Ecology: Population, Community, and Ecosystem Concepts (Lecture Notes)

Population ecology

• Focus of population ecology: track change over time by looking at gene pool makeup and assessing allele frequency change and genotype frequency change over time.
• Humans as a special case: we don’t always follow the same rules as other organisms, which is why this topic is framed with caveats when discussing humans.
• Main goal of population studies in this course: understand how populations change over time and what factors drive those changes.
• Key concepts introduced:
  • Allele frequency change
  • Genotype frequency change
  • Population size and its importance for genetics in a given environment
• Population size and sampling:
  • It can be difficult or impractical to count every individual in a population.
  • Counting methods rely on sampling and estimation rather than full enumeration.
• Counting example and discussion:
  • Counting earthworms example: use a quadrat (a defined square area) to sample a portion of the population.
  • The quadrat size used in the example is approximately 1.9 m on a side (about 1.9 m × 1.9 m).
  • Different plots are sampled by different students; extrapolation is used to estimate population size for the area.
  • Forester example: counting trees over a wide area is possible but still typically not done for every tree; estimation is common.
• Population change over time:
  • Example: Snow geese migrate long distances to Arctic breeding grounds; their population dynamics were discussed with a practical, non-mathematical approach for class.
  • A simple numerical example used in the discussion:
  • 7,500 individuals added to the population (births, immigration, etc.).
  • About 3,500 individuals removed (deaths, emigration).
  • Net gain:
    $\Delta N = B - D = 7500 - 3500 = 4000.$
• Rate of increase (r):
  • r is the rate of intrinsic increase; it indicates how quickly a population grows or declines.
  • In class, focus is on the slope of the population trajectory rather than precise numerical r values.
  • Interpretations:
  • If r = 0, there is zero population growth; the population is stable (no net gain or loss) over time.
  • If the slope is upward, growth occurs; the steeper the slope, the faster the growth.
  • If the slope is downward, the population is declining.
  • Positive r means growth; negative r means decline; some organisms have strategies to maximize r, others to maintain stability.
• Real-world factors limiting population growth:
  • Limiting factors (also called environmental resistance) keep population growth in check.
  • Common examples include resource limitation, waste accumulation, space, predation, disease, temperature, etc.
  • Waste accumulation is highlighted as a less obvious limiting factor, especially in smaller or closed systems (e.g., bacterial growth on a petri dish):
  • Waste products accumulate as population grows and can limit further growth.
  • Space limitation can also constrain population growth; when space and resources are scarce, growth slows or reverses.
• Disease and genetics as limiting factors:
  • If a disease disproportionately affects a majority of a population, it can wipe out a large portion of individuals.
  • Genetic factors can influence susceptibility to disease and other stressors, impacting population trajectories.
• Deer and other species: population growth dynamics can vary; some populations grow quickly and may require management to prevent problems.
• Summary takeaway for population ecology:
  • Populations are dynamic and influenced by births, deaths, immigration, emigration, and environmental constraints.
  • The balance of these factors determines whether a population grows, remains stable, or declines.
  • Practical study often relies on sampling and estimation rather than complete enumeration.
  • Understanding the slope of population change is more intuitive for this course than memorizing r values.

Community ecology

• Community ecology focuses on interactions among populations of different species that co-occur in the same area.
• It examines the web of interactions that shape community structure and dynamics.
• Key types of interactions include:
  • Predation (one species feeds on another)
  • Competition (organisms compete for the same resources)
  • Symbiotic relationships (mutualism, commensalism, parasitism)
• The content emphasizes how these interactions influence population sizes and species distributions within a community.
• The discussion highlights that species can indirectly affect one another through trophic interactions, resource use, and habitat modification.
• Example considerations:
  • Different populations may interact by predation or competition, influencing growth and survival rates of involved species.
  • In some cases, species from different populations can interact across the same landscape in ways that shape community structure (e.g., sharing resources, affecting each other’s survival).

Ecosystem ecology

• Ecosystem ecology is described as the largest, broadest level of ecological organization, integrating biological communities with their physical environment.
• Conceptual takeaways:
  • Ecosystems encompass the flow of energy and cycling of nutrients among organisms and their surroundings.
  • The spatial scale can vary from a backyard to a large landscape, but the idea is to understand how energy and nutrients move through the system.
• Mobility and geographic range:
  • The ability of organisms to move and disperse affects their potential range and interactions.
  • Boundary effects (e.g., political borders or geographic boundaries) can influence gene flow and interbreeding opportunities.
  • The discussion uses the idea that interbreeding cannot occur if populations are separated by geographic barriers or are too far apart (e.g., frogs in Wisconsin vs. Minnesota unless near a border and close enough for gene flow).
• Range size and territory size:
  • These measures depend on the organism's ecology and mobility.
  • Highly mobile organisms (e.g., many birds) may have large ranges and territories, with higher potential for interbreeding across broader areas.
  • Less mobile organisms may have smaller, more localized ranges.
• Practical implications for ecology:
  • Understanding population size and distribution informs studies of genetics, evolution, conservation, and management.
  • Across levels (population, community, ecosystem), interactions and environmental context shape the dynamics and resilience of natural systems.

Connections and takeaways

• Interconnectedness across levels:
  • Population changes influence community structure (e.g., how predator populations respond to prey abundance).
  • Community interactions and environmental conditions feed back into ecosystem processes (energy flow, nutrient cycling).
• Real-world relevance:
  • Population size estimation and understanding growth rates are essential for wildlife management and conservation.
  • Limiting factors such as waste accumulation and space are practical considerations in both natural and managed environments (e.g., breeding sites, habitat fragmentation).
• Life history strategies (mentioned at the end of the lecture):
  • The instructor indicates that future discussions will cover life history strategies, which describe how organisms allocate resources to growth, reproduction, and survival to optimize fitness in their environments.

Key terms and quick definitions

• Allele frequency: the proportion of a specific allele in a population’s gene pool.
• Genotype frequency: the proportion of individuals carrying a specific genotype in a population.
• Population size: the number of individuals of a given species in a defined area.
• Quadrat: a defined square sampling area used to estimate population size.
• B (births) and D (deaths): components used to estimate population change; often alongside immigration and emigration (I and E).
• Net population change: $\Delta N = B - D$ (when considering births and deaths in a simple example).
• Rate of intrinsic increase: $r = \frac{1}{N}\frac{dN}{dt}$, a measure of how quickly a population grows or declines.
• Environmental resistance / limiting factors: factors that limit population growth (e.g., space, resources, waste, disease).
• Life history strategy: a suite of traits that influence how an organism allocates energy to growth, reproduction, and survival.

Quick recap of examples discussed

• Snow geese: long-distance migration with Arctic breeding grounds, illustrating how migration and environmental factors influence population dynamics.
• Earthworms and trees: sampling via quadrats or plots to estimate population sizes when counting every individual is impractical.
• Population change illustration: a simple numerical example showing how births and deaths translate into a net gain or loss for the population.

Note: The instructor mentions that heavy statistical analyses of population dynamics are not a focus in this class; the emphasis is on understanding concepts through intuitive interpretation of trends and qualitative reasoning.