Tropical rain forests are home to most of Earth’s biodiversity. It is therefore paramount to understand the potential impacts human disturbances in the tropics have on biodiversity. Gibson et al. (2011) combine 138 studies to create a global meta-analysis on the impact of human disturbances on tropical biodiversity. The effect of humans differs throughout the world, but is present everywhere. Southeast Asian taxa show the largest sensitivity to human development, while birds are the hardest hit taxonomic group. However, not all species are negatively affected. Some mammals, specifically small mammals, benefited from human disturbances, and all mammals had a considerably lower sensitivity than birds. The resounding findings in this paper are the desperate need for more research to be done, especially in African tropics, as well as the need for conservation of primary forests. Secondary forests were potential habitats for biodiversity conservation, but this paper shows that secondary forests have substantially lower amounts of biodiversity than primary forests. Gibson et al. produce the first meta-analysis of biodiversity in the tropics, and reveal the great need for conservation efforts and for more research.—Mathew Harreld
Gibson L., Lee T.M., Koh L.P., Brook B.W., Gardner T.A., Barlow J., Peres C.A., Bradshaw C.J.A., Laurance W.F., Lovejoy T.E. 2011. Primary forests are irreplaceable for sustaining tropical biodiversity. Nature 478, 378–381.
The preservation of primary tropical forests is key to the sustainability of biodiversity, and the dramatic increase of human development in tropical forests could have a huge impact on biodiversity. As more studies are done on the effects on biodiversity from forest degradation, we might be able to find some answers as to how to stop them. Current literature shows varied results depending on the type of impacts studied, most often reporting studies of a specific response in a specific region. Gibson et al. attempt to piece together the puzzle of biodiversity in the tropics by analyzing 138 studies. Their goal to deliver a better overall understanding of what is causing biodiversity to change and how it is changing.
The 138 studies that were used in this study span the globe, with a focus on Central America and Southeast Asia. It is therefore important to keep in mind that the current data may be biased toward these two locations. The authors have discovered a huge gap in studies done in Africa, Central America, and India. There is a great need for more research in these areas, especially Africa since it is home to the second largest tropical forest.
The authors used the data presented in the papers to create a metric for understanding the effect of biodiversity. Putting together the specific effects on biodiversity, they developed a standard method for understanding each paper’s specific results, allowing them to compare each of the results by region, taxonomic group, metric, and disturbance type.
Even with this hole in the current data, the data on record reveal much. Southeast Asia, the focus of the Asian studies, is home to the most sensitive biota. The authors developed a metric of sensitivity by finding the median effect size, which was 0.51. Any numbers below zero reflected a benefit from the effects of human interference. The studies done in Southeast Asia reveal that there is an effect size of 0.95, which is much higher than any of the other regions studied. The authors suggest that this can be explained by the large increase in human development due to expansion of oil palm monoculture. Africa and South America showed effect sizes of about .35 and .42, respectively, while Central America showed an effect size of about 0.1. The stark difference between Asia, specifically Southeast Asia, and the rest of the regions demonstrates the urgent need for preventative measures against the adverse effects of human development.
Among taxonomic groups there was little difference between arthropods, birds, and plants. Each had an effect size of about 0.6 to 0.7. Mammals, however, showed an effect size of –12, suggesting that some mammals substaintally benefitted from some human disturbances. The authors suggest that this is due to the higher abundance of small mammals in degraded forests, perhaps be because small mammals tend to have a high tolerance to disturbance. Birds showed the most vulnerability of the four taxa, however here it is important to look at what disturbance type affected each taxonomic group most.
Birds were most sensitive to development of agricultural land, while plants are most sensitive to burned or shaded forests. Of the twelve-disturbance types, agricultural land use and abandonment had the largest impacts. The agroforestry and plantations (shaded and unshaded) were considerably lower, which is to be expected. The lowest effect was found in selective logging, however the value was still positive at 0.11. This finding is supported by other studies, which have found selective logging to preserve large numbers of local species. These findings suggest that selective logging is the best solution to preserve biodiversity. There is however still a large danger in logging. Logging and the long-term effects of logging roads through the forest have the potential to injure primary forests, increase risk of species extinction over the long-term, and further exacerbate already existing issues. So, these seemingly positive results should still be taken with a grain of salt.
Next the authors split the various measures of biodiversity into five response metrics: abundance, community structure and function, demographics, forest structure, and richness. The most common of these metrics are abundance and richness, used in over 75% of the papers surveyed. Richness and forest structure are the most sensitive to human disturbance, with effect sizes of 0.83 and 0.7, respectively. Community structure and function was the next largest impacted, while abundance came next at 0.19. Demographics showed little effect at all. The high level of richness was deemed conservative by the authors because it considered forest specialists and generalists equally. If only specialists were considered richness would be 1.16. The authors, therefore, deem that species richness is a good measure of forest value, and how urgently conservation acts are needed.
The need for more research is perhaps the most important finding of this study, especially in Africa. The other finding of great importance is that secondary forests are of little use in preserving biodiversity. It was believed that secondary forests may be a potential source of biodiversity habitat, but the authors have found secondary forests have significantly lower levels of biodiversity than do primary forests. The conservation of primary forests appear to be the only solution in our effort to preserve biodiversity.