Density-dependent Processes Research Articles

Growth rates of black caiman (Melanosuchus niger) and spectacled caiman (Caiman crocodilus) from two different Amazonian flooded habitats

Rates of growth and survival in wild populations are affected by the physical environment, biotic interactions, and density-dependent processes, such as growth and fecundity. However, the relative importance of these factors in long-lived reptiles is poorly understood. We analyzed growth rates of Melanosuchus niger and Caiman crocodilus coexisting in two areas of the Brazilian Amazon with very different environmental characteristics. Growth rates of Caiman crocodilus at the two sites were similar, but M. niger grew more slowly in the area with higher productivity and higher density of caimans. Growth rates of the same species from other sites and of the temperate-zone Alligator mississippiensis indicate large differences among sites, but little evidence that these differences are primarily due to differences in productivity or temperature. Demographic models used to estimate sustained yields from caiman harvests should take into account the likely importance of density-dependent growth.

Intercohort density dependence drives brown trout habitat selection

Habitat selection can be viewed as an emergent property of the quality and availability of habitat but also of the number of individuals and the way they compete for its use. Consequently, habitat selection can change across years due to fluctuating resources or to changes in population numbers. However, habitat selection predictive models often do not account for ecological dynamics, especially density dependent processes. In stage-structured population, the strength of density dependent interactions between individuals of different age classes can exert a profound influence on population trajectories and evolutionary processes. In this study, we aimed to assess the effects of fluctuating densities of both older and younger competing life stages on the habitat selection patterns (described as univariate and multivariate resource selection functions) of young-of-the-year, juvenile and adult brown trout Salmo trutta. We observed all age classes were selective in habitat choice but changed their selection patterns across years consistently with variations in the densities of older but not of younger age classes. Trout of an age increased selectivity for positions highly selected by older individuals when their density decreased, but this pattern did not hold when the density of younger age classes varied. It suggests that younger individuals are dominated by older ones but can expand their range of selected habitats when density of competitors decreases, while older trout do not seem to consider the density of younger individuals when distributing themselves even though they can negatively affect their final performance. Since these results may entail critical implications for conservation and management practices based on habitat selection models, further research should involve a wider range of river typologies and/or longer time frames to fully understand the patterns of and the mechanisms underlying the operation of density dependence on brown trout habitat selection.

Larval density dependence in Anopheles gambiae s.s., the major African vector of malaria.

Anopheles gambiae sensu stricto is the most important vector of malaria in Africa although relatively little is known about the density-dependent processes determining its population size. Mosquito larval density was manipulated under semi-natural conditions using artificial larval breeding sites placed in the field in coastal Kenya; two experiments were conducted: one manipulating the density of a single cohort of larvae across a range of densities and the other employing fewer densities but with the treatments crossed with four treatments manipulating predator access. In the first experiment, larval survival, development rate and the size of the adult mosquito all decreased with larval density (controlling for block effects between 23% and 31% of the variance in the data could be explained by density). In the second experiment, the effects of predator manipulation were not significant, but again we observed strong density dependence in larval survival (explaining 30% of the variance). The results are compared with laboratory studies of A.gambiae larval competition and the few other studies conducted in the field, and the consequences for malaria control are discussed.

The bioenergetics of density-dependent growth in Arctic char (Salvelinus alpinus)

We explored the mechanisms of density-dependent growth in Arctic char ( Salvelinus alpinus ) by comparing the energetics of growth, consumption, and activity obtained under three replicated density treatments in a large-scale enclosure (90 m2surface area) experiment. The enclosures permitted the entry of zooplankton and allowed char to feed on the bottom and at the surface of the lake. We found a negative (power) relationship between growth and density. Char consumption rate decreased linearly with increasing density. Growth efficiency was affected by fish density in a similar manner as growth rate. Finally, activity increased with fish density and was particularly high at high densities. Our findings illustrate the complexity of the relationships among consumption, activity, growth rates, and fish density and bring further evidence to the possible involvement of behavioural mechanisms in density-dependent processes, notably by modulating activity costs with density.

