There is growing evidence that many pollinators and plants are being triggered into earlier but not necessarily synchronous activity by the same temperature shifts associated with global warming. Find out what strategies you can implement in your own garden, orchard, or farm to enhance your plant pollinator habitat.
The consequences of global climate change could be dire for both pollinators and the plants they pollinate if pollinator habitats aren't adapted.
Photo courtesy Chelsea Green Publishing
With climatic uncertainty now “the new normal,” many farmers, gardeners, and orchardists in North America are desperately seeking ways to adapt how they grow food in the face of climate change. In his book Growing Food in a Hotter, Drier Land (Chelsea Green, 2013), author Gary Paul Nabhan, draws from the knowledge of traditional farmers and offers readers time-tried strategies on how to implement desert-adapted practices on their own land. In the following excerpt, learn the importance of plant pollinators and how to keep them in pace and in place with arid-adapted crops.
Buy this book from Chelsea Green Publishing: Growing Food in a Hotter, Drier Land.
What would you do if you showed up for a date, but after a few hours of waiting, you realized that your partner had arrived several hours earlier and then left? Well, that’s one of the potential scenarios for how climate uncertainty is likely to affect crop plant–pollinator interactions in the near future, if it has not already begun. The polite term for this temporal mismatch is asynchrony, but the colloquial expression is getting stood up!
There is growing evidence that many pollinators and plants are being triggered into earlier but not necessarily synchronous activity by the same temperature shifts associated with global warming. However, each partner may be responding differently to these shifts in space and in time, so much so that they are increasingly vulnerable to “ecological mismatches.” Even if plants and pollinators do respond to the same temperature cue, the strength of the response might differ. Certain plant pollinators and their host plants may have successfully interacted with one another for centuries, but now each of their phenologies or seasonal activities has begun to shift at rates somewhat independent of their partner’s. From a data bank of 1,420 kinds of pollinators that are known to visit some 429 kinds of plants, it has been predicted that climate-driven changes in flowering times will reduce floral resources for at least 17 percent and perhaps as many as half of all pollinators, resulting in diminished nutritional diversity within their diets.
While it has already been amply demonstrated that a few pollinators such as bumble bees are getting out of step with certain wildflowers, there is less consensus on whether the flowering of many crop plants has become increasingly asynchronous with their primary plant pollinators.
Nevertheless, it is worrisome that wildflowers such as glacier lilies are now flowering two to three weeks earlier than they did a couple of decades ago, for this may foreshadow what might soon happen to at least some food crops. At certain sites in the Rockies, bumble bees are waking up and emerging from mountain meadows later than they are needed to perform the bulk of pollination required by glacier lilies. Across the continent in Maryland, nectar flows from the flowers of trees are beginning a month earlier than they did in the past, as a result of warmer winter temperatures. At my own office on Tumamoc Hill in Tucson, hibiscus flowers are, on average, opening up and flowing with nectar a month earlier than when the first desert ecologists recorded their flowering in the early 1900s. But averages don’t tell the entire story; compared with the May 23 flowering at the onset of data taking by my predecessors around 1906, hibiscus flowers are sometimes opening up 70 days earlier than they did a century ago, perhaps due to the urban heat island effect exacerbating the influences of global warming.
At the same time, other reports suggest that the suites of available pollinators—and not the flowers themselves—show the most dramatic shifts in timing or in northward movements. Some of them are moving up mountainsides as much as 1,800 feet higher than in the past, while others are emerging several weeks earlier in the year, even as their associated flowers are blooming for shorter and shorter periods each season. In still another field report from the northeasternmost reaches of North America, scientists found that, over the past 130 years, the average springtime emergence of 10 different bee species has advanced as much as 9 to 12 days. Most worrisome is that much of this advance has taken place since 1970, and it closely parallels global increases in temperatures.
If we take some hints from what is already happening to the pollinators of plants native to wilderness landscapes, we can imagine that several different scenarios may eventually occur in agricultural landscapes:
• Due to warmer winters and longer growing seasons, a crop plant will break bud and flower in advance of most bees emerging at the same site, leading to either reduced fruit and seed set or starved bees.
• Due to warming on site, nearby, or farther south along a migratory corridor, pollinators arrive and then leave in advance of flowering, resulting in no seed or fruit set at all.
