Changes In Predation Pressure Research Articles

Personality, plasticity and predation: linking endocrine and behavioural reaction norms in stickleback fish

SummaryPredation plays a fundamental role in evolutionary processes, driving changes in prey morphology, physiology and behaviour. With organisms being increasingly exposed to rapid environmental changes, there is growing interest in understanding individual phenotypic plasticity in response to changes in predation pressure.Behavioural and physiological responses to predator exposure are of particular interest as differences in predation pressure are often reflected in correlated suites of behavioural and hormonal profiles across populations. Within populations, the association between endocrine profiles and behaviour is less understood and often lacking.Adopting a reaction norm approach and a repeated measures design, we assessed within‐population effects of changes in perceived predation risk on endocrinology and behaviour in three‐spined sticklebacks (Gasterosteus aculeatus). We repeatedly exposed subjects to a robotic model predator and assessed their behavioural response. The fish showed consistent behavioural profiles and were less active and shyer when predation risk was higher.Using non‐invasive waterborne hormone analysis, we assessed basal cortisol as well as the cortisol response to changes in predation risk. Individuals showed significantly higher cortisol levels following exposure to the model predator. Individual post‐predator exposure cortisol was repeatable but unrelated to behavioural responses. Accounting for between versus within‐subject effects, we found that basal cortisol and shyness were positively related within individuals, that is individuals overall were shyer on days they had higher cortisol levels. We also tested if basal testosterone predicted risky behaviour and found no evidence for this hypothesis.No individual differences in hormonal or behavioural responses to changes in predation risk were found, suggesting that individuals are not constrained by their personalities in their ability to cope with a potentially harmful threat.Overall, we show that individuals of different personalities are equally ‘flexible’ in their response to changes in predation pressure. Our study offers novel insight into consistent individual differences and plasticity in hormones and behaviour as well as their interplay within populations. Future studies should assess the applicability of these findings to other changes in the environment, as well as the effects of social context on endocrine and behavioural reaction norms.

Recovery in eastern Baltic cod: is increased recruitment caused by decreased predation on early life stages?

Cod (Gadus morhua) recruitment in the eastern Baltic Sea is influenced by predation on early life stages by sprat (Sprattus sprattus) and herring (Clupea harengus), which is considered as one of the mechanisms preventing cod recovery in the 1990s. In the light of improved cod recruitment in the second half of the 2000s, new analyses of stomach content of sprat and herring were conducted, to elucidate the contribution of changes in predation pressure on cod recruitment. Comparison of stomach contents of sprat and herring in 2004–2008 with data from the 1990s showed a similar diet composition in the two periods; however, changes were found in the ichthyoplankton abundance and composition in the diet, indicating reduced predation pressure on cod eggs in the most recent period. The abundance of cod eggs in the field, availability of other prey, and horizontal and vertical overlap between predator and prey were investigated as potential factors influencing cod egg predation.

A system dynamics approach to modelling multiple drivers of the African penguin population on Robben Island, South Africa

The African penguin (Spheniscus demersus) population in southern Africa has experienced rapid decline in the 20th century and as of 2010 is listed as “endangered” on the IUCN Red List. There is an urgent need for decision support tools to enable effective management of colonies. We present a system dynamics model of the penguin population on Robben Island, South Africa, that combines a demographic simulation with the modelling of multiple pressures including food availability and food competition by commercial fisheries, oil spills, predation by terrestrial and marine predators, and extreme climate events. The model is stochastic, stage-specific and resource-driven, and incorporates both well-defined, quantitative field data and qualitative expert opinion. Survival rates for eggs, chicks, immatures and adults were adapted from field data and an earlier model of this population to create a simulation of a stable population used in a variety of scenarios and sensitivity tests. The modelled population was found to be strongly driven by food availability and to a lesser degree by oiling and marine predation, while climate events and terrestrial predation had low impacts. Food biomass levels (small pelagic fish) in the penguins’ foraging area around the island (used during nesting) and further afield (used during the rest of the year) had an equal influence in driving population development in the short and long run. The impact of short-term (three years) fishing restrictions currently being trialled around the island was found to be generally beneficial to the modelled population, but easily masked by food-driven variability in population growth. The model produced population dynamics similar to those observed in 1988–2009 when immigration and a plausible change in predation pressure during this period were simulated. The model is being extended to other colonies to provide tools for specific management decisions and to enable the study of meta-populations by modelling migration between colonies. Our results suggest that improving food availability and mitigating the impact of oiling would have the highest beneficial impact on this penguin population.

