Connectivity by design: Enhancing functional connectivity for forest-dependent birds in tropical agroecosystems

Background/Question/Methods

Fragmentation of tropical landscapes often drives biodiversity loss in the tropics. Consequently, the conservation value of agricultural lands has received much attention recently. Studies in Mesoamerica have found that live fences and agroforests, which enhance structural complexity of habitat, can provide support for birds that would otherwise be locally extirpated. Four years of mist netting data collected at the CATIE center in Costa Rica suggest that complex agroforests and live fences foster farm-to watershed-scale habitat connectivity for forest-dependent birds. However, two questions have fueled debate regarding conservation value of tropical agricultural lands: (1) What is the optimal configuration of patches and corridors (e.g., agroforests and live fences) to facilitate habitat connectivity for forest-dependent bird species; and (2) Do these configurations differ from those needed to support disturbance-tolerant species (e.g. birds that thrive in open fields and plantation edges)? We modeled functional connectivity for six bird species with various levels of forest-dependency across 2000 ha of farmland and remnant forest patches. Using circuit theory, we calculated resistance values (based on mist netting data) to describe difficulty of movement for focal bird species. We modeled four agroecosystem configurations, altering the type of agroforests and number of live fences found between forest patches.

Results/Conclusions

Increasing the number of multistory, shade coffee plots and complex live fences enhanced functional connectivity for five of the six bird species; this was indicated by decreasing cumulative resistance values within the study area. Of the six bird species, the forest-dependent White-ruffed Manakin (Corapipo altera), experienced the greatest increase in functional connectivity with additional agroforests. Additionally, live fences oriented perpendicular to forest patches had higher conductance values than live fences oriented parallel to the same patches. In complex networks of corridors, orientation significantly influences the contribution of corridor segments (e.g., live fences bordering one side of a field) to functional habitat connectivity; adding corridor segments alters the cumulative resistance experienced by birds moving through the agroecosystem. In contrast, increasing the presence of complex live fences and shade coffee in our agroecosystem reduced functional habitat connectivity for the disturbance tolerant bird species, the Gray-crowned Yellowthroat (Geothlypis poliocephala). These findings emphasize the importance of corridor orientation and arrangement within agroecosystems to enhance functional connectivity for bird species of conservation concern. This circuit theory modeling approach can be used to inform strategies for re-incorporating trees into agroecosystems, such as planting placement for third party certifications that require increased tree cover.