Modeling shifts in resource use by lizards due to anthropogenic disturbance: An isotopic approach

Background/Question/Methods Historically, many ecosystems depended on fire to maintain habitat diversity and ecosystem integrity. Implementation of a fire suppression policy in the early 1900's reduced the frequency of wildfires and changed forest composition and structure throughout the US. Fire dynamics also changed as a consequence of fuel accumulation; high frequency, low intensity ground fires were replaced by infrequent, high intensity, crown fires. Recently, management policies have been revised to use prescribed burns for restoring forest ecosystem integrity, despite a number of criticisms on their effectiveness and ability to mimic natural processes. Disturbances can have many effects on animal populations, most notably, alterations of the physical environment. Such alterations can create ecological traps if the species are unable to adapt to changing resource bases; thrusting them on a path which could lead toward extinction. Human induced perturbations such as prescribed fires result in numerical and functional reductions in available vegetation that can have marked bottom-up trophic effects. One method of determining these effects is through modeling physical and biogeochemical (i.e., isotopes) parameters. Here, I demonstrate the applicability of applying isotopic models of resource use and diet specialization to predicting the effects of disturbance on lizard populations. Lizards are effective model organisms because reptiles typically respond to vegetation (and thus resource) changes rather than the disturbance itself (e.g., burning, logging). By comparing lizard populations among at least two regions, one in which remains undisturbed (control) and the other disturbed (experimental), one may determine how these habitat changes can affect resource use and trophic linkages. Specialist species relying on one primary trophic resource may risk extinction in disturbed habitats as BIC (isotopic variation between species) approaches TINW (total isotopic niche width) and WIC (isotopic variation within each species) remains constant. Specialists however that use constant proportions of different dietary sources should show shifts in dietary proportions with habitat change, as the BIC component decreases and WIC approaches zero. Generalist species then would show a BIC approaching zero and WIC approaching TINW.

Results/Conclusions

By modeling perturbation effects in this manner, I can predict the direct (changes within each trophic level) and indirect (changes among trophic levels) impacts of prescribed burning (and other) management practices. This is vital information for land managers and conservationists that plan to continue using prescribed burning. In addition, this technique will provide data on key traits linked with population dynamics in human altered environments.