Since quality is likely to vary among patches of habitat, it is important to consider how a low quality patch might affect a population. In this model, organisms occupy two patches of habitat. One patch, the source, is a high quality habitat that on average allows the population to increase. The second patch, the sink, is very low quality habitat that, on its own, would not be able to support a population. However, if the excess of individuals produced in the source frequently moves to the sink, the sink population can persist indefinitely. Organisms are generally assumed to be able to distinguish between high and low quality habitat, and to prefer high quality habitat. However, ecological trap theory describes the reasons why organisms may actually prefer sink patches over source patches. Finally, the source-sink model implies that some habitat patches may be more important to the long-term survival of the population, and considering the presence of source-sink dynamics will help inform conservation decisions.
Although the seeds of a source-sink model had been planted earlier, Pulliam is often recognized as the first to present a fully developed source-sink model. He defined source and sink patches in terms of their demographic parameters, or BIDE rates (birth, immigration, death, and emigration rates). In the source patch, birth rates were greater than death rates, causing the population to grow. The excess individuals were expected to leave the patch, so that emigration rates were greater than immigration rates. In other words, sources were a net exporter of individuals. In contrast, in a sink patch, death rates were greater than birth rates, resulting in a population decline toward extinction unless enough individuals emigrated from the source patch. Immigration rates were expected to be greater than emigration rates, so that sinks were a net importer of individuals. As a result, there would be a net flow of individuals from the source to the sink (see Table 1).
Pulliam's work was followed by many others who developed and tested the source-sink model. Watkinson and Sutherland presented a phenomenon in which high immigration rates could cause a patch to appear to be a sink by raising the patch's population above its carrying capacity (the number of individuals it can support). However, in the absence of immigration, the patches are able to support a smaller population. Since true sinks cannot support any population, the authors called these patches "pseudo-sinks". Definitively distinguishing between true sinks and pseudo-sinks requires cutting off immigration to the patch in question and determining whether the patch is still able to maintain a population. Thomas et al. were able to do just that, taking advantage of an unseasonable frost that killed off the host plants for a source population of Edith's checkerspot butterfly (Euphydryas editha). Without the host plants, the supply of immigrants to other nearby patches was cut off. Although these patches had appeared to be sinks, they did not become extinct without the constant supply of immigrants. They were capable of sustaining a smaller population, suggesting that they were in fact pseudo-sinks.
Watkinson and Sutherland's caution about identifying pseudo-sinks was followed by Dias, who argued that differentiating between sources and sinks themselves may be difficult. She asserted that a long-term study of the demographic parameters of the populations in each patch is necessary. Otherwise, temporary variations in those parameters, perhaps due to climate fluctuations or natural disasters, may result in a misclassification of the patches. For example, Johnson described periodic flooding of a river in Costa Rica which completely inundated patches of the host plant for a rolled-leaf beetle (Cephaloleia fenestrata). During the floods, these patches became sinks, but at other times they were no different from other patches. If researchers had not considered what happened during the floods, they would not have understood the full complexity of the system.
Dias also argued that an inversion between source and sink habitat is possible so that the sinks may actually become the sources. Because reproduction in source patches is much higher than in sink patches, natural selection is generally expected to favor adaptations to the source habitat. However, if the proportion of source to sink habitat changes so that sink habitat becomes much more available, organisms may begin to adapt to it instead. Once adapted, the sink may become a source habitat. This is believed to have occurred for the blue tit (Parus caeruleus) 7500 years ago as forest composition on Corsica changed, but few modern examples are known. Boughton described a source—pseudo-sink inversion in butterfly populations of E. editha. Following the frost, the butterflies had difficulty recolonizing the former source patches. Boughton found that the host plants in the former sources senesced much earlier than in the former pseudo-sink patches. As a result, immigrants regularly arrived too late to successfully reproduce. He found that the former pseudo-sinks had become sources, and the former sources had become true sinks.
One of the most recent additions to the source-sink literature is by Tittler et al., who examined wood thrush (Hylocichla mustelina) survey data for evidence of source and sink populations on a large scale. The authors reasoned that emigrants from sources would likely be the juveniles produced in one year dispersing to reproduce in sinks in the next year, producing a one-year time lag between population changes in the source and in the sink. Using data from the Breeding Bird Survey, an annual survey of North American birds, they looked for relationships between survey sites showing such a one-year time lag. They found several pairs of sites showing significant relationships 60–80 km apart. Several appeared to be sources to more than one sink, and several sinks appeared to receive individuals from more than one source. In addition, some sites appeared to be a sink to one site and a source to another (see Figure 1). The authors concluded that source-sink dynamics may occur on continental scales.
