Bottom-up, top-down, and within-trophic level pressures on a cactus-feeding insect

Abstract.  1. The relative importance of host-plants and predators in the population dynamics of herbivorous insects, and the frequency and intensity of inter-specific competition among herbivores, have both been intensively studied and debated. The joint effects of bottom-up, top-down, and within-trophic level interactions, however, have rarely been integrated in a single system.
2. I studied the dynamics of the cactus bug ( Narnia pallidicornis ), a specialist feeder on tree cholla cactus ( Opuntia imbricata ), in response to variable host-plant quality, spider predation, and interactions with cactus-feeding beetles ( Moneilema appressum ). Previous work suggests that cactus reproductive effort (the proportion of meristems allocated to reproduction) is an important component of host-plant quality for Narnia. I conducted a 2-year field experiment to test the hypotheses that Narnia abundance is positively related to host-plant reproductive effort, and that interactions with predators and putative competitors alter the shape of this relationship.
3. I found strong support for the first prediction (positive Narnia– plant quality relationship) in both years, but neither predator removal nor beetle exclusion had detectable effects on this relationship in either year. I conclude that the dynamics of this insect herbivore are driven predominantly from the bottom-up, and that available data from this work and from previous studies are too variable to permit broad generalisations for the combined effects of host-plants, predation, and competition on herbivore dynamics.     

Population dynamics of coral-reef fishes: Controversial concepts and hypotheses

Abstract Knowledge of processes that drive the local population dynamics of coral-reef fishes is important for managing reef fisheries, and for using these species as models for understanding the ecology of demersal marine fishes in general. However, the reef-fish literature is replete with poorly defined concepts and vague hypotheses regarding the issue of population dynamics. Dichotomous arguments, such as whether or not recruitment drives population dynamics, are misdirected because they fail to incorporate several important concepts. First, changes in local population size are driven by four demographic rates (birth, death, immigration and emigration), all of which must be studied to understand population dynamics. Second, all populations that persist do so because at least one of these demographic rates operates in a density-dependent way that is both sufficiently strong and appropriately time-lagged. Therefore, identifying the source(s) of direct density dependence is critical for understanding the limits to variation in population size (i.e. population regulation). Third, regulation does not imply a simple point equilibrium in population size; density dependence in populations of reef fishes is bound to lie within a field of stochastic variation, and thus be difficult to detect. Since its formal origin in 1981, the ‘recruitment limitation’ hypothesis for explaining local population dynamics in reef fishes has undergone ambiguous changes in definition that threaten its usefulness. ‘Recruitment, ‘originally defined as the appearance of newly settled fish on a reef, more recently is often measured months after settlement, thus confounding pre- and post-settlement processes. ‘Limitation, ‘ which originally referred to recruitment being so low as to preclude local populations from reaching densities where resources were limiting, is more recently defined as an absence of any form of density dependence after settlement. The most effective means of testing whether post-settlement mortality is in fact density-independent is to examine patterns of mortality directly, rather than indirectly by interpreting the shape of the relationship between initial recruit density and subsequent adult density within a cohort (the recruit-adult function). Understanding the population dynamics of coral-reef fishes will require a more equitable focus on all four demographic rates, be they density dependent or not, as well as greater attention to identifying sources of density dependence. Such a pluralistic focus necessitates integrated studies of both pre- and post-settlement processes conducted at multiple spatial and temporal scales. For example, recent evidence suggests that density-dependent pre-dation on new recruits that have settled among reefs at different densities may prove to be an important source of local population regulation, especially via the aggregative response of transient piscivores.     

Plant reproductive allocation predicts herbivore dynamics across spatial and temporal scales

Life-history theory suggests that iteroparous plants should be flexible in their allocation of resources toward growth and reproduction. Such plasticity could have consequences for herbivores that prefer or specialize on vegetative versus reproductive structures. To test this prediction, we studied the response of the cactus bug (Narnia pallidicornis) to meristem allocation by tree cholla cactus (Opuntia imbricata). We evaluated the explanatory power of demographic models that incorporated variation in cactus relative reproductive effort (RRE; the proportion of meristems allocated toward reproduction). Field data provided strong support for a single model that defined herbivore fecundity as a time-varying, increasing function of host RRE. High-RRE plants were predicted to support larger insect populations, and this effect was strongest late in the season. Independent field data provided strong support for these qualitative predictions and suggested that plant allocation effects extend across temporal and spatial scales. Specifically, late-season insect abundance was positively associated with interannual changes in cactus RRE over 3 years. Spatial variation in insect abundance was correlated with variation in RRE among five cactus populations across New Mexico. We conclude that plant allocation can be a critical component of resource quality for insect herbivores and, thus, an important mechanism underlying variation in herbivore abundance across time and space.     

