Latitudinal patterns of aquatic insect emergence driven by climate

Aim: Emerging aquatic insects link aquatic and terrestrial ecosystems across the Earth. Their diversity, abundance and functional importance mean their emergence is an important phenological event. Nevertheless, aquatic insect emergence is understudied at a global scale compared to other phenological events, despite changing phenology being one of the most significant ecological responses to climate change. Here, we quantitatively describe the global patterns, and key proposed drivers, of seasonal aquatic insect emergence, to further understand how these patterns might change in the future.

Location: Global.

Time period: 1950–2018.

Major taxa studied: Emerging aquatic insects.

Methods: We extracted monthly emergence data from 86 studies across 163 sites to construct 1053 annual emergence curves. We parameterised the curves using two complementary metrics of seasonality, which were modelled against geographic and climatic variables to determine the direct and indirect relationships between them. Results: We found clear global trends in aquatic insect emergence patterns across latitude and underlying climates. Between-month variation and temporal restriction of emergence increased from the equator to the poles, going from small, aseasonal fluctuations in the warm, thermally stable tropics to large, seasonal peaks at cooler, thermally unstable higher latitudes. While emergence trends were associated with gradients of precipitation, temperature was the dominant climatic driver of the latitudinal trend.

Main conclusions: These findings suggest that with climate warming, aquatic insects will emerge over longer periods, diluted in abundance and displaying less seasonal emergence patterns with smaller between-month fluctuations. This may result in disruption of ecosystem functions seasonally dependent on aquatic insects, such as riparian predation, pollination and disease transmission. The cross-ecosystem life cycle of aquatic insects means changes to their seasonal patterns of emergence will have impacts in both aquatic and terrestrial ecosystems. 

Methods
Further methodological details on data collection can be found within the published paper and its supplementary material.