2.1 Individuals, populations, communities and ecosystemsIB Environmental Systems and Societies HL: Revision notes

The biosphere is an ecological system made of individuals, populations, communities and ecosystems.

• An individual is a member of a species.
• A population is a group of organisms of the same species living in the same area at the same time, capable of interbreeding.
• A community is a collection of interacting populations within an ecosystem.
• A habitat is the location in which a community, species, population or organism lives.

Classification allows efficient identification and prediction of characteristics. Taxonomists use tools such as dichotomous keys, field guides, morphology and DNA analysis to identify organisms.

The distribution of a population depends on abiotic and biotic factors. Examples of abiotic factors are temperature, sunlight, pH, salinity, dissolved oxygen and soil texture.

A niche describes the particular set of abiotic and biotic conditions and resources upon which an organism or population depends.

Populations interact through herbivory, predation, parasitism, mutualism, disease and competition, with ecological, behavioural and evolutionary consequences.

Carrying capacity is the maximum population size determined by competition for limited resources. Population size is regulated by density-dependent factors (for example competition, predation, disease) and negative feedback.

Growth can be exponential (accelerating, with no limit) or limited by carrying capacity. Overshooting the carrying capacity can damage the resource base and cause a crash, as with the reindeer of St Matthew Island.

Limiting factors on human populations have increasingly been eliminated, with consequences for the sustainability of ecosystems. Carrying capacity cannot easily be assessed for humans because technology, trade and lifestyle change resource use.

Abundance can be estimated by random sampling, systematic sampling or transect sampling.

• Random quadrat sampling estimates population size of non-mobile organisms (for example plants, limpets).
• Capture-mark-release-recapture and the Lincoln index estimate population size of mobile organisms:

N = (n1 × n2) ÷ m2

where n1 = number first caught and marked, n2 = number in the second sample and m2 = marked individuals recaptured.

Assumptions: marks do not harm or alter the chance of capture, marked animals mix randomly, and there are no births, deaths or migration between samples.

Ecosystems are open systems: both energy and matter can enter and exit. Sustainability is a natural property of ecosystems, because they recycle matter and run on solar energy.

Human activity can lead to tipping points in ecosystem stability, after which the system shifts to a new state that may be hard to reverse.

Keystone species have a role in sustainability that is disproportionate to their abundance: removing them can change the whole ecosystem (for example sea otters, which control sea urchins and so protect kelp forests).

The planetary boundaries model indicates that changes to biosphere integrity have passed a critical threshold. To avoid critical tipping points, the loss of biosphere integrity needs to be reversed.

(HL)

• Clades: a cladogram groups organisms by shared ancestry and so illustrates evolutionary relationships. Advantages: it is based on evidence such as DNA sequences and shows how closely species are related.
• Difficulties classifying into the traditional hierarchy of taxa: species are hard to define (for example hybrids, asexual organisms), and boundaries between ranks are arbitrary and change as new evidence arrives.
• Fundamental niche: the full range of conditions and resources a species could use in the absence of competition. Realized niche: the narrower range actually occupied when biotic interactions such as competition are included. Example: Chthamalus and Semibalanus barnacles.
• Life cycles vary in reproductive behaviour and lifespan: some species breed once and die, others breed repeatedly; some produce many offspring with little care, others few with extended care.
• Knowing a species' classification, niche and life cycle helps us judge how vulnerable it is to human impacts and how fast it could recover.