Can Forest Fragmentation Benefit Tropical Birds?

Study challenges conventional wisdom on forest fragmentation
A recent study finds that forest fragmentation in Kenyan tropical cloud forest decreased nest predation of the white starred robin, Pogonocichla stellata, a native bird. Potential predators included mammals, snakes, and other birds. The study authors from the University of Ghent and the National Museum of Kenya found that White starred robin. Image credit, Alan Manson.predation decreased for nests closer to the forest edge.

The findings run counter to the prevailing logic in conservation science that increased fragmentation of habitat negatively affects native bird species by increasing nest predation. Past studies have supported the idea of the human influenced edge effect in which fragmentation creates larger edges with non-forest areas leading to greater predation by species that thrive in disturbance. The study authors conclude,

"Inverse edge effects on nest predation might be more common than previously believed, and may drive population dynamics of a broad array of forest species, especially in the Afrotropics, but also elsewhere. The possibility of inverse edge effects should therefore be carefully considered in avian population studies and when drafting conservation plans."

Bolstering their case is the fact that the study authors looked at live nests while past studies have largely looked at artificial nests - a method questioned by scientists for various reasons (e.g. aritificial nests often use chicken eggs which smaller predators are not able to eat).

Findings cause a stir. Scientists praise study... but with some reservations.
The study has caused a bit of a stir in the conservation community. After the findings were published in the Journal Animal Conservation, scientists authored three different commentaries in the same journal trying to put the study into perspective. The commentaries praised the importance of the study and the live-nest data. However, they also raised some questions, including the question of generalizablitity. Is this an unusual case in which a bird benefits from fragmentation, or is this indicative of more widespread phenomenon that will cause us to rethink our assumptions?

Douglas Robinson, a scientist at Oregon State University, states in his commentary,

"Acquiring data of the type Spanhove and colleagues have contributed is a huge, but worthwhile challenge. As we continue to move beyond documenting declines of species richness in fragmented landscapes and we venture into rigorous investigation of mechanisms causing population declines and extinctions, we will need to work together, use similar methods, and report results in standardized ways. Then, we will have a fighting chance to place all this variation in context, to see if we have more than a collection of case studies describing the range of variation, and to transform our understanding of why birds respond to tropical forest fragmentation in the ways that they do."

David Lahti's commentary, Why We Have Been Unable to Generalize About Bird Nest Predation takes the view that this study along with past nest predation research has eluded generalizablity because they have failed to adequately consider the role of the predator and it's interaction with the landscape and habitat. He states,

"Thus, perhaps the main reason why nest predation has eluded generalization is because researchers continue to focus on factors that are easier to measure (landscape, habitat, nest site and nest characteristics), while casting just a passing glance at factors that are more directly related to the process of nest predation (predator identity and behavior). The dynamics that influence nest predation are not really a particular kind of landscape or habitat or nest site per se, but rather an interaction between these things and the behavior of the particular nest predators that are active in a certain area. Wherever an interaction is driving a system, one term of the interaction commonly does not achieve significance by itself. Therefore, we should not expect to see universal edge or other fragmentation effects on nest predation, even if habitat fragmentation generally plays an important role."

Source:	Animal Conservation
Title:	Forest fragmentation relaxes natural nest predation in an Afromonte forest
Authors:	a, b) Toon Spanhove, a,b) Valerie Lehouck, b) P. Boets and b) Luc Lens
	a) National Museum of Kenya b) Ghent University, Belgium