Animals have shrunk and humans may be responsible – 09/11/2023 – Science

It is as if a large part of the animals and plants on Earth had been hit by a miniaturizing ray: since the 1960s, thousands of species have shrunk in size, indicates a study carried out by an international team of researchers. The effect is not universal, but there is good reason to believe that it is linked to different impacts of human action, with difficult-to-predict consequences on the functioning of the planet’s ecosystems.

Details of the discovery are described in an article in the latest edition of the specialized journal Science, one of the most important in the world. The study, whose first author is Portuguese scientist Inês Martins, from the University of St. Andrews (Scotland), worked with information collected about 4,292 species, belonging to six different groups of living beings: fish, vegetables, invertebrates, mammals , herpetofauna (reptiles and amphibians) and benthic marine organisms (those that live on the ocean substrate, such as corals, starfish, etc.), distributed in different regions of the globe.

The best represented on the list are fish, with almost 2,000 species monitored. The periods studied also vary, with surveys spanning from 5 to almost 60 years. In addition to analyzing what happened to the average size of individuals of each species, Martins and his colleagues also paid attention to the effects on sets of species that live in the same habitat.

“We can look at the effect of the replacement of species present and the changes in the average size of the set of species because some disappeared or appeared over time”, explained the Portuguese researcher to Sheet.

This aspect is important because of the web of interactions between different species. In many remaining areas of the Atlantic forest, for example, there are practically no jaguars, the largest Brazilian carnivore, left. Without having to compete with the big cat, medium-sized predators can very well multiply and even get bigger with the abundance of resources.

This would be an indirect effect of human action on the size of certain species. There are also more direct effects. In the case of hunting and fishing, for example, we know that it is common to capture larger individuals, as these can serve as a more abundant source of meat or other resources (such as elephant ivory).

Human interventions like this tend to cause the average size of the hunted or fished species to decrease over time. And this is not just because individuals with a natural tendency to grow less have a greater chance of having offspring (which, in turn, will also tend to have a more modest size). There is also the fact that capture pressure ends up favoring individuals that are able to reproduce earlier, spending less time to grow before reaching reproductive maturity, which also reduces the average size of the population.

These and other factors, such as the impact of climate change, probably help explain the results of the survey carried out for the study in Science. Two thirds of the species sets analyzed revealed a decrease in the average size of individuals. In almost 60% of cases, there is also a general decrease in size within each species.

But situations in which “miniaturization” is explained by the change in the composition of groups of species are also common — that is, the largest species have disappeared and the more modest ones have remained, which produces a smaller average size for everyone. This would be the case of forests that lose their jaguars and are left with only ocelots and wild cats, for example.

The situation in fish is the clearest of all groups, while there is more variability in effects (size reduction, increase or stability) in other groups of organisms.

“Due to other work, we know that the selection forces that act on the body size of organisms vary in space and time”, explains the Portuguese researcher. “In the case of marine fish, these changes are often linked to selective exploitation of large individuals by humans, warming and/or decreasing resource availability. We are still researching the factors driving the change, but it is likely that a combination of these factors results in the high variability in changing trends and prevalence of size decreases that we observed in our study.”