Despite the positive conservation impact of landscape corridors, concerns persist that these corridors can have unintended negative effects for animals.
Conservation corridors, areas of land that connect wildlife habitats that have been separated by human activity, are among the most popular tools for promoting biodiversity conservation, as they enable dispersal of native plants and animals and help maintain species richness. However, there is concern that these corridors may also have potential negative impacts that could outweigh their benefits.
This paper, published in Conservation Biology, reports on a study evaluating the prevalence of five potential negative effects of conservation corridors: dispersal of species antagonistic to conservation targets (e.g. predators or pathogens); enhancement of negative edge effects due to the creation of long and narrow corridors; spread and increased abundance of invasive species; spread of disturbances such as fires; and synchronization of population dynamics in connected patches, which increases the likelihood of simultaneous extinction.
The authors conducted a literature search for studies of corridor effects on dispersal, populations, or communities within patches connected by corridors. After applying additional strict criteria, they identified 33 studies that investigated one or more corridor effects. For each study they recorded the number of species studied and numbers of reported positive, negative, or neutral effects. They also conducted a meta-analysis on negative effects for categories with sufficient numbers of studies.
Results include the following:
- Antagonist effects were addressed in 17 papers. The number of studies showing negative effects was about half the number showing no effect, and two studies showed that corridors had positive effects. Meta-analysis showed that across all studies, effect sizes were not distinguishable from zero.
- Edge effects were investigated in 17 studies, some of which identified both negative and positive effects for different species. Overall, 8 papers identified negative effects, 6 identified positive effects, and 8 showed no effect. In total, 36 species were studied, 10 of which had negative responses, 11 of which had positive responses, and 15 of which had no responses. Meta-analysis confirmed the relatively equal division of positive and negative responses.
- Invasions of toxic species were investigated in 6 studies, none of which found evidence of negative corridor effects. The authors note that non-native species likely either remained where they were or spread regardless of corridors.
- Only one study investigated spread of disturbance by examining corridor effects on spread of fire. The study found that corridors increased fire intensity, which was actually beneficial because it accelerated restoration by promoting warm-season bunch grasses.
- Effects of corridors on population synchronization were investigated in 5 studies, 3 of which showed that corridors synchronize population dynamics and 2 of which showed no effects of corridors on synchronization.
In their discussion the authors state that “we found no consistently negative effect of corridors.” They note that their review showed no negative effects from increasing invasions or disturbances and showed mixed evidence for other effects. They further state that “the only negative effect of corridors that clearly reduces the population size or persistence of target species is the creation of edge, and even this effect is negative in only a fraction of cases,” and they suggest that such effects can be mitigated by “creating wider corridors or reducing contrast between corridors and surrounding matrix.”
While the authors conclude that “the weight of existing empirical evidence continues to show that the potential costs of corridors are outweighed substantially by their conservation benefits,” they also call strongly for additional studies on the subject, as their review identified a surprisingly small number. Additionally, many of the identified studies used small sample sizes, and nearly 80% took place at the same site, the Savannah River Site Corridor Experiment. The authors recommend additional studies on all five potential negative effects and in particular on effects from antagonists and edge creation, as well as larger population-level and long-term studies.
For advocates working on biodiversity conservation, the paper offers a positive, though tentative, endorsem*nt of the use of corridors for increasing biodiversity. While the current evidence indicates that corridors do more good than harm for target species, that evidence is fairly thin. Advocates in positions to do so may consider undertaking studies on the effects of corridors to add to these limited findings.