One of the more confusing issues involves identifying sources and sinks in the field. Runge et al. point out that in general researchers need to estimate per capita reproduction, probability of survival, and probability of emigration to differentiate source and sink habitats. If emigration is ignored, then individuals that emigrate may be treated as mortalities, thus causing sources to be classified as sinks. This issue is important if the source-sink concept is viewed in terms of habitat quality (as it is in Table 1) because classifying high-quality habitat as low-quality may lead to mistakes in ecological management. Runge et al. showed how to integrate the theory of source-sink dynamics with population projection matrices and ecological statistics in order to differentiate sources and sinks.
(high quality habitat)
|Stable or growing
|Stable or growing
|Stable or growing|
Avoided (or equal)
or trap patch
(low quality habitat)
|Declines to extinction
|Declines to stable size
|Declines to extinction|
Attractive (or equal)
Why would individuals ever leave high quality source habitat for a low quality sink habitat? This question is central to source-sink theory. Ultimately, it depends on the organisms and the way they move and distribute themselves between habitat patches. For example, plants disperse passively, relying on other agents such as wind or water currents to move seeds to another patch. Passive dispersal can result in source-sink dynamics whenever the seeds land in a patch that cannot support the plant's growth or reproduction. Winds may continually deposit seeds there, maintaining a population even though the plants themselves do not successfully reproduce. Another good example for this case are soil protists. Soil protists also disperse passively, relying mainly on wind to colonize other sites. As a result, source-sink dynamics can arise simply because external agents dispersed protist propagules (e.g., cysts, spores), forcing individuals to grow in a poor habitat.
In contrast, many organisms that disperse actively should have no reason to remain in a sink patch, provided the organisms are able to recognize it as a poor quality patch (see discussion of ecological traps). The reasoning behind this argument is that organisms are often expected to behave according to the "ideal free distribution", which describes a population in which individuals distribute themselves evenly among habitat patches according to how many individuals the patch can support. When there are patches of varying quality available, the ideal free distribution predicts a pattern of "balanced dispersal". In this model, when the preferred habitat patch becomes crowded enough that the average fitness (survival rate or reproductive success) of the individuals in the patch drops below the average fitness in a second, lower quality patch, individuals are expected to move to the second patch. However, as soon as the second patch becomes sufficiently crowded, individuals are expected to move back to the first patch. Eventually, the patches should become balanced so that the average fitness of the individuals in each patch and the rates of dispersal between the two patches are even. In this balanced dispersal model, the probability of leaving a patch is inversely proportional to the carrying capacity of the patch. In this case, individuals should not remain in sink habitat for very long, where the carrying capacity is zero and the probability of leaving is therefore very high.
An alternative to the ideal free distribution and balanced dispersal models is when fitness can vary among potential breeding sites within habitat patches and individuals must select the best available site. This alternative has been called the "ideal preemptive distribution", because a breeding site can be preempted if it has already been occupied. For example, the dominant, older individuals in a population may occupy all of the best territories in the source so that the next best territory available may be in the sink. As the subordinate, younger individuals age, they may be able to take over territories in the source, but new subordinate juveniles from the source will have to move to the sink. Pulliam argued that such a pattern of dispersal can maintain a large sink population indefinitely. Furthermore, if good breeding sites in the source are rare and poor breeding sites in the sink are common, it is even possible that the majority of the population resides in the sink.
The source-sink model of population dynamics has made contributions to many areas in ecology. For example, a species' niche was originally described as the environmental factors required by a species to carry out its life history, and a species was expected to be found only in areas that met these niche requirements. This concept of a niche was later termed the "fundamental niche", and described as all of the places a species could successfully occupy. In contrast, the "realized niche", was described as all of the places a species actually did occupy, and was expected to be less than the extent of the fundamental niche as a result of competition with other species. However, the source-sink model demonstrated that the majority of a population could occupy a sink which, by definition, did not meet the niche requirements of the species, and was therefore outside the fundamental niche (see Figure 2). In this case, the realized niche was actually larger than the fundamental niche, and ideas about how to define a species' niche had to change.
