Loss of animal seed dispersal increases extinction risk in a tropical tree species due to pervasive negative density dependence across life stages

Abstract

Overhunting in tropical forests reduces populations of vertebrate seed dispersers. If reduced seed dispersal has a negative impact on tree population viability, overhunting could lead to altered forest structure and dynamics, including decreased biodiversity. However, empirical data showing decreased animal-dispersed tree abundance in overhunted forests contradict demographic models which predict minimal sensitivity of tree population growth rate to early life stages. One resolution to this discrepancy is that seed dispersal determines spatial aggregation, which could have demographic consequences for all life stages. We tested the impact of dispersal loss on population viability of a tropical tree species, Miliusa horsfieldii, currently dispersed by an intact community of large mammals in a Thai forest. We evaluated the effect of spatial aggregation for all tree life stages, from seeds to adult trees, and constructed simulation models to compare population viability with and without animal-mediated seed dispersal. In simulated populations, disperser loss increased spatial aggregation by fourfold, leading to increased negative density dependence across the life cycle and a 10-fold increase in the probability of extinction. Given that the majority of tree species in tropical forests are animal-dispersed, overhunting will potentially result in forests that are fundamentally different from those existing now.

Monodominance in tropical forests: modelling reveals emerging clusters and phase transitions

“…A loss of dispersal vectors for large seeds due to human impact could be expected to reduce large-seeded species owing to dispersal limitation, and to increase spatial aggregation and, thus, negative density-dependence [51]. However, our results suggest that, at least at restricted spatial scales, the opposite might occur for some tree species which are able to use local feedbacks and therefore take advantage of their strongly clumped distribution.…”

“…Many tree species circumvent the trade-off between seed size and dispersal by producing highly rewarding seeds or fruits and relying on large-bodied seed dispersers able to ingest and disperse large seed loads over long distances [49,50]. Indeed, most tropical tree species (70-90%) are animal-dispersed [51,52], including many monodominant species (e.g. Dacryodes excelsa rsif.royalsocietypublishing.org J. R. Soc.…”

Overfishing disrupts an ancient mutualism between frugivorous fishes and plants in Neotropical wetlands

“…Frugivorous vertebrates are overhunted in tropical forests across the globe, which reduces plant recruitment, alters plant species composition, diminishes biodiversity, and causes numerous indirect changes to communities (Caughlin et al, 2015;Markl et al, 2012;Poulsen et al, 2013). As hunters prefer big prey, overharvesting is particularly problematic for large-seeded plant species, which require seed dispersal by sizable frugivores (Caughlin et al, 2015;Effiom et al, 2014).…”

Geographic consistency in the seed dispersal patterns of Taxus baccata L. in the Iberian Peninsula

“…Turdus merula and T. ilifrom collapsing (Farwig & Berens, 2012;Caughlin et al, 2015). Due to a low population growth rate associated with reduced recruitment, Taxus baccata is highly vulnerable to landscape degradation and climate change (Dovčiak, 2002;Thomas & Polwart, 2003;Abel et al, 2007).…”