Predator exposure alters stress physiology in guppies across timescales

In vertebrates, glucocorticoids mediate a wide-range of responses to stressors. For this reason, they are implicated in adaptation to changes in predation pressure. Trinidadian guppies (Poecilia reticulata) from high-predation environments have repeatedly and independently colonized and adapted to low-predation environments, resulting in parallel changes in life history, morphology, and behavior. We validated methods for non-invasive waterborne hormone sample collection in this species, and used this technique to examine genetic and environmental effects of predation on basal glucocorticoid (cortisol) levels. To examine genetic differences, we compared waterborne cortisol levels in high- and low-predation fish from two distinct population pairs. We found that fish from high-predation localities had lower cortisol levels than their low-predation counterparts. To isolate environmental influences, we compared waterborne cortisol levels in genetically similar fish reared with and without exposure to predator chemical cues. We found that fish reared with predator chemical cues had lower waterborne cortisol levels than those reared without. Comparisons of waterborne and whole-body cortisol levels demonstrated that populations differed in overall cortisol levels in the body, whereas rearing conditions altered the release of cortisol from the body into the water. Thus, evolutionary history with predators and lifetime exposure to predator cues were both associated with lower cortisol release, but depended on distinct physiological mechanisms.

A review on utilizing Bosmina size structure archived in lake sediments to infer historic shifts in predation regimes

Zooplankton are considered excellent indicators of aquatic food web structure, due to their role as grazers on primary producers and their sensitivity to predation by both planktivorous fish and invertebrates. Several key zooplankton taxa also leave identifiable remains that are often well-preserved in lake sediments, providing an opportunity to track changes in predation pressure over timescales of decades to thousands of years. For example, the small-bodied cladoceran zooplankter Bosmina (Branchiopoda, Crustacea) is often highly abundant in lake sediments, and because Bosmina often undergoes cyclomorphosis in response to fish and invertebrate predation, measurements of subfossil Bosmina features can be indicative of predation regime shifts. This review focuses on Bosmina cyclomorphic responses to varying predation regimes and the application of these principles to Bosmina subfossil remains to better understand long-term ecological changes occurring in lakes. We conclude that subfossil Bosmina size structure is a promising indicator of historic changes in predation pressure in response to fish introductions/extirpations/population

Rain reverses diel activity rhythms in an estuarine teleost

Activity rhythms are ubiquitous in nature, and generally synchronized with the day-night cycle. Several taxa have been shown to switch between nocturnal and diurnal activity in response to environmental variability, and these relatively uncommon switches provide a basis for greater understanding of the mechanisms and adaptive significance of circadian (approx. 24 h) rhythms. Plasticity of activity rhythms has been identified in association with a variety of factors, from changes in predation pressure to an altered nutritional or social status. Here, we report a switch in activity rhythm that is associated with rainfall. Outside periods of rain, the estuarine-associated teleost Acanthopagrus australis was most active and in shallower depths during the day, but this activity and depth pattern was reversed in the days following rain, with diurnality restored as estuarine conductivity and turbidity levels returned to pre-rain levels. Although representing the first example of a rain-induced reversal of activity rhythm in an aquatic animal of which we are aware, our results are consistent with established models on the trade-offs between predation risk and foraging efficiency.

Mate‐finding allee effect in spruce budworm population dynamics

AbstractAllee effects can cause populations to decline due to decreasing population growth rates with decreasing density and play a major role in population dynamics. Mate‐finding failure, a common mechanism contributing to demographic Allee effects, is usually difficult to demonstrate because of the arduous nature of sampling individuals at very low densities. In a rising outbreak of the eastern spruce budworm, Choristoneura fumiferana (Clemens) (Lepidoptera: Tortricidae), we used caged and tethered virgin females in traps to measure mating success over population densities ranging from deep endemic to outbreak conditions. We found that mating success increased with increasing population density, and that at endemic population densities, females experienced difficulties attracting males and mating, demonstrating for the first time a mate‐finding Allee effect in the spruce budworm. The relationship between population density and mating success is nonlinear. As population density increased, the proportion of mated females eventually reached a plateau and mating success was not 100% even at the highest moth densities, probably due to female reluctance to mate and perhaps interference competition by males for access to females. Both laboratory‐reared and wild females were equally effective in synthesizing pheromone, attracting males, and mating. Our results strongly suggest that a mate‐finding Allee effect is involved in maintaining low‐density spruce budworm populations below an Allee threshold where they fail to grow. Factors such as changes in predation pressure and immigration could help populations overcome this Allee threshold.

