Tropical forests are immensely species-rich. The question of what causes this diversity is a perennial one in tropical biology. In the 1970s Daniel Janzen and Joseph Connell independently came up with the same explanation - if the seeds or seedlings of more common species have a higher probability of being killed by a pest or pathogen (what is known as density-dependent mortality), then less common species will be favoured. If the organisms that are responsible for most seed and seedling mortality are specialists - if they focus on just a few plant species - then the pathogens and seed predators that specialise on common tree species should be more abundant (since there’s more food for them). Janzen was able to demonstrate this with a few species of beetle whose larvae fed on (and killed) seeds. When seeds of the Hog Plum (Spondias mombin) were abundant, female bruchid beetles laid their eggs on (and ended up killed) well over 90% of the seeds. When the seeds were scattered, mortality rates were reduced. Unfortunately, while there were several good anecdotes, there was little evidence of density-dependent mortality playing a role at the community level. In fact, there was evidence that trees were more likely to be clumped than scattered, a finding which was not in keeping with Janzen and Connell’s hypothesis.

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