An approach to management of natural resources that emphasizes how little is known about the dynamics of ecosystems and that, as more is learned, management will evolve and improve. Adaptive management is a systematic strategy that seeks to improve resource assessment, allocation, conservation, and maintenance by learning from management outcomes. Natural systems are very complex and dynamic, and human observations about natural processes are fragmentary and often inaccurate. Furthermore, much of the variability that affects natural populations is unpredictable and beyond human control. This combination of ignorance and unpredictability means that the ways in which ecosystems respond to human interventions are unknown and can be described only in probabilistic terms. Nonetheless, management decisions need to be made. Adaptive management proceeds despite this uncertainty by treating human interventions in natural systems as large-scale experiments from which more may be learned, leading to improved management in the future (Fig. 1). See also: Conservation of resources; Ecosystem; Environment; Environmental engineering

A model for the functioning of an agricultural system, with all inputs and outputs. An ecosystem may be as small as a set of microbial interactions that take place on the surface of roots, or as large as the globe. An agroecosystem may be at the level of the individual plant–soil–microorganism system, at the level of crops or herds of domesticated animals (Fig. 1), at the level of farms or agricultural landscapes, or at the level of entire agricultural economies. See also: Agriculture; Ecosystem

A natural process in which harmful or beneficial effects are caused by secondary metabolites that spread from a donor organism to a recipient and are produced by plants, algae, bacteria, and fungi. Allelopathy is a key ecological process and has been studied predominantly in plants (Fig. 1). The chemical compounds involved in allelopathy are referred to as allelochemicals and comprise almost all classes of organic chemical substances. Hans Molisch (1937) coined the term allelopathy from the Greek words allelon for mutual and pathos for harm or affection, based on his observation of the premature ripening of apples and pears that were stored together with fruits from early ripening varieties. Depending on the purpose, the early ripening effect could be regarded as beneficial or harmful. Historically, detrimental effects have made botanists aware of allelopathy. One of the first accounts of an allelopathic effect that is commonly observed, namely the zone of growth inhibition around walnut trees (Juglans species), was reported by Pliny (23–79 CE). Even earlier, Theophrastus (372–287 BCE) described allelopathic effects of weeds on crop plants, including the inhibition of growth of alfalfa (Medicago sativa) by pigweed (Amaranthus retroflexus). See also: Agricultural science (plant); Agricultural soil and crop practices; Agriculture; Botany; Chemical ecology; Plant growth

Any of the plant growth forms characteristic of the upper reaches of mountain slopes. Alpine vegetation refers to the plants that grow in mountainous areas above the limit of subalpine trees and other subalpine vegetation. In such environments, trees undergo gradual changes. These changes, although subtle at first, may become dramatic beyond the dense forest as the zone of transition leads into the nonforested zone of the alpine tundra. In varying degrees, depending on the particular mountain setting, the forest is transformed from a closed-canopy forest to one of deformed and stunted trees interspersed with alpine tundra species. This zone of transition is referred to as the forest-alpine tundra ecotone (a transition zone or geographic boundary between two different groups of plant or animal distributions). The trees within the ecotone are stunted and often shrublike, and they do not have the symmetrical shape of most trees within the forest interior. The classic image is one of twisted, stunted, and struggling individual trees clinging to a windswept ridge (Fig. 1). See also: Altitudinal vegetation zone; Ecological community; Ecology; Ecotone; Forest; Life forms of plants; Mountain; Tree; Tundra

An intergrading region on a mountain slope characterized by specific plant life forms or species composition, and determined by complex environmental gradients. Along an altitudinal transect of a mountain, there are sequential changes in the physiognomy (growth form) of the plants and in the species composition of the communities (Fig. 1). This sequential zonation of mountain vegetation has been recognized for centuries. Vertical zonation was fully developed as an ecological concept by the work of Clinton Hart Merriam with the U.S. Biological Survey of 1889. He described a series of life zones—land areas having uniform climate, soil, and plant and animal communities—on the slopes of the San Francisco Peaks in northern Arizona, based on characteristic species of the flora and fauna. Other patterns of plant physiognomic and community zonation have now been cataloged in mountain ranges throughout the world. See also: Ecology; Life zones; Mountain; Terrestrial ecosystem