Extreme climatic events drive mammal irruptions: regression analysis of 100‐year trends in desert rainfall and temperature

Extreme climatic events, such as flooding rains, extended decadal droughts and heat waves have been identified increasingly as important regulators of natural populations. Climate models predict that global warming will drive changes in rainfall and increase the frequency and severity of extreme events. Consequently, to anticipate how organisms will respond we need to document how changes in extremes of temperature and rainfall compare to trends in the mean values of these variables and over what spatial scales the patterns are consistent. Using the longest historical weather records available for central Australia – 100 years – and quantile regression methods, we investigate if extreme climate events have changed at similar rates to median events, if annual rainfall has increased in variability, and if the frequency of large rainfall events has increased over this period. Specifically, we compared local (individual weather stations) and regional (Simpson Desert) spatial scales, and quantified trends in median (50th quantile) and extreme weather values (5th, 10th, 90th, and 95th quantiles). We found that median and extreme annual minimum and maximum temperatures have increased at both spatial scales over the past century. Rainfall changes have been inconsistent across the Simpson Desert; individual weather stations showed increases in annual rainfall, increased frequency of large rainfall events or more prolonged droughts, depending on the location. In contrast to our prediction, we found no evidence that intra-annual rainfall had become more variable over time. Using long-term live-trapping records (22 years) of desert small mammals as a case study, we demonstrate that irruptive events are driven by extreme rainfalls (>95th quantile) and that increases in the magnitude and frequency of extreme rainfall events are likely to drive changes in the populations of these species through direct and indirect changes in predation pressure and wildfires.

Anthropogenic food patches and association patterns of Tursiops truncatus at Lampedusa island, Italy

Anthropogenic food patches in the marine environment, such as aquaculture farms and active trawlers, may impact on the behavior of marine mammals through modification of habitats, changes in predation pressure, or alterations in food distribution, availability, and predictability, affecting related social interactions and population demographics. This study examined patterns of association of a population of common bottlenose dolphins (Tursiops truncatus) around Lampedusa Island (Italy) during 1996‐ 2006 and tested the hypothesis that the trawl fishery and the presence of an aquaculture farm could affect such patterns. Here, we used measures of association between pairs of individuals to assess this impact on social unit composition/cohesion and some analytical techniques to describe the structure of dolphin social networks and temporal stability of associations. Association information for 71 regularly sighted individuals was obtained from photo-identification surveys within groups observed or not at ‘‘feeding stations.’’ We found association patterns between dolphins were nonrandom. The Lampedusa population seems to be arranged into 6 clusters and organized in communities composed of animals that were either never seen in association with feeding stations (N individuals) or those that are (Y individuals), although mixed assemblages were also recorded. Both communities showed long-term preferred companions, with different degrees of social cohesion—as resulted by network measures and temporal analysis. Delineating community structure at Lampedusa Island has offered basic information for further investigations in the area, also providing novel evidences on how disparities in association patterns between bottlenose dolphin

Spatio-Temporal Variation in Predation on Artificial Ground Nests: A 12-Year Experiment

Predation on artificial ground nests placed along four transects (alderwood, fallow, pine forest and fallow-forest edge) was studied in 1998–2009. On average, a 25.4% increase in overall predation rate was observed over the study period and increasing predation occurred at all sites. Mean multiannual predation rate differed up to 2.3-fold between transects and was highest in the alderwood and lowest in the fallow area. Despite that share of particular predators in depredation of nests was not random with respect to the studied habitats, high year-to-year variation in the pattern of predation was recorded. Between-year and between-transect changes in predation pressure were responsible for 48% of overall predation variability. Predation in one, randomly selected year was weakly related to averaged data from all 12 years, which indicates that short-term studies employing artificial nests can significantly over- or underestimate the contribution of particular species to overall predation.

