Ecological Relationship (Ecological Pyramid)
The study of food-chains and webs is a study of who eats what, and where they eat. A stepped population pyramid has been given in Fig.1.7. The ecological pyramid may be illustrated from the distribution of population in two ecosystems:
i) temperate grasslands, and
ii) temperate forest.
It may be observed from Figure 1.7 that there is a decreasing number of organisms at each successive higher trophic level. However, the base of the temperate forest pyramid is narrower, however, because most of the producers are large, highly productive trees and shrubs, which are outnumbered by the consumers they can support in the chain. In opposition to this the base of grassland is larger which tapers towards the primary, secondary and tertiary consumers.
A self-regulating association characterized by specific plant formations. It is usually named after the predominant vegetation and known as biome when large and stable.
The total dry mass of living organism on Earth or per unit area of landscape; also the weight of the living organisms in an ecosystem.
The ecosystems of the Earth are dynamic. They change in space and time. Each ecosystem operates in dynamic equilibrium, constantly adjusting conditions to maintain stability. The concept of change is thus, the key to the study of ecosystem stability.
Ecological succession occurs when older communities of plants and animals (usually simpler) are replaced by newer communities (usually more complex). Each successive community of species modified the physical environment in a manner suitable for the establishment of a later community of species. Changes apparently move toward a more stable and mature condition, to an optimum for a specific environment. This end product in an area is traditionally called the ecological climax, with plants and animals forming a climax community-a stable, self sustaining, and symbiotically functioning community with balanced birth, growth, and death.
Ecological succession often requires an initiating disturbance-for instance strong winds, a volcanic eruption, or a practice such as prolonged overgrazing. When existing organisms are disturbed or removed, new communities can emerge. At such times of transition, the interrelationship among species produce elements of chance, and species having an adaptive edge will succeed in the competitive struggle for light, water, nutrients, space, time, reproduction and survival. Thus, the succession of plant and animal communities is an intricate process with many interactive variables.
An area of bare rock and soil without any vestige of a former community can be a site for primary succession. The initial community is called pioneer community. It may occur at sites such as a new surface created by mass movements of land, in areas exposed by a retreating glacier, and active volcanic slopes where the lava has been cooled and solidified.
However, a succession from a previously functioning com-munity is more common. An area whose natural community has been destroyed or disturbed, but still has the underlying soil intact, may experience secondary succession. In terrestrial ecosystems, secondary succession begins with pioneer species and further soil development. A succession progresses, a different set of plants and animals with different niche requirements may adapt.
Ecosystem Stability and Diversity
Any ecosystem moves toward maximum biomass and stability to survive. However, the tendency for birth and death rates to balance and, the composition of species to remain stable, inertial stability, does not necessarily foster the stability to recover from change, resilience. Examples of stable communities include a redwood forest, a pine forest at a high elevation, and a tropical rain forest near the equator. Yet, cleared tracts recover slowly, and therefore have poor resilience.
Another aspect related to stability is diversity. The more diverse the species population (both in number of species and quantity of each species), the more risk is spread over the entire community, because several food sources exist at each trophic level. In other words, greater diversity in an ecosystem results in greater stability. An artificially produced community, such as fields of rice and wheat, is singularly vulnerable to failure due to weather or attack from insects or plant disease. A modern agricultural ecosystem is not only vulnerable to failure due its lack of ecological diversity, but also to practices such as harvesting and removing biomass from the land that interrupts the cycling of materials into the soil. This net loss of nutrient must be artificially replenished.
Humans simplify communities by eliminating diversity and in this way we place more ecosystems at the risk of unwanted change and perhaps failure. In some regions, simply planting multiple crops brings more stability to the ecosystem.
An association of plants and animals and their non-living environment in a water setting is known as aquatic ecosystem. Adaptation of organisms to aquatic environment also vary from place to place. Animals such as fishes live exclusively in water.
Frogs, crocodiles and aquatic birds are amphibious in nature. Again, some animals like echinoderms (starfish) live only in saline water. The aquatic ecosystem may be classified into (i) Marine ecosystem, (ii) Lakes and ponds ecosystem, and (iii) River ecosystem. A brief description of these ecosystems has been given in the following:
Marine ecosystem is different from the terrestrial ecosystems. On the basis of light, temperature, salinity, depth, latitude and water density, the marine ecosystem may be classified into the: (i) photic zone, and (ii) euphotic zone. The photic zone (photos=light), the sunlit layer of the ocean, extends in the tropics to a depth of approximately 200 metres and in productive mid-latitude water down to 100 meters. The upper half of the photic zone- the layer in which most biological productivity occurs-is called the euphotic zone (eu=good). Below the photic zone is the aphotic zone (a=without), the dark zone that extends to the bottom.