Spatial Autocorrelation in Farmland Grasshopper Assemblages (Orthoptera: Acrididae) in Western France

Agricultural intensification in western Europe has caused a dramatic loss of grassland surfaces in farmlands, which have resulted in strong declines in grassland invertebrates, leading to cascade effects at higher trophic levels among consumers of invertebrates. Grasshoppers are important components of grassland invertebrate assemblages in European agricultural ecosystems, particularly as prey for bird species. Understanding how grasshopper populations are distributed in fragmented landscapes with low grassland availability is critical for both studies in biodiversity conservation and insect management. We assessed the range and strength of spatial autocorrelation for two grasshopper taxa (Gomphocerinae subfamily and Calliptamus italicus L.) across an intensive farmland in western France. Data from surveys carried out over 8 yr in 1,715 grassland fields were analyzed using geostatistics. Weak spatial patterns were observed at small spatial scales, suggesting important local effects of management practices on grasshopper densities. Spatial autocorrelation patterns for both grasshopper taxa were only detected at intermediate scales. For Gomphocerinae, the range of spatial autocorrelation varied from 802 to 2,613 m according to the year, depending both on grasshopper density and on grassland surfaces in the study site, whereas spatial patterns for the Italian locust were more variable and not related to grasshopper density or grassland surfaces. Spatial patterns in the distribution of Gomphocerinae supported our hypothesis that habitat availability was a major driver of grasshopper distribution in the landscape, and suggested it was related to density-dependent processes such as dispersal.

Optimal design of experimental epidemics

We consider the optimal design of controlled experimental epidemics or transmission experiments, whose purpose is to inform the practitioner about disease transmission and recovery rates. Our methodology employs Gaussian diffusion approximations, applicable to epidemics that can be modeled as density-dependent Markov processes and involving relatively large numbers of organisms. We focus on finding (i) the optimal times at which to collect data about the state of the system for a small number of discrete observations, (ii) the optimal numbers of susceptible and infective individuals to begin an experiment with, and (iii) the optimal number of replicate epidemics to use. We adopt the popular D-optimality criterion as providing an appropriate objective function for designing our experiments, since this leads to estimates with maximum precision, subject to valid assumptions about parameter values. We demonstrate the broad applicability of our methodology using a diverse array of compartmental epidemic models: a time-homogeneous SIS epidemic, a time-inhomogeneous SI epidemic with exponentially decreasing transmission rates and a partially observed SIR epidemic where the infectious period for an individual has a gamma distribution.

Temporal variation in density dependent body growth of a large herbivore

Temporal variation both due to density dependent and density independent processes affect performance and vital rates in large herbivores. Annual fluctuations in climate affect foraging conditions and thus body growth of large herbivores during the short growing season in alpine habitats. Also, high animal densities on summer ranges may increase competition for food and reduce body mass gain. Yet, little is known about interactive effects of density and climate on alpine summer ranges, and the time scales these processes operate on. In this fully replicated landscape‐scale experiment, we kept domestic sheep at high and low densities over nine grazing seasons in an alpine habitat, and tested the relative role of density and annual variation for lamb body mass gain during summer and whether effects of density and annual variation interacted. We found that lambs at high density gained less mass over the summer season than lambs at low density. At short time scales the density effect interacted with annual fluctuations in body growth. We documented a long‐term temporal trend in body mass, consistent with the hypothesis that grazing effects affect habitat differentially at high and low density over years. At high density lamb autumn body mass declined during the first three grazing seasons and then stabilized, whereas body mass slightly increased over years at low density. This long‐term trend suggests accumulative density dependent effects from a biomass or quality reduction, and hence delayed food competition at high density and possibly facilitation at low density. Our experiment provides new insight into how density dependent effects on performance of a large herbivore depend on temporal scale of observation.

