Better forest 'simply' means more frugivores

Posted by Stu
 
We think our paper, just published in Forest Ecology & Management, has quite an important message, and one that should be encouraging, even in an archipelago with so little forest remaining and so many threatened species.

The study was part of Carmela Espanola’s PhD here at MMU and supervised by Nigel Collar, Aldrin Mallari and myself. She and her team walked around 500 km of transects, at 24 sites on the Philippine island of Luzon, looking for large avian frugivores. The target species included large pigeons, hornbills and parrots – birds crucial to proper functioning of forests in the tropics. If these seed dispersing birds disappear or become ‘functionally extinct’ then we might expect serious consequences for the health of forests.

The Imperial pigeons Ducula are key seed dispersers in southeast Asia's forests (Photo: Christine E. Telesforo)

What we did was to relate the presence of the different species to a series of habitat predictors, recorded at over 1,200 plots along the transects – these included things like altitude, canopy cover, and other indications of the quality of the forest. Basically, we asked what the difference was between plots that contained the frugivore species and those where the species was not found.

Survival for species like this Luzon Bleeding-heart is not just a matter of finding suitable habitat, but also avoiding hunting or capture (Photo: Carmela Espanola)

Our first finding was not so surprising - Frugivore species richness was highest in forest with large-girthed trees. But more interesting was that some small-scale agricultural disturbance was tolerated or even favoured by some species. World birders will know well this pattern – low intensity use of forests, which could be a little logging, or especially the creation of small mixed gardens within the forest, can be very good spots to see frugivorous birds. Importantly, our study showed that frugivore richness was highest in forests on flat ground, areas which are usually the first to be converted to agriculture. 

White-eared Brown-dove can tolerate seriously degraded forest, but still prefers good forest on Luzon (Photo: Pablo Espanola)

But I think the most important aspect of the study was our attempt to find non-linearities and other complexities in frugivore presence and richness responses along the habitat quality gradient. I, for one, felt sure that we would find some places along the disturbance gradient where there would be sudden rises in frugivore abundance – and, that in turn, management could be targeted at these points to gain disproportionate benefits for wildlife.

The forest disturbance gradient - importantly, we found linear increases in frugivore richness along this gradient.

We did not find that – rather, the probability of occurrence increased linearly along the forest quality/restoration gradient. While the precise benefits in terms of seed dispersal, and costs of management, at different points along the quality/restoration gradient are likely to be themselves complex, avian frugivores benefit proportionately from step improvements right along the gradient. Thus, any actions to improve forest quality on Luzon, from reforesting the most degraded lands to preventing degradation of relatively healthy forests, are likely to benefit frugivores. 

Carmela's PhD was funded by Loro Parque Fundacion. She now lectures at University of the Philippines.