3 Ecosystems: Concept, Structure and Functions – Part 3

Renuka Gupta

Contents

3.1 Ecosystem Development

3.2 Ecosystem regulation and stability

3.3 Ecosystem services

3.1 Ecosystem Development

An ecosystem is not static in nature. It grows and changes in its structure and function with time. These changes are very orderly and can be predicted, the process known as ecological succession. The ecological succession is defined as an orderly process of changes in the biotic community structure and function with time mediated through modifications in the physical environment ultimately culminating into a stable community known as climax. Clements (1916) while studying plant communities defined succession as “the natural process by which the same locality becomes successively colonized by different groups or communities of plants.” Odum (1969) preferred to call this process as ecosystem development. Ecosystem development may further be defined in terms of following parameters:

1. It is an orderly process of community development that involves changes in species structure with time. It is a directional process and thus predictable.
2. The succession is a community controlled process even though physical environment determines the patterns, rates of change and development.
3. It culminates in a stabilized ecosystem in which maximum biomass is maintained per unit of energy flow.

In the process of succession, the species present in an area will gradually change due to changes in the surrounding environmental conditions over a period of time. Each species is adapted to thrive and compete best against other species under a very specific set of environmental conditions. If these, then the existing species will be replaced by a new set of species which are better adapted to the new conditions. Ecological succession may also occur when environmental conditions suddenly and drastically change. A forest fire, wind storm, and human activities like agriculture all greatly alter the conditions of environment. The process is called as primary succession, if a community starts from the primitive substratum unoccupied by any other community. Secondary succession is referred to if a community starts on a previously built substratum. In such cases, the existing community might have disappeared due to sudden environmental changes such as fire, change in climatic factors, etc.

The process can be understood by an example of a bare land area where succession starts after a long period of rainfall and availability of nutrients.

Changes are noticed when a large bare area of land without any life form is left undisturbed or untilled and exposed to nature for a long time. Initially the soil is bare and totally exposed to sun and any rainwater would run off with little water being absorbed by the soil. In such conditions, such grasses will grow that would love the pounding sun and limited moisture available in soil. Once a blanket of grass thickens, it does not allow the rainwater to run off and increases the moisture content of soil.

Once the grass grows tall, it offers partial shade to plants that may grow between the clumps of grass. Therefore, a second plant species have a more congenial habitat and these plants would grow creating more shade and would retain more water. These changed conditions are not ideal for grass, so it dwindles in quantity.

Now these plants offer shade from the harsh sunlight and maintain adequate moisture level in the soil for another species to grow. Also, the dwindling of grasses provides the required space for this third plant species which may grow into larger bushes. These bushes, once grown tall, have to face sun unimpeded. However, these bushes may leave sufficient space in between which may be a suitable habitat for the growth of fourth plant species, i.e., large trees requiring adequate sunlight and water from soil and stifle the growth of the bushes beneath them.

earlier barren land is now full of several tall trees, few bushes, small plants and grass. This may be the equilibrium situation and hence may continue for a long time until any natural or manmade disturbance occurs. When this succession stabilizes, it is said to have reached a climax.

The study of these changes is important to understand the past and predict the future of ecosystem. As succession proceeds, changes occur not only in the biotic community but also in physical environment and overall structural and functional characteristics of ecosystems in a holistic manner. Thus, succession has been considered as ecosystem development that culminates in a stabilized ecosystem in which biomass and symbiotic function between organisms are maintained per unit of available energy flow. The process has been explained in detail in module 19 and 20 of this paper.

3.2    Ecosystem Regulation and Stability

All ecosystems regulate and maintain themselves under a set of environmental conditions. While evaluating ecosystem functioning, it is important to know whether an ecosystem is in a state of change or stable. In any ecosystem, any environmental stress tries to disturb the normal ecosystem functions. However, there is always a natural tendency of ecosystem to resist the change and maintain itself in equilibrium with the environment. This self-regulatory mechanism is known as homeostasis. `

An ecosystem is an open system that receives input from the environment and produces an output. The output from one sub – system may become the input for another sub – system in an ecosystem. For instance, food produced by plants becomes input for an autotrophic subsystem, which further provides an input to heterotrophic subsystem. Input from each subsystem controls the output of another subsystem. In open systems, when some portion of the output may be fed back as input to control the functioning, the system is called a cybernetic system (Fig 3.1).

