Globalization and climate change have accelerated the arrival and settlement of exotic species in new regions. These species, once naturalized, can develop a negative behavior becoming invasive alien species (IAS), which have been recognized as one of the greatest threats to biodiversity worldwide, negatively affecting different areas, from ecosystem services to economic activities and human and animal health. The increase in biological invasions is related to various causes such as the growing number of transport vectors, the invasiveness and degree of deterioration of host ecosystems or anthropogenic disturbances.
In particular, aquatic environments are an ideal scenario for the settlement of IAS since in many cases they are subject to high anthropogenic pressures that favor the arrival and settlement of these species. In addition, because they are very dynamic ecosystems, they allow for easy dissemination and propagation of individuals along their path. It is currently estimated that more than 20,000 IAS are present worldwide, with Europe being one of the most affected continents. In Spain, more than 300 exotic species have been detected in continental and estuarine aquatic systems alone. On the other hand, the management of invasive alien species also generates high costs in their control and eradication. It is estimated that the total reported costs linked to their management in Europe reached a value of 12 billion euros per year, while the cumulative costs are likely to reach 20 billion euros per year. These estimates, partly conservative, give an idea of the great impact these species have had on the local and national economy in recent decades.
One of the biggest problems associated with their management is that the early stages of invasion of these species are difficult to detect. Therefore, when they are detected, they are already widespread, creating a handicap in their elimination, and late detection works as a mechanism that favors their dispersal by human action by not taking preventive measures. For this reason, prevention, early warning and rapid response plans are advocated for the correct management of these species, since they are the only way to achieve success in their eradication. Currently, the processes carried out both by local administrations and by associations and companies dedicated to environmental quality consist, broadly speaking, of visual prospecting or indirect detection by means of standard procedures such as the analysis of phytoplankton, zooplankton or benthic macroinvertebrates. For certain fouling aquatic species, such as the zebra mussel(Dreissena polymorpha), there are techniques based on waiting for the colonization of an inert surface by the invasive species. In terrestrial environments, traps using pheromones or sugary attractants are also commonly used to capture invasive species with flying phases, such as the Asian wasp
Environmental DNA (eDNA) is the genetic material that different species release into the environment in which they live. This DNA can be captured by different procedures and allows, with a simple sample of water, air, soil or sediment, to detect most of the species living in that habitat. This simple principle gives rise to very powerful tools such as environmental DNA detection techniques. With respect to invasive species management, eDNA can be used in a number of ways, including detection of new introduced species, tracking the spread of an introduced species, or monitoring survivors of eradication efforts. Detection of invasive species in aquatic environments using eDNA has been studied extensively and includes groups as varied as vertebrates (e.g. northern pike(Esox lucius)), mollusks (e.g. zebra mussel
eDNA is a very useful tool in environmental management as it has a number of advantages over traditional invasive species detection techniques. Thanks to these new techniques, invasive species can be detected and identified even when there are only a few specimens in the sampled environment and none have been captured or seen. This is key for species whose ecological habits present cryptic stages of concealment in aquatic beds or for very small species that are difficult to detect with the naked eye by means of a visual survey. On the other hand, these techniques have a high specificity and a wide range of application, being their sampling a fast procedure with little or no intrusion in the environment, thus avoiding the transmission of invasive species from one geographical area to another, since some of them can remain attached to the sampling equipment, a common case in active sampling and traditional visual prospection.
Thanks to the high specificity of these techniques, very specific taxonomic identification resolutions are achieved at genus and species levels without the need for expert taxonomists for their identification. In addition, many exotic species are particularly difficult to identify by visual examination, especially juvenile or larval stages, so these techniques are essential for their correct specific determination. It is also a technique of rapid application and relatively inexpensive, since the field of molecular biology is evolving rapidly with a continuous improvement in accuracy and at a lower cost, given that it has a significant margin for optimization. It is likely that this technique will eventually become the new standard for the detection and monitoring of aquatic and terrestrial species, as has already occurred in the detection of certain species such as the great crested newt(Triturus cristatus) in the United Kingdom.
However, these techniques are not perfect, as they present a series of problems to be taken into account during their use in environmental management. The main problem is that most of the academic articles that develop these tools for the detection of invasive species tend to remain in the first levels of validation, with few reaching level 4 (depending on the different validation steps that the technique has passed for a species, a validation level of 1 to 5 is granted according to the validation scale developed by the DNAqua-Net consortium). It may happen that some of the primers (markers/primers) and published methodologies that are validated only at low levels, fail when attempting to validate them at higher levels. Also, if these primers are tested in other geographic areas, they may non-specifically amplify species local to the new area where they are to be applied.
eDNA detection techniques are species-specific, which means that each new species for which development is sought must go through a series of in silico (bioinformatically), in vitro (in the laboratory) and in situ (in the field) validation steps, in that order. In addition, for a validation assay to be considered, it must be tested in the geographic region of interest against related species in the area and with the sampling methodology that will ultimately be used. All this generates an added problem and that is that it may be the case that certain species do not present a sufficiently variable area of the genome that is able to differentiate between species and therefore a specific primer for the target species cannot be obtained. In addition, this technique is sensitive to the genetic variations of the species to be treated over time, so that species with very high rates of genetic variation may lose affinity for previouslyvalidated primers.
In conclusion, the use of environmental DNA(eDNA) has emerged as a promising tool for the detection of invasive species allowing early and specific detection. However, these techniques also present challenges, such as validation of genetic markers and sensitivity to genetic variations in target species. Despite these challenges, eDNA has the potential to revolutionize invasive species management by providing a more efficient and less intrusive tool for the detection and monitoring of invasive alien species in aquatic and terrestrial environments. Its continued research and development are critical to maximize its effectiveness and applicability in the conservation and protection of ecosystems.
