Spreading messages about invasives


Amanda D. Rodewald, School of Environment & Natural Resources, The Ohio State University, Columbus, OH 43210, USA.
E-mail: rodewald.1@osu.edu


Although control of invasive species remains a common part of ecological restoration efforts, there is a growing dialogue within scientific and conservation communities regarding positive influences of invaders and potential negative consequences of their removal. As one example, a recent Diversity & Distributions article cautions that removal of exotic and invasive honeysuckle (Lonicera spp.) may negatively affect populations of frugivorous birds and, therefore, may have undesirable ecological outcomes. In response, I share several insights from research in my lab on bird-honeysuckle interactions that show how honeysuckle disproportionately impacts birds of conservation concern and acts as an ecological trap even for generalist species. Although there is a real need to fully consider both positive and negative consequences of invasive species, if such research is not placed within the proper ecological context, we risk sending distorted or mixed messages to managers.

In their recent paper, Gleditsch & Carlo (2011) caution that removal of honeysuckle (Lonicera spp.), a genera of exotic and invasive woody shrubs now common in eastern North America, has the potential to negatively affect populations of frugivorous birds and, thereby, may have undesirable ecological and economic consequences. This research has been featured in the media under headlines such as, ‘Invasive plants can create positive ecological change’ (Penn State 2011) and ‘Birds loving honeysuckle invaders’ (Wall, 2011) and includes quotes from authors about honeysuckle’s contribution to restoring native biodiversity. The article and associated press coverage also prompted several managers of parks and natural areas in Ohio to contact me as they questioned their local efforts to control invasive plants. This reconsideration of management approaches appears to stem from comparatively light coverage and acknowledgement of the negative consequences of honeysuckle to native plants (Gould & Gorchov, 2000; Collier et al., 2002; Gorchov & Trisel, 2003; Miller & Gorchov, 2004), animals (Schmidt & Whelan, 1999; Watling et al., 2011), ecological services such as pollination (McKinney & Goodell, 2010), and even human vector-borne disease (Allan et al., 2010).

Because the article focused on birds, the intent of my letter is to share several findings from a decade of research on bird–honeysuckle interactions that may add a different perspective.

  • 1 Honeysuckle is associated with reduced numbers and lower productivity of some sensitive insectivorous birds that are more specialized in their habitat requirements and of regional conservation importance (Rodewald, in press; A.D. Rodewald unpublished data). One such example is the Acadian flycatcher (Empidonax virescens), which is a Neotropical migratory bird that avoids areas with dense honeysuckle (Bakermans & Rodewald, 2006).
  • 2 Although honeysuckle can promote high densities of certain avian species, birds that purportedly benefit tend to be generalists that are well adapted to use a wide variety of habitats within human-dominated landscapes (Leston & Rodewald, 2006; McCusker et al., 2010; Rodewald, in press). As a group, these birds are assigned low-priority status from conservation groups, such as Partners-In-Flight and North American Bird Conservation Initiative. Thus, from a conservation perspective, the native species benefited by honeysuckle are of lower priority than those species that may be negatively impacted.
  • 3 Even for those species that would seem to benefit, honeysuckle can act as an early-season ecological trap, such that abundance of birds and/or their demonstrated preferences for honeysuckle are not indicative of habitat quality. Previous research shows that birds actively selecting honeysuckle as a nesting substrate have lower annual reproductive output (i.e. numbers of young fledged) relative to birds using native plants for nesting (Rodewald et al., 2010). In fact, Northern cardinals (Cardinalis cardinalis) that choose to nest in honeysuckle for their first breeding attempt produce 20% fewer young over the entire breeding season compared with birds nesting in other substrates. The ecological trap results from the fact that honeysuckle increases risk of nest predation in understorey-nesting birds, particularly early in the breeding season (Schmidt & Whelan, 1999; Borgmann & Rodewald, 2004; Rodewald et al., 2010).
  • 4 Honeysuckle increases risk of brood parasitism by brown-headed cowbirds (Molothrus ater) for Acadian flycatchers, which are then unable to fledge their own young (Rodewald, 2009). Probably for this reason, cover by honeysuckle is the best predictor of, and negatively related to, annual reproduction by Acadian flycatchers (Rodewald, in press).
  • 5 Honeysuckle alters the selective environment in ways that may affect microevolutionary processes and sexually selected ornamental traits. For example, honeysuckle fruit likely contributes to the reduced usefulness of plumage brightness as a signal of male quality for cardinals in urban forests (Jones et al., 2010). Moreover, honeysuckle seems to create an evolutionary trap in rural landscapes such that the brightest cardinals that are in the best physical condition, secure the most highly preferred territories (i.e. those with dense honeysuckle), and breed earliest in the season ultimately produce the fewest young because of the high risk of nest predation in honeysuckle for early season nests (Rodewald et al., in press).

I am pleased to see that researchers are investigating unexpected consequences and possible ecological benefits of exotics and invasives rather than adhering blindly to the popular and narrow view that impacts are exclusively negative (Davis et al., 2011). I also agree entirely the point made by Gleditsch and Carlo and others (McCusker et al., 2010) that we need to better understand and carefully consider direct and indirect consequences of any restoration efforts involving removal of invasive plants. At the same time, as scientists, we need to set the appropriate ecological context for that point, especially when we are relying upon small sample sizes from limited spatial and temporal extents. Otherwise, we may inadvertently send distorted messages to conservation and management communities.


I thank T. Robison, J. McCormac and P. Rodewald for stimulating dialogue and helpful comments on this letter. I am grateful for the hard work of many talented students, field assistants and volunteers that have contributed to my research on plant–animal interactions. Research mentioned in this letter was supported by National Science Foundation (DEB-0340879, DEB-0639429 and REU Supplements to A.D. Rodewald), the Ohio Division of Wildlife and US Fish and Wildlife Service through the State Wildlife Grants Program and The Ohio State University.


Amanda Rodewald is a professor who studies responses of animal populations and communities to human activities at local and landscape scales. Her current work focuses heavily on understanding causes and consequences of altered species interactions because of anthropogenic disturbance, invasive species and resource subsidies.

Editor: Marcel Rejmanek