Negative resistance and resilience: A mesocosm experiment demonstrating consequences for biological recovery in restoration

Abstract

Traditionally, resistance and resilience are associated with good ecological health, often underpinning restoration goals. However, degraded ecosystems can also be resistant and resilient to disturbance (negative resistance and resilience) making them resistant to restoration. This may be a consequence of degraded communities becoming dominated by species whose traits enhance food-web stability, preventing recolonisation by other species and resulting in low biodiversity and poor ecosystem function. We present a mesocosm experiment demonstrating how presence of a degraded community hinders biological recovery. We established 12 stream mesocosms, each mimicking a physically healthy stream. Degraded invertebrate communities were established in half, mimicking the post-restoration scenario of physical recovery without biological recovery. We then introduced a biodiverse, healthy colonist community to all tanks, to see if presence of the degraded community impacted establishment of a healthy community. Colonists established less readily in the degraded community mesocosms. We observed decreases in abundance of sensitive taxa in the presence of the degraded community, potentially driven by changes in resources; algal biomass was reduced by degraded community presence, increasing resource competition. Reductions in abundances likely occur by drift, but also by changes in life history. For example, more Deleatidium mayflies emerged in the presence of the degraded community, and their nymphs were more developed at smaller sizes suggesting accelerated development. Since presence of a degraded community prevents colonist establishment and community recovery, we propose that degraded communities must first be destabilised to facilitate recovery, creating the space required for re-establishment of a healthy community.