Population Biology

Environmental Challenges

• Natural selection drives evolutionary adaptation to environmental conditions.
• Two main components of the environment:
  • Abiotic: Temperature, water, sunlight, soil.
  • Biotic: Other living organisms.
• Individual organisms adapt physiologically or behaviorally to changing conditions.
• Natural selection favors individuals that can survive a range of environmental conditions.

Populations

• Populations are groups of individuals in one place and time.
• Population ecology focuses on:
  • Population range: Where is the population located?
  • Dispersion: Pattern of spacing of individuals within the range.
  • Population size: How it changes through time.
  • Demographics: Composition of types of individuals within a population.

Population Range

• Northward Expansion of Trees: Illustrates how populations shift range due to environmental change.
• The environment changed after the glaciers retreated.
• Plant and animal populations expanded northward.
• Changes in climate allowed populations to live at higher elevations.

Dispersion = Spacing Patterns

• Random spacing: Individuals do not interact strongly; uncommon in nature.
• Uniform spacing: Behavioral interactions, resource competition.
• Clumped spacing: Uneven distribution of resources; common in nature.

Metapopulations

• Clumped distribution leads to partially separated subpopulations.
• Interaction may not be symmetrical.
• Subpopulations increase and send out dispersers.
• Small populations produce fewer dispersers.
• Subpopulations may face extinction or recover.

Source-Sink Metapopulations

• Source-sink metapopulations: Subpopulations in better areas (source) bolster populations in poorer areas (sink).
• Allows a population range to expand beyond the most successful areas.

Population Dynamics

• Population dynamics: Change in population size over time.
• Fitness: Ability to pass on genes to the next generation, requiring survival (low death rates) and reproduction (high birth rates).
• Tradeoffs often exist between survival and reproduction.

Body Size and Generation Time

• Population growth rate is affected by generation time, the average interval between generations.
• Examples illustrating the relationship between body size and generation time are provided (e.g., bacteria, insects, mammals, plants).

Age Structure

• Study birth and death rates as a function of age.
• Cohort: Group of individuals of the same age.
• Fecundity: Number of offspring produced in a standard time.
• Mortality: Death rate in a standard time.
• Age structure critically influences a population's growth rate.
• Age at first reproduction differs from lifespan.

Population Demography: Survivorship

• Survivorship curve: Percentage of an original population that survives to a given age.
• The slope of the curve indicates the relative rate of mortality.
• Different types of survivorship curves (Type I, II, and III) are associated with different life history strategies.

Cost of Reproduction

• Tradeoffs exist between survival and reproductive success.
• Data from bird species illustrates this tradeoff.

Survival of Offspring

• Balance between the number and size of offspring.
• Larger offspring have a greater chance of survival but require more energy to produce.
• Small offspring may have low survival rates.

Models of Population Growth

• Rate of growth: r = (b - d) + (i - e), where:
  • r = rate of population increase,
  • b = birth rate,
  • d = death rate,
  • i = immigration,
  • e = emigration (per capita).
• Biotic potential: r_i = (b - d), the intrinsic rate of natural increase for the population.
  • Innate capacity for growth.

Models of Population Growth

• Exponential growth model: Rate of growth is constant.
• Growth rate in numbers: \frac{dN}{dt} = rN, where:
  • N is the number of individuals in the population,
  • \frac{dN}{dt} is the rate of change over time.
• Results in unchecked exponential growth if ri > 0, extinction if ri < 0

Population Density

• Population size often described by population density.
• Carrying capacity: Symbolized by K, the maximum number of individuals the environment can support.
• Common Density-dependent factors include:
  • Resource shortages,
  • Disease,
  • Increased predation.

Population Growth

• Logistic growth model: Considers carrying capacity (K) in the equation for population growth.
• The equation for logistic growth is: \frac{dN}{dt} = rN(\frac{K - N}{K})

Logistic Growth

• Examples of logistic growth curves in fur seals and cladocerans.

