Illinois Natural History Survey - University of Illinois

INHS Reports Summer 2001

Changes in the Pollinator Guild of a Prairie Species in the Past 70 Years

Pollinators are an essential part of the reproductive biology of plants. In most cases, more than one pollinator is associated with a species. Pollinator guild is the term used to describe the group of species that will pollinate a plant. Pollinator guilds can be composed of a particular group of organisms, such as bees, or can be a mix of several groups, such as flies and beetles. Our knowledge of the composition of these pollinator guilds is limited, especially in the case of prairie plants. Even less information is available about how these guilds have changed over time. Changes in pollinator guilds can sometimes have a negative impact on the reproductive success of a species.

From 1998 to 2000 I studied the reproductive biology of the pale spiked lobelia (Lobelia spicata) at the Midewin National Tallgrass Prairie (Midewin). The pale spiked lobelia is a short-lived prairie perennial that blooms in northern Illinois from May to August. It is a gynodioecious species, which means that there are both hermaphroditic and female individuals. To understand the reproductive biology of this species it was necessary to identify its pollinator guild. Through personal observations, I determined that the main pollinators of pale spiked lobelia are bees.

During the course of this study I found a Ph.D. thesis by J.F.W. Pearson entitled "Studies on the Ecological Relationship of Bees in the Chicago Region." During 1930-31 Pearson visited and collected bees in several towns in the Chicago region (Ashburn, Chicago, Lemont, Matteson, New Lenox, Palos, Somonauk, South Chicago, Volo, Waukegan, Willow Spring, and Zion). He listed all the plant species associated with the bees, including the pale spiked lobelia. Because Midewin is just 10 miles from one of the towns that Pearson sampled, I decided to determine how the pollinator guild (i.e., bees) had changed over the past 70 years.

In 1999 I collected bees at Midewin and Dr. Wallace LaBerge (Illinois Natural History Survey) identified these specimens. I found that pale spiked lobelia at Midewin is pollinated by four bee species, Augochlorella striata being the main pollinator (see Table 1 and Fig. 1). Pearson had also found four bee species associated with pale spiked lobelia in the Chicago region (see Table 1). Only one common species, Ceratina dupla (Fig. 1), was found at Midewin and in the Chicago region. Pearson did not provide information about the main pollinator.

It can be argued that although bees are still the main pollinators of pale spiked lobelia, the pollinator guild has changed over the past 70 years. Only one species is still the same between Midewin and the Chicago region of the 1930s. However, it is possible that at Midewin I may have underestimated the pollinator guild of pale spiked lobelia. At Midewin the few species of bees that were found may reflect the habitat where the study was conducted--a degraded prairie that was heavily grazed in the past. Few prairie species can be found at the study site, and fewer are blooming when pale spiked lobelia is blooming. This may decrease the attractability of the site for pollinators, affecting the size of the pollinator guild. It is likely that if a native prairie is visited, the pollinator guild of this species will increase. However, it is interesting to note that Pearson also detected only four species in the Chicago region. This suggests that the pollinator guild of pale spiked lobelia is small.

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Table 1: Bees associated with pale spiked lobelia at the Midewin National Tallgrass Prairie and in the Chicago region (Pearson 1932).

Midewin National Tallgrass Prairie

Family--Genus/species
Andrenidae--Andrena wilkella (Kirby)
Halictidae--Augochlorella striata (Provancher
Anthophoridae--Ceratina dupla dupla Say
Apidae--Tetralonia hamata Bradley

Chicago Region

Family¹--Genus/species
Megachilidae--Alcidamea simplex^c
Ceratinidae^a--Ceratina dupla Say
Megachilidae--Megachile brevis Say
Eucerdae^b--Tetralonia dilecta (Cresson)

¹Current nomenclature: a) Anthophoridae, b) Apidae, c) Hoplitis simplex Cresson 1864 (Heriades).

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Finally, in another part of this study I found that both female and hermaphroditic individuals had a high reproductive output. This suggests that although the pollinator guild of pale spiked lobelia has changed over the last 70 years, this change does not appear to have had a negative impact on the reproductive output of the species.

Brenda Molano-Flores, Critical Trends Assessment Project and Center for Biodiversity

Charlie Warwick, editor