Planning for a transition zone in a time of climate change
Subtropical and temperate reefs are currently undergoing ‘tropicalisation’
Going from tropical to temperate reefs, species richness in corals and fishes declines, but that of algae, echinoderms and other invertebrates can increase
We should aim to conserve sites that consistently remain important for conservation through time
Range shifts and local extinctions from climate change will undoubtedly modify our ecosystems in the coming decades. These changes are likely to be evident first in the places where many species occur at the margins of their ranges and environmental tolerances (biogeographic transition zones).
Biogeographic transition zones, such as going from tropical to temperate shallow reef ecosystems, harbour a mix of organisms with different thermal tolerances and habitat associations. For example, on subtropical reefs you may find tropical and temperate fish species, tropical hard corals and temperate kelp species occurring together in the same habitat. These systems provide excellent natural laboratories to examine the forces that structure ecological communities along environmental gradients. They also provide a lens on how communities may change with climate change.
Subtropical and temperate reefs are currently undergoing ‘tropicalisation’. This involves several shifts including tropical coral species expanding their ranges poleward, tropical fishes overwintering on temperate reefs, and reefs changing from being kelp dominated to coral dominated. Exactly how these ecological transitions take place is poorly understood and the specifics will likely vary among species.
The challenges of managing transitional ecosystems range from working out what the key drivers and threats are (and the corresponding conservation goals), to setting up frameworks that incorporate transitional changes (in time and space) into conservation prioritisation. Over the past few years, CEED researchers, in collaboration with colleagues from the ARC CoE for Coral Reef Studies, have tackled these questions, and we continue to work towards better understanding and protecting subtropical reefs.
Subtropical reef environments are highly variable and can be too cold and dark for tropical species to thrive. On the gradient from tropical (low latitude) to temperate (high latitude) reefs, species richness in corals and fishes declines, but that of algae, echinoderms and other invertebrates can increase (Beger et al, 2014).
We investigated the processes that drive community organisation of corals in southeastern Australia, linking species abundances, co-occurrence patterns and species traits. We found that species traits influence the ability of corals to persist in these harsh environmental conditions at higher latitudes. In particular, structural traits linked to energy acquisition and physical stability may be particularly important for coral survival in these marginal environments (Sommer et al, 2014).
When thinking about the conservation of range shifts of reef species, in particular in subtropical reefs, these ecological findings are important to predict future change. Considering the responses of organisms to changing temperatures, but also the acidification of oceans, we should aim to conserve sites that consistently remain important for conservation through time (Beger et al, 2014; Makino 2014).
For example, in Japan we tested conservation priorities across predicted long-term distributional changes of corals that are expanding their distributions poleward. Developing a new method to connect planning units through time, we found that conservation targets could be largely achieved across three time slices (2010, 2050 and 2100) without needing to change conservation areas over time (Makino 2014). This work was based on predicted temperature trajectories across Japan, using different climate change scenarios.
We found that the robustness of conservation priorities over time on Japanese subtropical reefs can depend on which model scenario is used. The world is currently tracking towards the most severe scenario; this scenario was also the one where conservation priorities were most different (Makino 2015).
Subtropical transitional reefs are important conservation areas, and we are working towards influencing management outcomes as tropical, subtropical and temperate marine species respond to change.
Beger M, B Sommer, PL Harrison, SDA Smith, & JM Pandolfi (2014). Conserving potential coral reef refugia at high latitudes. Diversity and Distributions 20: 245-257. http://onlinelibrary.wiley.com/doi/10.1111/ddi.12140/full
Sommer B, PL Harrison, M Beger & JM Pandolfi (2014). Trait-mediated environmental filtering drives assembly at biogeographic transition zones. Ecology 95: 1000-1009. http://onlinelibrary.wiley.com/doi/10.1890/13-1445.1/abstract
Makino A, H Yamano, M Beger, CJ Klein, Y Yara & HP Possingham (2014). Spatio-temporal marine conservation planning to support high-latitude coral range expansion under climate change. Diversity and Distributions 20: 6-12. http://onlinelibrary.wiley.com/doi/10.1111/ddi.12184/abstract
Makino A, CJ Klein, HP Possingham, H Yamano, Y Yara, T Ariga et al. (2015). The effect of applying alternate IPCC climate scenarios to marine reserve design for range changing species. Conservation Letters 8: 320–328. http://onlinelibrary.wiley.com/doi/10.1111/conl.12147/abstract