Predicting local extinctions of Amazonian vertebrates in forest islands created by a mega dam
Introduction
Mega hydroelectric dams have become a major driver of forest habitat loss and fragmentation across several Amazonian river basins, with dozens of new major hydropower projects either planned or currently under construction (Finer and Jenkins, 2012, Fearnside, 2014). In Brazilian Amazonia, a total of 1,105,400 ha of pristine forests have already been inundated by eleven major hydroelectric dams (ECOA, 2012), but over 10 million ha of forests are expected to become permanently inundated following the planned construction of new dams (Fearnside, 2006). Assessments of the social and environmental impacts of large dams worldwide have so far primarily focused on flooding of indigenous territories and displacements of local communities (Esselman and Opperman, 2010), alterations in fluvial hydrology (Nilsson et al., 2005), augmented emissions of greenhouse gases (Almeida et al., 2013), and losses in fisheries and aquatic biodiversity (Barthem et al., 1991, Alho, 2011, Liermann et al., 2012, Palmeirim et al., 2014). In contrast, the performance of terrestrial vertebrate populations in tropical ecosystems affected by dams has received comparatively little attention (but see Terborgh et al., 1997, Cosson et al., 1999, Gibson et al., 2013, Benchimol and Venticinque, 2014). Given hugely escalating investments in hydropower infrastructure worldwide, impact assessments of mega-dams on terrestrial biodiversity in many terrestrial systems, including the Amazonian basin, are conspicuously missing.
As mega-consumers and apex predators, large-bodied vertebrates are often considered as good bioindicators of intact tropical forests, as they provide key ecological services for ecosystem dynamics and are sensitive to forest disturbance and hunting (Dirzo et al., 2014). Local extinctions from forest patches can induce a series of trophic cascades, promoting unexpected shifts in forest composition and function. For instance, predator-free land-bridge islands in Venezuela are typically denuded by hyperabundant herbivores, decimating seedling recruitment of canopy trees (Terborgh et al., 2001). Also, rising floodwaters drastically reduced vertebrate species diversity in newly formed islands compared to continuous forest areas only four years after French Guiana’s Petiti Saut Dam was built (Cosson et al., 1999), suggesting that isolation effects in true islands are more severe than in habitat patches surrounded by a non-water matrix.
Newly isolated vertebrate assemblages could undergo nonrandom drifts in species composition within tropical land-bridge islands following a long relaxation time, but this is a function of species-specific responses to patch- and landscape-scale variables and life-history traits. Vertebrate persistence in Neotropical fragmented landscapes is likely to be affected by landscape structure and the history of human disturbance (Michalski and Peres, 2005, Canale et al., 2012), with a range of species responses to habitat fragmentation. Rare, matrix-intolerant species unable to disperse amongst remnant patches are usually considered more extinction-prone in fragmented landscapes (Davies et al., 2000, Henle et al., 2004). Hence, some life-history traits can be excellent predictors of forest patch occupancy, and coupled with patch and landscape-scale site attributes, can help predict species survival within forest remnants and inform species-specific conservation guidelines.
Here, we assess how 34 terrestrial and arboreal vertebrate species responded to the insularization process induced by a major hydroelectric dam in lowland Central Amazonia, based on intensive, well-replicated field surveys in a large number of islands and neighbouring continuous forests. Specifically, we examine (1) the observed and estimated forest patch occupancy of each species (accounting for imperfect detection), assessing minimum critical area required to ensure their persistence; (2) how different patch, landscape and habitat quality metrics affect patterns of occupancy for individual species; and (3) the relative importance of landscape context and species traits in explaining pattern of local extinction across all islands. Based on these results, we predict the aggregate vertebrate species richness and composition across >3500 islands within the reservoir, pinpointing priority sites for conservation, and dissect how large hydroelectric dams affect terrestrial vertebrate diversity in lowland Amazonia.
Section snippets
Study area
This study was conducted within the Balbina Hydroelectric Reservoir (BHR), a man-made reservoir within the Uatumã River basin of central Brazilian Amazonia (1°48″S; 59°29′W). The Balbina Dam was completed in 1986 to supply hydropower to Manaus, the state of Amazonas capital city. The rising floodwaters inundated an area of 312,900 ha, resulting in the formation of 3546 land-bridge islands ranging in size from <1 to 4878 ha. In 1990, the lake became protected by the creation of the ∼940,000-ha
Forest island occupancy
A total of 34 species was recorded across all 40 survey sites (Table 2), including 29 mammal, four large terrestrial bird, and two tortoise species (see Fig. A1 for CT photos; data available from the Dryad Digital Repository). However, we failed to record several highly inconspicuous, nocturnal, or low-density species which almost certainly occurs in the study area (e.g. Coendou prehensilis, Cyclops didactylus, Atelocynus microtis, Speothus venaticus, Galictis vittata and Potos flavus). The 37
Discussion
A number of studies have addressed large-bodied terrestrial vertebrate populations in fragmented tropical forest landscapes (Michalski and Peres, 2005, Urquiza-Haas et al., 2009, Sampaio et al., 2010, Thornton et al., 2011a, Thornton et al., 2011b, Canale et al., 2012). However, these study areas are dominated by a terrestrial vegetation matrix of varying degrees of permeability as there are few opportunities to examine faunal assemblages in truly archipelagic landscapes where insular forest
Acknowledgments
We are grateful for the field assistance of Evanir, Evandro, Joelson, Naldo, Zé, Davi, Nina, Márcio and Aline within the Balbina archipelago, and C. Ramalheira during interviews. We thank Danielle Storck-Tonon for her assistance with the landscape metrics, and E. M. Venticinque for allowing us to use his photograph in Fig. 1. We are grateful to all staff from the Uatumã Biological Reserve for providing substantial financial and logistical support during fieldwork, and W. Endo and CENAP for
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