Human activities, such as agriculture, have broken up forests into smaller fragments. A danger of such fragmentation is that it may hinder the ability of critters living within the forest to move between sections.
This hinderance may limit their ability to obtain food; it may also place them at risk of extinction. This is especially true of critters that rarely venture out of the forest.
Cameron Gillies and Colleen St. Clair (University of Alberta, Canada) have investigated this threat for two bird species in tropical forests. They have shown that water-rich native forest corridors between forest fragments can aid the transit of birds which solely live within the forest.
The study area.
The scientists chose northwest Costa Rica as their study area. Here, agriculture and ranching has destroyed 75% of the native tropical forest landscape.
Consequently, very little of the native landscape remains. What does remain is patches of native riparian (water-rich) forest, and rows of trees (fencerows) that serve as pasture boundaries.
It is unknown whether many animal species utilize these forest corridors, or which one (riparian or fencerows) is more effective. Demonstrating such utilization would help encourage environmentalists and farmers to incorporate them into regional planning.
The birds.
The scientists studied two bird species in this work. Both are common birds, both eat insects, and both are territorial; i.e., they hold a specific territory year-round.
However, Barred Antshrikes (Thamnophilus doliatus) are found only in intact forests. Conversely, Rufous-naped Wrens (Campylorhynchus rifinucha) are found in both intact and fragmented forests.
This difference enabled the scientists to test the ability of forest specialist (antshrikes) and forest specialist (wrens) birds to return home, given different direct transit corridors.
Measuring bird transit.
The scientists captured and moved 30 birds of both species, inducing them to move back home (over a distance of 0.7 - 1.9 kilometers, well beyond their normal range). All birds were released in either riparian forest or fencerow habitat.
The difference was in the directly intervening habitat, that between their home and the release site. The directly intervening habitat was either pasture (24% tree cover), fencerows (15-30 meters wide, 31% tree cover), or riparian forest (50 - 150 meters wide, 40% tree cover).
Radio transmitters were used to track the specific trajectory of each bird back home. Birds were tracked for 10 days or until they arrived home, whichever as faster. The season (wet or dry) did not impact the observed results.
Upon release, both bird species made initial forays into their new surroundings, until an acceptable habitat and trajectory was found. This is similar to how other animals move prior to dispersal, such as squiirels, butterflies, and homing pigeons.
Return trajectory and habitat.
The two bird species exhibited different return trajectories, when faced with non-native habitat.
Eighty percent of the antshrikes who faced a fencerow or pasture direct path home used an indirect native forest trajectory for the majority of their return trip, and crossed half as many forest gaps as wrens. None of the antshrikes used fencerows for the majority of their return trajectory.
In contrast, only 1 out of the 13 wrens who faced a fencerow or pasture direct path home utilized an indirect native forest trajectory for the majority of their return trip.
This means that antshrikes (forest specialists) used a native forest trajectory to return home, even though this was an indirect route, and crossed fewer forest gaps as wrens. In contrast, wrens (forest generalists) used the direct trajectory to return home, even if it was non-native habitat.
Success and time of return.
The two bird species exhibited different return probabilities and times, when faced with non-native habitat.
Antshrikes were half as likely to return within ten days if the directly intervening habitat was pasture instead of native forest. The average time of return was 34 daylight hours, with less time required over direct native forest habitat.
In contrast, wrens returned with equal probability and time (26 daylight hours), regardless of the directly intervening habitat.
This means that antshrikes (forest specialists) were more likely to return home, and did so more quickly, when faced with direct intervening native forest habitat. In contrast, wrens (forest generalists) successfully found their way home, in equal time, no matter what the direct intervening habitat.
Both species were less likely to find their way home when the travel distance increased. They did not die; instead, they typically settled into a new territory when they could not find their way home.
Forest specialists depend more on native forest than forest generalists.
This data clearly demonstrates that antshrikes (forest specialists) depend on intact native forest habitat far more than wrens (forest generalists). Possible reasons for this discrepancy may be that forest specialists tend to avoid open areas that may expose them to attack by predators, and their greater need for quality habitat.
What this means is that forest specialists are more susceptible to extinction in fragmented habitats than forest generalists. This emphasizes the need to preserve as much contiguous native habitat as possible.
for more information:
Gillies, C. S.; St. Clair, C. C.
Riparian corridors enhance movement of a forest specialist bird
in fragmented tropical forest.
Proc. Natl. Acad. Sci. USA 2008, 105, 19774-19779.