|Streams contain a variety of types of algae that remove pollutants from the |
water. This image shows several species of algae similar to those used in
the study. (c) Danuta Bennett
He used 150 miniature model streams, which use recirculating water in flumes to mimic the variety of flow conditions found in natural streams. He grew between one and eight species of algae in each of the mini-streams, then measured each algae community's ability to soak up nitrate, a nitrogen compound that is a nutrient pollutant of global concern.
He found that nitrate uptake increased linearly with species richness. On average, the eight-species mix removed nitrate 4.5 times faster than a single species of algae grown alone.
'The primary implication of this paper is that naturally diverse habitats are pretty good at cleaning up the pollutants we dump into the environment, and loss of biodiversity through species extinctions could be compromising the ability of the planet to clean up after us,' said Cardinale.
Why are more diverse streams better pollutant filters? Niche partitioning, Cardinale said.
In the stream experiments, each algae species was best adapted to a particular habitat in the stream and gravitated to that location - its unique ecological niche. As more algae species were added, more of the available habitats were used, and the stream became a bigger, more absorbent sponge for nitrate uptake and storage.
Think of niche partitioning as a division of labour among specialist organisms.
'People as far back as Darwin have argued that species should have unique niches and, as a result, we should see a division of labour in the environment,' Cardinale said. 'But demonstrating that directly has proven very difficult.
'And so one of the primary contributions of this study is that I was able to nail the mechanism and show exactly why streams that have more species are better at removing these nutrient pollutants from the water,' he said.
In the experiments, the channels inside each flume were lined with a continuous slab of moulded plastic. The plastic provided a growth surface for the algae, and variations in the shape of the slab's surface created a variety of water features - riffles, pools and eddies, for example - found in real streams.
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