The role of fecundity and reproductive effort in defining life‐history strategies of North American freshwater mussels

Selection is expected to optimize reproductive investment resulting in characteristic trade‐offs among traits such as brood size, offspring size, somatic maintenance, and lifespan; relative patterns of energy allocation to these functions are important in defining life‐history strategies. Freshwater mussels are a diverse and imperiled component of aquatic ecosystems, but little is known about their life‐history strategies, particularly patterns of fecundity and reproductive effort. Because mussels have an unusual life cycle in which larvae (glochidia) are obligate parasites on fishes, differences in host relationships are expected to influence patterns of reproductive output among species. I investigated fecundity and reproductive effort (RE) and their relationships to other life‐history traits for a taxonomically broad cross section of North American mussel diversity. Annual fecundity of North American mussel species spans nearly four orders of magnitude, ranging from < 2000 to 10 million, but most species have considerably lower fecundity than previous generalizations, which portrayed the group as having uniformly high fecundity (e.g. > 200000). Estimates of RE also were highly variable, ranging among species from 0.06 to 25.4%. Median fecundity and RE differed among phylogenetic groups, but patterns for these two traits differed in several ways. For example, the tribe Anodontini had relatively low median fecundity but had the highest RE of any group. Within and among species, body size was a strong predictor of fecundity and explained a high percentage of variation in fecundity among species. Fecundity showed little relationship to other life‐history traits including glochidial size, lifespan, brooding strategies, or host strategies. The only apparent trade‐off evident among these traits was the extraordinarily high fecundity of Leptodea, Margaritifera, and Truncilla, which may come at a cost of greatly reduced glochidial size; there was no relationship between fecundity and glochidial size for the remaining 61 species in the dataset. In contrast to fecundity, RE showed evidence of a strong trade‐off with lifespan, which was negatively related to RE. The raw number of glochidia produced may be determined primarily by physical and energetic constraints rather than selection for optimal output based on differences in host strategies or other traits. By integrating traits such as body size, glochidial size, and fecundity, RE appears more useful in defining mussel life‐history strategies. Combined with trade‐offs between other traits such as growth, lifespan, and age at maturity, differences in RE among species depict a broad continuum of divergent strategies ranging from strongly r‐selected species (e.g. tribe Anodontini and some Lampsilini) to K‐selected species (e.g. tribes Pleurobemini and Quadrulini; family Margaritiferidae). Future studies of reproductive effort in an environmental and life‐history context will be useful for understanding the explosive radiation of this group of animals in North America and will aid in the development of effective conservation strategies.     

Reproductive cycle of the oviparous lizard Sceloporus jalapae,from Zapotitlán Salinas,Puebla, Mexico

Reproductive patterns vary widely among species and populations of squamates. In general, patterns can be divided into cyclic and acyclic. Cyclic patterns are common in tropical and temperate species of seasonal environments, while acyclic ones are characteristic of tropical species that inhabit less variable environments. We studied the reproductive cycle of Sceloporus jalapae, one of the smallest species of Sceloporus, in an arid environment at Zapotitlán Salinas, Puebla, Mexico. Both sexes mature in the first year following hatching and exhibit a seasonal reproductive pattern with maximum activity in spring and summer that appears to be influenced by temperature. Unlike small species of Sceloporus that inhabit pine–oak and tropical forests, S. jalapae exhibits an extended period of reproductive activity that allows females to produce at least two clutches of 3–7 eggs each, and contrary to other species of Sceloporus, in this species there is no correlation between female length and clutch size. The reproductive pattern of S. jalapae is like that shown by other oviparous species of the genus; nevertheless, some life history traits of this species are shared with oviparous and viviparous species of small size.     

Costs of reproduction can explain the correlated evolution of semelparity and egg size: theory and a test with salmon

Species’ life history traits, including maturation age, number of reproductive bouts, offspring size and number, reflect adaptations to diverse biotic and abiotic selection pressures. A striking example of divergent life histories is the evolution of either iteroparity (breeding multiple times) or semelparity (breed once and die). We analysed published data on salmonid fishes and found that semelparous species produce larger eggs, that egg size and number increase with salmonid body size among populations and species and that migratory behaviour and parity interact. We developed three hypotheses that might explain the patterns in our data and evaluated them in a stage‐structured modelling framework accounting for different growth and survival scenarios. Our models predict the observation of small eggs in iteroparous species when egg size is costly to maternal survival or egg number is constrained. By exploring trait co‐variation in salmonids, we generate new hypotheses for the evolution of trade‐offs among life history traits.     