Source–sink dynamics has also been incorporated into studies of metapopulations, a group of populations residing in patches of habitat . Though some patches may go extinct, the regional persistence of the metapopulation depends on the ability of patches to be re-colonized. As long as there are source patches present for successful reproduction, sink patches may allow the total number of individuals in the metapopulation to grow beyond what the source could support, providing a reserve of individuals available for re-colonization. Source–sink dynamics also has implications for studies of the coexistence of species within habitat patches. Because a patch that is a source for one species may be a sink for another, coexistence may actually depend on immigration from a second patch rather than the interactions between the two species. Similarly, source-sink dynamics may influence the regional coexistence and demographics of species within a metacommunity, a group of communities connected by the dispersal of potentially interacting species. Finally, the source-sink model has greatly influenced ecological trap theory, a model in which organisms prefer sink habitat over source habitat.
Land managers and conservationists have become increasingly interested in preserving and restoring high quality habitat, particularly where rare, threatened, or endangered species are concerned. As a result, it is important to understand how to identify or create high quality habitat, and how populations respond to habitat loss or change. Because a large proportion of a species' population could exist in sink habitat, conservation efforts may misinterpret the species' habitat requirements. Similarly, without considering the presence of a trap, conservationists might mistakenly preserve trap habitat under the assumption that an organism's preferred habitat was also good quality habitat. Simultaneously, source habitat may be ignored or even destroyed if only a small proportion of the population resides there. Degradation or destruction of the source habitat will, in turn, impact the sink or trap populations, potentially over large distances. Finally, efforts to restore degraded habitat may unintentionally create an ecological trap by giving a site the appearance of quality habitat, but which has not yet developed all of the functional elements necessary for an organism's survival and reproduction. For an already threatened species, such mistakes might result in a rapid population decline toward extinction.
In considering where to place reserves, protecting source habitat is often assumed to be the goal, although if the cause of a sink is human activity, simply designating an area as a reserve has the potential to convert current sink patches to source patches (e.g. no-take zones). Either way, determining which areas are sources or sinks for any one species may be very difficult, and an area that is a source for one species may be unimportant to others. Finally, areas that are sources or sinks currently may not be in the future as habitats are continually altered by human activity or climate change. Few areas can be expected to be universal sources, or universal sinks. While the presence of source, sink, or trap patches must be considered for short-term population survival, especially for very small populations, long-term survival may depend on the creation of networks of reserves that incorporate a variety of habitats and allow populations to interact.
Bacterivores are free-living, generally heterotrophic organisms, exclusively microscopic, which obtain energy and nutrients primarily or entirely from the consumption of bacteria. Many species of amoeba are bacterivores, as well as other types of protozoans. Commonly, all species of bacteria will be prey, but spores of some species, such as Clostridium perfringens, will never be prey, because of their cellular attributes.Copiotroph
A copiotroph is an organism found in environments rich in nutrients, particularly carbon. They are the opposite to oligotrophs, which survive in much lower carbon concentrations.
Copiotrophic organisms tend to grow in high organic substrate conditions. For example, copiotrophic organisms grow in Sewage lagoons. They grow in organic substrate conditions up to 100x higher than oligotrophs.Cross-boundary subsidy
Cross-boundary subsidies are caused by organisms or materials that cross or traverse habitat patch boundaries, subsidizing the resident populations. The transferred organisms and materials may provide additional predators, prey, or nutrients to resident species, which can affect community and food web structure. Cross-boundary subsidies of materials and organisms occur in landscapes composed of different habitat patch types, and so depend on characteristics of those patches and on the boundaries in between them. Human alteration of the landscape, primarily through fragmentation, has the potential to alter important cross-boundary subsidies to increasingly isolated habitat patches. Understanding how processes that occur outside of habitat patches can affect populations within them may be important to habitat management.Decomposer
Decomposers are organisms that break down dead or decaying organisms, and in doing so, they carry out the natural process of decomposition. Like herbivores and predators, decomposers are heterotrophic, meaning that they use organic substrates to get their energy, carbon and nutrients for growth and development. While the terms decomposer and detritivore are often interchangeably used, detritivores must ingest and digest dead matter via internal processes while decomposers can directly absorb nutrients through chemical and biological processes hence breaking down matter without ingesting it. Thus, invertebrates such as earthworms, woodlice, and sea cucumbers are technically detritivores, not decomposers, since they must ingest nutrients and are unable to absorb them externally.Dominance (ecology)
Ecological dominance is the degree to which a taxon is more numerous than its competitors in an ecological community, or makes up more of the biomass.
Most ecological communities are defined by their dominant species.
In many examples of wet woodland in western Europe, the dominant tree is alder (Alnus glutinosa).
