Ecological Adaptation, Community Dynamics, and Population Ecology Notes

Adaptation, Evolution, and Natural Selection

• Adaptations are inherited characteristics (traits) that allow a species to survive and reproduce successfully in its environment.

  • Polar bear adaptations: Heat-capturing fur, insulating fat layers, wide feet for swimming, and white hair that renders it nearly invisible to seal prey during the long, sunless arctic winter.

• Adaptation vs. Acclimation:

  • Adaptation: Changes occurring at the population level across generations as advantageous traits are inherited.

  • Acclimation: Changes occurring in an individual organism in response to an altered environment (e.g., sun exposure increasing dark skin pigment concentration). These temporary physiological adjustments are not passed on to offspring, though the capacity to produce pigments is inherited.

• Natural Selection:

  • Charles Darwin detailed this concept in his 1859 book On the Origin of Species by Means of Natural Selection.

  • Natural selection is the process whereby individuals with useful traits compete more effectively for limited resources, survive, and pass those advantageous traits to subsequent generations, while less suited individuals reproduce less successfully.

• Genetic Basis of Traits:

  • Traits are encoded in genes, which are specific portions of DNA. Organisms possess thousands of genes.

  • Random changes (mutations) occur during DNA replication. Only mutations occurring in reproductive cells are inherited by offspring; mutations in nonreproductive cells (such as cancers) are not passed down.

  • Most mutations have minor or negative effects on fitness, but occasionally a mutation helps an individual exploit new resources or survive changing conditions.

  • The accumulation of many small mutations over extended time periods leads to speciation.

Limiting Factors and Tolerance Limits

• Environmental Limits on Species Distribution:

  • An organism's survival depends on suitable levels of temperature, moisture, nutrient supply, soil/water chemistry, and living space.

  • Critical limiting factors constrain a species from expanding everywhere, categorized into:

  • Physiological stress from inappropriate levels of critical environmental factors (moisture, light, temperature, pH, specific nutrients).

  • Competition with other species.

  • Predation, including parasitism and disease.

  • Example: Gulf of Mexico, an infusion of nitrogen or phosphorus causes rapid algal blooms. As algae density increases, sunlight is blocked, making solar energy the limiting factor; unable to photosynthesize, the algae die off rapidly.

• Liebig's Law of the Minimum:

  • States that a single critical factor, rather than the total amount of resources, constrains where a species can live.

  • Saguaro Cactus Example: Although tolerant of intense heat, drought, full sun, and nutrient-poor soil, the saguaro is limited by freezing temperatures. A single winter night with temperatures below freezing for $12\text{ hours}$ or more kills growing tips. —> its northern boundary corresponds to a zone where freezing lasts less than half a day.

• Shelford's Law of Tolerance Limits:

  • Proposed by ecologist Victor Shelford (1877–1968).

  • States that each environmental factor has minimum and maximum limits (tolerance limits) beyond which a species cannot survive or reproduce. The single factor closest to these limits acts as the critical limiting factor.

  • Combined Factors: Multiple interacting factors usually dictate species distributions (e.g., intertidal mussels and barnacles are limited by temperature extremes, drying time between tides, salinity, competitors, and food availability).

  • Life Stage Differences: Tolerance limits can differ between young and adults.

  • Desert Pupfish: Adults tolerate temperatures between $0^\text{o}\text{C}$ and $42^\text{o}\text{C}$ and wide salinity ranges. Eggs and juveniles require temperatures between $20^\text{o}\text{C}$ and $36^\text{o}\text{C}$ and are killed by high salt levels, restricting reproduction to a narrow portion of the adult range.

• Indicator Species:

  • Organisms whose sensitivities provide information about environmental conditions.

  • Lichens are sensitive to sulfur dioxide pollution.

  • Eastern white pine is sensitive to ozone.

  • Trout require cool, clean, well-oxygenated water; their presence indicates overall stream ecosystem health.

Ecological Niche and Resource Partitioning

• Habitat vs. Ecological Niche:

  • Habitat: The physical place or set of environmental conditions in which an organism lives.

  • Ecological Niche: Describes both the functional role played by a species in a biological community and the full combination of environmental factors determining its distribution.

  • Elton's Definition (1927): British ecologist Charles Elton defined niche as a species' role in obtaining food, its relationships with other species, and its community services.

