Population Trends in the Face of Climate Change

Rapid warming is associated with population decline among terrestrial birds and mammals globally (2018) Spooner et al., Global Change Biology DOI: 10.1111/gcb.14361

The Indian Pond Heron, one species which could face population declines as a result of climate change
The Indian Pond Heron, one species which could face population declines as a result of climate change (Image Credit: Dr Raju Kasambe CC A-S 4.0)

The Crux

The term climate change is almost ubiquitous these days. Humans tend to concentrate on how the warming of certain parts of the globe will affect them, but the species we share the globe with also experience a myriad of effects at the hands of climate change. These include rising temperatures constricting the ranges of some species and concurrently extending the range of others, who can move into areas that were previously too cold for them.

Whilst the focus of climate change has often been on species range shifts, the effects on species abundances are less well studied. This paper attempts to quantify the effects of climate change on a large number of bird and mammal species, whilst accounting for other factors which could affect species abundances, like rates of land use by humans, species body size, and whether or not the animals are in a protected area.

How It Works

The paper took 18,000 animal populations (defined here as a group of individuals of the same species living within a given area) from an online database and narrowed them down to find suitable study populations. They had to have been monitored at least 7 times over a period of at least 5 years, the location had to be known so that climate data could be obtained, body mass data had to be available, and they had to know whether the population was inside or outside a protected area.

Once they had narrowed their list down to 987 populations of 481 species at 441 study sites, they then tested to see which variables of the following correlated with the population trends they saw.

  • Change in climate
  • Change in surrounding land use
  • Body mass
  • Whether or not the population was located in a protected area

Did You Know: Body Size and Bergmann’s Rule

Body size is an incredibly useful characteristic when classifying species, and there are a number of body size trends worldwide. In mammals and birds, larger species often occur at higher latitudes. There are many theories as to why this occurs, and they all may be equally valid. Carl Bergmann put forward that it may be because these animals have to deal with colder temperatures, so a lower surface-area-to-volume ratio may be helpful. More recent theories suggest that as species richness lowers as you move away from the equator, competition with other organisms becomes less intense, and species must no longer carve out smaller niches to avoid competition. They can then generalise, and grow larger.

Whatever the case, larger animals generally have longer gestation periods, and fewer young. Unfortunately, this makes them even more vulnerable to population fluctuations, and gives them a higher risk of extinction, especially when their range is restricted.

What They Found Out

Climate change was the most important variable for both birds and mammals, with an increased climate having a negative effect on population abundance. Bird populations inside protected areas underwent less severe declines, whereas being in a protected area did not seem to matter to mammals. For mammals, there were other factors which increased their rates of population decline, with increased land use by humans being the most telling. There was also some suggestion that larger mammals were more vulnerable to population decline, though the uncertainty here was high.

Problems?

Most of the problems in this study come from the slightly lopsided nature of the data. There were very few sites in tropical areas, where increased human land use could be doing significant damage. There were also disproportionate amounts of bird populations in Europe and mammal populations in Africa that were used, and 84.6% of the mammal populations were found inside protected areas. Biases in data like these can often sway data or obfuscate results.

brazilian_tea
Some species, such as the Brazilian Teal, may find their population growth accelerated in some parts of the world by climate change (Image Credit: Olaf Oliviero Riemer, CC A-S 3.0)

So What?

Well, this section is a little depressing. Often the papers we present have a conclusion with some interesting applications for local regions. However here, we are dealing with a global phenomena, and one that can’t easily be halted. The conclusion is obvious – if climate change continues at similar rates, populations are likely to decline at higher rates. Apart from the usual steps that we need to take to reduce climate change, human land use will need to get smarter.

There is a large degree of variation in the population results that was left unexplained though. So there needs to be more research, taking into account other anthropogenic factors like pollution, human disturbance, and disease. These could help us isolate the species most at risk of severe declines in the face of climate change.

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