15.1: Communities and Niches

The world ocean can be considered a single ecosystem, but it is too large and complex to be studied as a whole. Consequently, marine ecologists often separate the oceans into ecosystems that have common biological and physical characteristics. Because even the simplest of these is complex, marine ecologists study the relationships of species with each other and their environment at a number of different levels, ranging from ocean-sized ecosystems to individual species. These levels are generally not well defined or distinct from each other.

Within an ecosystem, species are distributed in a nonuniform way because of small-scale variations in the physical environment and competition and other interactions among species. Species are often clustered in their communities. The term community is inexact and can refer either to all organisms that coexist in a specific location or to groups of species that are found together in many different locations. Thus, a community can be all the species within an entire coral reef, or a specific species of sea fan and all species that commonly live in or on it wherever this sea fan is present.

The distribution of species and their relationships with each other and the physical environment can be studied at the species level. In such studies, a very useful concept is the biological niche. Each species has evolved to take advantage of certain characteristics of the physical environment. Hence, each species has a range of environmental variables within which it can survive. These variables include salinity, temperature, suspended sediment concentration, substrate grain size, light intensity, and nutrient and other chemical concentrations. The combination of the ranges of each of these environmental variables within which the species can survive is the species’ survival niche. Figure 15-2a shows how such a niche can be defined by two environmental parameters. In practice, niches are defined by many more parameters and cannot be fully represented in a two-dimensional diagram.

Figure 15-2. Ecological niches can be defined in several ways and by many parameters. These diagrams illustrate the concepts using just two such parameters: salinity and sediment grain size. (a) The fundamental niches of two bivalves are defined by the range of values of the two parameters defined by the blue areas. The survival niches are defined by a wider range of environmental conditions and include both the blue and green areas within each dotted line. Because the niches of these two species do not overlap, they would not occur in the same environment. (b) If the niches of the two species overlap, as they do in the red area, they may both occur in environments within this range of the two parameters, but they may also compete so that one species may be excluded from this part of its niche.

Within the survival niche of a particular species is a somewhat smaller range of environmental variables within which the species can both survive and successfully reproduce (Fig. 15-2a). This range is often referred to as the organism’s fundamental niche. Species rarely, if ever, occupy every area within the marine environment where environmental parameters are within their fundamental niche. Two factors restrict the distribution of a species within its fundamental niche. First, each species has a smaller niche within its fundamental niche in which it finds optimal conditions. Second, species with overlapping niches may compete, and one species may be excluded from parts of its fundamental niche (Fig. 15-2b). 

The interaction of species with each other and with the environment is wonderfully complex and may never be fully understood for all species in the oceans. Thus, descriptions of ecosystems in the discussion that follows represent only a cursory view of the factors that contribute to the amazing profusion and diversity of species in the oceans.