The rescue effect is a phenomenon which was first described by Brown and Kodric-Brown,Brown JH, Kodric-Brown A. 1977 Turnover rates in insular biogeography: effect of immigration on extinction. Ecology 58, 445– 449. (doi:10.2307/ 1935620) and is commonly used in

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 ...

dynamics and many other disciplines in

ecology Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps wi ...

. This populational process explains how the migration of individuals can increase the persistence of small isolated populations by helping to stabilize a metapopulation, thus reducing the chances of extinction.Richards, C. M. (2000). Inbreeding depression and genetic rescue in a plant metapopulation. American Naturalist, 155, 383– 394.Eriksson A, Elı´as-Wolff F, Mehlig B, Manica A. 2014 The emergence of the rescue effect from explicit within- and between-patch dynamics in a metapopulation. Proc. R. Soc. B 281: 20133127. http://dx.doi.org/10.1098/rspb.2013.3127 In other words, immigration can lead to the recolonization of previously

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

patches, promoting the long-term persistence of the network of populations.

Inner mechanics

The rescue effect is remarkably important in areas where the persistence of a species is threatened because of the increasing rates of

habitat destruction Habitat destruction (also termed habitat loss and habitat reduction) is the process by which a natural habitat becomes incapable of supporting its native species. The organisms that previously inhabited the site are displaced or dead, thereby ...

and fragmentation. If the distributional extent of a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

is reduced because of its habitat destruction, the

migration Migration, migratory, or migrate may refer to: Human migration * Human migration, physical movement by humans from one region to another ** International migration, when peoples cross state boundaries and stay in the host state for some minimum le ...

rate will decrease producing a decline in the abundance of another

population Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...

(of the same species) whose patch has been unaltered. This is a clear example, of how the reduction or elimination of interpatch movement and consequently the lack of the rescue effect, is directly related with the abundance and patch occupancy of a species. The same consequences on the abundance and distribution can be observed when different landscapes or patches are unconnected because of the lack of wildlife corridors.Wu J. and Vankat J.L. 1995. Island biogeography: Theory and applications. In: Nierenberg W.A. (ed.), Encyclopedia of Environmental Biology. Academic Press, San Diego, California, USA, pp. 371–379. Also the opposite situation can be observed when the rate of immigration per patch increases as the proportion of patches that are occupied increases, resulting in a positive relation between local abundance and the number of occupied patches. Consequently, metapopulation dynamics and

landscape connectivity In landscape ecology, landscape connectivity is, broadly, "''the degree to which the landscape facilitates or impedes movement among resource patches''". Alternatively, connectivity may be a continuous property of the landscape and independent of p ...

for the persistence of populations in fragmented landscapes are important factors to take into account when managing endangered species.

Relations with other theories and phenomena

The rescue effect is a common influence on many island populations, even if it seems not obvious due to their apparent lack of connection with the mainland. Nevertheless, a common principle applicable to any island, states that: whenever immigration rates are sufficiently high relative to extinction rates, islands that are closer to sources of dispersing species will have higher immigration rates, and hence lower extinction and turnover rates than more isolated islands. On the other hand, if immigration and

colonization Colonization, or colonisation, constitutes large-scale population movements wherein migrants maintain strong links with their, or their ancestors', former country – by such links, gain advantage over other inhabitants of the territory. When ...

rates are low relative to extinction rates, meaning more individuals of a population die comparing to the newly arrived ones, the rescue effect is small. It can also be concluded that the insular isolation should be inversely related to the turnover rate of the population inhabiting the island. Therefore it can be concluded that recolonization by conspecifics is an important mechanism, enabling some species to persist on islands. This is particularly true in species that represent early stages in insular taxon cycles and are characterized by species-area curves of shallow slope.Ricklefs RE, Cox GW. 1972. Taxon cycles in the West Indian avifauna. The American Naturalist 106: 195–219.Scott Wilson, Ann E. McKellar, Matthew W. Reudink, Peter P. Marra and Laurene M. Ratcliffe, Density-dependent immigration promotes population stability in a long-distance migratory bird, Population Ecology, 10.1007/s10144-017-0582-5, 59, 2, (169-178), (2017). Also a common sense concept linked to the chances of an effective rescue effect on islands, is the size of the island itself. This is called the ‘target‐area effect’, and it states that:The larger the area of the island the more likely a colonization is, either of active or passive immigrants.

Rescue effect and dispersal movements

Another common principle relating both with the rescue effect and the previously commented insular biogeography is the dispersal capabilities of a species. The rescue effect will be increased by the tendency, for those species that are good dispersers and hence have high immigration rates.

Rescue effect and fitness

A reduction in the

fitness (biology) Fitness (often denoted w or ω in population genetics models) is the quantitative representation of individual reproductive success. It is also equal to the average contribution to the gene pool of the next generation, made by the same individua ...

of a population is a direct consequence of its low diversity which is dependent on the expression of deleterious

recessive alleles In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...

. In large populations,

natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charle ...

keeps such alleles at a low frequency. Since they usually occur in

heterozygotes Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...

, they are masked by a copy of “normal alleles”. But in small populations, due to the

inbreeding Inbreeding is the production of offspring from the mating or breeding of individuals or organisms that are closely related genetically. By analogy, the term is used in human reproduction, but more commonly refers to the genetic disorders and o ...

, this deleterious alleles can drift to high frequencies and become expressed in

homozygotes Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...

. This homozygous (inbred) individuals often have lower chances of surviving and

fecundity Fecundity is defined in two ways; in human demography, it is the potential for reproduction of a recorded population as opposed to a sole organism, while in population biology, it is considered similar to fertility, the natural capability to pr ...

; reduced fitness. The natural process of migration, acts as a rescue effect by counteracting the fixation of the deleterious alleles and increasing the number of heterozygotes by importing novel alleles from other populations. This means that immigrants make a positive contribution to fitness over and above the demographic effects of simply adding more individuals, by bringing novel

alleles An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...

to the population. This rescue effect is most likely to occur if the recipient population is small, isolated, and suffering from inbreeding depression.Macdonald, D. W., & Willis, K. J. (2013). Key Topics in Conservation Biology 2. Hoboken, NJ, Estados Unidos: Wiley.

Rescue effect and environmental fluctuations

The effectiveness of rescue effect reducing the extinction risk via dispersal also depends on the correlation of environmental fluctuations experienced by different populations. If the correlation is high, all populations decline simultaneously reducing recolonization rates of empty patches. This means that if an environmental phenomena diminishes the distributions or abundances of many populations over a big geographical area at the same time, the probabilities of a rescue effect are very low.

Positive consequences

Some of this previously explained concepts can be very valuable and applicable when managing populations. In addition to human mediated dispersal through reintroduction and translocation, the dispersal of individuals (consequently promoting the rescue effect) can be ensured by restoring and conserving the habitat lying between existing populations, sometimes called the landscape matrix (term often used in

landscape ecology Landscape ecology is the science of studying and improving relationships between ecological processes in the environment and particular ecosystems. This is done within a variety of landscape scales, development spatial patterns, and organizati ...

Negative consequences

Increased connectivity may also have anti rescue effects when the spread of disease, parasites or predators is favoured. Also, gene flow can sometimes reduce local adaptation. With the arrival of new individuals (immigrants) contributing with their new genetic variability the genetic differentiation of insular population can be delayed or prevented, not allowing the adaptation of the population to their new habitat. It has been found evidence supporting the idea that the evolution of genetically distinct insular populations represents an entry into the taxon cycle, leading to the extinction of the insular endemic populations.

References

{{reflist Population ecology