Seasonal regulation and intraspecific competition in tropical forest marsupials: A 16-year study

Background/Question/Methods

A key question in population ecology concerns how populations are regulated in seasonal environments. In the tropics, many populations face their biggest challenges in the dry season due to limited food, low temperature and little rainfall. Although ignored in many theoretical models, most populations fluctuate seasonally, and a key focus in ecology remains to understand the interaction of factors that drive these dynamics. Seasonal changes are likely to be especially problematic for small, short-lived, seasonally breeding endotherms since they must balance their energy budgets between current maintenance and growth, as well as for reproduction in the next season. To investigate the effect of seasonality on population dynamics, we tested if (i) regulation is stronger in the dry season, (ii) surrogates of food availability positively affect wet season growth rates, and (iii) potential competition and intraguild predation have negative effects on dry season growth rates. To test these hypotheses, we studied five species of didelphidae marsupials in the Atlantic Forest, south-eastern Brazil, using a high-resolution 16-year dataset on 1276 individuals. Population growth rates in the dry and wet seasons were evaluated as functions of seasonal population sizes and exogenous variables. Seasonal feedback structure was analyzed with linear autoregressive models. 

Results/Conclusions

Four of five species were seasonally regulated. Strong first-order negative feedback in the dry season was found for two species, while for two others seasonal regulation was stronger in the wet season. Surrogates of food availability (rainfall and leaf litter) had little influence on population dynamics; warmer temperatures influenced growth rates in both seasons, with low growth of the smaller marsupial species occurring in colder conditions. The lack of evidence for delayed effects and no strong effect of interspecific competition and intraguild predator, or of the small mammal assemblage, suggest that populations were probably regulated by intraspecific competition for food resources and/or space. Although the study marsupials share common phylogeny and similar life histories, they varied in respect to the season when population regulation was strongest. Regulation occurred in both dry and wet seasons, with compensation processes in just one season being sufficient to allow population persistence. Our results demonstrate that seasonality plays a key role in driving marsupial population dynamics, and highlight the need to account for seasonality in demographic studies even in tropical environments.