Honors Biology Unit 18 Review: Population and Community Ecology

A community is every population of every species living and interacting in one area. Those interactions shape which species stay and how the community changes over time.

A keystone species has an effect far larger than its abundance. Sea otters eat sea urchins, so when otters disappear, urchin numbers explode and the urchins overgraze kelp, turning a kelp forest into a bare "urchin barren." Beavers act as keystone species too, because their dams create ponds that support fish, amphibians, and wetland plants.

Facilitation happens when one species changes the environment in a way that helps another. In deserts, nurse shrubs shade seedlings and hold moisture. Legumes host nitrogen-fixing bacteria that turn $N_2$ gas into ammonia, enriching the soil for nearby plants. The facilitating species doesn't have to benefit, which is how facilitation differs from mutualism.

Succession is the gradual change in which species live in an area. Primary succession starts with no soil, on fresh lava or rock left by a retreating glacier. Lichens and mosses break down rock and add organic matter, and building soil can take centuries. Secondary succession starts where a fire, flood, or logging removed the community but left the soil. After the 1988 Yellowstone fires, grasses and wildflowers returned within a year and lodgepole pines sprouted within a few years. In some forests, shade-tolerant trees later replace sun-loving early arrivals. Succession does not always follow one path or end in a single stable community.

Biodiversity is the variety of life at three levels: genetic, species, and ecosystem. At the species level, ecologists measure two things. Species richness is the number of species, and species evenness is how equally individuals are spread among them.

Worked example. Pond A has 100 frogs split 25, 25, 25, 25 among four species. Pond B has 100 frogs split 85, 5, 5, 5. Both have a richness of 4. Simpson's diversity index, $D = 1 - \sum p^2$, gives the chance that two independent picks with replacement are from different species. Here, $p$ is the fraction of individuals in each species. For Pond A, $D = 1 - 4(0.25^2) = 1 - 0.25 = 0.75$. For Pond B, $D = 1 - (0.85^2 + 3 \times 0.05^2) = 1 - (0.7225 + 0.0075) = 0.27$. Same richness, but Pond A is far more diverse because it is more even.

Diverse communities tend to be more stable because of functional redundancy. If one pollinator declines, other pollinators can still do the job.

Symbiosis is a close, long-term interaction between two species. Bees getting nectar while pollinating flowers is mutualism, orchids growing on tree branches is commensalism, and tapeworms in a host's gut are parasitism. Competition hurts both species, and the competitive exclusion principle says two species with identical niches can't coexist forever. Species often avoid this through resource partitioning, using different foods, times, or spaces. The fundamental niche includes the conditions and resources a species could use without limiting interactions with other species. Its realized niche is the portion it actually occupies, which can be restricted by competition and other interactions.

A population grows when births plus immigration outpace deaths plus emigration. In a closed population with a constant per capita growth rate, the exponential model is $\frac{dN}{dt} = rN$, where $N$ is population size and $r$ is the per capita birth rate minus the per capita death rate. When $r$ is positive, the population grows in a J-shaped curve.

Real environments have a carrying capacity ($K$). The logistic model, $\frac{dN}{dt} = rN\left(\frac{K-N}{K}\right)$, adds a braking term that shrinks toward 0 as $N$ approaches $K$, producing an S-shaped curve.

Worked example. Rabbits have $r = 0.5$ per year and $K = 1000$. At $N = 200$, exponential growth predicts $0.5 \times 200 = 100$ rabbits per year. Logistic growth predicts $100 \times \frac{800}{1000} = 80$. At $N = 900$, logistic growth drops to $0.5 \times 900 \times 0.1 = 45$. Growth peaks at $N = K/2 = 500$, where $0.5 \times 500 \times 0.5 = 125$ per year.

Density-dependent factors like competition, predation, and disease get stronger as a population gets crowded, which holds it near $K$. Density-independent factors like hurricanes and droughts can affect populations regardless of their density. Their effects do not have to be identical at every density. Survivorship curves show when individuals die. Type I species (humans) mostly die old, Type II (many songbirds) die at a steady rate, and Type III (oysters, sea turtles) mostly die young. The r-selection and K-selection model describes a life-history tradeoff. Some species produce many offspring with little investment in each, while others produce fewer offspring with more investment in each. Real species vary along this spectrum.