Ecological extinction is "the reduction of a species to such low abundance that, although it is still present in the community, it no longer interacts significantly with other species".[1]
Ecological extinction stands out because it is the interaction ecology of a species that is important for conservation work. They state that "unless the species interacts significantly with other species in the community (e.g. it is an important predator, competitor, symbiont, mutualist, or prey) its loss may result in little to no adjustment to the abundance and population structure of other species".[1]
This view stems from the neutral model of communities that assumes there is little to no interaction within species unless otherwise proven.
Estes, Duggins, and Rathburn (1989) recognize two other distinct types of extinction:
Global extinction is defined as "the ubiquitous disappearance of a species".[1]
Local extinction is characterized by "the disappearance of a species from part of its natural range".[1]
Paine first established the concept of a keystone species by studying the sea star.
Robert Paine (1969) first came up with the concept of a keystone species while studying the effects of the predatory sea starPisaster ochraceus, on the abundance of the herbivorous gastropod, Tegula funebralis. This study took place in the rocky intertidal habitat off the coast of Washington; Paine removed all Pisaster in 8m x 10m plots weekly while noting the response of Tegula for two years. He found that removing the top predator, in this case being Pisaster, reduced species number in the treatment plots. Paine defined the concept of a keystone species as a species that has a disproportionate effect on the community structure of an environment in relation to its total biomass. This keystone species effect forms the basis for the concept of ecological extinction.[2]
Examples
Sea otters maintain the overall biodiversity of the kelp forest community.
The potential role of the sea otter as the keystone predator in near-shore kelp forests was evaluated by Estes et al. in a 1978 study. They compared the Rat and Near islands in the Aleutian islands to test if "sea otter predation controls epibenthic invertebrate populations (specifically sea urchins), and in turn releases the vegetation association from intense grazing".[3] Estes and his colleagues found that different size structures and densities of sea urchins were correlated with the presence of sea otter populations, and because they are the principal prey of this keystone predator, the sea otters were most likely the main determinants of the differences in sea urchin populations. With high sea otter densities the herbivory of sea urchins in these kelp forest was severely limited, and this made competition between algal species the main determinant in survival. However, when sea otters were absent, herbivory of the sea urchins was greatly intensified to the point of decimation of the kelp forest community. This loss of heterogeneity serves as a loss of habitat for both fish and eagle populations that depend on the richly productive kelp forest environment. Historical over harvesting of sea otter furs has severely restricted their once wide-ranging habitat, and only today are scientists starting to see the implications of these local extinctions.[3]
The California spiny lobster, or Panulirus interruptus, is a keystone predator that has a distinct role in maintaining species diversity in its habitat. Robles (1987) demonstrated experimentally that the exclusion of spiny lobsters from the intertidal zone habitats led to the competitive dominance of mussels (Mytilus edulis and M. californianus). This results shows another example of how the ecological extinction of a keystone predator can reduce species diversity in an ecosystem. The threshold of ecological extinction has passed due to over fishing so that local extinctions of the California spiny lobster are common.[4]
Commercial oyster fishing had not affected the Chesapeake Bay ecosystem until mechanical dredges were utilized in the 1870s, resulting in overfishing of the oysters. The bay today is plagued by eutrophication due to algal blooms, and the resulting water is highly hypoxic. These algal blooms have competitively excluded any other species from surviving, including the rich diversity in faunal life that once flourished such as dolphins, manatees, river otters, sea turtles, alligators, sharks, and rays. This highlights the top-down loss of diversity commercial fishing has on marine ecosystems by removing the keystone species of the environment.[5]
Invasive species
The potential ecological extinction of guanacos (Lama guanicoe) and lesser rheas (Pterocnemia pennata) as a prey source for native omnivores and predators in the Argentine Patagonia was assessed by a study by Novaro in 2000. These native species are being replaced by introduced species such as the European rabbit, red deer, and domestic cattle; the cumulative damage from the increased herbivory by introduced species has also served to accelerate destruction of the already dwindling Argentine pampas and steppe habitats. This was the first study to take into account a large number of diverse predators, ranging from skunks to pumas, as well as conduct their survey in non-protected areas that represent the majority of southern South America. Novaro and his colleagues found that the entire assemblage of native carnivores relied primarily on introduced species as a prey base. They also suggested that the lesser rhea and guanaco had already passed their ecological effective density as a prey species, and thus were ecological extinct. It is possible that the niches of introduced species as herbivores too closely mirrored those of the natives, and thus competition was the primary cause of ecological extinction. The effect of introduction of new competitors, such as the red deer and rabbit, also served to alter the vegetation in the habitat, which could have further pronounced the intensity of competition. Guanacos and rheas have been classified as a low risk for global extinction, but this simplistic view of their demography does not take into account that they have already become functionally extinct in the Argentine Patagonia. Novaro and his colleagues suggest "this loss could have strong effects on plant-animal interactions, nutrient dynamics, and disturbance regimes..."[6] This is an example of how current conservation policy has failed to protect the intended species because of its lack of a functionally sound definition for extinction.[6]
Christian found Argentine ants to disrupt large seed dispersal mutualisms.
