Some preliminary results

Here are some preliminary results that I wrote up for a grant proposal that James and I submitted to the Galapagos Conservancy. We hope to go back to Pinta next summer to check on the tortoises and the changes in the plant community. With Matt Buff, web guru, we made a cool animation of the tortoises moving over the landscape - look for that soon on the Galapagos Conservancy website.

Tortoises dispersed quickly over the island, eventually covering over 815 hectares (minimum convex polygon area), or about 23% of the vegetated area of Pinta, within the first two months after introduction. Males covered more ground than females with the average maximum displacement from the introduction point over a two month period being 1173 110 meters for males versus 517 114 meters for females. Most of the tortoises do not seem to have settled into a “home range;” they are still making extended movements over short periods of time. In short, they are still exploring the island. This is consistent with other tortoise introduction efforts during which the tortoises did not settle into consistent home ranges until several months had passed after the introduction (Tuberville et al. 2005, Field et al. 2007).

Despite their exploratory behavior, the tortoises do appear to be making habitat choices, especially on an island-wide scale. The most striking example of this is the difference in habitat preference between tortoises with domed carapaces and tortoises with saddle-backed carapaces. Several of the tortoises have made more-or-less linear forays up the elevation gradient towards the peak of the island. The only tortoises that have exhibited this behavior and surpassed 450 meters of elevation have a domed carapace. This result is intuitive as the domed phenotype is more typically associated with higher elevations and habitats dominated by ferns, Zanthoxylem, and Chiococca. The saddle-backed tortoises typically did not exceed an elevation of 450 meters, which elevation corresponds to the limit of the cactus elevation gradient.

The release process appeared to involve no trauma to the tortoises; they appeared to display normal behavior almost immediately after the introduction. We spent 76 hours observing tortoise behavior. Tortoises spent, on average, 25% of the day feeding, which is comparable to other tortoise time activity budgets (Nagy and Medica 1986, Lagarde et al. 2008). We observed tortoises feeding on 28 plant species, but the most preferred species were Opuntia cactus, grasses, and forbs like Lantana and Justicia. Tortoises associated strongly with the cactus in their habitat within the first two weeks after introduction, being more likely to be within 2 meters of a cactus than a random point in the vicinity (x2 = 9.64, df = 1, p = 0.0019). Tortoises fed on fallen adult cactus pads, and also pushed down juvenile cactus to eat the trunks. A few individuals who found recently dead, large, adult cacti did not move from the cactus for weeks, feeding on all of the fallen pads.

While most of the cactus in the introduction area were not fruiting, a few tortoises that moved to the western “saddle” area of the island found fruiting cactus in abundance. We found cactus seeds in these tortoises’ feces, and found seeds of other species in 35% of the fecal samples that we dissected. This suggests the restoration of tortoise-associated seed dispersal phenomenon on Pinta. The tortoises created disturbance in the habitat by making trails as they travelled across the landscape and large swaths of matted down vegetation when they settled into a feeding area. We also observed other normal behaviors like bed-making and mating attempts, suggesting that the tortoises are adapting to their new environment.