population dynamics on the strength of the local numerical response of storks. We estimated variability, cycle amplitude, and periodicity (by means of direct and delayed density dependence) in 13 Czech and Polish vole populations. Cross-correlation between annual stork productivity and vole abundance, characterizing the strength of the local numerical response of storks, increased when the vole population fluctuated more and population cycles were shorter. We further show that the onset of incubation of storks was delayed during the years of higher vole abundance. We demonstrate that high reproductive flexibility of a generalist consumer in tracking the temporal dynamics of its resource is driven by the properties of the local resource dynamics and we discuss possible mechanisms behind these patterns.     

Barn Owl Productivity Response to Variability of Vole Populations

We studied the response of the barn owl annual productivity to the common vole population numbers and variability to test the effects of environmental stochasticity on their life histories. Current theory predicts that temporal environmental variability can affect long-term nonlinear responses (e.g., production of young) both positively and negatively, depending on the shape of the relationship between the response and environmental variables. At the level of the Czech Republic, we examined the shape of the relationship between the annual sum of fledglings (annual productivity) and vole numbers in both non-detrended and detrended data. At the districts’ level, we explored whether the degree of synchrony (measured by the correlation coefficient) and the strength of the productivity response increase (measured by the regression coefficient) in areas with higher vole population variability measured by the s-index. We found that the owls’ annual productivity increased linearly with vole numbers in the Czech Republic. Furthermore, based on district data, we also found that synchrony between dynamics in owls’ reproductive output and vole numbers increased with vole population variability. However, the strength of the response was not affected by the vole population variability. Additionally, we have shown that detrending remarkably increases the Taylor’s exponent b relating variance to mean in vole time series, thereby reversing the relationship between the coefficient of variation and the mean. This shift was not responsible for the increased synchrony with vole population variability. Instead, we suggest that higher synchrony could result from high food specialization of owls on the common vole in areas with highly fluctuating vole populations.     

Population‐level manipulations of field vole densities induce subsequent changes in plant quality but no impacts on vole demography

Grazing‐induced changes in plant quality have been suggested to drive the negative delayed density dependence exhibited by many herbivore species, but little field evidence exists to support this hypothesis. We tested a key premise of the hypothesis that reciprocal feedback between vole grazing pressure and the induction of anti‐herbivore silicon defenses in grasses drives observed population cycles in a large‐scale field experiment in northern England. We repeatedly reduced population densities of field voles (Microtus agrestis) on replicated 1‐ha grassland plots at Kielder Forest, northern England, over a period of 1 year. Subsequently, we tested for the impact of past density on vole life history traits in spring, and whether these effects were driven by induced silicon defenses in the voles’ major over‐winter food, the grass Deschampsia caespitosa. After several months of density manipulation, leaf silicon concentrations diverged and averaged 22% lower on sites where vole density had been reduced, but this difference did not persist beyond the period of the density manipulations. There were no significant effects of our density manipulations on vole body mass, spring population growth rate, or mean date for the onset of spring reproduction the following year. These findings show that grazing by field voles does induce increased silicon defenses in grasses at a landscape scale. However, at the vole densities encountered, levels of plant damage appear to be below those needed to induce changes in silicon levels large and persistent enough to affect vole performance, confirming the threshold effects we have previously observed in laboratory‐based studies. Our findings do not support the plant quality hypothesis for observed vole population cycles in northern England, at least over the range of vole densities that now prevail here.     

A new geographical gradient in vole population dynamics

A new geographical gradient in the dynamics of small rodents is demonstrated by analysing 29 time series of density indices of the common vole (Microtus arvalis) from Poland, the Czech Republic and the Slovak Republic. This gradient extends from more stable northerly populations in coastal Poland to more variable and cyclic populations in the southernmost parts of the Slovak Republic, and is hence a reversal of the Fennoscandian gradient. All studied variables (such as mean density, cycle amplitude, density variability and the coefficients in a second-order autoregressive model) exhibit consistent latitudinal variation. Possible underlying factors are discussed. In particular, we suggest that seasonality may be a key element in explaining the observed new gradient.     

