Effect of reduced impact logging and small-scale mining disturbances on Neotropical stream fish assemblages

Deforestation and mining are recognized as major threats to Amazonian biodiversity, but, in addition to the well known impacts of clear cutting and industrial mining, the impact of cryptic threats such as illegal small-scale gold-mining and reduced impact logging remain little known. Here, we quantify the impact of those cryptic disturbances on a set of 201 sites dispersed throughout French Guiana. The fish assemblages of 139 pristine forest sites were compared to 16 sites subjected to reduced impact logging (i.e. selective logging), 24 sites with ongoing small-scale gold-mining and 22 sites formerly mined for gold. Controlling for the environmental variability between sites showed the significant structuring effect of all disturbances on fish taxonomic structure, with a marked impact of gold-mining. This effect was of strong magnitude and remained significant after mining activity ceased. In contrast, the reduced impact logging effect remains of low magnitude, although significant. From a functional point of view, gold-mining drives species assemblages towards a decrease in the richness of small-sized stream habitat specialist species and favours larger ubiquitous species living in both streams and rivers. Reduced impact logging effect was slighter, and negatively affected only the richness of phytophagous species. These results, encompassing a variety of hydrographic basins, unambiguously show the detrimental effect of small-scale gold-mining on fish assemblages as well as the slight effect of reduced impact logging. Since gold-mining is one of the most widespread threats throughout the Amazonian region, particular care should be given to controlling this, often illegal, activity.