A method used by animals to conceal themselves from an enemy by appearing to be a section of the natural background. Animal camouflage is a strategy that animal organisms use to avoid or deflect the attacks of predators by misleading the latter's visual senses. With the use of camouflage, animals can blend in with their surroundings (Fig. 1). In general, visual camouflage is recognized as one of the commonest and most powerful forces in natural selection and is found in nearly every ecosystem on Earth; yet, curiously, it is one of the least-studied phenomena in biology. Camouflage patterns (resulting from pigmentation, which is the property of biological materials to impart coloration) have evolved in animals of every size and shape, including both invertebrates and vertebrates, and occur in aquatic and terrestrial habitats. Camouflage is even employed at night, which is a seemingly odd notion until it is realized that many nocturnal predators have specialized visual systems for seeing well under lighting conditions that humans cannot perceive. See also: Behavioral ecology; Physiological ecology (animal); Pigmentation; Predator-prey interactions; Protective coloration

A discipline within the field of animal behavior that focuses on the reception and use of signals. Animal communication could well include all of animal behavior because a liberal definition of the term signal could encompass all stimuli perceived by an animal. However, most research in animal communication deals only with those cases in which one member of a species generates a signal, defined as a structured stimulus, that subsequently influences the behavior of another member of the same species in a predictable way (intraspecific communication) [Fig. 1]. In this context, communication occurs in virtually all animal species, if only as a means by which a member of one sex finds its partner. In general, the field of animal communication includes an analysis of the physical characteristics of those signals believed to be responsible in any given case of information transfer. See also: Ethology; Instinctive behavior; Neuroethology; Physiological ecology (animal); Primate communication

An oxygen-depleted region in a marine environment. The dynamic steady state between oxygen supply and consumption determines the oceanic oxygen concentration. In regions where the rate of consumption equals or exceeds the rate of supply, seawater becomes devoid of oxygen and thus anoxic. In the open ocean, the only large regions that approach anoxic conditions are in the equatorial Pacific between 50 and 1000 m (165 and 3300 ft) depth and in the northern Arabian Sea and Bay of Bengal in the Indian Ocean between 100 and 1000 m (330 and 3300 ft) depth. The Pacific oxygen-depleted region consists of vast tongues extending from Central America and Peru nearly to the middle of the ocean in some places. In parts of this zone, oxygen concentrations become very low, 15 micromoles per liter (atmospheric saturation is 200–300 μmol/L). Pore waters of marine sediments are sometimes anoxic a short distance below the sediment–water interface. The degree of oxygen consumption in sediment pore waters depends upon the amount of organic matter reaching the sediments and the rate of bioturbation (mixing of the surface sediment by benthic animals). In shallow regions (continental shelf and slope), pore waters are anoxic immediately below the sediment-water interface; in relatively rapid sedimentation-rate areas of the deep sea, the pore waters are usually anoxic within a few centimeters of the interface; and in pore waters of slowly accumulating deep-sea sediments, oxygen may never become totally depleted. See also: Marine sediments

The watery belt surrounding the continent of Antarctica. Also called the Southern Ocean, the Antarctic Ocean includes the great polar embayments of the Weddell Sea and Ross Sea, the deep circumpolar belt of ocean between 50 and 60°S, and the southern fringes of the warmer oceans to the north. Its northern boundary is often taken as 30°S (see figure). As would be expected, the Antarctic is a cold ocean, covered by sea ice during the winter from Antarctica's coast northward to approximately 60°S. See also: Antarctica; Continent; Ocean

An ongoing current event in which a large number of living species are threatened with extinction or are going extinct because of environmentally destructive human activities. The Earth is presently in the midst of a mass extinction event. In the past, there have been five other similar large extinctions. Because of this, the current Anthropocene extinction is often referred to as the sixth extinction or the sixth great mass extinction; another term is the Holocene extinction because this event is taking place in the current Holocene Epoch, which covers the last 10,000 years of the Earth's history. However, as the present-day extinction event is anthropogenic (that is, related to or resulting from the influence of humans on nature), the favored term is Anthropocene extinction. This article presents an overview of the five previous large extinctions, the major human activities causing the Anthropocene extinction, and the trends in species extinction related to this present-day extinction event (see illustration). See also: Biodiversity; Endangered species; Extinction; Holocene; Mass extinctions; Paleobiodiversity; Paleoecology; Species concept