Coping with extinction: can plants cope with the loss of their dispersers?

In an era of widespread deforestation and habitat loss, we hear much about the problems that this causes for wildlife. The plights of orangutans, gorillas, lemurs and other charismatic species as they lose the forests on which they depend, and of apes and other wild animals as they are hunted in Africa for bushmeat, are often featured on wildlife programmes, and the protection of these animals, and the forests in which they live, is a concern of many.

But what is perhaps less well known to the public is the other side of the coin: just as wildlife depends on the survival of habitats such as tropical forests, so the health of the forests is in turn dependent on the animals living in them. The trees and other forest flora often depend on particular animals for aspects of their life-cycles such as pollination and seed dispersal, and if those animals are lost through hunting, or because the forest becomes too fragmented for them to survive, this can have profound effects on the ecosystem of those forest areas which do remain.

An interesting perspective on the possible effects of loss of wild animals on the plants they disperse comes from an open access paper published on PloS on 5 March by Guimarãe et al. This paper looks at the dispersal mechanisms of some neotropical fleshy-fruited plants in South America, and suggests that these plants may have evolved their fruit and seed traits for dispersal by large mammals which have long been extinct. While the survival of the plant species shows that other dispersal mechanisms can in some cases take over, they show impaired dispersal, with consequences for ecology and plant diversity. The dispersal patterns of ‘megafaunal fruits’ can give insights into the consequences of ongoing loss of wildlife on which other plant species depend.

The paper examines anachronistic seed dispersal patterns, which are described as interactions between animal frugivores and plants involving traits that are unfit for the current fauna. Megafaunal fruits are defined as fruits 4–10 cm in diameter with up to five large seeds, and fruits >10 cm diameter with numerous small seeds. It is suggested that present-day seed dispersal by scatter-hoarding rodents, introduced livestock, runoff, flooding, gravity, and human-mediated dispersal allowed survival of megafauna-dependent fruit species after extinction of the major seed dispersers. However, consequences for the species which have to rely on seed dispersal mechanisms other than those for which they evolved can include reduced dispersal distances, clumped spatial patterns, reduced geographic range, and limited genetic variation. By studying the effects of past extinctions on the population structure of living plants, the authors suggest that we can better predict the ecological effects of todays ongoing extinction of seed dispersers.

Some interesting previous work on the possible effects of wildlife loss on tropical forests was published last year in an issue of Biotropica which had a special section on the impact of hunting on tropical forests. Several papers in this section examined the implications of loss of game on seed dispersal. A paper by Peres and Palacios suggests that widespread hunting of frugivores in Amazonian forests may "produce a collapse of seed-dispersal services for dependent plant species" in some areas, altering plant community composition. Summarizing papers in the section, Wright and colleagues write that the research demonstrates "that hunting has pervasive effects on tropical forest plant communities altering levels of predispersal seed predation, primary and secondary seed dispersal, and postdispersal seed predation, which, in turn, alter seedling and sapling species composition." It is also suggested that loss of game animals leads to effects that are yet further removed from the direct impact of hunters on forest vertebrates. Possibilities include altered spatial patterns of seedlings and saplings and dramatic increases in recruitment to the adult stage among insect seed predators. (For an excellent blog on this journal section, see Mongabay.com from 1 May, 2007).

We at CABI were kindly sent the PloS paper by co-author Mauro Galetti, who co-edited an early CABI book on Seed Dispersal and Frugivory. CABI published another book on seed dispersal in 2007, which includes a chapter on the importance of large mammals on seed dispersal in the Pantanal of Brazil, co-authored by Galetti. The book ‘Seed Fate: Predation, Dispersal and Seedling Establishment‘ may also be of interest.

The Seed Abstracts subset of CAB Abstracts gives comprehensive coverage of seed biology of both wild and cultivated plants, including over 2100 abstracts on seed dispersal in forests. Recently indexed papers on seed dispersal by mammals, and the breakdown of the dispersal mutualism, include one by Kitamura et al. on forests in Thailand, and this paper in Biotropica by Cramer et al.

One thought on “Coping with extinction: can plants cope with the loss of their dispersers?

  1. Aerogarden Grower March 19, 2009 / 12:01 am

    Great post! It was very well researched and I enjoyed it very much. I bookmarked your site and will be back very soon, I look forward to reading some interesting posts! Thanks

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