Impacts of an Amazonian hydroelectric dam on frog assemblages

Abstract

About 90% of the Amazon’s energy potential remains unexploited, with many large hydroelectric dams yet to be built, so it is important to understand how terrestrial vertebrates are affected by reservoir formation and habitat loss. We investigated the influence of the construction of the Santo Antônio Hydroelectric dam on the Madeira River in southwestern Amazonia on the structure of frog assemblages based on samples collected in two years before the dam flooded (pre-stage) and one (post1-stage) and four years (post2-stage) after its construction. We surveyed five 500-ha plot systems three times during each stage; in the pre-stage we sampled 19 plots in low-lying areas that would be flooded by the dam, (from now called flooded pre-stage plots) and 45 plots in terra-firme forest (from now called unflooded pre-stage plots). At the post1-stage we sampled the 45 unflooded plots and in the post2-stage we sampled the remaining 39 unflooded plots. We detected frogs by active visual and acoustic searches standardized by both time and sampling area. Few species recorded in the pre-stage flooded plots were not found in the pre-stage unflooded plots or in stages after flooding. However, the composition of frog assemblages based on relative densities in flooded pre-stage plots did not re-establish in plots on the new river margins. In unflooded areas, frog assemblages were distinct among the flooding stages with no tendency to return to the original assemblage compositions even four years after the dam was filled. For the areas that were not flooded, there was an increase in species richness in 82% of the plots between the surveys before dam construction and the first surveys after dam completion, and 65% between the pre-stage and surveys four years after dam completion. Lack of understanding by the controlling authorities of the long-term effects of landscape changes, such as water-table rises, means that studies covering appropriate periods post construction are not required in legislation, but the data from Santo Antônio indicate that changes due to dam construction are either long-term or difficult to distinguish from natural fluctuations. Future environmental-impact studies should follow strict BACI designs.

A new nurse frog of the Allobates tapajos species complex (Anura: Aromobatidae) from the upper Madeira River, Brazilian Amazonia

“…Allobates kamilae appears to be restricted geographically to riparian forests of the upper Madeira River (UMR); its known range falls entirely within the influence zone of the Santo Antônio and Jirau dams, the two largest dams constructed in the Madeira Basin. Recently, Dayrell et al (2021) showed detrimental impacts of reservoir filling on A. kamilae (as Allobates sp. 1).…”

“…1). Before reservoir filling behind the Santo Antônio dam, the species was commonly recorded in several plots near riverbanks, but four years later it was found in only one plot ( Dayrell et al, 2021 , Fig. 3 ).…”

Capítulo 20: Impulsores e impactos de los cambios en los ecosistemas acuáticos

“…In a study of frogs at the Santo Antônio Dam on the Madeira River, the composition of species assemblages present near the natural river margin before reservoir flooding did not re-establish on the new margin up to four years after the reservoir was filled (Dayrell et al 2021). Frog species richness near the new margins increased by 82% one year after filling, but this percentage had declined to 65% by four years after filling and showed "no tendency to return to the original assemblage."…”

Dams Pose a Critical Threat to Rivers in Brazil’s Cerrado Hotspot

“…The conversion of lotic habitats into semi-lentic habitats creates novel environments that are inhospitable to many species, particularly those adapted to flowing waters (rheophilic) and seasonal flood pulses [17][18][19][20][21][22][23][24][25]. This can trigger important negative impacts on ecosystem functioning, with cascading effects on biodiversity and the livelihoods of local populations, both of which are adapted to the natural seasonality of the rivers [24][25][26][27][28].…”