A growing population is not always a healthy one.

When trying to save an endangered species, the most obvious method would be to increase the population size. However, what if I told you increasing population size wasn’t enough?

It's all about genetics. A small population suffers from small numbers of individuals but it also suffers from lower genetic diversity. Increasing the numbers of individuals in a small population would evidently increase the population size, but all those individuals would retain similar genetic material from the initial population. 

This loss of genetic diversity can increase risk of inbreeding depression, a phenomenon that occurs when close relatives. Starting from the basics of genetics, you have dominant and recessive alleles which describe how they are expressed rather than whether they are beneficial or harmful. When an individual inherits a dominant and a recessive allele for a particular trait, the effect of the dominant allele can mask that of the recessive one. This means that potentially harmful recessive alleles can remain hidden within a population when only one copy is inherited. When closely related individuals reproduce, their offspring have a greater chance of inheriting two copies of the same harmful recessive allele, allowing its effects to be expressed.

This becomes particularly concerning in small populations, where individuals are more likely to share similar genetics and have fewer unrelated mates available. Now, one may ask, ‘how do you prevent this from happening?’. Well, it depends entirely on the context. How inbreeding is managed differs depending on whether one is dealing with captive animals or wild animals. 

Inbreeding vs Outbreeding diagram. Source

In captive populations, animal populations can be more easily controlled by humans. Zoos and conservation programmes allow for close examination of genetics, usually through pedigrees and databases, to examine the relatedness between individuals and how genetic diversity is represented across the population. Using these resources helps zoologists and conservationists select breeding pairs that minimise relatedness and ensure that individuals whose genetics are underrepresented within the population have an opportunity to reproduce.

Managing wild populations is more complicated. The main difficulty is maintaining genetic diversity while interfering as little as possible with natural populations. That’s why, instead of handling breeding pairs directly, conservationists focus on maintaining gene flow between populations. Habitat fragmentation is a concern that indirectly creates smaller gene pools by splitting populations into smaller groups. As stated already, conservationists can mitigate the effects of habitat fragmentation by creating wildlife corridors to reconnect split populations. Wildlife corridors allow individuals to move between previously isolated populations, creating opportunities for breeding and allowing genes to flow between the populations. In cases where human interference is needed, conservationists may directly relocate animals between populations with the aim of increasing genetic diversity.

However, preventing closely related animals from mating is only part of the picture. A population may look like it's growing, but that doesn’t mean every individual of the population is contributing equally to the next generation. Therefore, counting the number of individuals in a population doesn’t necessarily mean the population is genetically healthy.

This ties back to the big picture, a growing population is not always a healthy population. The total size of a population is known as the census population size (N), for example, the census population size of 500 cats is 500 cats. Additionally, conservationists have denoted another term known as effective population size (N_e) which is a measure that reflects how effectively a population is passing genetic diversity from one generation to the next. Unlike census population size, it takes into account factors such as how many individuals successfully reproduce and how evenly they contribute offspring. This gives conservationists greater insight into the genetic health of a population than census numbers alone. Using the example of 500 cats, imagine that only a small fraction successfully reproduce. Although the census population size remains 500, its effective population size could be considerably smaller because relatively few individuals are contributing genes to future generations. 

Census population size (N) vs Effective population size (N_e). Source

Why does effective population size matter? A population with a smaller effective population size means genetic diversity can be lost more quickly than if the effective population size were to be larger. Therefore, conservationists cannot rely entirely on the population size as an indicator of genetic health.

Learning about effective population size changed the way I understood conservation. Initially, I thought saving a species meant simply increasing its population size. However, after my first year in Animal Care, I now understand that while population size plays a crucial role in conserving a species, it is not the only factor that determines the health of a population. Conserving species is also about preserving the genetic diversity that gives future generations the best chance of remaining healthy and adaptable.