The spread of invasive alien species is considered one of the greatest threats to the conservation of biodiversity in aquatic ecosystems and is particularly detrimental to native species. The major European rivers are among the areas most affected by their spread. An alien species is considered invasive if its population has exceeded the stage of exponential growth and is spreading rapidly, or if its presence causes or is likely to cause economic or ecological damage or harm to human health. The biological characteristics of alien invasive species that enable their successful spread include rapid growth, earlier sexual maturity, high fecundity, short life span and good tolerance to various environmental stresses to which they are exposed.

Alien species can enter new areas naturally. However, most aquatic species lack the means for active dispersal into isolated areas and instead utilise various transport vectors for passive dispersal, such as the wind carrying dried eggs of freshwater invertebrates. On the other hand, the increase in global travel and trade has created many more pathways for alien species to travel long distances, especially through the ballast water of large ships.

The biotic resistance hypothesis emphasises the high diversity of native species in the recipient community as a major obstacle to the success of the invasion. The physico-chemical and morphological integrity of river systems provides the best protection against the invasion of alien invasive species, and consequently the success of the invasion is negatively correlated with the diversity of the native community. For this reason, good ecological and chemical status of freshwater ecosystems is of paramount importance.

Previous studies in Croatian freshwater ecosystems have identified a total of 29 alien species of benthic macroinvertebrates. Thanks to the Biological Testing Service of the Main Water Laboratory of the Josip Juraj Strossmayer Water Institute, three species were identified for the first time as part of the systematic monitoring of water status (national monitoring) in Croatia, while other alien species were noticed due to their spread or colonisation of new rivers. This emphasises the importance of regular monitoring of water conditions in relation to the biological elements of water quality, as timely detection of changes in the environment or early identification of invasive alien species can slow their spread somewhat.