Multiple predators induce risk reduction in coexisting vole species

Large predators may affect the hunting efficiency of smaller ones directly by decreasing their numbers, or indirectly by altering their behaviour. Either way this may have positive effects on the density of shared prey. Using large outdoor enclosures, we experimentally studied whether the presence of the Tengmalm's owl Aegolius funereus affects the hunting efficiency of the smallest member of the vole-eating predator guild, the least weasel Mustela nivalis, as measured by population responses of coexisting prey species, the field vole Microtus agrestis and the sibling vole M. levis . We compared the density and survival probability of vole populations exposed to no predation, weasel predation or combined predation by a weasel and an owl. The combined predation of both owl and weasel did not result in obvious changes in the density of sibling and field vole populations compared to the control populations without predators, while predation by least weasel alone decreased the densities of sibling voles and induced a similar trend in field vole densities. Survival of field voles was not affected by predator treatment while sibling vole survival was lower in predator treated populations than in control populations. Our results suggest that weasels are intimidated by avian predators, but without changing the effects of predators on competitive situations between the two vole species. Non-lethal effects of intraguild predation therefore will not necessarily change competitive interactions between shared prey species.     

Fox predation on cyclic field vole populations in Britain

The diet of the red fox Vulpes vulpes L. was studied during three winter periods in spruce pklantations in Britain, during which time the cyclic field vole Microtus agrestis L. populations varied in abundance. Field voles and roe deer Capreolus capreolus L. were the two main prey species in the diet of the red fox. The contribution of lagomorphs to fox diet never exceeded 35% and species of small mammal other than field voles were of minor importance. The contribution of field voles was dependent on vole density. The non-linear density dependent relationship with a rather abrupt increase of field voles in fox did when vole density exceeded ca 100 voles ha⁻¹ was consistent with a prey-switching response. The contribution of field voles to fox diet during the low phase of population cycles was lower in Kielder Forest than in other ecosystems with cyclic vole populations. The number of foxes killed annually by forestry rangers was consistent with the evidence from other studies that foxes preying on cyclic small rodents might show a delayed numerical response to changes in vole abundance. Estimates of the maximum predation rate of the fox alone (200–290 voles ha⁻¹ of vole habitat year⁻¹) was well above a previously predicted value for the whole generalist predator community in Kielder Forest. Our data on the functional response of red foxes and estimates of their predation rates suggest that foxes should have a strong stabilising impact on vole populations, yet voles show characteristic 3-4 yr cycles.     

Temporally stable genetic variability and dynamic kinship structure in a fluctuating population of the root vole Microtus oeconomus

Genetic variability, kin structure and demography of a population are mutually dependent. Population genetic theory predicts that under demographically stable conditions, neutral genetic variability reaches equilibrium between gene flow and drift. However, density fluctuations and non‐random mating, resulting e.g. from kin clustering, may lead to changes in genetic composition over time. Theoretical models also predict that changes in kin structure may affect aggression level and recruitment, leading to density fluctuations. These predictions have been rarely tested in natural populations. The aim of this study was to analyse changes in genetic variability and kin structure in a local population of the root vole (Microtus oeconomus) that underwent a fourfold change in mean density over a 6‐year period. Intensive live‐trapping resulted in sampling 88% of individuals present in the study area, as estimated from mark–recapture data. Based on 642 individual genotypes at 20 microsatellite loci, we compared genetic variability and kin structure of this population between consecutive years. We found that immigration was negatively correlated with density, while the number of kin groups was positively correlated with density. This is consistent with theoretical predictions that changes in kin structure play an important role in population fluctuations. Despite the changes in density and kin structure, there was no genetic differentiation between years. Population‐level genetic diversity measures did not significantly vary in time and remained relatively high (H_E range: 0.72–0.78). These results show that a population that undergoes significant demographic and social changes may maintain high genetic variability and stable genetic composition.     

