Plant-bird interactions respond stronger to fragmentation at high than at low elevations

At 1000 m a.s.l., a paradise tanager (Tangara chilensis) consumes fruits of Myrsine coriacea.

By Marta Quitián, Vinicio Santillán, Carlos Iván Espinosa, Jürgen Homeier, Katrin Böhning-Gaese, Matthias Schleuning and Eike Lena Neuschulz 

Tropical mountain forests are one of the most diverse ecosystems of the world. However, many species living in mountain forests are affected by human land use, for instance by forest fragmentation, and are threatened by climate change. These drivers may cause disruptions of important ecosystem functions, such as the dispersal of plant seeds by animals. But, do effects of forest fragmentation on seed dispersal interactions differ along elevational gradients? And are lowland plant and bird communities more or less affected by forest fragmentation than highland communities?

The Podocarpus National Park and San Francisco Reserve in southern Ecuador cover an area of 147 280 ha undisturbed mountain rainforest, ranging from ca 1000 to 3500 m a.s.l.

Outside of the protected area, only forest fragments, mostly embedded in pastures, remain​.

In this study, we analyzed the effects of elevation and forest fragmentation on plant-frugivore interaction networks from the plant and from the bird perspective. Over a period of two years, we recorded seed removal by frugivorous birds in continuous and fragmented forests at 1000, 2000 and 3000 m a.s.l. in southern Ecuador.

We found that the number of effective bird partners of plants generally decreased with increasing elevation, indicating a decline in the redundancy of avian seed dispersers. Furthermore, bird specialization on specific plant partners increased towards high elevations, meaning that birds in the highlands partitioned their foraging plants more than those in the lowlands. Fragmentation had a relatively weak effect on interaction networks. However, we found that bird specialization significantly increased in fragmented forests at high elevations, indicating a stronger effect of fragmentation on plant-frugivore interactions at high compared to low elevations. We explain this pattern by the smaller bird species pool at high elevations, while the size of the plant species pool remained constant.

Marta Quitián and Vinicio Santillán observing birds on one of the few sunny days in the montane rain forest. The high elevation forests at 3000 m above sea level is almost permanently hidden by cloud caps.

Our study highlights that the vulnerability of seed-dispersal interactions to human disturbance is contingent on the properties of the ecological community, such as the local species pool and the ratio between consumer and resource species richness. Our findings also indicate that forest conservation efforts should aim at maintaining the consumer diversity in high-elevation forests because these systems are particularly vulnerable to human impact and are crucially important for the response of mountain biodiversity to climate change.

Comments