Ecosystems Revision Notes

An ecosystem is a functional unit where living organisms interact with each other and with their physical environment. It includes both biotic (living) and abiotic (non-living) components.

A population is a group of individuals of the same species living in a defined area, for example, a population of oak trees in a forest.

A community consists of all populations of different species living and interacting in the same area, such as all plants, animals, fungi, and microorganisms in a pond ecosystem together forming its community structure.  The habitat is the specific natural environment where an organism lives, like a pond for frogs or a desert for cacti.  A niche describes the role or function of an organism within its ecosystem, including how it obtains food, interacts with other species, and contributes to energy flow and nutrient cycling. For example, bees have the niche of pollinators in many ecosystems.

• Ecosystem = living organisms + physical environment interacting.
• Population = same species group in an area.
• Community = all species populations in an area.
• Habitat = natural living place of an organism.
• Niche = organism’s role/function in ecosystem.

Biotic factors are the living parts of an ecosystem. These include: producers (organisms that make their own food, mainly green plants), primary consumers (herbivores that eat producers), secondary and tertiary consumers (carnivores that eat other consumers), and decomposers (organisms like fungi and bacteria that break down dead material and recycle nutrients).

Abiotic factors are the non-living physical and chemical elements that influence ecosystems. Important abiotic factors include temperature, humidity, availability of water, oxygen concentration, salinity (salt content), light intensity, and pH (acidity or alkalinity). These factors affect the survival and distribution of organisms. For example, fish species vary depending on water temperature and oxygen levels.

• Biotic: producers, consumers (primary, secondary, tertiary), decomposers.
• Abiotic: temperature, humidity, water, oxygen, salinity, light, pH.

Organisms interact in various ways that affect their survival and reproduction. Competition occurs when two or more organisms require the same limited resource such as food, space, or light. For example, trees in a dense forest compete for sunlight.

Predation is an interaction where one organism (the predator) hunts and eats another (the prey), such as a fox hunting rabbits.

Pollination is a mutualistic interaction where pollen is transferred from one flower to another, often by insects like bees, enabling plant reproduction. This benefits both plants and pollinators.

• Competition: organisms vie for limited resources.
• Predation: predator hunts and eats prey.
• Pollination: transfer of pollen aiding plant reproduction.

Photosynthesis is the process by which green plants use chlorophyll to convert carbon dioxide and water into glucose and oxygen, using energy from sunlight. This process stores energy in chemical bonds of glucose, which can be used by the plant or other organisms.

The word equation for photosynthesis is: carbon dioxide + water → glucose + oxygen.

Respiration is the process by which organisms break down glucose in the presence of oxygen to release energy needed for cellular activities. The products are carbon dioxide and water, which are released back into the environment, completing a cycle with photosynthesis. The word equation for respiration is: glucose + oxygen → carbon dioxide + water + energy.

• Photosynthesis: carbon dioxide + water → glucose + oxygen (using chlorophyll and sunlight).
• Respiration: glucose + oxygen → carbon dioxide + water + energy.

Energy flows through ecosystems via food chains and food webs. A food chain shows a linear sequence of organisms where each is eaten by the next, for example: grass → rabbit → fox.

Food webs are interconnected food chains showing complex feeding relationships in a community.

Trophic levels represent the position of organisms in a food chain: producers at the first level, primary consumers at the second, and so on. Energy decreases at each trophic level due to loss as heat and metabolic processes, typically about 90% lost per level, so only about 10% is transferred to the next level, limiting the number of trophic levels in an ecosystem. Ecological pyramids illustrate this energy flow or the number of organisms at each level. Pyramids of numbers show the count of organisms, which can sometimes be inverted, while pyramids of energy always show a decrease in energy from producers upwards.

• Energy flows via food chains and webs.
• Trophic levels: producers → consumers (primary, secondary, tertiary).
• Energy transfer efficiency ~10% per trophic level.
• Ecological pyramids show numbers or energy at each level.

The carbon cycle describes how carbon atoms move through the environment, cycling between living organisms and the atmosphere.

Photosynthesis removes carbon dioxide from the air to produce glucose in plants.

Respiration by plants, animals, and decomposers returns carbon dioxide to the atmosphere by breaking down glucose for energy release. Decomposition of dead organisms also releases carbon back into the environment. Additionally, combustion of fossil fuels releases stored carbon as carbon dioxide, contributing to atmospheric carbon levels.

• Carbon cycles through photosynthesis, respiration, decomposition, combustion.
• Photosynthesis stores carbon in glucose.
• Respiration and decomposition release carbon dioxide.
• Combustion releases carbon dioxide from fossil fuels.