Common Methods for Tallgrass Prairie Restoration and Their Potential Effects on Bee Diversity
Alexandra Harmon-Threatt, Kristen Chin
Habitat restoration is consistently recommended as the primary method to limit the continual decline of pollinators (Dixon 2009; Potts et al. 2010; Winfree 2010). While non-target restoration can be effective at conserving pollinators (Williams 2011), it is imperative that we assess the methods used to restore and maintain land to maximize the benefit of restored areas to bee conservation. If bee conservation is to be either a primary aim or a secondary benefit of restoration projects, more research is needed to understand how bees respond to many of the most common practices conducted to restore habitat. Research in these areas will allow us to identify practices that may be the least detrimental to establishment and persistence of bees in prairie restorations.
While managers have little control over existing site characteristics, such as surrounding habitat, age, or previous habitat type—all of which may affect bee diversity within a site—they can control the methods used to establish and manage habitat. For example, tillage and herbicides, which are commonly used to clear land, are known to disrupt existing nests (Shuler et al. 2005) and potentially affect navigation for some species (Balbuena et al. 2015). Whether these patterns apply broadly to most bees is still largely unknown. Thus, it is important to expand research to more bee species to understand how many of the management methods affect bee diversity in general rather than one species in particular.
Plant diversity in tallgrass prairie is likely to be one of the most important features of restored habitats to helping increase bee diversity. But some of the most common methods used to establish plants, such as broadcasting, which is used by over 92% of managers, are known to have variable success rates dependent on season and site (Larson et al. 2011). If increasing forb diversity is most important for bees, it will be important to broadcast during the winter, which is currently done in about half of sites, and possibly to continuously add seed or to drill plant species known to be important for bees. More data on plants that are important for bee diversity are likely to be very important to improving these efforts.
Maintenance of the restored areas will also be critical to maintaining plant diversity and encouraging bee diversity. Over 90% of managers reported burning their sites, which is important for increasing forb diversity. Burning, which can adversely affect cavity-nesting bees, is expected to be good for bee diversity in general by increasing plant diversity and clearing plant debris. Nonetheless, burn intervals may be very important for creating a heterogeneous landscape; however, to date there has been little work to determine if there is an optimal burn frequency that allows cavity nesting bee species to persist, encourages plant growth, and clears land for ground-nesting bees. Other maintenance methods like mowing and haying are largely unexplored and many questions remain about the efficacy of cattle grazing (used by almost 17% of respondents) for helping to maintain bee diversity.