Incêndios de alta intensidade e seus efeitos sobre a comunidade de formigas arborícolas do Cerrado

Abstract

Fire is a historically important agent of disturbance in tropical savannas, maintaining habitat structure and community dynamics in these ecosystems. However, in many places, climate change and changes in land use and land cover have increased the frequency and intensity of savanna fires well above historical levels. Ants have been frequently used as model organisms to assess the biodiversity consequences of fire disturbance. Although most studies indicate that the ground-dwelling, savanna ant fauna is resilient to the effects of fire, relatively little is known about how arboreal ants (a dominant and highly diverse group in the tropics) respond to fire, especially those of high-intensity. In this context, we assessed the extent to which high-intensity fires affect the structure of arboreal ant communities in a Neotropical savanna (Cerrado). We asked the following questions: 1) Does fire affect the structure of the ant community foraging and/or nesting on individual trees? 2) Does the effect of fire vary between different species of host-trees, or in relation to the height of the flames that hit the host-tree? 3) Which are the ant species most affected by fire? 4) Is there evidence that the effects of fire are more pronounced after two consecutive rather after a single fire? 5) Are the effects of severe fires persistent over a period of two years? To answer these questions, we sampled the same focal trees (n = 169), once in 2010 and once in 2018 in a savanna reserve that suffered two severe burns, one in 2014 and the other in 2017. A subset of these trees (49 trees) were also sampled in 2013, 2015 and 2019 and we used this subset of trees to evaluate the changes in the structure of the ant community between the pre-fire (2010 and 2013) and the post-fire (2015, 2018 and 2019) surveys. As a temporal control, we sampled 55 trees in another reserve, once in 2011 and again in 2018, where the last fire took place in 2006. In total, I recorded 80 species of ants in the burned area and 57 in the unburned one. In the burned area, there was a significant temporal variation in species richness per tree, whereas in the control area there was not. Fire caused a decline in species richness of about 30%. Responses to fire varied among the most diverse ant genera. While the species richness of Camponotus varied little among survey years, that of Pseudomyrmex, Cephalotes declined sharply following the two fire events. Changes in species composition over a period of 7-8 years were much more marked in the burned than in the unburned area. In addition, in the burned area, the dissimilarity in species composition was much smaller between the two pre-fire surveys than between the pre and post-fire surveys, most notably the 2018 survey. The magnitude of the changes in species richness and composition per tree at the burned area did not differ between host-tree species or between trees of different sizes. However, I found a positive relationship between the height of the flames that hit the host-tree and the magnitude of changes in species richness and composition. Among the most frequent species in our sampling, 48.6% showed significant changes in abundance over time in the burned area, as compared to only 25% in the control area. The species most negatively affected by fire were Cephalotes pusillus, Pseudomyrmex gracilis and Tapinoma sp.04, whereas Camponotus melanoticus, Brachymyrmex sp.12 and Solenopsis sp.01 seem to be favored by fire. Overall, our results show that high-intensity fires cause significant changes in the structure of the arboreal ant community and that these effects are still evident two years after the last fire. Ant species richness declined significantly, with species with strictly arboreal habits being the most affected. Measures to mitigate the occurrence of these fires are necessary since, due to climate change, high-intensity fires are becoming increasingly more common in various regions of the world, including those naturally prone and adapted to fire, such as the Cerrado.