Unit 8: Ecology

24 terms. Say what each one means, then open the card to check.

• Endotherm

  An animal that keeps its body temperature up mainly with heat from its own metabolism, so it burns more fuel when its surroundings are cold.

  Example. A fictional arctic fox eats far more in the depths of winter than in summer, burning the extra food to hold its body near 38 degrees C.

• Ectotherm

  An animal whose body temperature is set mostly by its surroundings. It controls temperature by behavior, such as basking or seeking shade, and needs less food.

  Example. A fictional desert lizard basks on a rock in the morning until it is warm enough to hunt, then rests under a bush at midday.

• Taxis

  Moving in a set direction because of a stimulus: heading straight toward it or straight away from it. The stimulus might be light, a chemical or gravity.

  Example. Fictional flatworms placed in a dish crawl straight away from a lamp toward the dark side, a negative response to light.

• Kinesis

  A change in the speed or turning rate of movement in response to a stimulus, with no set direction toward or away from it.

  Example. Fictional beetle larvae crawl quickly in bright light but slow down and turn more often in shade, so over time they collect in shady spots.

• Photoperiodism

  A response of an organism, such as flowering in a plant, to the relative lengths of day and night.

  Example. A fictional marsh plant flowers only when nights are longer than 12 hours, so it blooms in autumn.

• Pheromone

  A chemical signal released by an animal that changes the behavior or body of other members of its species.

  Example. A female of a fictional moth species releases a scent at dusk, and males of her species fly upwind to find her.

• Trophic level

  A feeding step in a food chain: producers first, then primary consumers that eat them, then secondary consumers, and so on.

  Example. In a fictional pond, algae are producers, snails that graze algae are primary consumers, and fish that eat snails are secondary consumers.

• Net primary productivity

  The energy producers store in new biomass over a period: gross primary productivity (all energy captured) minus the energy producers use in their own respiration.

  Example. A fictional meadow captures 15,000 kilocalories per square meter per year and its plants use 9,000 in respiration, leaving 6,000 as net productivity.

• Energy transfer between levels

  Only a small share of the energy at one trophic level, roughly 10 percent though it varies, ends up in the next level. Most is lost as heat from respiration or never eaten.

  Example. If the plants in a fictional field store 10,000 kilocalories, the grasshoppers that eat them store only about 1,000.

• Decomposer

  An organism, such as a fungus or bacterium, that breaks down dead matter and wastes and returns nutrients such as nitrogen and phosphorus to the soil or water.

  Example. Fungi on fallen leaves in a fictional forest release nitrogen compounds that tree roots take up the next spring.

• Nitrogen fixation

  The conversion of nitrogen gas from the air into ammonia or ammonium, which plants can use. Only some bacteria and archaea can do it.

  Example. Bacteria living in nodules on the roots of a fictional clover turn nitrogen gas into ammonium, so the clover thrives in poor soil.

• Exponential growth

  Growth with no limits, in which the number added each year is proportional to the population size: dN/dt = rmax x N. It makes a J-shaped curve.

  Example. A fictional weed with rmax = 0.5 per year in an empty field adds 50 plants in a year at 100 plants and 500 in a year at 1,000.

• Carrying capacity

  How many of a species a place can feed and house year after year, given its food, water and space. Biologists call it K.

  Example. A fictional island has food and nest sites for about 400 gulls, so the gull population rises and then levels off near 400.

• Logistic growth

  Growth that slows as a population nears carrying capacity: dN/dt = rmax x N x (K - N) / K. It makes an S-shaped curve and is fastest at half of K.

  Example. Fictional yeast in a flask of broth multiply quickly at first, then more slowly, and level off as sugar runs low and wastes build up.

• Density-dependent factor

  A factor whose effect on birth or death rates grows stronger as a population gets more crowded, such as competition for food, disease or predation.

  Example. A fictional disease spreads faster in a crowded rabbit warren than in a sparse one, so it kills a larger share of the crowded population.

• Density-independent factor

  A factor that changes birth or death rates by the same share no matter how crowded a population is, such as a frost, flood or fire.

  Example. A fictional late frost kills about 30 percent of young plants in both a thin stand and a dense stand.

• Niche

  The role of a species in its community: the conditions it can live in and the resources it uses. Competition can shrink the part of its niche it actually uses.

  Example. A fictional warbler feeds on insects high in spruce trees, nests on low branches and breeds in early summer; all of that together is its niche.

• Competitive exclusion

  The idea that two species using the same limited resources in the same way cannot coexist for long; one eventually outcompetes the other.

  Example. Two fictional ground beetles eat the same seeds in the same strip of damp soil; in plots where researchers add the second species, the first slowly disappears.

• Mutualism, commensalism and parasitism

  Close interactions between species: in mutualism both gain; in commensalism one gains and the other is unaffected; in parasitism one gains and the host is harmed.

  Example. A fictional bee pollinates a flower while drinking its nectar (mutualism), while a fictional tick feeds on a deer’s blood (parasitism).

• Keystone species

  A species that matters to its community far more than its numbers suggest. Take it away and the mix of other species can change a great deal.

  Example. A fictional bat is the only pollinator of a desert cactus whose fruit feeds many birds and insects; where the bat disappears, the cactus fails to fruit and those animals become scarce.

• Simpson’s diversity index

  A measure of diversity that accounts for both the number of species and how evenly individuals are spread among them: 1 minus the sum of (n/N) squared.

  Example. A plot with 50 plants each of two species gives 1 - (0.25 + 0.25) = 0.5; a plot with 90 of one and 10 of the other gives 1 - (0.81 + 0.01) = 0.18.

• Invasive species

  A species brought into a new area, often by people, that spreads quickly and harms native species, frequently because its natural predators or diseases are absent.

  Example. A fictional vine planted in gardens escapes into a forest with no insects that eat it and smothers the native shrubs.

• Eutrophication

  Excess nutrients, often from fertilizer runoff, cause algae to bloom; when the algae die, decomposers use up dissolved oxygen and fish and other animals die.

  Example. After heavy rain washes farm fertilizer into a fictional lake, a green bloom spreads, and weeks later dead fish wash up along the shore.

• Ecological succession

  The slow turnover of which species live in a place after it is disturbed. Primary succession begins on bare rock with no soil; secondary succession begins where soil is left.

  Example. Grasses and wildflowers move onto a fictional field after a fire, then shrubs, then young trees over several decades.