The growth and industrial development of the global human population is increasing the consumption of natural resources. This produces multiple environmental pressures on biodiversity that endanger the provision of ecosystem services, such as insect pollination. Insect pollinators of crops and wild plants are threatened worldwide by land-use intensification (including habitat destruction and pesticide use), the spread of diseases and parasites, invasive species and climate change. The individual and combined effects of these pressures on insect pollination have important implications for long-term food security, human health and ecosystem function.
Are insect pollinators declining?
Current evidence from the northern hemisphere suggests widespread reductions in the diversity and abundance of many wild and managed pollinators. Parts of Northern Europe have witnessed declines in bee and hoverfly diversity (Biesmeijer et al., 2006, Keil et al., 2011). The extinction, lower numbers and reduced distribution of bumblebee (Cameron et al., 2011, Williams and Osborne, 2009, Bommarco et al., 2011) and butterfly (Forister et al., 2010, Warren et al., 2001) species are reported across Europe, North America and Asia. Moreover, despite a global increase in the number of managed honey bee colonies (Aizen and Harder, 2009), there have been extensive declines in wild, feral and managed honey bees across Europe and North America over several decades (Potts et al., 2010, Jaffe et al., 2010, vanEngelsdorp et al., 2011). Threats in tropical regions are real and pressing (Aizen and Feinsinger, 1994, Freitas et al., 2009) but data on decline is sparse in these regions.
What is clear is that so far much of the evidence for pollinator declines comes from developed countries where extensive anthropogenic environmental change has already occurred. Similar pressures (e.g. land-use change) are predicted to increase in developing regions (Sala et al., 2000) and it is likely that pollinator diversity and abundance will be affected in similar ways to that seen in the developed countries.
Pollinator declines are hard to detect because:
- There is a monitoring gap that makes detection and prediction of pollinator responses to environmental change difficult. Beside data on honey bees, there is a general lack of systematic monitoring of wild pollinators encompassing a range of geographic regions and different pollinator groups. This means that evidence of pollinator losses is mostly indirect, coming from studies of specific environmental impacts on particular pollinator groups at a certain time or place. The threat to pollinators has been inferred from the combined evidence of indirect studies and the few studies measuring pollinator declines over time.
- The picture is furthermore complicated as not all species of pollinator are equal. This means that species differences in vulnerability to environmental stresses, together with the multitude of biological interactions, tends to produce both winners (mostly species that are generalist in their habitat or food needs) as well as losers (often specialists) in the face of environmental changes (Cameron et al., 2011, Bommarco et al., 2011, Warren et al., 2001).