When Urban Pollinator Gardens Meet Native Plant Communities

Public concern about the state of bees and other pollinating insects has led to increased interest in pollinator gardens. Planting a pollinator garden is often promoted as an excellent way for the average person to help protect pollinators. And it is! However, as with anything in life, there can be downsides.

In many urban areas, populations of native plants remain on undeveloped or abandoned land, in parks or reserves, or simply as part of the developed landscape. Urban areas may also share borders with natural areas, the edges of which are particularly prone to invasions by non-native plants. Due to human activity and habitat fragmentation, many native plant populations are now threatened. Urban areas are home to the last remaining populations of some of these plants.

Concern for native plant populations in and around urban areas prompted researchers at University of Pittsburgh to review some of the impacts that urban pollinator gardens may have and to develop a “roadmap for research” going forward. Their report was published earlier this year in New Phytologist.

Planting a wildflower seed mix is a simple way to establish a pollinator garden, and such mixes are sold commercially for this purpose. Governmental and non-governmental organizations also issue recommendations for wildflower, pollinator, or meadow seed mixes. With this in mind, the researchers selected 30 seed mixes “targeted for urban settings in the northeastern or mid-Atlantic USA” to determine what species are being recommended for or commonly planted in pollinator gardens in this region. They also developed a “species impact index” to assess “the likelihood a species would impact remnant wild urban plant populations.”

A total of 230 species were represented in the 30 seed mixes. The researchers selected the 45 most common species for evaluation. Most of these species (75%) have generalized pollination systems, suggesting that there is potential for sharing pollinators with remnant native plants. Two-thirds of the species had native ranges that overlapped with the targeted region; however, the remaining one-third originated from Europe or western North America. The native species all had “generalized pollination systems, strong dispersal and colonization ability, and broad environmental tolerances,” all traits that could have “high impacts” either directly or indirectly on remnant native plants. Other species were found to have either high dispersal ability but low chance of survival or low dispersal ability but high chance of survival.

This led the researchers to conclude that “the majority of planted wildflower species have a high potential to interact with native species via pollinators but also have the ability to disperse and survive outside of the garden.” Sharing pollinators is especially likely due to super-generalists like the honeybee, which “utilizes flowers from many habitat types.” Considering this, the researchers outlined “four pollinator-mediated interactions that can affect remnant native plants and their communities,” including how these interactions can be exacerbated when wildflower species escape gardens and invade remnant plant communities.

photo credit: wikimedia commons

The first interaction involves the quantity of pollinator visits. The concern is that native plants may be “outcompeted for pollinators” due to the “dense, high-resource displays” of pollinator gardens. Whether pollinator visits will increase or decrease depends on many things, including the location of the gardens and their proximity to native plant communities. Pollinator sharing between the two has been observed; however, “the consequences of this for effective pollination of natives are not yet understood.”

The second interaction involves the quality of pollinator visits. Because pollinators are shared between native plant communities and pollinator gardens, there is a risk that the pollen from one species will be transferred to another species. High quantities of this “heterospecific pollen” can result in reduced seed production. “Low-quality pollination in terms of heterospecific pollen from wildflower plantings may be especially detrimental for wild remnant species.”

The third interaction involves gene flow between pollinator gardens and native plant communities. Pollen that is transferred from closely related species (or even individuals of the same species but from a different location) can have undesired consequences. In some cases, it can increase genetic variation and help address problems associated with inbreeding depression. In other cases, it can introduce traits that are detrimental to native plant populations, particularly traits that disrupt adaptations that are beneficial to surviving in urban environments, like seed dispersal and flowering time. Whether gene flow between the two groups will be positive or negative is difficult to predict, and “the likelihood of genetic extinction versus genetic rescue will depend on remnant population size, genetic diversity, and degree of urban adaptation relative to the planted wildflowers.”

The fourth interaction involves pathogen transmission via shared pollinators. “Both bacterial and viral pathogens can be transmitted via pollen, and bacterial pathogens can be passed from one pollinator to another.” In this way, pollinators can act as “hubs for pathogen exchange,” which is especially concerning when the diseases being transmitted are ones for which the native plants have not adapted defenses.

photo credit: wikimedia commons

All of these interactions become more direct once wildflowers escape gardens and establish themselves among the native plants. And because the species in wildflower seed mixes are selected for their tolerance of urban conditions, “they may be particularly strong competitors with wild remnant populations,” outcompeting them for space and resources. On the other hand, the authors note that, depending on the species, they may also “provide biotic resistance to more noxious invaders.”

All of these interactions require further investigation. In their conclusion, the authors affirm, “While there is a clear potential for positive effects of urban wildflower plantings on remnant plant biodiversity, there is also a strong likelihood for unintended consequences.” They then suggest future research topics that will help us answer many of these questions. In the meantime, pollinator gardens should not be discouraged, but the plants (and their origins) should be carefully considered. One place to start is with wildflower seed mixes, which can be ‘fine-tuned’ so that they benefit our urban pollinators as well as our remnant native plants. Read more about plant selection for pollinators here.


Year of Pollination: Bumblebees and Climate Change

Bumblebees, generally speaking, are having a rough time. In a world increasingly dominated by humans, some bumblebee species continue to thrive while many others are seriously struggling. Several are nearing extinction. A recent study involving 67 species of European and North American bumblebees concluded that climate change is having a major impact. Bumblebees do not appear to be migrating north in response to warming climates – a hesitation that could spell disaster.

