AP Bio Unit 8 Review: Ecology

Organisms detect changes in their internal state or external environment and respond through behavioral mechanisms (moving toward or away from a stimulus, changing activity patterns) or physiological mechanisms (hormonal changes, altered metabolism). These responses are shaped by natural selection because responses that improve survival and reproduction become more common over generations. Communication between organisms using visual, audible, tactile, electrical, or chemical signals can also change behavior and affect fitness.

• Taxis vs. kinesis: Taxis is directed movement toward or away from a stimulus; kinesis is a change in movement rate or turning frequency without a directional component.
• Photoperiodism: A plant's response to the relative lengths of light and dark periods, triggering seasonal events like flowering.
• Fight-or-flight response: A physiological response to a threat that increases heart rate and redirects energy to muscles, preparing the organism for immediate action.
• Cooperative behavior: Behaviors such as pack hunting or alarm calls that increase fitness for the individual and survival for the population.
• Innate vs. learned behavior: Innate behaviors are genetically programmed; learned behaviors are acquired through experience. Both can be favored by natural selection if they increase reproductive success.

Energy enters most ecosystems through photosynthesis by primary producers and flows through trophic levels: producers, primary consumers, secondary consumers, and so on. Only about 10% of energy transfers to the next trophic level because the rest is lost as heat during cellular respiration. Chemosynthetic organisms capture energy from inorganic molecules and support ecosystems without sunlight. Matter, unlike energy, is conserved and cycles through biogeochemical cycles including the carbon, nitrogen, phosphorus, and water cycles. Changes in energy availability, such as a reduction in producer biomass, ripple through all trophic levels.

• 10% rule: Approximately 10% of energy at one trophic level is available to the next, because the rest is lost as heat through metabolism.
• Gross vs. net primary productivity: GPP is total energy fixed by producers; NPP is GPP minus energy used by producers in respiration, representing energy available to consumers.
• Biogeochemical cycles: Pathways through which matter moves between biotic organisms and abiotic reservoirs such as the atmosphere, soil, and water.
• Chemosynthesis: Energy capture from inorganic molecules by organisms such as deep-sea vent bacteria, supporting ecosystems in the absence of sunlight.
• Decomposers: Bacteria and fungi that break down dead organic matter, returning nutrients to abiotic reservoirs and completing biogeochemical cycles.

Population size changes based on birth rate (B) and death rate (D): dN/dt = B - D. When resources are unlimited, populations grow exponentially at the maximum per capita growth rate: dN/dt = rmax N, producing a J-shaped curve. Exponential growth is a theoretical model; in practice, resources are finite and growth slows. Adaptations related to obtaining energy in a particular environment influence how quickly a population can grow.

• dN/dt = B - D: The basic population growth equation: change in population size equals birth rate minus death rate.
• dN/dt = rmax N: Exponential growth equation where rmax is the maximum per capita growth rate and N is current population size.
• J-shaped curve: The graph produced by exponential growth, showing accelerating population increase with no upper limit.
• rmax: The maximum per capita growth rate of a population under ideal conditions with no resource constraints.

As population density increases, density-dependent factors such as food competition, disease transmission, and predation pressure intensify and slow growth. Density-independent factors such as temperature extremes or natural disasters affect populations regardless of density. Together these limits produce logistic growth, where the population follows an S-shaped curve and levels off at carrying capacity K.

• Logistic growth equation: dN/dt = rmax N((K-N)/K): as N approaches K, the term (K-N)/K approaches zero and growth slows to zero.
• Carrying capacity (K): The maximum population size an ecosystem can sustain given available resources such as food, water, and space.
• Density-dependent factors: Limiting factors whose effect intensifies as population density increases, including competition, predation, and disease.
• Density-independent factors: Environmental conditions such as drought, fire, or frost that limit population size regardless of how crowded the population is.
• S-shaped curve: The graph of logistic growth, showing rapid initial increase that slows as N approaches K.

A community is all the populations of different species living and interacting in one area. Community structure is described by species composition (which species are present) and species diversity (richness plus evenness), quantified with Simpson's Diversity Index: 1 - sum of (n/N)^2. Species interactions including competition, predation, and symbioses (mutualism, parasitism, commensalism) determine how populations access energy and matter. Niche partitioning allows competing species to coexist by using different resources or the same resources in different ways. Trophic cascades occur when a change in one trophic level triggers effects through multiple other levels.

• Simpson's Diversity Index: Calculated as 1 - sum(n/N)^2, where n is the count of each species and N is the total count; values closer to 1 indicate higher diversity.
• Niche partitioning: Division of resources among species that reduces direct competition and allows coexistence in the same habitat.
• Trophic cascade: Indirect effects that ripple through a food web when a predator population changes, alternately increasing and decreasing populations at lower trophic levels.
• Mutualism, parasitism, commensalism: Symbiotic relationships classified by whether each partner benefits (+), is harmed (-), or is unaffected (0): mutualism is +/+, parasitism is +/-, commensalism is +/0.
• Competitive exclusion: When two species occupy the same niche, one will outcompete and eliminate the other; coexistence requires niche differentiation.

Ecosystems with higher biodiversity are generally more resilient to disturbance because functional redundancy means multiple species can perform similar roles. Keystone species have effects on community structure that are disproportionately large relative to their abundance. Removing a keystone species often triggers a trophic cascade that collapses the ecosystem. Producers, essential abiotic factors such as light and nutrient availability, and biotic factors all contribute to maintaining diversity.

• Ecosystem resilience: The ability of an ecosystem to absorb disturbance and return to its original structure and function, generally increasing with biodiversity.
• Keystone species: A species whose removal causes disproportionate disruption to ecosystem structure relative to its population size, such as a top predator controlling prey populations.
• Functional redundancy: Multiple species performing the same ecological role, so the loss of one species does not eliminate that function from the ecosystem.
• Abiotic factors: Nonliving components such as temperature, light, soil pH, and water availability that set the conditions for which species can survive in an ecosystem.

Disruptions interact with preexisting genetic variation in populations. Adaptations are genetic variations favored by selection that provide advantages in a particular environment. Mutations are random and not directed by environmental pressure, but selection acts on the variation they produce. Heterozygote advantage, as seen with sickle cell trait and malaria resistance, shows how environmental conditions can maintain genetic diversity. Invasive species such as kudzu and zebra mussels exploit new niches free of natural predators or competitors, outcompeting native species. Human activities including biomagnification of toxins like DDT and eutrophication from agricultural nutrient runoff cause ecosystem-level changes. Geological and meteorological events such as continental drift, El Nino, and global climate change also reshape habitat distribution.

• Heterozygote advantage: When the heterozygous genotype has higher fitness than either homozygous genotype, as in sickle cell trait providing malaria resistance in heterozygotes.
• Invasive species: Non-native organisms introduced intentionally or unintentionally that exploit a new niche without natural predators or competitors, often outcompeting native species.
• Biomagnification: The increasing concentration of a toxin such as DDT at higher trophic levels, because each consumer accumulates the toxin from all the prey it eats.
• Eutrophication: Excess nutrient input, typically nitrogen and phosphorus from agricultural runoff, causing algal blooms, oxygen depletion, and death of aquatic organisms.
• Mutations are not directed: Mutations arise randomly from replication errors or environmental damage; the environment selects among existing variants but does not cause specific adaptive mutations.