Science Behind the Photo #22

Andrew Purcell

The Insurance Hypothesis

Farming monoculture, such as in this corn field, can lead to decreased net ecosystem CO2 intake, meaning land becomes less useful in the fight against climate change. According to the insurance hypothesis: “Biodiversity insures ecosystems against declines in their functioning because many species provide greater guarantees that some will maintain functioning even if others fail”. Put simply, given the same set of environmental conditions, more diverse plant ecosystems will tend to photosynthesise at a greater rate than ecosystems with fewer species present. Obviously, forests are very diverse, whereas crop fields are usually monocultures, meaning that only the one species of plant is present. In fact, any other plant species present are usually classified as ‘weeds’ and are, hence, destroyed by farmers post-haste.

The idea behind ‘the insurance hypothesis’ is simple: when different species are present, they are able to fulfil a variety of different ecological niches within a given ecosystem. By contrast, with monocultures, all of the individual plants are competing for the resources held within one specific ecological niche. Consequently, the overall rate of photosynthesis in biodiverse ecosystems tends to be much higher than that of monocultures, which means biodiverse ecosystems, usually natural ones, are much better at helping us tackle climate change.

In addition, monocultures are much more susceptible to disease than biodiverse ecosystems are. Plant viruses are usually specialised toward attacking a particular species, genus, or family of plants. Consequently, it is possible for one viral strain to destroy an entire monoculture plantation of biofuel crops, thus reducing the photosynthesis rate of this area of land to zero. In stark contrast to this, should a virus destroy any given type of plant within a biodiverse ecosystem, the gap created will quickly be filled by the other plant species present, meaning that overall rate of photosynthetic carbon dioxide uptake will remain high. Thus, the clearance of forests, in order to make way for crop plantations, is not only detrimental in terms of the animal species lost, for whom this forest was their habitat, but it can also cause a net increase in atmospheric carbon dioxide levels over time.

References:

Naeem, S. & Li, S. (1997). Biodiversity enhances ecosystem reliability. Nature 390 pp 507-509.

Yachi, S. & Loreau, M. (1999). Biodiversity and ecosystem productivity in a fluctuating environment: The insurance hypothesis. Proceedings of the National Academy of Sciences of the United States of America 96 (4) pp 1463-1468.

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