  • Hutchinson's Definition (1957): American limnologist G. E. Hutchinson defined niche as a multi-dimensional space including physical/chemical condition ranges (temperature, light, acidity, humidity, salinity) and biological interactions (predators, prey, resource availability).

• Niche Breadth:

  • Generalists: Possess broad ecological niches and tolerate wide environmental ranges.

  • Example: Black bear (Ursus americana) is an omnivore ranging across most North American forested regions.

  • Pest/Weedy Generalists: Rats, cockroaches, and dandelions thrive across varied environments.

  • Specialists: Possess narrow ecological niches and exacting habitat requirements.

  • Example: Giant panda (Ailuropoda melanoleuca) evolved from carnivorous ancestors to feed almost exclusively on bamboo (a low-nutrient grass). It spends up to $16\text{ hours}$ daily eating and relies on a slow metabolism, slow movements, and low reproductive rate. It is endemic to the mountainous bamboo forests of southwestern China and highly endangered by forest destruction.

  • Example: Saguaro cactus is endemic to the Sonoran Desert and cannot persist in cooler or wetter climates.

• Cultural Expansion of Niche:

  • Socially complex species (elephants, chimpanzees, dolphins) expand their ecological niche by inventing and culturally transmitting new behaviors across generations.

• Competitive Exclusion Principle and Resource Partitioning:

  • Formulated by Russian biologist G. F. Gause (1910–1986): "Complete competitors cannot coexist."

  • Competitive Exclusion Principle: No two species can occupy the exact same ecological niche for long. The more efficient user excludes the other.

  • Resource Partitioning: Coexistence achieved when competing species adjust behavior or resource use to exploit different parts of the same resource.

  • MacArthur's Warblers: Ecologist Robert MacArthur demonstrated resource partitioning among woodland warbler species foraging in the same trees by specializing in different canopy heights and branch depths (inner vs. outer branches).

Speciation and Selection Pressure

• Speciation Mechanics:

  • Speciation is the evolutionary process by which new species arise.

  • Galápagos Finches: 13 current species derived from a single ancestral seed-eating finch species that accidentally colonized the isolated, volcanic archipelago (which was never connected to a continent).

• Modes of Speciation:

  • Allopatric Speciation: Speciation resulting from geographic isolation preventing interbreeding (e.g., finch populations isolated on different Galápagos islands).

  • Sympatric Speciation: Speciation occurring within the same geographic area.

  • Behavioral Isolation: Tree frogs Hyla versicolor and Hyla chrysoscelis inhabit identical areas in eastern North America but do not interbreed due to distinct mating calls.

  • Polyploidy: Ferns and other plants undergo sympatric speciation by doubling or quadrupling parental chromosome numbers, creating immediate reproductive incompatibility.

• Selection Pressures and Patterns:

  • Selection Pressure: Any factor that makes certain mutations advantageous.

  • Directional Selection: Shifts traits toward one extreme.

  • Geospiza fortis on Daphne Major: A 2-year drought reduced small seed availability. Finches with larger bills successfully cracked larger remaining seeds and dominated the population within $2\text{ years}$. When rains returned, beak size shifted back toward moderate sizes.

  • Additional Examples: Emergence of antibiotic-resistant bacteria and pesticide-tolerant insects.

  • Stabilizing Selection: Reduces trait variation, favoring intermediate phenotypes.

  • Disruptive Selection: Causes traits to diverge toward both extremes simultaneously, often driven by competition.

• Evolutionary Rates:

  • Slow Evolution: The American alligator has remained virtually unchanged for over $150 \times 10^6\,\text{years}$.

  • Rapid Evolution: Flu virus strains evolve every season; red squirrels in Arizona developed distinct traits within \n10,000\,\text{years} following post-glacial reproductive isolation.

Taxonomy and Biological Classification

• Taxonomy defined: The study of types of organisms and their evolutionary relationships, using cellular structure and genetic similarities to trace common ancestry.

• Three Domains of Life:

  • Bacteria: Single-celled organisms lacking a nuclear membrane.

  • Archaea: Genetically distinct from bacteria; adapted to survive extreme conditions (e.g., hot springs).

  • Eukarya: Organisms whose cells contain a membrane-bound nucleus.

• Four Kingdoms of Eukarya

• Binomial Nomenclature:

  • Binomial (scientific/Latin) names consist of Genus and species.

  • Eliminates ambiguity caused by common names (e.g., multiple yellow flower species called "buttercups", or Pinus resinosa being called red pine, Norway pine, or pine).

