Ecology Terms Starting With S
Ecology Glossary: S
Savanna
/ suh-VAN-uh / · Spanish savana (treeless plain)
Savanna is a tropical or subtropical biome dominated by grasses, with scattered drought-tolerant trees and shrubs, a pronounced alternation of wet and dry seasons, and annual rainfall typically between 250 and 900 millimeters.
Savannas receive most of their precipitation during a wet season lasting three to six months, followed by a dry season during which grasses cure and fire risk rises sharply. Scattered acacia, baobab, and other drought-resistant trees grow widely spaced, allowing enough sunlight to reach the ground for continuous grass cover. Fire, heavy grazing by large herbivores, and nutrient-poor soils all suppress tree recruitment and maintain grass dominance against the encroachment of closed woodland.
African savannas alone support the largest remaining assemblages of large terrestrial mammals on Earth, including wildebeest (Connochaetes taurinus) herds that exceed one million individuals during the Serengeti migration.
The Cerrado of central Brazil, the world's most biodiverse savanna, covers roughly 2 million square kilometers and contains approximately 11,000 plant species, about 44 percent of which are found nowhere else on Earth.
Savannas are dry forests with the trees removed. Grasses are the structural and functional dominant in savannas, and the open canopy is maintained by fire and herbivory rather than simply by low rainfall.
The Serengeti-Mara ecosystem in Tanzania and Kenya receives 500 to 800 millimeters of rain annually, concentrated in two wet seasons. Roughly 1.5 million wildebeest, 400,000 zebras, and 200,000 Thomson's gazelles complete an annual migration circuit of approximately 1,800 kilometers, tracking the flush of new grass growth across the savanna.
Secondary Production
/ SEK-un-dair-ee proh-DUK-shun / · Latin secundus, following; producere, to lead forward
Secondary production is the rate at which heterotrophic consumers convert ingested organic matter into new body biomass, representing the energy available to support higher trophic levels.
Calculating secondary production requires separating assimilated energy from the fraction lost to respiration and the fraction that passes through the gut undigested. Ecological transfer efficiency between trophic levels averages roughly 10 percent, so a grassland supporting 1,000 grams of plant biomass per square meter per year typically yields only about 100 grams of herbivore biomass and 10 grams of carnivore biomass over the same area and period. Freshwater stream ecologists use secondary production estimates to assess how much invertebrate biomass is available to support fish populations, a calculation central to fisheries management.
Production-to-biomass ratios vary widely across taxa, from less than 0.1 per year in large, slow-growing fish to more than 5 per year in small, fast-reproducing invertebrates.
Midge larvae (family Chironomidae) in productive lake sediments can achieve secondary production rates exceeding 10 grams of dry mass per square meter per year, making them among the most productive invertebrate communities measured in freshwater ecosystems and a critical food source for fish and waterfowl.
Secondary production means photosynthesis that occurs at a second stage. Secondary production is biomass synthesis by consumers, not by photosynthetic organisms; primary production is the term for organic matter synthesized from inorganic carbon by plants, algae, and cyanobacteria.
Zooplankton such as Daphnia in temperate lakes convert phytoplankton biomass into animal tissue during spring blooms, with production rates sometimes reaching 0.5 to 1.0 milligrams of carbon per liter per day. Planktivorous fish such as yellow perch (Perca flavescens) then harvest this zooplankton production, transferring roughly 10 percent of it into fish biomass.
Secondary Succession
/ SEK-un-dair-ee suk-SESH-un / · Latin secundarius (second) + successio
Secondary succession is the sequential recovery of a biological community following a disturbance that removes existing vegetation but leaves the soil, seed bank, and below-ground root systems largely intact.
Recovery begins within weeks as fast-growing annual weeds and grasses germinate from the surviving seed bank, rapidly covering bare ground and reducing erosion. Perennial herbs and shrubs establish within one to five years as soil organic matter accumulates and nutrient cycling resumes. Pioneer tree species such as quaking aspen (Populus tremuloides) or tulip poplar (Liriodendron tulipifera) invade within a decade, shading out early herbaceous colonizers through competitive displacement.
Late-successional forest communities typically develop within 50 to 200 years, far faster than the 1,000-plus years required for primary succession on bare rock or glacial till.
Following the 1988 Yellowstone fires, which burned roughly 320,000 hectares, lodgepole pine seedlings germinated at densities exceeding 300,000 per hectare in some burned areas within two years, demonstrating how rapidly secondary succession can begin even after severe fire.
Succession must always begin on bare rock with no soil. After fires, floods, agricultural abandonment, or windstorms, recovery starts from existing soil containing seeds, fungal networks, and root fragments that accelerate recolonization far beyond what primary succession allows.
