Laura Burkle

   Postdoctoral Research Associate 
   Washington University in St. Louis, Biology Department
   Ecology, Evolution & Population Biology


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pemigawasett
photo credit: Charlie DeTar
erigeron

lonicera

black canyon

Using historic and current plant-pollinator interaction webs to understand the roles of climate change, invasive species and land use change

Pollination is a critical ecosystem service -- over 90% of plants worldwide require animals for pollination, including one third of agriculturally important crop plants. Like many ecosystem services, pollination is being disrupted by anthropogenic activities. Climate change, invasive species, and land use change may alter the abundance, distribution, and phenology of plants and pollinators and disrupt their historic interactions. However, the degree to which these interactions are flexible at the community level over time (i.e., whether pollinators can shift their behavior to feed on new host plants when preferred species become extinct or no longer overlap in phenology) is not known. We currently lack a complete understanding of how human actions are affecting pollination, in part because we typically do not have historical information about plant-pollinator interactions prior to the disruption.

In the late 1800’s, the eminent entomologist Charles Robertson meticulously detailed all of the interactions between plants and pollinators in Carlinville, IL. Nowhere else is a community of this type so well documented. His dataset provides an unprecedented opportunity to explore the same area, recollecting these data to compare historic interaction networks with those that persist today. Through this comparison, we will begin to understand how species interactions are influenced by three major anthropogenic forces that have been acting on this system for over a century.

How have plant-pollinator visitation webs changed over time? Specifically,

1.      How has climate change (warming) altered the phenology of native and alien plants, and do these changes in phenology disrupt their interactions?

2.      Have alien plants and pollinators become integrated into the interaction network, and have these aliens “usurped” links compared to Richardson’s time? If so, can the network recover lost interactions when alien plants are removed?

3.      How has land use change, and the accompanying changes in native and alien species composition, affected plant-pollinator network structure?

This is a timely and important project since many global conservation organizations are concerned about a looming pollinator crisis. Preserving this ecosystem service will require an understanding of how human actions are affecting pollination networks.
garlic mustard

honey bee

corn
gothic

bee with pollen

bee larva

Bottom-up effects of nutrient enrichment on plants, pollinators, and their interactions

Nutrients play fundamental roles in biological systems, affecting plant growth and quality, community structure, and species interactions. Although the effects of nutrients on primary and secondary production have been well documented, their effects on mutualistic consumers have rarely been addressed. Nutrient addition could alter pollinator behavior via changes in floral reward quality and quantity. Moreover, if nutrients strongly affect characters important for pollination, nutrient enrichment may influence not only plant fitness and species composition but also the performance of consumers that rely on nectar and pollen to provision their offspring. Using flowering plants in subalpine meadows and their pollinators, my dissertation research focused on the degree to which nutrient addition affected producers, consumers, their interactions, and the mechanisms by which those responses occurred.

At the individual plant level, one year of fertilization affected floral traits, pollen receipt, and reproduction of individual plants. Plant life-history may influence responses to nutrient additions, with delayed effects in iteroparous perennials and immediate responses in monocarps.   (Burkle and Irwin 2009) PDF

At the community level, three years of nitrogen addition to plant assemblages showed that nitrogen increased plant productivity but did not always enhance the reproduction of flowering plants. High levels of nitrogen enrichment favored growth and reproduction of grasses, while low levels of nitrogen addition enhanced biomass, flower production, pollinator visitation, and reproduction of forbs. There was limited evidence of pollen limitation of female plant reproduction, and the direct effects of nitrogen addition on reproduction were relatively stronger than the indirect effects associated with changes in pollination.

In a food web context, annual variation in network structure and interactions, possibly due to fluctuations in pollinator populations, was greater than effects of nitrogen enrichment. Given that nitrogen addition can affect floral resource quality and quantity, I tested the degree to which solitary bee larvae are sugar limited to determine how nutrients might scale up to affect pollinator consumers. I found that nectar-sugar concentration limited larval growth. Together, this work suggests that nutrient enrichment can have bottom-up effects on plants and their mutualist pollinators; however, the direct effects of nutrients on plant reproduction outweighed the indirect effects via pollination.
gothic

bombyliid on potentilla

mating solitary bees