Box 5: How behavior can shape stressor effects on communities
Behavioural responses to stressors can have important indirect
implications for community structure and function (Figure 5). Adaptive
responses to stressors often involve shifts in foraging or antipredator
behaviour that change the functional role of organisms within the
food-web. For instance, in response to stress, herbivores may shift
their diet preference to select plants with higher rich, digestible
carbohydrates to achieve nutritional and homeostatic balance. This
selective herbivory not only changes the plant community composition,
but has consequences for nutrient cycling and energy flow, because it
alters the elemental composition and non-processed plant litter reaching
the detrital pool (Hawlena & Schmitz 2010). Importantly, however, such
behavioural responses to environmental stressors, and the corresponding
ramifications for communities, are largely mediated by how these
stressors alter species interactions, such as predator-prey dynamics,
competition and mutualisms (Miller et al. 2014; Thompson et
al. 2018). Understanding how stressors impact behaviorally mediated
species interactions is essential for predicting when synergistic
interactions will arise, but this fundamental aspect of ecology is often
absent from multiple stressor studies (Tylianakis et al. 2008;
Thompson et al. 2018).
If stressors hit predators harder than prey, stressors can benefit prey
and moderate both the individual and combined effects of stressors by
allowing prey to feed with reduced risk, and divert energy into other
life-history and physiological processes (Francis et al. 2009;
White et al. 2018). If stressors specifically inhibit keystone
predators, however, this can lead to the restructuring or collapse of
entire ecosystems (Breitburg et al. 1998; Rumschlag et al.2019). For example, the combination of organophosphate insecticides and
triazine herbicides dramatically increased trematode pathogens in pond
mesocosms, because these conditions favoured populations of the
intermediate host (snails) by reducing the top-down behaviorally- and
density-mediated effects of their predators and increasing periphyton
food abundance (Rumschlag et al. 2019). In contrast, if stressors
disproportionately change the behavior and feeding ecology of prey, for
instance, by increasing their foraging activity or reducing antipredator
responses, predation rates are likely to increase, leading to stronger
top-down effects on intermediate consumers (Shears & Ross 2010; Milleret al. 2014).
Single and multiple stressors can also directly and indirectly disrupt
mutualisms (Hegland et al. 2009; Schweiger et al. 2010).
Changes in the timing and spatial synchronicity of species behavioural
patterns in response to stress may lower species co-occurrence rates
leading to the deterioration of mutualistic interactions (Hegland et al.
2009, Schweiger et al. 2010, Figure III). Furthermore, negative impacts
of stressors on a single member of a mutualism can have large costs for
other members, including indirect beneficiaries of the mutualism
(Hegland et al. 2009; Schweiger et al. 2010; Barton &
Ives 2014). For example, the negative impact of agricultural pesticides
on honeybee foraging behaviour and colony health can reduce plant
numbers (via reduced pollination) and, in turn, have negative effects on
herbivores (Goulson et al. 2015; Tosi et al. 2017).
Alternatively, mutualisms can help buffer ecosystems against negative
effects of environmental stress (Thompson et al. 2018). For
instance, an ant-aphid mutualism can protect plants from indirect
effects of increasing temperatures by limiting positive-effects of
rising temperatures on the abundance of pest predator species (Marquiset al. 2014).
Multiple stressor effects on communities are difficult to predict but
are likely contingent upon the degree of tolerance and co-tolerance of
species and functional groups to those stressors (Vinebrooke et
al. 2004), as well as the trophic level(s) upon which these stressors
have the greatest impact. In situations when key functional groups or
species are particularly sensitive to stressors and are subsequently
reduced or eliminated from the community pool, pronounced behavioural
and numerical effects can propagate across trophic levels if the
ecosystem functionality of those groups is not replaced (Galic et
al. 2018; Dib et al. 2020). On the other hand, when remaining
species can compensate functionally for this loss, stressor effects tend
to be weakened across trophic levels, leading to more resistant
communities (Jackson et al. 2016).