Strong density-dependent survival and recruitment regulate the abundance of a coral reef fish

Debate on the control of population dynamics in reef fishes has centred on whether patterns in abundance are determined by the supply of planktonic recruits, or by post-recruitment processes. Recruitment limitation implies little or no regulation of the reef-associated population, and is supported by several experimental studies that failed to detect density dependence. Previous manipulations of population density have, however, focused on juveniles, and there have been no tests for density-dependent interactions among adult reef fishes. I tested for population regulation in Coryphopterus glaucofraenum, a small, short-lived goby that is common in the Caribbean. Adult density was manipulated on artificial reefs and adults were also monitored on reefs where they varied in density naturally. Survival of adult gobies showed a strong inverse relationship with their initial density across a realistic range of densities. Individually marked gobies, however, grew at similar rates across all densities, suggesting that density-dependent survival was not associated with depressed growth, and so may result from predation or parasitism rather than from food shortage. Like adult survival, the accumulation of new recruits on reefs was also much lower at high adult densities than at low densities. Suppression of recruitment by adults may occur because adults cause either reduced larval settlement or reduced early post-settlement survival. In summary, this study has documented a previously unrecorded regulatory mechanism for reef fish populations (density-dependent adult mortality) and provided a particularly strong example of a well-established mechanism (density-dependent recruitment). In combination, these two compensatory mechanisms have the potential to strongly regulate the abundance of this species, and rule out the control of abundance by the supply of recruits.     

Incorporating density dependence into the oviposition preference-offspring performance hypothesis

1. Although theory predicts a positive relationship between oviposition preferences and the developmental performance of offspring, the strength of this relationship may depend not only on breeding site quality, but also on the complex interactions between environmental heterogeneity and density-dependent processes. Environmental heterogeneity may not only alter the strength of density dependence, but may also fundamentally alter density-dependent relationships and the preference-performance relationship. 2. Here I present results from a series of field experiments testing the effects of environmental heterogeneity and density-dependent feedback on offspring performance in tree-hole mosquitoes. Specifically, I asked: (i) how do oviposition activity, patterns of colonization and larval density differ among habitats and among oviposition sites with different resources; and (ii) how is performance influenced by the density of conspecifics, the type of resource in the oviposition site, and the type of habitat in which the oviposition site is located? 3. Performance did not differ among habitats at low offspring densities, but was higher in deciduous forest habitats than in evergreen forest habitats at high densities. Oviposition activity and larval densities were also higher in deciduous forests, suggesting a weak preference for these habitats. 4. The observed divergence of fitness among habitats with increasing density may select for consistent but weak preferences for deciduous habitats if regional abundances vary temporally. This would generate a negative preference-performance relationship when population densities are low, but a positive relationship when population densities are high. 5. This study demonstrates that failure to recognize that fitness differences among habitats may themselves be density-dependent may bias our assumptions about the ecological and evolutionary processes determining oviposition preferences in natural systems.     

Contrasting effects of large density changes on relative testes size in fluctuating populations of sympatric vole species

Across species, there is usually a positive relationship between sperm competition level and male reproductive effort on ejaculates, typically measured using relative testes size (RTS). Within populations, demographic and ecological processes may drastically alter the level of sperm competition and thus, potentially affect the evolution of testes size. Here, we use longitudinal records (across 38 years) from wild sympatric Fennoscandian populations of five species of voles to investigate whether RTS responds to natural fluctuations in population density, i.e. variation in sperm competition risk. We show that for some species RTS increases with density. However, our results also show that this relationship can be reversed in populations with large-scale between-year differences in density. Multiple mechanisms are suggested to explain the negative RTS–density relationship, including testes size response to density-dependent species interactions, an evolutionary response to sperm competition levels that is lagged when density fluctuations are over a certain threshold, or differing investment in pre- and post-copulatory competition at different densities. The results emphasize that our understanding of sperm competition in fluctuating environments is still very limited.     

Is the shape of the density-growth relationship for stream salmonids evidence for exploitative rather than interference competition?