Species Interactions: Competition, Predation, and Symbiosis

• Competition Types:

  • Intraspecific Competition: Competition between members of the same species.

  • Direct combat (e.g., bull elk battling for territory/mates) is rare due to high injury risks; posturing is preferred.

  • Mechanisms to reduce intraspecific competition:

    1. Dispersal of young (seeds carried by wind/animals; juvenile animal migration).

    2. Strong territoriality driving offspring/intruders away.

    3. Resource partitioning between generations (e.g., monarch caterpillars eat milkweed foliage; adult butterflies sip nectar. Swimming crab larvae do not compete with adult bottom-dwellers).

  • Interspecific Competition: Competition between members of different species.

• Predation:

  • Ecological Definition: Any organism that feeds directly on another living organism, regardless of whether it kills the prey. Includes herbivores, carnivores, omnivores, parasites, and pathogens. Excludes scavengers, detritivores, and decomposers.

  • Herbivory: Predation practiced by grazing and browsing animals on plants.

  • Predator-Mediated Competition: A top competitor builds a large population, attracting predators that selectively hunt it down, thereby allowing weaker competitors to persist.

  • Starfish Removal Example: Removing the ochre starfish (Pisaster ochraceus) from Pacific intertidal zones caused common mussel (Mytilus californicus) populations to explode and crowd out other species.

  • Biological Pest Control Example: Cyclamen mites (Phytonemus pallidus) damage California strawberry crops. Naturally occurring or introduced predatory mites (Typhlodromus and Neoseiulus) control them. Pesticide spraying can exacerbate infestations by killing predatory mites.

  • Shift in Predators across Life Stages: Planktonic barnacle larvae are eaten by small fish; adult barnacles are protected by shells from fish but eaten by starfish and predatory snails. Herbivorous tadpoles metamorphose into carnivorous adult frogs.

• Evolutionary Arms Race and Adaptations:

  • Coevolution: Close, mutual evolutionary adjustment of two species.

  • Speed: On the East African Serengeti, cheetahs accelerate from $0\text{ to }72\,\text{kph}$ in $2\,\text{seconds}$ for surprise attacks, while Thomson's gazelles rely on stamina to outlast the cheetah.

  • Plant Defenses: Bark, spines, thorns, and chemical defenses (poison ivy, stinging nettle).

  • Mimicry Types:

  • Batesian Mimicry: A harmless, edible species evolves to resemble a toxic, dangerous, or unpalatable species.

    • Example: Harmless longhorn beetle mimics the black-and-yellow warning patterns of wasps.

    • Example: Harmless viceroy butterfly closely resembles the distasteful monarch butterfly.

  • Müllerian Mimicry: Two unpalatable or dangerous species evolve to look alike, reinforcing predator avoidance for both.

• Symbiosis:

  • Symbiosis: Non-antagonistic, intimate relationships where the fates of two or more species are linked.

  • Mutualism: Relationship where both species benefit.

  • Star Orchid and Moth: Charles Darwin predicted a moth with a $20\,\text{cm}$ proboscis existed to pollinate Madagascar's star orchid ($20\,\text{cm}$ nectary); confirmed and filmed in 1992.

  • Lichens: Mutualistic association between a fungus and a photosynthetic partner (alga or cyanobacterium).

  • Coral Reef Polyps and Algae: Polyps provide structural protection; photosynthetic algae supply nutrients.

  • Ants and Acacia Trees: Central/South American acacias provide shelter (hollow thorns) and food (nectar glands at leaf bases, protein structures on leaflet tips). Ants defend the tree against herbivores and trim competing surrounding plants.

  • Root Mycorrhizae: Fungi associated with plant roots in temperate forests facilitate mineral uptake.

  • Commensalism: One organism benefits while the other is neither helped nor harmed.

  • Epiphytes: Tropical mosses and bromeliads growing on trees receive moisture from rain and nutrients from dust/litter without harming or helping the host tree.

  • Parasitism: Parasite benefits at host expense (a specialized form of predation/symbiosis).

Keystone Species and Ecosystem Engineers

• Keystone Species:

  • Species whose impact on community structure and function is extraordinarily large relative to its abundance or biomass.

  • Tropical Fig Trees: Produce fruit continuously year-round at low rates, preventing frugivore starvation during dry seasons when other fruit is scarce.