Insights into life history theory: a brood size manipulation on a southern hemisphere species,Tachycineta leucorrhoa,reveals a fast pace of life

Life history traits exhibit substantial geographical variation associated with the pace of life. Species with a slow pace are expected to invest more in their future/residual reproductive value and are more common at tropical latitudes, whereas species from high latitudes, with a faster pace, are expected to prioritize the current reproductive effort. Most evidence supporting this pattern comes from studies conducted in tropical and north temperate species; very little is known about patterns in southern South American species. Here, we describe the life history of a southern swallow Tachycineta leucorrhoa and use an experimental approach to test their breeding strategy over four breeding seasons. We manipulated brood size for 105 nests of white‐rumped swallows to measure whether costs of reproduction were borne by adults or nestlings as alternative selection strategies towards maintaining residual or current reproductive value. Adults increased their feeding effort in enlarged broods, at least enough to maintain nestlings’ development/growth. In addition, adults decreased the number of visits to the nest (without having a negative effect on nestlings) in reduced broods. We did not detect differences in fledging success among treatments, suggesting there were no differences in nestlings’ survival. However, enlarged broods more frequently incurred in complete nest failure, suggesting only some adults were able to cope with increased costs of reproduction. We conclude this species is characterized by a fast pace of life similar to their northern congeners and less like its tropical ones. This is one of the first studies to use an experimental approach to test a life history hypothesis of pace of life using data from a southern South American species. We encourage researches to include southern species when evaluating latitudinal variations as we still do not have enough evidence to assume all southern subtropical species are indeed similar to tropical ones.     

Life history strategy of Leptinaria unilamellata (d'Orbigny, 1835) (Mollusca,Pulmonata, Subulinidae)

Abstract

Life history theory predicts that the patterns of resource allocation in animals are associated with different strategies, selected in the course of evolution. In the present study, the life history of Leptinaria unilamellata was characterized under laboratory conditions. We determined the growth, reproduction, and longevity patterns of this species and elucidated the strategy related to the development of embryos, through direct observations and examination of the morphology of the gravid uterus. Furthermore, we attempted to analyze the glycogen and galactogen contents of the albumen gland, digestive gland and cephalopedal mass in order to understand energy allocation to life history traits, for three life stages. Leptinaria unilamellata's life history is characterized by great longevity, a short juvenile phase, early sexual maturity, and repeated reproductive events, with little reproductive effort at each event and some mortality shortly after the first reproduction. In the terraria, we found juveniles but no eggs. However, the results of the anatomical study showed no morphological connection between the embryos and the parental organism. Thus, this species should be described as ovoviviparous rather than viviparous. Egg retention in the parent organism is the primary cause of the release of juveniles, instead of eggs, enabling the offspring to withstand environmental stress. The higher quantity of galactogen found in the adults' albumen gland, as compared to juveniles and senescent individuals, as well as the ratio of glycogen to galactogen, reveal the allocation of energy to reproduction rather than to growth. The remaining energy is directed to the maintenance of omeostasis. Such pattern was confirmed by the low levels of glycogen and galactogen observed in the senescent stage, compared to the juvenile and adult stages. In the life strategy of L. unilamellata, the distribution of the reproductive effort among many events associated with ovoviviparity indicates a long-term investment in reproductive success.     

Comparing latitudinal and upstream–downstream gradients: life history traits of invasive mosquitofish

