Lecture Notes: Organisms, Populations, and Ecosystem Dynamics

Review of Lecture 2: Organisms and Environmental Factors

• Species name; ecotone

  • Ecotone: a transitional zone between two ecosystems where species from both communities may occur and interact.

• Condition/resource survival curve

  • Describes how survival or performance varies across an environmental gradient.

  • Key regions: optimum range, zones of stress, and limits of tolerance.

• Liebig’s Law of the Minimum (the scarcest resource; long-term)

  • Growth or performance is limited by the required resource in shortest supply relative to demand.

• Blackman’s Law of limiting factors (short-term; process; the limitation can shift from one factor to another as conditions change)

  • The factor that currently limits a process can change as conditions change; multiple factors may influence outcomes depending on context.

• Matter in Living and Nonliving Systems

  • Matter exists as molecules/compounds; elements and compounds move through living and nonliving systems.

• Molecule/compound; N CHOPs

  • CHNOPS: Carbon (C), Hydrogen (H), Nitrogen (N), Oxygen (O), Phosphorus (P), Sulfur (S) – essential elements in biology.

• Organic vs inorganic

  • Organic: carbon-containing compounds typically produced by living organisms; inorganic: lacks carbon-hydrogen bonds or synthetic by living systems.

• The four spheres of earth’s environment

  • Atmosphere, Hydrosphere, Lithosphere, Biosphere

Matter and Energy in Living Systems

• Law of Conservation of Matter

  • Matter cannot be created or destroyed; it is conserved through chemical and biological processes.

• First Law of Thermodynamics

  • Energy cannot be created or destroyed; it can be transformed from one form to another.

• Second Law of Thermodynamics

  • In energy transformations, some energy becomes unavailable for work (often released as heat); entropy tends to increase.

• Photosynthesis; cell respiration

  • Photosynthesis: captures light energy to convert
    it into chemical energy stored in glucose, which serves as a primary source of energy for organisms.

  • Cellular respiration: uses glucose and oxygen to release energy, producing
    carbon dioxide and water as byproducts, which subsequently contributes to the cycling of nutrients within ecosystems.

• The biogeochemical cycling of C, N, and P

  • Carbon cycle, Nitrogen cycle, Phosphorus cycle: interconnected pathways moving elements through biotic and abiotic reservoirs.

• Bacterial denitrification (NO3− to N2)

  • Denitrification reduces nitrate to molecular nitrogen, returning N to the atmosphere.

  • Reaction (conceptual): $NO<em>3^- ightarrow N</em>2$

• Nitrogen fixation: N2 + 3 H2 → 2 NH3

  • Conversion of atmospheric nitrogen to ammonia, making nitrogen biologically available.

• Haber–Bosch process: N2 + 3 H2 → 2 NH3

  • Industrial nitrogen fixation enabling large-scale fertilizer production.

Nitrogen Fixation in Legumes and Review Question

• Nitrogen-fixing microbes live in legume root nodules

  • Symbiotic bacteria (e.g., Rhizobium) fix atmospheric N2 into bioavailable NH3 within root nodules.

• Review Question 1

  • Question: Some farmers in the Midwest rotate their crops from year to year, switching from soybeans to corn on the same fields. What is one advantage of doing this?

  • Options:
    A. Soybeans add large amounts of carbon dioxide to the soil, which helps the corn crop.
    B. Both crops require the same fertilizing supplies, so farmers save by buying fertilizer in bulk.
    C. Corn adds large amounts of phosphorus to the soil, which helps the soybean crop.
    D. The corn crop benefits from reactive nitrogen fixed through a biological process called nitrogen fixation and added to the soil by the soybean crop.

  • Answer: D

  • Rationale: Soybeans host nitrogen-fixing bacteria that convert N2 to ammonia, enriching soil nitrogen for subsequent corn growth, reducing fertilizer needs.

Nitrogen Cycling: Denitrification and Aquatic Systems

• Question 2: Bacterial denitrification is an anaerobic process that converts NO3− to __.

  • Options: A. N2 (nitrogen gas) B. NH4+ (ammonium) C. Organic nitrogen D. None of the above

  • Answer: A. N2

• Question 3: Biologically available nitrogen can be added to aquatic systems

  • Options:
    A. by nitrogen fixation and lost by denitrification
    B. by either anammox or denitrification and lost by nitrogen fixation
    C. by anammox and lost by either nitrogen fixation or denitrification
    D. by either nitrogen fixation or anammox and lost by denitrifiation

  • Answer: D

  • Note: Possible processes include nitrogen fixation and anammox adding bioavailable nitrogen; denitrification removes fixed N as N2 gas.

