Pervasive decline of subtropical aquatic insects over 20 years driven by water transparency, non-native fish and stoichiometric imbalance

Abstract

Insect abundance and diversity are declining worldwide. Although recent research found freshwater insect populations to be increasing in some regions, there is a critical lack of data from tropical and subtropical regions. Here, we examine a 20-year monitoring dataset of freshwater insects from a subtropical floodplain comprising a diverse suite of rivers, shallow lakes, channels and backwaters. We found a pervasive decline in abundance of all major insect orders (Odonata, Ephemeroptera, Trichoptera, Megaloptera, Coleoptera, Hemiptera and Diptera) and families, regardless of their functional role or body size. Similarly, Chironomidae species richness decreased over the same time period. The main drivers of this pervasive insect decline were increased concurrent invasions of non-native insectivorous fish, water transparency and changes to water stoichiometry (i.e. N : P ratios) over time. All these drivers represent human impacts caused by reservoir construction. This work sheds light on the importance of long-term studies for a deeper understanding of human-induced impacts on aquatic insects. We highlight that extended anthropogenic impact monitoring and mitigation actions are pivotal in maintaining freshwater ecosystem integrity.

Insect decline in Brazil: an appraisal of current evidence

“…In the first case, losses may have preceded the start of studies, whereas in the second, they were too short-term (4–5 years) to detect subtle trends. In a single 19-year study, aquatic insect abundance and diversity declined in sites whose water quality was compromised by dam construction and that were concomitantly invaded by non-native insectivorous fish [30].…”

Human pressure drives biodiversity–multifunctionality relationships in large Neotropical wetlands

“…1). Paraná includes 150 constructed dams 21 and faces the strongest human pressure among the four wetlands. The climate ranges from subtropical to tropical, with a mean annual temperature of 16 -29ºC and a mean precipitation of 1,300 -2,000 mm year-1 22 .…”

“…The eld surveys were designed to include lakes representing a wide range of climate, human pressure, and environmental conditions. They followed a standardized sampling protocol and the sampling effort was the same in all lakes 21 . In order to provide a comprehensive assessment of aquatic communities, we performed one sampling during the dry season and another during the wet season in each lake.…”

Recent trends in stream macroinvertebrates: warm-adapted and pesticide-tolerant taxa increase in richness

“…In recent years, many studies have reported declines in insect richness [1][2][3][4][5][6][7]. Nevertheless, it is difficult to generalize trends since regional, taxa-and ecosystem-specific patterns exist [8][9][10][11][12][13]. Although terrestrial insects appear to be declining in many areas, aquatic insects have more frequently been reported to be increasing [14].…”