Predator population dynamics under a cyclic prey regime: numerical responses,demographic parameters and growth rates

The consequences of cyclic fluctuations in abundance of prey species on predator continue to improve our understanding of the mechanisms behind population regulation. Among predators, vole‐eating raptors usually respond to changes in prey abundance with no apparent time‐lag and therefore contradict predictions from the predator–prey theory. In such systems, the interplay between demographic traits and population growth rate in relation to prey abundance remains poorly studied, yet it is crucial to characterize the link between ecological processes and population changes. Using a mechanistic approach, we assessed the demographic rates associated to the direct and indirect numerical responses of a specialist raptor (Montagu's harrier) to its cyclic prey (common vole), using long term data from two adjacent study sites in France. First‐year survival rates were weakly affected by vole abundance, probably due to the fact that Montagu's harriers are trans‐Saharan migrants and thus escape the vole collapse occurring in autumn–winter. Recruitment of yearling as well as breeding propensity of experienced adult females were strongly affected by vole abundance and at least partially shaped the trajectory of the breeding population. We argued that the strong density dependent signal detected in predator time series was mostly the phenomenological consequence of the positive direct numerical response of harriers to vole abundance. Accounting for this, we proposed a method to assess density dependence in predator relying on a cyclic prey. Finally, the variation in Montagu's harrier population growth rates was best explained by overwinter growth rates of the prey population and to a lesser extent by previous residual predator density.     

Moose recruitment in relation to bilberry production and bank vole numbers along a summer temperature gradient in Norway

The plant stress hypothesis states that plant stress factors other than herbivory improve herbivore performance due to changes in the content of nutritive or defensive compounds in the plants. In Norway, the bilberry (Vaccinium myrtillus) is important forage for the bank vole (Myodes glareolus) in winter and for the moose (Alces alces) in summer and autumn. The observed peaks in bank vole numbers after years with high production of bilberries are suggested to be caused by increased winter survival of bank voles due to improved forage quality. High production of bilberries should also lead to higher recruitment rates in moose in the following year. We predict, however, that there is an increasing tendency for a 1-year delay of moose indices relative to vole indices with decreasing summer temperatures, because low temperatures prolong the period needed by plants to recover in the vole peak year, and thus positively affect moose reproduction also in the succeeding year. In eight out of nine counties in south-eastern Norway, there was a positive relationship between the number of calves observed per female moose during hunting and a bilberry seed production index or an autumn bank vole population index. When dividing the study area into regions, there was a negative relationship between a moose-vole time-lag index and the mean summer temperature of the region. These patterns suggest that annual fluctuations in the production of bilberries affect forage quality, but that the effect on moose reproduction also depends on summer temperatures.     

Genetic structure of the cyclic fossorial water vole (Arvicola terrestris): landscape and demographic influences

Genetic structure can be strongly affected by landscape features and variation through time and space of demographic parameters such as population size and migration rate. The fossorial water vole (Arvicola terrestris) is a cyclic species characterized by large demographic fluctuations over short periods of time. The outbreaks do not occur everywhere at the same time but spread as a wave at a regional scale. This leads to a pattern of large areas (i.e. some hundreds of km2), each with different vole abundances, at any given time. Here, we describe the abundance and genetic structures in populations of the fossorial water vole. We use the data to try to understand how landscape and demographic features act to shape the genetic structure. The spatial variability of vole abundance was assessed from surface indices, collected in spring 2002 (April) in eastern central France. Genetic variability was analysed using eight microsatellite loci at 23 localities sampled between October 2001 and April 2002. We found some congruence between abundance and genetic structures. At a regional scale, the genetic disruptions were associated with both sharp relief and transition between an area of low abundance and another of high abundance. At a local scale, we observed a variation of the isolation-by-distance pattern according to the abundance level of vole populations. From these results we suggest that the dispersal pattern in cyclic rodent populations varies throughout the demographic cycle.     

Long-term pattern of population dynamics in the field vole from central Europe: cyclic pattern with amplitude dampening

The subject of population cycles is regarded as controversial due to a number of unsettled questions such as whether or not cyclic patterns are governed by the same processes at high and low latitudes in Europe. Recent evidence suggests that the dynamics at high and low latitudes share the common temporal pattern of vole dynamics referred to as collapsing population cycles. Despite concurrent interest, the key contention around the causal mechanisms that drive population cycles remains a hot topic in ecology. The aims of this study are to supplement information on the seasonal population dynamics of the field vole Microtus agrestis in the Czech Republic by analysing 25 years of time series data. By applying robust estimation procedures, we estimated several parameters to describe population dynamics, such as population variability, amplitude dampening, cycle period, order of the dynamics and the structure of density dependence. The parameters indicate that field vole dynamics in central Europe are highly variable, cyclic dynamics of order two, with peaks in abundance occurring regularly at intervals of 4–5 years. In addition to exhibiting population cycles, the field vole populations show a pattern of dampened amplitude as observed elsewhere in Europe, including northern latitudes. By analysing temporal trends in seasonal abundances, population growth rates and environmental temperatures, we did not obtain evidence to support the hypothesis that amplitude dampening results from the negative effect of increasingly mild winters on winter population growth rates.     

