A fantastic new paper published recently in Current Biology (Cinner et al. 2009) examines the correlations between a variety of human demographic, social, cultural and economic indicators and reef fish biomass. The somewhat surprising and novel result is that fish depletion is maximized at intermediate levels of human socioeconomic development. The authors, mostly based at James Cook University’s Centre of Excellence for Coral Reef Studies, surveyed 19 coastal communities adjacent to coral reefs in the Indian Ocean.
In fished sites, fish biomass was negatively related to human population density, but it was best explained by reef complexity and a U-shaped relationship with socioeconomic development. The biomass of reef fishes was four times lower at locations with intermediate levels of economic development than at locations with both low and high development.
An important underlying mechanisms was the changing availability of technologies as communities become wealthier:
…in low-development sites, technological constraints and social institutions may limit people’s exploitation of marine resources. Reduced dependence on marine resources, variable access to boats but increasing access to engines and other technologies, high use of spear guns, and a lack of customary management institutions characterize communities with intermediate levels of development.
One thing that didn’t surprise me was that human population density was only weakly related to fish biomass and wasn’t even included in the best fit model:
A key and surprising finding from this study is that the best model included the quadratic socioeconomic-development index and reef structural complexity, but did not include human population density.
There is a pretty big debate among coral reef and other marine scientists about whether human population size (and growth) per se is the ultimate problem (as my buddy The Natural Patriot believes) or whether the impacts of societies have a lot more to do with governance and policy, local choices and customs, and the application of technologies. As I have written about on ClimateShifts in the past, I think simply blaming the problem on human population growth is silly, i.e., condoms are not going to save coral reefs. Stephen Jameson published a provocative essay addressing this debate and I still think there is plenty of evidence to suggest that we cannot mitigate most threats to the ocean simply by limiting or reducing the number of people on earth. In the context of the Cinner et al paper, where would a human population cap put us along the development index?
One of the other things that struck me about the results was the relative weakness of the effect of the human indicators on fish biomass compared to unfished reefs in fully protected reserves (Fig 3): across a range of human population density and socio-economic indicators, people flat out totally deplete coral reef fish. I wonder what this means for the purported links between the development index, macroalgae, coral recruitment and reef resilience laid out by Bob Steneck in his companion piece (Steneck 2009; see the figure from Steneck’s article about the Cinner et al. paper below). Given the relatively small variance in fish biomass (only about 25% of the natural range) could there really be a predictable indirect effect of human development on coral populations? Maybe. I wonder if Cinner et al. also measured macroalgae and coral recruitment in their surveys.
I also wonder if the net impact of people living in societies with a high development index (>1) is just being displaced across a region or even globally via protein imports, i.e., does the net impact continue to increase even as the local impact decreases? Think of highly developed countries like the US and Japan that import a large portion of their animal protein. Such societies do have far higher protein consumption rates, so it seems plausible that they are simply eating fish from other peoples reefs! It would be interesting to examine fish imports, exports and consumption at a local societal scale across such a development gradient.
Another issue is how the other impacts to coral reefs, in particular fossil fuel consumption and global warming, are related to the development index. They must certainly be related, but are they linearly related? I bet it could even be a positive exponential function. In other words, how is the development index related to other taxa and indicators of reef health or state. For example, what about invertebrates? Might prey populations of these harvested fish species show the opposite relationship with socio-economic indicators? And what about corals? The enormous complication is that the impacts of many human activities (e.g., driving a car, using an electric clothes dryer, flying on an airplane to go on vacation, etc.) are regional to global and thus cannot be analytically related to local human socioeconomic parameters (i.e., the sample size of the response variable is essentially 1).
Like most transformative research, the Cinner et al. paper raises at least as many questions as it answers. I think it is sometimes hard to gague the future impact of resarch when it first comes out-it can be difficult to impossible to envision all the twists and turns a research program will take. But I put my money on this being a citation classic. It has already really influenced my thinking and future research plans!
Cinner, Joshua E, Timothy R McClanahan, Tim M Daw, Nicholas A.J Graham, Joseph Maina, Shaun K Wilson, Terence P Hughes (2009) Linking Social and Ecological Systems to Sustain Coral Reef Fisheries. Current Biology 19:206-21
Jameson SC (2008) Guest editorial: Reefs in trouble the real root cause. Marine Pollution Bulletin 56(9):1513-1514
Steneck, RS (2009) Marine Conservation: Moving Beyond. Malthus. Current Biology R117 DOI: 10.1016/j.cub.2008.12.009