A meta-analytic review of corridor effectiveness, Gilbert-Norton et al. 2010

Habitat fragmentation, a frequent consequence of habitat loss, is a primary threat to populations and species because isolated subpopulations are expected to experience reduced population viability and ultimately greater risk of extinction. Colonization and gene flow between habitat patches, however, can mitigate these effects [Gilbert-Norton 2010: 661].

This meta-analysis, consisting of 78 experiments from 35 studies, asked the question: Do ecological corridors increase movement between habitat patches, and how does that differ among taxa? The study’s results answer the first part of the research question affirmatively: “There was approximately 50% more movement between habitat patches connected by a corridor than between isolated habitat patches” [Gilbert-Norton 2010: 665].

Furthermore, corridors increase movement for all taxa. “Most corridors are created for terrestrial vertebrates, including birds, although our data suggest that invertebrates and plants also benefit from corridors” [Gilbert-Norton 2010: 665]. This study found that corridors work equally well for all taxa except birds, for whom the corridors were used less; however, birds still favored corridors compared to surrounding matrix.

While three quarters of the experiments showed corridors to be more effective for movement compared to the matrix landscape, 23% of experiments showed corridors were less effective. The authors suggest several explanations for this result. It’s possible that the “matrix habitat has been misclassified as nonhabitat for a study organism” [Gilbert-Norton 2010: 665], that the habitat quality of the corridor is not particularly high, or that the corridor is difficult to locate, given its small size compared to surrounding landscape. Furthermore, use of corridors varies by species.

That almost a quarter of the studies showed organisms used matrix habitat rather than corridors to move between habitat patches furthers the idea that although corridors may be used by many species, they are unlikely to be used by all species, and whether corridors are relevant for land managers may depend on the species of interest [Gilbert-Norton 2010: 665].

The authors also observed that organisms showed greater use of natural corridors (those existing prior to the study) compared to those created and maintained for the study. The real-world applicability of this, as the authors note, is that “it may be better to protect natural landscape features that function as corridors rather than attempting to create corridors” [Gilbert-Norton 2010: 667]. This highlights the importance of protecting natural or semi-natural lands from development.