ESS Topic 2 Lecture: Biosphere, Energy Flow, and Succession

A niche is the role an organism plays and the way it uses resources within an ecosystem. A species consists of organisms with similar form that can interbreed to produce fertile offspring. The binomial system names each species with a capitalized genus followed by a lowercase species epithet, both italicized. Decomposers (saprotrophs) break down dead material chemically, while detritivores physically ingest organic debris.

Species, Populations, and Competition

Carrying capacity (K) is the maximum number of individuals that an environment’s limited resources can sustain. Taxonomy organizes life into hierarchical categories, and binomial nomenclature provides a universal label for each species. A fundamental niche describes the full range of conditions an organism can tolerate, whereas the realized niche reflects the portion actually used after competition. Interspecific competition occurs between different species, while intraspecific competition happens among members of the same species. Competitive exclusion removes less adapted species from a locality.

Population Models

K‑strategists—organisms with long lifespans, few offspring, and parental care—follow an S‑shaped growth curve that levels off near K. R‑strategists—short‑lived organisms that produce many offspring and mature quickly—exhibit a J‑shaped curve that can overshoot resources before declining. Density‑dependent factors (e.g., disease, predation) intensify as population size grows, whereas density‑independent factors (e.g., fire, flood) affect populations regardless of density. Predator‑prey cycles illustrate how prey numbers rise when predators are scarce, prompting predator increases, which then suppress prey, leading to predator declines, and the cycle repeats.

Energy Flow and Food Webs

The first and second laws of thermodynamics govern energy transformations in ecosystems. Photosynthesis captures solar energy, while respiration releases it as heat. Trophic levels organize organisms by feeding position, and the 10 % rule states that roughly ten percent of energy moves from one level to the next; the rest is lost as heat, respiration, or excretion. Bioaccumulation describes the buildup of chemicals within a single organism over time, whereas biomagnification refers to increasing concentrations of those chemicals across successive trophic levels.

Biogeochemical Cycles

In the carbon cycle, carbon moves from the atmosphere to plants via photosynthesis, travels through food chains, and returns to the atmosphere through respiration, decomposition, or the combustion of fossil fuels. Human activities have altered the carbon budget, increasing atmospheric CO₂ and influencing climate. The solar constant delivers about 49 % of solar energy to the ground; plants absorb roughly 40 % of that, but only about 3.6 % is effectively used for photosynthesis.

Biomes, Climate, and Atmospheric Circulation

Precipitation and evaporation balance to a ratio of 1 in fertile soils. The tri‑cellular model of atmospheric circulation explains high rainfall at the equator and around 60° latitude, and dry conditions near 30° latitude where many deserts form. Climate describes long‑term, regional weather patterns, while weather refers to short‑term, local conditions. Climate change could raise global temperatures by up to 4.5 °C by the end of the century if mitigation does not occur, reshaping biomes worldwide.

Succession and Zonation

Zonation refers to spatial changes in community composition across a landscape, whereas succession describes temporal changes after a disturbance. Primary succession begins on newly created land—such as volcanic islands like St. Helens or glacial retreats in Glacier Bay, Alaska—where pioneer r‑strategists colonize bare rock, create soil through decay, and enable larger species to establish. Secondary succession follows disturbances that leave soil intact, such as fire. Biodiversity typically peaks before the climax community, the most stable ecosystem, is reached. Human activities can create a plagioclimax, an ecosystem that remains in an arrested state because ongoing disturbance (e.g., grazing) prevents progression to climax.