CAN CULLING TO PREVENT MONKEYPOX INFECTION BE COUNTER-PRODUCTIVE? SCENARIOS FROM A THEORETICAL MODEL

In the last two decades, monkeypox outbreaks in human populations in Africa and North America have reminded us that smallpox is not the only poxvirus with potential to cause harm in human populations. Monkeypox transmission is sustained in animal reservoirs, and animal–human contacts are responsible for sporadic outbreaks in humans. Here, we develop and analyze a deterministic epizootic (animal-based) transmission model capturing disease dynamics in an animal population, disease dynamics in an age-structured human population, and their coupling through animal–human contacts. We develop a single-patch model as well as a two-patch meta-population extension. We derive mathematical expressions for the basic reproduction number, which governs the likelihood of a large outbreak. We also investigate the effectiveness of culling strategies and the impact of changes in the animal–human contact rate. Numerical analysis of the model suggests that, for some parameter values, culling can actually have the counter-productive outcome of increasing monkeypox infection in children, if animal reproduction is a density-dependent process. The likelihood of this happening, as well as the prevalence of monkeypox in humans, depends sensitively on the animal–human contact rate. We also find that ignoring age structure in human populations can lead to overestimating the transmissibility of monkeypox in humans. The effectiveness of monkeypox control strategies such as culling can strongly depend on the details of demography and epidemiology in the animal reservoirs that sustain it. Therefore, to better understand how to prevent and control monkeypox outbreaks in humans, better empirical data from wild animal populations where monkeypox is endemic must be collected, and these data must be incorporated into highly structured theoretical models.

Experimental evidence of density dependent activity pattern of a large herbivore in an alpine ecosystem

Density dependent processes affecting foraging strategies may in turn influence vital rates and population regulation in large herbivores. Increased competition may lower both forage availability and quality, but whether the main activity constraint at high density is increased searching time or increased digestion time is poorly investigated. In a fully replicated landscape-scale experiment, we used long-term data (2003–2009) from domestic sheep grazing at high and low density (80 and 25 sheep km–2, respectively) on alpine summer ranges to test density dependence in allocation of time to feeding (moving) vs digestion (resting) activities and how this in turn affected body growth. Sheep at high density spent more time actively feeding than sheep at low density, but sheep moved shorter distances while foraging at high density. Increased activity levels at high density suggest that the main activity constraint at high density was availability of high-quality food increasing searching time and possibly reducing intake rates. Increased movement distances at low density is consistent with a higher selection for more productive vegetation types since high-quality patches are dispersed in the landscape. The alternative hypothesis, that food processing time increased at high density was not supported as it would have reduced overall activity levels. Individual activity levels increased body growth, but this was not sufficient to fully compensate for lower habitat quality leading to an overall reduced body growth at high density. Our experiment clearly documents changes in activity budgets and movement distances of a large herbivore at high population density, providing one potential behavioural mechanism of density dependent responses observed in vital rates.

Density-Dependent Processes in the Life History of Fishes: Evidence from Laboratory Populations of Zebrafish Danio rerio

Population regulation is fundamental to the long-term persistence of populations and their responses to harvesting, habitat modification, and exposure to toxic chemicals. In fish and other organisms with complex life histories, regulation may involve density dependence in different life-stages and vital rates. We studied density dependence in body growth and mortality through the life-cycle of laboratory populations of zebrafish Danio rerio. When feed input was held constant at population-level (leading to resource limitation), body growth was strongly density-dependent in the late juvenile and adult phases of the life-cycle. Density dependence in mortality was strong during the early juvenile phase but declined thereafter and virtually ceased prior to maturation. Provision of feed in proportion to individual requirements (easing resource limitation) removed density dependence in growth and substantially reduced density dependence in mortality, thus indicating that ‘bottom-up’ effects act on growth as well as mortality, but most strongly on growth. Both growth and mortality played an important role in population regulation, with density-dependent growth having the greater impact on population biomass while mortality had the greatest impact on numbers. We demonstrate a clear ontogenic pattern of change in density-dependent processes within populations of a very small (maximum length 5 mm) fish, maintained in constant homogeneous laboratory conditions. The patterns are consistent with those distilled from studies on wild fish populations, indicating the presence of broad ontogenic patterns in density-dependent processes that are invariant to maximum body size and hold in homogeneous laboratory, as well as complex natural environments.

Unravelling the effects of water temperature and density dependence on the spatial variation of brown trout (Salmo trutta) body size

This study looks at the relative influence of water temperature and density dependence on the spatial variation in body size of 126 brown trout ( Salmo trutta ) cohorts from 12 Iberian rivers over a 12-year period. Mean cohort mass and length of age groups 0+ to 2+ varied significantly among sampling sites because of the concurrent effect of water temperature and density dependence. Density in suitable habitat had a limiting role that influenced potential maximum growth of cohorts, and water temperature differentiated these cohorts in two groups of sites with high and low potential maximum growth. Water temperature had a positive cumulative effect on body size of all age classes. However, body size of age-0 trout was nonlinearly influenced by short-term exposure to extreme water temperature. Thus, extremely high temperatures became a limiting factor and had deleterious effects on growth. There were intracohort and intercohort effects of density dependence throughout the life span, which were mainly due to the density in the available suitable habitat of trout of the same age or older. The present study supports the hypothesis that both density-dependent and density-independent processes are crucial for the understanding of population dynamics and that their relative importance varies across scales of space and time.

