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Pollination Dynamics in a Changing Climate

June 8, 2012

Photo credit: ggallice, Flickr Creative Commons. CC BY 2.0.

The behavior of plants and their pollinators is often intimately and specifically tied to the environmental factors found within their habitats (Gonzales et al. 2009). This is important, given that it is well-documented that average global temperatures have risen around half of a degree Celsius in the last hundred years, and are predicted to continue rising (Solomon et al. 2007). Many plant and animal species are already showing earlier spring activity correlating with this rise temperature (Root et al. 2003, Hoover et al. 2012, Bartomeus et al. 2011). Since the ability of many flowering plants to reproduce is dependent on efficient animal pollination, changes in timing of cyclical, seasonal behavior (known as an organism’s phenology) have the potential to cause mismatches in the availability of plants and pollinators. But while this idea of a “phenological mismatch” has been considered a possible and probable result of climate change among ecologists for many years, it has yet to be definitively shown in action (Wilmer 2012). Another predicted effect of warming is a change in habitat ranges as temperatures increase, which could theoretically cause geographic disconnects between pollinators and plants (Franzén and Öckinger 2011). The fact that pollination dynamics in fragmented habitats can be especially vulnerable to environmental instability (and that such fragments are practically ubiquitous near people) means the possibility that climate change could negatively affect biodiversity—either directly or through compounding effects—is worth investigating.

Honeysuckle plants are flowering an average of ten days earlier compared to 1968 (Cayan et al. 2001). Photo credit: Lane 4 Imaging, Flickr Creative Commons. CC BY 2.0.

Phenological shifts
The potential for a mismatch between timing of events comes from the different ways that plants and pollinators respond to environmental cues. For example, bee emergence is often stimulated by the occurrence of a certain number of consecutive days above a threshold temperature, whereas plant response is commonly affected by day length (Wilmer 2012). However, while many plants and pollinators have shown earlier spring activity in response to warmer temperatures, most research so far suggests that these changes are happening more or less in sync with each other. For example, one large study of generalist pollination dynamics in northeast North America found that bumblebees are emerging in the spring an average of about ten days earlier than they did 130 years ago, which roughly parallels the shift seen in plants in the same area (Bartomeus et al. 2011). However, another study found that species with no apparent timing shifts were much less successful at attracting pollinators when early flowering was experimentally induced (Rafferty and Ives 2011); conversely, plants that were showing earlier flowering had no problem attracting pollinators and reproducing in the experiment. This suggests that plants that are dependent on certain pollinators may be under pressure to maintain concurrent timings, which could explain why we don’t see mismatches despite observing a multitude of phenological shifts across the globe.

Queen bumblebees emerge from hibernation after a certain number of “degree days,” or days above a required temperature threshold after winter. Photo credit: Rob Cruickshank, Flickr Creative Commons. CC BY 2.0.

Changes in species’ natural ranges
Another predicted effect of climate change on plant-pollinator relationships is a shift in the ranges where species live. A warming climate is expected to allow populations to expand into previously colder regions, such as higher elevations and/or towards the poles, which appears to be the general trend (Root et al. 2003). For example, butterfly and moth species richness has been increasing in the Arctic (which is experiencing temperature increases 2-3 times the global average), which would be expected from northward range expansion (Franzén and Öckinger 2011, Pateman et al. 2012). Shifting population ranges are not necessarily negative for ecosystems, however: if both a plant and its pollinators shifted together, there would be little to no loss of pollination services or reproductive success. But since animal pollinators are much more mobile than their stationary plant counterparts, they may be capable of shifting at faster rates (Wilmer 2012); this could potentially lead to an eventual loss of plant-pollinator overlap if warming continues. But while certain species, types of organisms, and communities have been found to make significant range shifts (Walther et al. 2002), so far there has not been conclusive evidence suggesting that plants have lost access to pollinators by this mechanism.

The arctic butterfly Aricia agestis (brown argus) has expanded its range northward almost 80 kilometers (Pateman et al. 2012). Photo credit: Silversyrpher, Flickr Creative Commons. CC BY 2.0.

While it is good news that no clear, isolated effects of climate on pollination have been found, it’s important to realize that these early studies are really just laying the groundwork for future research. So far, ecologists aren’t even necessarily sure which questions to ask, since serious inquiry into these problems has only occurred in the last few decades. Furthermore, the most informative studies in this field are long-term and examine real, natural habitats; these can be the most difficult (and expensive) types of studies. Once we begin considering the potential compounding effects of invasive species, habitat fragmentation, and the prediction of continued, accelerated warming, it is clear that future threats to pollination dynamics are not out of the question.

This post is the third in a series on pollination dynamics. View the others:
1. Plant-Pollinator Relationships
2. Habitat Fragmentation and Pollination Dynamics

References

1. Bartomeus, I., Ascher, J. S., Wagner, D., Danforth, B. N., Colla, S., Kornbluth, S., and Winfree, R. 2011. Climate-associated phenological advances in bee pollinators and bee-pollinated plants. Proceedings of the National Academy of Sciences, USA 108(51):20645-20649.

2. Franzén, M., and Öckinger, E. 2012. Climate-driven changes in pollinator assemblages during the last 60 years in an arctic mountain region in northern scandinavia. Journal of Insect Conservation 16(2):227-38.

3. Gonzalez, A.M.M., Dalsgaard, B., Ollerton, J., Timmermann, A., Olesen, J.M., Andersen, L., and Tossas, A.G. 2009. Effects of climate on pollination networks in the west indies. Journal of Tropical Ecology 25(5):493-506.

4. Hoover, S.E.R., Ladley, J.J., Shchepetkina, A.A., Tisch, M., Gieseg, S.P., and Tylianakis, J.M. 2012. Warming, CO2, and nitrogen deposition interactively affect a plant-pollinator mutualism. Ecology Letters 15(3):227-34.

5. Pateman, R. M., Hill, J. K., Roy, D. B., Fox, R., & Thomas, C. D. 2012. Temperature-dependent alterations in host use drive rapid range expansion in a butterfly. Science (New York, N.Y.) 336(6084):1028-1030.

6. Rafferty, N. E. and Ives, A. R. 2011. Effects of experimental shifts in flowering phenology on plant-pollinator interactions. Ecology Letters 14(1):69-74.

7. Root, T. L., Price, J. T., Hall, K. R., Schneider, S. H., Rosenzweig, C., and Pounds, J. A. 2003. Fingerprints of global warming on wild animals and plants. Nature 421(6918):57-60.

8. Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M., and Miller, H.L. 2007. Climate change 2007: The physical science basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge.

9. Walther, G., et al. 2002. Ecological responses to recent climate change. Nature 416(6879):389-395.

10. Willmer, P. 2012. Ecology: Pollinator-plant synchrony tested by climate change. Current Biology 22(4):R131-2.

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