The Impact of Climate Change on Pollinators

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About three-quarters of the world’s crops depend to some extent on the availability of external pollination to thrive, although because this figure does not include most cereal crops, such as corn, wheat, and rice, it only accounts for about one-third of total crop production.  The loss of pollination would not necessarily eliminate production of those crops, which are mainly fruits, vegetables, and tree nuts, but it would affect their yields and more importantly, their nutritional value.  According to a 2019 article in Forbes, between $235 and $577 billion worth of annual global food production relies on such services.

I’ve written in the past on the problems that have emerged in the commercial provision of pollination services through making available bee hives that are transported by truck throughout major specialty crop growing regions such as California, Texas, Michigan, and Georgia during the appropriate flowering seasons.  A phenomenon that has become known as ‘colony collapse disorder’ or CCD was first identified in 2006 in the United States, as a large number of honeybees disappeared or died outside of their home hives, with losses approaching 70 percent in some parts of the country.  CCD has also emerged as a problem in the commercial pollination sector in other developed countries, such as within several European Union member countries and Japan.  Commercial beekeepers primarily use honeybees for their hives, because they can generate revenue from the honey produced as well as the pollination services they provide, although bumblebee hives are sometimes used to pollinate specific crops, such as berries, melons, peppers, and tomatoes.

A variety of potential causes have been identified, including the use of certain classes of insecticides on crops, invasion of virus-carrying pests called varroa mites into hives, and stress on the insects from being dislocated from their home grounds.  Recent research suggests that CCD may be the result of a combination of these factors.  The European Union has banned outdoor use of a specific class of pesticides known as neonicotinoids that have been identified as a prime culprit in several studies, while the U.S.’s Environmental Protection Agency imposed some restrictions in 2019 on their use that are short of a ban.

However, there is an additional factor that is likely affecting not only the availability of commercial pollination services, but also pollination provided by wild species.  That factor is the disruptions being wrought in the natural environment by climate change, both in terms of the average increase in global temperature but also related phenomena such as more variable rainfall and more frequent severe weather events such as droughts and tropical storms/hurricanes.

So-called wild pollinators include not just other species of bees such as leafcutters and bumblebees, but also butterflies and beetles, as well as vertebrate species such as birds and bats–basically, any creature that flies and relies at least in part on flowering plants for food.  A recent study by the UN’s Intergovernmental Science Policy Platform on Biodiversity and Ecosystem Services (IPBES) found that up to 40 percent of the world’s pollinator species may be at risk of extinction, due at least in part to the impact of climate change.  Recent studies have shown that climate change has led to the migration of certain pollinator species toward new zones, such as higher altitudes or towards the poles, in search of milder conditions.  

This shift could have several implications.  The migration could lead the species in motion away from areas with economically important crops that are dependent on pollination, potentially reducing yields and nutritional content of those crops.  It could also lead them into areas where they will compete with native pollinator species for habitat, reducing the overall supply of wild pollination services in that region.

In North America, recent research suggests that climate change is having a devastating effect on the population of monarch butterflies.  Researchers examining data from 18,000 monarch counts in the United States, Canada and Mexico learned the species is extremely sensitive to weather conditions in its spring and summer breeding grounds.  The species has largely disappeared in the western U.S. since the 1980’s, and has dropped by nearly 80 percent in the eastern part of the country over the same period.  One hypothesis is that milkweed, which is the main source of food for this butterfly species, is reacting to warmer temperatures by increasing the level of the toxic substance cardenolide it produces as a defense mechanism from the plant's predators.  This change could be affecting the reproduction rate of monarchs, which typically plant their eggs on this plant.  The milkweed supply is also being reduced by the widespread use of herbicides, further threatening the monarch population.  The monarch butterfly is seen by entomologists as an indicator species, so what happens to it may foretell similar problems for other insects.

The health of bees in commercial colonies is also under threat from climate change, as these populations do not have the luxury of changing their migration patterns as their wild pollinator counterparts can.  Research suggests that severe weather events such as droughts and hurricanes, projected to occur more frequently as climate change advances, augment losses in bee colonies that are already occurring due to the CCD factors described above.  This is believed to occur both due to physiological stress on the insects and the adverse impact on the flowering crops in the neighborhood that the colony bees usually feed on when not deployed in fields with horticultural crops.  Heavy rains during the spring season can also inhibit the ability of bees to collect food for their offspring.

Studies in Brazil and California suggest that reduction of pollination can modestly reduce overall yield of the horticultural crops that rely partially or wholly on that service. Since the crops most dependent on pollination are citrus, berry, tomato and pepper crops that provide much of the natural Vitamin C and A in human diets, this problem could lead to serious vitamin deficiencies if not addressed.

 

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