What does it take to rescue a declining population?

11-03-2026

When a wildlife population begins to disappear, conservationists often face a difficult question: What can be done to stop it from going extinct? 

The answer may involve moving animals, restoring habitat, connecting isolated populations or helping a population adapt to a changing environment. But these approaches can work in different ways — and sometimes several can work at the same time. 

In a new review published in Trends in Ecology & Evolution, Purdue University researchers Mark Christie and Catherine Searle and Michigan State University researcher Sarah Fitzpatrick examine three ways a declining population can be “rescued”: demographic rescue, evolutionary rescue and genetic rescue.

Rather than treating these as three separate conservation strategies, the researchers argue that they should be understood as different biological processes toward the same goal: increasing a population’s growth or abundance enough to reduce its risk of extinction. The authors also emphasize that the three forms of rescue can overlap and occur simultaneously or one after another. 

Three paths to population rescue
Imagine a small population of animals living in an area where their numbers have been declining. The population could face challenges such as habitat loss, environmental change, inbreeding or a shortage of individuals available to reproduce.

One way to help would be to add more individuals to the population. This is known as demographic rescue. 

For example, conservationists might release individuals raised through a captive-breeding program or build a wildlife corridor that could allow individuals from another area to move into an isolated population. If those additional individuals survive and reproduce, the population can grow and become less vulnerable to extinction. 

A second possibility is evolutionary rescue. In this case, the environment changes in a way that threatens a population, but some individuals have genetic characteristics that help them survive under the new conditions. Over generations, those characteristics can become more common, allowing the population to adapt and continue to survive. 

The third pathway is genetic rescue, which occurs when translocation introduces new genetic variation into a population. This can be particularly important for small, isolated populations that have lost genetic diversity. New genetic variation can reduce some of the effects associated with inbreeding and provide additional variation for natural selection to act upon. 

Rescue is an outcome, not a single strategy
One of the central ideas in the review is that “rescue” should describe what happens to a population, rather than a particular action taken by people. 

For example, imagine conservationists move individuals from one population to another. 

At first, the action could produce demographic rescue simply by increasing the number of individuals. If the newcomers also introduce beneficial genetic variation, it could result in genetic rescue. Over time, that new genetic variation could help the population adapt to environmental conditions, contributing to evolutionary rescue. 

In other words, a single conservation action can potentially trigger multiple types of rescues.

The researchers say this interconnectedness is important because conservation decisions often involve trade-offs. An action that produces an immediate increase in population size may not necessarily provide the same long-term benefits as one that maintains or increases genetic diversity. Understanding how demographic, genetic and evolutionary processes interact can help managers consider both immediate population needs and the ability of a population to persist in the future.

Matching conservation strategies to the population
There is no single approach that will work for every declining population.

The researchers emphasize that conservation managers need to consider the condition of the population, how quickly it is declining, its genetic characteristics, environmental conditions and the potential benefits and risks of different interventions. 

That could mean adding individuals to a population, improving habitat connectivity so individuals can move naturally between populations, or considering assisted movement of individuals to introduce genetic variation. In other situations, allowing natural selection to act may be an important part of a population’s ability to persist as its environment changes. 

The goal is to understand which processes are most likely to help a particular population and how those processes might interact.

A framework for conservation managers
The researchers hope the review will provide a useful framework for wildlife and natural resource managers and other conservation practitioners as they develop strategies for declining populations.

“Rescue” does not necessarily mean choosing one of three options. Instead, conservationists can use an understanding of demographic, genetic and evolutionary processes to determine how different management actions could affect a population, both now and in the future. 

As species face increasingly rapid environmental change, that distinction could become more important. Helping a population survive today is one challenge. Giving it the genetic diversity and adaptive capacity to continue surviving tomorrow is another.

Understanding how those goals connect may help conservationists design strategies with a greater likelihood of long-term success. 

About the Department of Biological Sciences at Purdue University
The Department of Biological Sciences is the largest life sciences department at Purdue University. As part of Purdue One Health, we are dedicated to pioneering scientific discoveries and transformative education at the cutting edge of innovation. From molecules to cells, from tissues to organisms, from populations to ecosystems- we bring together multiple perspectives, integrating across biological scales to advance our understanding of life and tackle the world’s most pressing challenges. Learn more at bio.purdue.edu.

Written by: Alisha Willett, Senior Communications Specialist, amwillet@purdue.edu
Contributors: Cat Searle, Mark Christie