Insect pollinators ensure transfer of genetic material between plants (sexual reproduction) and maximisation of fruit sets and yields. However these beneficial insects are in decline worldwide, owing to the intensification in crop production of the 20th century (giving less diverse forage), misuse of pesticides and proliferation of various diseases. A driver of decline still under contention is global warming (more accurately climate change) as the effects are hard to predict and will be different for specific groups of pollination- an optimum foraging temperature for bees may be not be so good for pollinating moths, butterflies, hoverflies or birds.
Temperature too high?
Many insects are ectotherms, meaning they do no generate their own heat and need to bask in the sun to become warm enough. Pollinator groups such as butterflies have their distributions limited by low temperatures at high latitudes, so when the climate warms in these areas the generalist species (those that may feed on a wide breadth of flowers) are expected to expand their distribution northwards, but a colder winter temperature will cause their range to recede southwards. For bumblebees the relationship is not as clear- some species have retreated northwards (Bombus distinguendus) and others have retreated southwards (B. sylvarum). Other problems may arise for bumblebee queens that overwinter and emerge from dormancy to find their newly-found colonies is out of sync with their forage plants- this is known as a phenological mismatch or phenotypic asynchrony.
Out-of-sync with host plants?
Climate change is causing phenological advances (a delay) of flowering in plants which means insects have a shorter foraging season to feed and raise their young (either by feeding larvae or provisioning resources to eggs). Non-Apis bees (bees other than honeybees) in particular are shifting in relation to their host plants, with their queens even emerging from overwintering to find very few nectar plants have flowered yet, suggesting no clear pattern for phenological mis-matching. But some researchers argue that in robust pollinator networks there is an assemblage of multiple plant species for early emerging or late emerging pollinators to feed on, and vice versa. For specialist pollinators there is also a risk of spatial mismatches, where plants offering nectar and pollen shift their range and distribution much to the chagrin of specialist pollinators that must ‘track’ their host plant by migration (but this depends on dispersal ability, commonness of preferred nesting habitat). Generalists, however, will be able to take advantage of biodiversity of forage plants and will be affected less. In response to pressures to alter their diet, some bee species have even been rapidly evolving shorter tongues in order to feed from plants with shallower corollas. In 40 years, 2 alpine species of bumblebee (Bombus balteatus and B. sylvicola) have reduced their tongue length by 3 millimetres in response to a 60% decline in flower production.
Exacerbating other drivers of decline?
Climate change may interact synergistically with other causes of declines, for instance increased temperatures may speed up pathogen growth rates and lead to increased proliferation of bee parasites, such as varroa mite. Climate warming is speculated to increase the competition for resources between native bees and invasive ‘super-generalists’, possibly leading to extinction by competitive exclusion. Climate change may also cause development of agricultural methods that have an adverse effect on bees, such as devoting more land to growing crop monocultures (reducing florally diverse habitats) or increased use of pesticides.
What can be done?
Efforts are being made to help sustain pollinator diversity in agricultural landscapes, such as the compulsory planting of wildflowers through environmental stewardship schemes. However some researchers argue the existing biodiversity of food plants will ensure plant-pollinator phenological synchrony against climate change, and only very specialist feeders will be at risk. The rapid evolution of some at-risk bee species (such as those with shorter tongues) will also play a key role in recovery of pollinator populations. Efforts to mitigate the effects of climate change (such as reduction of emissions and geo-engineering solutions) would also reduce the subsequent effects on pollinator decline.
Bartomeus, I., Ascher, J. S., Wagner, D., Danforth, B. N., Colla, S., Kornbluth, S., & Winfree, R. (2011). Climate-associated phenological advances in bee pollinators and bee-pollinated plants. Proceedings of the National Academy of Sciences, 108(51), 20645-20649.
Burkle, L. A., Marlin, J. C., & Knight, T. M. (2013). Plant-pollinator interactions over 120 years: loss of species, co-occurrence, and function. Science,339(6127), 1611-1615.
Garibaldi, L. A., Steffan-Dewenter, I., Winfree, R., Aizen, M. A., Bommarco, R., Cunningham, S. A., … & Klein, A. M. (2013). Wild pollinators enhance fruit set of crops regardless of honey bee abundance. Science, 339(6127), 1608-1611.
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