Facultative mutualisms: A double-edged sword for foundation species in
the face of global change
Abstract
Ecosystems worldwide depend on habitat-forming foundation species that
engage in facultative mutualisms. Global change, however, is causing
rapid declines of foundation species-structured ecosystems, often
typified by sudden collapse. Although disruption of obligate mutualisms
involving foundation species is known to precipitate collapse (e.g.
coral bleaching), how facultative mutualisms (i.e. context-dependent,
non-binding reciprocal interactions) affect ecosystem resilience is
uncertain. Here, we synthesize recent advancements, and combine this
with model analyses supported by real-world examples, to propose that
facultative mutualisms may pose a double-edged sword for foundation
species. We suggest that by amplifying self-facilitative feedbacks by
foundation species, facultative mutualisms can increase foundation
species resistance to global change stressors. Simultaneously, however,
mutualism-dependency can generate or exacerbate bistability, implying a
potential for sudden collapse when the mutualism’s buffering capacity is
exceeded, while recovery requires conditions to improve beyond the
initial collapse point (hysteresis). Thus, our work emphasizes the
importance of acknowledging facultative mutualisms for conservation and
restoration of foundation species-structured ecosystems, but highlights
the potential risk of relying on mutualisms in the face of global
change. We argue that significant caveats remain regarding the
determination of these feedbacks, and suggest empirical manipulation
across stress gradients as a way forward to identify related nonlinear
responses.