The problem of assessing the viability of invasive species in the conditions of the steppe zone of Ukraine

This article proposes a completely new method of resolving the pressing global environmental problem of assessing the capacity of invasive organisms to adapt to new environmental conditions. A new three-step approach to the evaluation of vital and ecological functions of invasive species is recommended. In addition to classic species and population surveys, it was proposed to apply a stress-resistance biochemical assessment of invasive species. Stress resistance can be regarded as the main response of living organisms to changes in continuous environmental components. The obtained results will provide an opportunity to give a qualitative prognisis of what adaptive capacity an invasive species has, what precise ecological functions and for what time period it can perform in a new ecosystem.


Introduction
According to FAO and IUCN estimations, there is an annually increase in the number of animal species which move into new environments, atypical for them, due to direct or indirect human activity. A number of these animals after adaptation begin to compete with native species, invading the stable environmental features of different ecosystems. The result of such penetration can often have irreversible environmental consequences leading to significant biological impairment in the living activities of entire ecosystems, resulting in significant economic waste in various economic sectors. In European countries, a list of 435 quarantine species was drawn up, which have different danger statuses, both environmental and economic, because they annually cause direct economic losses. Nowadays, according to specialist research, the number of potentially invasive species that are able to penetrate into the territory of Ukraine is estimated as 1500 species. Abnormalities in the functioning of the natural ecosystem caused by the influence of invasive species can also bring direct and indirect risk to human health. At the the early XXI century, the challenge of risk assessment on penetration by invasive species and control of existing species is a matter of national security for of every contemporary state.
Research has found that invasive plant species are able to cause significant changes in the soil environment, which is evident in the reduction of pH levels in the soil solution, changes in C/N ratio, increase of N content (Lazzaro et al., 2004). Along with this, soil characteristics, particularly conditions of contemporary and past land use, are the determining factors of rooting of newer plant species (Csecserits et al., 2016). Changes in environmental conditions in consequence of infestation lead to decrease in biodiversity and biomass productivity of native species, as well as reduction of their toleration (Ruckli et al., 2004;Brygadyrenko, 2015aBrygadyrenko, , 2015b. However, invasive species often do not lead to deterioration of the environmental state during the first stages of their dispersion (Hulme et al., 2013); this can be determined by application of more sensitive mechanisms to invasion diagnostics compared with environmental changes, particularly use of molecular markers (Wolf et al., 2012).
The results of current researches are devoted to the studies of biology, ecology, and spreading of invasive species in multiple-purpose water bodies (Marlis et al., 2015), as well as the socio-economic consequences of biological invasions (Lotz and Allen, 2013). European scientists conduct the study of migration vectors and spreading of invasive species (Frances et al., 2016), but studies of species adaptive capabilities have almost never been performed.
Living systems are faced with a variety of stresses in the process of their continuous interaction with the environment (Stoliar and Lushchak, 2012). Environmentally induced stresses often activate the production of endogenous reactive oxygen species (ROS), most of which are generated as byproducts of tissue respiration. Thus, the permanent influence of stress factors can exacerbate the ROS-mediated oxidative damages. A large number of agricultural and industrial wastes enter the environment and, thereafter, pass into various living organisms, causing multiple changes in them. Some of effects involve enhancing the direct formation of reactive oxygen, while others can act indirectly, such as by binding to cell thiols and decreasing of antioxidant potential. Pollution of the aquatic environment especially affects fish. However, academic studies on the adaptive capabilities of invasive species have almost never been performed. So, the question of studying the antioxidant support network under development of oxidative stress is quite relevant.

Invasive plant species
Invasions by non-native species are the greatest global environmental problems in the modern age, which are of particular importance due to processes of biotic globalization (Davis, 2003). Invasive plants pose a significant hazard to biodiversity, ecosystem management, agriculture and forestry, etc. According to assessment of the Convention on Biological Diversity, invasions of non-native species are the second most important threat to biodiversity on the global level, following direct destruction of wildlife habitat (Mack et al., 2000).
In Ukraine, 95 plant species of adventive flora have been identified as the species with high invasive capacity (Protopopova et al., 2002). The most common of these are 21 species (Table 1) (Abduloeva et al., 2008).
In terms of occupied area, and thus in the impact on the local ecosystems, Robinia pseudoacacia ranks first among arboreal species. This species was introduced from South America. It was first introduced to Ukraine in the late XVIII century in Count Razumovsky's Park. In 1808, this plant was grown by I.N. Karazin in his estate in Kharkov region. In Ukraine, R. pseudoacacia came actively into cultivation in early 1920s (Vakulyuk and Samoplavskyy, 1998). The species occupies an especially large area within the steppe zone, as it is perfectly adapted to arid conditions. R. pseudoacacia is a rapid-growth species, and in the first year after planting it grows to 1.5 m, while its stool shoot can reach 2-3 meters in the first year within clear felled areas (Ukraynska …, 1999). R. pseudoacacia is not a soildemanding tree species. At the present time, under steppe conditions almost everywhere a significant expansion of the distribution of R. pseudoacacia is observed; this is related to its growth-inhibiting role in respect to other tree species, such as the oak and ash (Riabchenko, 2012). Invasion of R. pseudoacacia into native oak forests can lead to significant changes in forest-site conditions (Nascimbene et al., 2012). At the same time, R. pseudoacacia often compares poorly in moistening to steppe herbaceous native species. Some scientific researches (Abduloeva and Karpenko, 2008) have revealed a high allelopathic activity of R. pseudoacacia soluble exudates, which explains to some extent its occupation of new territories and displacement of other plant species. Significant expansion in distribution of R. pseudoacacia is also observed in other countries, particularly in Northern Italy there is expansion of the species into new territories (Radtke et al., 2013). In South Korea, expansion of R. pseudoacacia occurs within lowlands, and this phenomenon is not observed for uplands (Lee et al., 2004).

