Ecology and Classification

Ecology is the part of General Science with the fewest facts and the most structure. Learn the levels and the three relationships and most questions fall out.

Levels of organization

From smallest to largest:

Organism, population, community, ecosystem, biome, biosphere.

A community is all living things; an ecosystem adds the non-living. That distinction is asked directly: water, soil, temperature and light are part of an ecosystem and not of a community. The living parts are called biotic and the non-living abiotic.

Habitat and niche sound alike and are not. A habitat is where an organism lives - its address. A niche is its role there - its job: what it eats, what eats it, when it is active. Two species cannot share exactly the same niche for long; one outcompetes the other.

Food chains and energy flow

Enlarge

Zoom in

The roles:

Every food chain starts with a producer, and almost every one starts with sunlight captured by photosynthesis.

The ten percent rule

Only about 10 percent of the energy at one level reaches the next. The rest is lost as heat, or used for the organism's own living, or is in parts that are not eaten.

That explains two things the test asks:

Why food chains are short. After four or five steps there is not enough energy left to support another level.

Why there are fewer predators than prey. It takes a great deal of grass to support one hawk, and the numbers at each level shrink accordingly.

A question giving you 10,000 units of energy at the producer level and asking what reaches the tertiary consumer wants 10,000, then 1,000, then 100, then 10.

Chains against webs

A food chain is one path. A food web is all the chains in a community connected, and it is the more realistic picture, because most animals eat more than one thing.

Decomposers are in the web but not usually drawn in a chain. They return nutrients to the soil, which is what makes the cycle a cycle: energy flows through and is lost, but matter is recycled.

Biomass is the total mass of living things at a level. Like energy, it shrinks at each step up the pyramid.

A keystone species has an effect on its ecosystem far larger than its numbers suggest - remove it and the whole community changes. Sea otters are the classic case: they eat sea urchins, and without otters the urchins strip the kelp forest bare.

The cycles of matter

Energy flows through an ecosystem once; matter goes round and round. Four cycles are asked.

Water. Evaporation from oceans and lakes, transpiration from plants, condensation into clouds, precipitation back down, and runoff or groundwater returning it to the sea.

Carbon. Photosynthesis takes carbon dioxide out of the air; respiration, decomposition and burning return it. Burning fossil fuels releases carbon that was locked away for millions of years, which is why it raises atmospheric carbon dioxide.

Nitrogen. The air is 78 percent nitrogen, but plants and animals cannot use it in that form. The cycle is a chain of bacteria, and each step is asked:

Because the nitrifying bacteria need oxygen, waterlogged soil stalls nitrification - the hardest question in this family.

Phosphorus is the odd one out: it has no gas phase. It cycles slowly through rock, soil, water and living things, and never enters the atmosphere.

Populations

Exponential growth is what happens with unlimited resources: the more there are, the faster they increase, drawing a J-shaped curve. The maximum rate a population could grow at is its biotic potential.

Logistic growth is what actually happens: growth slows as food, space and other limiting factors run short, and the curve flattens into an S shape at the carrying capacity - the largest population the environment can support.

Species take two broad strategies. R-selected species reproduce quickly and have many offspring, most of which die young (insects, weeds). K-selected species have few offspring and invest heavily in each (elephants, humans).

Specialist species - those that depend on one food or one habitat - are the most vulnerable to extinction. A species found naturally in only one place is endemic to it; one brought in from elsewhere that spreads and does damage is invasive.

Succession

Succession is how a community rebuilds over time.

• Primary succession starts on bare rock with no soil at all, after a lava flow or a retreating glacier. Pioneer species - lichens and mosses - arrive first and slowly make soil.
• Secondary succession follows a disturbance that leaves the soil: a forest fire, a flood, an abandoned farm. It is much faster, because the soil is already there.

Either way it ends in a stable climax community, such as a mature forest.

Symbiosis

Three long-term relationships between species, distinguished by who gains and who loses.

Mutualism is both, commensalism is one and neither, parasitism is one at the other's cost. Questions give you a scenario and ask for the name.

Predation is not symbiosis. A predator kills and eats; a parasite lives on or in its host and usually does not kill it, because a dead host is no use.

Competition is a fourth relationship, where both are harmed because they need the same limited resource.

Biomes

Rainfall and temperature are what define a biome, not location. A cold desert is still a desert.

The tropical rainforest has the greatest biodiversity of any land biome, and the tundra the least - both asked.

Classification

Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species - broadest to narrowest.

Enlarge

Zoom in

The classic mnemonic runs on the first letters: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

A species is the narrowest rank, and it is defined as a group whose members can breed and produce fertile offspring. Fertile is the operative word: a horse and a donkey produce a mule, which is sterile, so they remain separate species.

Binomial nomenclature names an organism by its genus and species, with the genus capitalized and both italicized: Homo sapiens.

The kingdoms usually given at this level are animals, plants, fungi, protists and bacteria.

The vertebrate classes are sometimes asked by their Latin names: Mammalia (mammals), Aves (birds), Reptilia (reptiles), Amphibia (amphibians). The animal diversity lesson says what sets each apart.

Productivity

Primary productivity is the rate at which producers make new organic matter by photosynthesis. Gross primary productivity is all of it; net primary productivity is what is left after the producers' own respiration, so NPP = gross - respiration. With a gross of 100 and respiration of 60, the net is 40. Tropical rainforests have the highest net primary productivity of any land ecosystem; deserts and the open ocean are among the lowest.

What you can skip

Across all 2,104 General Science questions in our bank:

• The three domains by name. Archaea and Eukarya never appear, and "domain" only as the first word of the ranking. Know the order of the ranks.
• Biome trivia. Questions ask what defines a biome and which has the most and least biodiversity. Tropical rainforest and tundra carry most of it; you do not need average rainfall figures.
• Population math. No question asks you to calculate a growth rate. Knowing the J curve from the S curve, and what carrying capacity is, is enough.

Where people lose points

Saying energy is recycled. Matter is recycled; energy flows one way and is lost as heat.

Getting the ten percent rule backward and multiplying instead of dividing.

Calling predation parasitism.

Putting the ranks in the wrong order, especially class and order.

Confusing a community with an ecosystem.

Saying a food chain starts with a consumer. It starts with a producer.

Work one in under a minute

A food chain runs grass, grasshopper, frog, snake. If the grass holds 50,000 units of energy, roughly how much reaches the snake?

Ten percent per step, and the snake is three steps up.

• Grasshopper: 5,000
• Frog: 500
• Snake: 50

Three divisions by ten, one per arrow. Counting the arrows rather than the organisms is what keeps this straight.

Where this leads

The producers at the base of every food chain are doing the photosynthesis of the previous lesson, and the variation within a species is what evolution acts on.