Half measures don’t work when restoring marine forests

Would you recognise a desert if it was covered in water? What I mean by that is if somewhere that should be covered in forest were barren and empty, would you notice? A paper in PLOS One outlines why it matters.

Seaweeds (macroalgae) are the “trees” of the oceans, providing habitat structure, food and shelter for other marine organisms…

Kelp forest
Forest-like kelp at San Diego aquarium. Photo Swandieve/sxc.hu

It’s easy to overlook the importance of marine plants if you’re not a scuba diver. From the shore one patch of sea looks much the same as another. Obviously if you’re underwater then things look different. Towards Restoration of Missing Underwater Forests by Campbell et al. is a paper looking at the missing forests of Phyllospora comosa, a brown algae that should be found of the coast of Sydney.

The seaweed disappeared with increasing pollution from the city but, despite an increase in water quality, the forests have not returned. Why?

Campbell and her team transplanted Phyllospora into sites at Long Bay and Cape Banks near Sydney. They observed the algae to see how they survived. They also watched plants at the donor sites for comparison. The results were mixed.

They did well at Long Bay. Better than well, in fact. They were reproducing better than the control sites, which suggests that the only reason there weren’t Phyllospora at Long Bay is that there weren’t any. That’s tautological, but obviously in nature you get new Phyllospora from older Phyllospora. A colonisation effort in Long Bay would get the re-establishment of the seaweed started.

Things did not go so well at Cape Banks. Here Phyllospora did much worse than at Long Bay or the original populations. What this did see were that the transplanted algae were short and had a lot of bite marks from fish. What they suggest here is that the reason there isn’t Phyllospora on site is because there isn’t enough. Small colonies are suitable for snacking, but because they’re so small all the plants get damaged. A larger area might be so large that not all the plants suffer and that leaves enough for reproduction of the next generation.

They also found the new plants were concentrated in, or at the edge of the adult population. That suggests that the lone colonist plant will not flourish by itself, what matters isn’t just the plant but the whole community.

What I particularly liked about this paper is that there’s a classic example of scientists being scientists in it.

The disappearance of Phyllospora from reefs in Sydney coincided with a peak in high volume, near-shore sewage outfall discharges along the metropolitan coastline during the 1970s and 1980s (Coleman et al. 2008). Although causation has not been formally established, embryos of this species are particularly susceptible to pollutants commonly found in sewage, to the extent that they are used as a test species in standard ecotoxicological assessments.


They’re susceptible to pollutants. There were pollutants in the area, but that’s a correlation, not a proven causation. A causal link between pollution and the demise of the algae would be extremely convenient for anyone wanting to argue now is the time to restore the forests, and it’s not a ridiculous leap to make, but they still point out that it’s still not fully proven.

Correlation does not mean causation.
Image by Randall Munroe CC BY-NC.

What the paper shows is an example of discontinuity in ecosystems. The results show that it’s not simply a question of degree of forestation, but that you either have enough Phyllospora to make a viable forest system or you don’t. The amount you need might vary from place to place, but spending half the money isn’t going to give you half the result.

It’s also something that requires close examination. For plants that you don’t see from the shore, it’s easy not miss them when they’re gone. There are knock-on effects in how the loss of habitat affects other organisms. but that might appear a long way from the site where the root problem is.

Images

The Kelp Forest at an aquarium in San Diego. Photo © Swandieve/sxc.hu
Correlation. Image by Randall Munroe. This image licensed under a Creative Commons by-nc licence.