EXAM 3 BIO

1. Climate, Atmospheric Circulation, & Biome Distribution

Abiotic Drivers of Distribution and Abundance
- Solar Radiation and Latitudinal Variation:
  - Earth's spherical shape creates an uneven distribution of solar irradiance: sunlight strikes perpendicular at the equator ( $0^\circ$ ), maximizing energy per unit area, whereas at higher latitudes sunlight strikes at an oblique angle, distributing the same energy across a larger surface area.
  - Earth's axial tilt ( $23.5^\circ$ ) relative to its orbital plane causes seasonal variation in solar angle and day length across temperate and polar latitudes. Equators experience minimal seasonality in solar angle.
- Atmospheric Circulation (Hadley Cells):
  - Equator ( $0^\circ$ ): Intense warming causes surface air to heat, expand, decrease in density, and rise. As it ascends into the upper troposphere, it cools adiabatically. Because cooler air holds less water vapor, condensation triggers massive tropical precipitation.
  - Subtropics ( $30^\circ\text{N}$ and $30^\circ\text{S}$ ): The dry, cooled air moves poleward at high altitudes and descends around $30^\circ$ . As it descends, it compresses and warms adiabatically, drastically lowering its relative humidity and pulling moisture from the land, creating major global desert belts.
- Geological and Topographic Controls (Rain Shadow Effect):
  - Windward Slope: Moist maritime air driven by prevailing oceanic winds encounters mountain ranges and is forced upwards (orographic lifting). The rising air cools, condenses, and releases heavy precipitation, supporting lush forests.
  - Leeward Slope: As dried air crests the peak and descends the opposing side, it warms and absorbs ambient moisture, creating an arid rain shadow characterized by grasslands, scrublands, or deserts.
- Ocean Currents and Temperature:
  - Massive ocean gyres transport warm water from the equator toward the poles (e.g., Gulf Stream warming Western Europe) and cold polar water toward the equator (e.g., California Current cooling the US West Coast), strongly altering coastal climates relative to inland areas at the same latitude.
Predicting Terrestrial Biomes
- Terrestrial biomes are predicted primarily by mean annual temperature and mean annual precipitation, as well as the seasonal variation in both variables.
- Importance of Seasonality: Two regions can have identical mean annual temperatures and total precipitation but support distinct biomes. Constant year-round precipitation supports a temperate rainforest, whereas the same total precipitation concentrated into a single three-month monsoon season results in severe seasonal drought, favoring savanna or drought-adapted chaparral ecosystems.

2. Biogeography & Island Biogeography

Past vs. Present Controls on Distribution
- Historical (Past) Factors: Plate tectonics and continental drift (e.g., breakup of Pangaea) created physical dispersal barriers over evolutionary time, restricting lineages to specific biogeographic realms despite climatically suitable habitat existing elsewhere.
- Contemporary (Present) Abiotic Factors: Current climatic extremes, soil nutrient limitations, moisture availability, or temperature tolerances.
- Contemporary Biotic Factors: Negative interactions (interspecific competition, predation, parasitism, disease) or the absence of obligate mutualists (e.g., specific mycorrhizal fungi or pollinators) that prevent colonization.
MacArthur-Wilson Theory of Island Biogeography
- Predicts species richness at ecological equilibrium ( $\hat{S}$ ), where the rate of immigration of new species equals the rate of extinction of resident species.
- Immigration Rate ( $I$ ):
  - Governed primarily by distance from the mainland (isolation). Near islands have higher immigration rates than far islands.
  - Immigration curves decline as species richness on the island increases because arriving individuals are increasingly likely to belong to species already present.
- Extinction Rate ( $E$ ):
  - Governed primarily by island area (size). Small islands have higher extinction rates than large islands because they support smaller population sizes (vulnerable to genetic drift and demographic stochasticity) and offer fewer microhabitats/resources.
  - Extinction curves rise as species richness increases due to intensified interspecific competition and niche overlap.
- Equilibrium Predictions ( $\hat{S}$ ):
  - Highest Equilibrium Richness ( $\hat{S}$ ): Large, Near islands (maximized immigration, minimized extinction).
  - Lowest Equilibrium Richness ( $\hat{S}$ ): Small, Far islands (minimized immigration, maximized extinction).
  - Intermediate Richness: Large-Far and Small-Near islands.

