Alien species, invasive species: changing paradigms in the face of climate change?

Gatunki obce

Dynamic climate change should encourage a critical look at the environmental paradigms that were obvious until recently. This includes a rather rigid approach to how alien species are defined and the associated algorithms for dealing with their spread. According to the regulations, an alien species is one that has expanded its range as a result of human activity. Implicitly – it took place during the historical period.

From the point of view of a researcher-ecologist, such an accepted explanation is not useful. Its assumptions mean that assigning the label of invasive species to some taxon says nothing about its characteristics and possible role in the ecosystem in which it appeared. In addition, in most cases the assumptions made are unverifiable. The most important problem, however, is the danger of uncritically applying this definition to ecosystem management planning.

Alien species – non-functional definition

The definition of invasive species adds nothing to the functional approach to the organisms thus described. They can pose a real threat to local populations, their impact can be neutral or even significantly positive for the local biocenosis, if only by increasing its taxonomic and functional diversity and redundancy, thus providing greater stability and resilience of the system to disturbances.

Assigning the epithet “alien” to a species says nothing about whether it should be eliminated or protected. It is clear that its role in the new area must be watched closely, but this is true for any species expanding its range, including those considered native. In the case of the recent Oder disaster, it didn’t matter whether the so-called ” disaster” was a “disaster. The golden alga(Prymnesium parvum) was transferred to the river naturally – on the feathers of waterfowl, or artificially – in the ballast water of the Oder fleet. Verifying the information, whether it represented alien species or not (we will probably never know), was of no importance.

Unverifiability of definition assumptions

Difficulties in verifying the assumptions of the accepted definition describing alien species can arise for a great many reasons. First, the degree of human transformation of virtually all habitats means that any change in a species’ range can be attributed to some degree to anthropogenicity. Whether by removing or reshaping migration barriers, anthropogenic alteration of the migration routes of vector organisms, or possible direct entanglement of humans or their means of transportation. Accordingly, any new species in an area should, by definition, be treated as alien.

On the other hand, as the example of the Oder River disaster cited earlier shows, this has virtually no relevance to assessing the observed range expansion and its effects. Another problem with the definition under consideration is the contractual timing of settlement of new areas. It is worth noting that the vast majority of organisms treated as native in Poland did not occur in our area several thousand years ago, waiting for the next interglacial in the refugia. Looking back, we see the slow colonization of our country by numerous new species, species that are foreign at the time.

It is also worth noting that during this time Europe was also settled by humans, whose contribution to shifting the ranges of other species was probably important. And not just in the prehistoric period. In many cases, no one is able to determine to what extent their dispersion was entangled with at least the mass migrations of merchants along the amber route, documented since the 5th century. BC. It is likely that a sizable portion of the so-called “”B.C. native species have been imported by humans during the historical period.

Local classification of native species as invasive alien species

In the excellent description of the current definition of an alien species on the IOP PAN website, it is clearly stated that “even short-distance movements of individuals within the same country, to areas adjacent to the limits of the natural range, should be considered introductions.” This is logical, since national boundaries are arbitrarily drawn and do not coincide with either habitat or climate zone boundaries. An alien invasive species under this definition is perch, which is displacing trout from Bieszczady streams.

It can feed effectively in fast-flowing streams, but does not appear there in large numbers due to the lack of places to spawn effectively. Such places, studies show, are provided by man-made mountain dam reservoirs. The artificial lake enables the mass reproduction of perch, which migrate up the streams flowing into it, effectively competing for resources with the native brook trout population. It is worth mentioning that quite recently, during a conference discussion, a significant part of the assembled, recognized specialists in water protection, objected to the qualification of this example as an illustration of the action of an alien invasive species.

Naturalness of species range shifts and invasions

The last million years in the Northern Hemisphere have been characterized by the cyclical appearance of glacial periods and interglacials. Each time, this involved huge shifts in the ranges of species – their escape to interglacial refugia and the gradual repopulation of areas from which the glacier receded. It is clear that organisms differ significantly in their dispersion capabilities. The resting forms of some of them, such as plankton rowers, can travel hundreds of kilometers in a short period of time on bird feathers. Others, such as clams, are much less mobile.

It may have taken some as many as many thousands of years to reach the refugia to the habitats restored after the glacier receded. A good example is the bivalve mollusk Corbicula sp., which managed to return to the territory of present-day Poland during the Robbery and Lublin interglacials, which lasted several tens of thousands of years, and did not appear during the shortest, about 10,000 years long, Eemian interglacial.

According to many researchers, our ecosystems are still not “saturated” with species returning after the cessation of the last glaciation, so we are still in the phase of the natural return of species present with us during interglacial periods. More importantly, the emergence of Corbicula sp. in subsequent interglacials probably had the character of an invasion that died out after the biocenosis stabilized in the new structure.

