In the late 1990s Kyle Harms and others began to find evidence that supported the idea that density-dependent seedling mortality was able to contribute to species diversity (e.g., Harms et al. 2000). With that, we have two key factors in place:
- density dependent seed mortality is a mechanism by which diversity can be maintained, and
- density dependent seed mortality appears to be a mechanism by which diversity is maintained.
While Janzen was able to identify a few agents which were capable of playing a role in this, the actual cause in real communities remains poorly understood. In a paper which came out in the December issue of the journal Ecology, Timothy Paine and Harald Beck examined this question. Using seeds of 13 tree species and one liana, they looked at the effect of different size classes on mammals on the survival of seeds and seedling over the course of two years.
They use five different types of enclosures to regulate the mammals that had access to the seeds:
- cages that excluded all mammals
- cages that excluded medium and large mammals, but let small ones in
- cages that excluded large and small mammals, but let medium-sized ones in
- cages than excluded small mammals but let medium and large one in, and
- cages that allowed all types of mammals in.
They defined small mammals as those weighing less than 1 kg, medium as 1-12 kg and large as 20-200 kg.
They found that both small and medium-sized mammals “significantly reduced seed survival, seedling survival, and seedling density”, and that small mammals also increased species diversity. Large mammals had no detectable effect (in other words, none that couldn’t already be explained by the small and medium-size mammals). Thus, they were able to show that seed-eating mammals were able to increase species diversity in tropical forests through density-dependent mortality.
While it was interesting, the study raised some interesting questions. Why, for example, didn’t the large mammals have a larger impact on seeds and seedlings? In this area, large mammal biomass exceeds small and medium-sized mammal biomass (400, 57, and 14 kg/km2, respectively). Herds of white-lipped peccaries can have a locally huge effect as they root around through an area. But they tend to occupy defined territories. So any given area (and the seeds therein) is more likely to be investigated by a small mammal that a large one. This, the authors suggest, is the probable explanation for what happened in this study.
The study was carried out at the Cocha Cashu Biological Station in Peru’s Manu Maru National Park, one of the few field stations in the Neotropics that still supports abundant populations of large native mammals.
Paine, C.E., Beck, H. (2007). Seed predation by Neotropical rain forest mammals increases diversity in seedling recruitment. Ecology, 88(12), 3076-0387. DOI: 10.1890/06-1835.1
Harms K. E., Wright S. J., Calderon O., Hernandez A., Herre E. A. 2000. Pervasive density-dependent recruitment enhances seedling diversity in a tropical forest. Nature. 404: 493–495. DOI: 10.1038/35006630