Seed dispersal mechanisms play a fundamental role in the regeneration and continuation of community structure, and a recent study by Christian (2001) demonstrated a shift in the composition of the plant community in the South African shrublands following an invasion by the Argentine ant(Linepithema humile). Ants disperse up to 30% of the flora in the shrublands and are vital to the survival of fynbos plants because they bury the large seeds away from the dangers of predation and fire damage. It is also crucial for seeds to be buried, because nearly all seed germination takes place in the first season after a fire. Argentine ants, a recent invader, do not disperse even small seeds. Christian tested whether the invasion of the Argentine ant differentially effected small and large-seeded fauna. He found that post-fire recruitment of large-seeded flora was reduced disproportionately for large seeds in sites already invaded by Argentine ants. These initial low large-seed density recruitments will eventually lead the domination of small-seeded fauna in invaded habitats. The consequences of this change in community structure highlight the struggle for dispersal of large-seeded flora that have potential reverberations around the world because ants are major ecological seed dispersers throughout the globe.[7]
Modeling ecological extinction
Below a certain density threshold, flying foxes are no longer effective seed dispersers.
The McConkey and Drake (2006) study is unique because it was one of the first attempts to model a density-dependent threshold relationship that described ecological extinction. They studied a seed dispersal interaction between flying foxes and trees with large seeds on the tropical Pacific Islands. Insular flying foxes (Pteropus tonganus), are considered to be keystone species because they are the only seed dispersers that can carry large seeds long distances. The host-pathogen model by Janzen and Connell suggests that survivorship of seeds in the tropics greatly increases the further away from the parent tree it lands, and that trees require this dispersal in order to avoid extinction. In the pathogen latent environment of the tropics, seed dispersal only becomes more paramount to species survival. As hypothesized, McConkey and Drake found a threshold relationship between the Flying Fox Index (FFI) and the median proportion of seeds carried over five meters. Below the threshold of abundance seed dispersal was insignificant and independent of flying fox abundance; however, above the threshold, dispersal positively correlated with increased flying fox abundance (as measured by the FFI). Although they did not directly prove the cause for this relationship, McConkey and Drake proposed a behavioral mechanism. Flying foxes are known to be territorial, and in the absence of competition a flying fox will eat within one tree, effectively dropping the seeds right below it. Alternatively, if there is a high density of flying foxes feeding at one time (abundance above the threshold density) then aggressive behavior, such as stealing fruit from another individual's territory, will lead to longer average seed dispersal. In this way the seed dispersing flying fox has a disproportional effect on the overall community structure in comparison to their relative biomass. Modeling the effect of ecological extinction on communities is the first step to applying this framework into conservation work.[8]
While ecologists are just starting to get a grapple on the significant interactions within an ecosystem, they must continue to find an effective density threshold that can maintain the level of equilibrium species diversity. Only with this knowledge of where and to what extent a specific species interacts with its environment will the proper and most efficient levels of conservation work take place. This work is especially important on the limited ecosystems of islands, where there are less likely to be replacement species for specific niches. With species diversity and available habitat decreasing rapidly worldwide, identifying the systems that are most crucial to the ecosystem will be the crux of conservation work.[8]
Climate change
