Invasive species, disrupted chemical community dynamics and future adaptations: Commentary on Chabaane et al. 2015

Chabaane et al. (2015) describe the subtle disruptive chemical changes that an alien herbivore (a moth caterpillar, Spodoptera littoralis) may cause in a native community. This disruption is described across trophic links from the plant it eats (a mustard family species, Brassica rapa) to native herbivores (the cabbage white butterfly, Pieris brassicae) and their native natural enemies (parasitic wasps, Cotesia glomerata). This paper reinforces the concern that new insect species arriving in natural communities may potentially have a detrimental impact: inducing unexpected changes in plant chemistry; altering competitive interactions with other herbivores; and changing the dynamics of the natural enemies of such herbivores. Extreme disruptions by ‘invasive’ Lepidoptera include pest population outbreaks that may cause large-scale defoliations which affect the entire ecosystem by cascading chemical, physiological and ecological changes across all trophic levels. In consequence altering the functional dynamics in various ways (Redman & Scriber 2000; Kenis et al. 2009; Gandi & Herms 2010). The study by Chabaane et al. (2015) shows how subtle changes in chemical cues from alien herbivores may result in major changes with cascading responses of the native plant–herbivore–parasite community. Whether such chemically based disruptions such as those described by Chabaane et al. (2015) result in local population outbreaks, and whether such changes become part of the future community dynamics, depends on the intensity and variability of natural selection that occurs. All of these biotic interactions resulting from invasive species are shaped by a combination of ecological and evolutionary processes (Moran & Alexander 2014). The fundamental component at all levels (organism, population, community and ecosystem) is the underlying genetic variability, which both interacts with and shapes all aspects of the phenotypes (phytochemical, biochemical, behavioural, physiological and morphological traits) and the potential for rapid adaptation and introgressive gene flow (Brasier 2001; Scriber 2011; Franks & Hoffmann 2012; Pauls et al. 2013). Plant genotypes can shape communities and drive evolutionary change (‘community genetics’; Smith et al. 2011). Similarly, underlying genetics shape interactions and natural selection across different environmental gradients at the landscape and ecosystem levels (‘geographic mosaic of co-evolution’; Thompson 2005). Understanding the underlying chemical, ecological and genetic sources of evolutionary responses to invasive species in the face of rapid environmental changes (such as global warming) will become one of the most significant challenges in the next decade (Moran & Alexander 2014; Scriber 2014). There is clearly a need for additional studies such as Chabaane et al. (2015) because the impacts of introduced species may run contrary to expectation. Illustrated by the fact that the alien (potentially invasive) herbivore species caused disruptive impacts on the community via induced changes in plant chemistry. This occurred despite the fact that exotic herbivores themselves were not actually used as hosts by the native parasitoids. Chabaane et al. (2015) show that exotic herbivores may invade new environments where they can significantly alter the native communities. This is done via both direct (induced changes in plant phytochemistry or nutritional ecology; Scriber 2002; Gandi & Herms 2010) and indirect (cascading) impacts on species across trophic levels in the ecosystem (Redman & Scriber 2000; Dicke & Baldwin 2012; Stam et al. 2014). The chemical basis of these plant/herbivore/enemy interactions has historically focused on host plant resistance, herbivore counter-adaptations and chemical defenses derived from the plants or herbivores in a relatively recent field of chemical ecology (Price et al. 1980; Feeny 1992; Gols 2014). Changes in other abiotic factors (e.g. climate and local drought conditions, as well as stoichiochemistry; Mattson & Scriber 1987; Mattson & Haack 1987; Elser et al. 2000; Chunco 2014) can also greatly alter the chemical ecology of the community, including interactions of plants, herbivores and their enemies (predators, parasites, and pathogens: Stamp 1993; Ayres & Lombardero 2000; Singer & Stireman 2005). However, Chabaane et al. (2015) show that changes in the chemical environment are not always ecologically predictable. More ground-breaking research as described in the interactions of chemical components among native and exotics across local populations and trophic levels will be required in the future (Chabaane et al. 2015). However, the elucidation of the genetic basis of ecological ripples caused by allelochemical interactions will likely be the next major step in better understanding the autecological (organism & population) causes of synecological responses in the community and ecosystem (Whitham et al. 2003; Wheat et al. 2007; Stam et al. *Correspondence author: E-mail: scriber@msu.edu