Genetic rescue means deliberately adding individuals from another population to relieve inbreeding in a small, isolated one. The results can be rapid and large, which is why the method is used despite genuine uncertainties.
The problem it addresses
Isolated populations accumulate relatedness because every animal descends from the same few founders, and harmful recessive variants become common as a result.
Symptoms include poor sperm quality, small litters, skeletal abnormalities and heightened susceptibility to disease, all of which suppress growth even under good conditions.
Because these effects are genetic, habitat improvement alone cannot correct them. The population needs variation it no longer contains.
Why introduced individuals help so quickly
An immigrant carries different variants, so its offspring inherit one working copy of genes where the resident population carried two damaged ones.
The improvement appears in the first generation of hybrid offspring rather than gradually, which is why rescue attempts are often visible within a few years.
A small number of successful immigrants is sufficient. Once their descendants breed into the population, the introduced variation spreads without further intervention.
Choosing where the animals come from
Source populations are selected to be closely related enough that the two groups share an evolutionary history and an environment they are both adapted to.
Distant sources risk outbreeding depression, where offspring inherit combinations of traits suited to neither parental environment and perform worse than either.
Managers therefore prefer populations that were connected in the recent past and were separated by human action rather than by long geographic isolation.
What can go wrong
Introduced animals may fail to establish, being outcompeted, killed by residents or simply leaving, in which case the effort produces no genetic effect at all.
Disease transfer is a real hazard, since the source population may carry pathogens the recipients have never encountered, which is why quarantine and screening precede release.
There is also a conservation objection: mixing populations erases distinctiveness that some regard as worth preserving in itself, particularly where subspecies have been formally described.
Why it is not a permanent fix
Genetic rescue relieves the symptoms of isolation without removing the isolation, so relatedness begins accumulating again as soon as the intervention ends.
Populations that remain cut off therefore require repeated translocations indefinitely, effectively substituting management for the gene flow a connected landscape would supply.
This is the argument for treating corridors and connectivity as the underlying objective, with rescue as a holding measure while that work proceeds.