Potential for Competition Between Endemic Giant Tortoises and Introduced Cattle on Santa Cruz Island, Galapagos

Abstract

The Galápagos, an isolated cluster of small equatorial volcanic islands, have long been famous for their many unique endemic species, such as Galápagos giant tortoises. Unfortunately, these islands are now also increasingly notable for facing immense conservation challenges from introduced species, including domestic livestock. In this project, I investigate the differences in grazing preferences between introduced cattle (Bos taurus) and endangered Galápagos giant tortoises (Chelonoidis porteri) on Santa Cruz Island to assess the degree of competition between these two large herbivores.

Although studies have been performed on the diets of several major grazers and browsers on the Galápagos, none have investigated interspecific resource interactions between agricultural cattle and Galápagos tortoises. I use 158 twenty-minute grazing observations of Galápagos giant tortoises and 152 five-minute grazing observations of cattle that I performed in summer 2017 on a small private ranch in the agricultural highland district of Santa Cruz, in conjunction with ten tortoise fecal DNA samples, to determine what types of plants tortoises and cattle consume. I use this data to compare the dietary overlap between cattle and Galápagos tortoises and therefore to investigate their potential for competition. My results show that the dietary overlap between cattle and tortoises is significantly different, mostly due to the fact that cattle graze primarily on elephant grass (Pennisetum purpureum) while tortoises prefer to consume Sida, Axonopus, and other Poaceae species. This finding suggests that the potential for food resource competition between cattle and tortoises is lower than commonly suggested in the popular media.