Ecological Systems and Population Dynamics: An In-Depth Analysis

Posted on Dec 14, 2024 in Biology

Ecology and Distribution of Ecological Systems

Ecology is the study of interactions among organisms and their environment, including biotic (living) and abiotic (non-living) factors.

Levels of Study:

• Organismal: Focuses on individual organisms and their interactions with the environment.
• Population: Examines factors that regulate population growth and size.
• Community: Studies interactions among different species in a particular area.
• Ecosystem: Investigates interactions between communities and their environments, including both biotic and abiotic factors.
• Landscape: Explores spatial patterns and how they affect ecological processes.
• Global: Examines ecological phenomena at the scale of the biosphere, integrating global interactions and processes.

Key Global Processes:

• Hadley Cells: Help move heat and rain around the Earth, determining where it’s wet, dry, or windy. They can create rainforests near the equator and deserts at 30° latitude.
• Seasonality: How the weather changes during the year, like spring, summer, fall, and winter. This happens because the Earth is tilted and moves around the sun, so different places get more or less sunlight at different times.
• Abiotic factors (e.g., temperature, light, water) shape ecological systems.
• Biomes depend on temperature and precipitation (e.g., deserts, rainforests).

Global Processes: The tilt of the Earth influences seasons and temperature. Air and water currents, such as Hadley cells, affect climates.

Seasonality: How the weather changes during the year, like spring, summer, fall, and winter. This happens because the Earth is tilted and moves around the sun, so different places get more or less sunlight at different times.

AMOC Consequences: Flooding in the Southeastern US, frigid temperatures in Europe, and drought in Southern Africa.

Human Impact: Habitat destruction, species extinction, pollution, and climate change.

Topography: “Rain shadows”.

Biomes: Major life zones characterized by vegetation type or physical environment.

Terrestrial Biomes: Influenced by temperature, precipitation, fire, grazing, and soil nutrients.

Aquatic Biomes: Influenced by temperature, nutrients, and salinity.

Conservation Ecology: Aims to understand and prevent the extinction of vulnerable species.

Restoration Ecology: Works to restore the health of damaged ecosystems.

Population Ecology

Population: A group of individuals of the same species in one location at the same time.

Geographic Range: The total area a species inhabits.

Population Size (N): The total number of individuals.

Population Density (N/Unit Area): How crowded a population is.

Population Characteristics:

• Sex Ratio: Influences reproduction rates.
• Age Structure: Affects growth rates (e.g., more young individuals lead to faster growth).
• Density: High density often leads to competition, disease spread, or resource scarcity.

Quantifying Population Size:

• Full Census: Counting every individual.
• Sub-Sampling: Measure density in small areas and extrapolate. Population sizes can change over time.

Works well for sessile (non-mobile) organisms.

Population Dynamics: Populations change due to births (B), deaths (D), immigration, and emigration. Declining populations may indicate extinction risk.

Do you think these different population dynamics are related? Yes, factors like resource availability, environmental conditions, and species interactions influence population dynamics. Declining and growing populations are affected by these factors.

Multiplicative Population Growth: N = N + B – D, total population size at the next time step.

r = b – d, per capita growth rate.

Exponential Population Growth: dN/dt = rN

• r > 0: growth
• r < 0: decline
• r = 0: stable

K: Carrying capacity, an equilibrium. As size increases, growth slows and eventually stops when the population reaches a particular size (K).

Exponential Growth: No matter how small r is, eventually the population will grow.

Example: Whooping crane. Cannot last but possible in the short term.

Logistic Growth: Growth slows at carrying capacity (K).

Growth Models:

• Exponential Growth: Rapid increase, unlimited resources.
• Logistic Growth: Growth slows at carrying capacity (K).

Key Metrics:

• Population Size (N): Total number.
• Density (N/unit area): Crowding effect.
• Growth Rate (r): Net effect of birth rate (b) and death rate (d).
• Quantify: Census and sub-sampling.

Density Factors:

• Dependent: Resources, disease, predation.
• Independent: Weather, catastrophes.
• Example: Overcrowding reduces fecundity in rabbits.

Density-Independent Factors: Weather, natural disasters, recurring catastrophes (often enough to constrain the population).

Example: Hurricanes affecting coastal bird populations.

Early Growth: When N is small, K-N/K ≈ 1, and growth is close to exponential.

Slow Growth: As N approaches K, K-N/K decreases, slowing growth.

No Growth: When N = K, K-N/K = 0, and population growth stops (dN/dt = 0).

Red curve: Exponential growth.

Blue curve: Logistic growth with slowing as N nears K.

Important Equations:

• Exponential Growth: dN/dt = rN
• Logistic Growth: dN/dt = rN(K-N/K)

Allele Threshold: Critical population size below which growth becomes negative (N < A). Population dynamics: growth increases when N > A. Population collapses if N < A due to insufficient density for survival. Crucial for conserving small populations of endangered species where maintaining N > A is essential.

Human Population Growth

1. Exponential Growth: Human population size grows exponentially, with the highest rate (r = 2.2) in 1963.
2. Carrying Capacity: Technological advancements support 10.9 billion people but rely on finite fossil fuels, affecting ecosystems.

