How many organisms?
Count or estimate.
Population size is always tied to a place, time and sampling method.
Side 61
Ecology
A study of relationships across living systems. Ecology connects organisms, populations, communities and ecosystems through competition, cooperation, energy flow, nutrient cycling and environmental constraint.
organism→interaction→population→community→ecosystem
06ecological lenses
05interaction types
05stability questions
61Side
Population ecology asks how abundance changes.
Births, deaths, immigration and emigration interact with density, resources and environmental variation.
How quickly is abundance changing?
Births − deaths ± movement.
Growth can be rapid when resources are abundant and constrained as density rises.
How crowded is the population?
Individuals per area or volume.
Density changes contact, competition and disease transmission.
What constrains growth?
Food, space, predators, climate?
Limits can be density-dependent or density-independent.
Who makes up the population?
Age, stage, sex, genotype?
Demographic structure can determine future growth even at the same total abundance.
Species alter one another’s survival and reproduction.
Ecological interactions create feedbacks that reshape abundance and selection.
Shared limiting resource.
Competitors reduce one another’s access to resources.
One organism consumes another.
Predator and prey dynamics can generate cycles and behavioral adaptation.
One benefits while host is harmed.
Parasites often depend on keeping hosts alive long enough to transmit.
Both partners benefit.
Benefits can be obligatory or conditional on environment.
One benefits with little measured effect on the other.
Interaction categories can shift with context.
One species improves conditions for another.
Especially important in stressful environments.
Communities are structured by who is present and how they interact.
Diversity includes richness, relative abundance and functional difference.
Richness
How many species?
Species count alone ignores how common or rare each species is.
Evenness
How balanced are abundances?
Communities with the same richness can differ greatly in dominance.
Niche
How does a species use environment and resources?
Niches describe multidimensional ecological requirements and effects.
Keystone
Does one species have outsized effect?
Impact can be disproportionately large relative to abundance.
Trophic structure
Who consumes whom?
Food webs distribute energy and indirect effects through communities.
Ecosystems connect living communities to energy and matter.
Energy flows through; nutrients cycle among organisms, soils, water and atmosphere.
| Process | What moves? | Main direction | Key constraint |
|---|---|---|---|
| Primary production | Energy into biomass | Environment → producers | Light, water, nutrients, temperature |
| Consumption | Energy + matter | Prey/resource → consumer | Availability and assimilation |
| Decomposition | Organic matter | Dead material → decomposers/inorganic pools | Temperature, moisture, substrate quality |
| Nutrient cycling | N, P, C and others | Between biotic and abiotic reservoirs | Transformation rates |
| Respiration | Stored chemical energy | Organic molecules → usable cellular energy | Metabolic demand and conditions |
Ecological systems are historical.
Disturbance, colonization, extinction and succession change community composition through time.
Reset structure partially or strongly.
Fire, storms, grazing and human activity alter resources and mortality.
Communities reorganize after change.
Early and later species differ in colonization, growth and competitive ability.
New species enter.
Impact depends on traits, recipient community and environmental conditions.
Habitat becomes divided.
Isolation and edge effects can change movement, gene flow and extinction risk.
Species redistribute geographically.
Climate and land-use change can move suitable habitat.
New combinations emerge.
Historical associations can dissolve and recombine under changed conditions.
Stability has several meanings.
A system can resist change, recover quickly, persist through reorganization, or shift into a new state.
Resistance
How little does the system change after disturbance?
Recovery
How quickly does it return toward prior structure or function?
Redundancy
Can multiple species perform overlapping ecological functions?
Threshold
Can gradual pressure trigger a large state shift?
Adaptive capacity
Can composition or behavior change while core functions persist?
EcologyBegon, Townsend & Harper · ecological foundation
Community EcologyMorin · species interactions
Fundamentals of Ecosystem Scienceenergy and nutrient systems
Resilience ThinkingWalker & Salt · ecological resilience