Invader danger: lizards faced with novel predators exhibit an altered behavioral response to stress.
ABSTRACT Animals respond to stressors by producing glucocorticoid stress hormones, such as corticosterone (CORT). CORT acts too slowly to trigger immediate behavioral responses to a threat, but can change longer-term behavior, facilitating an individual's survival to subsequent threats. To be adaptive, the nature of an animal's behavior following elevated CORT levels should be matched to the predominant threats that they face. Seeking refuge following a stressful encounter could be beneficial if the predominant predator is a visual hunter, but may prove detrimental when the predominant predator is able to enter these refuge sites. As a result, an individual's behavior when their CORT levels are high may differ among populations of a single species. Invasive species impose novel pressures on native populations, which may select for a shift in their behavior when CORT levels are high. We tested whether the presence of predatory invasive fire ants (Solenopsis invicta) at a site affects the behavioral response of native eastern fence lizards (Sceloporus undulatus) to elevated CORT levels. Lizards from an uninvaded site were more likely to hide when their CORT levels were experimentally elevated; a response that likely provides a survival advantage for lizards faced with native predatory threats (e.g. birds and snakes). Lizards from a fire ant invaded site showed the opposite response; spending more time moving and up on the basking log when their CORT levels were elevated. Use of the basking log likely reflects a refuge-seeking behavior, rather than thermoregulatory activity, as selected body temperatures were not affected by CORT. Fleeing off the ground may prove more effective than hiding for lizards that regularly encounter small, terrestrially-foraging fire ant predators. This study suggests that invasive species may alter the relationship between the physiological and behavioral stress response of native species.
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ABSTRACT: We sampled blood from free-living spotted salamanders (Ambystoma maculatum) to test whether differences in the concentrations of a stress hormone (corticosterone) were associated with different qualities of breeding and migration habitat. Spotted salamanders are forest specialists that migrate to vernal pools to breed, and upland habitat degradation may have sub-lethal effects on animals that lead to population declines. An individual's level of physiological stress may function as a biomonitor for sub-lethal effects, and thus as a biomonitor for ecosystem quality. We compared unstressed (baseline) and stress-induced corticosterone concentrations in spotted salamanders: (1) at sites that differed in amount of forest loss; (2) during breeding migration across forest habitat versus pavement; (3) in microhabitats that varied in soil drainage and canopy cover. Removal of large amounts of terrestrial habitat surrounding a breeding pond was correlated with lower baseline (in males) and stress-induced corticosterone concentrations, which may indicate healthy individuals with a reduced ability to respond to additional stress or individuals experiencing chronic stress. Male salamanders migrating across pavement had elevated baseline corticosterone concentrations compared to animals migrating through a forest, consistent with an acute stress response. However, concentrations of corticosterone did not differ between individuals in microhabitats with canopy cover and well-drained soil versus those in microhabitats with no canopy cover and/or swampy soil. This endocrinological technique may be one useful measure of a population's health, helping to identify populations where further ecological study is recommended to evaluate conservation concerns.Animal Conservation 01/2003; 6(1):11 - 18. · 2.69 Impact Factor
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ABSTRACT: We examined patch use of deer mice foraging in the presence vs. the absence of red imported fire ants in a laboratory experiment. We tested the H = P + C + MOC patch use model (H, harvest rate; P, predation risk; C, metabolic costs; MOC, missed opportunity costs), an extension of the marginal-value theorem, by comparing in-patch harvest rates and giving-up densities (GUDs) in the presence and the absence of fire ants. The model predicts that foragers should seek patches that yield higher feeding rates in the presence of increased costs. GUDs also should increase, since they should correlate directly with quitting-harvest rates, as long as the forager experiences diminishing returns on its rate of resource harvest. We identified behavioral trade-offs made by deer mice when foraging in the presence of fire ants. In the presence of fire ants, mice harvested a greater proportion of seeds from, spent more time in, and made more visits to rich patches than to poor, whereas in the absence of fire ants, these variables did not differ between rich and poor patches. Mice also increased their in-patch harvest rate in the presence of fire ants, as predicted by the H = P + C + MOC model, but harvested patches to a lower GUD. Mice left patches at lower final seed densities, but were able to maintain a higher mean in-patch harvest rate, suggesting that mice foraged more efficiently in the presence of fire ants. When fire ants were present, deer mice spent more time on out-of-patch handling (fire ants were confined to patches). Despite increased costs or lost time due to taking seeds away from the ants for consumption, mice were able to harvest the same number of seeds in the same total time, at the same mean harvest rate. By biasing effort toward more energetically profitable rich patches and by increasing their in-patch harvest rate, deer mice were able to obtain the same net yield in the same time, despite increased costs. Read More: http://www.esajournals.org/doi/abs/10.1890/0012-9658(1997)078%5B0308:PUUPHE%5D2.0.CO%3B2Ecology 01/1997; 78(1):308-317. · 5.18 Impact Factor
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ABSTRACT: The negative impacts of biological invasion are economically and ecologically significant and, while incompletely quantified, they are clearly substantial. Ants (family Formicidae) are an important, although often overlooked, component of many terrestrial ecosystems. Six species of ants are especially striking in their global ability to invade, and their impacts. This paper focuses on the impacts of the most destructive of those species, the red imported fire ant (Solenopsis invicta), and focuses on impacts on native vertebrates. Red imported fire ants often become the dominant ant species in infested areas outside of their native range due to their aggressive foraging behavior, high reproductive capability and lack of predators and/or other strong competitors. The evidence suggests that mammals, birds and herpetofauna are vulnerable to negative impacts from fire ants, and some species are more likely to experience negative population-level impacts than other species. Assessing the ecological impacts of fire ants on wild animal populations is logistically difficult, and very few studies have combined replicated experimental manipulation with adequate spatial (>10 ha) and temporal (>1 y) scale. Thus, most studies have been observational, opportunistic, small-scale or 'natural' experiments. However, significant research, including an increase in experimental and mechanistic investigations, has occurred during the past decade, and this has led to information that can lead to better management of potentially affected species.American Midland Naturalist 01/2009; · 0.67 Impact Factor