Diet preference of roaming goats (Capra hircus) on columnar cacti in Bonairian scrublands
The introduction of goats to Bonaire in the 17th century, caused direct and indirect changes in both terrestrial and marine ecosystems and has led to serious alterations in the intricate interplay of island ecology. Direct effects such as the preference of goats for young and palatable plants has likely altered the plant communities from a dry forest towards the current dominance of thorny bushes and cacti, and overgrazing has led to cascading effects through soil erosion. The subsequent reduction of several crucial ecological and economical functions such as local climate control and recreation, are starting to deteriorate the ecosystem services that the inhabitants of Bonaire rely on, particularly with respect to tourism; a major source of income for the island. After centuries of goat herbivory, columnar cacti are some of the remaining species still present today on Bonaire. However, monitoring has provided evidence that cacti endure severe damage by herbivory. Studies conducted in different geographical areas have shown that goats commonly browse on cacti. Goat populations on Bonaire have remained high in recent decades, often causing irreversible damage to columnar cacti.
In this study, I investigated the diet preference of goats with respect to three columnar cacti; Subpilocereus repandus, Stenocereus griseus, and Pilosocereus lanuginosus, in relation to associational resistance, spine characteristics and foraging by the green iguana. I assessed the dietary preference through food choice experiments using choice options, such as with or without spines on cacti, with or without Opuntia to interfere with accessibility, and evaluation of additive damage effect from the native green iguana. The study consisted of a combination of field and enclosure experiments. Three different field experiments were simultaneously conducted within Washington Slagbaai national park during which diet preference, association resistance and additional browse damage were assessed. Goats were free to enter and leave the experimental locations during these field experiments. In enclosure experiments 18 goats were monitored in captivity during which the first 2 field experiments were replicated and herbivory reduction due to cacti spines was added as third enclosure experiment.
Beforehand, I assessed the spine characteristics of the three columnar cacti, and I observed that a higher density of spines was associated with a decline in spine thickness. This may indicate a trade-off between shade (density of spines) and defensive traits (thickness). After that, Opuntia and spines were tested as factors, and their presence was shown to have no reducing effect on goat herbivory. Field experiments with indigenous Iguana showed no observations of cactus biomass consumption. Therefore it is safe to assumed that the Iguana rarely if at all, browse on cacti, with the exception of their fruit and flowers.
During all field and enclosure experiments goats showed a diet preference for S. repandus over other cacti species, and this preference was not influenced by the removal of spines or the presence of Opuntia. It is not yet clear why goats prefer this cacti over other species, but we speculate that taste and its related nutritional value of this cacti species is higher. This study has shown that herbivory by Bonairian goats on cacti is worrisome. A 60% rate of browsing damage was observed within 48 hours of cacti placed in the field, and when restricted to compulsory enclosed conditions, 100% of available cacti were damaged within 24 hours. The findings in this report provide quantitative evidence of cacti consumption by Bonairian goats, which show severe goat herbivory on S. repandus and a cascading effect when this cactus is gone. When the S. repandus has been eaten goats will move to the S. griseus and P. lanuginosus. The large population of goats inhabiting Bonaire will lead to irreversible damage if not reduced. Similar reports have shown the detrimental effects goat populations can have on similar semi-arid ecosystems. I recommend a policy that combines the reduction or eradication of feral goats within the park, together with intensive education among human communities to raise social awareness and change goat husbandry practises on the island at large.