Aim Variation of life history traits along spatial gradients is poorly understood in invasive species and particularly in freshwater fish. We aimed to examine life history variation in a highly invasive fish (Gambusia holbrooki) along latitudinal and upstream–downstream river gradients and to assess the effects of age on this variation. We hypothesized similar responses in populations inhabiting environments more favourable to this species (lower latitudes and lower reaches of rivers). Location European rivers from southern Spain to southern France. Methods We sampled mosquitofish from the lowest reaches of ten river basins along 6° of latitude in the Mediterranean region and seven sites along the upstream–downstream gradient in three of the rivers. We examined abundance, population structure, size‐at‐age and other life history traits along these gradients. Results As hypothesized, lower reaches and lower latitudes both resulted in higher reproductive effort and lower body condition of mosquitofish. However, these patterns explained low per cent variation, were nonlinear and strongly depended on fish age. Independently of fish size, age groups differed in reproductive effort, in the gonadal weight–size relationship and its variation along spatial gradients. Mean size‐at‐age (or overall body size) did not vary with latitude (so the intra‐specific version of Bergmann’s rule or its converse does not apply) and in contrast increased upstream in rivers. Main conclusions Our findings suggest that for life history traits of freshwater organisms, river longitudinal variation plays a role as important as climate, with often differential effects. Our results also illustrate the poor knowledge of spatial variation of many life history traits, which precludes the understanding and prediction of biological invasions in a rapidly changing world.     

Life history patterns of new world monkeys

Allometric methods are used to investigate the life history patterns of New World monkeys. These patterns are compared to those found in other haplorhines and links with ecology are explored. The life history characteristics of the cebid monkeys suggest that these species are primarily slow breeding and slow developing for primates of their size. I suggest that these characteristics arose during a long period of adaptation to stable rainforest habitats. The capuchins and squirrel monkeys are a notable exception to the general rule that genera found outside primary forests will have a “fast” life history. In the marmosets and tamarins selection for small size and high birth rate has occurred, probably as a result of their adapting to more secondary habitats. It is this selection for rapid breeding, combined with the preadaptation of paternal care, that has probably led to twinning in these species. The evidence presented indicates that it is unlikely that twinning arose as a direct result of the mechanical constraints of small size but is more probably due to selection for increased reproductive output.     

Habitat and life history differences between two species of Gambusia

Life history strategies reflect trade-offs that tend to maximize fitness, such as investment in a few large or many small offspring. We compared life histories of two temperate livebearing fishes Gambusia affinis and G. nobilis, an endangered species which is virtually unstudied. The two species persist in environments that differ widely in abiotic and biotic factors in the same local area. Gambusia affinis were typically found in habitats with high productivity and wide fluctuations in temperature, salinity and dissolved oxygen, whereas G. nobilis occurred in more stable spring-fed habitats. We collected data on life-history traits: embryo mass, brood size (number of embryos), total maternal reproductive effort, population sex ratios, and size (mass and length) distributions of adults and juveniles. There was no difference between species in reproductive effort per brood, but they differed in investment strategy. Gambusia affinis females produced large broods with small embryos, whereas G. nobilis females produced broods of fewer, larger embryos. These differences in life history strategies reflect a tradeoff between individual productivity and differential mortality rates in different environments. At our field site G. affinis persists as an annual species with relatively high growth rates and corresponding reproductive patterns, whereas G. nobilis females have a slower reproductive tempo and may live multiple years.     

The value of structuring rarity: the seven types and links to reproductive ecology

Since 1981, 365 papers have cited a rarity matrix organized along three axes: geographic range (GR) (large vs. small), habitat specificity (HS) (specialist vs. generalist), and local abundance (LA) (dense vs. sparse). In the wider ecology literature, research on the association between plant species distributions and life history traits has mainly focused on a single axis such as GR. However, the internal structure of species ranges is widely recognized as important. In order to determine if identifying different types of rarity leads to alternative conclusions regarding the causes and consequences of rarity, we created a dataset linking the seven types of rarity matrix and to reproductive ecology traits. We found associations between the axes and these traits in a dataset of 101 rare plant species culled from 27 papers. Significant traits included mating system and seed dispersal mechanism. Species with small GR are more likely to have ballistic or wind dispersal than biotically-mediated dispersal (abiotic:biotic ratio 3:1). Habitat specialist species with small GRs are more likely to have outcrossing mating systems compared to habitat specialists of large GR (16:1). These results show that, within rare species, the structure of rarity is important (e.g. habitat specialization is different from small GR) and should be identified when determining basic mechanisms of plant distribution and abundance.     

Do life history traits account for diversity of polychaete annelids?

