Deciding which species to conserve is hard enough, but deciding how many of a species is a viable goal is an entirely different matter (Image Credit: Frank Wouters, CC BY 2.0, Image Cropped)

When talking about species conservation, my concern is always around how many individuals should there be in a population of species. What should be our numeric goal in re-establishing a species? Should the endangered anoa become as many as their domestic relative, the water buffalo, of which there are at least 3 million individuals in Indonesia? How about the songbirds? Should each species be as abundant as the chicken?

The Earth has a carrying capacity for each species, although we are yet to see it for modern humans. A carrying capacity is essentially the number of species a habitat can maintain before species start dying due to a lack of resources. Humans aside, some species tend to be in a certain range of population size. 

In ecology, this is part of what is called the ‘energy pyramid’: the higher your trophic level, the less you are in terms of abundance because of the low efficiency of energy transfer between different trophic levels. Because of this, carnivores tend to stay low in numbers, with herbivore populations generally larger (but not more than plants). As such, small population size is an inevitability for some species. So how do we come up with a population goal for the tigers, the rhinos, the condors, the ferrets, and many more species in the brink of extinction?

Introducing MVP

There is a relatively old concept called ‘minimum viable population’ (MVP). As the phrase suggests, it is about determining the minimum number of individuals that needs to be alive at a given time for a species to persist in the long term. This concept is useful and sad at the same time as it reminds us of the least likeable reason for conservation: compromise. It allows policy-makers to have a clear deterministic conservation goal while simplifying the issue of population persistence. Mark L. Shaffer in his paper in 1981 proposed the definition to be “the smallest isolated population that has a 99% chance of surviving in the next 1,000 year”, but in practice people have been choosing different probability and period thresholds.

How do people count MVP? There are several things that you need to know about a species to count MVP and this comes from the original formal definition: “the number of individuals required to have a specified probability of persistence over a given period of time”. Let’s say we know that an anoa can give birth to one individual only after they are sexually reproductive (around three years old). For each available female, we can expect one birth every three years. If all is well, the existing individuals can give birth to young anoas as long as they live. Therefore, we need an estimate for life expectancy. But wait, how fertile are older anoas? And how likely is a young anoa to survive to three years old? How likely is female or male anoa to be killed by a predator any given week?

There is a lot of information to account for when we want to know about the likelihood of a populations’ persistence, and these are included in a statistical model with quite a lot of simplifying assumptions. The hunting rate is assumed to stay the same, even though predators or humans might vary hunting regimes. Likewise, usually we keep population growth the same. Fertility. Expectancy. The list of things that usually are assumed to stay the same across time in our models goes on except for the probability of persistence. Despite these difficulties, this type of modelling remains the easiest way to assign long-term conservation targets among practitioners. 

Based on that crash course, you can probably already see that MVP is species-specific, not generalisable, and also dependent on the kinds of environment the species inhabits. However, often we don’t have the necessary information for a  proper estimation of an MVP, so many conservationists have used the 50/500 rule. It’s an infamous magic number that is set as the general rule of species persistence: conserve at least 50 to avoid population crashes in the future because of inbreeding depression and at least 500 to allow some adaptive potential to persist in the long term.

Should We Keep The Magic Number?

You should already be able to see a lot of flaws with this magic number. It can overestimate the numbers of individuals that are needed for long lived species that are able to persist at small numbers for quite a long time. Also, this undermines the value of many small-sized populations with high potential to help conserve their relatives in another place. There are some cases of New Zealand birds that are able to persist in small population sizes because the bad mutations that are supposed to crush their populations to extinction if their population stays small had been purged in the past. In other words, the evolutionary history of species needs to be accounted for.

The 50/500 rules has been criticised by many since its first publication also, but this magic number magically stands the trials of science. In a paper as recent as 2020, I found that this rule is still discussed for the post-2020 Biodiversity framework as a threshold of preserving the genetic diversity of many species. There are some updates, such as the effective population size has to be more than 500 (forget the 50 part), or at least 10% of the census size population. Effective population size is the number of individuals that likely contribute genetic diversity to the following generation or “genetically unique” within the census population size. The concept of preserving genetic diversity is not really straightforward (see my post here) and we need some kind of useful generalisation for the policy makers. This is the principle of promoting behavioural change in conservation: If you want people to consider something new, you have to make it easy for them.

Easy does not necessarily mean right, as compromise is the least likeable approach to conservation. We wish to have all the resources to keep them without having to resort to the minimum number. There is an increasing awareness to abandon such triage mindsets nonetheless. With the pandemic going on, the discussion has arisen on how prioritisation is actually undesirable came to the conservation world. Prioritisation is considered a conservation fallacy because it allows extinction while zero extinction should be what conservation pursues. The idea is still limited to choosing species to consider, but I am quite optimistic that as this get discussed more widely and frequently, people will think more about the number of individuals within each species also. We actually have the resources; what is left is whether we have the will.

I need to know though, do readers agree with the concept of ‘minimum viable population’? Don’t we want the populations to flourish instead of persisting? Let me know your thoughts in the comments!

Sabhrina Gita Aninta is a conservation geneticist currently pursuing her PhD at Queen Mary University of London to understand how genome-wide variation of the endemic pigs and buffalos from Southeast Asia could assist their conservation. Follow her Twitter here for an update of her work, along with a mix of conservation, biodiversity, evolution, but mostly various rants and random stuffs in Indonesian and English. You can find more of her work at Ecology for the Masses at her profile.