Definition of Biological Invasions (of species)

Lissette Cardenas Hinojosa | Jun. 2022
PhD in Biology and Applied Ecology

First of all, it is necessary to specify some essential concepts to understand the invasion process. We will call the species that inhabit a given territory native species, on the other hand, when they move from their range of original distribution to other habitats, we will call them non-native species (with synonyms: 'alien', 'exotic' or 'introduced'). Biological invasions occur frequently, however, only a small percentage of them are successful. When a non-native species manages to establish reproductive populations for several life cycles in the invaded areas, expand their distribution range and remain for long periods of time in these sites, they become species invasive.

Biological invasions and interactions between exotic and native species have been of great interest to ecologists, developing various hypothesis to explain this phenomenon and its effects on ecosystems.

factors and hypothesis Darwin and Elton

The well-known naturalist Charles Darwin was one of the pioneers in recording the ecological interactions between species during the invasion process and he was the one who proposed the so-called Darwin Naturalization Hypothesis, where he establishes a relationship between competitive intensity and the phylogenetic relationship of native and exotic species.

Years later, Charles Elton, who is recognized as the founder of the study of invasion ecology, proposed the Biotic Resistance Hypothesis, which considers the diversity of species as a determining factor during the invasion process, establishing that the communities more diverse have a lower degree of susceptibility to being invaded, since they consume all the means available in the environment, limiting the entry of exotic species.

Another determining factor for biological invasions is the absence of natural enemies such as predators, herbivores and pathogens, in the invaded sites (enemy release hypothesis). In this way, alien species reallocate resources (Energy) that they use in defense to favor relevant physiological processes such as growth and reproduction, promoting an increase in the biological adaptation of these species (hypothesis of ability increased competitiveness), which will be more efficient in expanding their range of distribution.

Several studies have shown the development of adaptive advantages of exotic species in invaded areas. For example, both in plants and in invasive algae, increases in the concentrations of secondary metabolites such as alkaloids and compounds have been recorded. allelopathic, versus their native congeners (hypothesis of novel weapons), realizing that the biochemical potential could be of great relevance when invading new territories.

How does the invasion process occur?

Biological invasions have been described for different taxonomic groups such as bacteria, plants, animals and fungi, both from terrestrial and marine environments. The invasion process is made up of four phases; dispersion, establishment, naturalization and expansion. It all begins with the transport or movement (dispersal) of the species from its native habitat to the arrival in new territories. Already in new areas, non-native species must be able to establish themselves in these previously unknown territories. For this they must withstand the prevailing environmental conditions and gain access to the necessary resources (food, space) to live. In addition, a single individual is unable to invade an area, therefore it needs to find a partner to reproduce and continue the invasion process.

Once established, a great challenge begins for non-native species, the naturalization stage, which depends mainly of a series of distinctive attributes of these, that allow them to overcome the new abiotic conditions and biotic. It is recognized that a species is naturalized when they are capable of originating reproductive populations during several life cycles in new territories, which finally allows them to expand their distribution range in the invaded areas and remain for long periods of time, becoming species invasive.

During biological invasions, various ecological interactions occur between non-native species and the invaded community. For example, facilitation is a mechanism that generates increases in the survival and population growth of invasive species. On the other hand, competition is one of the main mechanisms involved in the invasion process and that could limit, as well as favor the establishment, naturalization and expansion of invasive species in new ranges geographical.

What defines the invasion success of the species?

The ability of non-native species to establish themselves in new territories depends on abiotic factors such as resource availability and prevailing environmental conditions, as well as of biotic factors such as low rates of herbivory in the case of plant species and low competitive interactions between non-native species and native species of the community invaded In this context, the success of the invasion of exotic species will depend mainly on two ecological factors, the invasiveness of the invasive species (invasivness in English) and the invasibility of the receiving community (invasibility in English). English).

Invasiveness is defined as a set of distinctive attributes of invasive species that allow their colonization in new habitats, such as rapid growth, strategies of asexual reproduction, high rates of dispersion and pressure of propagules in the case of plant species and high phenotypic plasticity to respond to environmental variations in the areas invaded. On the other hand, invasibility corresponds to the characteristics of the recipient communities, such as the structure and evolutionary history and the network of interactions between these such as competition, facilitation, herbivory and predation In this way, communities can be resistant or susceptible to invasion, which can also be regulated by environmental factors such as availability of resources and the levels of disturbances of natural or anthropogenic origin, which can limit or favor the success of the invasion of the species non-native.

Impacts of biological invasions on ecosystems

In recent decades, the introduction of non-native species has increased considerably at the fortunately, a low percentage of these species manage to expand into new ranges geographical. In terrestrial environments, the main routes of introduction are the use of species for ornamental purposes, the animal trafficking, the release of pets, among others, while, in the marine environment, maritime traffic and the crop of commercially important species are the most common dispersal vectors.

Currently, biological invasions are one of the main causes of biodiversity loss at the local and global levels, due to that alter distribution patterns, abundance and richness of native species, affecting the composition of communities containers. On the other hand, there are other effects such as the alteration of nutrient cycles, which change the abiotic conditions of the ecosystems and that in the long term generates negative impacts on the species native. In addition, the presence and expansion of invasive species can produce biotic homogenization, changing drastically the landscape structure of the invaded areas, affecting, for example, activities such as sightseeing.

Everything described above has a direct impact on the ecosystem services provided by terrestrial and marine ecosystems, considerably harming the human settlements that live near invaded communities, even causing damage to the health of the population. This should lead us to reflect on the importance of maintaining natural ecosystems and not carrying out irresponsible actions such as the release of pets. On the other hand, it is urgent that the authorities of the different countries carry out actions to control invasive species, such as designing and implementing management and eradication of these species, applying solutions based on nature or developing biotechnological tools for the use of species invasive.