Old methods not so deep

By Andrew Baird

 

“It is time to move beyond this simple metric for greater ecological insight.”

In this week’s Ecology paper we outline a revolutionary new way to describe how corals occupy and use space.

The standard approach for describing a species depth preference is its depth range; that is the difference between the shallowest and deepest records of the species.

Our work suggests this standard approach is overly simplistic and offers limited ecological insight because most coral species have a broad depth range.

One of the most prominent features on coral reefs is a strong and consistent zonation of coral assemblage with depth. Predictable groups of species occupy discreet depths over a broad geographic range. This shouldn’t happen if species are more ‘generalist’ with their depth range.

We explored this paradox by modelling the abundance distributions for 110 coral species over a 45 m depth range in Kimbe Bay, Papua New Guinea. These models estimated two metrics to capture how a species uses space; peak depth (the depth at which the species is most abundant) and niche breadth (the depth over which the abundance remains at more than 50% of the maximum abundance). These models allow for an infinite number of ways to describe the way a species uses depth.

More than 85% of species occupy more than 50% of the available depth range when using the traditional metric of depth range. These corals can be considered depth-generalists.

However, when using modelled abundance distributions, only 19% of species span more than 50% of the available depth range.

Our work resolves the paradox by demonstrating that most species are actually depth specialists.

This ecological insight, gained by using this new approach, is further illustrated when comparing estimates for the number of species likely to be able to use deeper areas of the reef as a refuge from disturbances that disproportionately affect corals in the shallows—such as bleaching and cyclones. We call this the ‘the deep reef refugia hypothesis’.

So, based on depth range, 70% of coral species are found at both vulnerable (0–10 m) and potential refuge (>30 m) depths.

In contrast, only 40% of species have a refuge in depth when using the modelled niche breadth of the species.

Depth range is a trait that has been used to predict the vulnerability of species to climate change, the dispersal capacity of a species, and as a proxy for environmental tolerance. Our work suggests it is time to move beyond this simple metric for greater ecological insights.

PAPER: Roberts T, Bridge T, Caley M, Madin J, Baird A (2019). Ecology.  ‘Resolving the depth zonation paradox in reef-building corals’. DOI: http://dx.doi.org/10.1002/ecy.2761