Hunting wildlife changes tropical forests
Over-hunting in tropical forests changes the forest itself, say Rhett D. Harrison, Sylvester Tan, Joshua B. Plotkin, Ferry Slik, Matteo Detto, Tania Brenes, Akira Itoh and Stuart J. Davies
People have hunted in tropical forests ever since we have been defined as human, but with modern weapons hunting is rarely sustainable.
Our study in Lambir Hills National Park in Sarawak, Malaysia, has shown that intense hunting is driving substantial changes in the spatial structure and dynamics of the tree species in the park, and causing a constant decline in local tree diversity. To maintain critical ecosystem functions in tropical forests such as this, we recommend that increased efforts be made to protect and restore wildlife.
Hunting in tropical forests is indiscriminate and, beyond a concern for the future of many hunted species, understanding the functional consequences of declines in wildlife populations is important for the long-term conservation of tropical biodiversity.
But this isn’t easy because quarry includes species involved in many different ecological processes, including vertebrate predators, seed dispersers, seed predators and herbivores. Hence, it is difficult to deduce what the consequences of hunting might be for a tropical forest and, unsurprisingly, there is still considerable debate about what impact hunting has on ecosystem functions and the services they provide.
To help fill this gap, at least in part, we investigated the impact that hunting had on trees in Lambir Hills. In terms of the richness of tree species, the park is home to the most diverse forests known. Moreover, in 1984, when Lambir’s vertebrates were first surveyed, the park supported a complete fauna, including several rare and hunting-sensitive species.
However, in 1987 a road through the park was completed and in the early 1990s local urban bushmeat markets expanded dramatically.
By 1994, when the next surveys were conducted, one species of hornbill had been extirpated, that is, no longer found in the area, and many other species were rare.
By 1999, all large animals had either been extirpated or were extremely scarce. Over 20% of the larger (>1 kg) mammals and over 50% of the larger bird species had been locally extirpated. Amongst these, almost all important large frugivores, including gibbons, six of seven hornbill species, and imperial pigeons no longer occurred in the park; langurs, which are important seed predators, were extirpated around 1998; and wild pigs, which are extremely important seed and seedling predators, had also become very rare. The list goes on.
In 1992, a 52 hectare, ‘large-scale ecological dynamics’ plot was established in the park. This approximately coincided with the start of intense hunting and hence we were able to monitor changes in forest structure and composition caused by the hunting.
But will take several plant generations, potentially a century or more, before the full impact of defaunation at Lambir is realised. Hence, the 15-year period of our study corresponds to the initial impact of defaunation.
In the study, we found that during the same period that substantial proportions of the park’s mammals and larger birds were extirpated, the tree community changed profoundly.
Specifically, there was a large increase in the total number of stems, increased clustering among saplings of species with animal-dispersed seeds, a relative decline in recruitment rates among tree species with animal-dispersed seeds compared to those with abiotic dispersal mechanisms, and a reduction in local diversity across the plot.
These findings are supported by a series of predictions based on ecological theory and, hence, suggest that the results are relevant to a large proportion of the tropics.
We also considered the potential role of other factors, such as climate change or past large-scale disturbance. However, the decline in large vertebrates turned out to be the most likely factor causing the changes.
Over-hunting is causing declines in wildlife throughout tropical Asia and in most other tropical areas with high human populations and a relatively low proportion of remaining forest, including West Africa and the Atlantic forests of Brazil. Moreover, as roads open up other areas of tropical forest we can expect over-hunting to become an increasing problem elsewhere, such as in the Amazon or Congo basins, unless the process is countered by strong conservation measures.
To obtain a general understanding of the impact of over-hunting on tropical forests, long-term studies such as ours should be replicated in other forests, in different regions and with different hunting histories.
Our study concludes that ‘… vast areas of tropical forest, both inside and outside protected areas, are depleted of wildlife as a consequence of over-hunting. Our results strongly suggest this will have important consequences for ecological processes that structure plant diversity. Enhancing the protection of wildlife and restoring animal populations where they have been depleted will be essential for biodiversity conservation in tropical forest landscapes’.
Edited by Robert Finlayson
Read the article
Harrison RD, Tan S, Plotkin JB, Slik F, Detto M, Brenes T, Itoh A, Davies SJ. 2013. Consequences of defaunation for a tropical tree community. Ecology Letters. Available from http://dx.doi.org/10.1111/ele.12102.
This work is related to the CGIAR Research Program on Forests, Trees and Agroforestry