Spatial and dietary overlap between blackbuck (Antilope cervicapra) and feral horse (Equus caballus) at Point Calimere Wildlife Sanctuary, Southern India: Competition between native versus introduced species
摘要
Overgrazing by livestock is a serious threat to the biodiversity in the tropical forests of Asia. Feral horse, an introduced species at Point Calimere Wildlife Sanctuary, is a grazer and overlaps extensively in space and food resources with blackbuck, a native species, and thus could be a potential competitor. We have assessed, between December 2012 and June 2013, if the ecological conditions in the study site favour competition between these two species by assessing their spatial distribution pattern, population density, and dietary overlap. Spatial distribution pattern on direct sighting of both the species across habitats revealed that feral horses ranged mostly in the southern and southeastern parts of the sanctuary, in areas mostly with open grasslands, overlapping extensively in space with blackbuck, a grassland habitat specialist of the sanctuary. Population density estimated using line-transect distance sampling method showed 50 (95% CI 41.6–60.6) blackbucks and 21 (15.2–30.0) feral horses per km2 of the sanctuary. Dietary overlap assessed through site examination of feeding trails revealed that out of 14 plant species identified from blackbuck diet during the study period, 12 (86%) were also consumed by feral horses. Similarly, food plants such as Cloris parpata, a grass species, and herb Desmodiam dryflorae, the principal food of blackbuck, were also the major food plants of feral horses. Further, estimation of food consumption rate showed that feral horse consumed significantly more food per unit time (2.8 ± 0.08 g/min) than blackbuck (1.7 ± 0.07 g/min). Dietary overlap estimated using Schoener’s Index showed a significant overlap between the two species (overall 89% and habitat-wise 85–90%). An analysis of food preference using Jacob’s Preference Index revealed that all the preferred food plants of the blackbuck were equally preferred by feral horses indicating significant overlap in the diet species composition. Although the density estimate revealed a lesser number of feral horses per capita than blackbuck, the higher food consumption rate of the former coupled with larger body mass, could outweigh the higher density of blackbuck and the same is discussed as an effective ecological competitor. Based on the findings appropriate recommendations are made for the management of blackbuck in the sanctuary.