Organisms and Populations

1. Chapter at a glance

• Ecology studies interactions among organisms and between organisms and their abiotic environment at four levels: organisms, populations, communities and biomes.
• A population is a group of individuals of a given species that share or compete for similar resources in a defined geographical area and potentially interbreed.
• Populations possess attributes that individuals do not: birth rates and death rates (expressed per capita), sex ratio, age distribution (shown as age pyramids) and population density (N).
• Population density changes through four processes: natality and immigration increase density; mortality and emigration decrease it.
• Under unlimited resources, populations show exponential growth (J-shaped curve); under limited resources, they show logistic growth (sigmoid curve) limited by carrying capacity (K).
• Populations evolve life-history traits (e.g., breeding once vs many times, many small offspring vs few large) to maximise Darwinian fitness in their habitat.
• Interspecific interactions include mutualism (+ +), competition (− −), predation and parasitism (+ −), commensalism (+ 0) and amensalism (− 0).

2. Definitions and laws

• Population density (N): Size of a population; may be measured as numbers, biomass or per cent cover.
• Natality: Number of births during a given period in the population that are added to the initial density.
• Mortality: Number of deaths in the population during a given period.
• Immigration: Number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration.
• Emigration: Number of individuals of the population who left the habitat and gone elsewhere during the time period under consideration.
• Intrinsic rate of natural increase (r): Parameter chosen for assessing impacts of any biotic or abiotic factor on population growth.
• Carrying capacity (K): Maximum possible number of individuals of a species that a given habitat can support.
• Gause’s Competitive Exclusion Principle: Two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior one will be eliminated eventually.
• Resource partitioning: Mechanism by which competing species avoid competition by choosing different times for feeding or different foraging patterns.

3. Important diagrams and activities

• Figure 11.1 (age pyramids for human population): Shows age distribution of males and females; shape indicates whether population is growing, stable or declining.
• Figure 11.3 (population growth curves): Illustrates J-shaped exponential growth when resources are unlimited and sigmoid logistic growth when resources are limiting, with K marked.
• Figure 11.4 (fig tree–wasp mutualism): Demonstrates one-to-one co-evolved pollination and seed-feeding relationship.
• Figure 11.5 (bee pollinator on orchid): Shows co-evolution via sexual deceit in Ophrys orchid.

4. Common misconceptions and exam pitfalls

• Treating birth rate/death rate or sex ratio/age distribution as attributes of an individual organism rather than of the population.
• Assuming exponential growth continues indefinitely; text emphasises that logistic growth is more realistic once K is reached.
• Confusing predation with herbivory; text states herbivores are predators in the broad ecological sense.
• Believing competition occurs only between closely related species or only when resources are limiting; text gives counter-examples.
• Stating that an ideal parasite does not harm the host; text notes parasites reduce host survival, growth or reproduction.
• Overlooking that population density may be expressed as biomass or per cent cover, not only numbers.

5. Formula sheet