Orangutan seed dispersal effectiveness and spatial distribution patterns
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Primates have important ecological roles as seed dispersers and seed predators in tropical forests. Orangutans are large-bodied frugivores that consume a high diversity of plant species, however, relatively little is known about their ecological roles. Ecological interactions are critical processes for ecosystem dynamics, structures, and functions. This dissertation investigated Bornean orangutans’ (Pongo pygmaeus wurmbii) ecological roles by studying orangutan patterns of frugivory, seed dispersal, and seed predation, dispersed seed spatial patterns, and seed fate outcomes. This research was conducted at the Cabang Panti Research Station in Gunung Palung National Park, Borneo, Indonesia. Chapter 2 analyzes orangutan seed dispersal behavior using the seed dispersal effectiveness framework to identify which fruits orangutans are most effectively dispersing. Orangutans predated seeds more frequently than they spat or swallowed seeds. Additionally, the fruits preferred most by orangutans were highly predated. Despite the prevalence of seed predation, orangutans also frequently dispersed seeds, 71.8% of fecal samples contained seeds with a mean of 28 seeds (>2mm) per fecal sample. Chapter 3 models orangutan seed dispersal distances to understand how far orangutans are dispersing seeds across the landscape and if seeds are dispersed across habitat types. This study found orangutans dispersed seeds, on average, 400-650m. There were occasional long distance seed dispersal events, and the maximum dispersal distance was 2.2km. Both male and female orangutans dispersed seeds long distances (>1km). Unflanged male orangutans dispersed seeds the longest mean distances of the age-sex classes with the farthest mean maximum distances at the 80-hour gut retention time. In Chapter 4, the fate of seeds dispersed by orangutans is investigated along with the variation underlying the fate of dispersed seeds. Camera traps and seed tracking studies revealed the orangutan primary seed shadow was heavily reshaped post-dispersal. By 6-months post dispersal, most orangutan dispersed seeds (86-87%) had been removed, mostly by seed predators, and almost all of the remaining seeds (11-14%) had died. This dissertation reveals orangutans are involved in important ecological interactions. Orangutans disperse and predate high quantities of seeds from many plant genera, and the loss of the orangutan would likely negatively affect their natural ecosystems.