Small populations on small islands: what chance does an orchid have?

Premise of research. Small populations on small islands are intrinsically more vulnerable to population decline and extinction. Nevertheless, small native populations that occur on multiple islands may have life history characteristics that buffer impacts from novel disturbance regimes, and rather than populations contracting, they may be expanding. We monitored three populations of the orchid Brassavola cucullata from two Caribbean islands and asked what is the likelihood of population persistence.

Methodology. Over 3-4 years, we recorded growth, fruit production, herbivory, recruitment and mortality for all plants in each of our populations. We assessed persistence and predicted possible population changes using both population projection models (PPM) and integral projection models (IPM). Our results include a mixture of traditional (lambda, elasticities) and more recent indices (transient dynamics and non-linear sensitivities).

Pivotal results. Growth, reproduction and predicted population persistence varied among years and islands. IPM and PPM gave similar results. The overall trend is for a reduction in population growth rates, although population reactivity may buffer this pattern in the short-term (λ > 1). Populations would be extremely vulnerable to reduction if small plants dominated, yet even with an abundance of large plants, substantial reductions in population density are possible. Medium and larger plants contribute more to the persistence of the population, yet survival and growth of small individuals might have greater effect on lambda if retrogression is observed. To attain population stability, effective recruitment rates must increase dramatically.

Conclusions. Populations of perennial plants on small islands can fluctuate substantially suggesting a degree of vulnerability. While B. cucullata shows a general trajectory of decline, there are some signs of stability despite deforestation and herbivore activity. The outlook is precarious for the Saba population given the predominance of younger plants, and all three populations could decline if spasmodic recruitment fails to occur, which may happen if disturbance regimes change and the ongoing warming and drying trends persist.

Keywords: Brassavola cucullata - population dynamics - Lesser Antilles - transient dynamics - island conservation - Orchidaceae