A comparison of three methods for monitoring a keystone species (Alosa pseudoharengus) across its native and invasive range: direct capture, environmental DNA, and environmental effects.

Keystone species, or species with an outsized influence over their environment relative to their abundance, are important drivers of ecological processes. Their ecological importance makes them prime candidates for biological monitoring, both to preserve and restore their populations when facing decline, and to limit their spread as invasive species. To monitor species well requires cost and labor efficient methods that are capable of detecting the target species at low abundances. Environmental DNA (eDNA) has emerged as an alternative to traditional monitoring methods, but it has yet to be tested on many taxonomic groups and habitat types, and for others has been shown less effective than traditional sampling methods. Even less studied is the use of environmental or keystone effects as a monitoring tool for keystone species. Here, we study the effectiveness of three methods—eDNA, traditional purse seining, and environmental effects via mean zooplankton length—for monitoring the presence and abundance of an ecologically important keystone species, the alewife, Alosa pseudoharengus, in freshwater lakes. The alewife is a zooplanktivorous fish managed as a species of conservation concern along coastal New England, USA, and an invasive species throughout the Laurentian Great Lakes watershed. We sampled lakes throughout Michigan and Connecticut from 2018-2020 and compared the three monitoring methods along four axes: alewife presence/absence, alewife abundance, financial cost, and time efficiency. Our results suggest that monitoring alewife with environmental effects is more accurate, more cost efficient, and more time efficient than purse seining and environmental DNA. These results raise novel possibilities for efficiently and effectively monitoring keystone species such as alewife for the purpose of restoration or management.