Organisms and Populations — Core Principles

Core Principles

Ecology begins with the organism, an individual unit that adapts to its environment through morphological, physiological, or behavioral changes to cope with abiotic factors like temperature, water, light, and soil.

Organisms can be regulators (maintaining internal constancy) or conformers (allowing internal changes). Populations are groups of the same species in an area, characterized by density, birth rate (natality), death rate (mortality), sex ratio, and age distribution.

Population growth follows either an exponential (J-shaped, unlimited resources) or logistic (S-shaped, limited resources, reaching carrying capacity $K$) model. Interactions between different species within a community are diverse: predation (+/-), competition (-/-), parasitism (+/-), commensalism (+/0), mutualism (+/+), and amensalism (-/0).

Understanding these fundamental concepts is crucial for comprehending the broader ecological landscape and its dynamics.

Important Differences

vs r-selected vs. K-selected Species

Aspect	This Topic	r-selected vs. K-selected Species
Environment	Unstable, unpredictable	Stable, predictable
Population size	Fluctuates widely, often below K	Relatively stable, close to K
Offspring number	Many	Few
Offspring size	Small	Large
Maturity	Early	Late
Parental care	Little to none	Extensive
Lifespan	Short	Long
Examples	Insects, bacteria, annual plants	Humans, elephants, large trees

The distinction between r-selected and K-selected species highlights contrasting life history strategies adapted to different environmental conditions. R-selected species prioritize rapid reproduction and high offspring numbers to exploit ephemeral resources in unstable environments, often experiencing boom-and-bust cycles. K-selected species, conversely, invest heavily in fewer, larger offspring with extensive parental care, thriving in stable environments where competition is high and populations are maintained near the carrying capacity. This conceptual framework helps explain diverse reproductive patterns observed in nature.