Climate change has produced numerous shifts in the distributions and abundances of species. Thomas et al. (2004) went on to assess the extinction risk due to these shifts over a broad range of global habitats. Their predictive model using midline estimates for climate warming over the next 50 years suggests that 15–37% of species will be "committed to extinction" by 2050. Although the average global temperature has risen .6°C, individual populations and habitats will only respond to their local changes in climate.[9] Root et al. (2002) suggests that local changes in climate may account for density changes in regions, shifts in phenology (timing) of events, changes in morphology (biology) (such as body size), and shifts in genetic frequencies. They found that there have been an average phenological shift of 5.1 days earlier in the spring for a broad range of over a thousand compiled studies. This shift was also, as predicted, more pronounced in the upper latitudes that have concurrently had the largest shift in local average temperatures.[10]
While the loss of habitat, loss of pollinator mutualisms, and the effect of introduced species all have distinct pressures on native populations, these effects must be looked underneath a synergistic and not an independent framework. Climate change has the potential to exacerbate all of these processes. Nehring (1999) found a total of 16 non-indigenous thermophilic phytoplankton established in habitats northwards of their normal range in the North Sea. He likened these changes in range of more southerly phytoplankton to climatic shifts in ocean temperature. All of these effects have additive effects to the stress on populations within an environment, and with the additionally fragile and more complete definition of ecological extinction must be taken into account into preventative conservation measures.[11]
Implications for conservation policy
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Conservation policy has historically lagged behind current science all over the world, but at this critical juncture politicians must make the effort to catch up before massive extinctions occur on our planet. For example, the pinnacle of American conservation policy, the Endangered Species Act of 1973, fails to acknowledge any benefit for protecting highly interactive species that may help maintain overall species diversity. Policy must first assess whether the species in question is considered highly interactive by asking the questions "does the absence or loss of this species, either directly or indirectly, incur a loss of overall diversity, effect the reproduction or recruitment of other species, lead to a change in habitat structure, lead to a change in productivity or nutrient dynamics between ecosystems, change important ecological processes, or reduce the resilience of the ecosystem to disturbances?".[12] After these multitudes of questions are addressed to define an interactive species, an ecologically effective density threshold must be estimated in order to maintain this interaction ecology. This process holds many of the same variables contained within viable population estimates, and thus should not be difficult to incorporate into policy. To avoid mass extinction on a global scale unlike anyone has seen before, scientists must understand all of the mechanisms driving the process. It is now that the governments of the world must act in order to prevent this catastrophe of the loss of biodiversity from progressing further and wasting all of the time and money spent on previous conservation efforts.[12]
A keystone species is a species that has a disproportionately large effect on its natural environment relative to its abundance, a concept introduced in 1969 by the zoologist Robert T. Paine. Keystone species play a critical role in maintaining the structure of an ecological community, affecting many other organisms in an ecosystem and helping to determine the types and numbers of various other species in the community. Without keystone species, the ecosystem would be dramatically different or cease to exist altogether. Some keystone species, such as the wolf, are also apex predators.
This glossary of ecology is a list of definitions of terms and concepts in ecology and related fields. For more specific definitions from other glossaries related to ecology, see Glossary of biology, Glossary of evolutionary biology, and Glossary of environmental science.
Kelp forests are underwater areas with a high density of kelp, which covers a large part of the world's coastlines. Smaller areas of anchored kelp are called kelp beds. They are recognized as one of the most productive and dynamic ecosystems on Earth. Although algal kelp forest combined with coral reefs only cover 0.1% of Earth's total surface, they account for 0.9% of global primary productivity. Kelp forests occur worldwide throughout temperate and polar coastal oceans. In 2007, kelp forests were also discovered in tropical waters near Ecuador.
