DEMOGRAPHIC EFFECTS OF HABITAT FRAGMENTATION ON A TROPICAL HERB: LIFE-TABLE RESPONSE EXPERIMENTS

Abstract

Habitat fragmentation is a leading cause of extinction, with effects that may be particularly pronounced in tropical ecosystems. However, little is known regarding the demographic mechanisms underlying changes in abundance in fragmented landscapes. Us- ing six years of demographic data collected from .6600 individuals of the Amazonian understory herb Heliconia acuminata, we calculated population growth rate (l) in exper- imentally isolated 10-ha forest fragments, 1-ha forest fragments, and continuous forest. We then used life-table response experiment analyses to elucidate the mechanisms responsible for observed differences in l. On average, l 1.05 in continuous forest, while l 1i n both 1-ha and 10-ha fragments. However, while the differences in l between 10-ha fragments and continuous forest were largely attributable to the negative contribution of stage-specific fertility rates, reduced l in 1-ha fragments was due to both reductions in reproductive rates and changes in the rate of plant growth. Our results show that similar reductions in l in fragments of different sizes can be driven by distinct demographic mechanisms. Without comprehensive demographic data, attempts to mitigate the decline of populations in frag- mented landscapes could be unsuccessful because they might be focusing on inappropriate demographic targets.