Box 4: Marine ecosystems have many attributes distinct from those of terrestrial ecosystems that have important implications for understanding biodiversity.
DISTINCTIVE FEATURES OF MARINE ECOSYSTEMS
Marine and terrestrial ecosystems differ in significant ways that suggest that the ocean may respond to human perturbations in a fundamentally different manner from the land. Some of the unique attributes of marine ecosystems are listed below:
- Marine primary producers are represented by small and often mobile phyla. Terrestrial producers tend to be large and sessile. Marine producers are subject to fluid transport processes, can be spatially mixed, and can unexpectedly produce blooms that may be toxic.
- Large marine carnivores and grazers—top predators such as fish and sea stars—have a greater range of life-history characteristics than terrestrial counterparts. Most marine predators have planktonic and benthic life stages, each with unique environmental responses. Marine predators differ strikingly in their much higher reproductive output. This may buffer them from extinction due to overexploitation, but it also renders their populations far more variable and less predictable and makes them more vulnerable to threshold effects.
- When ocean and continental (aquatic and terrestrial) systems are compared, biomass is found to be thousands, to hundreds of thousands of times more dilute in the oceans, oceanic species interact trophically with more other species than continental species, the largest marine predators and prey are larger by one or two orders of magnitude, and the oceans are on average several to hundreds of times less productive than the continents.
- Distant marine habitats can be linked by dispersing larvae. Such systems are "open," and connections between benthic and planktonic life-history stages assume great significance, unlike most terrestrial systems.
- The higher order diversity of marine life is substantially richer: there are 13 unique marine animal phyla (as opposed to 1 unique land phylum). The existence of such a large number of unique phyla provides a compelling argument for the importance of the evolutionary history of life in the sea.
Key References: Steele (1985); May (1988); Steele et al., (1989); Strathmann (1990); Cohen (1994); Knowlton (1993).
species, and almost nothing about interactions among communities at different sites (e.g., S.A. Levin, 1992). Changes in climate and the flux of nutrients, plankton, or larval recruits all depend on regional oceanographic processes, often largely independent of local events (e.g., Butman, 1987; Roughgarden et al., 1988; Underwood and Petraitis, 1993). There are also nagging questions about minimum population sizes and critical areas for survival of species in the sea (e.g., Nee and May, 1992). None of these issues can be addressed at a local scale.