PS 25-88
Forest fragmentation disrupts the population dynamics of a late-successional tropical tree
Tropical late-successional tree species are at high risk of local extinction due to habitat loss and fragmentation. Population growth rates in fragmented populations are predicted to decline as a result of reduced fecundity, survival and growth. We examined the demographic effects of habitat fragmentation by comparing the population dynamics of the late-successional tree Poulsenia armata (Moraceae) in southern Mexico between a continuous and fragmented forests using integral projection models (IPMs) with two life stages, juveniles and adults, during 2010-12. We hypothesized: (1) reduced P. armata densities in the fragmented forest compared to continuous forest, (2) lower population growth rates for fragmented populations of P. armata, and (3) high elasticity values of population growth rate to adult survival and growth. Alternatively, unfavorable changes to plant fecundity in fragments may severely limit plant recruitment leading to reduced population growth rate. In 2010, four permanent 1-ha permanent vegetation plots were established within a continuous forest (64-ha) and four more plots were set up at four forest fragments. All juveniles and adults individually tagged for posterior identification in 2011 and 2012. We quantified survival, growth and reproduction of the studied individuals. In established juveniles, we measured basal diameter, whereas in adults we measured dbh.
Results/Conclusions
Forest fragmentation did not lead to differences in population density and, even resulted in a higher population growth rate (λ) in fragments compared to continuous forests. We observed a shift in the population size structure towards smaller individuals in the fragmented forest. Fragmented populations experienced a significant decrease in juvenile survival and growth compared to unaltered populations. Adult survival and growth made the greatest relative contributions to λ in both habitat types during 2011-12. However, the relative importance of juvenile survival and growth to λ was highest in the fragmented forest in 2010-11. A Life Table Response Experiment analysis revealed that positive contributions of adult fecundity explained most of the variation of λ between both habitats and annual periods. Habitat fragmentation had drastic effects on the dynamics of P. armata, causing the population structure to shift towards smaller sizes and likely jeopardizing long-term persistence of fragmented populations. This is consistent with previous work that late-successional trees are still present in large remnant patches at Los Tuxtlas, suggesting that resilience of the fragmented forest may not have been compromised. Developing a mechanistic understanding of how forest fragmentation affects plant population dynamics, as done here, will prove essential for the preservation of natural areas.