Global warming could disrupt freshwater food chains

An increase in temperature in water bodies can lead to a remarkable change in the life cycle of parasites, scientists at the Institute of Ecology and Evolution, named after the VI ONE, have discovered. Severtsov (IPEE) RAS. Due to global warming, it is possible to change the “host-parasite” chains, which will lead to a restructuring of the entire appearance of freshwater ecosystems.

In recent years, there has been a growing interest among scientists in how temperature affects the risk of transmitting infections to ecosystems or, in other words, the interaction of parasites and their hosts. To find out, scientists at the Institute of Economics and Ecology of the Russian Academy of Sciences studied how temperature affects the separation of the dispersal stages (nymphs) of two closely related species of parasitic flatworms (trematodes): Rhipidocotyle fenica campanula. The first intermediate host to these species of tremors is the toothless mollusk (Anodonta anatina), the second cockroach. And only the final owners – predatory fish – differ.

To conduct the experiment, the scientists collected mollusks from rivers and distributed them in flow tanks with three temperature regimes. The average temperature in these modes was 18 ° C (range 7-24 ° C), 15 ° C (7-20 ° C) and 13 ° C (6-18 ° C). The design of the experiment did not receive a constant mollusk temperature. Instead, it had to fluctuate according to seasonal changes, such as in natural ecosystems. This is reminiscent of the real world in terms of global warming.

The experiment lasted five months. Every two weeks, the researchers tested whether the mollusks secreted parasitic larvae (cercariae) and, if so, how many.

“We were interested in how temperature affects the duration of cercariae emission by mollusks, the time of onset and completion, the total number of isolated cercariae and other indicators. It turned out that these closely related species of tremors are very different in their temperature preferences. Cercariae of R. campanula began to be released at 9-10 ° C and a further increase in temperature only moderately increased their mollusk production. At the same time, the percentage of molluscs secreting cercariae was approximately the same in all temperature regimes. R. fennica proved to be much more thermophilic. The optimum temperature for isolating the stands for this podium was over 15 ° C and the dependence of the number of stands produced on the temperature was much stronger. In addition, the percentage of mollusks producing cercariae of this parasite increased with temperature, “says Mikhail Gopko, one of the study’s authors.

The scientists concluded that in typical summer latitude northern latitude temperatures, many people with R. fennica do not have time to accumulate enough scoring days to continue their life cycle. In this case, it turns out that an increase in average annual temperatures can yield far more benefits to R. fennica than to R. campanula. At high temperatures, the beginning and end of the cercariae production season shift closer to the beginning of the season. At similar temperatures, molluscs produced fewer cercariae in autumn than in winter.

“In other words, temperature is temperature and the ‘fatigue’ of the host organism imposes natural limits on the parasite’s ability to force its reproduction,” adds Ekaterina Mironova, co-author of the article.

The scientists concluded that due to global warming, the balance in the system R. fennica – roach – R. campanula may shift to the predominance of a more thermophilic type of parasite. Thus, an increase in temperature in water bodies can lead to a noticeable change in the life cycle of the parasite. Ultimately, given the incredible abundance of host-pest systems, this has the potential to change the whole face of freshwater ecosystems.

In their next study, the scientists plan to investigate how these changes will be synchronized with changes in the life cycle of the next parasite host, the fish.

What will happen to the changes in the “parasite-host” systems? Answered by Mikhail Gopko

There are at least two realistic scenarios for developing relationships between parasites and their hosts under global warming. According to the first of these, as the temperature rises, the parasites will gain an advantage over their hosts. Pests develop at a faster rate, which is important in changing conditions, and also have higher productivity at moderately high ambient temperatures. For aquatic organisms, such as fish, the situation is further complicated by the fact that at high temperatures their immune system begins to fail, which facilitates the entry of infections into the host organism.

It is possible that the benefits of overheating pests will only be temporary – most experiments on the effects of heating on pests are short-lived. In addition, parasites can even benefit from de-synchronizing the life cycles of the parasite and the host. Let’s say that heating will lead to the fact that the parasites will be ready to infect the host earlier in the season, but no one can guarantee that the host will already be “ready” for, say, migrating to a suitable part of the tank.

The second scenario can lead to very drastic changes in ecosystems. The fact is that parasites play an unusually important and specific role in communities. This is a type of glue that combines different food levels. When a cheetah rushes across the savannah, chasing Thompson’s gazelle or a greedy gull trying to snatch a child from the surface of the water, the false impression can be created that what is happening is the work of a predator and its prey. . In fact, parasites that weaken each side sometimes play an even more important role. De-synchronizing the life cycles of parasites and their hosts can lead to a “domino effect”, disrupting the food interactions between predators and their prey.

A new study by scientists supports the second view. However, the possibility of applying its effects to all “parasite-host” systems is ruled out. In addition, there is no information on how quickly changes in ecosystems will occur and whether nature will be able to adapt to them. As Mikhail Gopko notes, the impact of global warming on host parasite systems and ecosystems will generally be stronger as we approach the poles and the equator, but at temperate latitudes, things are unlikely to be so fatal.

The article Opposite temperature responses to the seasonal timing of cercariae elimination by Rhipidocotyle trematodes was published in the journal Parasitology.

A, B, cercariae, R. fennica and R. campanula, respectively. C, R. fennica cercariae that develops in the gonads of molluscs. Authors of the photo: A, B – Telervo Valtonen, C – Yoni Taskinen.

Dependence of the mean (for all molluscs producing cercariae on the day of observation) on the number of cercariae produced by temperature for R. fennica (A, B) and R. campanula (C, D). On the right, partial regression diagrams (variable diagram added) taking into account seasonality. Note the extreme points in the “under belly” of graph A. This is the production of cercariae during the autumn months, when the emission volume was lower at the same temperatures. The addition of a seasonality factor (the number of days from the start of the experiment) “corrects” the dependence and the points fit very well on the regression line (graph C). For R. campanula (bottom row), the situation is similar.


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