9: population dynamics

What determines population size?

Population size is determined by four opposing processes:

births and immigration increase population size, while deaths and emigration decrease it

What is population dynamics?

Study of how and why population size changes over time

with implications for extinction risk, endangered species, pest control, and resource management

Dispersal:

Why is dispersal important?

Dispersal moves individuals between areas, increasing or decreasing local population density and connecting populations

What are the main drivers of dispersal?

Climate change, food availability, and transport mechanisms

How does climate change drive dispersal?

Species shift ranges over time (e.g., tree movement tracked using pollen records)

What is a numerical response?

(Food availability) A change in predator population size in response to changes in prey abundance

How does transport influence dispersal?

Organisms may drift in flowing systems (rivers/streams) or actively move to new areas

What is the drift paradox?

Some organisms avoid being washed downstream by dispersing upstream during mobile life stages

Metapopulations:

What is a metapopulation?

A set of subpopulations connected by dispersal between habitat patches

What is a subpopulation?

A smaller population within a larger system, often maintained by immigration

How does patch size affect population size?

Larger patches support more individuals and have lower extinction risk

How does distance (isolation) affect populations?

Greater distance reduces dispersal, making it harder to maintain populations

Why is connectivity important?

It allows movement between patches, supporting population persistence

What is the effect of habitat fragmentation?

Increases edge habitat, decreases interior habitat, and often reduces biodiversity

Survival:

Why is survival important in population dynamics?

Survival rates directly influence population size and growth

What is a life table?

A table summarizing survivorship and mortality across different age groups

Life Tables:

What is a cohort life table?

Tracks a group from birth to death; accurate but difficult to obtain

What is a static life table?

Estimates survival using ages at death from many individuals; easier but less precise

What is age distribution?

The proportion of individuals in each age class within a population

Age Distribution:

What can age distribution reveal?

Past environmental conditions, population trends, and effects of climate or resource availability

Example of environmental impact?

High rainfall years increase young individuals; drought years increase mortality, with long-term effects

Survivorship:

What are the main survival patterns?

High survival of young (late-life mortality), constant mortality, and high early-life mortality

What is a Type I curve?

High survival in early/mid life, with mortality increasing in old age (common in large vertebrates)

What is a Type II curve?

Constant mortality rate across all ages (e.g., birds, turtles)

What is a Type III curve?

High mortality early in life, with few individuals surviving to adulthood (e.g., fish, many plants)

Population Growth:

How do life tables and fecundity schedules help?

They allow calculation of population growth parameters and prediction of future population trends

What is a fecundity schedule?

The reproductive output of individuals at different ages

What is net reproductive rate (R₀)?

Average number of offspring an individual produces over its lifetime

What does R₀ < 1 mean?

Population is declining because individuals are not replacing themselves

What is geometric rate of increase (λ)?

Ratio showing how population size changes from one time step to the next

What is generation time (T)?

Average time between the birth of parents and their offspring

What is intrinsic rate of increase (r)?

Per capita rate of population growth under ideal conditions

Applications:

How are life tables used in ecology and society?

Predict population declines, guide conservation of endangered species, estimate human mortality, and study aging to improve lifespan