How do the distribution and abundance of columnar cacti relate to microsite types and goat grazing pressure?Submitted by Peter Verweij on Wed, 04/20/2016 - 19:16
Exotic herbivores are able to disrupt entire ecosystems by competing with native species and feeding on native species. Especially, on islands without natural herbivores, the impact on native plant species can be strong. This has happened in Bonaire where goats have been brought by Spanish settlers centuries ago. Nowadays, the ecosystem of Bonaire is arid which is largely due to the timber industry and a history of keeping goats. Even though the timber industry has disappeared from
Bonaire, goats still roam around the entire island and are still part of the Bonairian way of life. In Washington Slagbaai National Park, feral goats are remnants of the previous use of the park. These goats have grown to large numbers and hence have a large impact on the vegetation of the park, which include the three columnar cacti species; Pilocereus repandus, Stenocereus griseus and Pilosocereus lanuginosus. These species define the landscape of the park. More importantly, these
columnar cacti are critical for various endemic frugivorous and nectarivorous birds and bats that depend on it for food and nesting opportunities. However, as the goats have continued to increase in numbers, they have started to forage on the bark of the columnar cacti. This foraging behavior is threatening the columnar cacti and indirectly the birds and bats that depend on it.
This study investigated the effect of goats on the columnar cacti population. This was done by collecting data on distribution, damage and abundance of columnar cacti in both Washington- Slagbaai National Park and Klein Bonaire. As I expected that facilitation by microsites could prove important in an arid ecosystem with grazers, microsites and abiotic amelioration by microsites was studied as well. As expected, goats impacted the columnar cacti populations in a negative way. Using Klein Bonaire as a control area where no goats have been roaming for almost 50 years, I found significant differences when comparing columnar cacti populations with Washington-Slagbaai National Park.
The population of P. repandus in WSNP is unhealthy with only 27.7% of the population being juvenile in WSNP-Limestone. In Klein Bonaire, the population is a lot healthier with 75.2% of the population being juvenile. The WSNP population of S. griseus is healthy and most common with nearly 9000individuals documented and 86% being juvenile and 95% being juvenile in Klein Bonaire. P. lanuginosus is the least abundant columnar cacti species in WSNP and is absent in Klein Bonaire. However, this is likely to be due to environmental stressors instead of herbivore pressure as P. lanuginosus is regenerating well with over 89% of the WSNP population being juvenile.
A significant difference on P. repandus also exists in the amount of damage between Klein Bonaire, where a mean damage of 4% was found, and Washington-Slagbaai National Park where the mean damage on P. repandus was 24%. For S. griseus, a mean damage of 19% was found in WSNP and a mean damage of 7% was documented in Klein Bonaire. However, this difference was not significant due to the small sample size. This was also the case with the proportion of dead adult S. griseus
which was 8.8% in WSNP and 0% in Klein Bonaire. As for P. repandus, the difference was significant with 10.2% of the adult P. repandus population being dead against only 1.9% in Klein Bonaire. These observations are likely to be attributed to foraging by goats as they seem to prefer P. repandus above the other two cactus species judging from the relatively high amount of trunk damage for P.
repandus (3.9% against 1% and 1.8% for S. griseus and P. lanuginosus respectively). As a result, P. repandus seems to be in direct danger, which is concerning as this columnar cactus is favored by birds, bats and people. Although the other cacti species are likely to be targeted by goats once P. repandus’ population has declined even more.
Furthermore, results show that increasing damage leads to a decrease fruit production, which links the goats indirectly with the native bats and birds which depend on the fruit in the dry season. The results on microsites of this study are in line with the theory on plant facilitation, as the more herbivory-vulnerable species P. repandus was facilitated ignificantly more often than S. griseus, which is a more herbivory-resistant species. All these results add up to conclude that the exotic goat has been affecting Bonaire’s columnar cacti in a way which has been detrimental to its native flora as well as its fauna. Therefore, stringent measures need to be taken to eradicate goats from Washington-Slagbaai National Park.