Life History and Age-Structured Populations

Vocabulary flashcards covering key terms and concepts related to life history strategies and age-structured populations in ecology.

Age-structured population

A population divided into distinct age groups to analyze how age-specific rates of birth, death, and reproduction influence overall population dynamics. These models are crucial for predicting population trends, informing resource management, and detecting age-specific vulnerabilities.

Life table

A summary of age-specific survival and reproductive rates within a population. Key components include: age class (x), survivorship (l_x), age-specific mortality (d_x), age-specific mortality rate (q_x), and age-specific fecundity (m_x). Life tables can be cohort (following a group) or static (sampling at a specific time).

Fecundity

The reproductive capacity of an organism, typically referring to age-specific fecundity (m_x), which is the average number of female offspring produced by a female of age x. It's vital for calculating R_0 and T.

Survivorship curve

A graph plotting the proportion of individuals surviving at each age (l_x vs. x). There are three main types:

1. Type I (Concave): High survival until old age (e.g., humans).
2. Type II (Diagonal): Constant mortality rate throughout life (e.g., some birds).
3. Type III (Convex): High mortality for the young, high survival for those reaching maturity (e.g., many insects).

Semelparity

A reproductive strategy where an organism has a single, often large, reproductive event followed by death (e.g., Pacific salmon). This is common in unpredictable environments.

Iteroparity

A reproductive strategy involving repeated reproductive events over an organism's lifespan (e.g., humans). This is favored in stable environments where adults have a high chance of surviving to reproduce again.

Allee effect

A phenomenon where individual fitness and population growth rate decrease at low population densities. This can be due to difficulties in mate finding, reduced cooperative benefits, or genetic problems, posing a risk of extinction.

Net reproductive rate (R_0)

The average number of female offspring produced by a female in her lifetime, calculated as the sum of (l_x \times m_x). If R_0 > 1, the population is growing; if R_0 = 1, it's stable; if R_0 < 1, it's declining. \qquad R_0 = \sum_{x=0}^{\omega} l_x m_x

Generation time (T)

The average age of mothers in a cohort when they give birth, representing the average time between generations. Shorter generation times lead to faster population growth. It can be estimated by: \qquad T = \frac{\sum_{x=0}^{\omega} x l_x m_x}{R_0}

K strategy

A life history strategy for stable environments, characterized by slow development, long lifespan, large body size, few large offspring, high parental investment, and density-dependent selection (e.g., elephants).

r strategy

A life history strategy for unpredictable environments, characterized by rapid development, short lifespan, small body size, many small offspring, low parental investment, and density-independent selection (e.g., bacteria, insects).

Life expectancy (e_x)

The average number of additional years an individual of a given age x is expected to live, calculated from life tables.

Reproductive value (v_x)

The expected contribution of an individual of age x to future population growth, considering current and future reproductive output and survival probabilities. It helps prioritize conservation efforts.

Survivorship schedule (l_x)

A component of a life table detailing the proportion of an initial cohort still alive at the beginning of each age interval x, measuring the probability of surviving from birth to age x.

Fecundity schedule (m_x)

A component of a life table specifying the average number of female offspring produced per female during a particular age interval x. It's crucial for calculating R_0.

Trade-offs in reproduction

The allocation of limited energy and resources by an organism among competing demands like growth, maintenance, and reproduction. Key trade-offs include current vs. future reproduction, number vs. size of offspring, and parental care vs. offspring number.

Population Growth Models

Mathematical frameworks to predict how population size changes over time.

1. Exponential Growth: Occurs with unlimited resources (J-shaped curve). Formula: \frac{dN}{dt} = rN or Nt = N0 e^{rt}.
2. Logistic Growth: Accounts for environmental limits (carrying capacity K) (S-shaped curve). Formula: \frac{dN}{dt} = rN(1 - \frac{N}{K}).

Carrying capacity (K)

The maximum population size that a given environment can sustain indefinitely, where births typically equal deaths in the logistic growth model.

Intrinsic rate of natural increase (r)

The maximum potential per capita rate of population growth under ideal conditions. A higher r indicates a faster-growing population. It can be approximated by: \qquad r \approx \frac{\ln(R_0)}{T}

Density-dependent factors

Factors that limit population growth more strongly as population density increases, intensifying with crowding. These are usually biotic (e.g., competition, predation, disease) and regulate population size towards K.

Density-independent factors

Factors affecting population growth regardless of density. These are typically abiotic (e.g., natural disasters, extreme weather, pollution) and can cause population fluctuations but don't regulate towards K.

Life History Traits

A set of characteristics influencing an organism's survival and reproduction, shaped by natural selection through evolutionary trade-offs. Key traits include age at first reproduction, offspring number/size, reproduction frequency, lifespan, and parental care.

Mortality rate (q_x)

The probability that an individual alive at the beginning of an age interval x will die during that interval. Calculated as deaths (d_x) divided by individuals alive at the start (n_x): \qquad q_x = \frac{d_x}{n_x}