Stress and demographic decline: a potential effect mediated by impairment of reproduction and immune function in cyclic vole populations

The stress response is initially adaptive, operating to maintain homeostasis. However, chronic long-term exposure to stressors may have detrimental effects. We proposed that chronic stress may be a major factor in demographic vole cycles, inducing decline in high-density populations. We monitored four populations of the fossorial water vole Arvicola scherman, which undergo pluriannual demographic cycles in the Jura Mountains (France). Sampling was conducted during the high densities and the decline. We measured fecal corticosterone metabolites (FCMs) to assess stress levels and injected phytohemagglutinin to estimate the cell-mediated immune response. We demonstrated that stress levels increase between the high densities and the decline in most of the vole populations. At the individual level, FCM concentrations varied with reproductive status and body condition. During the outbreak, we observed significantly higher levels of FCM concentrations in nulliparous females than in females that had previously reproduced. Moreover, a significant negative correlation was observed between concentrations of FCMs and both immunocompetence and body condition during population decline. These results might reflect an impairment of the female reproductive capability in high densities and accelerated senescence affecting immune function during decline, both arising from chronic stress.     

Cowpox virus infection in natural field vole Microtus agrestis populations: delayed density dependence and individual risk

1. Little is known about the dynamics of pathogen (microparasite) infection in wildlife populations, and less still about sources of variation in the risk of infection. Here we present the first detailed analysis of such variation. 2. Cowpox virus is an endemic sublethal pathogen circulating in populations of wild rodents. Cowpox prevalence was monitored longitudinally for 2 years, in populations of field voles exhibiting multiannual cycles of density in Kielder Forest, UK. 3. The probability that available susceptible animals seroconverted in a given trap session was significantly positively related to host density with a 3-month time lag. 4. Males were significantly more likely to seroconvert than females. 5. Despite most infection being found in young animals (because transmission rates were generally high) mature individuals were more likely to seroconvert than immature ones, suggesting that behavioural or physiological changes associated with maturity contribute to variation in infection risk. 6. Hence, these analyses confirm that there is a delayed numerical response of cowpox infection to vole density, supporting the hypothesis that endemic pathogens may play some part in shaping vole cycles.     

Regulation of root vole population dynamics by food supply and predation: a two-factor experiment

This paper reports the effects of food supply, predation and the interaction between them on the population dynamics of root voles, Microtus oeconomus , by adopting factorial experiments in field enclosures. This two-factor experiment proved the general hypothesis that food supply and predation had independent and additive effects on population dynamics of root voles. The experimental results proved the following predictions: (1) predation reduced population density and recruitment significantly; (2) food supply increased population density; (3) predation and food supply influenced spacing behavior of root voles separately and additively: Exposure to predation reduced long movements of root voles between trapping sessions; additional food supply reduced aggression level and home range size of root voles. Less movement of individuals that exposed to predators possibly reduced their opportunity of obtaining food and lessened population survival rate, which led population density to decrease. Smaller home range and lower aggression level could make higher population density tolerable. The interactive effect of predation and food on home range size was highly significant (P=0.0082<0.01). The interactive effect of food and predation on dispersal rate was significant (P<0.01). From the experimental results, we conclude that the external factors (predation, food supply) were more effective than internal factors (spacing behavior) in determining population density of root voles – under the most favorable external conditions (−P, +F treatment), the mean density and mean recruitment of root vole population was the highest; under the most unfavorable external conditions (+P, −F treatment), the mean density and mean recruitment of root vole population was the lowest.     