  • Pacific Kelp Forest Ecosystem: Giant brown kelp (Macrocystis pyrifera) forms dense structural habitat. Sea urchins graze kelp on the seafloor; sea otters eat sea urchins. Otter removal allows urchins to overgraze and destroy kelp forests.

  • Other Examples: Vegetation-clearing elephants, predatory ochre sea stars, frog-eating salamanders, and mycorrhizal root fungi.

• Ecosystem Engineers:

  • Species that physically alter or transform their physical environment.

  • Examples: Beavers (dam building), sea urchins, earthworms, and whales (nutrient redistribution across marine ecosystems).

Population Dynamics and Mathematical Growth Models

• Exponential Growth Model:

  • Continuous growth without limits, producing a J-shaped curve when graphed.

  • Differential Equation:     $\frac{dN}{dt} = rN$

  • $d$ represents change.

  • $N$ represents population size.

  • $t$ represents time.

  • $r$ represents intrinsic capacity for increase (average individual contribution to growth).

  • Rate Dynamics:

  • If $r > 0$, population increases.

  • If $r < 0$, population shrinks.

  • If $r = 0$, growth rate $\frac{dN}{dt} = 0$

• Carrying Capacity ($K$) and Population Oscillations:

  • Carrying Capacity ($K$): Maximum number or biomass of a species that a given habitat can sustainably support over time.

  • Overshoot: When a population exceeds $K$, resource depletion raises death rates above birth rates, causing a population crash or dieback.

  • Predator-Prey Cycles: Canada lynx (Lynx canadensis) and snowshoe hare (Lepus americanus) exhibit a \n10\,\text{year} coupled oscillation cycle, documented in Hudson Bay Company fur trading records and mathematically described by the Lotka-Volterra model.

• Logistic Growth Model:

  • Describes population growth that slows as it approaches carrying capacity ($K$), producing an S-shaped (sigmoidal) curve.

  • Differential Equation:     $\frac{dN}{dt} = rN \times \frac{K - N}{K}$

  • The feedback term $\frac{K - N}{K}$ calculates the remaining unutilized fraction of carrying capacity:

  • When $N < K$, growth rate is positive.

  • When $N > K$, growth rate becomes negative.

• Population Density Controls:

  • Density-Dependent Factors: Effects intensify as population density increases (food scarcity, stress, disease, predation).

  • Example: Overcrowded house mice ($> 1,600/\text{m}^3$) produce $5.1\text{ offspring/litter}$ versus uncrowded mice ($< 34/\text{m}^3$) producing $6.2\text{ offspring/litter}$.

  • Density-Independent Factors: Affect mortality regardless of population size, typically non-biological events (droughts, freezes, floods, landslides, human habitat destruction).

Reproductive Strategies ($r$ and $K$ Selection)

• Comparative Trait Matrix:

  • $r$-Selected Species:

  1. Short life

  2. Rapid growth

  3. Early maturity

  4. Many, small offspring

  5. Little parental care and protection

  6. Little investment in individual offspring

  7. Adapted to unstable environments

  8. Pioneers, colonizers

  9. Niche generalists

  10. Prey

  11. Regulated mainly by extrinsic factors

  12. Low trophic level

  • Survivorship: Type III curve (high initial juvenile mortality; e.g., female clams releasing $1 \times 10^6\,\text{eggs}$).

  • $K$-Selected Species:

  1. Long life

  2. Slower growth

  3. Late maturity

  4. Few, large offspring

  5. High parental care or protection

  6. High investment in individual offspring

  7. Adapted to stable environments

  8. Later stages of succession

  9. Niche specialists

  10. Predators

  11. Regulated mainly by intrinsic factors

  12. High trophic level

  • Survivorship: Type I curve (high survival across early/middle life; e.g., elephants maturing at $18\text{ to }20\,\text{years}$, conceiving every $4\text{ to }5\,\text{years}$, with $18\text{ month}$ gestation and $60\text{ to }70\,\text{year}$ lifespan).

• Intermediate Strategies: Type II survivorship curves exhibit equal mortality risks across all age classes.

Community Structure, Diversity, and Landscape Ecology

• Diversity vs. Abundance:

  • Diversity: Number of different species present per unit area. Highest near the equator and lowest near the poles.

  • Abundance: Number of individuals of a particular species. Low in tropics; extremely high near poles (e.g., Arctic mosquitoes).

