Diversity in the deep blue seas

Most research carried out on the diversity of species has been with land animals. According to these investigations, diversity is greater in life-forms with medium productivity - defined as the relation between energy consumed and biomass produced.

But these patterns linking diversity and productivity in land animals are questionable when dealing with smaller beings as, for example, in the case of those at the lowest scale of the marine food cycle. According to some research, these species do not follow the pattern of animals analysed with respect to diversity.

In this research undertaken by Xabier Irigoien, on the other hand, it is shown that, in the case of algae - phytoplankton - these do, in fact, reach the maximum point of diversity with medium productivity. According to these results, it could well be that many of the patterns used in terrestrial ecology may also be applicable to marine ecology.

To arrive at this conclusion, the AZTI researcher undertook studies of more than 350 samples taken from various oceans in the world. In these analyses, the dominant pattern linking diversity and productivity is unimodal. In this pattern, when the productivity of the
species is small, its diversity is also small; when productivity is medium, diversity is maximum and, when the productivity increases, the diversity diminishes once again. In other words, the diversity has a triangular pattern.

Why unimodal pattern?

In the case of phytoplankton there could be a number of reasons why diversity is maximum when productivity is medium. The history of the community, the distribution of habitats, the struggle to find food and resources, the relationship between different species, the abundance or lack of food, the size of the species, etc. could be the cause of this diversity pattern. For example, if on the coast the waters are rich in nutrients and the productivity is high, there will be many predators and, thus, only those species of phytoplankton best adapted and prepared to face up to the predators will survive. The result is scant diversity. On the other hand, if the waters lack sufficient nutrients and productivity is small, few species will survive and, once again, diversity will be low. When the nutrients are sufficient and the productivity is medium, is when diversity amongst the phytoplankton is at its maximum.

Moreover, this pattern is even more interesting if zooplankton is analysed. Zooplankton feeds on phytoplankton and, according to Irigoien's research, diversity amongst both life-forms is not related. This is not very usual amongst land animals, given that greater diversity amongst large prey usually translates into the same diversity amongst the predators.

Differences in size of species on which other species feed could also be the cause of the difference between land and marine animals. Specifically, phytoplankton is diminutive but the complexity of marine hydrodynamics is great.

So, what defines diversity amongst zooplankton? According to the AZTI researcher, just as occurs with phytoplankton, the struggle for nutrients and adaptation to predators are key. Thus, there is still much to investigate in the theory of the predator and prey and it could even be that that, in the end, this relation is discarde as far as diversity is concerned.

Elhuyar Fundazioa

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Diversity in the deep blue seas

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