11.4 Niches, Adaptation and Biodiversity

A habitat is the physical place where an organism lives. A niche is the species' mode of existence: the resources it uses, conditions it tolerates, timing and location of activity, feeding relationships and interactions with other organisms. Two species may share a habitat yet occupy different niches by feeding at different heights, times or on different sizes of prey.

Community structure emerges from many interactions among populations and with abiotic conditions. Temperature, water availability, salinity, pH, light, oxygen and substrate can limit distribution. Predation, herbivory, parasitism, mutualism and competition further modify which parts of a physically suitable range are occupied. No isolated list of adaptations fully predicts a community because each species changes the conditions experienced by others.

For an abiotic variable, a species often has an optimum range where survival and reproduction are greatest, zones of physiological stress on either side, and limits beyond which it cannot persist. Distribution reflects several variables simultaneously, so a correlation with one factor does not prove that factor is causal. A species absent from a suitable temperature might be excluded by salinity, competitors or lack of dispersal.

Quadrats sample organisms in fixed areas and are appropriate for plants or slow-moving organisms. Random quadrat positions estimate abundance without deliberately favoring convenient patches. A transect samples along an environmental gradient; quadrats can be placed at regular intervals for an interrupted belt transect or continuously for a belt transect. Measuring an abiotic variable at the same positions permits a test of association between conditions and distribution.

Sampling design determines the claim that can be made. Replicates reduce the effect of local anomalies, standardized quadrat size makes counts comparable, and sufficient coverage improves precision. Percentage cover may be more useful than counts for clonal grasses, while frequency records how many quadrats contain a species. Mark–release–recapture is better suited to mobile animals. Uncertainty and detection bias must be acknowledged.

The fundamental niche is the potential mode of existence permitted by a species' adaptations in the absence of restricting biotic interactions. The realised niche is the mode actually occupied after competition and other interactions. It is normally equal to or smaller than the fundamental niche. Removal experiments can reveal the distinction: if a species expands after a competitor is removed, the previously unoccupied area was physiologically tolerable but biotically restricted.

On rocky shores, a barnacle may tolerate a broad vertical range when alone but be confined to the upper shore when a faster-growing competitor occupies lower rocks. The lower boundary is then set by competition, while the upper boundary may be set by desiccation tolerance. This demonstrates why distribution boundaries can have different causes even within one species.

Competition occurs when organisms require the same limiting resource. Intraspecific competition is often intense because members of one species have highly similar needs; interspecific competition occurs between species. The competitive exclusion principle predicts that two species with identical niches cannot coexist indefinitely under stable conditions: even a small advantage allows one to reduce the shared resource below the other's requirement.

Coexistence becomes possible when niches differ. Species may partition food size, feeding height, activity time, rooting depth or microclimate. Character displacement can increase trait differences where competitors occur together. The principle does not claim that any overlap causes extinction; partial overlap can persist when resources fluctuate, predators prevent dominance or each species has an advantage under different conditions.

Classic culture experiments show that two Paramecium species can each grow alone on the same resource, while one declines when they are cultured together. Such a result supports competition only if alternative differences in conditions are controlled. Natural systems add spatial refuges, multiple resources and predators, so experimental conclusions should be applied at the scale and conditions actually tested.

Adaptations may be structural, physiological or behavioral. Desert succulents store water, reduce leaf surface, use thick cuticles and deploy broad shallow roots that capture brief rainfall. Marram grass rolls leaves, shelters stomata inside humid grooves, carries hairs that slow air movement and uses deep, spreading roots to reach water and stabilize sand. Each feature changes a particular transfer or mechanical problem; naming the feature without its effect is incomplete.

Mangroves tolerate waterlogged, saline substrates through combinations of salt exclusion at roots, salt secretion by leaves, supportive spreading roots and aerial roots that improve gas exchange in anoxic sediment. Emperor penguins combine insulation and physiological control with huddling behavior, rotating positions so the energetic cost of the exposed edge is shared. An organism is adapted by inherited population history, not because an individual deliberately develops whatever feature the habitat requires.

Metabolic tolerance also defines niches. Obligate anaerobes are harmed by atmospheric oxygen and rely on anaerobic pathways; facultative anaerobes use oxygen when available but can switch to fermentation or anaerobic respiration; obligate aerobes require oxygen for sustained ATP production. These terms describe metabolic constraints and should not be confused with whether an organism happens to occupy deep water, soil or an intestine.

Divergent evolution produces increasing differences among descendants of a common ancestor, often as they adapt to different niches. Homologous structures preserve a common structural origin despite different functions: the humerus, radius, ulna and wrist bones of vertebrate forelimbs are rearranged for grasping, swimming, running or flight. Adaptive radiation is rapid divergence into multiple forms that exploit newly available niches.

Convergent evolution occurs when distantly related lineages independently acquire similar solutions under similar selection pressures. Streamlined shark and dolphin bodies and water-storing forms in cacti and euphorbias are analogous at the relevant functional level. Analogous structures share function but not origin. A bird wing and insect wing are analogous as wings, while the bones within a bird wing are homologous to other tetrapod forelimbs. The comparison level must be stated.

Island colonization can combine founder effects, ecological opportunity and reproductive isolation. A small ancestral population carries limited variation. As numbers grow, competition and different food resources favor different beak forms or behaviors. Restricted gene flow lets gene pools diverge, and reproductive isolation eventually preserves multiple species. Adaptive radiation therefore links population genetics, selection, speciation and niche partitioning.

Biodiversity can be considered at genetic, species and ecosystem levels. Species richness is the number of species; evenness describes how equally individuals are distributed among them. Two communities can have the same richness but different diversity if one is dominated by a single species. Genetic diversity gives populations alternative alleles that may support response to future change, while ecosystem diversity captures variation among habitats and ecological processes.

Simpson's diversity index combines richness and evenness by measuring how strongly individuals are concentrated within species. The exact convention must be checked because textbooks use several related forms; for the reciprocal form shown here, larger values mean greater diversity. The index allows habitats or dates to be compared using one standardized calculation, but results still depend on sampling effort, season, identification accuracy and spatial scale.