Diversity is Strength

The following article was written as an exercise of scientific journalism by Natalie DeGroot, a SH student, as part of the coursework for
BL4268– Conservation & Management of Biodiversity.

Diversity is Strength
How Climate Change is Threatening the Marine Environment

You think polar bears are the only animals affected by climate change? Think again. In our oceans, climate change is causing rapid reorganisation of groups of fish species, altering what scientists call ‘assemblage composition’. This results in the erosion of marine biodiversity. In other words, climate change threatens marine organisms as well as terrestrial ones. It’s just harder to see the damage.

For the past 29 years, scientists working off the west coast of Scotland collected and analysed data on groundfish assemblages. They found that assemblages in cooler northern locations were beginning to resemble those in warmer southern locations in terms of species identity. This is called spatial homogenization.

Over the same period, the ocean temperature in the North Atlantic rose by 1°C – a rise undoubtedly caused by human driven climate change. Temperatures in the north rose faster than those in the south, explaining the changes occurring in the assemblage composition.

When distinctly different communities become more similar, this is called biotic homogenization. When this occurs, a few species become winners, while many are losers. We humans are largely responsible for this; we drive native species (the losers) out of their habitats, and in extreme cases, to extinction. These species are then replaced by a few generalist species (the winners), which results in reduced biodiversity.

So, if you think of an assemblage as a neighbourhood, previously diverse neighbourhoods of fish are beginning to look the same. This isn’t just boring; it’s dangerous. Less diverse assemblages are more vulnerable to ecological breakdown. As is the case with human communities, diversity is strength.

The Dangers of Homogenization

The problem of biotic homogenization is prevalent in terrestrial ecosystems as well as marine ecosystems; it has been observed in a wide range of terrestrial organisms, from insects to birds. Urbanisation is, for instance, a prime cause of biotic homogenization; some animals and plants thrive in the presence of humans. The perfect example is the house sparrow.

When urbanisation occurs, native species which cannot survive in cities are driven out and replaced by species which can. These changes to the terrestrial environment seem serious because they’re right in front of us. After all, when we see a fox in the city, it’s difficult to ignore.

The urban fox: a winner in a homogenized environment

The urban fox: a winner in a homogenized environment

For problems at sea, however, it’s a case of out of sight, out of mind. In fact, the crisis at sea is arguably worse. Studies of plankton movements in the North Atlantic, for instance, indicate that climate change is causing more rapid changes in the marine than in the terrestrial environment.

Previous studies have focused largely on how we directly cause biotic homogenization in the marine environment by introducing non-native species into communities for purposes such as aquaculture and fishing.

This direct impact has been revealed in a study in the United States that showed that pairs of states (say Arizona and Montana) now have an average of 15.4 more fish species in common than they did before the Europeans arrived.

But what concerns us here is not direct impact but indirect impact, namely climate change. Fish are extremely sensitive to fluctuations in ocean temperature; if these fluctuate too much for a given species, that species will be forced to relocate, or face extinction. Herring and cod are notable among fish that exhibit this strong sensitivity.

As we know, certain fish species are targeted by the fishing industry more than others. Entire fishing communities depend on one species for their livelihood. If that species is being driven away due to climate change it can be a disaster.

Take, for instance, cod – a fish we love with chips. Atlantic cod is highly sensitive to temperature fluctuations; between 1961 and 2001, it was found that North Sea cod had moved its geographic range some 100km north to reach cooler temperatures. In other words, they couldn’t stand the heat.

What would Scotland be without cod? Warming oceans are continually forcing Atlantic cod further north.

What would Scotland be without cod? Warming oceans are continually forcing Atlantic cod further north.

A Hidden Problem

When cod are forced to move north in order to survive, the community they leave becomes less diverse and more similar to surrounding assemblages. This problem of biotic homogenisation has long been recognised but its seriousness has been masked by assemblages rapidly reorganising so that the number of species in an assemblage stays constant. In fact, however, biotic homogenization might be a far greater problem than the loss of individual species.

A diverse planet is a healthy planet. The more homogenised fish populations become, the more vulnerable they are. The problem of homogenisation might not become obvious until it is too late.

Biotic homogenization is arguably one of the greatest threats to marine biodiversity, but the problem can be fixed. The conclusions of marine biologists studying this problem are not all doom and gloom. “As long as species are not globally extinct”, they argue, “homogenization is potentially reversible.” But for that to happen, we need to change.

Sources

Anon, 2014. Climate change forces fish to migrate or face extinction. Herald Scotland [online] Available at:
[Accessed: 9/10/2015]

Gregory, B., Cristophe, L., & Martin, E. (2009). Rapid biogeographical plankton shifts in the North Atlantic Ocean. Global Change Biology, 15(7), 1790-1803.

Lockwood, J. L., Brooks, T. M., & Mckinney, M. L. (2000). Taxonomic homogenization of the global avifauna. Animal Conservation, 3(01), 27-35.

Magurran, A. E., Dornelas, M., Moyes, F., Gotelli, N. J., & McGill, B. (2015). Rapid biotic homogenization of marine fish assemblages. Nature Communications, 6.

McKinney, M. L., & Lockwood, J. L. (1999). Biotic homogenization: a few winners replacing many losers in the next mass extinction. Trends in ecology & evolution, 14(11), 450-453.

Olden, J. D., Poff, N. L., Douglas, M. R., Douglas, M. E., & Fausch, K. D. (2004). Ecological and evolutionary consequences of biotic homogenization. Trends in Ecology & Evolution, 19(1), 18-24.

Rahel, F. J. (2000). Homogenization of fish faunas across the United States. Science, 288(5467), 854-856.

Rose, G. A. (2005). On distributional responses of North Atlantic fish to climate change. ICES Journal of Marine Science: Journal du Conseil, 62(7), 1360-1374.

Pictures

Canadian Museum of Nature. Atlantic Cod. [online] available at: [Accessed: 9/10/2015]

The Guardian (2015) Urban Fox. [Online] available at: [Accessed: 9/10/2015]

The Guardian (2013) [online] available at: [Accessed: 9/10/2015]