Energy and Ecosystems

Ecology is the study of how living organisms interact with each other and with their environment. An ecosystem is a major ecological unit. For example a forest consists of all living organisms living there as well as the soil, rocks, water etc.

• Ecosystem = a natural unit of living (biotic) components in a given area, as well as the non-living (abiotic) factors with which they interact.
• Community = the different populations of species that live in a habitat
• Decomposers = microbes, bacteria and fungi that obtain nutrients from dead orgainsms and faeces. They complete the process of decomposition started by detritivores
• Detritivores = organisms such as earthworms which feed on small fragments of organic debris, called detritus, made up of non-living organic material such as faeces, fallen leaves and the remains of dead organisms
• Habitat = the particular area occupied by a population which is the place where an organism lives. It has biotic and abiotic features which seperate it from other habitats
• Saprobionts = include bacteria and fungi that feed by secreting enzymes on the food substrate absorbing the resulting products of this extracellular digestion

Features include :-

Edaphic features relate to the soil and include all its physical and chemical characteristics

Climatic features include light, temperature, moisture, salinity and particularly the stability or variability of these

Microhabitats are small localities within a habitat, each with its own particular conditions

An ecological niche is the place of each species in an ecosystem. This is not only the physical space which it occupies but the role which it carries out within the community and its inter-relationships with other species as well. In the long-term, two species cannot occupy the same niches in a specific habitat otherwise they compete with each other

Nutrition is the process by which organisms obtain energy to maintain life functions and matter to create and maintain structure.

These are obtained from Nutrients.

Most Autotrophic organisms use the simple organic materials, carbon dioxde and water, to manufacture energy - containing complex organic compounds during Photosynthesis.

Heterotrophic organisms consume complex organic food material for Respiration to produce energy in the form of ATP.

The study of the flow of energy through ecosystem is known as Ecological Energetics. The energy from the sun enters the Food Chain at the producer level during Photosynthesis and is the source of energy for the ecosystems.

• The Sun's Energy is passed from one feeding or trophic level to another through the Ecosystems
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• Energy is passed along a hierarchy of trophic levels with primary producers (plants) at the starting point of the chain
• All animals are consumers
• Primary consumers eat plants and are also called herbivores
• Secondary consumers are carnivores that feed on primary consumers
• Tertiary consumers are carnivores that eat other consumers
• Eventually the energy leaves the system as Heat

• Green plants trap solar energy and manufacture sugars from simple raw materials by the process of photosynthesis
• Herbivores (primary consumers) are animals which feed on plants
• Carnivores are animals that feed on other animals
• Each of these groups forms a feeding or trophic level with energy passing from each level to a higher one as material is eaten
• Only a small quantity of the total energy that reaches the plant as light is incorporated into plants tissues. As energy is passed along the food chain there is a large loss at each level
• At each level energy is lost through Respiration, and through the Excetion of Waste products, so the energy passed on is reduced
• The sequence from plant to herbivore to carnivore is a food chain and is the route by which energy passes between trophic levels
• It is the loss of energy at each level which limits the length of a food chain so the number of links in a chain is normally limited to four or five
• On death of producers and consumers, some energy remains locked up in the organic compounds of which they are made. Detritivores and decomposers feed as saprobionts and contribute to the recycling of nutrients.

• The ultimate source of energy for ecosystems is the sun from which energy is released in the form of electromagnetic waves
• A good deal of the solar energy is reaching the Earth's atmosphere does not penetrate it. It is reflected or absorbed and radiated back into space by the ozone layer, dust particles and clouds
• Also about 90% of the energy reaching the surface of the Earth is reflected by vegetation, soil and water or absorbed and radiated to the Earths atmosphere as Heat
• This means that only about 10% is left for producers to make use of. Therefore, only a small part of the total energyreaching the Earth's atmosphere enters ecosystems
• The quantity absorbed by plants varies considerably at different latitudes
• Of the energy entering a plant only 1% to 5% is utilised by the plant, the rest is lost, partly by reflection and partly by evapouration.

This is the % of energy at one trophic level which is incorportaed into the next trophic level. The rate at which energy passes into the animals at each trophic levels is about 10% of that entering the previous level. This is called Gross Ecological Efficiency (GEE).

This value differs from one ecosystem to another with some highest values around 40%, occuring in oceanic food chains.

Some lowest values, around 1%, are found in ecosystems where most of the animals are birds and animals.

Example:

In a particular food chain, if 15,00KJ of energy enters the primary consumer level and 1,500KJ passes to the secondary consumer level, then: -

Gross Ecological Efficiency  =  (15000/1500)  X 100 = 10%

The energy flowing from one organism to another in the food chain originates as sunlight. A large proportion of the energy that falls on a plant is not absorbed:-  (Assume that 100 units of energy per unit time reach the leaves of a crop plant)

• Unit of Energy - What happens to the Energy?
• 50  - The wrong wavelength, that is, the photosynthetic pigments absorbing mainly light at the red and blue parts of the spectrum
• 10 - Some of the light is reflected and some passes straight through the leaf, that is, it is transmitted
• 30.8 - Lost in the process of Photosynthesis and Evaporation
• 9.2 - Incorporated into plant products such as glucose
• 3.7 - Used up in the process of Respiration

Photosynthetic Efficiency indicates the ability of a plant to trap light energy = Quantity of energy incorporated into product / Quantity of energy falling on the plant

This is normally less than 10% and depends on the external factors such as temperature, light intensity.

