The Dry Season Shuffle: Gorges Provide Refugia for Animal Communities in Tropical Savannah Ecosystems.

J Sean Doody,Graeme Sawyer,Domenic D’Amore,Geoff Kay,Kim Hands,Steve Wilson,Colin Mchenry,Michelle Quayle,Christina Castellano,Ryan Ellis,Michael Bass,Simon Clulow,David Rhind,Jordi Moya-Larano

doi:10.1371/journal.pone.0131186

Abstract

In the wet-dry tropics, animal species face the major challenges of acquiring food, water or shelter during an extended dry season. Although large and conspicuous animals such as ungulates and waterfowl migrate to wetter areas during this time, little is known of how smaller and more cryptic animal species with less mobility meet these challenges. We fenced off the entire entrance of a gorge in the Australian tropical savanna, offering the unique opportunity to determine the composition and seasonal movement patterns of the small vertebrate community. The 1.7 km-long fence was converted to a trapline that was deployed for 18-21 days during the early dry season in each of two years, and paired traps on both sides of the fence allowed us to detect the direction of animal movements. We predicted that semi-aquatic species (e.g., frogs and turtles) would move upstream into the wetter gorge during the dry season, while more terrestrial species (e.g., lizards, snakes, mammals) would not. The trapline captured 1590 individual vertebrates comprising 60 species. There was a significant bias for captures on the outside of the fence compared to the inside for all species combined (outside/inside = 5.2, CI = 3.7-7.2), for all vertebrate classes, and for specific taxonomic groups. The opposite bias (inside/outside = 7.3, N= 25) for turtles during the early wet season suggested return migration heading into the wet season. Our study revealed that the small vertebrate community uses the gorge as a dry season refuge. The generality of this unreplicated finding could be tested by extending this type of survey to tropical savannahs worldwide. A better understanding of how small animals use the landscape is needed to reveal the size of buffer zones around wetlands required to protect both semi-aquatic and terrestrial fauna in gorges in tropical savannah woodland, and thus in ecosystems in general.

Highlights

The wet-dry tropics are characterized by distinct wet and dry seasons and are found on every major continent
Understanding how animals respond to dry season conditions is important—we would expect prolonged dry conditions to impact population dynamics and shape an organism’s physiology, ecology and behavior [3,4,5]
There was a significant bias for captures on the outside of the fence compared to captures on the inside of the fence (p < 0.001), with an overall ratio of outside/ inside (O/I) of 5.2 (Table 2)

Summary

Introduction

The wet-dry tropics are characterized by distinct wet and dry seasons and are found on every major continent. The wet-dry seasons drive an intra-year ‘boom to bust’ cycle which requires highly specialized ecological traits to evolve [1, 2]. Understanding how animals respond to dry season conditions is important—we would expect prolonged dry conditions to impact population dynamics and shape an organism’s physiology, ecology and behavior [3,4,5]. Revealing these responses is critical for predicting the impacts of anthropogenic influences on species and population persistence. Climate change models predict increases in the length and intensity of dry seasons over large areas such as the Amazon Basin [6]

Methods

Results

Conclusion