Flowering asynchrony and mating system effects on reproductive assurance and mutualism persistence in fragmented fig–fig wasp populations

Plants and animals may experience reproductive Allee effects in fragmented populations, and obligate pollination mutualisms may be especially sensitive to extinction risk via this density-dependent process. In this study we examine how a shift from within-crown reproductive synchrony to asynchrony influences reproductive assurance through contributions to selfing and outcrossing in small, spatially isolated populations of Ficus. The research focuses on the monoecious fig F. petiolaris and consists of phenological analyses and genetic assessments of selfing and outcrossing for populations located in Baja California's Sonoran Desert. Phenological censuses of eight populations revealed within-crown asynchrony in 44% of reproducing trees, with 16% having sufficient overlap of male and female flowering phases to permit selfing via the cycling of pollinating fig wasps within natal trees. In mating system analyses of two of these populations, however, multilocus outcrossing rates (t(m)) were indistinguishable from 1. This result, combined with low levels of inbreeding, indicates selfing to be absent or at best a minor contributor to reproductive assurance. The results indicate that the fitness benefits of within-crown asynchrony lie not with selfing, as commonly asserted, but with increased opportunities for outcross pollen transmission and receipt, changing our understanding of the mechanisms by which reproduction is facilitated and extinction risk minimized in naturally fragmented Ficus populations. Given the role of fig fruit as a keystone food resource in many tropical environments, trait variation leading to reproductive assurance in figs, such as within-crown asynchrony, has broader ecosystem-level implications.

Paradigm lost: how parasite control may alter pattern and process in human helminthiases

Knowledge of pattern and process in helminth population biology is mostly based on the endemic equilibrium state that characterises infections before control. Current large-scale intervention programmes aim at controlling/eliminating helminth infection, transmission and morbidity. As a result, age-infection profiles will be modified; immune responses will be affected; underlying parasite distributions may become more aggregated; density-dependent regulatory processes will relax; and the contribution to transmission and morbidity of different host population groups will shift. There is an urgent need to understand how the paradigm that has guided parasite population biology research is changing under chemotherapy-based control programmes for this research to continue supporting parasite control efforts effectively.

Cyclic voles and shrews and non-cyclic mice in a marginal grassland within European temperate forest

Cyclic population dynamics of small mammals are not restricted to the boreal and arctic zones of Eurasia and North America, but long-term data series from lower latitudes are still less common. We demonstrated here the presence of periodic oscillations in small mammal populations in eastern Poland using 22-year (1986–2007) trapping data from marginal meadow and river valley grasslands located in the extensive temperate woodland of Białowieża Primeval Forest. The two most common species inhabiting meadows and river valleys, root vole Microtus oeconomus and common shrew Sorex araneus, exhibited synchronous periodic changes, characterised by a 3-year time lag as indicated by an autocorrelation function. Moreover, the cycles of these two species were synchronous within both habitats. Population dynamics of the striped field mouse Apodemus agrarius was not cyclic. However, this species regularly reached maximum density 1 year before the synchronized peak of root voles and common shrews, which may suggest the existence of interspecific competition. Dynamics of all three species was dominated by direct density-dependent process, whereas delayed density dependent feedback was significant only in the root vole and common shrew. Climatic factors acting in winter and spring (affecting mainly survival and initial reproduction rates) were more important than those acting in summer and autumn and affected significantly only the common shrew. High temperatures in winter and spring had positive effects on autumn-to-autumn changes in abundance of this species, whereas deep snow in combination with high rainfall in spring negatively affected population increase rates in common shrew.