1. Empirical studies show that average growth of stream-dwelling salmon and trout often declines with increasing density in a characteristic concave relationship. However, the mechanisms that generate negative density-growth relationships in populations in natural streams are not certain. 2. In a recent study, Imre, Grant & Cunjak (2005; Journal of Animal Ecology, 74, 508-516) argue that density-dependent growth due to exploitative competition for prey causes the negative density-growth relationships for stream salmonids. They argue that the concave shape of empirical density-growth relationships is consistent with a simple model of exploitative competition and not consistent with interference competition for space. 3. We use a simple model to show that competition for space can yield concave density-growth relationships consistent with the empirical pattern when individuals compete for foraging sites that vary spatially in quality and lower-quality sites predominate. Thus, the predictions of the exploitative competition and spatial competition models overlap. 4. The shape of the density-growth relationship does not differentiate between candidate mechanisms underlying density-dependent growth for stream salmonids. Our results highlight the general problem with determining the mechanism driving an ecological process from patterns in observational data within the context of linking population demographics to habitat structure and animal behaviour.     

Density-dependent resistance of the gypsy moth <Emphasis Type="Italic">Lymantria dispar</Emphasis> to its nucleopolyhedrovirus,and the consequences for population dynamics

The processes controlling disease resistance can strongly influence the population dynamics of insect outbreaks. Evidence that disease resistance is density-dependent is accumulating, but the exact form of this relationship is highly variable from species to species. It has been hypothesized that insects experiencing high population densities might allocate more energy to disease resistance than those at lower densities, because they are more likely to encounter density-dependent pathogens. In contrast, the increased stress of high-density conditions might leave insects more vulnerable to disease. Both scenarios have been reported for various outbreak Lepidoptera in the literature. We tested the relationship between larval density and disease resistance with the gypsy moth (Lymantria dispar) and one of its most important density-dependent mortality factors, the nucleopolyhedrovirus (NPV) LdMNPV, in a series of bioassays. Larvae were reared in groups at different densities, fed the virus individually, and then reared individually to evaluate response to infection. In this system, resistance to the virus decreased with increasing larval density. Similarly, time to death was faster at high densities than at lower densities. Implications of density–resistance relationships for insect–pathogen population dynamics were explored in a mathematical model. In general, an inverse relationship between rearing density and disease resistance has a stabilizing effect on population dynamics.     

Exploring density-dependent relationships in demographic parameters in populations of birds at a large spatial scale

The importance of density-dependent processes in natural populations is widely accepted, but the issue of the shape of density-dependent relationships (such as influenced by vagueness, or time-delay) remains unresolved. We explored the density-dependent relationships in demographic parameters for 12 species of birds in Britain using large-scale, long-term data sets. We predicted that a negative relation between density and demographic parameters should be observed for the stable species, whereas the decreasing or increasing species should display a positive relation if the environment changes progressively through time bringing about a continuous change in density dependence. Our prediction was verified for nine species out of 12; however, we observed, for the three remaining species, a significant decrease of survival rates through time that seems to be involved in a long-term population decline. In all cases where a density-dependent relation was found, we observed an important variance around the relation. In one case, we showed that this variance increased significantly with density. We found evidence for time-delayed effects of density dependence both for survival and breeding performance. In two species, our results suggest the existence of complex interactions (compensatory mechanisms) between survival and breeding performance or between the different components of breeding performance.     

