Don't Wake the Sleeper Species!
Michael J Spear, Jake R Walsh, Anthony Ricciardi, M Jake Vander Zanden, The Invasion Ecology of Sleeper Populations: Prevalence, Persistence, and Abrupt Shifts, BioScience, Volume 71, Issue 4, April 2021, Pages 357–369, https://doi.org/10.1093/biosci/biaa168.
Summary written by Ayodelé O’Uhuru, edited by Audrey Barker Plotkin
Summary
Sleeper populations are established nonnative populations that persist at low abundance but could become invasive when triggered by an environmental change. Spear et al. (2021) emphasize the concept of sleeper populations rather than sleeper species - in other words, a species could be invasive in one area, established and unlikely to ever be invasive in another, and established and a potential sleeper in yet another area. Most established populations of introduced species exist at low abundance, but we tend not to study them until after a population change has occurred. Thus, identifying low-abundance nonnative populations and understanding the environmental factors that could cause an abrupt shift is an important research priority. Spear et al. use the example of the invasive spiny water flea in Lake Mendota, WI to show how easy it can be to miss small populations of introduced species even when dedicated monitoring is in place. This example helps us understand the challenges of identifying and managing sleepers. The first challenge is discovering the presence of an introduced species in a new ecosystem, and the second challenge is figuring out what environmental conditions could trigger a population explosion. With climate change creating novel environmental changes, more work is needed to understand and reduce triggers that could awaken sleeper populations.
Take home points
Sleepers are populations of introduced species that currently persist at low abundance, but have the potential to become invasive if the environment changes.
Some categories of environmental triggers that can lead to abrupt shifts in abundance are food web changes, completed mutualism (e.g. a newly introduced mutualist enables population growth), threshold responses (e.g. newly favorable climate), and stochastic environmental driver (e.g. a disturbance event).
Gradual changes in climate can lead to abrupt shifts in species populations, for example, if temperature crosses a lethal tolerance level it could lead to a rapid increase (or decrease) in population growth.
Management implications
Keep an eye out for population changes in established, non-native species. Established species that are known to be invasive in other regions might be more likely to be sleepers.
Improving ability to detect rare and nonnative species and monitoring their presence (using molecular approaches eDNA, community science, and remote sensing) can improve detection of sleeper populations.
Understand environmental triggers that cause populations to increase, and focus on minimizing those interactions in areas where sleeper populations already exist.
The same species could be invasive in one region, non-invasive in another, and have sleeper populations in a third. Geography matters when determining potential invasiveness.
Keywords
Invasive species; sleeper species; persistence; abrupt shifts; impact studies; climate extremes; biological invasion; climate change; competitiveness; invasive plant; model; management efficacy; risk assessment