• Due to different triggers activating both pollinators and crop plants in the spring, there is only partial overlap in the timing of flowering and pollinator availability, leading to reduced fruit and seed set, as well as limited nutritional rewards for the pollinators.
• Due to extremely hot springs and summers, bees or other animals pollinate the crop’s flowers, but the flowers still abort due to heat stress.
• Due to unfavorable weather that limits their nutrition and reproduction year after year, more and more pollinators are starved to death, their populations decline, and their species become endangered.
If you are skeptical that these scenarios may currently or eventually apply to crop pollinators, look again at recent reports in science journals and newspapers. Bumble bee species known to pollinate both wildflowers and crop plants are declining in several regions of North America, with some of them becoming so rare that they have been nominated for the US Endangered Species List. While climate change is not the only factor causing their declines, it is one more contributor to “the perfect storm” that has ravaged pollinator abundance over a single human generation.
It came as no surprise to seasoned beekeepers that their honeybee colonies would suffer as a result of the 2011–12 drought that hit two-thirds of North America. With less nectar and pollen to forage, it is clear that less honey was produced, and likely that pollination services were diminished. It prompted the New York Times’s Harvey Morris to repeat an adage attributed to Albert Einstein: “If the bee disappears from the face of the earth, man would have no more than four years left to live.”
Most fruit and vegetable crops require pollination by animal vectors at levels sufficient enough to set seed, ripen fruit, and allow them to mature as fully formed, nutritionally rich foods. An apple tree that lacks adequate pollination may lack its full array of 10 germinable seeds per fruit, and its fruits may be shriveled or misshapen. In fact, at least some populations of most plant species in the world currently suffer from inadequate pollination due to pollinator scarcity.
In attempting to avoid such problems—by adapting to climate change through maintaining enough pollinators in your foodscape—there are four general rules of thumb to keep in mind:
• It is best to invest in pollinator-habitat-enhancing practices that offer redundancy through the presence of a diversity of wild and domesticated pollinators, rather than simply investing in efforts to keep around a single pollinator species—whether it is a honeybee, bumble bee, butterfly, or bat.
• That said, enhancing a few pollinators that are uniquely matched to your crop mix—for instance, blue orchard bees for an orchard of mixed fruits, alfalfa leaf cutter bees for a pasture of leguminous forages, monarchs for milkweeds and their kin, or squash and gourd bees for your pumpkin patch—will never hurt. Honeybees, of course, are terrific generalists and remain valuable for diversified CSA farms and many other settings.
• The ideal situation to achieve is one in which wild or cultivated nectar plants are planted early enough so that they may begin to bloom on your land in advance of your crops, and maintain enough floral resources to keep a critical mass of bees in your field or orchard through the collective flowering times of all your crops.
• Pesticides and herbicides are likely to disrupt wild pollinators with body sizes smaller than honeybees even more than they disrupt managed honeybee colonies.
But just how do we keep native wild bees in place and in pace with the flowering of crop plants? I conducted field studies as part of the Forgotten Pollinators Campaign’s teams of researchers in four parts of North America, and we determined that the close proximity of healthy wild habitat adjacent to fields and orchards was far more effective than sowing rows of pollinator-attracting forage plants in the fields, or constructing bee boxes or other nesting sites in an orchard. We found that in riparian habitats adjacent to a mix of forage pastures, vegetable fields, and orchards, both migratory pollinators and other invertebrate pollinators in fragmented habitats benefited from this type of linkage. Our initial results have been validated by more recent studies, which also recommend that the width of these corridors of agro-habitats should be greater than one home range. They also provide an economic incentive to preserve wildland habitats in larger food-producing working landscapes. In Canada, for example, it has been demonstrated that higher yields associated with higher populations in “bee meadows” adjacent to fields makes it cost-effective to convert a third of the acreage on a farm in field crops back to wildland habitats or diverse cultivated meadows.
However, immediately along the edges of fields and orchards—in and near the hedges and fencerows we call fredges—there is much that food producers can do to accommodate a wide variety of pollinators, especially bees. Different kinds of bees nest in barren but often unplowed ground; in hollow stems or pithy sticks; or in dead trunks, woody flower stalks, or fence posts.
In addition to taking care of roosting, nesting, and (for butterflies) larval host plants, the key issue is to ensure pollinator diversity and abundance. The availability of sequentially flowering nectar plants in these bee meadows and other wildland habitats can often ensure this abundance and diversity, which in turn favors better rates of pollination and fruit-set.