An abandoned agricultural field in the eastern United States typically supports crabgrass and pigweed within the first growing season, transitions to goldenrod and aster meadow within three to five years, and sees shrubs such as sumac and hawthorn establish within a decade. Closed-canopy forest with shade-tolerant species such as sugar maple (Acer saccharum) may not develop for 80 to 150 years after abandonment.
Species Diversity
/ SPEE-sheez dy-VER-sih-tee / · Latin species + diversitas (variety)
Species diversity describes both the number of species in an area and how evenly individuals are spread among those species.
A community has higher diversity when it has many species and no single species dominates too strongly. In ecosystems, species diversity affects how organisms interact with resources, habitats, and environmental change. Shannon diversity index quantifies this balance mathematically: a value above 3.0 indicates a highly diverse community, while values below 1.5 often signal degraded or disturbed ecosystems.
Tropical rainforests achieve the highest species diversity on land, with a single hectare of Amazonian forest containing up to 300 tree species compared with 10 to 20 in a temperate deciduous forest of equivalent area. Intermediate levels of disturbance, such as occasional fire or flooding, often maximize diversity by preventing competitive exclusion , a relationship documented across grasslands, rocky intertidal zones, and coral reefs.
The Amazon rainforest contains over 16,000 tree species in an area smaller than the continental United States. A single hectare in Ecuador can hold more tree species than exist in all of North America.
The site with the most individuals is always most diverse. A site dominated by one species may have low diversity even with many organisms.
A coral reef with many fish species and balanced abundances has high species diversity. A reef dominated by one fish species would be less diverse.
Species Richness
/ SPEE-sheez RICH-nes / · Latin species + Old English rice (wealth)
Species richness is the total count of different species living in a particular habitat or ecosystem, such as the 400 bird species found in the Amazon rainforest.
Species richness is a simple numerical count of all different species in a defined area, measured independently of their population sizes or frequencies. A pond containing 10 frog species has higher richness than one containing 3 species, regardless of whether one species dominates numerically. Richness varies predictably across environmental gradients, with tropical regions typically supporting 5 to 10 times more species per area than temperate regions.
Biodiversity metrics like the Shannon index incorporate both richness and evenness to provide more complete descriptions of community structure.
Species richness is simply the number of species in an area. It does not describe how common or rare each species is.
Rare Birds →Richness and diversity are always identical. Diversity often includes both richness and evenness, so two communities with the same number of species can have very different diversity values if one is dominated by a single abundant species.
A pond with ten frog species has higher frog species richness than a pond with three frog species, even if one species makes up 95 percent of all individuals in the richer pond. Panama's Barro Colorado Island, a 15.6-square-kilometer forest fragment created by dam flooding in 1914, has lost at least 60 bird species since isolation , a species richness decline of roughly 25 percent , driven by the loss of large frugivores and insectivores that require more territory than the island provides. Island biogeography theory predicts equilibrium richness from the balance of colonization and extinction rates, and Barro Colorado has tracked this prediction closely enough that it remains a benchmark dataset used in conservation planning worldwide.
Sustainability
/ suh-stayn-uh-BIL-ih-tee / · Latin sustinere (to hold up) + -ability
Sustainability is the responsible use of resources so present needs can be met without damaging the ability of future generations to meet their needs.
Sustainability requires harvesting renewable resources at rates matching regeneration and minimizing waste of non-renewable resources to preserve options for future generations. Sustainably managed fisheries apply harvest limits and seasonal closures that allow breeding stocks to replace catches, maintaining population stability over decades. Sustainable forestry removes less than net annual growth, preserving ecosystem function and timber supply.
Economic sustainability requires that resource extraction generates income sufficient to support communities without depleting natural capital.
The Brundtland Commission's 1987 definition of sustainable development , meeting present needs without compromising the ability of future generations to meet their own needs , has been cited in over 20,000 peer-reviewed articles and incorporated into the national legislation of more than 100 countries. Gro Harlem Brundtland, the Norwegian prime minister who chaired the commission, later estimated that implementing the report's recommendations would require redirecting approximately 2 percent of global GDP annually toward renewable energy and ecosystem restoration.
Sustainability means never using natural resources. It means using resources at rates and in ways that can continue over time.
A sustainably managed fishery sets catch limits at 50 to 60 percent of maximum sustainable yield, allowing fish populations to reproduce faster than they are removed. The North Sea cod stock collapsed in the 1990s when catches exceeded 90 percent of maximum sustainable yield for over a decade; rebuilding the population required a near-complete fishing moratorium between 2001 and 2006 and cost the industry an estimated 400 million euros in lost revenue. By contrast, the Alaskan pollock fishery, managed with annual stock assessments and catch limits set below maximum sustainable yield since 1976, remains the world's largest single-species fishery at roughly 1.5 million tonnes per year while maintaining a biomass above the target reference point.