Why Bees Are So Vulnerable to Environmental Stressors
Résumé
Bee populations are declining in the industrialized world, raising concerns for the sustainable pollination of crops. Pesticides, pollutants, parasites, diseases, and malnutrition have all been linked to this problem. We consider here neuro-biological, ecological, and evolutionary reasons why bees are particularly vulnerable to these environmental stressors. Central-place foraging on flowers demands advanced capacities of learning, memory, and navigation. However, even at low intensity levels, many stressors damage the bee brain, disrupting key cognitive functions needed for effective foraging, with dramatic consequences for brood development and colony survival. We discuss how understanding the relationships between the actions of stressors on the nervous system, individual cognitive impairments, and colony decline can inform constructive interventions to sustain bee populations. Bees Are Exposed to Multiple Environmental Stressors Bees are ecologically and economically vital pollinators for both wild and cultivated flowers. Presently many populations are in decline [1-4], while demand for pollination[ 2 8 5 _ T D $ D I F F ]-dependent crops continues to rise, generating understandable alarm and debate about the possibility of an emerging 'pollination crisis' [5]. Many causal factors have been identified, including a range of pathogens and parasites [6,7], human-induced stressors such as pesticides [8-10], and other forms of environmental degradation [11]. Very few of these stressors can be considered new, but many have increased in intensity over the past decade in much of the industrialized world. Our objective in this review is to consider why bees are particularly sensitive to these environmental stressors, even at low levels, and why their populations are now declining. Bees, with the exception of parasitic species, raise their brood in a single defensible nest [12]. We argue that, in these insects, central-place foraging on ephemeral, dispersed, and highly variable floral resources places particularly heavy demands on cognitive capacities. Individuals must learn to forage at an energetic profit, locate high-quality feeding sites, efficiently handle flowers, and navigate back to the nest to provision their brood with the right mix of nectar and pollen. The cognitive capacities underpinning these complex behaviors require optimal development and function of central brain structures as well as precisely regulated plasticity of brain circuits necessary for learning, memory, and navigation [13,14]. These brain systems are very easily disrupted, and it is especially problematic that many pesticides found in floral resources directly target key neural pathways [15,16]. Pathogens and nutritional deficits also compromise cognitive functions [17,18]. Even mild damage to the brain can significantly reduce foraging performance, thus rendering bees especially vulnerable to these environmental stressors. In social species, such as honey bees, bumblebees, and stingless bees, efficient division of labor and coordination of tasks across nest mates provide buffering against environmental stressors because individuals share a fortress-factory stocked with stored resources [19]. However, this
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