New model can predict what the relative numbers of large and small species should be in different areas of the ocean.
In the sea, body size determines position in the food chain and who eats who to a much greater extent than on land. This in turn provides a number of clear and relatively simple rules for determining how many small and large animal organisms there can be within a given marine area. Now a new study has demonstrated that these rules can also explain diversity, i.e. the number and sizes of the species that inhabit the area. In the longer term, this may provide one of the long-sought-after keys to better understand the regulation of biodiversity and how to preserve it which is a goal of the EU’s marine strategy and a high priority in international treaties that seek to protect marine life.
Number predictor
While marine diversity has been studied for many years, there are surprisingly few explanations as to why there are generally more small species than large, and a poor understanding of why species richness or biodiversity across the vast majority of animal classes increase as you move from the poles to the equator.
Professor in Fisheries Biology at DTU Aqua, Henrik Gislason, elaborates:
"Many researchers have attempted to explain these patterns, but without any great success. Although we are aware of numerous differences between north and south, for example that seasonal temperature variations in northern and southern waters result in uneven food production, while production is more evenly distributed over the year at the equator, no-one has been able to determine just how this affected biodiversity. Now, however, we have constructed a model – based on fundamental principles for how marine animals feed on each other – that can predict what the relative numbers of large and small species should be in different areas and how these numbers depend on differences in temperature and the size of the area considered.”
The bigger the fish the fewer the species
Henrik Gislason was part of a research team led by US mathematician Dan Reuman that worked together to combine and further develop two explanatory models. One, the size spectrum model, developed by Ken H. Andersen and Jan Beyer at DTU Aqua, predicts how many individuals there should be of a given size, while the other, the neutral biodiversity model, can calculate how many species there should be in an area of ocean when you know the number of individuals.
“At a global level, the combined model predicts that the relationship between diversity and the body mass of species should be a straight line on a log-log plot with a slope of roughly -0.5, which is surprisingly accurate given that our empirical studies of the sea give a straight line with a slope of -0.56,” explains Henrik Gislason.
This concordance is evidence that the theory holds up in practice. Furthermore, if the researchers include differences in the size and temperature of the sea areas in the model, it can also predict how the relative numbers of large and small species change from area to area – and these predictions also closely reflect reality. The results have been published in the Journal of Animal Ecology.
Enhanced insight into biodiversity and sustainability
Combining the two models has provided the basis for a new unified theory that stretches from the physiology, food intake and growth of organisms to biodiversity at a global level. This makes the work an important step forward for fundamental research, though the full potential of it goes much further.
Species are currently disappearing at an alarming rate, and preserving biodiversity is a key element in the EU’s marine strategy and in many international marine protection treaties.
Unfortunately, only a tiny fraction of the world’s oceans have been studied intensively enough for us to know how many species they contain. The new model and future developments of it will allow us to make more precise calculations of the sea’s biodiversity; for example in areas where we know the number of species within particular classes, such as the number of fish species, but would like an estimate of how many other species are present.