Mitigating the Effects of Heat on Urban Pollinators
This piece originally appeared in the Fall 2018 edition of Xerces’ biannual publication Wings. Click to view the full Fall 2018 issue.
In June, in Sacramento, California, I participated in the presentation of a Xerces short course on creating urban pollinator habitat. Before the class started, we spent some time netting pollinators to show to the participants. It wasn’t long before we had found sweat bees, wool carder bees, mining bees, bumble bees, and, of course, honey bees—all in the middle of the city.
Indeed, many urban environments are home to large and diverse communities of native bees. A recent study by Paul CaraDonna and colleagues shows that bumble bees have higher survival and reproduction rates in towns and cities compared to farmland. But we also know that bees are sensitive to temperature, and that temperatures in cities are higher than they are in more natural areas, due primarily to large expanses of impervious surfaces, such as concrete and asphalt, that cause an “urban heat island” effect.
Climate change will continue to bring higher temperatures and greater extremes in weather, as well as increases in the frequency and intensity of heat waves. Thanks to the urban heat island effect, these variations will be exacerbated in cities in ways that may spell trouble for bees. Increasing temperatures can affect bee performance by changing phenology (the timing of biological events such as the rate of development or the date of emergence), and reducing survival rates, body mass, fat storage, and reproduction—and can ultimately result in fewer bees. Research by April Hamblin and colleagues at North Carolina State University, for example, showed that bee abundance in the city of Raleigh declined 41 percent with every 1˚C increase in temperature.
Pollinators are affected by climate change in a variety of ways. Some species will shift their ranges to track more optimal conditions, provided that they have habitat to move to and through. Other shifts could be problematic if pollinator emergence times or rates of development become misaligned with the flowering times of their host plants. Plants will also respond to the changing climate, and alterations in available flora, including plant diversity and community composition, are likely to have knock-on effects on pollinator populations. Increasing temperatures and greater drought frequency can affect the number of flowers a plant produces as well as the amount of nectar and pollen in each flower, thereby reducing the number of pollinators that an area can support. Moreover, the effects of climate change will combine with other drivers of species declines, such as habitat loss, pesticide use, and disease, which can interact in unexpected ways that may be difficult to predict. The combined interplay of multiple drivers may lead to outsized negative effects on pollinators.
Notwithstanding the potential for the effects of climate change to be amplified in cities, urban habitats can also present unique opportunities. Several years ago, I came across Michael Rosenzweig’s book Win-Win Ecology, in which he points out that nature preserves alone are not sufficient to safeguard the Earth’s biodiversity and suggests that we should change the way we design human habitats so that those areas serve to protect biodiversity as much as possible. In other words, we have to get better at sharing spaces. Urban environments provide a perfect opportunity to put this idea into practice.
What actions can we take to help urban pollinator communities become more climate-resilient? The first is to create and preserve habitat. Enhancing and restoring habitat is crucial to protecting biodiversity because high-quality patches, particularly when connected, have many benefits for wildlife. Connections enable individual insects to move among populations, increasing genetic diversity and perhaps helping to rescue declining populations. These patches also provide habitat for species to move to and through as they shift their ranges in response to climate change. Finally, increasing habitat and connectivity can allow populations to grow, which is important because small populations are more likely to disappear than are large ones. You can create habitat by adding a pollinator garden to your yard or balcony. If you don’t have a yard, consider talking to local park managers about installing pollinator gardens. Many schools, churches, and businesses are also planting gardens on their properties, and you can offer them encouragement and support.
Your pollinator habitat will be most beneficial if it contains a variety of different flowers and includes native species. Aiming to have a few different species blooming at all times from early spring to late fall will support a diversity of pollinators and may help buffer the effects of timing mismatches between pollinators and host plants. It is ideal to incorporate native plants into your garden as much as possible; they typically are better adapted to local conditions, and research indicates that they may attract more native bees than do ornamental and nonnative plants. Check out Xerces’ pollinator plant lists to find ideas for native plants that will attract pollinators in your area. Be sure not to forget butterflies and moths and to provide host plants for their caterpillars.
Native bees will benefit from the inclusion of nesting habitat in your pollinator garden. Leaving some bare soil will be helpful for ground-nesting bees, while retaining snags and downed logs, and growing pithy-stemmed shrubs such as elderberry, help to create natural sites to harbor wood-nesting and cavity-nesting bees.
Reducing the use of pesticides should be part of planning your garden or habitat, including avoiding the use of plants that have been pre-treated with neonicotinoids (widely used insecticides that are absorbed by plants and remain in their tissues long after application). Pesticide reduction is important for mitigating the effects of climate change, because multiple stressors can build upon each other. While exposure to some particular pesticide may not be lethal to pollinators, exposure to that pesticide in combination with a heat wave or reduced flower availability may become deadly. Reducing pesticide use in our communities helps to alleviate one of those stressors for pollinators as well as other organisms, making these populations more climate-resilient.
Beyond creating habitat, look for ways to mitigate the urban heat island effect. Cities can reduce temperatures by installing rooftop gardens and by planting more vegetation and trees. In some regions, painting rooftops white can help. Policies that promote green infrastructure—ecoroofs, bioswales, etc.—in your community can mitigate some of the heat island effect as well, all of which can help pollinators and other wildlife. Depaving—removing impervious surfaces and replacing them with gravel or plants—reduces temperatures, increases infiltration when it rains, and has the added advantages of lowering flood risk and lessening the amount of pollutants that make it into our waterways. The organization Depave has a variety of resources for people interested in reducing the amount of impermeable surfaces in their communities.
These steps can help offset some of the negative effects of climate change on urban pollinators, but it’s also imperative that we work to constrain the magnitude of climate change itself. A team of scientists from Britain’s University of East Anglia and Australia’s James Cook University, led by Rachel Warren, analyzed the effects of limiting future warming to 1.5˚C, 2˚C, and 3.2˚C above preindustrial levels. Recently published in Science, their study found that insect biodiversity is strongly affected by the amount of warming. If temperatures rise 3.2˚C—the level we will reach if no further emissions reductions are made beyond those set for 2030 in the Paris Agreement—then an estimated 49 percent of all insects will suffer a reduction in range of more than half. The effect of restricting warming to 2˚C or 1.5˚C dramatically cuts the number of insects predicted to suffer such reductions: at 2˚C, it is 18 percent of insects, and at 1.5˚C, just 6 percent. Reduction in range and the corollary loss of available habitat is an important metric because reduced range means smaller populations, which make a species more vulnerable to extinction. You may be wondering how such a seemingly small temperature difference can have such large effects, but it’s important to remember that these differences are not actually small. The numbers do not refer to changes in average local temperatures, but to changes in average global temperatures. To give context to those numbers, the difference in average global temperature between a warm period and an ice age is only around 5°C.
Recently, I was talking with our director of communications and outreach, Matthew Shepherd, about our work toward boosting the climate resilience of pollinators in California’s Central Valley by increasing habitat connectivity. Matthew pointed out that we’re not just connecting habitats, we’re also connecting people and communities. He’s right about how important this is, because the challenges posed by global climate change are enormous—and those challenges can only be met by a large number of people working together to overcome them. There are small actions that we can take every day to make a difference and, over time, our small actions, combined with those of our neighbors, add up to become substantial. It is valuable to keep this idea in mind as we each find ways to reduce our carbon footprints and advocate for action. By coming together with others in our communities who care about climate change—and working to increase the numbers of those who care—we will be able to bring about the changes that are needed before it’s too late for our pollinators.
Angela Laws has a background in climate change research and is a monarch and pollinator ecologist with Xerces’ endangered species program.