Ecological interactions in altered landscapes

Tropical forests are remarkable and unique not only for their exceptionally high biodiversity, but also the variety and complexity of interactions among the species within these forests. Interactions between species might have asymmetric benefits wherein one species gains while the other loses, such as when predators hunt prey. Other interactions can be mutualistic with benefits for both partners. For example, when insects visit flowers for nectar and pollinate them in the process, both parties benefit. Networks of interactions can have profound impacts on the wider ecosystem, and beyond. For example, fruit-eating birds and mammals dispersing seeds away from parent trees, and rodents and microbes damaging and destroying seeds under parent trees, are thought to explain the high diversity of trees one encounters in tropical forests. Our research investigates how forest degradation and land use change are altering plant-animal interactions, and what implications this might have for forests and ecosystem benefits we derive from them.

Frugivory and seed predation in forest fragments

The fragmentation of tropical forests affects wildlife, but not all species respond the same. Large-bodied species such as hornbills tend to decline in abundance, while smaller-bodied species such as rodents tend to increase in degraded fragments compared to large, intact forests. Similarly, we are finding that not all plant-animal interactions respond the same to fragmentation. For example, visits to fruiting trees by frugivorous birds decrease in smaller and more isolated fragments in the case of certain tree species, but not others. In another set of experiments, the seeds of a few tree species experienced higher levels of seed predation in fragments than intact forests, but certain species with less palatable seeds did not display this pattern. These studies are beginning to indicate how tree dispersal and regeneration are being affected, and which tree species are more at risk of regeneration failure in fragmented rainforests.

Frugivory, seed dispersal and coffee

Coffee farms have the potential to attract a wide variety of fruit-eating wildlife. These include mammals such as civets that feed on coffee berries, as well as several bird and mammal species foraging in farms that maintain figs and other fruiting species as shade trees. Our research is finding that an abundant and diverse suite of frugivorous birds forage in native fruiting trees on coffee farms, although there are a few significant absences such as the Yellow-browed Bulbul – a forest-specialist species that is endemic to the Western Ghats. This research partly motivates our restoration efforts that aim to leverage coffee farms as seed sources for native tree species, while promoting and diversifying native shade trees on farms. Another line of research focuses on frugivory and seed dispersal of coffee. We are studying which mammals consume coffee and disperse its seeds, whether other tree species get dispersed in the process, and what impacts such dispersal can have on forests in coffee-growing landscapes.

Nocturnal insects and coffee pollination

Like many crops, coffee yields improve when insects cross-pollinate their flowers. Bees and other diurnal insects are recognized as important pollinators of coffee, and studies have shown that insect activity and pollination are enhanced when there are forest patches nearby. At the same time, we know far less about how nocturnal insects – mainly moths and beetles – are faring, and whether they contribute to coffee pollination. We have initiated studies that aim to document nocturnal insects across seasons and years in rainforests and coffee farms at a long-term monitoring site in the central Western Ghats. We are also evaluating the role of nocturnal insects in coffee pollination by conducting experiments that exclude insect visitors to coffee flowers at different times of the day. Preliminary results suggest that coffee farms have fewer nocturnal insects than the rainforest, and that many varieties of nocturnal insects do visit coffee flowers, but we so far see no clear evidence of coffee pollination by nocturnal insects.

Collaborators

Mahesh Sankaran

Smitha Krishnan

Meghna Krishnadas

Alumni: Abhishek Gopal, Rama Narayanan, Sanath R M