  • Geographical Comparison: Greenland possesses $56\,\text{breeding bird species}$ (glacier-free for only $10,000\,\text{years}$), while Colombia possesses $1,395\,\text{species}$ in one-fifth the area.

• Spatial Distribution Patterns:

  • Random: Individuals settle wherever resources occur and chance permits.

  • Uniform: Results from territoriality or fierce resource competition (e.g., nesting seabirds spaced just out of reach of neighbors; sagebrush roots exuding toxic chemicals creating bare circles of ground).

  • Clustered/Clumped: Grouping for defense, hunting, or resource access (e.g., fish schooling, wolf packs, blackbird flocks, baboon troops, mountain treeline evergreen groves).

• Stratification:

  • Vertical Stratification: Structural layering based on light, temperature, moisture, or pressure gradients (most pronounced in tropical rainforests and aquatic zones).

• Landscape Structure and Edge Effects:

  • Core Habitat: Large interior patch area free from external edge influences.

  • Example: Northern spotted owls require over $1,000\,\text{ha}$ of intact core coniferous forest habitat in the Pacific Northwest to successfully raise young without barred owl competition.

  • Edge Effects: Microclimatic transition zones where adjacent communities blend (sunnier, hotter, drier, wind-damaged zones subject to generalist predator invasion).

  • Patch Geometry: Irregular or narrow patches are entirely consumed by edge effects, whereas square or compact patches preserve core interior habitats.

Ecosystem Productivity, Complexity, and Resilience

• Community Complexity and Guilds:

  • Complexity: Number of trophic levels and the species diversity within each level.

  • Ecological Guilds: Functional groups of species feeding in similar ways (e.g., fruit-eaters, leaf-nibblers, root-borers, sap-suckers).

• MacArthur's Resilience Hypothesis:

  • Proposed by ecologist Robert MacArthur (1930–1972).

  • Asserts that higher community complexity provides greater resilience against environmental disturbances because redundant species can fill functional roles if others are stressed or eliminated.

• Primary Productivity:

  • Primary Productivity: Rate of organic biomass generation via photosynthesis, measured in $\text{g}/\text{m}^2/\text{year}$.

  • Net Primary Productivity (NPP): Total biomass produced minus biomass consumed during cellular respiration.

  • High NPP Ecosystems: Tropical rainforests, coral reefs, estuaries.

  • Low NPP Ecosystems: Deserts (moisture-limited), tundra/high mountains (temperature-limited), open ocean (nutrient-limited).

  • Energy Processing Efficiency: Temperate oak forests absorb only $50\%$ of midsummer light, using $99\%$ of absorbed energy for respiration and transpirational cooling (transpiring thousands of liters of water daily to produce a few kilograms of sugars).

Ecological Succession and Disturbance Dynamic

• Historical Paradigms of Succession:

  • Organismic/Relay Model: Championed by F. E. Clements (1874–1945), building on J. E. B. Warming and Henry Chandler Cowles. View that communities develop in predictable, fixed relay stages toward a stable, climate-determined climax community acting like a maturing single organism.

  • Individualistic Model: Proposed by H. A. Gleason (1882–1975). View that community composition is open to chance, representing temporary, variable assemblages of individual species establishing based on independent colonization and tolerance capacities (now supported by most modern ecologists).

• Types of Succession:

  • Primary Succession: Colonization of bare, soil-less substrates (sandbars, bare rock, lava flows) by hardy pioneer species (lichens, mosses, microbes). Organisms facilitate later succession by forming soil from organic debris.

  • Secondary Succession: Community re-development following disturbance, utilizing remaining biological legacies (soils, seed banks, root stocks).

• Disturbance and Ecosystem Dynamics:

  • Ecological Disturbance: Any force disrupting established patterns of species diversity, abundance, community structure, or ecosystem function.

  • Regime Shift: A permanent transition to an entirely new community state following severe disturbance or altered background conditions (e.g., Michigan's Kingston Plains logging/fires created an enduring dry "stump prairie").

  • Competitive Suppression: Periodic disturbances reset dominant competitors (e.g., shade-tolerant maples crowding out oaks/hickories until windstorms or fires open up the canopy).

  • Fire-Adapted Ecosystems: Grasslands, chaparral, savannas, and coniferous forests depend on regular fire intervals for seed germination (e.g., lodgepole pine Pinus contorta cones opening after the 1988 Yellowstone fires).