Behavioral Reduction of Infection Risk Kiesecker, Joseph M.; Skelly, David K.; Beard, Karen H. ...
Proceedings of the National Academy of Sciences - PNAS,
08/1999, Letnik:
96, Številka:
16
Journal Article
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Evolutionary biologists have long postulated that there should be fitness advantages to animals that are able to recognize and avoid conspecifics infected with contact-transmitted disease. This ...avoidance hypothesis is in direct conflict with much of epidemiological theory, which is founded on the assumptions that the likelihood of infection is equal among members of a population and constant over space. The inconsistency between epidemiological theory and the avoidance hypothesis has received relatively little attention because, to date, there has been no evidence that animals can recognize and reduce infection risk from conspecifics. We investigated the effects of Candida humicola, a pathogen that reduces growth rates and can cause death of tadpoles, on associations between infected and uninfected individuals. Here we demonstrate that bullfrog (Rana catesbeiana) tadpoles avoid infected conspecifics because proximity influences infection. This avoidance behavior is stimulated by chemical cues from infected individuals and thus does not require direct contact between individuals. Such facultative modulations of disease infection risk may have critical consequences for the population dynamics of disease organisms and their impact on host populations.
The rank abundance of common and rare species within ecological communities is remarkably consistent from the tropics to the tundra. This invariant patterning provides one of ecology's most enduring ...and unified tenets: most species rare and a few very common. Increasingly, attention is focused upon elucidating biological mechanisms that explain these species abundance distributions (SADs), but these evaluations remain controversial. We show that college basketball wins generate SADs just like those observed in ecological communities. Whereas college basketball wins are structured by competitive interactions, the result produces a SAD pattern indistinguishable from random wins. We also show that species abundance data for tropical trees exhibits a significant-digit pattern consistent with data derived from complex structuring forces. These results cast doubt upon the ability of SAD analysis to resolve ecological mechanism, and their patterning may reflect statistical artifact as much as biological processes.
A fundamental goal of ecology is to understand the factors that influence community structure and, consequendy, generate heterogeneity in species richness across habitats. While niche-assembly (e.g. ...species-sorting) and dispersal-assembly mechanisms are widely recognized as factors structuring communities, there remains substantial debate concerning the relative importance of each of these mechanisms. Using freshwater snails as a model system, we explore how abiotic and biotic factors interact with dispersal to structure local communities and generate regional patterns in species richness. Our data set consisted of 24 snail species from 43 ponds and lakes surveyed for seven years on the Univ. of Michigan's E. S. George Reserve and Pinckney State Recreation Area near Ann Arbor, Michigan. We found that heterogeneity in habitat conditions mediated species-sorting mechanism to drive patterns in snail species richness across sites. In particular, physical environmental variables (i. e. habitat area, hydroperiod, and canopy cover), pH, and fish presence accounted for the majority of variation in the species richness across sites. We also found evidence of Gleasonian structure (i. e. significant species turnover with stochastic species loss) in the metacommunity. Turnover in snail species distributions was driven by the replacement of several pulmonate species with prosobranch species at the pond permanence transition. Turnover appeared to be driven by physiological constraints associated with differences in respiration mode between the snail orders and shell characteristics that deter molluscivorous fish. In contrast to these niche-assembly mechanisms, there was no evidence that dispersal-assembly mechanisms were structuring the communities. This suggests that niche-assembly mechanisms are more important than dispersal-assembly mechanisms for structuring local snail communities.
An important contribution to infectious disease emergence in wildlife is environmental degradation driven by pollution, habitat fragmentation, and eutrophication. Amphibians are a wildlife group that ...is particularly sensitive to land use change, infectious diseases, and their interactions. Residential suburban land use is now a dominant, and increasing, form of land cover in the USA and globally, contributing to increased pollutant and nutrient loading in freshwater systems. We examined how suburbanization affects the infection of green frog (
Rana clamitans
) tadpoles and metamorphs by parasitic flatworms (
Echinostoma
spp.) through the alteration of landscapes surrounding ponds and concomitant changes in water quality. Using sixteen small ponds along a forest-suburban land use gradient, we assessed how the extent of suburban land use surrounding ponds influenced echinostome infection in both primary snail and secondary frog hosts. Our results show that the degree of suburbanization and concurrent chemical loading are positively associated with the presence and burden of echinostome infection in both host populations. This work contributes to a broader understanding of how land use mediates wildlife parasitism and shows how human activities at the household scale can have similar consequences for wildlife health as seemingly more intensive land uses like agriculture or urbanization.
