Biology, SS 1
Topic: Functioning Ecosystem
Introduction
An ecosystem is a basic functioning unit in nature. It is made up of living organisms (plants and animals) and their non-living environment. The biotic or living components such as the producers and consumers interact in their environment resulting in the ecosystem being a functional unit.
Autotrophs, Heterotrophs and Decomposers
Autotrophs
Autotroph is an organism that serves as a primary producer in a food chain. Autotrophs obtain energy and nutrients by harnessing sunlight through photosynthesis (photoautotrophs) or, more rarely, obtain chemical energy through oxidation (chemoautotrophs) to make organic substances from inorganic ones. Autotrophs do not consume other organisms; they are, however, consumed by heterotrophs.
Energy Production
Autotrophs produce their own energy by one of the following two methods:
- Photosynthesis – Photoautotrophs use energy from sun to convert water from the soil and carbon dioxide from the air into glucose. Glucose provides energy to plants and is used to make cellulose which is used to build cell walls. E.g. Plants, algae, phytoplankton and some bacteria. Carnivorous plants like pitcher plant use photosynthesis for energy production but depend on other organisms for other nutrients like nitrogen, potassium and phosphorous. Hence, these plants are basically autotrophs.
- Chemosynthesis – Chemoautotrophs use energy from chemical reactions to make food. The chemical reactions are usually between hydrogen sulfide/methane with oxygen. Carbon dioxide is the main source of carbon for Chemoautotrophs. E.g. Bacteria found inside active volcano.
Heterotrophs
Heterotrophs are organisms that survive by feeding on organic matter produced by or available in other organisms. It is an organism that consumes other organisms in a food chain, hence, they are called consumers. In contrast to autotrophs, heterotrophs are unable to produce organic substances from inorganic ones. They must rely on an organic source of carbon that has originated as part of another living organism. Heterotrophs depend either directly or indirectly on autotrophs for nutrients and food energy.
Decomposers
Decomposers are organisms that feed on dead organisms and other decaying organic materials. Fungi and bacteria are decomposers in energy transformation in an ecosystem. They are responsible for breaking down the complex organic compounds into simple nutrients. There are different types of decomposer organisms, which are responsible for returning simpler nutrients to the soil to be used by plants — and so the energy transformation cycle continues.
Food Chain and Food Web
Every organism needs to obtain energy in order to live. For example, plants get energy from the sun, some animals eat plants, and some animals eat other animals.
Food Chain
A food chain is the sequence of who eats whom in a biological community (an ecosystem) to obtain nutrition. In other words, food chain is defined as a feeding relationship involving the transfer of energy through food from producers to consumers. A food chain starts with the primary energy source, usually the sun. The next link in the chain is an organism that makes its own food from the primary energy source — an example is photosynthetic plants that make their own food from sunlight (using a process called photosynthesis) and chemosynthetic bacteria that make their food energy from chemicals in hydrothermal vents. These are called autotrophs or primary producers.
Some eat the autotrophs; these organisms are called herbivores or primary consumers — an example is a grasshopper that eats grass.
The next link in the chain is animals that eat herbivores – these are called secondary consumers — an example is a rat that eat grasshopper.
The next link in the chain is animals that eat the secondary consumers – these are called tertiary consumers – an example is a snake the eat rat
In turn, these animals are eaten by larger predators — an example is an owl that eats snakes.
The tertiary consumers are eaten by quaternary consumers — an example is a hawk that eats owls. Each food chain ends with a top predator and animal with no natural enemies (like an alligator, hawk, or polar bear).
The arrows in a food chain show the flow of energy, from the sun or hydrothermal vent to a top predator. As the energy flows from organism to organism, energy is lost at each step.
Trophic Levels
The trophic level of an organism is the position it holds in a food chain.
Primary producers (organisms that make their own food from sunlight and/or chemical energy from deep sea vents) are the base of every food chain – these organisms are called autotrophs.
Primary consumers are animals that eat primary producers; they are also called herbivores (plant-eaters).
Secondary consumers eat primary consumers. They are carnivores (meat-eaters) and omnivores (animals that eat both animals and plants).
Tertiary consumers eat secondary consumers.
Quaternary consumers eat tertiary consumers.
Food chains “end” with top predators, animals that have little or no natural enemies.
When any organism dies, it is eventually eaten by detrivores (like vultures, worms and crabs) and broken down by decomposers (mostly bacteria and fungi), and the exchange of energy continues.
Some organisms’ position in the food chain can vary as their diet differs. For example, when a bear eats berries, the bear is functioning as a primary consumer. When a bear eats a plant-eating rodent, the bear is functioning as a secondary consumer. When the bear eats salmon, the bear is functioning as a tertiary consumer (this is because salmon is a secondary consumer, since salmon eat herring that eat zooplankton that eat phytoplankton, that make their own energy from sunlight).