Biogeographic variability in ecosystem engineering: patterns in the abundance and behavior of the tube-building polychaete Diopatra cuprea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 447:1-13 (2012) - DOI: https://doi.org/10.3354/meps09658 FEATURE ARTICLEBiogeographic variability in ecosystem engineering: patterns in the abundance and behavior of the tube-building polychaete Diopatra cuprea Sarah K. Berke* Smithsonian Environmental Research Center, 647 Contees Wharf Rd, Edgewater, Maryland 21037, USA Present address: Department of the Geophysical Sciences, University of Chicago, 5734 S. Ellis Ave, Chicago, Illinois 60637, USA *Email: skberke@gmail.com ABSTRACT: Ecosystem engineering is an important process influencing the structure and function of eco­systems worldwide. Geographic variation in a species’ engineering processes can thus have important implications for spatial heterogeneity in ecosystem function. In this study I explore geographic variability in the engineering properties of an important organism in marine ­sedimentary habitats, the polychaete Diopatra cuprea. In the Northwest Atlantic, D. cuprea facilitates benthic communities by creating predation refugia and substrate—both density-dependent processes—and by actively attaching macroalgae to its tube. D. cuprea’s ecosystem-engineering properties thus arise from both its abundance and its behavior. I show that both components of D. cuprea engineering decrease precipitously at low latitudes. I assess the relationship between these patterns and a suite of environmental parameters, finding that northern versus southern sites cluster separately in multivariate environmental space and that temperature, dissolved oxygen, and pH may contribute to both D. cuprea abundance and the biomass of facilitated algae. I present evidence that southern D. cuprea populations may be ­affected by post-recruitment mortality more severely than northern populations. Finally, I experimentally demonstrate that D. cuprea’s failure to facilitate algae in its south­ern range is not due to latitudinal differences in algal availability, acceptability or retain­ability, but rather is a behavioral shift driven by still-unknown factors. This study demonstrates dramatic geographic variability in both D. cuprea’s density—with strong implications for its density-dependent engineering functions—and D. cuprea’s algal facilitation. Disentangling the mechanisms underlying such variability is an important goal for understanding spatial heterogeneity in the structure and function of North­west Atlantic shallow-water sedimentary systems. KEY WORDS: Benthic habitats · Biogenic habitat structure · Biogeographic comparison · Facilitation · Infauna · Intertidal macroalgae · Intertidal sediments · Northwest Atlantic Full text in pdf format Information about this Feature Article Supplementary material NextCite this article as: Berke SK (2012) Biogeographic variability in ecosystem engineering: patterns in the abundance and behavior of the tube-building polychaete Diopatra cuprea. Mar Ecol Prog Ser 447:1-13. https://doi.org/10.3354/meps09658Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 447. Online publication date: February 13, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Building the evidence base for ecological impact assessment and mitigation

Building the evidence base for ecological impact assessment and mitigation

Settlement, mortality and growth of the asari clam (Ruditapes philippinarum) for a collapsed population on a tidal flat in Nakatsu, Japan

Although fluctuation and decline in bivalve populations have been reported worldwide, the underlying processes are not yet fully understood. This lack of understanding is partly due to an absence of demographic information for the early post-settlement period. This is the case particularly for annual production of the asari clam (also commonly known as the Manila clam, Ruditapes philippinarum) in Japan, which has greatly decreased in recent years. A remarkable decrease has been observed in the Nakatsu tidal flat, where current yields are less than 0.02% of the maximum yield. Possible explanations for this decline are: 1. limitation on recruitment due to overfishing; and 2. the demographic processes of growth and mortality have been altered by environmental changes, such as rise in seawater temperature or decrease in phytoplankton abundance. However, because of a lack of demographic information (e.g., the initial densities of larval settlement and mortality and growth rates post-settlement), the reasons for the decline, and the relative importance of each period in the life cycle in determining population abundance, remain unclear. Despite the decline, we observed high levels of recruitment of 0-year-class clams on the Nakatsu tidal flat in spring 2005, where more than 10,000 individuals m−2 3–5mm in shell length, estimated to have settled during the previous autumn, were observed. To obtain demographic information on the Nakatsu clams, we investigated two factors. First, we investigated the distribution of the 0-year-class clams and their rate of change in density as a combination of mortality, emigration and immigration on the whole tidal flat after a year. Second, we investigated the rate of change in the density and growth of clams after settlement in the center of the flat for 3years. The rate of decrease in the density of the 0-year-class clams over the whole tidal flat after a year was greater at the stations where the initial density was higher. This suggests that density-dependent processes such as predation or competition may affect population levels. In the center of the flat, the initial density of settlement was more stable than the rate of decrease after settlement. These results suggest that the clam population on this tidal flat is probably suppressed by variable but high mortality rates after settlement, not by recruitment limitation.