Density-Dependence in Song Sparrows

• Demonstrates how the number of young per female decreases as the number of breeding adults increases, illustrating density-dependent regulation.

r-K Theory

• Logistic growth model highlights two different strategies for evolutionary success (maximizing population).
  • Max growth rate r.
  • Carrying Capacity K.

K-selected populations (increase K)

• Adapted to thrive when the population is near its carrying capacity and push K even higher.
• Costs of reproduction are high.
• Individuals must compete and utilize resources efficiently.
• Lots of parental investment to ensure success.
• Can lower reproductive rates.

r-Selected Populations (increase r)

• Populations far below carrying capacity.
• Evolutionary success by rapid population growth (increase r).
• Resources abundant.
• Costs of reproduction are low.

r vs. K

• Most natural populations show adaptations that are some mix of r- and K-selected traits.
• Stable environments favor K selection - largest populations for organisms exquisitely tuned to environment.
• Unstable environments favor r selection - largest populations for organisms that can recover rapidly after a disaster.

Other patterns of population dynamics

• Allee effect: Growth rates increase with population size due to benefits of large groups.

Density-Independent Effects

• Rate of growth of a population can be limited by factors unrelated to the size of the population.
  • External environmental aspects: cold winters, droughts, storms, volcanic eruptions.
• Fluctuations in the number of pupae of various moth species.

Population cycles

• North American snowshoe hare has a 10-year cycle.
• Two factors generate this cycle:
  • Food plants.
  • Predators.
• Density-dependent but from biotic interactions with other species
• Community ecology

Human Population Growth

• Earth's rapidly growing human population constitutes a challenge to the future of the biosphere.
• The world ecosystem is already under stress.
• Thomas Malthus: Essay on the Principle of Population - inspiration for Darwin's idea of natural selection.

Human Population Growth

• What is K for the human population?
• Changes since the 1700s allowed humans to escape logistic growth.
• Human population has grown exponentially.
  • Birth rate has remained unchanged.
  • Death rate has fallen dramatically.

History of Human Population Size

• World population growth rate is declining.
  • High of 2.0% in 1965 to 1970.
  • 1.1% in 2015, but still an increase of 80 million people per year.

History of Human Population Size

• Uneven distribution of population growth among countries.

Population Pyramid

• Bar graph displaying the number of people in each age category.
• Kenya's population could double in 30 years, whereas Sweden's will remain stable.

Humanities future population growth is uncertain

• Projections vary based on fertility rates (high, medium, low).

What matters in population growth

• Education and women's autonomy play significant roles in shaping population growth.

Consumption

• Wealthiest 20% 86% consumption of resources and produces 53% of CO2 emissions.
• Poorest 20% is responsible for 1.3% consumption and 3% CO2 emissions.
• Ecological Footprint: productive land required to support each individual.

Pandemics and Human Health

• Pandemic: widespread disease outbreak that extends over multiple countries
  • Bubonic plague (1346 to 1353)
  • Spanish flu (1918)
  • COVID-19 (2020)
• Epidemic: more localized outbreak

Factors affecting Ro

• Ro depends on:
  • Transmissibility of pathogen
  • Degree of immunity in the population
  • Degree of vaccination in the population
  • Behavioral effects that affect transmissibility
• Ro for COVID-19 in unvaccinated population = 1.4-2.4
• Ro for measles in unvaccinated population = 12-18

Herd Immunity

• As the proportion of those with immunity in a population increases, transmission declines.
• When Ro<1, the disease eventually disappears.
• Herd immunity occurs when the proportion of immune individuals is large enough to stop the spread of the disease (often around 70%).

Ro is not uniform

• Degree of spread is not uniform over the population.
• Subpopulations may harbor the pathogen and keep it alive and cause it to break out into rest of population.
• Vaccination.
• Not an independent choice per individual.
• Difficult balance.

Pathogens Evolve

• Natural selection favors mutations that cause a pathogen to more effectively infect new individuals.
• Can evolve to:
  • Become more contagious.
  • Overcome host's immunity.
• More cases = more chances for mutation.