Biodiversity in water and on land

Habitats in the sea, in freshwater, and on land differ dramatically in species composition and diversity. Of the roughly 1.5 million known species of macroscopic organisms on earth, the modern ocean — despite its much larger area and volume — supports only about 15% of species, whereas terrestrial environments account for about 80% of species, and freshwater for the remaining 5% (Figure 1). Even when taking into account previously undetected biodiversity in all of the physical realms revealed through molecular techniques, these differences appear to be robust — certainly among multicellular organisms. A closer look at these differences reveals two striking patterns: first, it’s only relatively recently, in the last 100 million years or so, that the number of species on land has come to vastly exceed the diversity in the seas; second, despite some transitions and overlaps in distribution, many major clades of plants, animals, fungi, and microbes occur primarily in only one, or two, of the three realms (Table 1). Although there are exceptions, particularly among fish and some decapod crustaceans (many crabs and shrimp), this observation, together with abundant phylogenetic and fossil evidence, implies that evolutionary transitions among these realms are typically infrequent and apparently difficult. These contrasts in diversity continue to puzzle both ecologists and evolutionary biologists, and possible explanations must span multiple spatial and temporal scales. There are at least two fundamental questions that must be answered: first, to what extent do the presently observed differences in diversity reflect historical differences in rates of diversification or biases in the rate of transitions between realms? Second, why are there so few transitions among the great realms? Given the enormous challenges in estimating speciation and extinction rates, for most groups, it remains unclear whether there are truly differences in rates of diversification across the three realms, and this remains an area of very active research. Nevertheless, we can speculate on some of the major factors that collectively influence diversification.