In temperate bogs, the dominant vegetation is usually species of Sphagnum moss.
Tidal swamps in the tropics are usually dominated by species of mangrove (Rhizophoraceae)
Some sea floor communities are dominated by brittle stars.
Exposed rocky shorelines are dominated by sessile organisms such as barnacles and limpets.Ecological trap
Ecological traps are scenarios in which rapid environmental change leads organisms to prefer to settle in poor-quality habitats.
The concept stems from the idea that organisms that are actively selecting habitat must rely on environmental cues to help them identify high-quality habitat. If either the habitat quality or the cue changes so that one does not reliably indicate the other, organisms may be lured into poor-quality habitat.Feeding frenzy
In ecology, a feeding frenzy occurs when predators are overwhelmed by the amount of prey available. For example, a large school of fish can cause nearby sharks, such as the lemon shark, to enter into a feeding frenzy. This can cause the sharks to go wild, biting anything that moves, including each other or anything else within biting range. Another functional explanation for feeding frenzy is competition amongst predators. This term is most often used when referring to sharks or piranhas. It has also been used as a term within journalism.Landscape genetics
Landscape genetics is the scientific discipline that combines population genetics and landscape ecology. It broadly encompasses any study that analyses plant or animal population genetic data in conjunction with data on the landscape features and matrix quality where the sampled population lives. This allows for the analysis of microevolutionary processes affecting the species in light of landscape spatial patterns, providing a more realistic view of how populations interact with their environments. Landscape genetics attempts to determine which landscape features are barriers to dispersal and gene flow, how human-induced landscape changes affect the evolution of populations, the source-sink dynamics of a given population, and how diseases or invasive species spread across landscapes.Landscape genetics differs from the fields of biogeography and phylogeography by providing information at finer temporal and spatial scales (i.e., at the level of individual genetic variation within a population). Because it focuses on sampling individuals, landscape genetics has the advantage of not having to subjectively define discrete populations prior to analysis. Genetic tools are used to detect abrupt genetic differences between individuals within a population and statistical tools are used to correlate these genetic discontinuities with landscape and environmental features. The results of landscape genetics studies have potentially important applications to conservation biology and land management practices.Lithoautotroph
A lithoautotroph or chemolithoautotroph is a microbe which derives energy from reduced compounds of mineral origin. Lithoautotrophs are a type of lithotrophs with autotrophic metabolic pathways. Lithoautotrophs are exclusively microbes; macrofauna do not possess the capability to use mineral sources of energy. Most lithoautotrophs belong to the domain Bacteria, while some belong to the domain Archaea. For lithoautotrophic bacteria, only inorganic molecules can be used as energy sources. The term "Lithotroph" is from Greek lithos (λίθος) meaning "rock" and trōphos (τροφοσ) meaning "consumer"; literally, it may be read "eaters of rock". Many lithoautotrophs are extremophiles, but this is not universally so.
Lithoautotrophs are extremely specific in using their energy source. Thus, despite the diversity in using inorganic molecules in order to obtain energy that lithoautotrophs exhibit as a group, one particular lithoautotroph would use only one type of inorganic molecule to get its energy.Mesotrophic soil
Mesotrophic soils are soils with a moderate inherent fertility. An indicator of soil fertility is its base status, which is expressed as a ratio relating the major nutrient cations (calcium, magnesium, potassium and sodium) found there to the soil's clay percentage. This is commonly expressed in hundredths of a mole of cations per kilogram of clay, i.e. cmol (+) kg−1 clay.Metacommunity
An ecological metacommunity is a set of interacting communities which are linked by the dispersal of multiple, potentially interacting species. The term is derived from the field of community ecology, which is primarily concerned with patterns of species distribution, abundance and interactions. Metacommunity ecology combines the importance of local factors (environmental conditions, competition, predation) and regional factors (dispersal of individuals, immigration, emigration) to explain patterns of species distributions that happen in different spatial scales.
