The global decrease in the diversity and density of wild fauna, which is a very well documented phenomenon, is one of the main challenges of nature protection. The loss of diversity and population decline is observed in virtually all taxonomic groups of vertebrates. A decrease in numbers is often preceded by a change in the population structure (e.g. sex proportions) and individual changes related to survival and reproduction. 

“That is why it is so important to identify patterns related to the change process. This helps to predict the future dynamics of the entire population,” said one of the study’s authors Stanisław Bury from the Institute of Zoology and Biomedical Research of the Jagiellonian University, an activist of the Herpetological Society NATRIX. He conducted research with Bartłomiej Zając from the Institute of Environmental Sciences of the Jagiellonian University.

Bury and Zając have been conducting observations of Polish snakes for years. This time, together with their colleagues, they decided to check whether in the last few decades there had been changes in the grass snake (Natrix natrix) population around Kraków that could indicate that something disturbing was happening to it. They described their findings inEnvironmental Science and Pollution Research (https://doi.org/10.1007/s11356-021-16128-y).

Scientists focused on the body size of snakes and sex proportions in the population. The comparative material for their results was Włodzimierz Juszczyk's book Płazy i gady krajowe('Domestic amphibians and reptiles') published in the 1970s.

Bury said: “We watched snakes in little transformed habitats in Kraków and the Niepołomice Forest, because that was where the very well-known herpetologist Włodzimierz Juszczyk collected his data. For a long time, his book was considered the bible of Polish amphibians and reptiles. The author included a huge amount of data, although - unfortunately for us - he presented them in a quite general manner, ignoring important information and a number of quantitative data. This was a challenge we had to face.”

Researchers started by checking if and how the body size of the snakes had changed. Bury said: “This was of interest to us because we know that size decreases in many different species of animals under the influence of human presence in the environment.

“The reasons for this phenomenon vary. For example, in fish it can be the result of overfishing, because it is easier to catch large individuals, so they suffer the fastest decline. In the case of many other animals, including snakes, large individuals are an easier prey; they are simply easier to spot, capture and kill. As we all know, people do not like snakes, so they often suffer this fate. It is the same with mammals: large individuals are an easier target for poachers.”

Another factor that can affect the gradual decrease of body size of a given species is climate warming. “Small body size allows to get rid of excess heat from the body easier and faster. It can be beneficial in changing environmental conditions,” said Bury.

He added that there can be many reasons for a potential decrease in body size. And while in the case of many other organisms this problem is now better researched and explained, in the case of reptiles there are almost no reports on this issue.

Bury said: “This was our main motivation: to try to determine if something has happened to Kraków snakes over the years. Our results show that something has actually changed.”

The analysis of the results showed that over the years there has been a clear decrease in body size of grass snakes and this effect affected both sexes. In addition, researchers noticed that the current share of both sexes does not deviate from 1:1, while 40 years ago females significantly outnumbered males. This means that the number of females in the population has decreased over the years.

Bury continued: “The observed changes probably result from several factors, the most important of which are the following two.

“Firstly, road traffic increased significantly in the examined area, and the human presence increased. These are factors with a known negative impact on the survival of snakes, especially larger ones, because they are an easier target for people. Females that reach larger body size were probably more often victims of eradication and because of this their numbers decreased. 

“Secondly, as demonstrated by previous studies, the population of amphibians, which are the main food of grass snakes, has decreased significantly. And the food deficiency disrupts growth and over time can cause a decrease of the final size achieved by organisms.”

Both of these factors affect females and males. The results of another study (https://academic.oup.com/cz/article/66/2/217/5520939?login=false) show, however, that females can react more strongly than males. In this case, the researchers focused on sites in strongly urbanized areas in Kraków. They included Zakrzówek (famous for the occurrence of the smooth snake, which is unusual considering that this place is located only a few kilometres from the centre of a large city) and the Libana quarry (famous for having been a location for Schindler's List). 

Zając said: “Besides, it is also a very interesting natural area, and one surrounded by busy streets, so one can expect that the city's pressure strongly affects it.”

In these two places, the researchers observed that the sizes of female body were comparable to the sizes of male body, while at sites with a lesser extent of transformation (the Niepołomice Forest and the riparian woodland in Przegorzały near Kraków) - as befits the grass snake - females had larger bodies than males. Why was the change of female sizes greater in urban areas? Are they more susceptible to the impact of the urban environment and why?

Zając said: “When we compared the population of grass snakes from the Niepołomice Forest with those from Kraków, it turned out that among the city snakes there were no large females at all. And the grass snake is a species with a very clear sexual dimorphism. Under normal conditions, females and males differ in the size of the body, with the former being much larger and, sometimes exceeding one meter in length. We have indeed found such females in the Niepołomice Forest, I see them in the Bieszczady Mountains and other large forest complexes. But there were no such specimens in the locations surrounded by street traffic. This was very clear.”

According to the study authors, this can be attributed to a higher reproductive cost of females. Limited access to resources can reduce survival after laying eggs, increased mobility associated with the search for appropriate places to lay eggs further increases mortality on the roads, and females must put a lot more energy (compared to males) to the production of their gametes and their development after fertilization.

Zając said: “Generally, producing offspring is a much higher cost for females. In fact, it is entirely transferred to this sex, so the energy demand of females is much greater. As a result, females are more sensitive to a decrease in resources in the environment.

“The fact that there are no large females in densely built-up Kraków may also be due to the fact that in the city they may not be able to reach a more advanced age, and therefore full size, because as larger individuals they are more often killed by humans.”

There is also a third factor that affects an overall decrease in the size of urban grass snakes over the years: the changing climatic conditions. It has been repeatedly confirmed that higher temperatures are conducive to lower body size. In addition, a hotter and longer summer may translate into an even lower food availability, because during the day of the amphibians hide from the heat in hard to reach places, remaining out of the reach of snakes.

The researchers said: “Water bodies dry up and amphibians disappear with them. And even if they are still present in the environment, their activity is heavily reduced during drought, and most of the time they remain deeply hidden, beyond the reach of snakes. Since there is less food, there is a lack of resources for tissue building and growth. In this situation, it is not about the inability to reach a more advanced age, but about the lack of building material that allows to achieve full body size.

“We have reached two conclusions: in the past 40 years, grass snakes have become smaller, and urban pressure in the city seems particularly unfavourable for females.”

The question arises whether the grass snake population is generally in decline? “Since Juszczyk's data were not sufficiently precise (we do not know over how many years he collected his snakes or exactly in what area he conducted research), we cannot strictly determine changes in the size of the entire population,” the researchers said, adding: “But we have our assumptions of course and they are as follows: the grass snake population has almost certainly decreased over the past 40 years'. (PAP)

author: Katarzyna Czechowicz

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