Taking ‘Favourable Conservation Status’ out of the ‘too difficult’ box – the science bit

Favourable Conservation Status is a concept enshrined in international, European and national nature protection laws which aims to identify what good looks like for habitats and species, and therefore what conservationists should be aiming to achieve. Principal Conservation Scientist, Gilliant Gilbert discusses a paper published yesterday in Plos One where we present a whole new measure for quantifying favourable population size for breeding bird species.

How do you establish Favourable Conservation Status?

Attempting to define Favourable Conservation Status for habitats and species doesn’t seem, at first glance, to be either difficult or controversial. Instead it looks like an opportunity to set favourable benchmarks for species and habitats which will allow them to thrive into the future in the face of an as yet unknown threat.

The problem with setting these benchmarks within a legal framework is that the whole method must be robust and defendable, fit all species, and not place them at a future disadvantage (to learn more about the political implications of this paper, you can read our policy blog here).

Defining ‘favourable’ from a scientific and ecological point needs a set of parameters to outline what a healthy, robust and thriving population or habitat is. Certain parameters are already used regularly by organisations, such as population decline, range contraction, population size and localisation. As an example, the IUCN use these throughout their Red List of Threatened Species.

Enter – a new measuring stick

In a paper published yesterday in Plos One, we present a new measure for estimating a minimum threshold of favourable population size for breeding bird species. This measure builds on a current extinction prevention concept known as minimum viable population (MVP), the smallest possible number of individuals, forming a population which can survive without becoming extinct.

Calculating the MVP is done through modelling but doesn’t allow for the potential risk of that species continuing to decline into the future. In this paper, we suggest a new measure which multiples the MVP to allow for this. By using a Multiplier, combined with the MVP value, we can reasonably calculate the number of individuals needed to safeguard the species from extinction, even from unknown threats in the future, such as climate change. An important condition of using this minimum threshold measure is that it must be one of several measures used as part of a decision making process to decide on favourable conservation status.  

This Multiplier has been developed using real population declines data of UK bird species, as well as population decline estimates going back at least 200 years. In the example below, we use a generic Multiplier figure of 15.8 to calculate population size for Bearded tit Panurus biarmicus.

As we can see, the MVP needed to ensure a 90% chance of survival 100 years into the future would normally be 726 individuals, based on what we know/have estimated from previous declines. However, by including the Multiplier, we see that this should be 11,471 individuals and therefore immediately categorises this species as having an Unfavourable status and requiring continued conservation effort.

On the flip-side, if a species’ current population size is above its MVP Multiplier estimate, it passes this minimum threshold criteria, but this minimum threshold does not, on its own tell us what the Favourable Population size is that should be used as a conservation target. An estimate of favourable population size to be used as a target for conservation purposes must also take account of potential habitat and potential distribution, and a target measure which combines these parameters has not yet been robustly described.

For species, Favourable population size is the most difficult parameter to calculate, and determining how many is favourable within a legal framework is an uncomfortable responsibility. Our calculation may be useful when used as intended – with other measures such as favourable range or distribution, within a decision-making process.

The minimum threshold measure described by our new method provides a minimum population size required for UK breeding bird species to ensure that they are sufficiently large not only to survive, but also to withstand future shocks, be those the result of climate change, or other impacts as yet unforeseen.