The effect of tree plantation structure and composition on matrix permeability for tropical forest understory birds: Are native tree species always better?

Background/Question/Methods

The effects of tropical rainforest fragmentation on species persistence are largely mediated by matrix permeability and population connectivity across agricultural matrix habitats. Structurally complex agro-ecosystems like tree plantations may improve population connectivity between remnant forest fragments, if structural complexity increases the probability of organisms crossing or persisting in matrix habitats. Evidence that plantation habitat improves population connectivity for animal biodiversity in fragmented areas is mixed, and appears to depend on forest understory structure and canopy composition. The impact of these two factors on population connectivity has not been experimentally examined in tropical tree plantations. In this natural experiment across the landscape of northern Costa Rica, I evaluate how differences in matrix habitat structure and composition affect the movement of forest-dependent birds into wooded matrix habitats. For four forest understory-nesting bird species (Myrmeciza exsul, Henicorhina leucosticte,Thryothorus nigricapillus, and Glyphorhynchus spirurus), I measured boundary softness, a metric of animal movement across forest-matrix boundaries, as a first step in measuring potential population connectivity. Animal movements were compared between tropical forest controls and three forest-adjacent habitats: pastures with varying tree cover, native tree plantations with varying shrub cover, and exotic tree plantations with varying shrub cover. I used playback experiments to assess bird willingness to cross forest boundaries into tree plantations and pastures.  During the breeding season, I measured distance of intrusion into the matrix from the forest edge in response to playbacks. 

Results/Conclusions

My preliminary analysis indicates that, as predicted, all understory bird species were more likely to enter and penetrate further into tree plantations with shrubs, and they entered native and non-native plantations equally.  Entrance into pastures was species-specific: only the species with greater mean foraging heights (T. nigracapillus and G. spirurus) entered pastures.  For all species, entrance distance was positively related to tree and shrub densities. These results imply that management of shrub density within tree plantations might be critical to the movement of understory species, and that even non-native tropical tree plantations may have significant positive effects on the movement of forest birds.