Modeling the Effects of Animal Movements and Behavior on Spatial Patterns of Seed Dispersal in Fragmented Landscapes

Abstract

Habitat loss and the resulting fragmentation can alter animal movements and disrupt animal seed dispersal mutualisms; however, these effects on spatial patterns of seed dispersal are not well understood. To explore the effects of habitat loss and fragmentation on seed dispersal distances and seed dispersion (aggregation), I created a spatially-explicit, individual-based model of an animal dispersing seeds (SEADS – Spatially-Explicit Animal Dispersal of Seeds). For Chapter 1, I ran SEADS in a theoretical landscape of 0-90% habitat loss to determine the effects of habitat loss on three animal traits relevant to seed dispersal: movement distance, gut retention time, and time between movements. SEADS results revealed a complex interaction involving all animal traits and habitat loss on dispersal distances and dispersion, driven by a novel underlying mechanism of fragment entrapment. Unexpectedly, intermediate habitat loss could increase dispersal distances and dispersion relative to low and high habitat loss for some combinations of animal traits. In Chapter 2, I developed a new method (TriAD) to estimate animal locations from triangulation data to improve triangulations for toucans. Location estimates calculated with TriAD averaged 23-63 m closer to true locations for three of four distance classes compared to the traditional method (Lenth’s maximum likelihood estimator, LMLE). Additionally, over 99% of 570 triangulations converged for TriAD compared to 85.8% for the LMLE. For toucan data, 98.0% of 4445 triangulations converged in TriAD compared to 88.6% for the LMLE. In Chapter 3, I parameterized SEADS with data from toucan movements and gut retention trials collected in Costa Rica to evaluate the effects of landscape, body size, and social organization on seed dispersal. The larger, less social species dispersed seeds 17-28% farther and in less aggregated patterns than the smaller, social species. This pattern was driven by longer movements in the larger species, suggesting that body size is more important than social organization for seed dispersal in our system. Habitat use and home range sizes limited dispersal distance and dispersion for both species. Findings from our simulations illustrate the importance of separating and quantifying the effects of disperser behaviors and landscape effects that shape seed deposition in fragmented areas.