Biogeochemical Cycles: Carbon, Nitrogen, Phosphorus — Roles and Human Impact

• Comparing the cycles (summarized statements from slide)

  • Carbon is mainly found in the atmosphere and is directly taken in by plants (via photosynthesis).

  • Nitrogen and phosphorus are limiting factors in many ecosystems.

  • Bacteria perform many steps in the nitrogen cycle.

  • Phosphorus has no significant atmospheric component.

  • All three cycles have been sped up by human actions (e.g., fossil fuel combustion, fertilizer use, soil erosion).

Lecture 3: Populations and Communities

Learning Objectives

• Dynamics of Natural Populations

• Limits on Populations

• Community Interactions

• Evolution as a Force for Change

• Implications for Management by Humans

Examples: Populations in Nature

• African elephants have largely declined due to habitat loss, poachers, drought from climate change, etc.

• Sodom apples have increased; Weedy species toxic to grazers increasing as well.

• Economic and management implications for habitat and species interactions.

Population Growth and Equilibria

• Population growth definition:

  • Growth = Births + Immigration − (Deaths + Emigration)

• Equilibrium (steady state):

  • Births + Immigration = Deaths + Emigration

• Population growth curves:

  • Graphs showing: how fast a population could grow, how many are present now, and what the future size could be.

Carrying Capacity and Growth Models

• Carrying capacity (K): the maximum population a habitat can support without being degraded.

• Three models of population growth (brief statements):

  • Exponential growth: population’s doubling time remains constant.

  • Example: It takes 2 days to go from 8 to 16 and from 1000 to 2000.

  • Logistic growth: As the population approaches K, growth slows.

Real-life Population Dynamics

• Real-world patterns include population explosions and crashes due to environmental resistance and biotic potential.

• Maximum rate of population growth occurs halfway to K.

Ending Population Model (Discrete Logistic Framework)

• Ending population formula (discrete-time logistic model):

  • Starting population: N_0

  • Reproductive rate: r

  • Carrying capacity: K

• Key Concepts: Biotic Potential vs Environmental Resistance

• Biotic Potential

  • Reproductive rate; ability to migrate/disperse; ability to invade new habitats; defense mechanisms; goes up with higher reproduction and dispersal.

• Environmental Resistance

  • Factors reducing growth: limited food, water, habitat; adverse weather; predators; disease; parasites; competitors; conditions vary over time.

• Population size results from the balance between Biotic Potential and Environmental Resistance.

Reproductive Strategies

• r-strategists (r-selected)

  • Produce many offspring; little parental care; rapid reproduction; short lifespans; bloom-and-bust dynamics; examples: oyster, housefly.

• K-strategists (K-selected)

  • Lower biotic potential; care for offspring; stable environments near carrying capacity; larger, longer-lived; examples: elephants.

Limiting Factors

• Density-dependent factors: effects vary with population density (e.g., predation, food availability).

• Density-independent factors: effects do not vary with density (e.g., fire, frost).

• Human activities can influence both types but often act as density-dependent regulators when they directly affect resource availability or survival.

Regulation of Population Size

• Only density-dependent factors can regulate a population toward equilibrium in many models.

• Top-down regulation: control by predation (e.g., predators reducing herbivore populations).

• Bottom-up regulation: control by resource scarcity (e.g., food limits prey populations).

• Examples:

  • Predators regulate deer populations (top-down).

  • Grass availability regulates bighorn sheep (bottom-up).

Critical Number and Biodiversity Loss

• Critical number: the minimum population size required for survival and recovery.

• If a population falls below this number, survival and reproduction decline, increasing extinction risk.

• Endangered species: populations near the critical number.

• Humans are major drivers of biodiversity loss; not all declines are density-dependent.