Population regulation by post-settlement mortality in two temperate reef fishes

 Input of individuals dispersing into open populations can be highly variable, yet the consequences of such variation for subsequent population densities are not well understood. I explored the influence of variable input (”supply”) on subsequent densities of juveniles and adults in open local populations of two temperate reef fishes, the bluebanded goby (Lythrypnus dalli) and the blackeye goby (Coryphopterus nicholsii). Variable recruitment was simulated by stocking a natural range of densities of young fishes on replicate patch reefs. Density and mortality of the stocked cohorts were followed over time, until the fishes reached maturity. Over the first day of the experiments, mortality of both species was significantly density-dependent; however, there was still a very strong relationship between density on day 1 and density on day 0 (i.e., simulated recruitment was still an excellent predictor of population density). At this point in the study, the main effects of density-dependent mortality were to reduce mean densities and variation about the mean. Over the period from the start of the experiments until the time when maturity was reached by each species (about 1 and 3 months for Lythrypnus and Coryphopterus, respectively), mortality was strongly density-dependent. Such strong density-dependent mortality virtually eliminated any linear relationship between adult density and ”recruit” density. However, for both species, the relationship between these two variables was well fit by an asymptotic curve, with the asymptotic density of adults equal to c. 3/m² for Coryphopterus, and c. 10/m² for Lythrypnus. Natural recruitment (via settlement of larvae) to the reefs over the period of the study (9 months) was above the asymptotic densities of adults for the two species, even though the study did not encompass the periods of peak annual recruitment of either species. This suggests that adult populations of these two gobies may often be limited, and regulated, by post-settlement processes, rather than by input of settlers. Other studies have shown that mortality of the two species is density-independent, or only weakly density-dependent, on reefs from which predators have been excluded. Hence, it appears that predators cause density-dependent mortality in these fishes. Received: 26 November 1996 / Accepted: 5 April 1997     

Temporal changes in the strength of density-dependent mortality and growth in intertidal barnacles

1. In demographically open marine systems, the extent to which density-dependent processes in the benthic adult phase are required for population persistence is unclear. At one extreme, represented by the recruitment limitation hypothesis, larval supply may be insufficient for the total population size to reach a carrying capacity and density-independent mortality predominates. At the opposite extreme, populations are saturated and density-dependent mortality is sufficiently strong to reshape patterns established at settlement. 2. We examined temporal variation in the way density-independent and density-dependent mortality interact in a typical sessile marine benthic invertebrate, the acorn barnacle Semibalanus balanoides (L.), over a 2-year period. 3. Recruitment was manipulated at two high recruitment sites in north Wales, UK to produce recruit densities covering the range naturally found in this species. Following manipulation, fixed quadrats were monitored using digital photography and temporal changes in mortality and growth rate were examined. 4. Over a 2-year period there was a clear, spatially consistent, over-compensatory relationship between the density of recruits and adult abundance indicating strong density-dependent mortality. The strength of density dependence intensified with increasing recruitment. 5. Density-dependent mortality did not operate consistently over the study period. It only operated in the early part of the benthic phase, but the pattern of adult abundance generated was maintained throughout the whole 2-year period. Thus, early life-history processes dictated adult population abundance and dynamics. 6. Examination of the natural recruitment regime in the area of study indicated that both positive and negative effects of recruitment will occur over scales varying from kilometres to metres.     

Recruitment limitation: A theoretical perspective

Abstract A theoretical analysis of the concept of recruitment limitation leads to the conclusion that most populations should he regarded as jointly limited by recruitment and interactions between individuals after recruitment. The open nature of local marine systems does not permit avoidance of density-dependent interactions; it simply may make them more difficult to detect. Local populations consisting of settled organisms may not experience density-dependent interactions under some circumstances, but the entire species population consisting of the collection of local populations and their planktonic larvae must have density-dependent dynamics. Any local population of settled individuals can escape density dependence if sufficient density dependence occurs among planktonic larvae or within other local populations. Common conceptions of density dependence are too narrow, leading too often to the conclusion that it is absent from a system. It is equally wrong to expect that density-dependent interactions after settlement determine local population densities independently of recruitment. Special circumstances allowing density dependence to act strongly and quickly are needed before density dependence can neutralize the effects of recruitment. Recruitment limitation and density-dependent interactions therefore should not be regarded as alternatives but as jointly acting to determine the densities of marine benthic populations. Moreover, the interaction between fluctuating recruitment and density dependence is potentially the most interesting feature of recruitment limitation. For example, this interaction may be an important diversity-maintaining mechanism for marine systems.     

Influence of a large late summer precipitation event on food limitation and grasshopper population dynamics in a northern Great Plains grassland

The complex interplay between grasshoppers, weather conditions, and plants that cause fluctuations in grasshopper populations remains poorly understood, and little is known about the ecological processes that generate grasshopper outbreaks. Grasshopper populations respond to interacting extrinsic and intrinsic factors, with yearly and decadal weather patterns and the timing of precipitation all potentially important. The effects of initial and increasing grasshopper densities on grasshopper survival and reproductive correlates were examined at a northern mixed-grass prairie site through manipulations of grasshopper densities inside 10-m2 cages. High-quality grass growth occurred after a 9.1-cm mid-August rain. Reduced proportional survival was apparent in the two higher density treatments before the rain, indicative of food-limited density-dependent mortality. However, the large late summer rainfall event mediated the effects of exploitative competition on demographic characteristics because of the high-quality vegetation growth. This led to weaker effects of food limitation on survival and reproduction at the end of the experiment. The results indicate a direct link between weather variation, resource quality and grasshopper population dynamics led to a severe grasshopper outbreak and show that infrequent large precipitation events can have significant effects on population dynamics. Additional research is needed to examine the importance of infrequent large precipitation events on grasshopper population dynamics in grassland ecosystems.     

