Predictive modelling

Previous attempts at mapping the vegetation of the Christoffel national park on the island of Curaçao were done in times of intense grazing pressure and are likely not valid anymore after the removal of goats from the park because grazers have a significant effect on the native vegetation of the island ecosystems. In 2018, a 2-year fieldwork campaign was started to revisit the sampling points of Bokkestijn & Slijkhuis (1987) with the aim of remapping the vegetation communities and studying the change that occurred in the last decades. This thesis aims to assess the changes in vegetation distribution and use the newly acquired data to predict plant species richness across the entire national park at a high resolution using a macroecological modeling strategy. A trend of secondary vegetation succession has been found since 1985, with an increase in the coverage of trees, orchids, and bromeliads and a decrease in grasses and herbs. The large-scale recovery of the native vegetation is found especially on the coast and midland of the park, while the Christoffel mountain and its surroundings have remained relatively stable. An aerial photograph interpretation of the vegetation communities found significant dependence of vegetation communities on elevation and slope aspects. High-resolution plant species richness prediction models were built and it was found that elevation and slope aspects have the most predictive weight. Little research has been done on high-resolution species richness prediction models; however, it is shown that these models can be utilized to characterize the variables influencing species distribution at high resolution and local scale, with comparable accuracy to coarser prediction models.

Abstract:

Species invasions have a range of negative effects on recipient ecosystems, and many occur at a scale and magnitude that preclude complete eradication. When complete extirpation is unlikely with available management resources, an effective strategy may be to suppress invasive populations below levels predicted to cause undesirable ecological change. We illustrate this approach by developing and testing targets for the control of invasive Indo-Pacific lionfish (Pterois volitans and P. miles) on Western Atlantic coral reefs. We first developed a size-structured simulation model of predation by lionfish on native fish communities, which we used to predict threshold densities of lionfish beyond which native fish biomass should decline. We then tested our predictions by experimentally manipulating lionfish densities above or below reef-specific thresholds, and monitoring the consequences for native fish populations on 24 Bahamian patch reefs over 18 months. We found that reducing lionfish below predicted threshold densities effectively protected native fish community biomass from predation-induced declines. Reductions in density of 75- 95%, depending on the reef, were required to suppress lionfish below levels predicted to over-consume prey. On reefs where lionfish were kept below threshold densities, native prey fish biomass increased by 50-70%. Gains in small (<6cm) size classes of native fishes translated into lagged increases in larger size classes over time. The biomass of larger individuals (>15cm total length), including ecologically important grazers and economically important fisheries species, had increased by 10-65% by the end of the experiment. 

Crucially, similar gains in prey fish biomass were realized on reefs subjected to partial and full removal of lionfish, but partial removals took 30% less time to implement. By contrast, the biomass of small native fishes declined by more than 50% on all reefs with lionfish densities exceeding reef-specific thresholds. Large inter-reef variation in the biomass of prey fishes at the outset of the study, which influences the threshold density of lionfish, means that we could not identify a single rule-of-thumb for guiding control efforts. However, our model provides a method for setting reef-specific targets for population control using local monitoring data. Our work is the first to demonstrate that for ongoing invasions, suppressing invaders below densities that cause environmental harm can have a similar effect, in terms of protecting the native ecosystem on a local scale, to achieving complete eradication.