Concern over species declines has prompted researchers to use historical data as a basis for comparison with present-day information from the same sites to assess changes in presence/absence ...distributions. A review of the literature revealed that these resurveys typically lasted for 1 or 2 years, and many were based on museum records or other data relying on known historical presences. Using data on nine amphibian species from a set of 32 ponds at the E. S. George Reserve (ESGR) in Michigan, we evaluated the importance of the duration of a resurvey and the type of historical data used (information on historical presences and absences vs. historical presences only). We compared data we collected between 1996 and 2000 with information from the same ponds collected between 1967 and 1974. By systematically degrading the 1996-2000 data, we determined that a resurvey lasting 1 year would yield an estimated 45% decline in the number of presences, whereas a resurvey lasting 2 years would yield an estimated 28% decline. In contrast, a 5-year resurvey would yield an estimated 3% decline in the number of presences. In addition, when our historical data were limited to known presences in the past, even a 5-year resurvey yielded an estimated 30% decline in the number of presences. Our results suggest that estimates of decline and distributional change can be extremely sensitive to the duration of resurvey effort and the type of historical data used. The pattern we found in analyses of ESGR data is echoed in published studies in which multiple-year resurveys tended to yield smaller estimates of decline than single-year resurveys. Based on our findings, we suggest that future resurveys extend for long enough to estimate the value of additional data and that geographic scales of inference be chosen based on the amount and quality of historical information.
Ectotherms use behaviour to buffer effects of temperature on growth, development and survival. While behavioural thermoregulation is widely reported, localized adaptation of thermal preference is ...poorly documented. Larval amphibians live in wetlands ranging from entirely open to heavily shaded by vegetation. We hypothesized that populations undergo localized selection leading to countergradient patterns of thermal preference behaviour. Specifically, we predicted that wood frog (Rana sylvatica) larvae from closed canopy ponds would be more strongly temperature selective and would prefer higher temperatures than conspecifics from populations found in open canopy ponds. In a study of six breeding ponds in north‐eastern Connecticut, USA, these predictions were upheld. The countergradient, microgeographical variation in thermal preference documented here implies that wood frog populations may have diverged rapidly in the face of contrasting selection pressures. Rapid, behaviourally mediated responses to changing thermal environments have important implications for understanding population responses to climate change.
When animals are more active they encounter both more food and more predators. Thus, activity rates mediate a trade-off between growth rates and predation risk. Models of the trade-off generally, but ...not exclusively, predict reduced activity when resource availability increases or when predation risk increases. In a laboratory setting, we videotaped larvae of four species of ranid frogs (bullfrog, Rana catesbeiana; green frog, R. clamitans; leopard frog, R. pipiens; and wood frog, R. sylvatica). Changes in activity level in response to changes in food and predator density were measured. Overall, species reduced both the proportion of time active and swimming speed with increases in resource level and predator density. These effects were additive. Regardless of food level, additional predators reduced activity levels similar amounts in all four species. Larger animals, which are less vulnerable to predation, were more active than smaller animals. Leopard frog and wood frog larvae, which are characteristic of more temporary waters, responded more strongly to variation in food levels than did bullfrog and green frog larvae.
Prior studies have shown that macrogeographic gradients in temperature associated with latitude and altitude can lead to countergradient patterns of variation in a number of taxa: individuals from ...colder environments are known to grow or develop faster than their conspecifics from warmer environments when placed in a common setting. In this study, I hypothesized that countergradient variation also is important at microgeographic scales. The wood frog, Rana sylvatica, breeds in open‐canopied, temporary wetlands as well as those heavily shaded by vegetation. Shading leads to cooler thermal environments that are associated with embryonic development rates as much as 50% slower than those in unshaded wetlands. Wetlands with contrasting canopy environments are often found within tens or hundreds of meters of each other. In a common garden experiment, embryos from nearby natural wetlands displayed countergradient variation: individuals collected from shaded wetlands developed up to 12% faster than those collected from relatively unshaded wetlands. The results of this study suggest that the concept of countergradient variation may be extended to small scales of space. In addition, the rate and scale of vegetation dynamics (the agent of wetland shading) imply that divergence in development among residents of nearby wetlands may be relatively rapid, on the order of decades.
Highlights • We measured estradiol in metamorphosing froglets from forested and suburban ponds. • Suburban ponds were serviced either by septic systems or sanitary sewers. • Estradiol was elevated in ...male froglets from ponds in septic, but not sewered, neighborhoods. • This is evidence that endocrine disruption differs among septic and sewer neighborhoods.
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