Can the enemy release hypothesis explain the success of invasive alien predators and parasitoids?

Abstract

Biological invasions are ecologically and economically costly. Understanding the major mechanisms that contribute to an alien species becoming invasive is seen as essential for limiting the effects of invasive alien species. However, there are a number of fundamental questions that need addressing such as why some communities are more vulnerable to invasion than others and, indeed, why some alien species become widespread and abundant. The enemy release hypothesis (ERH) is widely evoked to explain the establishment and proliferation of an alien species. ERH predicts that an alien species introduced to a new region should experience a decrease in regulation by natural enemies which will lead to an increase in the distribution and abundance of the alien species. At the centre of this theory is the assumption that natural enemies are important regulators of populations. Additionally, the theory implies that such natural enemies have a stronger regulatory effect on native species than they do on alien species in the introduced range, and this disparity in enemy regulation results in increased population growth of the alien species. However, empirical evidence for the role of the ERH in invasion success is lacking, particularly for invertebrates. Many studies equate a reduction in the number of natural enemies associated with an alien species to release without studying population effects. Further insight is required in relation to the effects of specific natural enemies on alien and native species (particularly their ability to regulate populations). We review the role of ecological models in exploring ERH. We suggest that recent developments in molecular technologies offer considerable promise for investigating ERH in a community context.

Ecological genetics of invasive alien species

“…This approach was recently used to identify Israeli acute paralysis virus as a potential agent of colony collapse disorder in honeybees, Apis mellifera (L.) (Hymenoptera: Apidae) (Cox-Foster et al 2007), and to characterise microbes associated with the primary pest of bees, the ectoparasitic mite Varroa destructor (Anderson and Trueman) (Mesostigmata: Varroidae) (Cornman et al 2010). In principle, the same approach could be used to characterize the community of parasitic organisms living within or on an IAS, and therefore to test the prediction of the enemy release hypothesis (Roy et al 2011b) that there should be lower infection levels in alien populations compared to native populations of the same host species.…”

Predators and parasitoids of the harlequin ladybird, Harmonia axyridis, in its native range and invaded areas

“…The success of H. axyridis as an invader is often attributed to weak control of its populations by natural enemies, with the enemy release hypothesis (ERH) usually being quoted to explain this low level of biological control (Roy et al , 2011aBerkvens et al 2010;Firlej et al 2012;Comont et al 2014;Haelewaters et al 2017). ERH states that an alien species invading a new geographic range will be less affected by natural enemies compared to native species, leading to population build-up of the alien species in its invasive range.…”

Hesperomyces virescens (Fungi, Ascomycota, Laboulbeniales) attacking Harmonia axyridis (Coleoptera, Coccinellidae) in its native range

“…Based on the presented records, we consider H. virescens a historically globally distributed species that also attacks H. axyridis in its native range. Furthermore, we hypothesize that native H. virescens infection was lost when beetles were imported into North America, for example, because sampled specimens were ''clean'' (free from infection) [37], and in line with the natural enemy release hypothesis [7,8]. After a time lag reinfection with local H. virescens occurred, the first evidence of this was recorded in 2002 [17].…”

“…The enemy release hypothesis predicts that an invasive species in its new environment will experience a decrease in regulation by natural enemies, resulting in a competitive advantage for the alien species over native species [7,8]. While it has been suggested [9] that there is no support of natural enemies exerting regulatory effects on coccinellid populations, a recent study provides an overview of parasites and pathogens with focus on the resilience of H. axyridis compared to other Coccinellidae [10].…”