There are four theoretical frameworks, or unifying themes, that each detail specific mechanistic processes useful for predicting empirical community patterns. These are the patch dynamics, species sorting, source–sink dynamics (or mass effect) and neutral model frameworks. Patch dynamics models describe species composition among multiple, identical patches, such as islands. In this framework, species are able to persist on patches through tradeoffs in colonization ability and competitive ability, where less competitive species can disperse to unoccupied patches faster than they go extinct in others. Species sorting models describe variation in abundance and composition within the metacommunity due to individual species responses to environmental heterogeneity, such that certain local conditions may favor certain species and not others. Under this perspective, species are able to persist in patches with suitable environmental conditions resulting in a strong correlation between local species composition and the environment. This model represents the classical theories of the niche-centric era of G. Evelyn Hutchinson and Robert MacArthur. Source-sink models describe a framework in which dispersal and environmental heterogeneity interact to determine local and regional abundance and composition. This framework is derived from the metapopulation ecology term describing source–sink dynamics at the population level. High levels of dispersal among habitat patches allows populations to be maintained in environments that are normally outside the species environmental range. Finally, the neutral perspective describes a framework where species are essentially equivalent in their competitive and dispersal abilities, and local and regional composition and abundance is determined primarily by stochastic demographic processes and dispersal limitation. The neutral perspective was recently popularized by Stephen Hubbell following his groundbreaking work on the unified neutral theory of biodiversity.Metapopulation
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".A metapopulation is generally considered to consist of several distinct populations together with areas of suitable habitat which are currently unoccupied. In classical metapopulation theory, each population cycles in relative independence of the other populations and eventually goes extinct as a consequence of demographic stochasticity (fluctuations in population size due to random demographic events); the smaller the population, the more chances of inbreeding depression and prone to extinction.
Although individual populations have finite life-spans, the metapopulation as a whole is often stable because immigrants from one population (which may, for example, be experiencing a population boom) are likely to re-colonize habitat which has been left open by the extinction of another population. They may also emigrate to a small population and rescue that population from extinction (called the rescue effect). Such a rescue effect may occur because declining populations leave niche opportunities open to the "rescuers".
The development of metapopulation theory, in conjunction with the development of source-sink dynamics, emphasised the importance of connectivity between seemingly isolated populations. Although no single population may be able to guarantee the long-term survival of a given species, the combined effect of many populations may be able to do this.
Metapopulation theory was first developed for terrestrial ecosystems, and subsequently applied to the marine realm. In fisheries science, the term "sub-population" is equivalent to the metapopulation science term "local population". Most marine examples are provided by relatively sedentary species occupying discrete patches of habitat, with both local recruitment and recruitment from other local populations in the larger metapopulation. Kritzer & Sale have argued against strict application of the metapopulation definitional criteria that extinction risks to local populations must be non-negligible.Finnish biologist Ilkka Hanski of the University of Helsinki was an important contributor to metapopulation theory.Mycotroph
A mycotroph is a plant that gets all or part of its carbon, water, or nutrient supply through symbiotic association with fungi. The term can refer to plants that engage in either of two distinct symbioses with fungi:
Many mycotrophs have a mutualistic association with fungi in any of several forms of mycorrhiza. The majority of plant species are mycotrophic in this sense. Examples include Burmanniaceae.
Some mycotrophs are parasitic upon fungi in an association known as myco-heterotrophy.Organotroph
An organotroph is an organism that obtains hydrogen or electrons from organic substrates. This term is used in microbiology to classify and describe organisms based on how they obtain electrons for their respiration processes. Some organotrophs such as animals and many bacteria, are also heterotrophs. Organotrophs can be either anaerobic or aerobic.
Antonym: Lithotroph, Adjective: Organotrophic.Perceptual trap
A perceptual trap is an ecological scenario in which environmental change, typically anthropogenic, leads an organism to avoid an otherwise high-quality habitat. The concept is related to that of an ecological trap, in which environmental change causes preference towards a low-quality habitat.Planktivore
A planktivore is an aquatic organism that feeds on planktonic food, including zooplankton and phytoplankton.Recruitment (biology)
In biology, especially marine biology, recruitment occurs when a juvenile organism joins a population, whether by birth or immigration, usually at a stage whereby the organisms are settled and able to be detected by an observer.There are two types of recruitment: closed and open.In the study of fisheries, recruitment is "the number of fish surviving to enter the fishery or to some life history stage such as settlement or maturity".Relative abundance distribution
In the field of ecology, the relative abundance distribution (RAD) or species abundance distribution describes the relationship between the number of species observed in a field study as a function of their observed abundance. The graphs obtained in this manner are typically fitted to a Zipf–Mandelbrot law, the exponent of which serves as an index of biodiversity in the ecosystem under study.Species homogeneity
In ecology, species homogeneity is a lack of biodiversity. Species richness is the fundamental unit in which to assess the homogeneity of an environment. Therefore, any reduction in species richness, especially endemic species, could be argued as advocating the production of a homogenous environment.