Four views of how life in the sea and life on land correspond. A companion to Two trees, one shore.
Eight jobs every living world needs. Each has a land lane and a sea lane, coloured by the group that held the job. The roles persist; the holders change. Shaded bridges between lanes mark the same group holding the same job on both sides at once, and hatching marks lineages that came back from the other realm.
Kin on opposite sides of the shore, measured on the same scales. Sizes run on a log scale, so each gridline is ten times the last. Blue is the sea, brown is land.
Diversity through time, land above the shoreline and sea below, split into the regimes that dominated each era. The shapes are redrawn from published curves and each realm is scaled to its own present, so read the rises, crashes and handovers, not the heights. Move across the chart to read the mix at any moment.
Every living species can be counted, but the past survives only as fossils in rock that erodes and gets buried with age. This "pull of the Recent" (Raup 1979) inflates diversity near today. Correcting for uneven sampling (Alroy and colleagues, 2008) shrinks the sea's post-Palaeozoic rise to a modest increase: switch on the corrected curve to compare.
The pulses are real: bursts after new ways of living open up, crashes at extinctions, then new regimes. On land the climb is steeper and keeps going, fed by flowering plants and insects and by continents breaking apart into separate regions. Land overtook the sea in species around 110 million years ago (Vermeij and Grosberg 2010), and today roughly four in five named species live on land or in fresh water.
The family-tree ladder (species up to phylum) and the ecological ladder (a lake up to both realms worldwide) are what Eldredge and Salthe called the dual hierarchy. Eldredge's "sloshing bucket" links them: the bigger the upheaval, the higher the rank of the groups that replace the losers. Events near the band fit; events off it are the interesting ones.
Sea and land are not two separate experiments. Land changes the sea through what washes off it, the sea feeds land through oxygen and what animals carry ashore, and one shift in climate or geography can push both at once.