Invasive invertebrate species
Invasive insects are a serious hazard to the local ecosystems and economy. In Ukraine, according to the International Convention on Quarantine and Plant Protection and the International Standards for Phytosanitary Measures (ISPMs No. 19), lists of quarantine species were compiled. Today in Ukraine 218 species of quarantine organisms are listed in the National List of Regulated Hazardous Organisms, among which 98 species are insects. In the current list, all organisms are divided into three groups: -A-1 Quarantine organisms absent from Ukraine; -A-2 Quarantine organisms with restricted distribution in Ukraine; -Adjustable non-quarantine hazardous organisms. Species belonging to the A-2 group are the greatest environmental and economic hazards (Table 2) because of their annual irruption. The last 20 years have seen an intense process of fauna transformation in the Dnieper reservoirs. Invasion and distribution of invertebrates occurred (Pligin et al., 2013, Semenchenko et al., 2015. During the period 2000-2015 alone the species composition of the benthos fauna was enrichened by 6 species (Table 3).  The brackish and freshwaters of the Ponto-Caspian region. Shortly before 1946, it spread across continental Europe by both intentional (for fish feeding) and unintentional introductions, and arrived in the coastal brackish waters of the Baltic and the North Sea. It is soon to be expected on the Mediterranean coast. The westward spread occurred mainly through multiple invasion waves along waterways of the southern corridor, from the Danube Delta, through the Main-Danube Channel, and in the River Rhine down to the North Sea In 2000, in the downstream area of the Samara river an amphipod species new for Ukraine was recorded, S. ambulans. In 2001, in the Kilchen river, was registered the species R. kischineffensis, untypical for the steppe of Ukraine and newer for the Dnieper basin, which also expanded its range to the Samara river floodplain. In 2002, in Zaporizhzhya Reservoir, a catch of the mitten crab E. sinensis has been recorded; further reports of catching this species came in 2003 (Kakhovske Reservoir) and 2010-2015 (lower reach of Zaporizhzhya Reservoir) (Novitskiy, 2010).
In the spring of 2009, the Holland crab R. harrisii was found in the Zaporizhzhya Reservoir. In 2015 specimens of marbled crayfish P. fallax f. virginalis were first observed (Novitskiy, 2010).

Invasive vertebrate species
During all stages of existence of the Zaporizhzhya Rservoir, fish fauna has undergone significant transformation. Currently, 52 fish species belonging to 14 families inhabit the reservoir. The number of fish species in the Dnieper in its current form as a chain of huge reservoirs is the same as when it was a free-flowing river, but the composition of fish species has changed radically because of the establishment of new fish species (Fedonenko at al., 2008) (Table 4).
The fauna structure is considerably influenced by the complex of ecological factors that generated changes in ichtyocoenosis. The increase in number of species is associated with various events. Firstly, after disappearance of the Dnieper Rapids and rise of water salinisation, the natural colonizing process began (Fedonenko at al., 2008), resulting in expansion of southern species to the reservoir, such as Alosa pontica, Gasterosteus aculeatus, Syngnathus abaster nigrolineatus, Aterina pontica, Clupeonella cultriventris, Benthophiloides brauneri.
Secondly, some species of fish have been introduced in the reservoir for the purpose of fishery management implementation: Hypophthalmichthys molitrix and Carassius gibelio. In addition to the stocking of the Far-Eastern complex of herbivorous fish, Pseudorasbora parva has become estab-lished in the reservoir; this species unlike white amur and silver carp has acclimatized and expanded its distribution throughout the Zaporizhzhya Reservoir and tributary systems (Bulakhov at al., 2008;Fedonenko at al., 2008).
The emergence of new species was related also to the deliberate release of fish. In such a manner, Lepomis gibbosus appeared in water bodies of Dnipropetrovsk region; the species has adapted successfully and widely enlarged its range (Fedonenko at al., 2015). Because this introduced species is a predator, it can be potentially damaging for valuable commercial fish because it feeds on invertebrates, and occasionally eggs and young fish. At present, about 31% of the fish species in the Zaporizhzhya Reservoir are introduced. Such changes in the reservoir ichthyofauna composition can harm rational fishing, because introduced species are overwhelmingly the food competitors for the young of commercial fish species.

