Abstract
Biodiversity-rich forests in tropical Southeast Asia are being extensively logged and converted to oil palm monocultures. In addition, extreme climatic events such as droughts are becoming more common. Land-use change and extreme climatic events are thought to have synergistic impacts on aquatic biodiversity, but few studies have directly tested this. A severe El Niño drought in Southeast Asia in early 2016 caused 16 low-order hill streams across a land-use gradient encompassing primary forest, logged forest and oil palm plantations in Sabah, Malaysia, to dry up into series of disconnected pools. The resulting disturbance (specifically, increased water temperature and decreased dissolved oxygen concentration) tolerated by the fish during the drought exceeded any worst-case scenario for climate change-induced warming. We quantified the biomass, density and movement of the dominant freshwater fish species, Nematabramis everetti (Cyprinidae), in these streams across this land-use gradient before, during, and after the 2016 El Niño drought period. Density of N. everetti was significantly lower in logged forest streams than primary forest or oil palm streams, and the biomass of individuals captured was lower during drought than prior to the drought; however, there was no change in the biomass density of individuals during drought. The distance moved by N. everetti was significantly lower during and after the drought compared to before the drought. We detected a significant antagonistic interaction on biomass of captured fish, with the magnitude of the drought impact reduced land-use intensified. Populations of N. everetti were surprisingly resilient to drought and seem most affected instead by land-use. Despite this resilience, it is important to monitor how this widespread and abundant species, which provides an important ecosystem service to local human communities, is affected by future land-use and climate change, as logging, deforestation and conversion to plantation monocultures continue across Southeast Asia.
Highlights
Freshwater ecosystems have been severely affected by anthropogenic impacts and are the most threatened ecosystems globally (Thomsen et al 2012)
Density of N. everetti was significantly lower in logged forest streams than primary forest or oil palm streams, and the biomass of individuals captured was lower during drought than prior to the drought; there was no change in the biomass density of individuals during drought
With continued deforestation and conversion to oil palm plantations in Southeast Asia, our study provides crucial insight into how these land-use changes will interact with extreme climatic events to impact freshwater ecosystems
Summary
Freshwater ecosystems have been severely affected by anthropogenic impacts and are the most threatened ecosystems globally (Thomsen et al 2012). It is important to understand how these multiple stressors (land-use and climate change) interact and impact aquatic biodiversity. Land-use change can significantly impact freshwater ecosystems. Land-use changes can affect freshwater ecosystems through changes in sediment load, nutrient runoff, canopy cover and temperature, and leaf litter and woody debris inputs (Luke et al 2017a). These impacts change the microhabitats that fish inhabit and can affect taxonomic or functional groups differently (e.g. Newcombe and Macdonald 1991, Jones et al 1999, Sazima et al 2006)
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