Pathways of introduction are the numerous ways through which species, with the help of human activity, travel from one location to a new location. We distinguish them into two main categories: unintentional and intentional pathways. 

Intentional pathways include cases such as releases for biological control, hunting, fishing activities, releases for conservation, erosion control or environmental enhancement. Unintentional pathways include escape from confinement, transport as a contaminant, transport as a stowaway, corridors, and spontaneous spread.

The Guidelines for Interpretation of CBD Categories of Pathways for Introduction of Invasive Alien Species help us understand these pathways. The Convention on Biological Diversity (CBD) categorizes unintentional pathways in more detail, as presented below:

Priority Pathways of Introduction

Most invasive species are unintentionally introduced, making the management of these pathways critical for preventing their spread. 

Slovenia has identified three priority subcategories of unintentional pathways:

• Natural dispersal of invasive alien species that were introduced through other pathways: 

This subcategory includes species that spread into new regions through natural dispersal, meaning spreading spontaneously without human involvement. They spread from regions where they are non-native and were introduced through one of the other pathways.

Species are considered alien when they are introduced to areas outside their natural range. Pathways of introduction include intentional release, escape from confinement, transport as a contaminant, transport as a stowaway, and corridors. Once introduced, alien species can independently spread to neighbouring regions, which describes this pathway. Although these species now spread naturally, they were originally introduced to an area through human activity, which enabled them to overcome a geographical or biological barrier. This subcategory also includes species that arrived as contaminants with migratory species like birds, fish and ungulates. These species migrate independently of humans but act as vectors for transferring alien species, for example, through mud on their feet, feathers, or in their digestive systems.

• For ornamental purposes, excluding horticulture: 

This subcategory refers to the escape of plants from enclosed or controlled environments where they were introduced for ornamental purposes. It includes species with special structural and colour characteristics suitable for landscaping. The global movement of these species for municipal parks and private gardens has been facilitated by various forms of trade. Particularly included are species kept by hobbyist collectors and those used in landscape architecture for decorative purposes. These species may accidentally escape into (semi-) natural environments. Commercial horticulture, where plants are cultivated in nurseries or greenhouses, does not fall under this subcategory. Neither do ornamental aquarium species, which have their own subcategory.

• Pet/aquarium/terrarium species (including live food for such species): 

This subcategory includes the escape of species from enclosed or confined environments where collectors or hobbyists keep them for recreation, companionship, entertainment or sale. This pathway is part of the “Escape from confinement” category, specifically addressing unintentional escapes of species introduced into an area to be kept in enclosed or controlled environments (IUCN, 2017).

This pathway includes:

• Pets, aquarium, and terrarium animals kept by collectors and hobbyists (including various animal groups, not just typical vertebrate pets),
• Species kept and bred by private collectors and enthusiasts for sale to other collectors,
• Species used as live food for pets, such as mealworms, grasshoppers, and fruit flies,
• Aquarium and terrarium plants and other organisms (algae, fungi, etc.) used in aquaristic and terrarium culture.

For centuries, collectors have gathered alien and exotic species for hobbies, companionship, or exchange. However, escapes from confined spaces are inevitable, and species capable of surviving and establishing themselves may become invasive. The development of the internet has greatly simplified the purchase and exchange of organisms, leading to a significant increase in the trade of pets and aquarium species. As a result, this pathway has gained importance in recent decades (IUCN, 2017).

This subcategory includes various escapes and releases, which can occur independently or with the help of their owners (adapted from IUCN, 2017):

1. • Accidental pet escapes,
   • Escapes due to inadequate enclosures that allow species to escape,
   • Intentional and irresponsible releases by owners who no longer wish to care for their pets (for example, the release of unwanted aquatic species, such as red-eared slider turtles in Slovenia),
   • Ignorant or irresponsible disposal of aquarium waste through water systems into rivers, lakes, and the sea,
   • Releases of organisms during improper or irresponsible cleaning of aquariums and terrariums,
   • Releases of animals by some animal rights activists,
   • Release or use of animals intended as food for other animals in open systems.