Can inverse density dependence at small spatial scales produce dynamic instability in animal populations?

All else being equal, inversely density-dependent (IDD) mortality destabilizes population dynamics. However, stability has not been investigated for cases in which multiple types of density dependence act simultaneously. To determine whether IDD mortality can destabilize populations that are otherwise regulated by directly density-dependent (DDD) mortality, I used scale transition approximations to model populations with IDD mortality at smaller “aggregation” scales and DDD mortality at larger “landscape” scales, a pattern observed in some reef fish and insect populations. I evaluated dynamic stability for a range of demographic parameter values, including the degree of compensation in DDD mortality and the degree of spatial aggregation, which together determine the relative importance of DDD and IDD processes. When aggregation-scale survival was a monotonically increasing function of density (a “dilution” effect), dynamics were stable except for extremely high levels of aggregation combined with either undercompensatory landscape-scale density dependence or certain values of adult fecundity. When aggregation-scale survival was a unimodal function of density (representing both “dilution” and predator “detection” effects), instability occurred with lower levels of aggregation and also depended on the values of fecundity, survivorship, detection effect, and DDD compensation parameters. These results suggest that only in extreme circumstances will IDD mortality destabilize dynamics when DDD mortality is also present, so IDD processes may not affect the stability of many populations in which they are observed. Model results were evaluated in the context of reef fish, but a similar framework may be appropriate for a diverse range of species that experience opposing patterns of density dependence across spatial scales.     

What changed the demography of an introduced population of an herbivorous lady beetle?

1. The population dynamics of an introduced population of Epilachna niponica (Lewis) (Coleoptera: Coccinellidae) was investigated for a 7-year period following its introduction to a site outside of its natural range. A population from Asiu Experimental Forest was introduced to Kyoto University Botanical Garden, 10 km south of its natural distribution.
2. Arthropod predation was much lower in the introduced than in the source population. As a result of the lower predation in the Botanical Garden, larvae reached densities five times higher than in the Asiu Forest and host plants were frequently defoliated. Food shortage caused larval deaths from starvation and increased dispersal.
3. The density of the introduced population was much more variable than that of the source population. The variation in population density in both the introduced and source populations is limited by density-dependent reduction in fecundity and female survival. However, variation in the introduced population's density was increased due to host plant defoliation that resulted in overcompensating density-dependent mortality. In years with high larval density plants were defoliated and this increased adult mortality during the prehibernation period. Besides, the density-dependent regulatory mechanisms that produce population stability were weaker in the introduced population than in the source population.     

Spatial synchrony in population fluctuations: extending the Moran theorem to cope with spatially heterogeneous dynamics

The recent interest in the spatial structure and dynamics of populations motivated numerous theoretical and empirical studies of spatial synchrony, the tendency of populations to fluctuate in unison over regional areas. The first comprehensive framework applied to spatial synchrony was probably the one elaborated by P. A. P. Moran back in 1953. He suggested that if two populations have the same linear density-dependent structure, the correlation between them will be equal to that between the local density-independent conditions. Surprisingly, the consequences of violating the assumption that the dynamics of the populations are identical has received little attention. In this paper, making the assumption that population dynamics can be described by linear and stationary autoregressive processes, I show that the observed spatial synchrony between two populations can be decomposed into two multiplicative components: the demographic component depending on the values of the autoregressive coefficients, and the correlation of the environmental noise. The Moran theorem corresponds to the special case where the demographic component equals unity. Using published data, I show that the spatial variability in population dynamics may substantially contribute to the spatial variability of population synchrony, and thus should not be neglected in future studies.     

Salmonid population dynamics: stability under weak density dependence?