There are over 250 species of bumblebees worldwide (46 are found in North America north of Mexico). Unlike other bees, whose diversity is greatest in Mediterranean climates, bumblebee diversity is highest in cool, temperate climates and montane regions. The majority of bumblebee species are native to the Northern Hemisphere; a few species are native to South America, and a handful of species from Europe have been introduced to New Zealand and Tasmania. Some species of bumblebees, such as the polar bumble bee (Bombus polaris) and the forest bumble bee (Bombus sylvicola), can be found in extreme cold climates and are among a select group of pollinators found in such areas.

The field guide, Bumble Bees of North America, by Paul Williams, et al. provides this description:

“Bumble bees are very hairy bees with combinations of contrasting bright colors, mostly black and yellow, sometimes with various combinations of red or white. They have two pairs of wings that are usually folded back over the abdomen while they are foraging on flowers, or hooked together as a single unit when in flight. Bumble bees also have slender elbowed antennae, and females of the pollen-collecting species have the hind tibia expanded, slightly concave, and fringed with long hairs to form a pollen basket or corbicula.”

Most bee species are solitary insects; bumblebees, like honeybees, are social insects. Unlike honeybees, bumblebee colonies begin with a new queen each year. New queens, after mating in late summer, overwinter in a protected area and emerge in the spring. They then search for food and a nesting site. Suitable nests include abandoned rodent dens,  the bases of bunchgrasses, hollow logs, and human-made structures. They build up a colony of workers which maintain the nest and forage for food and other resources. As the season comes to a close, the queen produces males and new queens. The new queens mate, go into hibernation, and the rest of the bumblebee colony dies off.

Brown-belted Bumblebee (Bombus griseocollis) - photo credit: wikimedia commons

Brown-belted Bumblebee (Bombus griseocollis) – photo credit: wikimedia commons

Bumblebees face numerous threats, both natural and human-caused. Despite their defensive sting, they are regularly eaten or attacked by various mammals, birds, and invertebrates. They are also host to a variety of pests, parasites, and pathogens, some of which have been introduced or exacerbated by human activities. The commercial bee industry is particularly at fault for the spread of certain maladies. Other major threats include loss of habitat and excessive and/or poorly timed use of insecticides. One looming threat that new research suggests is especially concerning is climate change.

A group of researchers from various institutions looked at the historical ranges of 67 species of bumblebees in Europe and North America over a 110 year period. They “measured differences in species’ northern and southern range limits, the warmest or coolest temperatures occupied, and their mean elevations in three periods relative to a baseline period.” They found that on both continents bumblebees are not tracking climate change by expanding their northern range limits and that their southern range limits are shrinking. They also observed that within the southern range limits, some bumblebee species have retreated to higher elevations. They investigated land use changes and pesticide applications (in the US only) to determine the effect they had on the results. While these things certainly affect populations on an individual level, climate change was determined to be the most important factor that lead to nearly universal range contractions of the bumblebees in this study.

The question then is why are they not tracking changing climates the same way that many other species of plants and animals have already been observed doing? Bumblebees evolved in cooler climates, so shrinking southern range limits is not as surprising as the bumblebees’ delay in moving north. Many factors may be contributing to this phenomenon including lack of specialized habitats beyond their historical ranges, daylength differences, and population dynamics. The researchers call for further investigation in order to better evaluate this observed “range compression.” They also suggest experimenting with assisted migration of certain bumblebee colonies, which in general is a controversial topic among conservation biologists. (Read more about this study here.)

Buff-tailed Bumblebee (Bombus terrestris) - photo credit: wikimedia commons

Buff-tailed Bumblebee (Bombus terrestris) – photo credit: wikimedia commons

The loss of bumblebees is concerning because they play a prominent role in the various ecosystems in which they live. They are prolific and highly effective pollinators of both agricultural crops and native plants, and they are also a major component in the food web. Some species of plants “prefer” the pollination services of bumblebees, such as those in the family Solanaceae. Many plants in this family benefit greatly from buzz pollination – a process in which a bumblebee (or occasionally bees of other species) grabs hold of the flower and vibrates its body, dislodging the pollen.

Participating in bumblebee conservation is simple. It’s similar to any other kind of pollinator conservation. Just learning about the pollinators in your region and being mindful of them can make a big difference. If you own or rent property and have space for a garden (even if its just a few containters on a patio), choose plants that provide food for bumblebees, including spring and summer bloomers. If you live in North America, this Xerces Society publication and the field guide mentioned above are great resources that can help you determine which plants are best for your region. Additionally, if you are working in your yard and happen upon a hibernating queen or a bumblebee nest, do your best not to disturb it. It may disrupt your gardening plans for a season, but the bumblebee sightings and the pollination service they provide will be worth it.

One family of plants in particular that you should consider representing in your yard is the legume family (Fabaceae). Bumblebees are commonly seen pollinating plants in this family, and because these plants have the ability to convert nitrogen in the air into fertilizer, their pollen is especially rich in protein. In his book, A Sting in the Tale, Dave Goulson describes the relationship between bumblebees and legumes:

“From a bumblebee’s perspective, legumes are among the most vital components of a wildflower meadow. Plants of this family include clovers, trefoils and vetches, as well as garden vegetables such as peas and beans, and they have an unusual trick that allows them to thrive in low-fertility soils. Their roots have nodules, small lumps inside which live Rhizobium, bacteria that can trap nitrogen from the air and turn it into a form usable by plants. … This relationship gave legumes a huge advantage in the days before artificial fertilizers were widely deployed. Ancient hay meadows are full of clovers, trefoils, vetches, meddicks and melilots, able to outcompete grasses because they alone have access to plentiful nutrients. Most of these plants are pollinated by bumblebees.”

More information about bumblebees and bumblebee conservation:

Bumblebee Conservation Trust

Bumble Bee Watch

BugGuide (Bombus)

The Xerces Society – Project Bumble Bee