Corals in a marginal environment rely on establishment over dispersal

Brooding species Stylophora pistillata. Photo credit Erika Woolsey.

By Erika Woolsey and Sally Keith

Lord Howe Island (31.5°S) is the southernmost coral reef in the world, sitting 1000 km south of Australia’s Great Barrier Reef (GBR), and has long been known to have an unusual coral assemblage. Here, we sought to discover the underlying reasons for the sharp change in coral assemblage structure across this biogeographic border using a robust quantitative framework. To fully appreciate the difference in assemblage across the border, we moved beyond presence–absence and collected species abundance data from two sites along the GBR and from Lord Howe Island itself. We combined these data with traits of individual coral species (downloaded from coraltraits.org) that are proxies for environmental tolerance, evolutionary processes, competition and dispersal in a mixed effects regression model.

Location of three islands surveyed off the east coast of Australia. Land is shown in yellow, corals reefs in black. Latitude and longitude (WGS84) markings are provided in the inset.

The best predictor of high abundance for a species on Lord Howe Island was whether it is a brooder, i.e. reproduces through internal fertilization. Brooders represent around 80% of live coral cover at Lord Howe. This abundance contrasts strongly with the Great Barrier Reef, which is dominated by species that release eggs and sperm into the water column for external fertilization. This disparity suggests that local retention of larvae might be vital to survival in this high latitude environment.

View from Lord Howe Island lagoon. Photo credit Erika Woolsey.

Upon release, brooded larvae are immediately competent to settle on the natal reef and develop into a new coral colony, whereas larvae from spawning species require 3–4 days before becoming capable of settling and metamorphosing into adults. In this time, the spawn is likely to be flushed away from Lord Howe Island and doomed to the open ocean. As a result, Lord Howe Island has become a ‘land of brooders’, where the abundance of a given coral species appears to be determined by its capacity to establish and maintain a self-sustaining population, over and above its capacity to disperse. Secondarily, abundant coral species were also more strongly associated with traits that indicate tolerance of low light and temperature, which would enable them to further succeed in sub-optimal conditions.

Dr Woolsey surveying the Lord Howe Island reef, dominated by brooding species. Photo credit Sally Keith.

We hope that our findings encourage other researchers addressing biogeographical questions to collect abundance data explicitly. During fieldwork, it was striking to see that spawning species common on the GBR could be present in Lord Howe Island lagoon, but as single, non-breeding colonies. Therefore, abundance data was essential to effectively describe the Lord Howe Island assemblage, and identify characteristics of species that allow them to survive and become successful in this marginal environment. Our paper therefore demonstrates the promise of abundance data to tease out potential explanations for biogeographical patterns that would otherwise be missed.

Star plots of relative abundance for the twelve species with the highest mean relative abundance across all islands. 'Other species' includes one brooder on all islands plus a varying number of species with a spawning reproductive mode. Size of the segment represents the mean relative abundance of a given species. Green segments are brooding species, yellow to pink segments are spawning species, and white segments are all other species observed in our surveys. Specific species for each segment are labeled in the key at the bottom of the panel. Lizard Island (a) has a high relative abundance of spawning species and other species, the latter indicating its high diversity relative to the other two islands.

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