Disturbance In Tropical Forest Ecosystems: Perspectives From East Africa
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Disturbance causes the rapid death of vegetal biomass and major shifts in the structure of biological communities. It can play an important role in the maintenance of biodiversity, but human activities have altered disturbance regimes and threaten the integrity of entire ecosystems, especially of tropical forests. I studied the effects of human-aided disturbance in a poorly known habitat: the montane forests of East Africa. In a large scale survey covering three forests in a region of >40,000 km2, I observed the effects of subsistence activities (wood collection) of nomadic herders. I showed that birds respond individualistically to small scale human disturbance. Even among forest specialists, not all species are negatively affected by disturbance. Therefore, disturbance could be conceived more as a management tool than as a factor inevitably threatening the diversity of tropical forest bird communities. Then, I developed this hypothesis in a large scale experiment where I created 12-m radius canopy gaps in an old growth forest. This is currently the only long-term experimental study of canopy perforation and internal forest edges in a tropical forest. Results show that small canopy gaps support higher abundance, higher species richness and different species composition of forest specialist birds. Therefore, canopy gaps are important to maintain the diversity of forest specialist birds. Second, I showed that small-sized gaps generate extensive edge effects that affect vegetation and abundance of animals on an area eight times larger than the size of the gaps. Thus, small changes in the frequency of gap formation could potentially affect disproportionately large amounts of habitat even in unfragmented forests. Finally, I moved to the Eastern Usambara mountain of Tanzania, one of the biodiversity hotspots of Africa. Here, selective logging, ended in 1985, facilitated the spread of the introduced tree Maesopsis eminii (Rhamnaceae). I found that heavily invaded tracts of forest have much lower abundances of rare and threatened birds, while numerically common species are not affected. Maesopsis is one of the few cases of invasive trees in a mainland tropical forest. This species could become one of the most damaging invasive plants of the world.