Organisms and Populations
Ecology is the study of the interactions among organisms and between an organism and its physical (abiotic) environment. It is concerned with four levels of biological organisation - organisms, populations, communities and biomes. This chapter focuses on the organism and population levels.
Organism and its environment
The most ecologically relevant abiotic factors are temperature (most important; varies with latitude and altitude), water (most important for aquatic life; affects productivity on land), light (needed for photosynthesis and many physiological responses) and soil (grain size, pH, minerals decide vegetation and the animals it supports).Organisms cope with stressful conditions in four ways:
- Regulate: maintain a constant internal environment (homeostasis), e.g. birds, mammals, some plants.
- Conform: internal state changes with the external environment (majority of animals and nearly all plants).
- Migrate: move temporarily to a more hospitable area, e.g. winter migratory birds.
- Suspend activity: form spores, seeds, or enter dormancy/hibernation/aestivation/diapause.
An adaptation is any morphological, physiological or behavioural attribute that helps an organism survive and reproduce in its habitat (e.g. Allen's Rule - shorter limbs in cold climates).
Populations
A population is a group of individuals of a species in a defined area, sharing resources and potentially interbreeding. Populations have attributes individuals lack: birth rate (natality), death rate (mortality), sex ratio, and age distribution (age pyramid). Population size = population density (N), measured as number, per cent cover or biomass.Population growth
Density changes through four processes: natality and immigration (increase) and mortality and emigration (decrease). Nt+1 = Nt + [(B + I) - (D + E)]- Exponential growth (unlimited resources): dN/dt = rN, giving a J-shaped curve; integral form Nt = N0 e^(rt). Here r is the intrinsic rate of natural increase.
- Logistic growth (limited resources): dN/dt = rN[(K - N)/K], giving a sigmoid curve that levels off at the carrying capacity (K). Also called Verhulst-Pearl logistic growth; considered more realistic.
Population interactions (Table 11.1)
- Mutualism (+/+): both benefit - lichens, mycorrhizae, fig-wasp.
- Competition (-/-): both suffer - flamingoes and fishes, Balanus and Chathamalus.
- Predation (+/-): predator benefits, prey harmed - tiger and deer.
- Parasitism (+/-): parasite benefits, host harmed - Cuscuta, brood parasitism.
- Commensalism (+/0): one benefits, other unaffected - orchid on mango, cattle egret, clownfish.
- Amensalism (-/0): one harmed, other unaffected.