Those who have climbed Africa’s highest mountains have likely noticed how the plants and animals present gradually change, as one moves from tall lowland forest to moist, mid-elevation forest with a low canopy, then into grassy alpine meadows, and lastly, onto cold, windy, and rocky mountain peaks. We see these changes because, as we move across the landscape, physical conditions (e.g. geology, soil type, temperature, precipitation) change, and so also the species adapted to different environmental niches, as determined by the varying conditions. Thus, one by one, the species present at one location are replaced by new species better suited to the new conditions. We can see how the whole landscape changes in response to dynamic biotic and abiotic components of the environment (Figure 3.5). The variety of life resulting from these environmental changes is what gives rise to ecosystem diversity.
Ecosystem diversity describes the full variety of ecosystems of an area, while the term “ecosystem” describes all the organisms in an area, as well as the physical and chemical environment with which those organisms interact. An important component of any ecosystem is its biological community (or ecological community), defined as all the living individuals, populations, and species of a place, as well as all the biological interactions among those organisms. The abiotic (or physical) environment, especially climate, energy, and nutrients availability, greatly affects the structure, composition, and characteristics of an area’s biological community (or biotic environment), and ultimately the type of ecosystem present (Figure 3.6). For example, water that evaporates from leaves, the ground, and other surfaces may later become rain or snow that provides drinking water that sustains life. Sunlight energy, in turn, enables photosynthetic plants (or primary producers) to grow; the energy from the sun is later transferred to animals that eat the plants (herbivores, or primary consumers), and then to animals that eat other animals (carnivores, or secondary consumers). The physical environment similarly affects aquatic ecosystems. For example, in freshwater stream, the biological community present is determined in large part by the physical characteristics of the stream, including water chemistry, temperature, flow rate, and substrate.
At local scales, biological communities themselves can play prominent roles in altering the physical environment. For example, the trees present in a forest ecosystem can influence wind speed, light, humidity, soil chemistry, and temperature. Likewise, marine biological communities, such as kelp forests, seagrass beds, and coral reefs, can affect water temperature, water chemistry, sunlight penetration, and wave energy.
Within a biological community, individual species have specific ecological roles and have different requirements for survival. These roles and requirements enable different species to coexist, and in cases of interdependency, necessitate that they do so. For example, a given plant species may grow only in one type of soil, be pollinated by one type of insect, or have its seeds dispersed by only one type of animal. If any one of these requirements restricts the population size or distribution of that plant, it is considered a limiting resource. Even animal dung, usually considered a waste product, may become a limiting resource to species that rely on it for feeding and breeding. For example, studies from Côte d’Ivoire and Southern Africa have linked dung beetle population declines to the extirpation of large herbivores such as elephants and buffaloes (Nichols et al., 2009).
Environmental conditions that regulate the abundance of limiting resources may change over time. Consequently, many ecological communities can undergo major shifts in their composition over time. This is particularly prominent during ecological succession, which describes the gradual process during which ecosystems change after a disturbance. Consider, for example, an old-growth forest that is cleared by a logging operation. Shortly after clearing and abandonment, the soil absorbs more sunlight, resulting in high temperatures and low humidity during the day. These early stages present an ideal environment for pioneer species, such as sun-loving butterflies, annual herbs, and grasses, with wind-dispersed seeds. In a few years’ time, the early successional herb-field or grassland transition to a scrubland, which accommodates a new suite of species. As the shrubs mature, forest trees germinate in the shade provided by the shrubs. Over the course of decades, as the forest trees mature, the forest canopy is gradually re-established which, in turn, provide opportunities for species characteristic of mid- and late-successional stages, such as shade-tolerant wildflowers of moist soils. Eventually, after many decades, climax species representative of mature forests, such as birds that nest in the holes of dead trees, start colonising the area.