Reasons for Pollinator Decline

Intensification of Land-use

Rapid urbanisation and expansion of agricultural lands have resulted in fragmentation and shrinking of many natural habitats. Pollinators rely on these natural habitats for forage and nesting resources. Although this affects almost all the species of pollinators, it is more catastrophic for specialised species as they are the most vulnerable to habitat change. Moreover, not all species can locate and move between dispersed resources in different landscapes with ease. The intensification of land use often leads to local and regional extinction of certain pollinator species, thereby altering the structure and function of plant-pollinator communities.

Intensive crop management often includes the use of pesticides that can potentially harm pollinators. It is observed that species richness tends to be lower where pesticide loads and cumulative exposure risks are high. Widely used systemic pesticides (e.g. neonicotinoids) spread throughout plant tissues and can accumulate in plant nectar and pollen, and thereby producing sublethal adverse effects on pollinator performance and behaviour. This can, in turn, impair the brain function and the intellectual ability for foraging workers to relocate the hive in honeybees and reduce the foraging performance, growth rate and queen production of bumblebee colonies.




Climate Change

Plant and pollinator ranges are shifting, which is causing changes in pollinator populations that inhabit the edges of their species’ climatic range. This has resulted in the pollinators being more susceptible to population declines and even extinction as a result of climate change. Apart from changing distributions, climate change can alter the synchrony between plant flowering and pollinator flight periods. This affects the specialist pollinators more severely, but it can also reduce the diet versatility of generalists. For instance, climate change could curtail the bumblebee foraging season by reducing the availability of early or late season forage for queens establishing colonies.




Alien Species

The invasive plant species may co-opt pollinators and come to dominate plant-pollinator interactions by providing abundant foods for those pollinators that are preadapted to exploit them. The alien species may compete for or facilitate native plant pollination.


Invasive species : Lantana camara: Source - Wikipedia


Pests and pathogens

The decline of honey bees in the modern world can is mainly due to pests and pathogens. The varroa destructor mite is the primary vector of many viruses which is responsible for honey bee colony losses. The decline of bumblebee species associated with pathogens makes the pollination network vulnerable to collapse, which might have serious ecosystem consequences.


It is to be noted that there is no single overriding cause of pollinator decline but an array of reasons that cumulative harm. Land-use intensification and its associated effects alongside with diseases have been driving pollinator loses. Climate change, paired with globalisation aids in extending this to developing regions. This results in increasing the translocation of plants, pollinators, pests and pathogens worldwide.


Climate Change. + Habitat Fragmentation

Pollinators under climate change may migrate and colonise new regions with favourable climatic range. This increases the abundance and diversity of the recipient colony. However, this adaptive method may be inhibited by habitat loss and fragmentation. Therefore, low connectivity between habitat remnants is likely to reduce population sizes and increase the extinction rate of pollinators that are poor dispersers or habitat specialists. This again leads to a skewed population of specialists and generalists. Combined with this, the decrease in the food availability due to rapid land use and intensively managed landscapes which further stress pollinators.


Nutrition + Pathogens

The constant change in land-use patterns has caused a decline in the abundance of flowering plants and food they provide to pollinators. This has catastrophic consequences as pollinators require an optimum nutrition balance for growth and reproduction. Furthermore, infections caused by parasites increase the metabolic demand of the pollinators. For example, worker honey bees that are infected with a gut parasite called Nosema ceranae causes an increase in their daily carbohydrate intake, which is again limited due to rapid land-use changes. This poor nutrition leads in the reduction of immunity of the honey bees, making it more vulnerable to pathogen lead infections.




Pesticides + Pathogens

The combined effect of pesticides and pathogens have physiological implications in both individual and colony levels. Recent studies have shown that there is an increase in mortality of worker honey bees due to additive and synergistic interaction between N ceranae infection and sublethal doses of a neonicotinoid or phenylpyrazole pesticide.

The neonicotinoid-N ceranae interaction also reduces the activity of an enzyme which is used to sterilise the colony food stores and broods and to combat pathogen transmission by worker bees. Therefore, this again has the potential of causing adverse effects to the colony (which studies have proved in specific experiments).


Photo source : Shutterstock


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REFERENCE

[5]A. Vanbergen and t. Initiative, "Threats to an ecosystem service: pressures on pollinators",Frontiers in Ecology and the Environment, vol. 11, no. 5, pp. 251-259, 2013. Available: 10.1890/120126 [Accessed 1 November 2020].

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