Determining species viability by stress resistance indices
Today, the problem of antioxidant protection is very relevant. But, despite the fact that quite a lot is already known about antioxidant system functioning and adjustment, many questions remain unanswered. For example, it remains unclear how the introduced species respond to the impact of various factors and to the stress they experience when they enter a new environment, and why their cells use different protective adapting systems to the same factor of influence (Halliwell, 2007). Oxidative stress is a condition when formation of reactive oxygen species (ROS) prevails over the processes of their disposal, resulting in a major disruption of the main vital processes (Hansen, 2006;Lushchak, 2011). Reactive oxygen forms cause many different damages through oxide modification of lipids, proteins, DNA, and other components. They are by-products of cellular aerobic metabolism, or results of many xenobiotic functions.
Along with antioxidants, reactive oxygen species constitute a system of cellular redox signaling, which is an integral element of the overall signaling system of both cells and the whole organism (Sies, 1991). At the same time, imbalance between ROS formation and their degradation by antioxi-dants can lead to damage to biopolymers and lipids and, finally, to cell death. Induction of ROS-dependent pathways of signal transduction influenced by various external factors may cause the activation of the antioxidant system and thus improve organism resistance to stressors of various nature in different conditions of existence. Nevertheless, the mechanisms of interreaction of various components in the antioxidant system still remain unclear. Regardless of a longevity duration that can be days or decades, O 2 is dangerous for all living organisms, whether plants, insects, and vertebrates. For example, insects are not protected from the harmful ROS effect occurring at O 2 reduction. Insects can be particularly prone to oxidation stress (Felton, 1995). On reaching a new environment, plants are exposed to osmotic and ionic stresses that along with saline stress can cause the development of secondary oxidation stress. Oxidation stress is defined as short-term or prolonged increasing of steady-state ROS concentration, which causes disturbance of cell metabolism and its regulation processes as well as damaging the cellular compartments (Apel, 2004). Antioxidants play the key role in protection both plants and animals against the effects of stress factors; antioxidants can be represented by low-molecular compounds and antioxidant enzymes (Blokhina, 2003;Gill, 2010).
Superoxide dismutase and catalase are enzymes of the first line of defense against ROS. Superoxide dismutase (SOD, F.K. 1.15.1.1) is a permanent component of the antioxidant system. SOD catalyzes the dismutacion O 2 • to H 2 O 2 . Activity of SOD is related to redox-active metal ion in the molecular active center of the enzyme, and, depending on the enzyme type, that ion may be manganese, ferrum or cuprum, which is involved in the process of radical neutralization. At the first step, one-electron oxidation takes place, and the oneelectron reduction at the second step. These reactions do not require an external source of oxidation-reduction equivalents, so they are independent components of the antioxidant system (Kohen, 2002).
Catalase (F.K. 1.11.1.6) is an enzyme that is present in most anaerobic cells, and it catalyzes the transformation reaction of hydrogen peroxide to water and oxygen. In animal tissues, catalase is localized in the cytoplasm and peroxisomes. Catalases represents a large group of oxido-reductases, which are divided into three subgroups, depending on the physical and biochemical properties (Kohen, 2002;Halliwell, 2007). Most aerobic organisms contain catalase except some algae and parasitic helminths (Imlay, 2003).
Different redox groups can be used as indicators of changes in cell oxidation-reduction balance (Hansen, 2006;Lushchak, 2011). In the cell, there are three basic oxidationreduction systems. The eukaryote basic reduction systems supporting cell oxidation-reduction balance includes the glutathione-dependent system (GSH/GSSG) (Anderson, 1998). Glutathione acts as the most important intracellular lowmolecular thioic antioxidant. The importance of glutathione in the cell is determined by its antioxidant properties. Actually glutathione not only protects the cell from such toxic agents as free radicals, but also determines the redox status of the intracellular environment generally (Anderson, 1998;Lushchak, 2012).

Conclusion
We propose a completely new advanced technique aimed at addressing the modern global environmental target of evaluation of potential adaptive capacities of invasive species in new environments. Unlike its analogues, the proposed three-step approach in evaluation of life activity and ecological functions of invasive species has been developed. In addition to the classic species and population surveys, it was proposed to carry out biochemical evaluation of stress resistance in invasive species. After all, stress resistance can be considered as the primary reaction of living organisms to changes of stable environment components. The results will provide the opportunity to make a more accurate forecast concerning adaptation opportunities in invasive species, and exactly what kind of ecological functions and for which time period such species will be able to perform in the ecosystems new to them.