And the Distribution of Organisms

Although there are 270000 species of land plants (Hammond 1995), a series of filters eliminates most of these species from any given site and restricts the actual vegetation to a relatively small number of species (Fig. 1). Many species are absent from a given plant community for historical reasons. They may have evolved in a different region and never dispersed to the site under consideration. For example, the tropical alpine of South America has few species in common with the tropical alpine of Africa, despite similar environmental conditions, whereas eastern Russia and Alaska have very similar species composition because of extensive migration of species across a land bridge connecting these regions when Pleistocene glaciations lowered sea level 20000-100000 years ago.

Of those species that arrive at a site, many lack the appropriate physiological traits to survive the physical environment. For example, whalers inadvertently brought seeds of many weedy species to Svalbard, north of Norway, and to Barrow, in northern Alaska. However, in contrast to most temperate regions, there are currently no exotic weed species in these northern sites (Billings 1973). Clearly, the physical environment has filtered out many species that may have arrived but lacked the physiological traits to grow, survive, and reproduce in the Arctic.

Biotic interactions exert an additional filter that eliminates many species that may have arrived and are capable of surviving the physical environment. Most plant species that are transported to different continents as ornamental or food crops never spread beyond the areas where they were planted because they cannot compete with native species (a biotic filter). Sometimes, however, a plant species that is introduced to a new place without the diseases or herbivores that restricted its distribution in its native habitat becomes an aggressive invader, for example, Opuntia ficus-indica (prickly pear) in Australia, Solidago canadensis (golden rod) in Europe, Cytisus scoparius (Scotch broom) in North America, and Acacia cyclops (red-eyed wattle) and A. saligna (orange wattle) in South Africa. Because of biotic interactions, the actual distribution of a species (realized niche, as determined by ecological amplitude) is more restricted than the range of conditions

Figure 1. Historical, physiological, and biotic filters that determine the species composition of vegetation at a particular site.

Figure 1. Historical, physiological, and biotic filters that determine the species composition of vegetation at a particular site.

where it can grow and reproduce (its fundamental niche, as determined by physiological amplitude)

Historical, physiological, and biotic filters are constantly changing and interacting. Human and natural introductions or extinctions of species, chance dispersal events, and extreme events such as volcanic eruptions or floods change the species pool present at a site. Changes in climate, weathering of soils, and introduction or extinction of species change the physical and biotic environment. Those plant species that can grow and reproduce under the new conditions or respond evolutionarily so that their physiology provides a better match to this environment will persist. Because of these interacting filters, the species present at a site are simply those that arrived and survived. There is no reason to assume that the species present at a site attain their maximal physiologically possible rates of growth and reproduction (Vrba & Gould 1986). In fact, controlled-environment studies typically demonstrate that a given species is most common under environmental conditions that are distinctly suboptimal for most physiological processes because biotic interactions prevent most species from occupying the most favorable habitats (Fig. 2).

Given the general principle that species that are present at any site reflect their arrival and survival, why does plant species diversity differ among sites that differ in soil fertility? Typically, this diversity increases with decreasing soil fertility, up to a maximum, and then declines again (Grime 1979, Huston 1994). To answer this question, we need detailed ecophysiological information on the various mechanisms that allow plants to compete and coexist in different environments. The information that is required will depend on which ecosystem is studied. In biodiverse (i.e., species-rich), nutrient-poor, tropical rainforests, with a wide variation in light climate, plant traits that enhance the conversion of light into biomass or conserve carbon are likely to be important for an understanding of plant diversity. In the biodiverse, nutrient-impoverished sandplains of South Africa and Australia, however, variation in root traits that are associated with nutrient acquisition offers clues to understanding plant species diversity.

Figure 2. Biomass production of two hypothetical species (x and y) as a function of resource supply. In the absence of competition (upper panels), the physiological amplitude of species x and y (PAx and PAy, respectively) defines the range of conditions over which each species can grow. In the presence of competition (lower panels), plants grow over a smaller range of conditions (their ecological amplitude, EAx and EAy) that is

Figure 2. Biomass production of two hypothetical species (x and y) as a function of resource supply. In the absence of competition (upper panels), the physiological amplitude of species x and y (PAx and PAy, respectively) defines the range of conditions over which each species can grow. In the presence of competition (lower panels), plants grow over a smaller range of conditions (their ecological amplitude, EAx and EAy) that is constrained by competition from other species. A given pattern of species distribution (e.g., that shown in the bottom panels) can result from species that differ in their maximum biomass achieved (left-hand pair of graphs), shape of resource response curve (center pair of graphs), or physiological amplitude (right-hand pair of graphs). Adapted from Walter (1973).

Was this article helpful?

0 0

Post a comment