Gross Primary Production (GPP) is the rate at which products, such as glucose are formed. A substantial quantity of gross production is used up in respiration by the plant. GPP - Respiration = NPP      9.2 - 3.7 = 5.5 units

That which is left over after Respiration is called the Net Primary Productivity (NPP). This represents the food avaliable to primary consumers. In crop plants this represents the yield which may be harvested.

• Secondary productivity is the rate at which consumers accumulate energy in the form of cells or tissues
• Biological productivity is the rate at which biomass accumulates in an ecosystem
• Biomass is the dry weight of organic matter comprising a group of orgamsims in a particular habitat

Biological Productivity has two components:-

• Primary Productivity - the production of new organic matter by green plants 
• Secondary Productivity - the production of new organic matter by consumers

Consumers have a conversion efficiency of about 10%, that is, for every 100 grams of plant material taken in or ingested, only about 10 grams is incorporated into herbivore biomass.

This means that only part of NPP of the ecosystem is transferred to the primary consumers. Herbivores are not able to eat all the vegetation avaliable to them.

Consider a cow feeding on grass in a field:-

• Some of the plant material is not eaten by the cow, Cattle grazing a field will eat the grasses and edible weeds but do not eat the roots and often leave the woody parts of the plants
• Cows feed on plant material which contains cellulose which they are unable to digest. This passes out of the body as faeces containing a high proportion of undigested matter. Energy is used by decomposers
• Some of the food material in the field is being eaten by other herbivores, such as rabbits

Much of the energy in the food consumed by a cow is lost mainly by two processes:-

• Respiration - about 30% lost as heat
• Excretion - about 60% lost in urine and faeces

Farmers increase the productivity of their livestock by keeping the animals in barns during winter months. Herbivores have a lower secondary productivity than carnivores. That is, carnivores are more efficient at energy conversion than herbivores.

They have a much higher secondary productivity. This is because their protein-rich diet is more readily and efficiently digested. Only about 20% of the energy intake is lost in the faeces and urine of carnivores compared with a loss of about 60% in herbivores.

Carnivores absorb almost twice as much energy per unit mass of food compared with herbivores. The loss of energy at each trophic level gives ecological pyramids their characteristic shape.

The number of organisms, their biomass or the amount of energy contained in each trophic level can be represented in diagrams with a bar for each level. These are knwon as pyramids. They provide a quantitative account of the feeding relationships in a community. The most accurate way to represent the feeding relationships in a community is to use a pyramid of energy.

A pyramid of energy shows the quantity of energy transferred from one trophic level to the next, per unit area or volume, per unti time. This represents the total energy requirement of each successive trophic level in a food chain. As material passes up through the food chain energy is lost in respiration as heat, and in excretion, so the size of bars decreases sharply. Since only some of the energy is passes on from one level to the next, energy pyramids are never inverted as in biomass pyramids.

The use of a set period of time means that an energy pyramid overcomes the problem which arise when ecosystems are compared simply counting or measuring the standing crop of organisms.

Pyramids of energy enable eases of comparison of the efficiency of energy transfer to be made from one trophic level to the next between different communities.

However obtaining the data can be complex and difficult.

The distribution of species does not necessarily remain the same over a long periods of time. Ecosystems are dynamic and subject to change. Organisms and their environments interact, if one changes so does the other. A change in the environment affects the organisms, and a change in the organisms affect the environment.

An area of bare ground does not remain free of vegetation for long. Weeds are usually the first plants to grow, followed by grasses taller than plants. Provided the soil conditions are suitable, over a very long period of time the bare ground ould eventually become woodland. Succession is the change in structure and composition of species over time.

• Primary Succession- refers to the introduction of plants/animals into areas that have not previously supported a community. Eg. bare rock, or site of volcanic eruption
• Secondary Succession - refers to the reintroduction of organisms into a bare habitat previously occupied by plants and animals. If the original vegetation is removed. Eg. by fire or by tree felling, the area rapidly becomes re-colonised by a succession of different plants and animals.

In any area, over time, new organisms replace existing ones, that is, species diversity increases until a stable state is reached. All successions usually involve changes in community strcture and function untill a community reaches a climax of succession knwon as Climax Community, for example a mature woodland. This is a stable community which undergoes no further change. The different stages in a succession are known as seres.

Consider the colonistaion of bare rock. The first organisms to colonise the bare rock are algae and lichens. These plants are called pioneer species and form a pioneer community. Lichens slowly erode the rock. This together with weathering of the rock and the accumulation of dead and decomposing organic material leads to he formation of a primitive soil.

Wind blown spores allow mosses to appear and as the soil develops grasses become established. As the soil builds up deep-rooted shrubs appear. Over a very long time trees, such as oak, become established. This is known as the climax community. This is a stable condition dominated by long-living plants.

It should be noted that a community consists of animals as well as plants and that the animals have undergone a similar succession dictated by the plant types present at each stage.

In a secondary succession, seeds, spores and organs of vegetative reproduction may remain in the soil and dispersal of plants and migration of animals will assist in colonistation of the habitat.

Human interference can affect a succession and may prevent the natural development of the climax community:-

• Grazing by sheep
• Heather moorland management by controlled burning for grousing shooting
• Farming of land
• Deforestation and soil erosion.