Edge effects in tropical dry forests of Madagascar: additivity or synergy?

• Published in: Landscape ecology Vol. 32; no. 2; pp. 327 - 341
• Main Authors: Malcolm, Jay R., Valenta, Kim, Lehman, Shawn M.
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01-02-2017; Springer Nature B.V
• Subjects: Biomedical and Life Sciences; Dry forests; Ecology; Environmental Management; Forests; Habitat fragmentation; Land degradation; Landscape Ecology; Landscape/Regional and Urban Planning; Life Sciences; Nature Conservation; Research Article; Sustainable Development; Tropical forests; Madagascar; ISW, Indian Ocean, Madagascar; PSE, New Zealand; Forest fragmentation; Additive edge effects; Synergy between area and edge effects; Edge effects; Tropical dry forest; Madagascar; Species–area relationships; Area effects
• ISSN: 0921-2973; 1572-9761
• DOI: 10.1007/s10980-016-0453-z

Context
The study of habitat fragmentation is complex because multiple, potentially synergistic, ecological processes may be acting simultaneously. Further, edge effects themselves may be complex in that additivity from multiple edges can give rise to heterogeneous nearest–edge gradients.
Objectives
We used heat diffusion as a proxy for additive edge effects in two study landscapes in order to test whether two key observations recently attributed to synergy between edge and area effects could be more simply explained by additivity; namely, steeper edge gradients in larger fragments and variation in slopes of species–area relationships as a function of distances to fragment edges.
Methods
We sampled forest structure in northwestern Madagascar at various distances from the edge in fragments and continuous forest and used an inverse modelling approach to parameterize the model. In addition, we applied the model to data from a published study of beetle communities in fragmented forests in New Zealand.
Results
With increasing proximity to edges, woody stem densities decreased and, as predicted, smaller fragments had lower stem densities and less steep edge gradients than larger ones. The model successfully predicted shifts in species–area relationships as a function of nearest–edge distances for beetle species, although observed richness for forest specialists in the smallest fragments was lower than predicted.
Conclusions
Two key observations attributed to synergy between edge and area effects were explained by edge additivity. The model is particularly useful in that it can help to disentangle the complex sets of processes acting in fragmented landscapes.