Despite being at the bottom of the food chain, plants may be able to significantly alter the ecological patterns in their habitats. Generally, ecosystems are studied from a "top-down" perspective: top predators limit the numbers of herbivores, which in turn allows plants to flourish. In a paper published in Science last week, researchers showed that plants can also affect these patterns from the bottom up.

Milkweeds are a family of plants fed upon by a species of aphid, and these aphids serve as prey for a range of invertebrate predators. By manipulating soil nutrient levels and predator access in 16 species of milkweeds, the researchers tried to determine whether milkweed success—here measured via plant biomass—was always determined top-down, by the influence of predators.

Results show that milkweeds face an evolutionary trade-off and can adopt one of two strategies: they can invest their energy in growing quickly and taking advantage of rich soil, or in developing resistance to the aphids. Depending on which strategy was in operation, the milkweed species differed in how important top-down effects were; plants that were most responsive to changes in soil fertility were also the species in which the presence of predators had the biggest positive effect on plant biomass. In other words, the plants that sacrifice herbivore resistance for growth rely more heavily on predators for protection than plants with the alternative, aphid-resistant strategy do.

Apparently, less-resistant milkweed species still played an active role in the ecosystem, as they produce more volatile organic compounds called sesquiterpenes that can actually draw predators to the plants, providing some protection from the aphids. This study suggests that, while top-down effects are important, bottom-up effects based on evolutionary trade-offs in plants may be an underestimated source of variation in ecological communities.

Science, 2010. DOI: 10.1126/science.1184814 (About DOIs).