INTRODUCTION
Bees are the leading pollinators in terrestrial ecosystems and are thus essential providers of the ecosystem service of pollination: the transfer of pollen from the stamens (male organs) to the pistils (female organs) of the flower. Pollination is vital both for the agricultural industry and for natural ecosystems. Therefore, pollinators are central to global biodiversity: if pollination failed, ecosystems would collapse and the world would lose many food sources, among other consequences.
Bees are the quintessential pollinators, but they are currently under intense pressure. Some negative factors are the reduction of flower biodiversity, continued pesticide exposure, and contact with parasites accidentally spread during human trade. The situation is likely to worsen in the future because of climate stress and the disappearance of green plots due to insatiable urbanization. Several agencies predict the human population will reach 9 billion in 30 years. This circumstance and the corresponding landscape conversion will make the role of bees even more committed to human survival than at present.
As in the case of most insects, climate change will also have a profound impact on bee biodiversity. Yet, predicting these effects is difficult as changes are ultimately the sum of multiple factors acting together and at the same time. Numerous scientific papers agree that habitat loss will be the most universal and impactful factor, with invasive species, parasites, and pesticide use following in order of importance.
The problem with this lack of studies is the scarcity of good data on the abundance and distribution of different species of bees, especially wild bees. In addition, there are species still lacking a scientific description. Without this basic knowledge, it is hard to estimate bee populations’ decline and generate relevant conservation strategies.
Honey bees, especially the most widely used species, such as the Western honey bee (Apis mellifera), and wild bees, such as bumblebees (Bombus terrícola, Bombus dahlbomii and Bombus occidentalis) have been in decline in recent decades. Scientists already consider some species extinct, as in the case of Bombus franklini, without positive records since 2006. Although studies show the regression of domestic and wild bees, data are incomplete because of the great diversity of taxa and their complex responses to environmental change. Data on pollinators from some regions are also lacking, given that most available data come from America and Europe. We need good answers to numerous questions, so we assume forthcoming studies from all continents. Specifically, we need to assess the population status of the different species of bees to predict their response to the impending climate change. Assessments should cover bee abundance (whose knowledge is incomplete), population dynamics, and the factors that could influence bees’ survival. Such factors include the use of pesticides, changes in land use, urbanization, habitat loss, the impact of parasites and invasive species, interactions between these factors, etc.
THE PRICE OF CROPS AND POLLINATORS
The vast majority of crops in natural conditions depend on insects for pollination. More specifically, bees pollinate approximately 73% of the world’s cultivated plants, while the rest -in decreasing order- are pollinated by flies, bats, wasps, beetles, birds and butterflies.
In return for helping plants reproduce, pollinators get food (nectar, pollen or both). This mutualistic relationship (both parties involved obtain a benefit) has endured and evolved over centuries through the coevolution of structural and functional characteristics, such as the length of the pollinators’ tongues in correspondence with the depth of the flowers’ tubes. This mutualism has sustained most terrestrial ecosystems, both natural and artificial (agroecosystems).
The fraction of world crops requiring animal pollination has increased by about 45%. As a consequence, demand is outstripping supply.
Nature underpins all human activities and life. The goods and services it provides are vital for our well-being and present and future economic and social development. Pollination is one of the essential ecosystem goods and services for the proper functioning of natural systems and also for food production. Its importance is paramount also in monetary terms since its total economic value in crops worldwide is estimated at 156 billion annually. With this increased dependence on pollination and a scenario of climate change -marked by a decrease in bee biodiversity and its physiological and behavioural effects, such as reduced pollinator efficiency- some economic sectors are extremely concerned.
In addition, we must highlight that the agents that carry out most crop pollination worldwide are wild pollinators rather than domestic honey bees. For this reason, increasing honey bee numbers alone in a climate change scenario is unlikely to provide a good enough answer to the pollination crisis. Also, even if it were a good solution, relying on one species alone would be an unnecessarily risky strategy.
EFFECTS OF CLIMATE CHANGE: BEES, PLANTS AND THEIR POSSIBLE LACK OF ADJUSTMENT
Honey bees (Apis sp.) are the leading domestic pollinators because of their high foraging potential and are currently the most economically valuable crop pollinator globally. Honey bees have a wide geographical distribution encompassing a diversity of climates: as a consequence, honey bees are genetically diverse and have great potential for adaptation. They are, in addition, among the few bee species whose adults remain active during winter and regulate hive temperature. For these reasons, much of the research on pollinators’ decline and the associated economic losses has focused on this group of organisms. Here is a breakdown of the potential effects of climate change on honey bees:
- Climate change happens too fast, and despite bees’ adaptive potential, their populations would not have enough time to adapt. As a result, they would move to higher latitudes and altitudes where they could find more suitable climate conditions. Likewise, plants would face the same problem and might respond similarly. At this point, there are two possible scenarios:
- Pollination could still happen if the plant and its pollinator move together in space and time. However, this flower-pollinator coupling could be severely compromised and lost, as it depends on the phenology (life cycle phenomenon, e.g. the reproductive season) of the crop as well as on insects and climate change.
