Ecology Terms Starting With I
Ecology Glossary: I
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Indicator Species
/ in-dih-KAY-ter SPEE-sheez / · Latin indicare (to indicate) + species
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.
The Index of Biotic Integrity, developed by fisheries biologist James Karr in 1981, formalized the use of fish communities as indicators of river health by scoring 12 biological attributes simultaneously. This approach revealed degradation that chemical water-quality tests alone had missed, and versions of the index are now used by environmental agencies across North America and Europe.
One indicator species tells the entire story of an ecosystem's health. Ecologists use multiple species indicators because each responds to different environmental stressors, and no single species captures the full range of conditions that determine ecosystem condition.
Lichens are widely used as indicators of air quality in urban and industrial areas. Mapping surveys in cities such as London and Los Angeles have shown that lichen species richness drops sharply within 2 to 5 kilometers of major emission sources, with pollution-sensitive foliose and fruticose species disappearing first while crustose species persist at higher pollution levels.
Invasive Species
/ in-VAY-siv SPEE-sheez / · Latin invadere (to invade) + species
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.
The brown tree snake (Boiga irregularis), accidentally transported to Guam after World War II, drove nine of the island's twelve native forest bird species to extinction within decades. Guam now spends more than one million dollars per year on snake control, including dropping acetaminophen-laced dead mice from helicopters to poison snakes near forest edges.
Every non-native species is invasive. A species is invasive only if it spreads beyond introduction sites and causes measurable ecological, economic, or public health damage.
Zebra mussels (Dreissena polymorpha) invaded the Great Lakes in the late 1980s, likely arriving in ballast water from European ships. A single adult filters up to one liter of water per day, and dense colonies of up to 700,000 individuals per square meter strip phytoplankton from the water column, collapsing food webs that native fish and invertebrates depend on.
Island Biogeography
/ EYE-land by-oh-jee-OG-ruh-fee / · Old Norse ey (island) + Greek bios + geo + graphein
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 theory of island biogeography directly shaped modern conservation biology by providing a quantitative framework for designing nature reserves. Jared Diamond applied MacArthur and Wilson's principles in the 1970s to argue that large, connected reserves retain more species than small, fragmented ones, a debate that still drives reserve design today.
Island biogeography applies only to ocean islands. Mountaintops, desert springs, forest fragments, and even rooftop gardens function as habitat islands, and ecologists use the same equilibrium model to predict species richness in all of them.
The Galapagos Islands vary from about 14 square kilometers (Genovesa) to roughly 4,600 square kilometers (Isabela), and this size difference correlates with the number of resident land bird species on each island. Darwin's finches show the clearest pattern, with larger islands supporting more species and more morphological diversity among beak shapes adapted to local food resources.