Ecology Terms Starting With I

Indicator species is a species whose presence, absence, abundance, or physiological condition reflects the quality or state of the environment it inhabits.

Indicator species provide measurable evidence of environmental conditions by responding predictably to specific pollutants, water quality changes, or habitat disturbance. Lichens decline in areas with elevated sulfur dioxide pollution because they absorb gases and particles through their entire surface and lack the protective cuticle that vascular plants use to limit uptake. Aquatic macroinvertebrates like mayfly nymphs (order Ephemeroptera) indicate good water quality because they cannot tolerate dissolved oxygen concentrations below about 4 milligrams per liter or elevated heavy metal concentrations.

Scientists use indicator species alongside chemical measurements and physical surveys because a single species responds to only a subset of possible stressors and provides limited information about ecosystem condition.

An introduced species lives outside its natural range. Humans moved it there. The move may be deliberate or accidental. Many introduced species never establish or cause harm. Some spread and damage native species, ecosystems, or economies. Those harmful ones are called invasive species. IUCN regional lists usually mark introduced populations Not Applicable.

People move species for farming, pets, sport, and pest control. Others travel accidentally in cargo, ballast water, or soil. Many introductions never form self-sustaining wild populations.

Those that do establish are described as naturalized. A smaller share spread widely and cause measurable harm. Introduced predators and competitors can push native species toward extinction.

Island wildlife is especially exposed to such arrivals. Early detection makes removal cheaper and more likely to succeed. Border biosecurity checks aim to stop new arrivals entirely.

Invasive species are non-native organisms that spread rapidly in a new environment and reduce the survival of native species by competing for resources, transmitting disease, or preying on them.

Natural enemies such as predators, parasites, and pathogens that kept a species in check in its home range are typically absent in the invaded region, allowing populations to grow unchecked. Cheatgrass (Bromus tectorum), introduced to the American West in the late 1800s, now dominates millions of hectares and alters fire regimes by producing dense, dry biomass that burns more frequently than native vegetation. Some invasive species reshape physical habitat rather than simply outcompeting natives; the common carp (Cyprinus carpio) uproots aquatic vegetation and increases water turbidity, degrading conditions for native fish and waterfowl.

Economic costs of invasive species in the United States alone exceed 120 billion dollars annually, according to estimates published by Pimentel and colleagues in 2005.

Island biogeography is the study of how island size and distance from a source of colonists determine the number of species that can persist on an island at equilibrium.

Robert MacArthur and E.O. Wilson formalized the theory in their 1967 book, proposing that species number on an island reflects a dynamic balance between immigration and extinction rates. Large islands support more species because they offer greater habitat diversity and larger population sizes that buffer against local extinction.

Distant islands receive fewer colonists because most dispersers cannot cross wide stretches of open water or inhospitable terrain. Wilson and his student Daniel Simberloff tested the theory experimentally in the late 1960s by fumigating small mangrove islands in Florida Bay and tracking recolonization, finding that species counts returned to predicted equilibrium levels within about two years.

The IUCN Red List ranks species by extinction risk. The International Union for Conservation of Nature runs it. Experts assess species against set criteria and assign categories. The nine categories run from Extinct to Not Evaluated. Criteria use population size, decline rate, and range size. Critically Endangered, Endangered, and Vulnerable count as threatened. The list began in 1964.

The full category scale has nine steps. They are Extinct, Extinct in the Wild, and Critically Endangered. Next come Endangered, Vulnerable, Near Threatened, and Least Concern.

Species lacking enough information are classed as Data Deficient. Species never assessed are labeled Not Evaluated. Short codes run from EX to NE.

Assessors apply five criteria, labeled A through E. Criterion B uses extent of occurrence and area of occupancy. Version 3.1 of the criteria has applied since 2001.