Evaluating the Impact of Introduced Spotted Deer (Axis axis) on Forest Floor Herpetofauna of Andaman Islands

Abstract

It is common knowledge that organism in an ecosystem, are connected to each other through trophic levels. Even though the importance of interactions among trophic levels is well established in theory, demonstration of such interactions is not always easy. Over the years, studies that focus on the trophic interactions among starkly different taxonomic groups have come to the fore. These studies have furthered our understanding of ecosystems by demonstrating relationships between trophic levels so apart, the connection among which may not be apparent at first. Along these lines, the situation of introduced spotted deer in the Andaman Islands presented an ideal opportunity to understand the potential effect of a mammalian invasive herbivore on native, insectivorous forest floor herpetofauna. I hypothesised that herbivory is likely to depress folivorous arthropod abundance, which in turn may lead to a decline of insectivorous forest floor herpetofauna. Additionally, reduction in vegetation cover may render the habitat unsuitable for herpetofauna and make them vulnerable to predation. The objectives of this study were to evaluate the effect of chital on the forest floor herpetofaunal abundance and to determine the pathway of interaction between them. A contrasting effect of herbivory by chital on reptiles and on amphibians in the Andaman Islands was observed during the dry season. Forest floor reptiles, which included agamids, geckos and skinks showed reduced abundance in the presence of chital in comparison to an island where chital was absent. This effect of chital on reptiles was found to be mediated by vegetation cover. Chital significantly reduced the vegetation cover below their maximum browse height (1.5 m) in the Islands and which in turn led to a reduction in reptile abundance. Although, it was not clear if any of the observed species was benefitted in the presence of chital, the semi-arboreal Coryphophylax subcristatus appeared to be affected. Amphibian abundance on the other hand seemed to be unaffected by the use of the habitat by chital. Litter arthropods influenced the densities of amphibians the most. This study brought to light a pathway of indirect interaction between a mammalian herbivore and insectivorous herpetofauna. In doing so it raised conservation concern about the capability of an introduced species to alter an island ecosystem drastically and acutely impact several endemic fauna.