Behavioral responses to seasonality by two Sceloporine lizard species from a tropical dry forest

Abstract We investigated behavioral responses of two lizards, Sceloporus melanorhinus and S. utiformis, to environmental seasonality in a tropical dry forest of western Mexico. We conducted focal observations of individuals in the field during both the rainy and dry seasons and compared perch use, foraging and social behaviors and displacement activity within and between species. Our results indicate that seasonal changes occur in the behavior and activity of both male and female S. melanorhinus and S. utiformis. Feeding frequency, social displays, and distances travelled increased significantly in both species during the rainy season. These seasonal changes tended to be more pronounced in the arboreal species; S. melanorhinus. Surprisingly, female S. melanorhinus perched higher than males during the rainy season. Most of the observed changes are similar to the responses to seasonality observed in other lizards, both for species that exist within our study area, and for species that inhabit other seasonal environments in tropical and temperate ecosystems. Thus, the seasonal patterns in behavior documented in this study may represent a general set of behavioral strategies that allow lizards to better adapt to seasonal fluctuations in their environment, such as changes in predation pressure, variation in resource levels, and differences in insolation, temperature, and moisture levels.

Elevational Patterns of Diversity and Abundance of Eusocial Paper Wasps (Vespidae) in Costa Rica

ABSTRACTWe used a standard sampling protocol to measure elevational patterns of species richness and abundance of eusocial paper wasps (Hymenoptera: Vespidae) in Costa Rica. The sample transect of six sites spanned approximately 2000 m in elevation from lowland to montane forest. Species accumulation curves and species richness estimates both document a low elevation peak in paper wasp species richness at 50 and 300 m asl, with a decline in species richness at higher elevations. Comparison of species composition among elevations revealed strong species turnover from a rich lowland fauna to a depauperate, but distinct, montane fauna. We also observed a general trend toward a greater abundance of paper wasps at higher elevations, a pattern not usually observed in eusocial insects. Army ant species that prey on paper wasps declined in abundance with elevation across the sample transect, a pattern that has been observed at other sites. We discuss the possibility that elevational changes in predation pressure affect variation in paper wasp abundance and species richness. Eusocial paper wasp species employ one of two modes of colony founding, independent and swarm founding. We found that the total abundance of individual swarm‐founding wasps was higher at all elevations than the abundance of independent‐founding wasps, supporting previous suggestions that Neotropical swarm founders are more successful ecologically.

Maternal age and spine development in the rotiferBrachionus calyciflorus: increase of spine length with birth orders1

Summary1. Maternal effects have long been known to influence phenotypic plasticity in rotifers. Females inBrachionus calyciflorusand several other species produce long‐spined offspring when the predatory rotiferAsplanchnais present;B. calyciflorusalso develops short spines when food concentrations are low. These spines protect against predation and decrease food threshold concentrations.2. Some strains ofB. calyciflorusdevelop long spines even in the absence ofAsplanchnaand other environmental stimuli. We demonstrate in this study that spine length in such cases is dependent on the age of the mother.3. In strains from Florida and Georgia, offspring spine length increased significantly with birth order, sometimes to lengths formerly observed only in the presence ofAsplanchna. Significant variation in this trait was found among and within clones of a strain. Offspring body size also increased with maternal age. This is the first time maternal age has been shown to affect rotifer morphology.4. These birth‐order effects may have important ecological implications and explain phenotypic plasticity and polymorphism in body size and spine length in populations when predators are absent and food concentrations are high. They may be a bet‐hedging mechanism to assure adaptation to rapid changes in predation pressure or food conditions.

Climate Change and Trophic Response of the Antarctic Bottom Fauna

BackgroundAs Earth warms, temperate and subpolar marine species will increasingly shift their geographic ranges poleward. The endemic shelf fauna of Antarctica is especially vulnerable to climate-mediated biological invasions because cold temperatures currently exclude the durophagous (shell-breaking) predators that structure shallow-benthic communities elsewhere.Methodology/Principal FindingsWe used the Eocene fossil record from Seymour Island, Antarctic Peninsula, to project specifically how global warming will reorganize the nearshore benthos of Antarctica. A long-term cooling trend, which began with a sharp temperature drop ∼41 Ma (million years ago), eliminated durophagous predators—teleosts (modern bony fish), decapod crustaceans (crabs and lobsters) and almost all neoselachian elasmobranchs (modern sharks and rays)—from Antarctic nearshore waters after the Eocene. Even prior to those extinctions, durophagous predators became less active as coastal sea temperatures declined from 41 Ma to the end of the Eocene, ∼33.5 Ma. In response, dense populations of suspension-feeding ophiuroids and crinoids abruptly appeared. Dense aggregations of brachiopods transcended the cooling event with no apparent change in predation pressure, nor were there changes in the frequency of shell-drilling predation on venerid bivalves.Conclusions/SignificanceRapid warming in the Southern Ocean is now removing the physiological barriers to shell-breaking predators, and crabs are returning to the Antarctic Peninsula. Over the coming decades to centuries, we predict a rapid reversal of the Eocene trends. Increasing predation will reduce or eliminate extant dense populations of suspension-feeding echinoderms from nearshore habitats along the Peninsula while brachiopods will continue to form large populations, and the intensity of shell-drilling predation on infaunal bivalves will not change appreciably. In time the ecological effects of global warming could spread to other portions of the Antarctic coast. The differential responses of faunal components will reduce the endemic character of Antarctic subtidal communities, homogenizing them with nearshore communities at lower latitudes.

