11.2 Ecosystem Interactions & Human Impact

What Is an Ecosystem?

An ecosystem is a community of living things together with the non-living environment they live in - the water, soil, sunlight, air and temperature - all interacting as one system. A rock pool, a wetland, a eucalypt forest and even a school compost heap are all ecosystems. The living parts are called the biotic factors and the non-living parts the abiotic factors, and ICAS questions often ask you to tell them apart: a fallen log's moisture is abiotic; the fungi growing on it are biotic.

Within an ecosystem, every species has a habitat - the place where it lives - and a niche - its role or "job" in that place. A koala's habitat is eucalypt forest; its niche includes eating gum leaves, being most active at night and rarely needing to drink. Two species can share a habitat but rarely share exactly the same niche, because if they needed exactly the same food at exactly the same time, one would eventually push the other out.

How Organisms Interact

Ecosystem interactions are a named topic on Paper F, and three kinds cover most of what you will meet at school level:

Competition can happen within one species (two male kangaroos contesting the same mob) or between species (native parrots and introduced bees both nesting in tree hollows). Predation drives the food-web changes from the previous section: predator and prey numbers rise and fall in linked cycles, because more prey feeds more predators, and more predators then reduce the prey. Mutualism is cooperation: flowering plants get pollinated, and the birds or insects get food - remove one partner and the other suffers, which is why mutualism matters so much in conservation.

Reading a change in an ecosystem

When a question describes an ecosystem changing - say a drought shrinking a wetland - trace it through both biotic and abiotic parts. Less water (abiotic) means fewer water plants (biotic producers), which means less food and shelter for fish and frogs, which means less food for herons. Ecosystem thinking is really just food-web thinking with the non-living environment added in.

Human Impact: Pollution

The impact of pollution is a named topic on ICAS Paper D. Pollution is the release of harmful substances into the environment faster than nature can break them down. Know the three main types and their effects:

• Air pollution - smoke, exhaust fumes and industrial gases. Effects include breathing problems in people and animals, and acid rain, which forms when gases mix with water in the atmosphere and then falls onto forests and lakes, harming leaves and aquatic life.
• Water pollution - factory chemicals, fertiliser running off farms, oil spills and litter. Fertiliser causes algal blooms: the nutrients make algae multiply explosively, the algae block sunlight, and when they die and rot the bacteria use up the oxygen in the water, so fish suffocate.
• Soil pollution - dumped chemicals, pesticides and landfill. Poisons in soil are taken up by plants and then pass along the food chain, becoming more concentrated at each level, which is why top predators suffer most.

The step where pollution enters a food chain is a favourite exam angle: a toxin may be nearly harmless to a plant at low levels but builds up until it sickens the eagle at the top.

Introduced Species: An Australian Story

An introduced species is one brought by people - deliberately or accidentally - into an ecosystem where it did not evolve. Australia has painful examples. Rabbits, released for hunting in 1859, multiplied into millions, stripped grazing land bare and caused soil erosion. Foxes, also introduced for hunting, are a major predator of small native mammals and ground-nesting birds. Cane toads, brought to Queensland in 1935 to eat sugar-cane beetles, ate almost everything except the beetles; their poison kills native predators such as goannas and quolls that try to eat them.

Introduced species succeed for a common reason: they arrive without their natural predators, competitors or diseases, so nothing holds their numbers down. That phrase - "no natural predators" - is the key explanation ICAS looks for.

Habitat Clearing and Conservation

Habitat clearing - cutting down bushland for farms, roads and suburbs - is the biggest threat to Australian wildlife, because removing the habitat removes the food, shelter and breeding sites every species in it depends on. Conservation responses include national parks and reserves, wildlife corridors that reconnect isolated patches of bush, breeding programs for endangered species, and controlling introduced predators. Replanting native vegetation restores both the producers at the base of the food web and the niches built on top of them.

Interpreting environmental data

ICAS loves a data table with a story. Imagine fish counts in a river at three sites: 340 fish upstream of a factory, 85 fish just downstream, and 210 fish two kilometres further downstream. The pattern tells the story: numbers crash right below the factory and partly recover further down as the pollutant is diluted. The sound conclusion is that the factory is releasing something harmful - not that fish "prefer" upstream water, and not that the cause must be fishing, which would affect all sites. When you read environmental data, compare like with like, find where the pattern changes, and match the change to the event in the stimulus.