It’s a Matter of Scale
Image Credit: Kevin Gill, CC BY 2.0, Image Cropped
No consistent effects of humans on animal genetic diversity worldwide (2020) Millette et al, Ecology Letters, https://doi.org/10.1111/ele.13394
As a species, we humans have had enormous negative effects on the planet, and we have talked about many of these issues and how they relate to ecology on many separate occasions here on Ecology for the Masses (see here, here, and here). A key implication of these human-induced changes to our planet are that many organisms are threatened with extinction, which can be bad for us as well (looking at you insect apocalypse).
Having said all of that, a lot of the work that has been done in this area has focused on specific groups (like the charismatic koala). By doing so, we run the risk of not understanding the global pattern but instead draw conclusions based off of local patterns. While we sometimes must make these kind of generalizations, this is not always a good idea. For example, we cannot look at the health of animal populations in New York City and make statements about the entirety of all of the animal populations in North America. To get around that issue, today’s authors investigated, on a global scale, if humans were having a global impact on animal genetic diversity.
What They Did
Analyzing anything at a global scale is a massive undertaking, and as such the authors tested many thousands of specimens from four groups of animals: birds, fish (inland and coastal), mammals, and insects. They wanted to specifically test two hypotheses, the first being that animal populations in areas with a large amount of human land use and high human population densities would have the lowest genetic diversity. Their second prediction was that this pattern would be the most apparent at smaller spatial scales, in other words if one local population was affected it would show lower genetic diversity than a group of multiple populations of that same species, which may be experiencing different levels of human impact.
To measure genetic diversity the authors used CO1 sequences (part of mitochondrial DNA that is passed from mothers to their offspring) collected from 1980 to 2016, all of which had associated location and time data.
Using those data allowed the authors to not only track how a given population changed over a period of 36 years, but they could also compare those data to another population of the same species in a different area.
Did You Know: Genetic Diversity
Genetic diversity is one way to measure the health of a population, and populations with higher diversity are healthier populations. Populations suffer when their genetic diversity falls below a certain point because it limits how much they can respond to a variety of stressors, such as disease and climate change. If a population has a high genetic diversity, chances are at least some individuals will be immune or resistant to a disease that kills everyone else.
When these events happen though, diversity is reduced because only the ones that survived are able to reproduce. These “bottleneck” events can result in a group of organisms that is now much more likely to show some deleterious effects of their low genetic diversity. Case in point, almost every single German Shepard has bad hips because of the genetic bottleneck brought on through human-controlled breeding.
What They Found
Surprisingly, there were no clear effects of human land use intensity on the population genetic diversity. Any effects on population genetic diversity were dependent on the animal group, and these effects were either statistically non-significant or weak. This lack of a clear pattern applied to both large and small spatial scales. The best predictor of genetic diversity among populations was actually the amount of geographic distance between populations, not the amount of human land use intensity.
Despite including data from aquatic animals that lived near land, the author’s excluded areas that were purely aquatic (i.e. further from land). While this makes sense within the realm of their hypothesis covering changes in land use affecting animal genetic diversity, it ignores how changes in land use affect aquatic environments. For example, here in Arkansas one very contentious issue is the hog farms and how the chemical runoff pollutes the local rivers and lakes.
Today’s paper is a great example of what science is all about: building off of what has been done before. By expanding upon what previous groups have done, this study was able to provide evidence for the nuance involved in understanding patterns in nature.
It’s also a great example of publishing “negative results”, or the lack of a pattern that was expected. Often, results like these where there aren’t clear patterns or the hypothesis that was expected isn’t supported aren’t as “sexy” as a big flashy result (think of the insect apocalypse study, people are more likely to talk about that than if there wasn’t a so-called apocalypse). That being said, there is no such thing as a negative result, and publishing papers like these help us avoid overestimating the true effects of what we are studying.