A metapopulation consists of a group of spatially separated populations of the same species which interact at some level. The term metapopulation was coined by Richard Levins in 1969 to describe a model of population dynamics of insect pests in agricultural fields, but the idea has been most broadly applied to species in naturally or artificially fragmented habitats. In Levins' own words, it consists of "a population of populations".
An apex predator, also known as a top predator, is a predator at the top of a food chain, without natural predators of its own.
An urchin barren is commonly defined as an urchin-dominated area with little or no kelp. Urchin grazing pressure on kelp is a direct and observable cause of a "barren" area. However, determining which factors contribute to shifting a kelp bed to an urchin barren is a complex problem and remains a matter of debate among scientists.
Seed predation, often referred to as granivory, is a type of plant-animal interaction in which granivores feed on the seeds of plants as a main or exclusive food source, in many cases leaving the seeds damaged and not viable. Granivores are found across many families of vertebrates as well as invertebrates ; thus, seed predation occurs in virtually all terrestrial ecosystems. Seed predation is commonly divided into two distinctive temporal categories, pre-dispersal and post-dispersal predation, which affect the fitness of the parental plant and the dispersed offspring, respectively. Mitigating pre- and post-dispersal predation may involve different strategies. To counter seed predation, plants have evolved both physical defenses and chemical defenses. However, as plants have evolved seed defenses, seed predators have adapted to plant defenses. Thus, many interesting examples of coevolution arise from this dynamic relationship.
Trophic cascades are powerful indirect interactions that can control entire ecosystems, occurring when a trophic level in a food web is suppressed. For example, a top-down cascade will occur if predators are effective enough in predation to reduce the abundance, or alter the behavior of their prey, thereby releasing the next lower trophic level from predation.
Ecological release refers to a population increase or population explosion that occurs when a species is freed from limiting factors in its environment. Sometimes this may occur when a plant or animal species is introduced, for example, to an island or to a new territory or environment other than its native habitat. When this happens, the new arrivals may find themselves suddenly free from the competitors, diseases, or predatory species, etc. in their previous environment, allowing their population numbers to increase beyond their previous limitations. Another common example of ecological release can occur if a disease or a competitor or a keystone species, such as a top predator, is removed from a community or ecosystem. Classical examples of this latter dynamics include population explosions of sea urchins in California's offshore kelp beds, for example, when human hunters began to kill too many sea otters, and/or sudden population explosions of jackrabbits if hunters or ranchers kill too many coyotes.
In ecology, a community is a group or association of populations of two or more different species occupying the same geographical area at the same time, also known as a biocoenosis, biotic community, biological community, ecological community, or life assemblage. The term community has a variety of uses. In its simplest form it refers to groups of organisms in a specific place or time, for example, "the fish community of Lake Ontario before industrialization".
An ecological cascade effect is a series of secondary extinctions that are triggered by the primary extinction of a key species in an ecosystem. Secondary extinctions are likely to occur when the threatened species are: dependent on a few specific food sources, mutualistic, or forced to coexist with an invasive species that is introduced to the ecosystem. Species introductions to a foreign ecosystem can often devastate entire communities, and even entire ecosystems. These exotic species monopolize the ecosystem's resources, and since they have no natural predators to decrease their growth, they are able to increase indefinitely. Olsen et al. showed that exotic species have caused lake and estuary ecosystems to go through cascade effects due to loss of algae, crayfish, mollusks, fish, amphibians, and birds. However, the principal cause of cascade effects is the loss of top predators as the key species. As a result of this loss, a dramatic increase of prey species occurs. The prey is then able to overexploit its own food resources, until the population numbers decrease in abundance, which can lead to extinction. When the prey's food resources disappear, they starve and may go extinct as well. If the prey species is herbivorous, then their initial release and exploitation of the plants may result in a loss of plant biodiversity in the area. If other organisms in the ecosystem also depend upon these plants as food resources, then these species may go extinct as well. An example of the cascade effect caused by the loss of a top predator is apparent in tropical forests. When hunters cause local extinctions of top predators, the predators' prey's population numbers increase, causing an overexploitation of a food resource and a cascade effect of species loss. Recent studies have been performed on approaches to mitigate extinction cascades in food-web networks.
