Bird Communities in Transition: The Lago Guri Islands

Abstract

We report on the bird communities of a set of 12 7‐yr‐old forested land‐bridge islands in Lago Guri, a 4300 km2 hydroelectric impoundment in the State of Bolivar, Venezuela. Birds were censused on all islands and at mainland control sites by spot mapping in 1993, and via point counts in 1995. Instead of orderly “nested sets” of species on landmasses of graded size, the species composition of small (≈1 ha) and medium (11–12 ha) islands was highly variable. Spot mapping substantiated the occurrence of 58 species of forest‐nesting birds, collectively, on the 11 small and medium islands, which supported means of only 9 and 12 resident species, respectively. No species was found on all islands, and only five species were found on as many as 7 of the 11 small and medium islands. The mean number of islands per resident species was 3.0 for the 11 islands. Transients of many species were detected on all islands, but were more frequent on near (≤0.5 km from a larger landmass) than on far islands (≥1.0 km from a larger landmass). A large majority of the populations on the 1‐ha islands consisted of a single pair, implying that few such populations could have survived since isolation without new colonizations to replenish individuals lost to dispersal and mortality. Notwithstanding low species numbers, both census methods indicated that avian densities were approximately twice as high on the 1‐ha islands as on the mainland. In contrast, two of three medium‐sized islands supported anomalously low densities of approximately one‐fifth the mainland level. Both islands retain relict populations of capuchin monkeys (Cebus olivaceus). One hundred percent of artificial nests set out on one of these islands were raided, whereas no more than 30% of nests were lost on any of the other islands or on the mainland. For small and medium islands, we conclude that the founding communities present when the waters of Lago Guri reached their final level have already collapsed and been reconstituted. Species loss on some islands may have been accelerated by relict populations of predators, such as capuchins. Changes in composition thus appear to have been driven by a combination of biological (nest predation) and stochastic processes (high turnover).

Assemblage‐level responses of phyllostomid bats to tropical forest fragmentation: land‐bridge islands as a model system

“…Population-and assemblage-level responses in low-contrast systems have been examined mainly for plants, birds, and rodents (e.g. Adler & Seamon, 1991;Leigh et al, 1993;Terborgh et al, 1997;Asquith & Mejia-Chang, 2005). For bats, on the other hand, nearly all studies on fragmentation effects save those of Cosson et al (1999) and have been conducted in rather young, low-contrast systems sensu Watson (2002) (e.g.…”

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

“…On Barro Colorado Island, Panama, high avian nest predation has been attributed in part to increased small mammalian nest predators such as white-nosed coatimundi (Nasua narica), whitefaced capuchin monkeys (Cebus capucinus), and opossums following isolation from the mainland, and subsequent loss of large carnivores, when the Panama Canal was built (Loiselle & Hoppes, 1983;Sieving, 1992;Wright et al, 1994). On the Lago Guri islands in Venezuela, former hilltops isolated from nearby mainland by a hydro-electric reservoir, anomalously low bird densities were associated with abundant olive capuchin monkeys (Cebus olivaceus), which depredated all artificial nests (Terborgh et al, 1997). Olive baboons (Papio anubis) also dramatically increased in Ghana following steep declines in lions (Pantera leo) and leopards (Pantera pardus), negatively impacting smaller ungulates, primates, rodents, and birds, as well as human livestock, pets, and crops (Brashares et al, 2010).…”

“…This altered nutrient regime drove a ricocheting bottom-up effect, in which the positive indirect effects of howler monkey density (and concomitant increased nutrient availability) on bird species richness exceeded the negative, direct effects of island area on bird species richness (Feeley & Terborgh, 2008). Indeed, birds maintained territories 3 to 5 times smaller on islands with abundant howler monkeys than on the adjacent mainland (Terborgh et al, 1997). While the herbivore hyperdensities are commonly attributed to the absence of all large and mid-sized predators from the islands (Terborgh et al, 2001(Terborgh et al, , 2006, the very small size of the islands, extensive edge effects, and refuge effect during reservoir inundation also likely contributed (S.J.…”

Predator-prey interaction between two threatened species in a Brazilian hotspot

“…Egg predation was observed for various species of capuchin monkeys, such as Sapajus apella, Cebus capucinus, C. albifrons and C. olivaceus Boinski 1992, Riehl andJara 2009). Moreover, studies with artificial nests have revealed high rates of egg predation in habitat islands harbouring capuchins (Terborgh et al 1997). In Costa Rica, Cebus capucinus are known to regularly search for nests during the nesting seasons, and to feed on eggs of many species of birds (Fedigan 1990).…”