Lobón-Cerviá and Rincón analysed the dynamics of a stream-resident brown trout ( Salmo trutta ) population using stock-recruitment relationships, and reported no indication of density dependence but strong effects of water discharge on juvenile production. The authors concluded to have demonstrated an example of a population where "… endogenous density-dependent mechanisms played, at most, a limited role" for population dynamics (p. 643), and that the population persists with "… little or no operation of endogenous regulatory mechanisms" (p. 641). Further, they "… question the extent to which generalizations claiming that most natural animal populations should be subject to the operation of regulatory feedback mechanisms hold for fish populations" (p. 644). If these conclusions were accurate, it should radically alter our view of fish population dynamics and how such populations should be managed. However, here I argue that these conclusions should be treated cautiously, and suggest alternative interpretations which are more consistent with theory and previous empirical research. Furthermore, using the study of Lobón-Cerviá and Rincón as an illustrative example, I suggest that additional insight into population dynamics may be attained by shifting the focus away from the conventional absolute distinction between density-dependent and -independent factors towards studies that address the potential for interactions between external forces and density.     

Population stability in relation to resource availability in an introduced population of an herbivorous lady beetle

Mechanisms responsible for population stability in relation to resource availability were studied in an introduced herbivorous lady beetle,Epilachna niponica. The introduced population was relatively constant over a seven-year study period. Egg density was related to the variation in host-plant abundance in different years, and was highly stabilized during the period from reproductive adult to egg stage. Two density-dependent processes were identified in the reproductive season: (1) density-dependent reduction in fecundity and (2) density-dependent increase in female mortality and/or emigration, all of which operated early in the season. As a result, temporal variability in cumulative egg density was greatly reduced by mid-May, by which time approximately40% of total eggs were laid. A field cage experiment demonstrated that egg-laying of individual females was largely limited by resource availability even at low levels of leaf herbivory. Since movement activity of ovipositing females increased in a density-dependent manner, inter-plant movement is more likely to cause density-dependent reduction in fecundity and female loss, due to enhanced energy expendiditure. The introduced population was less stable than the source population, probably because of decreased inter-plant movement of females and the unlikelihood of egg resorption, both of which contribute significantly to the temporal stability ofE. niponica population densities.     

Dynamics of two Montana grasshopper populations: relationships among weather,food abundance and intraspecific competition

The population dynamics of two grasshoppers (Melanoplus femurrubrum and M. sanguinipes) were studied using experimental microcosms over 8 years at a Palouse prairie site in Montana. Grasshopper density, survival and reproduction in the experimental populations responded in a density-dependent fashion to natural and experimental changes in food availability for all grasshopper developmental stages, both within and between all years. We observed that field populations of the grasshoppers at the site exhibited density, survival and reproductive responses similar to the experimental populations over the period of the study. Because we could not identify any differences between the field and microcosm environments or the grasshopper individuals in them, we contend that field populations were ultimately limited by food within and between years. Density-dependent food limitation occurred for all age categories over the entire summer, because food abundance declined relative to grasshopper food requirements over the summer. Food limitation occurred between years, because in years with the lowest food abundance, the populations produced more hatchlings for the next year than could be supported by the highest observed food abundance. Finally, the observed annual changes in food abundance were correlated with the annual variation in weather (rainfall and temperature), which indicated that the long established relationship between grasshopper densities and weather conditions does not imply population limitation by density-independent processes.     

Searching for the physiological mechanism of density dependence: does corticosterone regulate tadpole responses to density?

Density-dependent population regulation is important in many natural populations, and in those populations, high population density is a likely stressor. In amphibians, previous laboratory studies with tadpoles suggest that corticosterone, the main glucocorticoid stress hormone, is one of the key regulators of density-dependent growth. To test this relationship in natural settings, we manipulated wood frog (Rana sylvatica) tadpole density at three levels in outdoor mesocosms and used a capture stress protocol to examine the hormonal stress response. In addition, we used the same capture protocol in six natural ponds (three high density and three low density). In the mesocosms, there was an increase in corticosterone levels in tadpoles following 1 h of confinement at weeks 1, 2, and 5. However, while tadpoles maintained at higher densities were smaller after metamorphosis, density did not alter mean levels of corticosterone obtained during confinement, and baseline levels of corticosterone did not differ between the densities. Similarly, in natural ponds, density did not correlate with baseline corticosterone or mean corticosterone levels obtained during confinement. We suggest that the physiological response to density may vary across the range of natural densities and that the role of corticosterone may be limited to periods of extreme high density, such as during pond-drying events.