Carla Essenberg

B.A. in philosophy and music, Saint Olaf College (2001)

Curriculum vitae

Dissertation committee: John Rotenberry (chair), Rick Redak, Kurt Anderson

Dissertation title: Effects of flower density on visitation rates and species composition of pollinators: Non-linearity and scale-dependence

Research

I am broadly interested in the ecology and evolution of species interactions, especially plant-pollinator relationships. For my dissertation, I am exploring the effects of flower density on how often flowers are visited by pollinators and which groups of pollinators visit them. The answers to these questions have important implications for plant ecology and conservation. If high-quality pollinators prefer to visit flowers in dense patches and populations, then plants in sparse areas may suffer reduced reproductive success. Such a decline in fitness at low densities, known as an Allee effect, makes a species more vulnerable to extinction.

Past studies have found a variety of relationships between flower density and visitation rates, so I am trying to identify factors that determine when flower visitation rates go up or go down with flower density, and when flower density might not matter. Two of the possibilities that I am investigating are that the effect changes as flower densities go up, and that the effect depends on spatial scale. My research thus far provides support for the both of these possibilities. I have shown that under an ideal free distribution, flower density will have its strongest effect on per-flower pollinator visitation at the lowest flower densities. At higher densities, visitation may change little or even decline with increases in flower densities. A field experiment that I carried out with the annual plant Holocarpha virgata confirms this pattern. The effect of flower density on visitation in this species is also scale-dependent: I found contrasting effects of large-scale (12.5 ha) versus local-scale (4m²) flowerhead density on per-flowerhead visitation by all visitors and by the most common visitor, Melissodes lupina.

In addition to changes in visitation rates, flower density can also influence the species composition of the animals that visit a plant's flowers. For instance, for H. virgata the proportion of visits by honeybees goes up and the proportion by M. lupina goes down with local flowerhead density. Shifts in pollinator species composition with flower density are currently poorly understood but could be important to plant ecology, because some flower visitors are much better pollinators than others. I shall be carrying out simulation modeling to identify possible explanations for these shifts in pollinator composition.