PS 10-119 - Growth-defense correlations differ within vs among species

Abstract: Defending against herbivores is assumed to be costly, resulting in tradeoffs between growth and defense. While growth-defense trade-offs are commonly documented across plant species, certain defensive traits are more costly than others. It is also assumed that tradeoffs that occur among species also occur within species, although emerging evidence shows mixed evidence for tradeoffs within species. Here we test the hypothesis that growth-defense should be negatively correlated across species but positively correlated within species. Additionally, we predicted that certain defensive compounds should be more costly, and show stronger negative correlations, than others. We grew five species of plants that originated from arid grasslands in Montana and from mesic tallgrass prairies in Wisconsin. Plants were grown in a fully a reciprocal transplant design, with common gardens located within each region. After two years, we measured plant growth and a suite of plant defensive traits.

We found that tradeoffs were common among species, while patterns were more mixed within species. However, the type of defense and the resource environment also modified the strength of these correlations. Among species, there was a strong tradeoff between total phenols and plant size, but only in the mesic garden. This pattern seemed to be cause by greater plasticity in growth between the gardens, where phenol production was reduced for fast-growing species when grown under mesic (compared to arid) conditions. For total terpenes, there was a weak tradeoff with plant size, but only in the arid garden. This pattern suggests that the harsher conditions in the arid garden may make producing terpenes more costly in terms of reduced growth. Within species, patterns were more variable, with no correlations between growth and defense being common. Overall, this work contributes to reconciling differences between inter- and intraspecific strategies of plant defense.