The Interplay Between Life History and Environmental Stochasticity: Implications for the Management of Exploited Lizard Populations

SYNOPSIS. The sustainable use of wild species by local peopleis emerging as an important conservation strategy. The premiseis the economic value of species will justify their own preservationas well as the habitats they occupy. However, the lack of naturalhistory and demographic information for the majority of speciesbeing exploited or with potential uses presents challengingproblems for implementing sustainable use programs. Each yearin Argentina and Paraguay, an average of 1.9 million tegu lizardsof the genus Tupinambis are exploited for their skins. In spiteof the importance of tegus as a resource, their biology is poorlyknown and their populations have never been managed. The lifehistory of Tupinambis, like that of other exploited lizards,is characterized by a relatively long life span, a large clutchsize, several years of growth before reproduction, and highmortality of hatchlings. Importantly, the mortality of young-of-the-yearand the proportion of females reproducing each year are bothprobably strongly influenced by interannual environmental variation.Whenthese parameters were randomized in life table projections tosimulate the effects of environmental stochasticity, the populationgrowth rate was highly sensitive to environmental fluctuations.Monte Carlo simulations of different harvest strategies showedthat estimates of population growth rates were overwhelminglyinfluenced by environmental variation and the number of yearsincluded in the growth rate estimate, even in the face of seeminglylarge changes in adult mortality that would result from populationmanagement. These results are both encouraging and cautionaryfor Tupinambis conservation. On the one hand these patternscan help explain how Tupinambis populations may have persistedin spite of high and variable harvest levels during many years,but conversely, stochastic effects make it difficult to evaluatethe effects of conservation measures. Size and sex can be determinedfrom harvested skins, and pilot studies suggest that analysesof the annual harvest can provide valuable information for evaluatinglong-term population trends.