3. Population Ecology & Growth Dynamics

Demographic Parameters & Per Capita Rates
- Population size change is dictated by:
  $\Delta N = (B - D) + (I - E)$
  where $B$ is births, $D$ is deaths, $I$ is immigrants, and $E$ is emigrants.
- Per Capita Rate of Increase ( $r$ ):
  $r = b - d$
  where $b$ is the per capita birth rate and $d$ is the per capita death rate.
  - If $r > 0$ , the population is growing.
  - If $r = 0$ , the population is at zero population growth (stable).
  - If $r < 0$ , the population is declining.
- Intrinsic Rate of Increase ( $r_{\max}$ ): The maximum potential growth rate under non-limiting, optimal environmental conditions.
Fitness Trade-Offs & Resource Allocation
- Organisms have a finite energetic and metabolic budget (Principle of Allocation).
- Fecundity vs. Parental Investment: Producing large clutches or litters requires energy to be divided thinly, resulting in low nutrient investment per offspring, lack of parental care, and low juvenile survivorship (Type III survivorship curve). Conversely, producing few offspring allows high investment and high survivorship (Type I survivorship curve).
Mathematical Models of Population Growth
- Exponential Growth (Density-Independent):
  $\frac{dN}{dt} = r_{\max} N$
  - Growth rate is independent of population density, yielding a $J$ -shaped curve.
  - Characteristic of colonizing founder populations or recovering populations following severe disturbance, provided resources remain unlimited.
- Logistic Growth (Density-Dependent):
  $\frac{dN}{dt} = r_{\max} N \left(\frac{K - N}{K}\right)$
  - Incorporates Carrying Capacity ( $K$ ), the maximum sustainable population size supported by the habitat.
  - As $N$ approaches $K$ , the term $\frac{K - N}{K}$ approaches $0$ , causing population growth to level off into an $S$ -shaped (sigmoid) curve.
  - If $N = K$ , $\frac{dN}{dt} = 0$ .
- Mechanisms of Density-Dependence: Intraspecific competition for limiting resources (food, space, light), accumulation of metabolic wastes, elevated disease transmission, and increased predator attraction as density rises.

4. Community Ecology & Interspecific Interactions

Classification of Species Interactions
- Competition ( $-/-$ ): Both species experience a fitness cost due to mutually exploitative or interference competition over a shared limiting resource.
- Consumption ( $+/-$ ): One organism gains energy/nutrients at the expense of the other (predation, herbivory, parasitism).
- Mutualism ( $+/+$ ): Both species experience reciprocal increases in evolutionary fitness (e.g., mycorrhizal fungi and plant roots).
- Commensalism ( $+/0$ ): One species benefits while the host experiences negligible fitness impact (e.g., epiphytic orchids on tree branches).
- Amensalism ( $-/0$ ): One species is harmed or destroyed while the other is unaffected (e.g., large ungulate trampling understory herbs).
Niche Theory & Competitive Dynamics
- Fundamental Niche: The total theoretical range of environmental conditions (abiotic and biotic) that a species can physiologically tolerate and survive in without competitors.
- Realized Niche: The actual, restricted portion of the fundamental niche an organism occupies in nature due to limiting biotic interactions such as competition and predation.
- Symmetric vs. Asymmetric Competition:
  - Symmetric: Competing species suffer equal reductions in fitness.
  - Asymmetric: One species suffers a substantially greater fitness reduction than the other.
- Competitive Exclusion Principle: Two species with identical ecological niches cannot coexist indefinitely if resources are limiting. The superior competitor will drive the inferior competitor to local extinction.
- Mechanisms Facilitating Coexistence:
  - Resource Partitioning (Niche Differentiation): Evolutionary divergence in resource use (timing, microhabitat, prey size) that minimizes niche overlap.
  - Character Displacement: Evolutionary change that occurs when two similar species inhabit the same environment (sympatry), leading to morphological divergence (e.g., bill depth divergence in Darwin's finches) to reduce competitive pressure.