Time stress, predation risk and diurnal–nocturnal foraging trade-offs in larval prey

Insect larvae increase in size with several orders of magnitude throughout development making them more conspicuous to visually hunting predators. This change in predation pressure is likely to impose selection on larval anti-predator behaviour and since the risk of detection is likely to decrease in darkness, the night may offer safer foraging opportunities to large individuals. However, forsaking day foraging reduces development rate and could be extra costly if prey are subjected to seasonal time stress. Here we test if size-dependent risk and time constraints on feeding affect the foraging–predation risk trade-off expressed by the use of the diurnal–nocturnal period. We exposed larvae of one seasonal and one non-seasonal butterfly to different levels of seasonal time stress and time for diurnal–nocturnal feeding by rearing them in two photoperiods. In both species, diurnal foraging ceased at large sizes while nocturnal foraging remained constant or increased, thus larvae showed ontogenetic shifts in behaviour. Short night lengths forced small individuals to take higher risks and forage more during daytime, postponing the shift to strict night foraging to later on in development. In the non-seasonal species, seasonal time stress had a small effect on development and the diurnal–nocturnal foraging mode. In contrast, in the seasonal species, time for pupation and the timing of the foraging shift were strongly affected. We argue that a large part of the observed variation in larval diurnal–nocturnal activity and resulting growth rates is explained by changes in the cost/benefit ratio of foraging mediated by size-dependent predation and time stress.

Investigating shellfish deposition and landscape history at the Natia Beach site, Fiji

The relationship between environmental variation and subsistence practices is a central point of discussion in much Oceanic archaeology. While human predation can significantly reduce prey populations, environmental variation also contributes to reductions in prey abundance, possibly leading to increased human competition and resource scarcity. At the Natia Beach site, Nacula Island, Fiji, geoarchaeological evidence suggests that coastal progradation began soon after initial occupation of the coastal plain. Additionally, at approximately 650 BP a marked increase in clay and silt deposition occurred. Changes in coastal geomorphology may be explained by landscape response to regional Mid-Holocene sea level fall combined with human induced soil erosion due to upland settlement. Smaller scale environmental changes associated with climate variability may have also played a role. Additionally, landscape change appears to have had a measurable impact on local nearshore mollusks that are sensitive to high levels of water turbidity. Minor evidence of human exploitation is observable in this shellfish assemblage, although changes in predation pressure may have allowed shellfish populations to recover. Increased ceramic diversity and fortified settlements also appear at approximately 650 BP on Nacula and other parts of Fiji. The suite of changes at Natia may be explained by processes of regional and local environmental changes, and human adaptation in terms of subsistence, spatial organization, and competition.

Fall diets of alewife, rainbow smelt, and slimy sculpin in the profundal zone of southern Lake Ontario during 1994–2005 with an emphasis on occurrence of Mysis relicta

In Lake Ontario, factors including the collapse of the burrowing amphipod, Diporeia spp., changes in the distribution and composition of the prey fish community, and occurrence of exotic cladocerans Bythotrephes longimanus and Cercopagis pengoi have led to changes in predation pressure on the remaining native profundal macroinvertebrate, Mysis relicta. We conducted a diet study on three important prey fishes, alewife (Alosa pseudoharengus), rainbow smelt (Osmerus mordax), and slimy sculpin (Cottus cognatus) in 2002–2005 at depths 75–130 m along the south shore of Lake Ontario to evaluate the current role of Mysis in the diets of these species relative to earlier studies and previously unpublished data from 1994–1995. Mysis have remained an important prey item for alewife, rainbow smelt, and slimy sculpin in the profundal zone of southern Lake Ontario through 2005, indicating that the population has been able to sustain itself following the collapse of Diporeia. Although the occurrence of Diporeia in prey fish diets was minimal in 2003–2005, Bythotrephes longimanus and Cercopagis pengoi played an important role in the diet of alewife and a minor role in the diet of rainbow smelt, and may actually serve to mitigate predation pressure on Mysis, particularly in years when they are very abundant. Conversely, without Diporeia, the benthic slimy sculpin was primarily reliant on Mysis as a prey item and would be most vulnerable to a decline in the Mysis population.

