ORGANISMS AND POPULATIONS

Ecology studies interactions among organisms and with the abiotic environment. This chapter concentrates on population attributes, growth and interactions between species.

A population is a group of individuals of one species in a defined geographical area that share or compete for similar resources and potentially interbreed. It has attributes that an individual does not: natality, mortality, sex ratio, age distribution and density (N).

• Density may be expressed as number, biomass or percent cover. Relative measures, such as fish caught per trap, are often sufficient.
• Population size rises through births and immigration and falls through deaths and emigration: N_t+1 = N_t + [(B + I) - (D + E)].
• Age pyramids show pre-reproductive, reproductive and post-reproductive groups: triangular means growing, bell-shaped stable and urn-shaped declining.

With unlimited food and space, population growth is exponential: dN/dt = rN. Here r is the intrinsic rate of natural increase, a measure of the inherent capacity to grow.

• Natural resources are ultimately finite. Logistic growth is therefore more realistic: dN/dt = rN ((K-N)/K), where K is carrying capacity.
• Exponential growth gives a J-shaped curve; logistic growth gives an S-shaped curve.
• Life-history traits maximise Darwinian fitness under particular abiotic and biotic constraints. Some species breed once, others repeatedly; some have many small offspring, others few large ones.

Species in a habitat do not exist alone. Their interactions may benefit, harm or leave either partner unaffected.

• Predation transfers energy across trophic levels and regulates prey. Predators can maintain diversity; removal of Pisaster starfish led to loss of many invertebrate species.
• Prey avoid predators through camouflage, poisons or other defences. Plants defend against herbivores with thorns and chemicals.
• Competition lowers fitness in presence of another species. Gause's principle states that close competitors for the same resource cannot coexist indefinitely; competitive release follows removal of a superior competitor.

• In parasitism, parasite benefits and host is harmed. Ectoparasites live on host surfaces (lice, ticks); endoparasites live inside. Parasites often have high reproductive capacity and complex life cycles.
• In commensalism, one benefits and the other is unaffected: orchid on mango, cattle egret with cattle, and clown fish with sea anemone.
• In mutualism, both benefit: lichen, mycorrhiza and fig-wasp association. Mycorrhizal fungi improve plant nutrient absorption; plants give carbohydrates.
• Fig and wasp show close co-evolution: the wasp pollinates while laying eggs, and developing seeds nourish larvae.

• Population attributes include density, natality, mortality, sex ratio and age distribution.
• Births and immigration increase N; deaths and emigration decrease it.
• Unlimited resources produce exponential growth; finite resources produce logistic growth toward K.
• Predators regulate prey; competition lowers fitness; parasites harm hosts.
• Commensalism benefits one partner, while mutualism benefits both.