Roaming livestock distribution, densities and population estimates for St. Eustatius, 2013.
The problem of roaming livestock is a major impediment to agricultural development and nature conservation on St. Eustatius, as it also typically is on other islands in the region. In support of a government-led culling program, we here conducted a baseline study of livestock abundance and distribution on the island in the final quarter of 2013. Population density of cattle, goats, sheep and chickens were estimated along 33.5 km of permanent trails, representing six different habitat zones. Each of the 13 trails was assessed five times. The results show overall high densities of chickens, goats and cattle. Clear and statistically significant livestock density differences were found in different zones of the island. The two most ubiquitous species of feral farm animals were goats and chickens which were found in all habitat zones. Island population estimates (± 1 SE) based on habitat-specific detection curves for goats is: N = 2470 ± 807. For chickens, habitat-specific detection-curves were insufficiently distinct to affect population estimates and the island population size estimate for chickens is: N = 2248 ± 668.
Cattle and sheep were more restricted. Our estimate for sheep numbers is only crude 1300 ± 992 and only indicates a minimum count for the island of about 300 sheep. As cattle are large animals and dependent on man-made trails for their movement through the terrain, population size estimates for cattle extrapolated using the Distance approach were found to lead to an excessively high mean population estimate (N = 1012 ± 458). Our best estimate, based on tag-resighting rates for cattle is: N = 600 animals, which does fall within 1 SE for density estimation. So, while the established transects are a useful tool for monitoring livestock density, the counts for cattle should not be used to extrapolate population size.
The density of roaming small ruminants (ie, goats and sheep) are currently at levels considered excessive for sustainable range management in other semi-arid landscapes. Our estimates for goat density per km2 and combined population size for the wooded habitats of the Northern Hills and the Quill where the terrestrial national parks are established are as follows: d = 109 ± 27 and n = (1323 ± 329). Such livestock densities cause soil degradation, loss of organic matter, reduced water retention and erosion in semi-arid rangelands. Therefore the results stress the need to cull, restrict and better manage the roaming livestock herds of the island. Of these, goats are the most problematic due to their habit of preferring steep terrain and cliffs. These are more vulnerable to erosion and harbour higher densities of rare species due to micro-habitat availability.
Complementary counts of cattle by LVV along the same network of trails show that over the last year cattle abundance has not appreciably declined, notwithstanding the ongoing removal efforts. Therefore cattle needs to be removed at a higher rate and/or longer period than achieved to date, to be able to effect a measurable population decline. As a final note we point to the high density of feral chickens on Statia. Chickens are aggressive omnivores capable of impacting small terrestrial animals and seedling regeneration. Their effect, particularly on the rainforest plants and animals of the upper Quill slopes and Quill crater deserves further assessment.