Research

Nick Rasmussen phenological shifts that affect species interactions

Figure 1. Shifts in A) mean and B) variation of phenological event.

The timing of the seasonal life-history events of species, or phenologies, is an important driver of community dynamics in many habitats. We know that patterns in phenologies naturally vary among years, and that phenologies of species all over the world are changing in response to climate change. The outstanding question is how does phenological variation affect the outcome of species interactions?

With my research, I seek to understand how shifts in two features of phenology affect species interactions, the mean and the variation around the mean (Fig. 1). Specifically, I am interested in (1) how shifts in mean timing of phenological events affect interspecific interactions, (2) how shifts in the variation of phenological events affect intra- and interspecific interactions, and (3) whether the effects of these phenological shifts on species interactions scale up to alter community structure and ecosystem functioning.

To answer these questions, I use a community assembly framework. This is because many of the phenological events that contribute to annual cycles of community assembly involve species “arriving” to the habitat (e.g., migration, birth, emergence from dormancy). When organisms arrive, they encounter and interact with organisms that arrive both before and after them, including conspecifics and heterospecifics. This can cause priority effects, whereby  early arrivers affect the survival, growth, and fitness of late arrivers, and the strength of these priority effects can vary among years with shifts in phenologies.

I use an experimental approach in my research. For my dissertation, I studied the assembly of aquatic communities of insect and amphibian larvae using outdoor mesocosms (Fig. 2A). As a postdoc, I currently study the assembly of the specialist herbivore community of milkweed (Asclepias spp.) plants (Fig. 2B). In both of these systems, I construct replicated communities, perform manipulations of arrival phenologies, and quantify the effects on the demographic rates of species, structure of communities, and features of ecosystem functioning.

Nick_Rasmussen_Experiments

Figure 2. Array of mesocosms for studying pond community assembly (A) and array of milkweed plants for studying herbivore community assembly (B).

Progress to date indicates that variation in phenologies can alter the strength, symmetry, and type of interactions that occur among species, and these effects on species interactions can scale up to alter the relative abundances of species, whether species can coexist, and in some cases even ecosystem-level processes.

Results of this work help us to understand the mechanisms that link phenological shifts to consequences for species interactions. This is important not only for understanding the dynamics of natural communities but also for predicting the effects of climate change on these communities.