Genetic basis of the trade-off between offspring number and quality in the bank vole

One of the main tenets of modern life-history theory is the negative relationship (trade-off) between the number and quality of offspring produced. Theory predicts a negative genetic correlation between these traits since both are closely related to fitness of individuals. However, the genetic basis of the trade-off has only been tested to a limited extent in natural populations. We examined whether size and quality of offspring are negatively related to litter size in the bank vole Clethrionomys glareolus. First, we found a significant negative phenotypic correlation between the number and size of offspring at birth in both laboratory and field populations of the bank vole. Second, a larger size at birth decreased the maturation age of female offspring in the laboratory, and increased the probability of breeding and the size of the first litter in the field. Furthermore, manipulation of offspring size at weaning indicated that structural effects of birth size in mammals have a more profound effect on the expression of life-history traits than weaning size. Finally, in addition to the phenotypic negative correlation between the number and size of offspring, we found evidence for a negative genetic correlation between these two traits, which confirms the genetic basis of the trade-off. This negative genetic covariation may have considerable effects on the rate and direction of evolution of the two related life-historical traits.     

How predation and landscape fragmentation affect vole population dynamics

Background

Microtine species in Fennoscandia display a distinct north-south gradient from regular cycles to stable populations. The gradient has often been attributed to changes in the interactions between microtines and their predators. Although the spatial structure of the environment is known to influence predator-prey dynamics of a wide range of species, it has scarcely been considered in relation to the Fennoscandian gradient. Furthermore, the length of microtine breeding season also displays a north-south gradient. However, little consideration has been given to its role in shaping or generating population cycles. Because these factors covary along the gradient it is difficult to distinguish their effects experimentally in the field. The distinction is here attempted using realistic agent-based modelling.

Methodology/Principal Findings

By using a spatially explicit computer simulation model based on behavioural and ecological data from the field vole (Microtus agrestis), we generated a number of repeated time series of vole densities whose mean population size and amplitude were measured. Subsequently, these time series were subjected to statistical autoregressive modelling, to investigate the effects on vole population dynamics of making predators more specialised, of altering the breeding season, and increasing the level of habitat fragmentation. We found that fragmentation as well as the presence of specialist predators are necessary for the occurrence of population cycles. Habitat fragmentation and predator assembly jointly determined cycle length and amplitude. Length of vole breeding season had little impact on the oscillations.

Significance

There is good agreement between our results and the experimental work from Fennoscandia, but our results allow distinction of causation that is hard to unravel in field experiments. We hope our results will help understand the reasons for cycle gradients observed in other areas. Our results clearly demonstrate the importance of landscape fragmentation for population cycling and we recommend that the degree of fragmentation be more fully considered in future analyses of vole dynamics.     

Modeling spatiotemporal dynamics of vole populations in Europe and America

The mathematical models proposed and studied in the present paper provide a unified framework to understand complex dynamical patterns in vole populations in Europe and North America. We have extended the well-known model provided by Hanski and Turchin by incorporating the diffusion term and spatial heterogeneity and performed several mathematical and numerical analyses to explore the dynamics in space and time of the model. These models successfully predicted the observed rodent dynamics in these regions. An attempt has been made to bridge the gap between the field and theoretical studies carried out by Turchin and Hanski (1997) [23] and Turchin and Ellner (2000) [24]. Simulation experiments, mainly two-dimensional parameter scans, show the importance of spatial heterogeneity in order to understand the poorly understood fluctuations in population densities of voles in Fennoscandia and Northern America. This study shed new light upon the dynamics of voles in these regions. The nonlinear analysis of vole data suggests that the dynamical shift is from stability to chaos. Diffusion driven model systems predict a new type of dynamics not yet observed in the field studies of vole populations carried out so far. This has been termed as chaotic in time and regular in space (CTRS). We observed CTRS dynamics in several simulation experiments. This directs us to expect that dynamics of this animal would be de-correlated in time and simultaneously mass extinctions might be possible at many spatial locations.     

