12.3 Ecological Relationships and Populations

A population consists of organisms of one species living in the same area at the same time with the possibility of interbreeding. A community is all interacting populations in an area. Intraspecific relationships occur within one species; interspecific relationships occur between species. Their effects can be summarized using signs: competition harms both participants (−/−), predation, herbivory and parasitism benefit one while harming another (+/−), mutualism benefits both (+/+), and commensalism benefits one while having no measurable effect on the other (+/0).

These signs describe net outcomes under stated conditions, not every event. A mutualist may pay a cost to supply its partner but receive a larger benefit overall. Competition may be weak enough for coexistence, especially if species divide resources by place, time or method. Relationships also alter distribution: a species may be absent from physically suitable habitat because a competitor excludes it, or present beyond its unaided dispersal range because a partner enables survival.

Sessile or slow-moving organisms can be sampled with quadrats. Investigators first define the study area, choose a quadrat size suited to organism scale and place sufficient replicates using random coordinates when estimating abundance without spatial bias. Counts can be reported as density, frequency of occurrence or percentage cover. Tiny quadrats miss large-scale patches; very large quadrats reduce replication for a fixed effort. A pilot study can compare how estimated mean and variance stabilize as quadrat size and number increase.

A belt transect samples consecutive or regularly spaced quadrats along an environmental gradient. It is systematic rather than random and is appropriate when the research question concerns zonation with distance, elevation, salinity, exposure or another changing factor. The abiotic variable should be measured at the same positions as the organisms. An association between abundance and one variable can still be confounded by another variable changing along the same line.

Sampling quality depends on an unambiguous counting rule, consistent species identification and adequate replication. Edge organisms may be counted only when they touch two preselected sides. Percentage cover estimates can use a point grid. Random placement reduces investigator choice, but a random sample can still be unrepresentative by chance; confidence improves through more independent samples. Repeating many quadrats in one patch is pseudoreplication if conclusions are meant to apply to the whole habitat.

Mobile animal populations can be estimated by capture–mark–release–recapture. In the first sample, organisms are captured, marked harmlessly and returned. After enough time for marked individuals to mix through the population, a second sample is captured and the marked fraction recorded. If marked animals are thoroughly mixed, the proportion marked in the second sample should approximate the proportion marked in the whole population.

The Lincoln estimate assumes a closed population during sampling, no mark loss, no effect of marking on survival or capture, equal capture probability and enough mixing. Immigration or births tend to dilute the marked fraction and can inflate the estimate; emigration or deaths of marked animals can do the same. Trap-shy marked animals are under-recaptured, whereas trap-happy animals are over-recaptured. A very small recapture count makes the quotient unstable, so repeated sampling and confidence intervals are preferable.

Population size changes through births, deaths, immigration and emigration. A newly established population may show a lag phase while individuals mature and acclimatize. With abundant resources and a high reproductive rate, it can enter exponential growth, producing a J-shaped curve because the number added per interval rises with the number already reproducing. Exponential growth cannot continue indefinitely in a finite environment.

As density rises, limiting resources are divided among more organisms and density-dependent effects intensify. Competition, infectious disease, parasitism and sometimes predation reduce per-capita population growth. The curve decelerates toward a stationary phase around carrying capacity, the maximum average population that the environment can sustain over time. Carrying capacity is not fixed: seasonal food, habitat structure, disturbance and environmental engineering can move it.

Real populations may overshoot carrying capacity because reproduction responds after resource depletion begins, then decline and oscillate. Density-independent events such as fire, frost or storms can affect populations regardless of density, although vulnerability may still differ among individuals. A decline phase can follow resource exhaustion, waste accumulation, disease or environmental change. A smooth sigmoid is therefore an idealized expectation, not a compulsory life history.

Competition occurs when organisms require the same limiting resource and use by one reduces availability to another. Intraspecific competition is often intense because conspecifics have very similar requirements. It contributes to density dependence and differential reproductive success. Interspecific competition can reduce population size, restrict a realized niche or cause local competitive exclusion if two species depend on exactly the same limiting resource under constant conditions.

Coexistence can arise through resource partitioning: species use different food sizes, microhabitats, soil depths or times of day. Character displacement can strengthen differences where competitors coexist. To test competition, investigators can compare each species alone and together while controlling other conditions. A negative field association is not sufficient because the two species may prefer different abiotic conditions. A chi-squared test can assess whether co-occurrence differs from independence, but statistical association still needs biological interpretation.

Allelopathy is chemical inhibition of one species by another. Plants may release compounds from roots, leaves or decaying litter; fungi and bacteria also produce chemicals that suppress neighbors. Reduced germination near a plant does not prove allelopathy because shade, water uptake and soil differences are alternatives. A strong investigation isolates an extract, uses appropriate solvent and concentration controls, standardizes seeds, replicates treatments and ideally identifies effects under field-realistic conditions.

Predator and prey populations can show linked oscillations. More prey supports predator survival and reproduction after a time lag; more predators then raise prey mortality; reduced prey later lowers predator numbers. Similar cycles do not prove that predation is the only driver because vegetation, climate and disease may also vary. Cross-correlation and manipulative evidence help identify the lag and mechanism.

Prey defenses include concealment, warning coloration, mimicry, armor, toxins, vigilance, rapid escape and group behavior. Predators counter with improved detection, pursuit, detoxification or cooperation. Plants use physical and chemical defenses against herbivores, while herbivores may evolve detoxification and selective feeding. This reciprocal selection is coevolution only when each species imposes selection on the other; parallel change alone is insufficient.

Cooperative intraspecific behavior can increase hunting success, predator detection, offspring care, thermoregulation or access to information. It can also increase competition, disease transmission and conflict over reproduction. Kin selection can favor behavior that helps relatives carrying shared alleles, while reciprocal cooperation can be maintained when partners interact repeatedly and cheaters can be detected. Describing a benefit does not remove the need to account for costs.

Mutualisms can organize whole communities. Mycorrhizal fungi expand the absorbing network of plant roots and receive carbohydrates; pollinators move pollen while obtaining food; coral symbionts exchange protected habitat and inorganic nutrients for photosynthetic products. Dependence makes these systems efficient but can also create vulnerability if one partner is lost. The net effect may change with nutrient availability, temperature or life stage.

A keystone species has an effect on community structure disproportionately large relative to its abundance or biomass. Sea otters limit sea urchins and indirectly protect kelp forests; large herbivores can maintain habitat mosaics; ecosystem engineers create physical habitat used by many species. Removing a keystone can trigger a trophic cascade. Keystone is not a synonym for abundant, endangered or top predator, and the claim should be supported by removal, recovery or strong comparative evidence.