Concerns about negative effects of corridors have been raised for decades. A principal concerns is that corridors could facilitate the spread invasive species. The reasoning is that the same principles that support corridors for the benefit of native species could benefit invasive species as well (to the detriment of native species). However, while ample positive effects (e.g., movement, gene flow, population size) of conservation corridors have been found for native species, none have been found for invasives, until recently.
Working in the experimental landscapes of the Savannah River Site Corridor Experiment, SC, USA, we measured how open corridors affect the invasive fire ant, Solenopsis invicta, and assemblages of co-occurring native ants. Fire ants are considered one of the most problematic invasive species in the United States and they are quickly becoming established across the world. An important aspect of the biology of fire ants that is central to this study pertains to the two social forms of fire ants: monogyne and polygyne, defined by the number of reproductive queens per colony. Monogyne colonies contain a single egg-laying queen, whereas polygyne colonies contain multiple egg-laying queens. The two social forms differ in important ways that affect their use of corridors and impact on native co-occurring ant species. First, impacts of the polygyne social form are more severe than those of the monogyne form because polygyne invasions result in very high population densities that can devastate native ants. The impact of monogyne invasions is comparatively less severe. Second, dispersal abilities between the social forms differ drastically. Monogyne queens disperse in mating flights at heights >100 m, and establish spatially independent colonies (sometimes several km away from their natal colony). In contrast, polygyne queens are more limited in their dispersal abilities, typically establishing new colonies within several meters of their natal colony.
Fire ants had invaded all our experimental landscapes. By chance, however, some landscapes were invaded by polygyne and some by monogyne fire ants. Here, luck met opportunity. In landscapes dominated by polygyne fire ants, we found that fire ant abundance was higher and, as result, native ant diversity was lower in habitat patches connected by corridors than in unconnected patches. Conversely, in landscapes dominated by monogyne fire ants, corridors had no effect on fire ant abundance and native ant diversity. This suggests that the high dispersal abilities of monogyne fire ants allowed them to colonize habitat patches regardless of the presence of a corridor and that the low dispersal abilities of polygyne fire ants allowed them to benefit from corridors. Polygyne fire ants dominated recently created landscapes, suggesting that these corridor effects may be transient. We are currently doing a follow-up study to determine this.
This study indicates that corridors can facilitate invasion and it highlights the importance of considering the impact and dispersal abilities of invasive species when assessing corridor utility. The reason that more studies have not found corridors to spread invasive species may be because invasive species often have high dispersal capabilities (like the monogyne social form of fire ants). Since species with strong movement abilities are likely not dispersal limited, corridors are unlikely to help them spread. More studies on species with different dispersal abilities and within different habitats would provide further guidance on when corridors are likely to spread invasive species. By better understanding the positive and negative effects of corridors, we can learn how to maximize positive effects and minimize negative effects.
Resasco J., N.M. Haddad, D. D. Shoemaker, J. L. Orrock, L. A. Brudvig, E. I. Damschen, J. J. Tewksbury, and D. J. Levey. 2014. Landscape corridors can increase invasion by an exotic species and reduce diversity of native species. Ecology 95:2033–2039.