Impact of goats on the original vegetation on Bonaire
Impact of goats on the original vegetation on Bonaire
Rapport van Wageningen UR waarin een zo onderbouwd mogelijk beeld van de geitenhouderij op Bonaire wordt gegeven
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.
Presentation summarizing results of MSc theses of various students
Feral goats (Capra hircus) are introduced but very successful herbivores found in areas all over the Caribbean island Bonaire. Within the Washington-Slagbaai national park, STINAPA is currently taking measures in order to control the goat population. Research was requested to provide a scientific background on the impact of feral goats in the park. This field experiment included the analysis of 13 areas where goats had been excluded for a period of 8 years. This study revealed the negative impact of feral goats on the vegetation of the Washington-Slagbaai national park. Recovery of the vegetation in the exclosures was found to be significantly higher in comparison with areas accessible for goats. Vegetation rejuvenation within the exclosures increased dramatically for tree species such as Capparis odoratissima, Randia aculeata and Guaiacum officinale. Direct and indirect positive relations with goat presence were observed for Opuntia wentiana and Croton flavens.
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.
Feral livestock grazing has long been recognized to have a negative effect on the ecosystem of the Washington Slagbaai National Park, Bonaire. Because of this STINAPA, the management of the national park, started a goat-catching project. In this study, the population density of feral livestock was estimated for the park and the Labra-Brasil area using the Distance method. The results indicate a goat density of 2.7 goats per hectare in the national park, corresponding to an abundance of about 11000 goats. Looking at the population structure of goats, around 20% are young animals and there are twice as many females as there are males. The population density of the other animal species was much lower. The Opuntia distribution was also assessed in the study area, a density dependent relation between Opuntia density and goat density was found. Finally, seasonal differences in diet composition of goats were observed; goats seem to become less specific and eat only cacti in the dry period. The study concludes with several recommendations. On the short-term, the priority STINAPA should be to control the goat population, preventing further damage to ecosystem. For this a more effective way of catching the goats should be used. Next to bringing down goat population density, it is also important to monitor the development of the other animal populations so the goat-catching project can be adjusted in time. Eventually the goal is to restore the original ecosystem of Bonaire. This will require more research and monitoring but most importantly a lower feral livestock density.
The presence of feral goats (Capra hircus) can have detrimental effects to island ecosystems where native plants have evolved in the absence of these animals. Feral goats are implicated in habitat destruction and alteration of species composition on sensitive island ecosystems. In the absence of population control, goats have become the ecologically dominant species on many islands, with the results that numerous endemic or native plant species have been extirpated, or are threatened by excessive grazing. It is demonstrated that the removal of goats can lead to rapid recovery of suppressed vegetation.
Nine excluders and nine controls were established randomly in three areas of the Quill. Data is collected annually every April. The size of each excluder and control is approximately 3m2. Overall a slight difference is apparent between the Quill control and excluder sites in 2014, with more plants present in the excluder plots than in the control plots.
Retrieved from STENAPA