Feral goats shift keystone cacti communities in Caribbean Islands

Vegetation on islands is particularly vulnerable to overgrazing by introduced herbivores as plants have evolved under lower herbivore pressure than on the mainland. We conducted a combination of field experiments and surveys to study the foraging behavior of feral goats and their impact on the composition, abundance and condition of keystone columnar cactus species. On the goat-browsed Caribbean island of Bonaire, 76.6 % of the adult cacti showed bark damage. The degree of bark damage on individual cacti was negatively correlated with the probability of bearing fruits and the number of fruits carried. Foraging experiments, in both field and enclosure settings, showed that goats preferentially consume the Subpilocereus repandus cacti. Experimental spine removal did not change goat foraging preference for this species. Overgrazing by feral goats reduced recruitment and increased mortality, shifting species relative abundance and population structure of columnar cacti species. S. repandus had a population structure characterized by a large proportion of adults, and low proportion of juveniles and resprouting individuals. We subsequently show that cactus communities have largely recovered from such impacts on the nearby island of Klein Bonaire where goats have been controlled for about half a century. Our findings reveal how overgrazing by feral goats shapes the community structure of columnar cactus species and imply that goat eradication is an essential step for the recovery and conservation of the original columnar cacti communities of Caribbean dry seasonal forests. • Overgrazing by feral goats threatens columnar cacti conservation in Caribbean islands. • Feral goats increase mortality and reduce recruitment of columnar cacti populations. • Goat foraging preference does not depend on cactus spinescence. • Feral goats shift the composition and distribution of columnar cactus species in dry Caribbean islands. • Eradication of feral goats can lead to recovery of cactus populations.