Abstract. Within many phylogenetic assemblages, a pattern of domination has been observed: one or a few clades have had many more speciation events or fewer extinctions than other clades in a particular assemblage. We investigated this phenomenon in the polychaete annelids. Polychaetes comprise ∼9000 described species classified in over 70 families and exhibit a great variety of life history strategies. Our goal was to test whether diverse polychaete families are characterized by species with short generation times, high reproductive output, small body size, or with planktotrophic larval development. Each of these factors has been advanced as cause for high diversity in other taxonomic assemblages. Here, we establish that the diversification pattern of polychaete families is non-random, but the data collected show no significant correlations between familial diversity and several life history traits including age at first reproduction, life span, body size, fecundity, and egg size. Pairwise comparisons of sister families do not reveal any trends between familial diversity and any of the life history traits. The great variability of life history traits within polychaete families may explain the lack of significant results; perhaps no trends are seen because polychaete life history traits cannot be generalized at the family level.     

Size-independent age effects on reproductive effort in a small, short-lived fish

1. Age-related changes in reproductive effort have been predicted by theoretical models and observed in a wide range of organisms. However, for indeterminate growers such as fish, an allometric relationship linking gonad weight to body size is commonly observed. There is often a positive linear relationship when these variables are log-transformed, which by implication reduces the influence of age on reproductive effort.
2. Contrasting with this usual pattern, we report a nonlinear relationship between gonad weight and fish size (after log-transformation) in mosquitofish ( Gambusia holbrooki ), clearly resulting from age changes. The declining rate of increase of gonad mass as a function of body size revealed a higher reproductive effort for younger individuals relative to size.
3. This size-independent age effect on reproductive effort was predicted based on previous studies of mosquitofish and is certainly related to their particular life-history strategy, combining an early maturation and short lifespan with the physiological costs of reproduction and over-wintering. Our findings probably apply to other small, short-lived species with similar life history.     

Growth and reproduction of New Zealand

In New Zealand, as elsewhere, research on rare species has been dominated by autecological studies of individual threatened species. Limitations of this approach are that it involves no comparison with related common species which may have similar traits, and that the minimal sample size prevents generalisation about causes and consequences of rarity. We report on experimentally determined growth and reproductive traits of 10 rare and common Acaena(Rosaceae) species from two taxonomic sections (sect. Ancistrum and sect. Microphyllae). We examined the relationship between rarity or commonness and relative growth rate, mode of vegetative expansion, morphology/presentation of reproductive structures and reproductive allocation. Rarity and commonness were defined according to geographic range size, measured as the number of 10-km grid squares containing at least one record of the species. There were tendencies across both taxonomic sections for species with large range size to have higher relative growth rates and in section Microphyllae, faster lateral expansion. Among section Ancistrum species, common species tended to produce inflorescences for a shorter period and held their capitula higher above the canopy, but other reproductive attributes showed little association with range size. In section Microphyllae all reproductive traits tended to be positively associated with range size. This was mainly due to the single very common species having high fecundity. The lack of strong patterns among our results may reflect insufficient sample size or that the rare species represent different types of rarity.     

Coevolution of life history traits and morphology in female subterranean amphipods

Subterranean species show a distinct morphology, yet the adaptive significance of some traits, like body size and shape, is poorly understood and cannot be explained solely by distinct environmental conditions (darkness, less food). We predicted that in females some morphological changes may have co‐evolved with life history traits, and that co‐evolving life history traits provide at least part of the explanation for evolutionary changes of morphology. Using museum material we tested this prediction on the subterranean amphipod genus Niphargus. We studied six species found in springs and eight species found in cave lakes. We treated them as two ecologically distinct groups, and the major ecological differences between them were the availability of nutrients and the water currents. Cave species were found to be larger and stouter (as inferred from the shape of coxal plates, which are part of the marsupium), they had larger eggs and lower reproductive effort per brood, whereas the egg number and brood volume if corrected for the body size were not different. Using phylogenetic independent contrasts, we found a positive correlation between body shape and egg volume, a positive correlation between body size and egg volume, and a negative correlation between body size and reproductive effort per brood. We tentatively conclude that evolutions of morphology and life histories are functionally connected and that co‐evolving traits contribute to overall selective regime.     

Territorial behaviour of the male johnny darter,Etheostoma nigrum

Synopsis A field study was undertaken to describe the territorial behaviour of male johnny darters (Etheostoma nigrum) over the reproductive cycle. While defending eggs, males performed fewer Lateral Displays and lost fewer encounters with conspecific males than in the pre-egg phase. As the reproductive cycle progressed males were less responsive to females and performed relatively fewer courtship and more aggressive displays. Nesting males apparently shifted from a sexual to a parental phase shortly after acquiring eggs.Frequency of response, responsiveness (number of responses/number of intrusions x 100) and maximum distance of response (MDR) of male johnny darters were highest early in the reproductive cycle and decreased over the egg-guarding phase. The patterns of territorial defence exhibited by male johnny darters over the reproductive cycle were similar to those of other species of fish with paternal care.     

