AP Environmental Science:: Unit 3 - Populations

A population, in the ecological sense used throughout this unit, is defined as:

- **A)** Every living organism present within a given ecosystem, regardless of species
- **B)** The nonliving physical/chemical components of a given environment, with no reference to any living organism
- **C)** A group of individuals of the same species living in the same geographic area at the same time, capable of interbreeding with one another
- **D)** A concept that applies only to human beings, with no meaningful applicability to any other species

The exponential population growth model describes a population that grows:

- **A)** At a constant per-capita growth rate with no limiting factors, producing a characteristic J-shaped growth curve that accelerates continuously over time as the population itself grows larger - this model assumes unlimited resources and is therefore most realistic only over a short time period or when a population is first colonizing a new, resource-rich environment
- **B)** At a rate that levels off as the population approaches the environment's carrying capacity, producing a characteristic S-shaped curve, a description that instead corresponds to the logistic growth model discussed next, not exponential growth
- **C)** At a constant absolute number of new individuals added per year regardless of the population's own current size, rather than a constant per-capita rate
- **D)** In a pattern that has no meaningful, describable mathematical relationship to population size, growth rate, or time at all

The logistic population growth model, generally considered more realistic than the exponential model over the longer term, differs from exponential growth in that logistic growth:

- **A)** Never levels off at any population size, continuing to accelerate indefinitely regardless of resource availability, the same core assumption that instead defines exponential growth, not logistic growth
- **B)** Incorporates the effect of limited resources by slowing the population's growth rate as it approaches the environment's carrying capacity (K) - producing a characteristic S-shaped (sigmoidal) growth curve that levels off once the population reaches K, rather than the unchecked, continuously-accelerating J-shaped curve produced by the exponential model
- **C)** Applies only to producer/plant populations, with no meaningful applicability to any animal population
- **D)** Describes population decline exclusively, with no meaningful applicability to any population growth scenario

The carrying capacity (K) of an environment, a central concept in the logistic growth model discussed above, is best defined as:

- **A)** The single largest population size that has ever been recorded for a given species anywhere on Earth, regardless of the specific environment being considered
- **B)** A population's maximum theoretical per-capita growth rate under conditions of unlimited resources
- **C)** A value that has no meaningful, describable relationship to resource availability, population size, or environmental limits of any kind
- **D)** The maximum population size that a given environment can sustainably support indefinitely, given the available resources (food, water, space, and other necessities) in that environment - a population that temporarily exceeds its environment's carrying capacity (an overshoot, discussed further below) will generally experience an increased death rate and/or decreased birth rate that brings the population back down toward K over time

A population overshoot, in which a population temporarily exceeds its environment's carrying capacity (K, discussed above), is frequently followed by a sharp population decline or "die-off" primarily because:

- **A)** A population above K is, by definition, consuming resources faster than the environment can regenerate them, so once those resources become sufficiently depleted, the population's death rate rises and/or its birth rate falls until the population size falls back toward (or, if the environment itself was damaged by the overshoot, even below) the original carrying capacity
- **B)** A population above K always continues growing indefinitely without any subsequent decline of any kind, the general opposite of an overshoot's own actual, well-established typical outcome
- **C)** Carrying capacity itself always increases automatically and immediately in direct proportion to any increase in population size, eliminating any possibility of a population ever truly exceeding it
- **D)** A population overshoot has no meaningful, describable relationship to resource depletion, death rate, or birth rate of any kind

Density-dependent limiting factors, which regulate population size, are best distinguished from density-independent limiting factors in that density-dependent factors:

- **A)** Affect a population's growth rate identically regardless of that population's own current size or density, a description that instead corresponds to density-independent factors, not density-dependent factors specifically
- **B)** Apply only to producer/plant populations, with no meaningful applicability to any animal population
- **C)** Have an effect on a population's growth rate that intensifies as the population's own density increases - examples include competition for limited food/resources, predation, and the spread of disease (which spreads more easily in a denser, more crowded population) - by contrast, density-independent factors (such as a severe storm, wildfire, or extreme temperature event) affect a population's growth rate by roughly the same proportion regardless of the population's current density
- **D)** Have no meaningful, describable relationship to population size, density, or growth rate regulation of any kind

Which of the following is the best example of a density-independent limiting factor on a wild population?

- **A)** Increased competition for limited food resources as a population becomes more crowded
- **B)** A severe, unseasonal early frost that kills a large proportion of a population regardless of how crowded or sparse that population currently is
- **C)** A disease that spreads more rapidly and severely through a denser, more crowded population than through a sparser one
- **D)** Increased predation pressure specifically triggered by a rising population density

An r-selected species (sometimes called an r-strategist) is best characterized by a life-history strategy that emphasizes:

- **A)** Producing very few offspring per reproductive event, but investing very heavily in parental care for each one, the same general life-history pattern that instead defines a K-selected species, not an r-selected species
- **B)** A relatively long overall lifespan combined with delayed sexual maturity and reproduction
- **C)** A population size that remains consistently very close to its environment's carrying capacity (K) at essentially all times
- **D)** Producing a very large number of offspring per reproductive event, with relatively little parental care invested in each individual offspring, rapid sexual maturity, and a relatively short overall lifespan - this strategy favors rapid population growth, making r-selected species well-suited to unstable, unpredictable, or newly-available environments where a population is far below carrying capacity