Study on Neighbor Effects of Three Dominant Species in the Evergreen-deciduous Mixed Forest in Jiugongshan Natural Reserve

Neighbor effects of three dominant species Cyclobalanopsis glauca,Castanopsis eyrei and Quercus serrata var.brevipetiolata in an old-growth evergreen-deciduous mixed forest in Jiugongshan Natural Reserve were examined by nearest neighbor distance statistics,and the importance of competition/facilitation and implications for local species co-existence were explored.There were no lower limits of nearest neighbor distance for any species.The size(e.x.DBH) of Cyclobalanopsis glauca was positively correlated with its intraspecific nearest neighbor distance(p=0.029),but the size of Quercus serrata var.brevipetiolata was negatively correlated with its interspecific nearest neighbor distance(p= 0.040).No significant correlations were detected between size of Castanopsis eyrei and either of its interspeci-fic/intraspecific nearest neighbor distance(p≥0.360).Intraspecific nearest neighbor distances differed insignificant from interspecific ones for all species(p≥ 0.122).Our results imply that local competition has not been fully sharpened in the old-growth evergreen-deciduous mixed forest in Jiugongshan Natural Reserve.Neighbor effects were species-specific,with intraspecific competition in Cyclobalanopsis glauca being evident,competitive/facilitative effects of Castanopsis eyrei unobvious if not negligible,and dependence of Quercus serrata var.brevipetiolata on interspecific facilitation over competition.The insignificant differences between interspecific and intraspecific nearest neighbor distance for all species suggest that stochastic processes rather than negative density-dependent processes drove local species co-existence.

Seal–cod interactions on the Eastern Scotian Shelf: Reconsideration of modelling assumptions

The cod stock on the Eastern Scotian Shelf (the 4VsW management area) collapsed in the early 1990s, and has experienced high natural mortality since then, while the abundance of grey seals foraging in the area has doubled about every seven years since the 1960s. The causes of the high cod natural mortality are not well understood, but seals are not considered to have played a significant role. This study takes a fresh look at the impact of seals on 4VsW cod abundance. Abundance trends of the Sable, Eastern Shore and Gulf seal herds which forage on the Scotian Shelf are estimated to 2020. The Sable herd is projected to stabilize at about 350,000 individuals. If their exponential growth continues, the Eastern Shore and Gulf herds could in aggregate reach about 200,000 individuals. However, density dependent processes are likely to slow population growth of these two herds sometime in the coming decade. Total annual food consumption of the three herds is estimated. In 2010, in excess of 550,000 t of fish are consumed annually by the Sable and Eastern herd, with the Gulf herd consuming about 138,000 t. The literature on seal diets is summarized and three scenarios of type II predator–prey functional response are defined. In an ADAPT analysis of the 4VsW cod stock, seals are treated as an additional fishing gear sector under two scenarios: respectively a “flat-top” and “domed” partial recruitment vectors. Model results infer that seals have contributed to increases in natural mortality since the late 1980s, and have contributed to the lack of recovery of the stock since 1993. However, predictions by the functional models are not consistent with estimate of recent increases in abundance of cod in trawl surveys. Present levels of grey seal abundance have not occurred on the Scotian Shelf since at least the 1800s.

Seasonal variation in the functional response of a coral-reef piscivore alters the inverse density-dependent mortality of its prey

Density-dependent processes are critical for regulating species' populations, and piscivory of coral-reef fishes is frequently density dependent. However, the mechanism driving this density-dependent mortality is poorly understood, but may be caused by changes in a predator's feeding rate at different prey densities (its functional response). An aquarium experiment replicated in winter and summer examined the functional response after 22 and 47 h of Cephalopholis cruentata feeding on Halichoeres pictus. With the exception of summer data after 47 h (density-independent mortality), mortality was inversely density dependent across all prey densities and increased with higher summer temperatures. The absence of an asymptotic pattern of inverse density-dependent mortality was caused by type II (summer) or dome-shaped type IV (winter) functional responses, with the benefits of schooling likely to cause the low mortality rates at higher prey densities. Predators' functional responses may underlie the inverse density-dependent mortality reported in field studies of aggregating fishes.