The first appearance of the slender goby(Neogobius fluviatilis) in Lake Roś followed a similar pattern, locally dominating the littoral, displacing native demersal species such as goatfish and piscivores. After a few years, the grandmother’s dominance was suddenly broken. Perches appeared in large numbers in the littoral, which probably learned to hunt gobies successfully. This was accompanied by the return of native species. Thus, we observed a rather smooth transfer of the gobies, formally representing alien species, into the local trophic network from the Pontocaspian refugium, to which they retreated during the last glaciation.

Impact of climate change on species range shifts

The effect of Europe’s changing climate over the last million years is not only the flight of warm-loving species to the refugia of the ice ages, but also the migration of cold-loving species in the wake of the shifting tundra frontier to the north. The current, very dynamic climate change brings similar consequences. We are facing a natural disappearance of species in Poland, whose ranges are moving northward. The lack of an influx of thermophilic species from the south would mean a drastic reduction in biodiversity. Leaving aside the demand, raised by many scientists, that biodiversity should not be treated as a goal in environmental management (after all, in many habitats we are just protecting low biodiversity), a strong reduction in biodiversity would probably make our ecosystems much less stable and less resilient to expected climatic disturbances and anthropopressure.

In summary, we should consider accepting the complete reconstruction of our biocenoses, that is, both the local disappearance of cold-loving species and the influx of “foreign” warm-loving species. Which ones? Certainly those occurring in the last, exceptionally warm Eemian interglacial and returning from glacial refugia, so, for example, from the most controversial examples, the thermophilic cyanobacteria such as Raphidiopsis raciborskii or the aforementioned Corbicula bivalves. There is a lack of reliable work on this, but perhaps species moving into areas they inhabited in their past, which were once part of a similar trophic network, show less tendency to invade than by appearing in areas where they did not occur in recent interglacials.

An example is the variegated crayfish Dreissena polymorpha, which causes dangerous invasions in North America and, returning to Europe from the Pontocaspian refugium, has been recognized as an admittedly alien but non-invasive species, and in places even considered useful and locally introduced to effectively control phytoplankton biomass.

In order to better realize the need to accept the fundamental reconstruction of biocenoses associated with climate change, it is useful to look at climate predictions for areas of Poland. In the near future, from occurring in our country back in the 20th century. climate, defined according to Köppen’s categories as continental humid with mild summers, will move to subtropical humid climate. It has so far been found on a small portion of the Azure coast in Europe, in addition to large areas in the southern United States, southeastern China and eastern Australia, among others. It is easy to see that the nature of the flora and fauna of these areas, including the aquatic ones, is far from what we still observe in Poland.

In conclusion

The purpose of these brief reflections was by no means to call for a sudden change of paradigms in environmental protection, but only to try to signal the need for a “refreshing” discussion of them. A dynamically changing world requires a change of perspective on many fundamental issues, including those describing alien species. Most worrisome in this context is the tendency to unreflectively use recognized definitions and concepts, without awareness of the substantive content they carry. It is fully understood in an administrative environment that expects scientists to provide simple and unambiguous definitions and algorithms for proceeding. In the scientific community, however, it should not be right.

The author is a doctoral candidate, hydrobiologist, ecologist, graduate of the Faculty of Biology at the University of Warsaw. Scientifically, it deals with the effects of environmental factors on organisms and the consequences of these effects on the functioning of biological systems. In practice, he also deals with the protection and restoration of lakes and the protection of river ecosystems. He is head of the Department of Hydrobiology at the University of Warsaw, head of the Laboratory of Water Protection and Reclamation at the Center for Biological and Chemical Sciences at the University of Warsaw, chairman of the Warsaw Branch of the Polish Hydrobiological Society and member of the Climate Council at the UN Global Compact Network Poland.

In the article, I used, among other things. From the works:

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  2. Bayar A.S., Yılmaz M.T., Yücel İ., Dirmeyer P. 2023. CMIP6 Earth system models project greater acceleration of climate zone change due to stronger warming rates. Earth’s Future, 11, e2022EF002972.
  3. Bylak A., Kukuła K., Easton R. et al. Reservoirs facilitate colonization of river catchments by a native invasive fish through provision of pelagic larval rearing habitat. Biol. Invasions, 25, 1541-1559.
  4. Gomes C., Sousa R., Mendes T., Borges R., Vilares P., Vasconcelos V. et al. 2016. Low Genetic Diversity and High Invasion Success of Corbicula fluminea (Bivalvia, Corbiculidae) (Müller, 1774) in Portugal. PLoS ONE 11(7): e0158108.
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