Microhabitat distribution and demography of two Florida scrub endemic plants with comparisons to their habitat-generalist congeners

Abstract

I evaluated hypotheses regarding the nature of habitat specialization by comparing the microhabitat distribution and demography of L. cernua and P. basiramia, two Florida rosemary scrub habitat specialist species, with their habitat generalist congeners, L. deckertii and P. robusta. Specifically, I addressed the following two hypotheses: (1) that habitat specialist species may occur in a narrower range of microhabitat conditions than habitat generalist species, and (2) that demographic parameters of habitat specialist species may be more variable than those of their habitat generalist congeners. For each pair of congeners, I compared the microhabitat distributions, variation in vital rates and population growth rates, and extinction probabilities under different climate regimes to evaluate these hypotheses. Both rosemary scrub specialist species occurred in a narrower range of bare sand microhabitat conditions than their habitat generalist congeners. Rosemary scrub specialists were significantly more likely to occur in sites with high percentage bare sand, whereas microhabitats of generalists were more variable with respect to percentage bare sand. Recruitment and survival rates of both rosemary scrub specialist species were more temporally variable than those of their habitat generalist congeners; however, plant growth rates of rosemary scrub specialist species were less variable than those of their generalist congeners. Rosemary scrub specialist species also exhibited greater temporal variation in population growth rates than their habitat generalist congeners. Both rosemary scrub specialist species had higher probabilities of quasi-extinction than their generalist congeners under every climate modeling scenario. The narrower microhabitat requirements and greater temporal variability of demographic parameters of L. cernua and P. basiramia distinguish them from their habitat generalist congeners. The restriction of P. basiramia and L. cernua to microhabitats with high percentage bare sand may limit their distribution to rosemary scrub habitat. Greater temporal variability in recruitment, survival, and population growth rates in L. cernua and P. basiramia may be associated with specialization on a narrower range of environmental conditions in these rosemary scrub specialist species. Greater temporal variability of demographic parameters in these rosemary scrub specialist species may make them more vulnerable to extinction than could be predicted solely from availability of suitable rosemary scrub habitat.