APES Unit 3 Notes: Species Strategies and the Dynamics of Population Growth

Niche

A species’ role in an ecosystem, including what it eats, where it lives, when it’s active, interactions with other organisms, and tolerance to conditions like temperature and moisture.

Generalist species

A species with a broad niche that can use many resources and tolerate a wide range of environmental conditions.

Specialist species

A species with a narrow niche that depends on a limited set of resources or specific environmental conditions.

Habitat fragmentation

The process of breaking a large habitat into smaller patches, which can simplify ecosystems and often harms biodiversity—especially for specialists.

Invasive species (general pattern)

A non-native species that spreads in a new area; many successful invaders are generalists because they can use many resources and tolerate varied conditions.

r-selected species

Species with life-history traits that maximize growth rate: early reproduction, many offspring, and relatively low investment (little energy) per offspring; common in variable or disturbed environments.

K-selected species

Species with life-history traits suited to living near carrying capacity: fewer offspring, higher parental care, later maturity, and longer lifespan; common in stable, predictable environments.

Intrinsic rate of increase (r)

The per capita population growth rate under ideal conditions; used in exponential and logistic growth models.

Carrying capacity (K)

The maximum population size an environment can sustain over time without degrading the resources the population depends on; varies with environmental conditions.

Limiting factor

Any environmental condition or resource that restricts population growth (e.g., food, water, space, disease, nesting sites, waste buildup).

Density-dependent factor

A limiting factor whose impact increases as population density increases (e.g., competition for food, disease spread).

Density-independent factor

A factor that affects populations regardless of population density (e.g., droughts, floods, extreme temperatures, many natural disasters).

Negative feedback (population context)

A regulating process where increasing population size reduces resources per individual, lowering birth rates and/or raising death rates as a population approaches K.

Survivorship curve

A graph showing the proportion of individuals from a cohort that are still alive at each age, revealing when mortality is highest (early, constant, or late).

Cohort

A group of individuals born around the same time, tracked through life in survivorship analysis.

Type I survivorship curve

High survival through early and middle life with most mortality occurring at old age; common in many K-selected species with parental care.

Type II survivorship curve

Roughly constant mortality risk across the lifespan, so individuals are about equally likely to die at any age.

Type III survivorship curve

Very high mortality early in life, but those that survive to adulthood often live much longer; common in many r-selected species with many offspring and little parental care.

Parental care (life-history trait)

Investment by parents in protecting or raising offspring; generally increases early survival and is more common in K-selected strategies.

Population change equation (population bookkeeping)

A relationship showing how population size changes: ΔN = (B + I) − (D + E), where births and immigration add individuals and deaths and emigration remove individuals.

Exponential growth

Population growth under effectively unlimited resources, where growth rate is proportional to current population size (often modeled as dN/dt = rN), producing a J-shaped curve.

Logistic growth

Population growth that slows as resources become limiting and the population approaches K (often modeled as dN/dt = rN(1 − N/K)), producing an S-shaped curve.

Bottom-up control

Regulation of population sizes by resource availability at lower trophic levels (e.g., nutrients/producers limiting herbivores, which then influence predators).

Overshoot

When a population temporarily exceeds the environment’s long-term carrying capacity, often due to time lags in feedback from resource depletion.

Dieback

A population decline that can follow an overshoot, especially when resources have been degraded and can no longer support the previous population size.