Ninhos de Atta sexdens (Hymenoptera: Formicidae) podem afetar a estrutura da assembleia de artrópodes do solo na Mata Atlântica?

RESUMO: As formigas-cortadeiras são consideradas os herbívoros dominantes da Região Neotropical e, portanto, estudos a respeito dos seus múltiplos efeitos sobre outros organismos e processos ecossistêmicos são relevantes. O presente estudo tem como objetivo determinar se ninhos de Atta sexdens (Linnaeus, 1758) podem afetar a estrutura da assembleia de artrópodes do solo em um fragmento de Mata Atlântica. Além disso, a fim de determinar os prováveis mecanismos que explicam a possível modificação na estrutura da fauna de artrópodes, foram testadas duas hipóteses: I) Hipótese da Serrapilheira: A. sexdens reduz a disponibilidade de serrapilheira nas proximidades do ninho; II) Hipótese dos Fatores Abióticos: A. sexdens modifica as condições microclimáticas ao redor do seu sítio de nidificação causando aumento da temperatura e luminosidade e redução da umidade relativa do ar. O estudo foi conduzido na Reserva Ecológica de Guapiaçu, junto à cidade do Rio de Janeiro, Brasil. Coletamos amostras de serrapilheira a intervalos de oito metros da borda dos ninhos de A. sexdens, ao longo de um transecto linear de 32 metros, para extração da assembleia de artrópodes e para estudar a estrutura da serrapilheira. Também medimos as temperaturas do ar e do solo, a luminosidade e a umidade relativa do ar onde coletamos as amostras de serrapilheira. Não detectamos qualquer efeito da presença do ninho sobre a riqueza, abundância e composição de artrópodes do solo no fragmento florestal estudado. Esse resultado foi provavelmente influenciado pelo fato de que os ninhos de A. sexdens estudados não afetam a disponibilidade de serrapilheira e as condições microclimáticas ao seu redor. Assim, concluímos que os múltiplos efeitos que as formigas-cortadeiras podem ter em uma floresta devem ser dependentes da espécie.

Habitat Modification by the Leaf-Cutter Ant, Atta cephalotes, and Patterns of Leaf-Litter Arthropod Communities

Ecosystem engineers are profoundly important in many biological communities. A Neotropical taxonomic group considered to have engineering effects is the Formicidae (ants). Leaf-cutter ants (LCAs), in particular, which form extensive colonies of millions of individuals, can be important ecosystem engineers in these environments. While the effects of LCAs on plant community structure and soil chemistry are well-studied, their effects on consumers are poorly understood. Therefore, we examined the indirect effects of the LCA Atta cephalotes L. on the leaf-litter arthropod community. We compared abundance and diversity patterns at ant nests to areas distant from nests, utilizing both a factorial design and gradient analysis for both nocturnal and diurnal arthropods. We found that arthropod abundance and diversity was significantly lower for multiple taxonomic groups and trophic levels near leaf-cutter nests, and this pattern was strongest at night. Exceptions to this pattern included two morphospecies of Collembola that were more abundant on nests, suggesting some specialization for these species. For the gradient analysis, abundance increased exponentially for most groups of arthropods. However, for the dominant arthropod species, the amphipod Cerrorchestia hyloraina Lindeman, a quadratic function was the best fit curvilinear model for abundance. It appeared that C. hyloraina had maximal abundance at the transition between nest site and less disturbed forest. These results indicate that LCA activity has a strong effect on the leaf-litter arthropod community, adding to spatial heterogeneity within neotropical forests. These effects may translate into changes in important ecological processes such as nutrient cycling and food web function.

Diet and reproductive function in wild female chimpanzees (Pan troglodytes schweinfurthii) at Kibale National Park, Uganda

Human female reproductive function is highly sensitive to current energetic condition, indicating adaptation to modulate reproductive effort in accordance with changing ecological conditions that might favor or disfavor the production of offspring. Here, we test the hypothesis that reproductive capacity in female chimpanzees is likewise limited by current energetic condition. We used 12 years of data on wild chimpanzees (Pan troglodytes schweinfurthii) in the Kanyawara community of Kibale National Park, Uganda, to examine the relationship of dietary quality, as assessed by fruit components of the diet, to the occurrence of sexually receptive females, concentrations of ovarian steroid hormones, and timing of conception. We found that the frequency of females having sexual swellings was positively related to the consumption of drupe fruits. Estrogen levels of both cycling and noncycling females increased during seasonal peaks in the consumption of drupe fruits. When average fruit consumption remained high across months, females conceived more quickly. These results support the hypothesis that cycling and conception in chimpanzees are contingent upon high energy balance, and they indicate that the availability of fruit is a key variable limiting reproductive performance in chimpanzees. Chimpanzees appear to share with humans a reproductive system that is primed to respond to proximate levels of energy acquisition.