Why are there so few evolutionary transitions between aquatic and terrestrial ecosystems?

Insects and flowering plants have rarely invaded the yea. Explanations for this have traditionally centered on the unique shortcomings of these groups in the marine environment. We show, however, that transitions among terres trial, freshwater, and marine environments are infrequent in all major plan t and animal clades except tetrapod vertebrates. In general, well-adapted i ncumbents are at a competitive advantage over would-be invaders from a phys ically different habitat. Data on the times and places of transition are co nsistent with our contention that evolutionary transitions among physically different environments are most likely when incumbents in the recipient en vironment exist in a regime of low-intensity competition and predation, as in terrestrial communities of the middle Paleozoic or the land biotas of oc eanic islands. Freshwater environments, in which inferred intensities of pr edation are lower than in most marine and terrestrial environments; offer l ess biotic resistance to invaders than do communities in the sea or on land . Most invaders respond to novel physical circumstances by shutting down th eir metabolic machinery, and therefore add to their subordinate status as c ompetitors with active incumbents. Only active tetrapods, particularly thos e with high and endothermically driven rates of metabolism, have successful ly overcome this limitation. (C) 2000 The Linnean Society of London.