Benjamin J. Saunders

Climate-driven regime shift of a temperate marine ecosystem

No turning back? Ecosystems over time have endured much disturbance, yet they tend to remain intact, a characteristic we call resilience. Though many systems have been lost and destroyed, for systems that remain physically intact, there is debate as to whether changing temperatures will result in shifts or collapses. Wernburg et al. show that extreme warming of a temperate kelp forest off Australia resulted not only in its collapse, but also in a shift in community composition that brought about an increase in herbivorous tropical fishes that prevent the reestablishment of kelp. Thus, many systems may not be resilient to the rapid climate change that we face. Science, this issue p. 169 Rapid warming tropicalizes a temperate kelp forest. Ecosystem reconfigurations arising from climate-driven changes in species distributions are expected to have profound ecological, social, and economic implications. Here we reveal a rapid climate-driven regime shift of Australian temperate reef communities, which lost their defining kelp forests and became dominated by persistent seaweed turfs. After decades of ocean warming, extreme marine heat waves forced a 100-kilometer range contraction of extensive kelp forests and saw temperate species replaced by seaweeds, invertebrates, corals, and fishes characteristic of subtropical and tropical waters. This community-wide tropicalization fundamentally altered key ecological processes, suppressing the recovery of kelp forests.

Tropical herbivores provide resilience to a climate-mediated phase shift on temperate reefs

Climate-mediated changes to biotic interactions have the potential to fundamentally alter global ecosystems. However, the capacity for novel interactions to drive or maintain transitions in ecosystem states remains unresolved. We examined temperate reefs that recently underwent complete seaweed canopy loss and tested whether a concurrent increase in tropical herbivores could be maintaining the current canopy-free state. Turf-grazing herbivorous fishes increased in biomass and diversity, and displayed feeding rates comparable to global coral reefs. Canopy-browsing herbivores displayed high (~ 10,000 g 100 m(-2) ) and stable biomass between 2006 and 2013. Tropical browsers had the highest abundance in 2013 and displayed feeding rates approximately three times higher than previously observed on coral reefs. These observations suggest that tropical herbivores are maintaining previously kelp-dominated temperate reefs in an alternate canopy-free state by grazing turfs and preventing kelp reestablishment. This remarkable ecosystem highlights the sensitivity of biotic interactions and ecosystem stability to warming and extreme disturbance events.

Nesting behaviour of a temperate damselfish (Parma mccullochi) and its influence on algae

Many pomacentrid fishes spawn demersally, often onto nest sites of filamentous algae. The temperate Western Australian pomacentrid P. mccullochi spawns onto such nest sites. During preliminary observations nest sites with visibly different algal structures were observed, with the filamentous algal cover of some nests being greatly reduced compared to others. These observations led us to hypothesise that algal management by this species would lead to a large change in the algal structure of nest sites, through more intense disturbance processes than have been described in tropical herbivorous pomacentrids when maintaining their algal feeding areas. Behavioural observations and collection of algae from nest sites revealed that in late spring and summer P. mccullochi maintained and defended a nesting site that was populated by filamentous algae. Fish with nests defended the territory more intensively than fish without nests, at a cost of reduced feeding rates. The algae within the nest sites progressed through a series of three stages: prepared nests without eggs, nests with eggs and cropped nests. These three stages were characterised by variation in the biomass of red filamentous algae, which was highest on nests with eggs. The nest algae were severely cropped back after hatching and most of the biomass of red filamentous algae was removed. On this small spatial scale during late spring and summer, the effect of algal manipulation by this temperate pomacentrid in an algal canopy dominated reef system, is comparable to the strongest effects of the tropical pomacentrids that maintain species poor, high biomass feeding areas in coral reef systems.