Universal Mechanism Controlling Biodiversity

Nature publishes the first comprehensive overview of the biodiversity patterns of phytoplankton, the tiny plants that float on the surface of the sea, on 24 June.

An international research team, partly funded by the Natural Environment Research Council, found striking similarities between biodiversity patterns on land and in the oceans prompting the conclusion that there is a universal mechanism controlling biodiversity. The oceans, by far the largest ecosystem on the planet, are the least understood.

The worldwide study of phytoplankton found that biodiversity appears to be dependent on the biomass in a particular habitat. Biomass, the total mass of all living organisms in a habitat, is a good indicator of the energy held in a system and hence it’s productivity. This finding is important because it is consistent with biodiversity on land.

The researchers discovered that phytoplankton biodiversity is greatest at medium levels of biomass. That is, if there is a huge phytoplankton bloom in a particular area it will probably be dominated by a single species, (or by a very small number of species). If there is little growth in a low-productive area, that too will be dominated by a single species or a limited number of species.

Phytoplankton species reach their maximum number at an intermediate biomass. These findings could be due to the fact that in waters of low biomass, or productivity, there are not enough nutrients for the survival of many species. On the other hand, in highly productive waters, with massive plankton blooms, there is not much light available but numerous predators, and only the strongest competitors survive.

Researchers did discover some discrepancies between biodiversity on land and ocean. They found only a very weak link between phytoplankton biodiversity and the biodiversity of their main consumers, zooplankton, the tiny animals that feed on phytoplankton. This is contrary to biodiversity on land. They suggest size is the key factor here, not the marine environment.

The amazing biodiversity of plants in tropical rainforests is accompanied by a similarly astonishing biodiversity of animals feeding on them. The size and complexity of trees in a forest provide many niches for smaller organisms to exploit encouraging diversity on land. Phytoplankton have no such complexity and contribute little structure to their environment. The team suggest that the size of primary producers, such as phytoplankton, may be an important factor in the way ecological communities are organised.

The researchers arguments for a universal mechanism controlling biodiversity are strengthened by the finding that biodiversity patterns in phytoplankton are remarkably similar throughout the world’s ocean despite obvious differences in environmental conditions – temperature, light, nutrients, currents and mixing.

The most important universal processes determining plankton biodiversity are nutrient availability, shading, and grazing by zooplankton species.

Phytoplankton are at the bottom of the marine food web, and hence have a major impact on marine resources including fish populations. Knowledge on the biodiversity of phytoplankton is important since some plankton species are highly edible whereas others are not, just like green grass and thistles on land.

Phytoplankton could also play a major role in climate change as they absorb huge quantities of carbon dioxide. During recent years various experiments have been carried out to fertilize the oceans with iron in order to stimulate phytoplankton productivity and carbon dioxide uptake. These experiments induced major shifts in plankton species.

This research is a major milestone in the understanding and prediction of plankton species composition and their role in global processes.