The system shows tolerance or resistance only within a maximum and minimum range, which is its range of tolerance known as homeostatic plateau. It responds to inputs and has outputs, and set two types of system response – negative feedback and positive feedback.

Negative feedback mechanisms are deviation counteracting mechanisms which try to bring system back to its ideal conditions. They provide stability to ecosystem. All ecosystems have hundreds of negative feedback loops that keep every part of the system within the bounds necessary for proper functioning of system. Negative feedback keeps things the same. When part of a system changes too much from what it should be, other parts of the system change in a way that reverses the change in the first part. The function of negative feedback is to keep the parts of a system within limits that are necessary for survival.

Predator-prey interactions are examples of negative feedback. Predator (tiger) feeds on prey (deer) and reduces its population. This shows the interaction of predator with its prey through a negative feedback. If the predators are more, they will reduce the prey population to the extent that enough food is not available for the predators. So, some of them will die due to lack of food. Hence, the predator is reduced and in turn, prey population will increase. The negative effect of predators on prey prevents an uncontrolled growth of a predator’s population, thereby stabilizing the population sizes of both predator and prey. If the negative feedbacks are weak or absent, population cycles can amplify and lead to extinction of one or both of the interacting species.

Homeostasis is an example of negative feedback in biological systems. Homeostasis is the control of an organism’s internal physical and chemical conditions within limits required for the organism’s survival. For example, if body temperature increases above 37° C, negative feedback reduces the body temperature by reducing metabolic heat generation and increasing heat loss from the body (more blood supply to the skin and more sweating). If body temperature decreases below 37° C, negative feedback increases the body temperature by increasing heat generation (shivering) and decreasing heat loss (less blood supply to the skin and less sweating). Keeping the body temperature close to 37° C is essential for a person’s survival (Fig. 3.2).

The positive feedbacks are the deviation accelerating mechanisms. When part of a system increases, another part of the system changes in a way that makes the first part increases even more. There is positive feedback whenever A has a positive effect on B, and B has a positive effect on A (Fig 3.3). Positive feedback is a source of instability; it is a force for change.

So the positive feedback mechanisms add to stress conditions and tend to take the system away from the optimal conditions. Exponential growth is an example of positive feedback. Exponential population growth occurs when there is surplus of food, space and other resources that allows a plant or animal population to grow without limit. More population leads to more growth, and more births lead to an increasing population. In recent years, the human population exponential growth is reducing the ecosystem carrying capacity to provide the resources and absorb pollution. Human beings should try to keep the ecosystems within the homeostatic plateau. They should not contribute to positive feedbacks otherwise the ecosystem will collapse.

Ecosystem stability can be understood in context of resistance and resilience. The resistance is the ability of the system to resist the forces that tend to disrupt its state of equilibrium. On the other hand, resilience is the ease with which the system returns to its original equilibrium state following any perturbation. For example, a forest ecosystem with large biotic structure may better resist a fire outbreak than a grassland ecosystem with smaller biotic structure. Nevertheless, burnt grassland can quickly recover to its original state than a burnt forest that may take hundreds of years to recover (Fig. 3.4).

Fig. 3.5: Different Ecosystem Services

1. Provisioning services: These services include the products obtained from ecosystems such as food, fresh water, wood, fibre, genetic resources and medicines.

2. Regulating services: These include the services provided by plants, animals, fungi and micro-organisms such as pollination of crops, prevention of soil erosion, maintenance of soil fertility, pest control, water purification, climate regulation, natural hazard regulation, and waste management.

3. Habitat services: The habitat services include the importance of ecosystems to provide habitat for living organisms (plants, animals and micro-organisms) and to maintain the viability of gene-pools.

4. Cultural services: These are the non-material benefits that people obtain from ecosystems such as spiritual enrichment, intellectual development, recreation and aesthetic values, tourism.

Some examples of key services provided by ecosystems are described below:

1. Local climate and air quality: The forests influence rainfall and water availability both locally and regionally. They also regulate air quality by absorbing pollutants from the atmosphere.

2. Waste management: Ecosystems such as wetlands filter waste produced by human activities and act as a natural buffer to the surrounding environment. The bio-degradable waste is broken down through the biological activity of microorganisms in the soil. Thereby the level of overall pollution is reduced.

3. Carbon sequestration and storage: Ecosystems regulate the global climate by storing and sequestering greenhouse gases. The plants remove carbon dioxide from the atmosphere in the process of photosynthesis and effectively lock it away in their tissues. In this way, forest ecosystems are carbon stores, and thus reduce impacts of global warming.

