​​​​​Marine biodiversity is increasingly threatened by human activities such as fishing, species introductions and climate change. We know that conservation of biodiversity is important for maintaining the function and structure of marine ecosystems and models have recently been constructed to predict how reductions in biodiversity will affect their production. But the basic understanding of the processes generating the spatial and temporal patterns in diversity we observe in the oceans is still far from complete.    

Nature is ripe with patterns in species diversity and abundance across time and space, and biology has provided a wealth of hypotheses to explain these patterns. Species richness declines from low to high latitudes in both the sea and on land, a pattern that has been known for more than three centuries. More than 32 hypotheses have been put forward to explain it, without being able to identify a single set of winning hypotheses that can link the pattern to the underlying processes. Species richness has furthermore been observed to decline with body size, but here we seem to know the cause. Size spectrum theory can be used to predict of how species abundance in the sea should scale with body size, and in combination with assumptions about speciation, extinction and migration, size spectra can be used to predict the diversity spectrum, the relative richness of large and small species. The first attempt to model the diversity spectrum has shown not only that a significant part of the variation in the size composition of marine pelagic species can be predicted from relatively simple assumptions, but also that species richness should scale with maximum body weight raised to a power of app. -0.5 on the global scale. A prediction confirmed by an analysis of the number of fish species of different size that has been identified.

Fisheries science has access to a gold mine of data from trawl surveys where not only species absence/presence but also fish abundance has been recorded. Over the last decades a wealth of information about fish species density and distribution has become accessible, making it possible to analyze how fish community structure and species richness vary over time and space. These analyses reveal striking similarities across large areas in the size structure of density and richness and should allow us to test the hypotheses proposed to explain the patterns. Results from a dataset of trawl hauls obtained over an area from Guinea in Africa to the Scotian shelf show that a surprisingly large proportion of the variation in fish species richness can be explained by temperature, size, and density.

Recent advances in palaeontology, palaeogeography and genetics have increased the understanding of how extinctions and diversification have driven marine evolution in the past. In the future this understanding should be placed in a common framework to explain how geological events and environmental conditions affect ecological and evolutionary processes to generate the biodiversity we observe in the oceans and to improve the strategies we use to protect it. Given the availability of long time series of survey data and being aware of the regularities we observe in marine biodiversity I am confident that marine science can provide answers to how ecosystem diversity and function is linked. ICES could play an important role in this process by facilitating the necessary interdisciplinary research.