- Climate change could affect the quality of the floral environment, increasing or decreasing the capacity and development of bee colonies and affecting their life cycle. As a consequence, pollination would decline.
- These temporal (phenological) mismatches between plants and bees could create serious consequences: fewer insect visits would reduce pollen production and availability. Bees would experience food shortages and become weaker and more vulnerable to pathogens. In addition to climatic and nutritional stress, pesticides could further increase plant vulnerability as the effects of pathogens would be more pronounced, eventually affecting bees’ biological fitness (survival and reproduction).
- Climate change will lead to atmospheric instability and thus to episodes of floods and droughts and higher global temperatures. Will plants be able to survive the rapid onset of these conditions? If the answer is yes, would the pollen and nectar be suitable for bees? One example that this may not be the case is that, in rainy weather, honey bees avoid acacia flowers because their nectar is too diluted. Also, we know that in dry spells, pollen and nectar production is much reduced, and their nutritional quality is impoverished. This is important because it is the young that need a diet of pollen.
- Moreover, bees cannot store food properly during intervals of extreme heat. Thus, winter food availability will not be enough, and worker bees will be incapable to feed the young (which should become workers the following spring). In addition, adults would have a weakened immune system.
ONE MORE STRESS: PESTICIDES
Bees have been exposed to pesticides through the intensification of agriculture despite altering their biological efficacy. This situation is likely to amplify in the future, and bees will therefore be exposed to pesticides routinely enough to suffer lethal or sublethal effects.
At the individual level, continued exposure can affect both detoxification mechanisms and immune responses, making bees more susceptible to parasites. Common pesticides such as imidacloprid, fipronil or thiacloprid have prejudicial effects on adult bees’ learning and orientation. They can also produce an ailment in nurse bees transmissible to larvae. Those pesticides -which belong to the group of neonicotinoids- are neurotoxins, i.e. they affect the nervous system of insects.
Colonies would also be adversely affected as pesticides have been associated with disease outbreaks. This would therefore indicate that increased infection may affect tolerance to agricultural pesticides. Thus, interactions between pathogens and pesticides may contribute to increased mortality of bee colonies and other pollinators.
HABITAT LOSS AND URBAN BEES
The effects of habitat loss will be visible in the long term. It is one of the factors with the highest impact on this decline in bees as it is the habitat where they have to find adequate floral resources and nesting sites.
With increasing urbanisation, the loss, fragmentation and destruction of natural habitats are accelerating, and we need more and better studies of bee communities found in cities. It is a field still largely unknown, and urban environments could become an opportunity to mitigate the loss of pollinators, provided authorities apply a sustainable city model and adequate conservation measures.
We need city designs containing enough suitable habitats to support natural bee populations. It must happen sooner or later, so the sooner, the better.
BEE PROTECTION AND SUSTAINABLE POLLINATION
The previous paragraphs should make clear that we must take measures to stop bees’ populations decline. Numerous factors are involved, and it can be hard to draw straightforward conclusions. Yet, this does not mean that conservation strategies need to be complicated. Such measures must consider the types of land use and socio-economic development of the region. Listed below are some generalist measures that would help bees:
- Enhance the biodiversity of floral resources: set up pots and gardens with suitable floral species, plant flowers at the margins of fields, or leave patches of natural habitat between crops or near farmland. These measures would promote crop pollination and bee biodiversity.
- Education and outreach: to expose the problem of running out of bees and to bring the public closer to nature, which is becoming increasingly urbanised.
- Territorial planning: sustainable cities. A city with sufficient green spaces and urban and vertical gardens would be compatible with social and economic activity and the environment. After all, these spaces will also be necessary to provide oxygen and thermoregulate the air in cities.
- Reduce pesticides: they are actively used, disproportionately and without reason, on many occasions. It is highly desirable to develop short- and long-term risk assessment programmes, an application protocol for each habitat and pesticide, an efficacious advisory system and incentives for the farming community.
- Provide suitable nesting sites, whether structures, cavities, bare ground or simply by increasing native floral diversity. City planning should consider this measure too.
- More studies and monitoring programmes: we lack knowledge on several fundamental issues. We need more efficient monitoring methodologies and methods to estimate the diversity, abundance and distribution of different bee species. Finally, it is desirable to build indices or a warning system to indicate how far we are from this pollination crisis to take respective measures as well as possible.
- Prevent introductions of non-native bees and pathogens: control of trade in bee species and implementation of strict quarantine controls for each movement. We must prevent the introduction of non-native bee species at all costs. For this reason, the relevant policies must be tightened.