Role of individual dispersal in genetic resilience in fluctuating populations of the gray‐sided vole Myodes rufocanus

Population densities of the gray‐sided vole Myodes rufocanus fluctuate greatly within and across years in Japan. Here, to investigate the role of individual dispersal in maintaining population genetic diversity, we examined how genetic diversity varied during fluctuations in density by analyzing eight microsatellite loci in voles sampled three times per year for 5 years, using two fixed trapping grids (approximately 0.5 ha each). At each trapping session, all captured voles at each trapping grid were removed. The STRUCTURE program was used to analyze serially collected samples to examine how population crashes were related to temporal variability, based on local‐scale genetic compositions in each population. In total, 461 and 527 voles were captured at each trapping grid during this study. The number of voles captured during each trapping session (i.e., vole density) varied considerably at both grids. Although patterns in fluctuations were not synchronized between grids, the peak densities were similar. At both grids, the mean allele number recorded at each trapping session was strongly, positively, and nonlinearly correlated with density. STRUCTURE analyses revealed that the proportions of cluster compositions among individuals at each grid differed markedly before and after the crash phase, implying the long‐distance dispersal of voles from remote areas at periods of low density. The present results suggest that, in gray‐sided vole populations, genetic diversity varies with density largely at the local scale; in contrast, genetic variation in a metapopulation is well‐preserved at the regional scale due to the density‐dependent dispersal behaviors of individuals. By influencing the dispersal patterns of individuals, fluctuations in density affect metapopulation structure spatially and temporally, while the levels of genetic diversity are preserved in a metapopulation.     

典型草原区布氏田鼠的活动节律及其季节变化

采用标志重捕、染色标记和野外观测法研究典型草原区布氏田鼠野外实验种群,采用4 种活动模式指数(日活动差异指数θ,昼行性指数λ,峰型指数;活动强度指数η)分析田鼠昼夜活动节律及其季节变化。结果表明,布氏田鼠全年12 个月的昼行性指数λ 均高于0.542,为典型的昼行性种类。其活动节律的季节变化主要与环境温度有关:1、2、3、4、11、12 月,气温较低,布氏田鼠的活动模式呈典型的单峰型;5、6、7 月气温较高,为典型的双峰型;而8、9、10 月则属于单双峰过渡类型。田鼠的峰型指数 在冬季最低,夏季最高,与月均气温呈显著的正相关关系(r=0.89,n = 12,P<0.05)。活动强度指数η与气温也存在显著的正相关(r=0.94,n = 12,P < 0.05)。5～9 月,布氏田鼠的活动强度最高,每日活动时间在6 h 以上;冬季布氏田鼠的日活动量骤降,很少外出活动,尤其在1～2 月,布氏田鼠日均地面活动时间不足1min。另外,室内实验表明,布氏田鼠具有在黑暗环境中活动能力,但在野外条件下,夜间活动极少发生。     

Coyote Abundance,Sheep Predation,and Wild Prey Correlates Illuminate Mediterranean Trophic Dynamics

ABSTRACT Sheep predation by coyotes (Canis latrans) is a major problem for sheep producers in North America. Solutions are facilitated by a basic understanding of the trophic dynamic context of this problem, one that likely varies geographically in important qualitative ways. Little is known about vertebrate trophic dynamics in Mediterranean ecosystems, where prey are diverse and their biomass is strongly influenced multi-annually by variable rainfall. We used long-term data sets from north-coastal California, USA, to investigate whether wild prey fluctuations caused immediate negative effects on sheep predation via a reduction in the coyote functional response or delayed positive effects on sheep predation via a numerical response by coyote predators. Because we could not measure prey biomass directly, we used variables associated with lower trophic levels (e.g., annual plant productivity, vole abundance, rainfall) as proxies for wild prey biomass. Coyote population growth rate was positively correlated with lower-trophic-level variables of the previous year, suggesting a numerical response, and sheep (ad F + lambs) predation was positively correlated with coyote abundance in the current year. Sheep predation also was negatively correlated with lower-trophic-level variables of the current year, suggesting an immediate buffering effect of wild prey on sheep predation. Together, coyote abundance and lower-trophic-level variables explained 47% of the multi-annual variation in sheep kills. The negative pathway between lower-trophic-level variables and sheep predation was stronger than the positive pathway, possibly due to the erratic nature of multi-annual fluctuations in lower-trophic-level variables, which could prevent the numerical response from reaching its full potential. Monthly analyses revealed a type III functional response of coyotes to lambs, which is expected to enhance buffering effects of wild prey on sheep predation. Our findings suggest the dominant effect of wild prey biomass on sheep predation by coyotes in this Mediterranean-type community is as a buffer.