Exploitative Competition Between Invasive Herbivores Benefits a Native Host Plant

Abstract

Abstract. Although biological invasions are of considerable concern to ecologists, relatively little attention has been paid to the potential for and consequences of indirect interactions between invasive species. Such interactions are generally thought to enhance invasives' spread and impact (i.e., the ''invasional meltdown'' hypothesis); however, exotic species might also act indirectly to slow the spread or blunt the impact of other invasives. On the east coast of the United States, the invasive hemlock woolly adelgid (Adelges tsugae, HWA) and elongate hemlock scale (Fiorinia externa, EHS) both feed on eastern hemlock (Tsuga canadensis). Of the two insects, HWA is considered far more damaging and disproportionately responsible for hemlock mortality. We describe research assessing the interaction between HWA and EHS, and the consequences of this interaction for eastern hemlock. We conducted an experiment in which uninfested hemlock branches were experimentally infested with herbivores in a 2 3 2 factorial design (either, both, or neither herbivore species). Over the 2.5-year course of the experiment, each herbivore's density was ;30% lower in mixed-vs. single-species treatments. Intriguingly, however, interspecific competition weakened rather than enhanced plant damage: growth was lower in the HWAonly treatment than in the HWA þ EHS, EHS-only, or control treatments. Our results suggest that, for HWA-infested hemlocks, the benefit of co-occurring EHS infestations (reduced HWA density) may outweigh the cost (increased resource depletion).

Direct and indirect effects of alien insect herbivores on ecological processes and interactions in forests of eastern North America

“…This study suggests that alien herbivorous insects can indirectly exert both top-down and bottom-up impacts on native fauna. Preisser and Elkinton (2008) documented competitive interactions between two alien insect specialists of hemlock: hemlock woolly adelgid and elongate hemlock scale (Fiorinia externa Ferris). The densities of both species were reduced by 30% in mixed-species relative to single-species trials.…”

Variation in Plant Defense against Invasive Herbivores: Evidence for a Hypersensitive Response in Eastern Hemlocks (Tsuga canadensis)

“…A second invasive herbivore, the elongate hemlock scale (Fiorinia externa; EHS), also feeds on eastern hemlocks in North America (McClure, 1977). Although both species are sapsucking insects that are considered pests, they have different impacts on their shared host (Preisser and Elkinton, 2008). In a multi-year experiment, Miller-Pierce et al (2010) found that infestation with HWA, but not EHS, significantly decreased tree growth.…”

Competitive interactions between co-occurring invaders: identifying asymmetries between two invasive crayfish species

“…The presence of multiple invaders in an ecosystem may change interaction outcomes which would occur between one invader and native species (Preisser and Elkinton 2008). Several studies so far have demonstrated different interaction outcomes that might arise from the presence of multiple invaders, which range from negative (e.g.…”