A review of the reproductive bionomics of aquatic gammaridean amphipods: variation of life history traits with latitude,depth, salinity and superfamily

Life history traits (mean and maximum body length of females, number of embryos per brood = brood size, embryo diameter, number of broods per female, lifespan of females) for 302 populations of aquatic gammaridean amphipods, representing 214 species in 16 superfamilies, were reviewed. The variation of these traits, of lifetime potential fecundity (i.e. the number of embryos produced per female lifespan) and of reproductive potential (i.e. the number of embryos produced per female per year), with temperature (latitude), depth, salinity and superfamily, was investigated by various univariate and multivariate methods. Gammaridean amphipods comprise semelparous and iteroparous populations and species, with semiannual, annual, biannual or perennial life cycles. However, most gammarideans studied so far are iteroparous annuals. Body length explains most of the variation in brood size and embryo diameter. The reproductive potential may be increased by increasing body size for a constant breeding frequency, by increasing brood size at the expense of smaller embryos, by increasing breeding frequency for a constant lifespan at the expense of smaller individual broods and/or embryos, and by increasing longevity for a constant breeding frequency and brood size. Combinations of these different options constitute the life history patterns of gammarideans, which vary across superfamilies, latitude and depth, and cannot simply be explained by variations in body length. High latitude species were generally characterized by biannual or perennial life histories, large body size, delayed maturity, and single or few broods with many, relatively large embryos; converse sets of traits characterized low latitude species. Deep-living species had relatively smaller broods and embryos than their shallow-living relatives, yet did not produce more broods. However, different superfamilies dominated in different habitats. The importance of natural selection relative to phylogenetic (historical) and physiological constraints in the forging of these patterns is discussed.     

Sociogenetic structure, kin associations and bonding in delphinids

Social systems are the outcomes of natural and sexual selection on individuals' efforts to maximize reproductive success. Ecological conditions, life history, demography traits and social aspects have been recognized as important factors shaping social systems. Delphinids show a wide range of social structures and large variation in life history traits and inhabit several aquatic environments. They are therefore an excellent group in which to investigate the interplay of ecological and intrinsic factors on the evolution of mammalian social systems in these environments. Here I synthetize results from genetic studies on dispersal patterns, genetic relatedness, kin associations and mating patterns and combine with ecological, life history and phylogenetic data to predict the formation of kin associations and bonding in these animals. I show that environment type impacts upon dispersal tendencies, with small delphinids generally exhibiting female-biased philopatry in inshore waters and bisexual dispersal in coastal and pelagic waters. When female philopatry occurs, they develop moderate social bonds with related females. Male bonding occurs in species with small male-biased sexual size dimorphism and male-biased operational sex ratio, and it is independent of dispersal tendencies. By contrast, large delphinids, which live in coastal and pelagic waters, show bisexual philopatry and live in matrilineal societies. I propose that sexual conflict favoured the formation of these stable societies and in turn facilitated the development of kin-biased behaviours. Studies on populations of the same species inhabiting disparate environments, and of less related species living in similar habitats, would contribute towards a comprehensive framework for the evolution of delphinid social systems.     

Life history of Bulimulus tenuissimus (D'Orbigny, 1835) (Gastropoda,Pulmonata, Bulimulidae): effect of isolation in reproductive strategy and in resources allocation over their lifetime

Despite the great diversity of tropical land snail species, the life history strategies of the great majority of them are unstudied. We studied reproduction, growth and survival patterns of the Brazilian species Bulimulus tenuissimus (D'Orbigny, 1835), and verified the effect of isolation on such patterns. We analyzed aspects of the life cycle of snails maintained in groups and in isolation. For both treatments, we determined the duration of the juvenile, adult and senescent stages. Growth pattern, life-time reproductive output, reproductive output during adult and senescent stages and longevity, were also verified. Isolation prolonged the duration of the juvenile stage, causing a decrease in life-time reproductive output and longevity. The reproductive pattern of the species is seasonal and, in grouped snails, three breeding periods occurred during their lifetime. The isolated snails reproduced by self-fertilization, and only reproduced once in their lifetime, indicating a significant change in reproductive strategy in the isolated individuals. Thus, isolation resulted in changes in energy allocation to growth, reproduction and survival. The results indicate that in adverse environmental conditions, life history traits can enhance the capacity for adaption.     

