Invasive plant species have been observed altering plant-pollinator webs and devastating plant communities by out-competing native plant species. Despite these observations, little is known about the direct effects invasive plants have on native bees, or if these effects are positive, negative or neutral.
This project focuses on understanding the direct effects of invasive plants on a native bee species. By identifying use, preference and nutrition of native and invasive plants, I hoped to better understand how the encroachment of invasive plants are influencing native bee pollen resource use and nutrition.
This information will be used to improve hedgerows in agricultural areas and to inform land-managers of the most important floral resources for conservation of native bees.
Global pollinator declines threaten the sexual reproduction of an estimated 90% of flowering plant species and 70% of crop species (Klein et al. 2007; Kearns et al. 1998; Potts et al. 2010; Ashman 2004). In species that require or benefit from animal-mediated pollination, these declines can result in pollen limitation and seed limitation that have demographic consequences on plant reproduction (Knight et al. 2005; Young et al. 2005; Aguilar et al. 2006) and threaten food security, ecosystem stability and the provision of valuable ecosystem services (Allen-Wardell et al. 1998; Potts et al. 2010). The rapid decline of European honeybee (Apis mellifera) colonies, the primary agricultural pollinators, from Colony Collapse Disorder threatens to drastically impact food production and has intensified the need to seek alternate pollinators, such as native bees (Winfree et al. 2007). However, native pollinators are also declining (Biesmeijer et al. 2006), placing even greater need on understanding and conserving this crucial ecosystem service.
Although conserving pollinators has become a major area of research, especially in agricultural areas, one major limitation is a lack of specific information on pollinator needs and preferences. Using Bombus vosnesenskii as a model species, we determined plant preferences in natural environments and identified characteristics of plants that influence preference.
1) Quantify the available pollen resources in grassland sites.
2) Identify pollen sources chosen by bumble bees.
3) Evaluate the characteristics of the plants chosen by bumble bees.
4) Identify the native plants that are essential to bumble bees throughout their life cycle.
5) Quantify changes in bumble bee abundance after improving hedgerows.
6) Provide information to farmers, researchers and societies to improve conservation, pollination and restoration efforts, especially oriented toward these important agricultural pollinators, bumble bees.
Throughout the project, I worked entirely with Bombus vosnesnenskii. I originally wanted to work with both B. vosnesenskii and B. californicus but could not find sites with sufficient numbers of both. During the summers of 2009 and 2010 plants and bumble bees were measured in eight grassland sites every two weeks.
To determine the pollen sources available to bumble bees, I first quantified the available plant resources using a quadrat sampling method; 50 1m2 quadrats evenly spaced along a grid throughout the site and sampled for flowering vegetation. All flowers or inflorescences within the quadrats appearing to have receptive stigmas or productive anthers were counted. A list of all species blooming within a site was also recorded to account for any species not found within quadrats, so that they were not falsely recorded as having zero availability. Floral availability was then rescaled based on the amount of pollen produced by each plant. Pollen production was estimated by counting the pollen on five mature buds using a digital particle counter called ImageJ (NIH). Morphological measures were also made of all available flowering species and additional pollen samples were collected for protein and amino acid analysis.
To determine the pollens collected by bumble bees, I removed pollen loads from the bodies of 20 bumble bees in each site and identified 300 grains of each pollen load. Preference of pollens was then determined for each site date using a complex hierarchical modeling procedure developed by Perry de Valpine.
Although Bombus vosnesenskii is considered a generalist pollinator, we found it had distinct preferences for some plants over others. We were able to determine that preferences of Bombus vosnesenskii are better described by course scales of measurement than the fine scale analysis we developed. This may reflect the importance of visual factors in identifying resources rather than the resource available. From our preference analysis we also found that often weedy plants are important to the diets of native pollinators, and thus care should be taken to make sure that when exotics are removed functionally and nutritionally plants are chosen to replace them.
Our analysis of the plants that were in the sites discovered that plants most visited by Bombus vosnesenskii were of higher protein quality, had a different amino acid profile and made more pollen than species that were available but not collected from. We also found that invasive plants as a group are not uniformly bad for native bees, and instead that invasive plants that are collected are more similar to native plants that are collected. This is a significant discovery because it is the first time that native and invasive plants were compared within the same study site and native plants that were not collected were also included in the analysis.
Educational & Outreach Activities
I am currently preparing three articles from this work for submission to peer-reviewed journals. Unfortunately the data analysis for this project became very difficult, and that delayed publication. I hope to have all of them published before the end of 2012. I was able to participate in five outreach events with a variety of practitioners and will continue to participate in them during 2012. There were additional outreach programs Xerces wanted to complete but could not be done before the completion of this funded project.
A number of new discoveries about how bees use resources and what factors are important in determining which resources they use were made during this project. Preference had not previously been determined for pollinators using standardized methods, and while it is often assumed invasive plants are bad for pollinators, we found that often invasives can provide nutrition that natives cannot. These findings could potentially change the way we work to conserve bees as well as greatly improve our conservation efforts. This is the first project to use pollen availability to determine preferences of pollinators and to look at the role of proteins and amino acids in the selection of resources.
Although we initially hoped to provide farmers with specific recommendations to improve hedgerows, our results implied that further research is needed to be able to make proper recommendations. Our project determined that not all native plants can provide the nutrition needed to sustain populations of bumble bees, and that visitation results alone do not well represent the plants most needed by bees. This suggests that more care needs to be taken to choose plants for hedgerows. We are still working to come up with simple ways to determine plants that would be good for hedgerows. Additionally, farmers have many concerns about plants that go in hedgerows, such as the amount of maintenance that they require that could limit the adoption of some of our possible recommendations.
Areas needing additional study
The method we developed for determining if plants are preferred by bees needs to be tested by more species before we can know if it is robust. I am also trying to determine if there is a simple way to estimate the nutritional content of plants that can be used to more easily make recommendations for bee plants. This project has also pushed my work into looking at landscape factors that may impact bee communities.