Small mammal responses to Amazonian forest islands are modulated by their forest dependence

Abstract

Hydroelectric dams have induced widespread loss, fragmentation and degradation of terrestrial habitats in lowland tropical forests. Yet their ecological impacts have been widely neglected, particularly in developing countries, which are currently earmarked for exponential hydropower development. Here we assess small mammal assemblage responses to Amazonian forest habitat insularization induced by the 28-year-old Balbina Hydroelectric Dam. We sampled small mammals on 25 forest islands (0.83-1466 ha) and four continuous forest sites in the mainland to assess the overall community structure and species-specific responses to forest insularization. We classified all species according to their degree of forest-dependency using a multi-scale approach, considering landscape, patch and local habitat characteristics. Based on 65,520 trap-nights, we recorded 884 individuals of at least 22 small mammal species. Species richness was best predicted by island area and isolation, with small islands (< 15 ha) harbouring an impoverished nested subset of species (mean ± SD: 2.6 ± 1.3 species), whereas large islands (> 200 ha; 10.8 ± 1.3 species) and continuous forest sites (∞ ha; 12.5 ± 2.5 species) exhibited similarly high species richness. Forest-dependent species showed higher local extinction rates and were often either absent or persisted at low abundances on small islands, where non-forest-dependent species became hyper-abundant. Species capacity to use non-forest habitat matrices appears to dictate small mammal success in small isolated islands. We suggest that ecosystem functioning may be highly disrupted on small islands, which account for 62.7% of all 3546 islands in the Balbina Reservoir.

Marked compositional changes in harvestmen assemblages in Amazonian forest islands induced by a mega dam

“…Contrary to our expectations, none of the local, patch and landscape variables were included in the best models explaining harvestmen species richness on islands. In the Balbina archipelagic landscape, island size was a strong positive predictor of species richness in other taxonomic groups ─ including birds (Aurélio-Silva et al, 2016), small mammals (Palmeirim et al, 2018) and large mammals (Benchimol & Peres, 2015a). Although SAR is one of the few ironclad rules in ecology (Matthews et al, 2016), we found that area failed in explaining richness patterns for harvestmen assemblages, the absence of this relationship has been documented before for this group in Amazonian upland forest sites where microhabitat structure and availability per unit area also explained patterns of harvestmen species composition but not species richness in harvestmen assemblages (Colmenares et al, 2016).…”

The Effect of Three Gorge Project on the Small Mammals in Yangtze River of China

“…Meanwhile, habitat fragmentation due to the construction of the TGD had led to further changes in species composition and biodiversity, nevertheless, the species richness of island and mainland rodent communities was not significantly different [29]. Along-term observations in Central Brazilian Amazonia showed small mammal assemblages further exhibited a nested structure in insular habitat, smaller and more isolated islands exhibited depauperate small mammal assemblages, in which some species were typically either missing or persisted at very low abundances [46]. So, it may need to have a long period of monitoring in TGRA.…”

“…It was not determined that the exact factors were had led to the observed changes in the small mammal community. Understanding how biodiversity is affected by hydropower projects is then of critical importance for management actions [46]. Therefore, future research should focus on determining to what extent and in what way the TGP has contributed toward changing the structure of the small mammal community inhabiting the up and downstream of TGD.…”

“…A number of permanent and seasonal islands had been formed. After habitat insularization, the ecosystem functioning might be highly disrupted on small islands [46]. The investigation of the species composition and distribution of rodent populations at two geographic locations in the TGRA showed that the species richness of island and mainland rodent communities was not significantly different at either site, but the dominant species differed significantly between island and mainland habitats: the total rodent density on the islands was significantly higher than that of mainland areas [29].…”

Marked compositional changes in harvestmen assemblages in Amazonian forest islands induced by a mega dam

“…Contrary to our expectations, none of the local, patch and landscape variables were included in the best models explaining harvestmen species richness on islands. In the Balbina archipelagic landscape, island size was a strong positive predictor of species richness in other taxonomic groups ─ including birds (Aurélio- Silva et al, 2016), small mammals (Palmeirim et al 2018) and large mammals (Benchimol & Peres, 2015a).…”