Winter 2015 Ecesis, Volume 25, Issue 4
Invertebrates can be effective indicators of the consequences of non-native plant invasions due to the important functional roles that they play in ecosystems, including nutrient recycling and energy flow, pollination, seed dispersal, and the maintenance of plant and animal community structure. Analysis of invertebrate responses to plant invasions can also help to guide the management and restoration of ecosystems, as they are both relatively easy to sample and respond rapidly to environmental changes. Although the abundance of a plant invader in a given area is likely to be a key determinant of its impact to invertebrate diversity, the nature of this relationship is unclear. Recent studies of bird and invertebrate diversity have found a decline after intermediate densities of the invader are reached; if such a pattern is common, the thresholds found will aid land managers in planning and prioritizing their restoration activities.
A meta-analysis approach can be used to combine multiple studies and thus look for overall trends while achieving greater statistical power. We conducted a meta-analysis of plant invasion effects on invertebrate richness, incorporating invader vegetative cover as a variable likely associated with these effects, and comparing restored to intact plots as controls to assess the success of our restoration efforts. We found fifty-two studies that were eligible for testing using a meta-analysis approach (means, variances and sample sizes were reported), and used natural breaks in the data to develop the following cover classes: <10%, 10-30%, 30-50%, 50-70%, 70-90%, and >90%.
Because different invaders have different effects on invertebrate food resources — such as native plants, invertebrate prey, and leaf litter — and vary in their levels of specialization, these guilds likely vary in their responses to plant invasion. Therefore to further understand impact patterns, we also reviewed published information on responses isolated by feeding guild, including herbivores, parasitoids, predators, detritivores, and pollinators.
The results of the meta-analysis are presented in Figure 1. We found that invertebrate species richness was 31% lower in areas dominated by exotic plants than in areas dominated by native plants. While invader cover at classes below 70% had little impact on invertebrate richness, above this threshold the effects were negative. One likely cause of this threshold is a decline in diversity of other plant species when an invader comes to dominate. Studies have shown that arthropod diversity is positively related to plant species richness, presumably because of effects on structural and food diversity as well as abiotic variables (e.g., temperature, moisture).
Interestingly, restored plots were more species-rich than intact native plots relative to invaded plots, but with greater variability. This may be because early-colonizing invertebrates such as adult butterflies can be attracted to more open, sunny restored areas disturbed by earth moving, invasive plant removal, and outplanting. On the other hand, higher invertebrate richness in restored than intact native areas could be the result of even higher plant richness and cover in restored than remnant intact habitats, suggesting that such restoration efforts are successful.
Studies have shown that herbivores can be more negatively affected by plant invasions than other feeding guilds and reflect changes in plant diversity. This is particularly true for herbivore specialists, which can only tolerate the chemical and physical defenses of specific plants to which they have adapted. While only slightly more than half of the studies addressing herbivore richness as a whole reported a negative effect of exotic plant species, specialist herbivores were slightly more negatively affected than herbivores as a whole, with four of the six studies that investigated level of specialization finding that alien plants hosted a greater proportion of generalist to specialist herbivore species. The scale of the study also appears to be an important factor in the level of observed impact. Stand-scale studies, which typically investigated invaders that had become dominant in the system, were far more likely to find that exotic plant species had a negative effect on herbivores than plant-scale studies, which often compared exotic plants to their native congeners. In the latter studies, the local presence of a closely related native plant may have increased the likelihood that associated specialist herbivores also used the introduced plants. Lastly, it is also significant that several studies noted non-native herbivores colonizing the alien plant.
Predators and parasitoids (“natural enemies”) as a whole respond positively to the abundance and diversity of their herbivore prey which, in turn, are related to the abundance and diversity of their host plants. They also rely on vegetation structural and compositional heterogeneity as well as habitat or landscape diversity, which provide a refuge from their own predators along with supplementary food such as nectar, pollen, fungi, and plant fluids. Yet because the effects of plant diversity dampen with increasing trophic level, predators as well as parasitoids are less affected by changes in plant diversity than herbivores. This explains our equivocal findings, where nearly equal numbers reported lower or higher invertebrate richness in invaded habitats. Our results also suggest that their responses are both context- and taxon-specific.
In contrast, detritivores — if they are typically generalists, as has been suggested — may benefit from the dense plant litter produced by many plant invaders. Yet only two of the four studies that we found showed a consistent positive response by detritivores to plant invasions. It may be that detritivores are responding more to invader-mediated changes in habitat structure or disturbance regime. Alternatively, detritivores may be more specialized than commonly thought.
Pollinators, which consume nectar and pollen and include both specialists and generalists, may, like herbivores, be more negatively affected by the decreases in plant diversity commonly associated with plant invasions. Accordingly, we found that pollinator species richness was lower on exotic than native plants in eight of nine identified studies. Multiple reviews and meta-analyses have concluded that the presence of invasive plants has a negative effect on pollinator visitation rates to native species flowering at the same time, lowering the seed set of those native species. Fortunately, the removal of invasive species typically restores pollinator richness to levels found in areas dominated by native plant species.
These results show that plant invaders are negatively impacting invertebrate biodiversity, and that these effects are particularly negative once 70% cover of the invader has been reached. They also indicate that varying responses by different feeding guilds are modulating the overall effect of plant invaders on invertebrate biodiversity, and that this diversity can be recovered through habitat restoration. With this knowledge, we can begin to predict the consequences of plant invasions and restoration efforts by considering both the extent of the plant invasion/restoration, and the effects of the given plant invader on plant diversity and habitat structure.
The full report on this study has been published as a dissertation chapter; journal publication is awaiting revision with additional studies. — by Denise A. Knapp, Karen A. Nikolakakis, and Tom Dudley