3. Demographic Transition:

   • Pre-Industrial: High birth and death rates.
   • Transitional: Decline in death rates precedes birth rate decline.
   • Industrial: Low birth and death rates.

Historical Stages:

1. 12,000 Years Ago (Hunter-Gatherer Era): Nomadic lifestyle. High infant mortality and short lifespans. Minimal environmental impact.
2. 10,000–8,000 Years Ago (Agricultural Revolution): Settled lifestyle enabled stable food supplies and storage. Increased births, decreased deaths.
3. 1346–1353 (Bubonic Plague): Pandemic killed 75–200 million globally. Vector-borne (fleas) and aerosol transmission.
4. 1650–Present: Advances in hygiene, medicine, and food production. Exponential population growth.

Population Dynamics:

1. Growth Patterns: Time to add 1 billion people has dramatically shortened (e.g., 130 years for the 2nd billion, 12 years for the 7th billion).
2. Population Momentum: Future growth driven by the current age structure.
3. Declining Growth Rate (r): From 2.2 (1963) to 1.05 (2019).

Expected Peak Population: 10.9 billion by 2100.

Major Impacts:

1. Environmental: Habitat loss, species extinction, ecosystem degradation. Americans (5% of the population) use 25% of global resources.
2. Social: Political instability due to resource shortages. Inequalities in resource distribution.

Solutions to Reduce Growth:

1. Promoting reproductive choice and gender equality.
2. Increasing education and healthcare access, especially for women.
3. Advocating for sustainable lifestyles.

1. Fertility Rates: Declining globally due to wealth, education, and healthcare improvements.
2. Ecological Footprint: Average sustainable footprint ≈ 2 ha/person; Americans ≈ 10 ha/person.

1. What drives population momentum? A large younger population ensures future growth despite declining birth rates.
2. Why hasn’t growth slowed? Technological innovations keep increasing carrying capacity.

Metapopulations

Metapopulations: Groups of subpopulations connected by emigration and immigration. Subpopulations may go extinct but can be recolonized. Connectivity is crucial for dynamics.

Biodiversity Measures

1. Richness: Total number of species.
2. Abundance: Number of individuals.
3. Relative Abundance: Proportion of a species compared to others.
4. Composition: Species present.

Succession

Primary Succession: Starts with no prior biological community (e.g., new volcanic islands).

Secondary Succession: Soil remains intact post-disturbance (e.g., fire, flooding).

Stages:

1. Pioneer Species: High dispersal, tolerate harsh conditions, weak competitors.
2. Mid-Successional Species: Soil improvement, better competitors.
3. Climax Community: Superior competitors, a stable community.

Outcome for Succession: Often a return to the original community.

Disturbance

Natural or anthropogenic events altering ecosystems.

Effects: Local extinctions. Abiotic changes (e.g., increased light, soil loss).

Intermediate Disturbance Hypothesis: Moderate disturbance maximizes species diversity.

Island Biogeography

Theory: Proposed by MacArthur and Wilson.

Colonization Rate (I): Declines as species increase.

Extinction Rate (E): Increases with more species.

Equilibrium (S): Colonization = Extinction.

Key Factors:

1. Distance Effect: Farther islands have fewer species.
2. Area Effect: Larger islands support more species.

Applications: Islands can be physical or habitat fragments (e.g., “sky islands”).

Case Studies:

• Edith’s Checkerspot Butterfly: Habitat limited to serpentine soil.
• Krakatau: Recolonization showed species recovery over decades.
• Mount St. Helens: Primary vs. secondary succession post-eruption.

Species Interactions

Intraspecific Interactions: Within the same species.

Interspecific Interactions: Between different species.

Competition: A mutually negative interaction between individuals resulting from resource limitation.

• Intraspecific: Within a species, individuals of the same population or species compete for limiting resources (e.g., space, mates, territory, dominance, food).
• Interspecific: Individuals of different species competing for the same resources. Competition is strongest when species have overlapping niches.

Niche: Multidimensional description of a species’ use of resources and role in the environment.

• Abiotic: Temperature, soil moisture, light.
• Biotic: Interactions like competition and herbivory.

Competitive Exclusion Principle: Two species with identical niches cannot coexist. Interspecific competition leads to a decrease in the resource pool. Two species competing for the same limiting resource will differ in their competitive ability; one will eventually eliminate the other locally.

Competition Outcomes:

Gause’s Experiments: Paramecium species show reduced population densities or extinction due to competition. Conclusion: If two competing species coexist, they have lower equilibrium population densities than either would alone. Interspecific competition causes one species to go extinct.

Fundamental Niche: Potential abiotic conditions required for a species to persist, for survival.

Realized Niche: Actual niche occupied by a species when biotic interactions occur.

Exploitation: Deplete the resource before others can use it.

Interference: Decrease other’s ability to use the resource.

Intraspecific competition > Interspecific competition.

Example: Each species reduces its own per capita growth rate more than it reduces that of other species.

Limiting Similarity: Species must be sufficiently different to coexist.

Niche Differentiation: Evolutionary change in resource use in response to competition. Individuals that don’t compete leave more offspring.

