Ecology Terms Starting With R

R-selected species produce many offspring quickly, investing little energy per individual offspring, and typically experience high juvenile mortality rates.

R-selected species often generate hundreds to millions of offspring per reproductive event, with juvenile mortality reaching 99 to 99.9 percent in many insect and fish populations. Sexual maturity arrives early, sometimes within days to weeks, so populations can double rapidly when resources are temporarily abundant. Small body size and lifespans under one year characterize most r-selected organisms, from annual plants to mayflies.

Unpredictable or frequently disturbed environments favor this strategy because individual survival is unreliable, and population persistence depends on numerical output rather than parental investment.

Realized Niche is the actual range of environmental conditions and resources a species uses when competition, predation, parasitism, and other biotic interactions are present.

Without competitors or predators, a species can occupy its full fundamental niche, the complete set of conditions it can physiologically tolerate. Biotic interactions compress this potential into a smaller realized niche by excluding the species from portions of its habitat or resource spectrum. The American barnacle (Chthamalus stellatus) can survive across a broad intertidal zone, but competition from the larger barnacle Semibalanus balanoides pushes it upward into the high intertidal, where Semibalanus cannot tolerate desiccation.

Joseph Connell’s classic 1961 experiments on Scottish rocky shores demonstrated this compression by removing Semibalanus and observing Chthamalus expand downward into its full fundamental niche.

Reforestation is the deliberate or natural reestablishment of tree cover on land that was previously forested but lost its trees through logging, fire, agriculture, or other disturbance.

Reforestation proceeds either through active planting of seedlings or through passive natural regeneration, in which surviving seeds, root systems, and soil seed banks drive recovery without human intervention. Species selection strongly influences outcomes; planting fast-growing exotic monocultures can suppress native understory plants and reduce wildlife habitat value compared with diverse native plantings. Restored forests in the Brazilian Atlantic Forest, one of the world’s most threatened biomes, can recover roughly 80 percent of native bird species within 40 years when planted with a mixture of native tree species.

Soil stabilization, carbon sequestration, and watershed protection all improve as canopy cover and root networks develop over decades.

Refugia are localized areas where populations of species persist through periods of regional environmental change that render surrounding habitats unsuitable.

These sheltered areas maintain temperature, moisture, or resource conditions that differ enough from the surrounding regional terrain to buffer resident populations against drought, glaciation, fire, or rapid climate shifts. Mountain valleys, north-facing slopes, deep springs, and cave systems can all function as refugia by preserving microclimatic stability. During the last glacial maximum approximately 20,000 years ago, temperate tree species survived in refugia along the southern margins of European and North American ice sheets, then recolonized northward as glaciers retreated.

Modern climate change is shrinking many existing refugia while creating new ones at higher elevations and latitudes, reshaping where species can persist.

Relict is a species or population that persists in a restricted area today after once occupying a much broader geographic range, typically because environmental or climatic changes eliminated it from most of its former distribution.

Relict species are living survivors of lineages that were far more widespread in the past, and their current distributions often map onto refugia where historical conditions have been preserved. The dawn redwood (Metasequoia glyptostroboides) was known only from Cretaceous and Eocene fossils spanning North America and Eurasia until living trees were discovered in a remote valley in Sichuan, China, in 1944. Populations of the horseshoe crab (Limulus polyphemus) along the Atlantic coast of North America represent a morphologically conservative lineage that has changed little in roughly 450 million years, persisting while related groups disappeared.

Studying relict distributions gives ecologists and paleontologists direct evidence of how past climates and habitats differed from present conditions.

Resource partitioning occurs when competing species reduce direct competition by using shared resources in different ways, at different times, or in different locations.

When two or more species require the same limiting resource, natural selection favors individuals that shift their use of that resource away from the zone of greatest overlap with competitors. Nile perch and tilapia in African rift lakes feed at different water depths, reducing direct dietary competition despite sharing the same body of water. Anole lizards on Caribbean islands use different perch heights and branch diameters, allowing several species to forage in the same tree without monopolizing the same microhabitat.

This division of resources permits coexistence that would otherwise end in competitive exclusion, where the superior competitor eliminates the weaker one entirely.

Riparian forests are bands of tree and shrub vegetation growing along rivers, streams, and other watercourses, where seasonal flooding, elevated soil moisture, and proximity to flowing water create ecological conditions distinct from surrounding upland habitats.

These forests form linear corridors that connect otherwise fragmented upland habitats, allowing wildlife to move across habitats that would otherwise be impassable. Vegetation shifts from water’s edge inward through emergent aquatic plants, moisture-tolerant willows and alders, floodplain hardwoods, and transitional upland species, creating a steep gradient of plant communities within a narrow band. Cottonwood-dominated riparian corridors along the Colorado River once extended for hundreds of kilometers and supported nesting populations of southwestern willow flycatchers (Empidonax traillii extimus), a federally endangered bird that depends almost entirely on this habitat type.

Nutrient and sediment interception by riparian root systems and leaf litter can reduce nitrogen loading to adjacent streams by 50 to 90 percent compared with unforested stream banks.