Climate change puts pollinators out of sync

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One of the biggest concerns, as the climate and environment around us changes, is the continuing decline of pollinators. In the UK, insect pollinators are estimated to be worth £430 million each year for their role in pollinating our crops. Research published last December provides strong evidence that temperature rises associated with climate change are negatively impacting on the relationships between plants and their pollinators.

Comparing museum specimens with wild bees and orchids, researchers at The University of East Anglia investigated the delicate mutualism between the early spider orchid (Ophrys sphegodes) and its pollinator the solitary miner bee Andrena nigroaenea.The orchid tricks the miner bee into pollinating its flowers by mimicking the appearance and smell of a female miner bee. The orchid’s flower appears at the same time as the bees emerge and exudes the same sex pheromone that female miner bees release to attract a male. The poor males are tricked into “pseudocopulation” with the flower, which transfers pollen. Multiple mistakes like this and the miner bee has helped the orchid to reproduce, even if it has failed to do so itself!

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Under natural conditions, the orchid flowers are carefully timed to emerge when the male miner bees emerge – the female bees are usually a little later appearing, so the orchids make use of this short window where they do not have to compete with real females for the affection, and pollination skills, of the male miner bees.

However, using well kept museum specimens at the Natural History Museum, London, the British Museum and Oxford University Museum of Natural History, dating back to 1893, the researchers showed that for every 1&degC increase in temperature, the orchids emerged on average 6 days earlier, but the bees emerged on average 11 days earlier. Most importantly, the effect on male and female miner bees was not equal – male miner bees emerged 9 days earlier but females (who usually emerge later than males) gained 15 days for every degree increase in temperature. The overall effect of this is a mismatch between the flower and its pollinators. Now, the male bees emerge first, search for females and find none to mate with. A few days later, the orchids begin to flower, but the female bees are also emerging now as well. The orchid flowers simply can’t compete with the real deal, and may be suffering severe reductions in their pollination success as a result.

pollinator003This is not an isolated phenomenon. Similar mismatchings in the timing of events, known as phenology have been identified in oak trees, nettles, puffins, eels, butterflies and moths, to name a few. For example, winter moths (Operophtera brumata) carefully time the emergence of their caterpillars to coincide with the bud burst of flowers in the oak trees (Quercus robur) where they live. This is crucial to their survival – emerge to early and the caterpillars starve, emerge to late and they are relegated to inferior food sources such as leaves. Warming springs over the last thirty years have lead to a mismatch, however, with caterpillars routinely emerging earlier than the bud burst. Since 1975, this has lead to a gap of up to 20 days between caterpillar emergence and oak tree bud burst – catastrophic for the winter moth!

Similarly, recent research investigating phenology in North Sea fish and seabirds has revealed mismatching is occurring here too, in response to increasing sea surface temperature. Many sea birds feed on Lesser Sand Eels (Ammodytes marinus), such as Razorbills (Alca torda), Common Guillemots (Uria aalge), European Shags (Phalacrocorax aristotelis), and Atlantic Puffins (Fratercula arctica). Increasing temperatures have caused a shift in development of sand eels, causing them to develop more slowly. Although sea birds are also showing delays in development, their shift has been less marked than those of the eels, meaning that seabird parents are tending to bring home smaller eels to feed their growing chicks. Researchers estimated that this mismatch is resulting in a loss of between 2000 and 5000 joules of energy per fish. Shags, for example, took home an average of 4.8 kJ less per fish in 2005 than 20 years earlier, representing a loss of over 70%!

pollinator002Why do these mismatches occur? Phenologial mismatches reveal the differences in plasticity and evolvability in different species and particularly, different trophic levels. Larger animals, and those higher in the food chain, tend to have a longer generation time and evolution is therefore slower to respond to changes in climate. Butterflies, moths, bees and eels reproduce more rapidly than trees, plants and seabirds. The extent to which behaviour and development are plastic (able to change in response to environmental conditions), is therefore crucial in slower-breeding animals in determining the extent to which the are able to track climate change. The mechanisms that influence a species’ plasticity is still relatively poorly understood, however it is expected that plasticity will be favoured by evolution for species living in frequently changing environments. Species that breed slowly in relatively stable environments may therefore be the most at risk from climate change, particularly if they are part of a mutualistic relationship with another (faster-breeding) species.

If this type of climate-change driven mismatching is a common phenomenon in plant-pollinator interactions, then continued climate change will bring about progressive disruptions to the natural pollination systems that provide almost all of our food. Over 75% of all our food crops rely upon animal pollination, and there are already major concerns over declines observed in bees and other pollinators due to disease, pesticides and habitat loss. Disrupting the natural mutualisms between plants and pollinators could have dire consequences for our native pollinators, and our future food security.

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