Character Displacement: Evolved change in species attributes resulting from past competition. Example: Galapagos finches.

Niche Partitioning: Species living together use resources slightly differently, enabling them to coexist in a community.

Consumption: A +/- interaction where one organism feeds on another.

Types: Predation (lethal or sublethal), herbivory, parasitism.

Impacts: Ecological: Alter population density and community structure. Evolutionary: Drives natural selection.

Species Interactions

Consumption Interactions (+/-)

1. Predation

   • Predator-prey dynamics often lead to population control.
   • Prey defenses and predator surmount/tolerate defenses.
   • Factors for Coexistence: Habitat heterogeneity, low predator density, prey switching.
   • Example: Lynx and hare cyclical population dynamics.
   • Habitat is heterogeneous: Differing in kind or mixed.
   • Prey Switching: Switch to a new prey species.
   • Cyclical dynamics: Hares show.

2. Herbivory

   • Herbivores consume parts of plants but rarely the whole plant.
   • Plant Defenses: Physical (spines, thorns), chemical (toxins like tannins, caffeine, opium, nicotine).

3. Parasitism

   • Parasite benefits at the host’s expense.
   • Specialized life cycles, overlapping with hosts.
   • Behavioral manipulations by parasites (e.g., Toxoplasma in mice, trematodes in frogs).

Adaptations in Predator-Prey Relationships

• Predator Adaptations: Detection, capture, processing (e.g., bolas spider with sticky webs).
• Prey Defenses:
  • Camouflage: Cryptic coloration.
  • Warning Signals: Aposematic coloration (warning coloration).
  • Mimicry:
    • Batesian: Harmless species mimics a harmful species; the mimic evolves a similarity to a harmful/unpalatable species (model), like the eastern coral (venomous) and scarlet king snake (non-venomous).
    • Müllerian: Harmful species evolve to resemble each other (monarch and viceroy butterflies).

Keystone Predators

• Control dominant competitors, promoting biodiversity (e.g., sea stars controlling mussel populations).
• The predator maintains species diversity by preferentially attacking a dominant competitor.

Parasitism

• Transmission: Key to parasite fitness. Benefits at host expense by draining nutrients from the host while living in or on it.
• Examples:
  • Behavioral Manipulations: Parasites altering host behavior for their benefit (e.g., hairworms in grasshoppers).
  • Parasitoids: Insects that consume hosts from within.
  • Interesting because it highlights the complexity of species interactions and natural selection. Parasites rely on hosts for survival, reproduction, and transmission, often exerting strong evolutionary pressures on their hosts to develop defenses. This creates a dynamic “arms race” between parasites and hosts.
  • 40% of all species are parasites.
  • Parasite: An organism that associates with a host and feeds off bodily resources.
  • Pathogen: A parasite, small organisms like bacteria and other organisms.
  • Infectious Disease: The negative fitness effects of infection with a parasite or a pathogen.
  • Isopod in dish eats, then replaces, fish tongue.
  • Example: Brood parasite: Manipulate the social behavior of another species to complete the life cycle. Parasitic on other birds’ nests.
  • Behavioral Manipulations: Strategies used by parasites to alter the behavior of their hosts in ways that increase the parasite’s chances of survival and transmission.
  • Trematodes in Frogs: Cause frogs to grow extra legs, making them more likely to be eaten by fish (the parasite’s next host).
  • Toxoplasmosis in Mice: Reduces the mouse’s fear of predators, increasing the chance of being eaten by a cat (the parasite’s definitive host).
  • Risk: Parasites depend entirely on their hosts for survival, reproduction, and transmission. If the host dies, becomes resistant, or declines in number, the parasite’s survival is at risk. Parasites are often highly specialized, which limits their adaptability, and they face challenges like evolving host defenses and the complexity of finding new hosts. Despite these risks, parasitism is successful because it efficiently exploits host resources.

Mutualisms (+/+):

• Species benefit each other while acting in self-interest.
• Examples:
  • Pollination: Animals pollinate plants while collecting nectar.
  • Defense: Clownfish and sea anemones.
  • Farming: Ants farming fungi.
  • Nutrient Exchange: Mycorrhizal fungi and plant roots; Rhizobia bacteria fixing nitrogen for legumes.
  • Mutualisms are widespread and critical for ecological stability (e.g., nitrogen fixation in plants).

  • Nitrogen Fixation: The process by which certain bacteria convert atmospheric nitrogen (N₂) into forms usable by plants, such as ammonia (NH₃). This happens in symbiotic relationships, like between legumes and Rhizobia bacteria, where bacteria fix nitrogen in exchange for carbon from the plant. It’s crucial for soil fertility and the global nitrogen cycle.

  • Selection favors traits in species A that benefit individuals of species A, and selection favors traits in species B that benefit individuals of species B.
  • Mutualisms are common and important in ecology.
  • Mutualisms have had major evolutionary consequences: legumes, corals, pollination.

Hares, Lynx, and Plants: The hare population is influenced by both food availability (plants) and predation (lynx), while lynx numbers are directly tied to prey availability (hares). This interplay demonstrates the complexity of predator-prey and resource-consumer dynamics.