Marine ecology and biodiversity

Understanding the nature of ocean life and the patterns of its diversity represents a difficult challenge. Not only are there technical difficulties involved with studying life under water (high pressure, need for oxygen tanks, lack of light), there is an urgency to develop a greater understanding of marine life as links between ocean processes and the larger patterns of terrestrial life become more well known. Our current understanding of oceanic life is based on three principal concepts: size, complexity, and spatial distribution.

Our knowledge about the size of the ocean's domain is grounded in three great discoveries of the past few centuries. When Magellan first circumnavigated the earth he inadvertently found that the oceans were a continuous water body, rather than a series of discrete bodies of water. Some time later, the encompassing nature of the world oceans was further clarified by the discovery that the oceans were a chemically uniform aqueous system. All of the principal ions (sodium, chloride, and sulfate) exist in the same concentrations. The third discovery, still underway, is that the ocean is composed of comparatively immense ecological systems. Thus in most ways the oceans are a unified system which is the first defining characteristic of the marine environment .

There is, however, a dichotomy between the integral nature of the ocean and the external forces played upon it. Mechanical, thermodynamic, chemical, and biological forces create variation through such things as differential heating, Coriolis force, wind, dissolved gases, salinity differences, and evaporation. The actions in turn set controls in motion which move toward physical equilibrium through feedback mechanisms. Those physical changes then interact with biological systems in nonlinear ways, that is, out of synchronization with the external stimuli and become quite difficult to predict. Thus, we have the second broad characteristic of the oceans, complexity.

The third major aspect of ocean life is that life itself is sparse in terms of the overall volume of the oceans, but locally productive systems can create immense populations and/or sites with exceptionally high species diversity. Life is arranged in active layers dictated by nutrients and light in the horizontal planes, and by vertical current (downwelling and upwelling) in the vertical planes. Life decreases through the depth zones from the epipelagic zone in the initial 328 ft (100 m) of the water column to the bottom layers of water, and then jumps again at the benthic layer at the water-substrate interface. Life also decreases from the littoral zones along the world's shorelines to the open ocean, interrupted by certain areas with special life supporting systems, like floating sargasso weed beds.

In the past twenty years the focus of conservation has shifted to include not only individual species or habitats, but to a phenomenon called biological diversity, or biodiversity for short. Biological diversity encompasses from three to four levels. Genetic diversity is the level of genotypic differences within all the individuals that constitute a population of organisms; species diversity refers to the number of species in an area; and community diversity to the number of different community types in a landscape. The highest level, landscape diversity has not frequently been applied in aqueous environments and will not be discussed here.

Commonly, species diversity is interpreted as biological diversity, and since few marine groups, except marine mammals, have had very much genetic work done, and community functions are only well known from a few systems, it is the taxonomic interpretation of diversity that is most commonly discussed (e.g., species or higher taxonomic levels such as families and classes, orders and phyla). Of all the species that we know, roughly 16% are from the seas. General diversity patterns in the sea are similar to those on land, there are more smaller than larger species, and there are more tropical species than temperate or polar species. There are centers of diversity for specific taxa, and the structure of communities and ecosystems is based on particular patterns of energy availability. For example, estuary systems are productive due to importation of nitrogen from the land, coral reefs are also productive, but use scarce nutrients efficiently by specially adapted filter feeding mechanisms. Abyssal communities, on the other hand, depend on their entire energy supply from detritus fall from upper levels in the ocean. Perhaps the most specifically adapted of all life forms are the hydrothermal vent communities that employ chemosynthesis rather than photosynthesis for primary production. Water temperature, salinity, and pressure create differing ecosystems in ways that are distinctly different from terrestrial systems. In addition, the boundaries between systems may be dynamic, and are certainly more difficult to detect than on land.

Most marine biodiversity occurs at higher taxonomic levels, while the land holds more species, most of them are arthropods. Most of the phyla (32 of 33) that we now know are either marine, or both marine and non-marine, while only one is exclusively non-marine. Thus most of the major life plans exist in the sea.

We are now learning that the number of species in the ocean is probably underestimated as we discover more cryptic species, very similar organisms that are actually distinct, many of which have been discovered on the deep ocean floor. This is one of the important diversity issues in the marine environment. Originally, the depths were cast as biological deserts, however, that view may have been promoted by a lack of samples, the small size of many benthic invertebrates, and the low density of benthic populations in the deep sea beds.

Improved sampling since the 1960s changed that view to one of the ocean floor as a highly species diverse environment. The deep sea is characterized by a few individuals in each of many species; rarity dominates. Whereas, in shallow water benthic environments, there are large, dense populations dominated by a few species. At least three theoretical explanations for this pattern have been made. The stability-time hypothesis suggests that ocean bottoms have been very stable environments over long periods of time. This condition causes very finely tuned adaptations to narrow niches, and results in many closely related species. The disturbance or "cropper" hypothesis suggests that intense predation of limited food sources prevents populations from reaching high levels and because the food source is dominated by detrital rain, generalist feeders abound and compete for the same food, which results in only small differences between species. A third hypothesis is that the area of the deep sea bed is so large it supports many species, following from generalizations made by the species area relationship concept used in island biogeography theory. The picture of species number and relative rarity is still not clearly understood.

In general, some aspects of marine biology are well studied. Rocky intertidal life has been the subject of major ecological research and yielded important theoretical advances. Similarly, coral reefs are the subject of many studies of life history adaptation and evolutionary biology and ecology . Physiology and morphology research has used many marine animals as examples of organisms' functions under extreme conditions. This new found knowledge is timely. Up until now we have considered the oceans as an inexhaustible source of food and a sink for our wastes, yet we now realize they are neither. Relative to the land, the sea is in good ecological condition, but to prevent major ecological problems in the marine environment we need to increase human knowledge rapidly and manage our behavior toward the oceans very conservatively, which is a difficult task under the conditions where the ocean is treated as a common resource.

[David Duffus ]