Enemy escape: A general phenomenon in a fragmented literature?

Abstract

Many populations are thought to be regulated, in part, by their natural enemies. If so, disruption of this regulation should allow rapid population growth. Such “enemy escape” may occur in a variety of circumstances, including invasion, natural range expansion, range edges, suppression of enemy populations, host shifting, phenological changes, and defensive innovation. Periods of relaxed enemy pressure also occur in, and may drive, population oscillations and outbreaks. We draw attention to similarities among circumstances of enemy escape and build a general conceptual framework for the phenomenon. Although these circumstances share common mechanisms and depend on common assumptions, enemy escape can involve dynamics operating on very different temporal and spatial scales. In particular, the duration of enemy escape is rarely considered but will likely vary among circumstances. Enemy escape can have important evolutionary consequences including increasing competitive ability, spurring diversification, or triggering enemy counteradaptation. These evolutionary consequences have been considered for plant–herbivore interactions and invasions but largely neglected for other circumstances of enemy escape. We aim to unite the fragmented literature, which we argue has impeded progress in building a broader understanding of the eco-evolutionary dynamics of enemy escape.

Testing the Enemy Release Hypothesis on tall-statured grasses in South Africa, using Arundo donax, Phragmites australis, and Phragmites mauritianus as models

“…For example, the monophagous gall-forming flies, Lipara (Diptera: Chloropidae) were introduced along with invasive P. australis from Europe into North America and it was found that in some areas of the adventive range the invasive P. australis genotypes had higher attack rates from Lipara than in the ancestral range (Allen et al ., 2015). Lastly, it may be important to consider that for A. donax release from herbivory may not be the leading factor for its invasive success; some plants are instead largely controlled by ecological factors (Keane & Crawley, 2002; Colautti et al ., 2004; Prior et al ., 2015; Mlynarek et al ., 2017).…”

The ecological and evolutionary importance of nectar‐secreting galls

“…Gall inducers suffer from extremely high rates of parasitoid mortality, and traits that improve their ability to resist parasitoid attack, even temporarily, should still be strongly selected for, given that even temporary or incomplete enemy escape can lead to diversification (Stone and Schönrogge , Mlynarek et al. ). Even in communities that are not top‐down regulated, mortality‐inducing agents can still act as selective mechanisms.…”

Biotic interactions limit the geographic range of an annual plant: herbivory and phenology mediate fitness beyond a range margin

“…Rapid phenology is commonly observed in arid systems, and this has long been presumed to be due to selection to escape the late season drought and unpredictable hydric environments of arid areas (Aronson et al 1992;Thuiller et al 2004;Levin 2006;Volis 2007). "Phenological escape" from insect herbivory has been shown for multiple plant taxa (Pilson 2000;Krimmel & Pearse 2016;Mlynarek et al 2017), but mammalian herbivore control on plant phenology and distributions in arid environments remains relatively unexplored.…”