Seed predation and seedling herbivory as factors in tree recruitment failure on predator-free forested islands

Abstract

Sapling mortality exceeds recruitment for many species of trees in the dry forest of 16-y-old islets in Lago Guri, Venezuela. Failure of sapling recruitment is potentially attributable to the aberrant animal communities of these islands. Predators of vertebrates are absent and densities of pollinators and seed dispersers are substantially reduced in comparison to the nearby mainland. In contrast, predators of invertebrates, rodents and generalist herbivores (leaf-cutter ants, howler monkeys, common iguanas) are present at greatly elevated densities. Given these distortions in the animal community, recruitment failure of saplings could potentially be attributable to several causes: reduced pollination or seed dispersal, excessive seed predation or seedling herbivory. Two of these hypotheses are tested herein. The seed predation hypothesis predicts greater seed removal in the presence of hyperabundant rodents, and the seedling herbivory hypothesis predicts reduced seedling survival in the presence of hyperabundant herbivores. Seed removal trials were conducted with 18 species of forest trees, using both exposed and lightly buried seeds. Seed removal was not generally higher on islands supporting hyperabundant rodents, contrary to the seed predation hypothesis. Seedlings exposed to herbivores for 4 mo suffered the highest mortality on small islands supporting hyperabundant rodents and leaf-cutter ants. Saplings survived equally well in cages open to arthropods + rodents as in cages open only to arthropods, suggesting that there was no additive effect of rodents on seedling mortality. Current evidence points to excessive seedling herbivory by arthropods, especially by leaf-cutter ants, as the principal cause of recruitment failure on predator-free Lago Guri islets.

Seed predation by rodents and safe sites for large-seeded trees in a fragment of the Brazilian Atlantic forest

“…Empirical and experimental evidence from tropical forests in Central America suggest that small rodents prey preferentially on small seeds and it has been proposed to be the process driving abundant recruitment of large-seed trees in Los Tuxtlas, despite the hyper-abundance of the small rodent Heteromys desmarestianus (Dirzo and Mendoza, 2007). Conversely, Lopez and Terborgh (2007) did not detect disrupted seed removal/predation by overabundant rodents on predator-free artificial islands. These authors argued that seedling herbivory by leaf-cutting ants may Table 1 for the log-linear model fitted to the number of seeds within three categories of seed fate.…”

“…It is particularly true for heavily defaunated forest fragments or predator-free patches with their distorted vertebrate communities in which small rodents tend to be overabundant (see Terborgh et al, 2001;Dirzo and Mendoza, 2007;Lopez and Terborgh, 2007). Currently, the role played by small rodent communities as seed predators remains neglected (i.e., few studies); and unclear since rodent abundance, as well as rates of seed harvesting, dispersal and predation by rodents are greatly affected by a myriad of factors such as seed size (Lopez and Terborgh, 2007), fruit pulp availability (Guimarães et al, 2005) nest site availability (Briani et al, 2001), habitat features (Talamoni and Dias, 1999;Fleury andGaletti, 2006), season (DeMattia et al, 2004), abundance of rodent predators (Dirzo and Mendoza, 2007), patch size and hunting pressure . Here we investigate the role played by the small rodent Oryzomys oniscus as a large-seed predator in a remnant of the Atlantic forest, northeast Brazil.…”

Amphibians on Amazonian Land‐Bridge Islands are Affected More by Area Than Isolation

“…Few studies have been conducted on real islands, such as those created by river damming (but see Terborgh et al 1997, Feeley 2003, Lambert et al 2003, Terborgh et al 2006, Lopez & Terborgh 2007, Brandão & Ara ujo 2008, Gibson et al 2013. The construction of hydroelectric dams has substantially reduced forested areas in tropical regions, with concomitant losses in biodiversity (Fearnside 2002, Wu et al 2004, and references there in), and many new dams are planned for tropical areas (Wu et al 2004).…”

Human-Altered Mesoherbivore Densities and Cascading Effects on Plant and Animal Communities in Fragmented Tropical Forests

“…As a result, seedling and sapling densities have dramatically decreased due to high mortality and low recruitment, creating "ecological meltdown" (Terborgh et al, 2001(Terborgh et al, , 2006. Mortality and recruitment of both small and large saplings on these islands exceeded comparable rates on the mainland by a factor of 2, primarily due to leaf-cutter ant herbivory (Lopez & Terborgh 2007;Terborgh et al, 2006). Though leaf-cutter ants are clearly not mesoherbivores, this example demonstrates the magnitude of the effects herbivores can have on tropical forest plant communities.…”