Sea otter conservation began in the early 20th century, when the sea otter was nearly extinct due to large-scale commercial hunting. The sea otter was once abundant in a wide arc across the North Pacific ocean, from northern Japan to Alaska to Mexico. By 1911, hunting for the animal's luxurious fur had reduced the sea otter population to fewer than 2000 individuals in the most remote and inaccessible parts of its range. The IUCN lists the sea otter as an endangered species. Threats to sea otters include oil spills, and a major spill can rapidly kill thousands of the animals.
Defaunation is the global, local, or functional extinction of animal populations or species from ecological communities. The growth of the human population, combined with advances in harvesting technologies, has led to more intense and efficient exploitation of the environment. This has resulted in the depletion of large vertebrates from ecological communities, creating what has been termed "empty forest". Defaunation differs from extinction; it includes both the disappearance of species and declines in abundance. Defaunation effects were first implied at the Symposium of Plant-Animal Interactions at the University of Campinas, Brazil in 1988 in the context of Neotropical forests. Since then, the term has gained broader usage in conservation biology as a global phenomenon.
A mesocarnivore is an animal whose diet consists of 50–70% meat with the balance consisting of non-vertebrate foods which may include insects, fungi, fruits, other plant material and any food that is available to them. Mesocarnivores are from a large family group of mammalian carnivores and vary from small to medium sized, which are less than fifteen kilograms. Mesocarnivores are seen today among the Canidae, Viverridae (civets), Mustelidae, Procyonidae, Mephitidae (skunks), and Herpestidae. The red fox is also the most common of the mesocarnivores in Europe and has a high population density in the areas they reside.
Island ecology is the study of island organisms and their interactions with each other and the environment. Islands account for nearly 1/6 of earth’s total land area, yet the ecology of island ecosystems is vastly different from that of mainland communities. Their isolation and high availability of empty niches lead to increased speciation. As a result, island ecosystems comprise 30% of the world’s biodiversity hotspots, 50% of marine tropical diversity, and some of the most unusual and rare species. Many species still remain unknown.
Intraguild predation, or IGP, is the killing and sometimes eating of a potential competitor of a different species. This interaction represents a combination of predation and competition, because both species rely on the same prey resources and also benefit from preying upon one another. Intraguild predation is common in nature and can be asymmetrical, in which one species feeds upon the other, or symmetrical, in which both species prey upon each other. Because the dominant intraguild predator gains the dual benefits of feeding and eliminating a potential competitor, IGP interactions can have considerable effects on the structure of ecological communities.
Empty forest is a term coined by Kent H. Redford's article "The Empty Forest" (1992), which was published in BioScience. An "empty forest" refers to an ecosystem that is void of large mammals. Empty forests are characterized by an otherwise excellent habitat, and often have large, fully grown trees, although they lack large mammals as a result of human impact. Empty forests show that human impact can destroy an ecosystem from within as well as from without.
Salt marsh die-off is a term that has been used in the US and UK to describe the death of salt marsh cordgrass leading to subsequent degradation of habitat, specifically in the low marsh zones of salt marshes on the coasts of the Western Atlantic. Cordgrass normally anchors sediment in salt marshes; its loss leads to decreased substrate hardness, increased erosion, and collapse of creek banks into the water, ultimately resulting in decreased marsh health and productivity.
Mediterranean California is a Level I ecoregion of North America designated by the Commission for Environmental Cooperation (CEC) in its North American Environmental Atlas. The region is present only in California and Baja California.
The green world hypothesis, or HSS, proposes that predators are the primary regulators of ecosystems: they are the reason the world is 'green', by regulating the herbivores that would otherwise consume all the greenery.
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