Comparing life histories of shrews and rodents

Small insectivores and rodents, despite similarities in body size and attributes scaling to body size, exhibit significant differences in other properties, including many life history traits. In this article major differences between life history traits of the two taxa are reviewed, with an indication of contrasting selection pressures related to somewhat different body size, as well as to differences in metabolic rates, diet and exposure to predation. Additionally, since the life history differences between small mammals are particularly well pronounced in highly seasonal habitats, the winter ecology of shrews and rodents is compared. Finally, the two different reproductive strategies typical for soricine shrews and small nonhibernating rodents, are presented. In conclusion, it is proposed that the reproduction delayed to the second calendar year of life in shrews is the result of selection for traits ensuring successful survival in winter, a period that is more perilous for shrews than for rodents. In rodents, in contrast, opportunistic reproduction is the most prominent characteristic which also helps to maximize their reproductive output. This ability for high reproduction seems to be the main antipredatory measure selected for in rodent evolution.     

Patterns of life history variation in the Funariales,Polytrichales and Pottiales

Abstract

A combination of Principal Component and Cluster Analyses is used to test for recurring suites of life history variation among 357 species of the Funariales, Polytrichales, and Pottiales. About 85% of among-species covariation in life expectancy, spore size and number, and sexuality is summarized by three Principal Components. Cluster Analyses, using species scores on these three derived variables, identify six groups of species based on similar grades of covariation in life history traits. These groups also differ significantly in water relations, gametophyte size, modes of spore dispersal, and prevalence of asexual reproductive propagules, and apparently are ecologically coherent, at least with regard to spatio-temporal scales of habitat availability and heterogeneity.

A number of differences emerge in comparisons of Principal Component Analyses performed separately for each of seven families. In particular, the distribution of overall variance among successive components varies greatly, suggesting that the number of biologically independent axes of life history specialization differs among families. Likewise, differences in loadings of the life history variables on Principal Components indicate that their patterns of covariation change as a function of familial membership.

In the Pottiales, the likelihood of producing sporophytes decreases with increasing life expectancy, and also shows a negative association with the production of asexual propagules. The likelihood of producing asexual propagules is higher among dioicous species and those not known with gametangia than among monoicous species, and is also positively associated with life expectancy. Alternative historical and ecological explanations are offered to account for these patterns of association.

These results support the idea that recurring suites of life history traits occur among moss species, and the patterns of life history variation appear to be associated with particular sets of environmental circumstances. The influence of phylogenetic history on life history variation is also strong. Until explicitly phylogenetic approaches can be employed, the extent to which observed patterns are adaptive rather than historically contingent must remain unresolved.     

Patterns of variation in life history among South American fishes in seasonal environments

Summary Ten traits related to life history theory were measured or estimated for 71 freshwater fish species from two locations in the Venezuelan llanos. Multivariate statistics and cluster analysis revealed three basic endpoint patterns bounding a two-dimensional continuum. A suite of attributes associated with parental care and aseasonal reproduction appeared to correspond to an equilibrium strategy. A second group of small fishes was distinguished by traits associated with rapid colonizing ability: early maturation, continuous reproduction, and small clutches. The third basic pattern was associated with synchronized reproduction during the early wet season, high fecundity, absence of parental care, and breeding migrations. A subset of mostly small fishes exhibiting little or no parental care, small clutches, and two to four month reproductive seasons was intermediate between the opportunistic (rapidly colonizing) and seasonal strategies. All ten life history variables showed significant effects of phylogeny. The cluster of species corresponding to the equilibrium group was dominated by siluriform fishes and perciforms of the Cichlidae. The opportunistic cluster was dominated by cyprinodontiform and characiform fishes, whereas the seasonal cluster contained primarily characiform and siluriform fishes. Seven of nine traits were significantly correlated with body length. The three reproductive patterns are interpreted as being adaptative with respect to relative intensity and predictability of temporal and spatial variation in abiotic environmental parameters, food availability, and predation pressure.