Quantifying the evidence for ecological synergies

Abstract

There is increasing concern that multiple drivers of ecological change will interact synergistically to accelerate biodiversity loss. However, the prevalence and magnitude of these interactions remain one of the largest uncertainties in projections of future ecological change. We address this uncertainty by performing a meta-analysis of 112 published factorial experiments that evaluated the impacts of multiple stressors on animal mortality in freshwater, marine and terrestrial communities. We found that, on average, mortalities from the combined action of two stressors were not synergistic and this result was consistent across studies investigating different stressors, study organisms and life-history stages. Furthermore, only one-third of relevant experiments displayed truly synergistic effects, which does not support the prevailing ecological paradigm that synergies are rampant. However, in more than three-quarters of relevant experiments, the outcome of multiple stressor interactions was non-additive (i.e. synergies or antagonisms), suggesting that ecological surprises may be more common than simple additive effects.

Uncovering emergent interactions in three-way combinations of stressors

“…However, from a basic rather than applied perspective, there is no reason to place primacy on synergism because the percentage of antagonistic interactions, in both drug-drug and ecological-driver effects, is roughly equal to the percentage of synergistic interactions. (In drug studies, 26% were found to be synergistic and 37% antagonistic [35]; in terrestrial systems, 35% synergistic and 42% antagonistic [39]; and in marine systems, 36% synergistic and 38% antagonistic [51]). Moreover, our study suggests that antagonism becomes more frequent than synergism when searching for emergent higher-order interactions.…”

“…For instance, studies of how multiple predators affect prey population dynamics may provide a strong correspondence because the survival rates of prey species are analogous to the growth rates of bacteria in multiple drug environments (e.g. [39]) and appear in the multiple predator effect (MPE) metrics in the same exact form as in our metrics for net interactions (i.e. the deviation from additivity (DA) measure) below.…”

Untangling interactions: do temperature and habitat fragmentation gradients simultaneously impact biotic relationships?

“…Contemporary ecological research recognizes the importance of studying the simultaneous impact of multiple drivers of global environmental change [6][7][8][9], and our results suggest that focusing on a single driver will provide only restricted insight into the impacts of global changes on species and their interactions. Here, we found evidence that temperature (using latitude as a surrogate) and the degree of habitat fragmentation (represented simultaneously by proximity to fragment edges and the area of forest fragments) both exerted influences on the density and trophic interactions of all levels of a tri-trophic food chain in New Zealand.…”

“…In the wider literature, non-additive impacts of multiple drivers are also considered to dominate studies that have investigated the simultaneous impacts of multiple drivers [2,7]. Although these non-additive interactions are often assumed to be synergistic in nature [11,53], several meta-analyses have found similar numbers of studies displaying antagonistic interactions between drivers [2,7]. In one of these reviews, community-level studies largely exhibited antagonistic interactions, whereas studies at population level largely exhibited synergistic responses [2].…”

“…Despite this, global environmental change research is a young branch of ecological knowledge [3], and only a few studies have empirically investigated [4,5], conceptually modelled [6] or reviewed [2,7,8] the effects of simultaneous pressures explicitly.…”