5. Ecosystem Ecology, Energy Flow, & Nutrient Cycling

Energy Transformation and Primary Production
- Primary producers convert radiant solar energy into chemical bond energy (sugar) through photosynthesis:
  $6CO_2 + 6H_2O + \text{Solar Energy} \rightarrow C_6H_{12}O_6 + 6O_2$
- Gross Primary Productivity (GPP): The total amount of solar energy converted into chemical energy per unit area per unit time.
- Net Primary Productivity (NPP): The energy remaining after primary producers satisfy their own cellular respiration requirements ( $R$ ):
  $\text{NPP} = \text{GPP} - R$
- NPP represents the actual biomass available for consumption by heterotrophic organisms.
Trophic Structure & Energy Dissipation
- Trophic Levels: Primary Producers $\rightarrow$ Primary Consumers (herbivores) $\rightarrow$ Secondary Consumers $\rightarrow$ Tertiary Consumers. Detritivores and decomposers consume non-living organic detritus across all trophic levels.
- Energy Flow vs. Nutrient Cycling:
  - Energy flows unidirectionally through ecosystems and cannot be recycled; with each transfer, energy dissipates as metabolic heat into the environment.
  - Nutrients cycle continuously through closed biogeochemical loops between abiotic and biotic reservoirs.
- 10% Rule (Pyramid of Productivity): Approximately $10\%$ (range $5\text{--}20\%$ ) of energy stored as biomass at one trophic level is successfully converted into biomass at the next level. Roughly $90\%$ is lost to cellular respiration, heat loss, excretion, and unconsumed biomass. This thermodynamic constraint restricts food chains to $4\text{--}5$ trophic levels.
Biogeochemical Cycles & Human Perturbations
- Carbon Cycle:
  - Reservoirs: Sedimentary rocks (largest), deep ocean, fossilized biomass (coal, petroleum), living biomass, and atmospheric $CO_2$ .
  - Fluxes: Photosynthesis fixes inorganic carbon; respiration, decomposition, and combustion release it back to the atmosphere.
  - Human Perturbation: Burning fossil fuels and deforestation release ancient sequestered carbon, elevating atmospheric $CO_2$ and driving global climate change and ocean acidification.
- Nitrogen Cycle:
  - Reservoirs: Atmosphere ( $N_2$ , composing $\sim 78\%$ of atmospheric gas, biologically unavailable to plants), soil organic matter, aquatic sediments.
  - Fluxes: Biological nitrogen fixation by diazotrophs (converting $N_2$ to $NH_3/NH_4^+$ ), nitrification (oxidizing to $NO_2^-$ and $NO_3^-$ ), plant assimilation, and microbial denitrification (returning $N_2$ to the atmosphere).
- Phosphorus Cycle:
  - Reservoirs: Sedimentary rock formations and marine sediments (lacks a major atmospheric component). Weathering slowly releases phosphate ( $PO_4^{3-}$ ) to soils and aquatic systems.
- Cultural Eutrophication (Human Impact on N & P):
1. Agricultural runoff discharges concentrated synthetic fertilizers ( $N$ and $P$ ) into aquatic ecosystems.
2. Rapid exponential population explosion of algae and cyanobacteria (algal blooms).
3. Dense surface blooms block sunlight, causing submerged aquatic plants to die.
4. Algae deplete available nutrients and die, generating massive amounts of organic detritus.
5. Aerobic heterotrophic decomposers (bacteria) proliferate and rapidly consume dissolved oxygen ( $O_2$ ) via cellular respiration.
6. Hypoxia ( $< 2\,\text{mg}\,\text{L}^{-1}\,O_2$ ) or anoxia leads to mass die-offs of fish and benthic invertebrates, forming ecological "dead zones."