The role of consumer ressource thophic interactions in determining vegetation distribution and ecotones

Background/Question/Methods

Although it is widely recognized that ecological interactions are important to maintain ecosystem properties, landscape models of vegetation dynamics usually account for a small number of species and/or a single trophic level. In particular, distribution models rarely consider large herbivores as key drivers, and reciprocal effects of vegetation on herbivore populations are even less commonly included. However, it is possible that geographical limits of some species or ecosystems such as the treeline or the temperate-boreal forest transition are the result of trophic interactions.

Here we propose analysing how accounting for the interaction between browsers and vegetation is likely to modify the distribution of main vegetation types along an environmental gradient. We examine two cases: (1) How might grazers and/or browsers affect treeline ecotones? (2) What role do browsers mainly feeding on temperate trees play in determining the transition between temperate and boreal forests?

We used simple mathematical models to represent the transition between major vegetation states at the landscape level. The herbivores' dynamics were modelled using a metaphysiological approach. We also included selective feeding behaviour depending on resource availability and mediation by climatic conditions. We compared the distribution of the main vegetation states over climatic gradients in absence of the herbivores, with either the grazers or the browsers, and finally with both of them.

Results/Conclusions

Our analysis suggest that herbivore populations, through complex trophic interactions, might be a significant factor limiting species ranges and defining ecotones. We also find that the distribution of vegetation might not be directly limited by physiological constraints related to climatic conditions, but instead by interactions with large herbivores. In addition to fire, this could explain the historical abundance of open environments in central Europe and shed a new perspective on the treeline distribution in mountainous areas. In a context of climate change, these results suggest that it is not possible to anticipate the response of the vegetation while ignoring its interactions with other trophic levels.