2.1 Individuals, populations, communities, and ecosystemsSyllabusFirst assessment 2026Topic2.1LevelHL
What you’ll learn30 learning objectivesChoose one objective for a focused lesson, or study the complete topic.2.1.1Biosphere composition• Ecological system composed of individuals, populations, communities, ecosystems• Parts of Earth where life existsSyllabus objective2.1.2Species definition• Individual organism is member of a species• Biological species concept: can interbreed and produce fertile offspringSyllabus objective2.1.3Classification of organisms• Allows efficient identification and prediction of characteristics• Binomial nomenclature: genus (capitalized) + species (lowercase)Syllabus objective2.1.4Taxonomist tools• Dichotomous keys, specimen comparison, DNA surveysSyllabus objective2.1.5Population definition• Same species in same area at same time, capable of interbreedingSyllabus objective2.1.6Distribution factors• Abiotic: non-living physical factors• Biotic: living componentsSyllabus objective2.1.7Abiotic factors examples• Temperature, sunlight, pH, salinity, dissolved oxygen, soil textureSyllabus objective2.1.8Niche• A niche describes the particular set of abiotic and biotic conditions and resources upon which an organism or a population depends• An ecological niche is the role of a species in an ecosystem• The niche comprises all biotic and abiotic interactions that influence the growth, survival and reproduction of a population, including how food is obtained• Include: some of the parameters of a niche for a named speciesSyllabus objective2.1.9Population interactions• Populations interact in ecosystems by herbivory, predation, parasitism, mutualism, disease and competition, with ecological, behavioural and evolutionary consequences• Consider: one example of each relationship and consider how the relationships influence the population dynamics of the interacting populations and the selective pressures involvedSyllabus objective2.1.10Carrying capacity• Carrying capacity is the maximum size of a population determined by competition for limited resources• Include: examples of resources that may affect carrying capacity, including biotic and abiotic factorsSyllabus objective2.1.11Density-dependent regulation• Population size is regulated by density-dependent factors and negative feedback mechanisms• Density-independent factors may influence population size, but density-dependent factors tend to regulate populations around carrying capacity• In addition to competition for limited resources, include the increased risk of predation and the transfer of pathogens in dense populations• These are examples of negative feedback returning a population to equilibriumSyllabus objective2.1.12Population growth curves• Population growth can either be exponential or limited by carrying capacity• If there are no limiting factors, population growth follows a J-curve (exponential growth)• When density-dependent limiting factors start to operate, the curve becomes S-shaped• Consider S-curves and boom-and-bust patterns, such as reindeer on St Matthew IslandSyllabus objective2.1.13Human population limiting factors• Limiting factors on the growth of human populations have increasingly been eliminated, resulting in consequences for sustainability of ecosystems• Include: the effects of elimination of natural predators, technological advances, and degradation of the environmentSyllabus objective2.1.14Human carrying capacity• Carrying capacity cannot be easily assessed for human populations• This is because of the broad and changing ecological niche of humans• Include: the idea of populations achieving equilibrium within ecosystems, but human populations being less limited due to mobility of resources• The expansion of the human niche also takes place through technological advances and changes in consumptionSyllabus objective2.1.15Estimating population abundance• Population abundance can be estimated using random sampling, systematic sampling or transect samplingSyllabus objective2.1.16Quadrat sampling• Estimate population size for non-mobile organisms• Measures: percentage cover, frequency, abundanceSyllabus objective2.1.17Capture-mark-release-recapture• Estimate population size for mobile organisms• Lincoln index: Population = (M × N) / RSyllabus objective2.1.18Community definition• Collection of interacting populations within ecosystemSyllabus objective2.1.19Habitat definition• Location where community, species, population, or organism livesSyllabus objective2.1.20Ecosystems as open systems• Energy and matter can enter and exit• Community + physical environment interactionsSyllabus objective2.1.21Ecosystem sustainability• Natural property of ecosystems• Inputs balanced by outputs in steady-state• Some persist for millions of years (e.g., tropical rainforests)Syllabus objective2.1.22Human impacts on stability• Can lead to tipping points• Example: Amazon deforestation reduces transpiration → reduced rainfallSyllabus objective2.1.23Keystone species• Keystone species have a role in the sustainability of ecosystems• There is a disproportionate impact on community structure of keystone species and the risk of ecosystem collapse if they are removed• Consider: two examples• For example, purple sea stars controlling mussel populations on the North Pacific coast that would otherwise overwhelm the ecosystem; elephants feeding on shrubs and treesSyllabus objective2.1.24Biosphere integrity boundary• The planetary boundaries model indicates that changes to biosphere integrity have passed a critical threshold• There is an interrelationship between ecosystems and species diversity• Disturbance of ecosystems due to human activity has led to loss of biosphere integrity• Extinction rates provide evidence that the planetary boundary for biosphere integrity has been crossedSyllabus objective2.1.25Reversing biosphere integrity loss• To avoid critical tipping points, loss of biosphere integrity needs to be reversed• Ecosystem damage and loss of species can be slowed by protecting the integrity of ecosystems• Protecting ecosystems ensures the preservation of the niche requirements essential for the ongoing survival of a speciesSyllabus objective2.1.26(HL)—Clades and evolutionary classification• Clades show evolutionary relationships within a taxonomic group• All clade members evolved from a common ancestorSyllabus objective2.1.27(HL)—Limits of traditional taxa• Traditional hierarchy: kingdom, phylum, class, order, family, genus, species• This hierarchy does not always match evolutionary divergence patternsSyllabus objective2.1.28(HL)—Fundamental and realized niches• Fundamental niche: conditions/resources where a species could survive and reproduce without limiting factors• Realized niche: actual mode of existence shaped by adaptations and competition• Examples: Connell barnacle species; brown and green anolesSyllabus objective2.1.29(HL)—Life cycles and r/K strategies• Life cycles vary in reproductive behaviour and lifespan• K-strategists: stable communities, few offspring, high survival chance• r-strategists: rapid colonization, many offspring, limited survival provisionSyllabus objective2.1.30(HL)—Human impacts on species• Classification, niche requirements and life cycles help explain human impacts• Examples: climate-driven plant life-cycle changes affecting animal life cyclesSyllabus objective
