AP Biology Unit 8 Essentials

Without limits, a population grows exponentially, producing a J-shaped curve — but no real environment has unlimited resources forever. Logistic growth is the realistic model: growth slows as a population approaches its environment's carrying capacity, the maximum size that can be sustainably supported. Density-dependent factors like competition, predation, and disease intensify as a population gets more crowded, while density-independent factors like natural disasters limit growth regardless of population size. Together these forces explain why most populations stabilize rather than grow forever.

Energy enters an ecosystem as sunlight, gets captured by producers, and flows upward through trophic levels as organisms eat one another. But at each transfer, roughly 90% of the energy is lost as metabolic heat — only about 10% becomes available to the next trophic level. This single rule explains why energy pyramids are pyramid-shaped, why there are far fewer top predators than producers, and why food chains rarely extend past four or five levels. Unlike energy, matter (carbon, nitrogen, water) is recycled through biogeochemical cycles rather than flowing through and being lost.

Communities are held together by a web of interactions — competition, predation, mutualism, commensalism, and parasitism — that shape which species can coexist and in what numbers. Some species, called keystone species, have an outsized effect on community structure relative to their abundance. Greater biodiversity generally makes an ecosystem more resilient to disturbance, because more species means more functional redundancy. When a disruption hits — a wildfire, an invasive species, a changing climate — high-biodiversity ecosystems tend to recover more readily than low-biodiversity ones.