A network of many food chains is called a food web. Food web is defined as a complex feeding relationship among organisms in the same environment with two or more inter-related food chains.
Numbers of Organisms:
In any food web, energy is lost each time one organism eats another. Because of this, there have to be many more plants than there are plant-eaters. There are more autotrophs than heterotrophs, and more plant-eaters than meat-eaters. Although there is intense competition between animals, there is also interdependence. When one species goes extinct, it can affect an entire chain of other species and have unpredictable consequences.
Equilibrium
As the number of carnivores in a community increases; they eat more and more of the herbivores, decreasing the herbivore population. It then becomes harder and harder for the carnivores to find herbivores to eat, and the population of carnivores decreases. In this way, the carnivores and herbivores stay in a relatively stable equilibrium, each limiting the other’s population. A similar equilibrium exists between plants and plant-eaters.
The number of organisms in a food chain can be represented graphically in a pyramid. Each bar represents the number of individuals at each trophic level (feeding level) in the food chain.
In this example a large number of caterpillars living in a single oak tree provide food for several blue tits, which in turn are consumed by a sparrowhawk.
The pyramid of numbers usually shows that the number of organisms at each trophic level gets smaller towards the top. This particular case is an exception – one tree provides food for many caterpillars.
Pyramid of Biomass
A pyramid of biomass is a more accurate indication of how much energy is passed on at each trophic level.
Biomass is the mass of living material in each organism multiplied by the total number of organisms in that trophic level. This makes it easier to compare the food value of a small number of large organisms with a large number of small organisms. Pyramids of biomass usually are a true pyramid shape.
The biomass in each trophic level is always less than the trophic level below. This is because biomass is a measure of the amount of food available. When animals eat, only a small proportion of their food is converted into new tissue, which is the food for the next trophic level. Most of the biomass that animals eat is either not digested, or used to provide the energy needed for staying alive.
Processes of Ecosystems
The diagram with the plants, zebra, lion, and so forth illustrates the two main ideas about how ecosystems function: ecosystems have energy flows and ecosystems cycle materials. These two processes are linked, but they are not quite the same.
Energy Flows and Material Cycles
Energy enters the biological system as light energy, or photons, is transformed into chemical energy in organic molecules by cellular processes including photosynthesis and respiration, and ultimately is converted to heat energy. This energy is dissipated, meaning it is lost to the system as heat; once it is lost it cannot be recycled. Without the continued input of solar energy, biological systems would quickly shut down. Thus the earth is an open system with respect to energy.
Elements such as carbon, nitrogen, or phosphorus enter living organisms in a variety of ways. Plants obtain elements from the surrounding atmosphere, water, or soils. Animals may also obtain elements directly from the physical environment, but usually they obtain these mainly as a consequence of consuming other organisms. These materials are transformed biochemically within the bodies of organisms, but sooner or later, due to excretion or decomposition, they are returned to an inorganic state. Often bacteria complete this process, through the process called decomposition or mineralization.
During decomposition these materials are not destroyed or lost, so the earth is a closed system with respect to elements. The elements are cycled endlessly between their biotic and abiotic states within ecosystems.
The Transformation of Energy
The transformations of energy in an ecosystem begin first with the input of energy from the sun. Energy from the sun is captured by the process of photosynthesis. Carbon dioxide is combined with hydrogen (derived from the splitting of water molecules) to produce carbohydrates (CHO). Energy is stored in the high energy bonds of adenosine triphosphate, or ATP.
Energy Loss in the Ecosystem
The solar radiation is used by plants during photosynthesis. Some are lost to the earth’s surface while some are stored in plants’ parts stems, leaves, etc. and are not used up.
First Law of Thermodynamics
The law states that energy can neither be lost nor created especially as it is converted from one form to another.
Second Law of Thermodynamics
The law states that no energy transformation is 100% efficient, since some are lost in form of heat.
Decomposition in Nature
Decomposers are organisms, mainly bacteria and saprophytes responsible for the breaking down of dead organic materials which could be of plants or animals origin. These decomposers are grouped into two classes:
- Micro-decomposers: These are small or microscopic organisms that can cause decay, e.g. certain bacteria and fungi.
- Macro-decomposers: These are bigger organisms that can cause decay of dead organic materials, e.g. earthworms, termites, snails, mushroom, toad stools, etc.
Process of Decomposition
The decomposers secrete enzymes onto their food source such as decaying plant. These enzymes break down complex organic compounds (food) like carbohydrates and proteins into simple soluble inorganic compounds.
Roles of Decomposers in Ecosystem
- Decomposers play major roles in the ecosystem in the following ways:
- They enrich the soil with nutrients required for plant growth
- They contribute to environmental pollution
- Decomposition is useful in the making of cheese and yogurt
- They also prevent an unsightly accumulation of remains and wastes of living organisms on earth surface.