Species Interactions: Table of Major Interaction Types

• Interaction types and effects (A = species A, B = species B):

  • Predation: A (+), B (−)

  • Competition: A (−), B (−)

  • Mutualism: A (+), B (+)

  • Commensalism: A (+), B (0)

  • Amensalism: A (0), B (−) [one organism harms the other while being unaffected]

  • Neutralism: A (0), B (0)

• Examples:

  • Predation: Wolves eat moose

  • Mutualism: Lichens (fungus/alga)

  • Commensalism: Water buffalo/Egret relationship

  • Neutralism: A fish and a distant plant in the same habitat

Predator–Prey Dynamics and Keystone Species

• Predators can regulate prey populations (e.g., Isle Royale wolf-moose fluctuations; other environmental factors also influence outcomes).

• Keystone species are crucial for maintaining ecosystem structure. Their removal can cause cascades and allow less-competitive species to flourish or lead to ecosystem collapse.

  • Example: Sea stars (predation on bivalves maintains community diversity).

Intra- and Interspecific Competition

• Intraspecific competition: within the same species; can lead to self-thinning and density-dependent regulation; territoriality as an adaptation to stabilize populations.

• Interspecific competition: between species; Competitive exclusion principle: complete competitors cannot coexist in the same niche.

• Resource partitioning: differentiation of resource use to reduce competition; e.g., grasses with different root depths or warblers foraging at different heights.

• Resource partitioning in plants: different root depths allow access to different soil water pools.

• Resource partitioning in warblers: several species feed at different heights/foraging strategies to minimize competition (illustrated by height-specific foraging data).

• Character displacement (finches): when similar species co-occur, differences in morphology (e.g., beak size) evolve to reduce competition; observed in Galápagos finches.

Speciation and Geographic Isolation

• Speciation: evolution of new species from ancestral populations.

• Mechanism (example with foxes): subpopulations spread and experience different selective pressures; isolation leads to divergence.

• Prerequisites for speciation:

  • Geographical separation

  • Different selective pressures

• Darwin’s finches (Galápagos) illustrate speciation driven by divergent selection on beak morphology in response to food types.

• Geographic isolation is fundamental to speciation; plate tectonics and climate changes create barriers and drive diversity across zoogeographic regions.

• Historical context: Pangaea and continental drift created barriers that led to geographic isolation and diversification across six zoogeographic regions.

Human Management Implications

• Keystone species are integral to ecosystem stability; their removal can trigger cascades and collapse.

• Introduced species can disrupt ecological balance (e.g., zebra mussels in the Great Lakes).

• When assessing management strategies, consider environmental resistance and biotic potential to predict responses to changes and interventions.

Notation and Formulas Used in These Notes

• Growth update (discrete logistic model):

  • $N<em>{t+1} = N</em>t + r N<em>t \bigl(1 - rac{N</em>t}{K}\bigr)$

• Nitrogen fixation (biological):

  • $ext{N}<em>2 + 3 ext{H}</em>2 <br>ightarrow 2 ext{NH}_3$

• Denitrification (example):

  • $NO<em>3^- ightarrow N</em>2$

• Photosynthesis (balanced form):

  • $6CO<em>2 + 6H</em>2O + ext{light energy} <br>ightarrow C<em>6H</em>{12}O<em>6 + 6O</em>2$

• Cellular respiration (balanced form):

  • $C<em>6H</em>{12}O<em>6 + 6O</em>2 <br>ightarrow 6CO<em>2 + 6H</em>2O + ext{energy}$

Quick Reference Highlights

• Liebig’s Law of the Minimum emphasizes the limiting resource in the long term.

• Blackman’s Law highlights shifting limiting factors across processes or conditions.

• Four environmental spheres: Atmosphere, Hydrosphere, Lithosphere, Biosphere.

• Biogeochemical cycles are accelerated by human activities (e.g., fertilizer use, fossil fuel burning).

• Carrying capacity (K) is the habitat’s maximum sustainable population size.

• Maximum growth rate occurs near half of K in the logistic model.

• r-strategists vs K-strategists describe two broad life-history strategies with different reproductive strategies and stability.

• Density-dependent vs density-independent factors influence population regulation differently.

• Top-down vs bottom-up regulation describes predator-driven vs resource-driven control of populations.

• Competition, predation, mutualism, commensalism, amensalism, and neutralism describe the spectrum of interspecific interactions.

• Resource partitioning and character displacement explain how species coexist and diverge in morphology or behavior.

• Speciation requires geographic isolation and divergent selection.

• Keystone species and invasive species have outsized impacts on ecosystem structure and function.