Plant range expansion and biotic interactions: An experimental approach

Background/Question/Methods

Climate change is expected to initiate a process of poleward range expansion for many plant species. However, those plants which germinate beyond their current range will be exposed to new biotic communities, which will contain different suites of herbivores and pathogens.  The ability of these plants to establish in new communities will depend in part on whether they encounter biotic resistance or enemy release.  Ultimately, the survival of long distance dispersing individuals will help determine the rate and success of range expansion.

To investigate these factors we conducted a seedling transplant experiment and quantified the interactions between migrant plants and existing communities. In this study we address the following questions: 1) Do migrant seedlings experience less herbivory and disease in their new ranges than native seedlings? 2) Are migrant species exposed to less herbivory and disease than in their native range? 3) How important is this to plant growth and survival?

To investigate these questions, ten species of woody plants were planted within and beyond their current distributional ranges and foliar herbivory, pathogen activity, growth, and survival were monitored for two years. Seedlings were also planted across light and moisture gradients at each site to separate the effects of environmental covariates that could also affect establishment of the seedlings. Data were analyzed in a Bayesian framework.

Results/Conclusions

Herbivory rates at northern sites were higher for most species, but were especially high for some migrant species including Carya glabra and Robinia psuedoacacia. Herbivory had significant effects on seedling survival, and although the results are highly species specific, we show examples of how biotic resistance is preventing the establishment of certain southern species in northern communities. In contrast, pathogen activity was significantly lower for plants occurring beyond their current range; migrant seedlings had an average of 4.9% of their leaf area affected by foliar pathogen activity over the growing season, compared to 9.4% for indigenous plants. In contrast, exotic invasive plants had foliar pathogen levels of 1.4%.

Our results demonstrate that biotic interactions will be an important factor influencing plant range expansion. This study highlights the importance of including biotic interactions in predictions of plant range shifts.