2010 Award for Excellence in Tropical Biology & Conservation

The Association for Tropical Biology and Conservation and the Editors of Biotropica proudly announce the winner of the 2010 Biotropica Award for Excellence in Tropical Biology and Conservation, presented to the author of a paper published in Biotropica during 2009. We recognize an outstanding contribution based on original research conducted in tropical regions. Criteria includes clarity of presentation, strong basis in natural history, well-planned experimental and/or sampling design, and novel insights gained into critical processes that influence the structure and functioning of tropical biological systems.

The 2010 Award is presented to Ezekiel Edward, Pantaleo K. T. Munishi, and Philip E. Hulme for their paper entitled “Relative roles of disturbance and propagule pressure on the invasion of humid tropical forest by Cordia alliodora (Boraginaceae) in Tanzania” published in Biotropica 41(2): 171–178.

Invasive exotic species are widely regarded as one of the most serious threats to biodiversity worldwide and have been the subject of research for decades. While invasive species have wreaked havoc on many tropical dry forests and grasslands, few exotic species appear to have become naturalized in tropical wet forests. Indeed, many studies have concluded that these forests appear more resistant to invasion than other biomes, perhaps due to some combination of structural complexity and high biodiversity. However, the conventional wisdom is that this resistance to invasion will be weakened by natural or anthropogenic disturbance, which is a key factor facilitating the establishment and spread of invasive species in other regions. If so, the key to invasion in tropical wet forests is the synergistic effects of disturbance and the arrival of propagules. And that is the crux of the problem: rarely do we know when or how many individuals arrive in a site, making it difficult to account for propagule pressure when assessing the relative importance of the multiple factors promoting invasion and establishment.

Edward, Munishi, and Hulme have circumvented this problem in a most creative way, by studying a deliberate introduction. In the early 1900s, a 50-hectare plantation of 210 Cordia alliodora trees was established in the Amani Botanical Garden in Tanzania. Cordia alliodora is a fast-growing hardwood tree native to the Americas and valued for its timber; it produces prodigious numbers of wind-dispersed seeds and is now the dominant species in the forest mosaic surrounding the Amani Garden. Using transects that radiated from the plantation into the surrounding landscape, Edward et al. were able to disentangle the effects of propagule pressure (measured as distance from the invasion), local diversity, forest structure, and disturbance on C. alliodora density and population structure. The results were clear. Distance from source populations were the most important variable correlated with density, suggesting propagule pressure plays the fundamental role in promoting the invasion of Cordia. Disturbance undoubtedly plays a role as well, as Cordia abundance, especially that of seedlings, was positively associated with disturbance. However, they correctly point out that disturbance and other habitat attributes are often confounded with propagule pressure, since degraded areas tend to be close to the site of invasion. By creatively taking advantage of a unique natural experiment, and with careful and detailed fieldwork coupled with robust statistical analyses, the authors were able to disentangle these confounded factors and reveal the clear and dominant signal of propagule pressure. Though it remains to be seen to what extent their conclusions can be generalized to other species and locations, their excellent study is a template other researchers can use to evaluate their conclusions. Congratulations to Edward et al. for simultaneously advancing the field of invasion biology and refocusing our attention on an emerging and overlooked issue in tropical biology and conservation.

You can read their perspective on the paper here.

Emilio Bruna