Unit 1: The Living World: Ecosystems

Ecosystem

A defined area where biotic (living) and abiotic (nonliving) components interact as a system through exchanges of energy and matter.

Biotic components

Living or once-living parts of an ecosystem (e.g., plants, animals, fungi, bacteria, decomposers, detritivores).

Abiotic components

Nonliving physical and chemical factors in an ecosystem (e.g., sunlight, temperature, water, soil, dissolved oxygen, salinity, pH, nutrients).

Population

All individuals of one species in a given area that interact with each other.

Community

Multiple interacting populations (different species) living in the same area.

Habitat

The place an organism lives (its “address”).

Niche

An organism’s role (its “job”): how it gets energy, uses resources, fits into food webs, and interacts with biotic and abiotic factors.

Generalist species

A species with a broad niche that can use a wide range of resources and tolerate many conditions; often persists through change.

Specialist species

A species with a narrow niche that relies on specific resources or conditions; often more vulnerable to disturbance and extinction risk.

Symbiosis

A close, long-term biological interaction between two different organisms (often different species).

Mutualism

A symbiotic relationship in which both species benefit.

Commensalism

A symbiotic relationship in which one species benefits and the other is unaffected.

Parasitism

A relationship in which one organism benefits while the other is harmed (the host).

Predation

An interaction in which a predator hunts, kills, and eats prey.

Competition

An interaction where organisms vie for limited resources such as food, mates, or territory; can occur within or between species.

Resource partitioning

A way competing species coexist by dividing resources (using them in different ways, places, or times), reducing direct competition.

Law of Tolerance

A species’ existence, abundance, and distribution depend on its tolerance to physical and chemical (abiotic) factors; outside tolerance limits, survival/reproduction decline.

Limiting factor

Any environmental factor that restricts population growth, abundance, or distribution.

Density-dependent factor

A limiting factor that becomes stronger as population density increases (e.g., disease, competition, predation).

Density-independent factor

A limiting factor that affects populations regardless of density (e.g., drought, storms, fire, temperature extremes).

Carrying capacity

The maximum population size an environment can sustain over time given available resources and limiting factors.

Negative feedback loop

A feedback that counteracts change and tends to stabilize a system (reduces the original disturbance).

Positive feedback loop

A feedback that amplifies change; “positive” means self-reinforcing, not beneficial.

Energy flow

The one-way movement of energy through trophic levels; energy enters (usually as sunlight) and is ultimately lost as heat.

Matter cycling

The movement and reuse of atoms (nutrients) among organisms and abiotic reservoirs (air, water, soil, rocks) through biogeochemical cycles.

Second Law of Thermodynamics

As energy is transferred or transformed, more becomes unusable (often dispersed as heat), helping explain why energy does not cycle in ecosystems.

Decomposer

An organism (often bacteria or fungi) that breaks down dead biomass and waste, returning nutrients to soil/water and supporting continued primary production.

Trophic level

An organism’s feeding position in a food chain or food web (how many steps it is from the start of energy input via producers).

Food web

A network of interconnected food chains showing multiple feeding relationships and energy transfer pathways in an ecosystem.

10% rule

A rule of thumb that only about 10% of energy at one trophic level becomes new biomass available to the next level; most is lost through metabolism and heat.

Bioaccumulation

A chemical builds up in an organism over time because intake exceeds elimination (especially for persistent, fat-soluble pollutants).

Biomagnification

Increasing concentration of a pollutant at higher trophic levels because predators consume many contaminated prey.

Gross primary productivity (GPP)

The total rate at which producers capture energy via photosynthesis (total energy fixed).

Net primary productivity (NPP)

The rate of energy stored as producer biomass after respiration; NPP = GPP − R, and it is the energy available to consumers and decomposers.

Biogeochemical cycle

The movement of a chemical element through living organisms (bio), Earth’s crust/soil/rocks (geo), and air/water via chemical processes.

Nitrogen fixation

Conversion of atmospheric N2 into biologically usable forms (mainly ammonia/ammonium), performed by certain bacteria (e.g., Rhizobium in legumes), lightning, or industrial processes.

Nitrification

A bacterial process converting ammonia/ammonium to nitrite and then nitrate.

Denitrification

A process where anaerobic bacteria convert nitrate to gaseous nitrogen forms (e.g., N2), returning nitrogen to the atmosphere (can also produce N2O).

Phosphorus cycle

The movement of phosphorus through rocks, soils, water, and organisms; it lacks a major atmospheric gas phase and often cycles slowly via weathering and sedimentation.

Ocean acidification

A decrease in ocean pH when CO2 dissolves in seawater; can disrupt coral reef formation and reduce carbonate availability for shells/skeletons.

Evapotranspiration

The combined transfer of water from land to the atmosphere via evaporation plus plant transpiration.

Aquifer

A geologic formation that stores water in quantities sufficient to support a well or spring.

Water table

The level below which the ground is saturated with water.

Aquifer depletion

A drop in groundwater levels when pumping exceeds recharge, lowering the water table and potentially causing shortages and other impacts.

Saltwater intrusion

The movement of saltwater into freshwater aquifers, often caused by overpumping coastal groundwater.

Dissolved oxygen (DO)

Oxygen dissolved in water that many aquatic organisms need; influenced by temperature, mixing, photosynthesis (adds), and respiration/decomposition (removes).

Biodiversity

The variety of life in an area (including genetic, species, and ecosystem diversity) linked to ecosystem resilience and function.

Species richness

The number of different species present in a community or ecosystem.

Species evenness

How evenly individuals are distributed among the species in a community.

Ecosystem services

Benefits humans receive from functioning ecosystems, including provisioning, regulating, supporting, and cultural services.