Are plant populations in fragmented habitats recruitment limited? Tests with an Amazonian herb


Decreased recruitment is hypothesized to be a primary mechanism driving the local extinctions of plant species from fragmented landscapes. A critical but untested assumption of this claim is that reductions in fecundity by individual plants actually result in demographic vulnerability at the population level. I tested this ‘‘recruitment limitation hypothesis’’ using three years of census data from 13 populations of the Amazonian understory herb Heliconia acuminata, which I integrated with empirical estimates of seedling establishment into matrix demographic models. I asked: (1) How much recruitment is necessary for ␭ Ͼ 1? (2) What are the projected values of ␭ for H. acuminata populations in fragments and continuous forest, given empirical estimates of seedling emergence? (3) What are the actual values of ␭ for populations in fragments and continuous forest, and why might projected and estimated values of ␭ differ? (4) What other demographic stages contribute to ␭, and how are these affected by fragmentation? Simulation models suggest that extreme reductions in recruitment are necessary for population declines, and empirical estimates of seedling establishment were frequently below these thresholds. As a result, Heliconia acuminata populations in fragments are projected to shrink at a rate of 1–1.5% per year, while those in continuous forest are projected to grow 2.3–4% per year. Annual censuses, however, indicated populations in both continuous forest and fragments grew at rates well in excess of those projected by matrix models. This discrepancy is due to higherthan-predicted seedling numbers. While elasticity analyses indicated transitions related to growth and survivorship actually made the greatest proportional contribution to ␭, these results suggest that dispersal into fragments is common and helping populations overcome the negative demographic consequences of reduced seedling establishment. Additional demographic studies of plant populations in fragmented areas are urgently needed to identify other potential mechanisms responsible for population declines. Particular attention should be paid to appraising the effect of fragmentation on plant growth and survivorship, as altering these life-history stages may have the most serious consequences for population growth rates.