Alternative states in the phytoplankton of Lake Kinneret, Israel (Sea of Galilee)

1. Through analyses of a 34-year record of phytoplankton, zooplankton and physicochemical parameters from Lake Kinneret, Israel, we show that distinct and persistent phytoplankton assemblage states occurred from winter to summer. 2. The most obvious characteristic of these states was the presence or absence of a spring bloom of the dinoflagellate, Peridinium gatunense. 3. Analyses of the data within the framework of the alternative states model revealed a possible complex triggering mechanism, and system hysteresis. 4. A change in zooplankton biomass and body size coincident with changes in predation pressure associated with the collapse of the Kinneret Bleak, Acanthobrama terraesanctae, fishery appeared to be the ‘slow changing’ variable in the context of the alternative states model. Alternative phytoplankton states were only possible after this variable crossed a threshold in 1993–94, following the collapse of the fishery. 5. When alternative states were possible, some physicochemical parameters and the structure of the zooplankton assemblage appeared to control which phytoplankton state emerged in a given year. In years without a P. gatunense bloom, important physicochemical parameters in winter included low NO3 loading, high water temperature, high water level, a deeper thermocline, low transparency, high concentrations of NO3 and Cl in the epilimnion, and low concentration of epilimnetic total phosphorus. In addition, the cladoceran Chydorus sphaericus and adults of the copepod Mesocyclops ogunnus were observed in winter in years without a bloom. 6. Zooplankton biomass and body size of some taxa have recovered since the 1993–94 collapse of the fishery, yet incidence of both phytoplankton states in Lake Kinneret was still possible. Within the framework of the alternative states model, this suggests that the slow changing variable threshold where alternative states became possible is different from the threshold where alternative states will no longer be possible. In other words, the system is characterised by a hysteresis.

Effects of small-scale isolation and predation on fish diversity on experimental reefs

A positive correlation is often found between fish species diversity on coral reefs and their degree of isolation. We examined whether isolation creates differences in fish assemblages among reefs through changes in predation pressure. First, small artificial reefs were placed at increasing distances from a naturally continuous reef, over a sloping bottom. Species richness and density of each species increased with isolation. Next, artificial reefs were relocated, together with all their fish inhabitants, closer to the natural reef. Following relocation, resident fishes of the artificial reefs exhibited a sharp decline in numbers, while predatory fish density increased. Both video obser- vations of the relocated artificial reefs and the correlation between piscivore aggregation and decline of the resident fishes revealed that the cause of decline was the strong predation around the artificial reefs. The decline was density-dependent, such that the per-capita rate of decline was higher on arti- ficial reefs with higher fish density. This study shows that the impact of piscivores on resident fish is modified by small-scale isolation. The results suggest that the high density of fish on small, isolated reefs is enabled by low predation.

Changes in individual quality during a 3-year population cycle of voles.

In small mammal populations with multiannual oscillations in density, the occurrence of large individuals in the peak phase (the "Chitty effect") is a typical feature, but mechanisms behind this phenomenon have remained unclear. We analysed long-term data sets collected in western Finland between 1984 and 1992 to: (1) find out how the body size and body condition of voles (Microtus agrestis, M. rossiaemeridionalis, Clethrionomys glareolus) and shrews (Sorex araneus) was associated with the 3-year population cycle of voles, and (2) relate the quality (body condition) of the individuals to changes in the biotic environment in order to detect how the different hypotheses about the mechanisms behind the Chitty effect can explain the observed variation. In the 3-year cycle studied, the mean body size and quality were strongly related to density oscillations in voles but not in sympatric shrews. Voles were lean in the decline phase but very stocky in the summer of the peak phase. This pattern appeared to be mainly caused by changes in body condition or body shape rather than mere size (body length). The quality of voles appeared to be delayed density dependent, especially in autumn when the dominant time lag was 12months. Previous vole density was strongly related to changes in the environment (activity of specialist predators, production of hay until early summer). We suggest that the previous density of voles mainly affects the quality of voles indirectly through changes in the biotic environment, and that the proximate cause behind the Chitty effect is the combined effect of changes in predation pressure and availability of food.