4. Maintenance of genetic diversity: Genetic diversity is the variety of genes between and within species. Genetic diversity distinguishes different breeds or races from each other and provides the basis for locally well-adapted cultivars and a gene pool for developing commercial crops and livestock. Some areas are having high species richness and endemism which add more genetic diversity than others and are known as biodiversity hotspots.

5. Moderation of extreme events: Extreme weather events or natural hazards include floods, storms, tsunamis, avalanches and landslides. Ecosystems and living organisms create buffers against natural disasters, thereby preventing possible damage. For example, wetlands can soak up flood water whilst trees can stabilize slopes. Coral reefs and mangroves help protect coastlines from storm damage.

6. Pollination: Animals, wind and water pollinate plants and trees which is essential for the development of fruits, vegetables and seeds. Animal pollination is an ecosystem service mainly provided by insects, birds and bats.

7. Aesthetic and cultural appreciation: Language, knowledge and the natural environment have been intimately related throughout human history. Biodiversity, ecosystems and natural landscapes have been the source of inspiration for much of our art, culture and science.

8. Medicinal resources: Ecosystems and biodiversity provide many plants used as traditional drugs and medicines. They provide the raw materials for the pharmaceutical industries. All ecosystems are a potential source of medicinal resources.

9. Erosion prevention and maintenance of soil fertility: Soil erosion is a key factor in the process of land degradation and desertification. Plant cover provides a vital regulating service by preventing soil erosion. Soil fertility is essential for plant growth and agriculture and well functioning ecosystems supply the soil with nutrients required to support plant growth.

Despite the ecological and economic importance of these services, ecosystems and the biodiversity are being degraded and lost at an unprecedented scale. One major reason for this is the value of ecosystems to human welfare are poorly understood and not fully recognized in every day planning  and decision-making. In simplistic economic terms, the value of ecosystem services is larger than the global economy. Ecosystem services go beyond the direct economic benefits derived from exploitation of very specific ecosystem functions such as timber from forests. It is ecosystems ongoing capacities to provide a stream of life supporting and life enhancing services that are vital to human well being. Many of these services are non-market services by virtue of their inherent characteristics eg. both the atmospheric ozone layer, and the climate stability provided by the global carbon cycle, cannot be owned by anyone who can control their use by others. Furthermore, the costs of externalities of economic development (e.g. pollution, deforestation) are usually not accounted for, while inappropriate tax and subsidy (incentive) systems encourage the over-exploitation and unsustainable use of natural resources and other ecosystem services at the expense of the poor and future generations.

Summary

1. The ecological succession is defined as an orderly process of changes in the biotic community structure and function with time mediated through modifications in the physical environment ultimately culminating into a stable community known as climax.
2. The succession is a community controlled process even though physical environment determines the patterns, rates of change and development.
3. All ecosystems regulate and maintain themselves under a set of environmental conditions. There is always a natural tendency of ecosystem to resist the change and maintain itself in equilibrium with the environment. This self-regulatory mechanism is known as homeostasis.
4. An ecosystem is an open system that receives input from the environment and produces an output. The output from one sub – system may become the input for another sub – system in an ecosystem. In open systems, when some portion of the output may be fed back as input to control the functioning, the system is called a cybernetic system.
5. The ecosystems respond to inputs and have outputs, and set two types of system response – negative feedback and positive feedback.
6. Negative feedback mechanisms are deviation counteracting mechanisms which try to bring system back to its ideal conditions.
7. The positive feedbacks are the deviation accelerating mechanisms. When part of a system increases, another part of the system changes in a way that makes the first part increases even more. Positive feedback is a source of instability.
8. Ecosystem stability can be understood in context of resistance and resilience. The resistance is the ability of the system to resist the forces that tend to disrupt its state of equilibrium. On the other hand, resilience is the ease with which the system returns to its original equilibrium state following any perturbation.
9. The specific ecosystem functions that are directly or indirectly beneficial to human beings are called ecosystem services.
10. There are four groups of ecosystem services: i) Provisioning services ii) Regulating services iii) Habitat services and iv) Cultural services.

References

• Clements, F.E. (1916). Plant succession: an analysis of the development of vegetation. Washington, DC: Carnegie Institution of Washington.
• Odum, E.P. (1969). The strategy of ecosystem development. Science. 164: 262-270.