Unit 8 · Ecology

The decision that organizes this unit

Mechanism route and repair branches

1. Main route: Define the organism, population, community, ecosystem, spatial boundary, and time interval relevant to the measured variable.
2. Main route: Label arrows as energy, matter, feeding, facilitation, inhibition, or another stated interaction before following their direction.

• Diagnostic cue: Food-web arrows are read from consumer to food. Wrong branch: The arrow is interpreted as who eats whom instead of resource or energy transfer. Repair: Translate each arrow into energy or biomass moves from the resource organism toward the consumer before predicting a cascade.

1. Main route: For populations, distinguish abundance, total growth rate, per-capita growth, births, deaths, immigration, emigration, and carrying capacity.

• Diagnostic cue: A population near carrying capacity is said to contain no births or deaths. Wrong branch: Zero net growth is confused with zero demographic events. Repair: Separate births and immigration from deaths and emigration; opposing flows can balance while individuals continue entering and leaving.

1. Main route: Apply the perturbation to its direct target, then trace indirect effects through consumers, resources, competitors, or habitat changes.
2. Main route: Use abundance, diversity, or productivity metrics only within comparable sampling effort and state what the metric does not measure.

• Diagnostic cue: More species automatically means a larger diversity index in every comparison. Wrong branch: Richness and evenness are collapsed. Repair: Use the supplied formula and relative abundances, then compare samples only when totals and sampling effort are compatible.

Load-bearing representation lab

Every arrow begins at the resource and ends at the consumer receiving energy. Grass feeds both rabbit and grasshopper; grasshopper feeds frog; and hawk receives energy through both the rabbit route and the frog route. The labeled transfers show successive reduction along a pathway—for example, 100 units at the basal link become 10 at the next consumer, and the frog-to-hawk route carries 1. That direction convention can feel reversed if an arrow is mistaken for the predator's action on prey. Read the arrowhead before naming the interaction, then follow complete paths to determine which species can transmit an indirect effect to the hawk. Energy moves through the network and is ultimately dissipated as heat; it is not recycled in the same sense as matter atoms. The figure does not claim that every trophic transfer is exactly ten percent in nature or that the two hawk inputs occur at identical rates. It supplies a clean accounting example in which resource-to-consumer direction, path reachability, and the displayed energy amounts must all agree.

Population size rises rapidly at intermediate abundance and then bends toward the carrying-capacity line near K = 1000. The steepest section occurs around half of K, where enough individuals are reproducing but density-dependent limitation is not yet maximal. As N approaches K, total growth slows because the per-capita contribution of births minus deaths declines with crowding or resource limitation. Reaching carrying capacity does not mean births and deaths stop; it means their population-level effects balance on average so net change approaches zero. A frequent error places maximum growth at K simply because population size is largest there, confusing abundance with the derivative dN/dt. Trace slope along the S-shaped curve, distinguish slope from height, and use the asymptote to identify the equilibrium. The graph illustrates one logistic model with a constant K and r. It does not capture overshoot, time-varying resources, age structure, or stochastic events, so the smooth approach should not be treated as a universal population trajectory. The supported mechanism is density dependence, not an assertion that every limiting factor has been measured.