Types of dispersal paths

In the „world conquest” of invasive species the paths on which they get to new habitats play a significant role.

The term invasive alien species (IAS) refers to organisms that conquer new habitats far away from their original home due to intentional or accidental introduction. Human intervention is thus a basic requirement for the spread of invasive species, thus the term “dispersal path of invasive species” not only refers to a concrete physical path but also to dispersal mechanisms playing role in the appearance of a species in a new location.

Paths and ways of reaching the new site

Of course identification of the geographical path is a basic requirement, because this is the path where the invasive species travels from its natural habitat to the site where it counts as alien after turning up. This path depends implicitly on species characteristics, ecological tolerance, biogeographic aptitudes and the dispersal potential resulting from all these alike, however, the most important factor in the case of invasive species, human intervention also facilitates overcoming natural barriers. Especially important are the medium or physical path, the so called passage – like roads, rivers, artificial channels – and the transporting “device” itself, the so called vector – be it a vehicle, clothing or even a living animal – assisting the dispersal of the species. Historical background explaining the intentional or accidental introduction of the species has to be taken into consideration as well.

It is worth to examine the dispersal mechanisms of individual species from the side of human intervention. Here one of the main factors to be considered is purposefulness, that is, was the introduction deliberate or if not, in what ways did human intervention influence dispersal of the given species. Despite the fact that mankind has become aware of the potential of alien species to become invasive, introduction is purposeful in a large amount of cases. This may have several reasons, but usually the problem arises from the fact that the invasive character of the introduced species could not be foreseen or the consequences of it were underestimated. A typical example for this is the settling and reproduction at an unwanted rate of individuals released for angling or hunting. Even more important is the escape of species introduced on purpose but kept and reproduced under control. Examples of latter are plant individuals escaping plantations and animal species straying from farms and then establishing self-sustaining populations in nature (e.g. coypu).

Transport of goods or people goes along with moving huge amount of materials and it easily comes to pollution of plant reproductive material or fry by unwanted organisms or parasites. Pollution has to be distinguished from “being a stowaway” when a species gets to a new site through the contribution of transportation (a vector) – e.g. sticking to the side of ships or getting accidentally into wrapping materials or into the luggage of tourists.

A category by itself is when a new dispersal medium or physical path, a kind of dispersal passage is created by human activity. This usually goes along with the creation of some kind of linear structure like roads, bridges or channels. These often cause big problems by linking sites that had been separated by an impenetrable barrier for the species living there before.

Spontaneous dispersal has to be distinguished from all above mentioned variations, when following introduction to a new site alien species overcome natural barriers and conquer new territories without human intervention. Species dispersing spontaneously also arrive to sites where they are seen as alien by one of the pathways described above. However, due to their characteristics enhancing dispersal (e.g. active locomotion, high tolerance against variable environmental effects, good reproductive potential), following this these species are able to populate new habitats without further human intervention.