Department of Botany and Zoology, Masaryk University, Kotlářská 2, CZ-611 37 Brno, Czech Republic
Each plant species evolved in a specific habitat and adapted to its abiotic environment where it co-existed with other species occurring regularly in that habitat. Once a species is introduced to a new area, it will likely spread especially in habitats that are similar to those in which it grows in its native range. Comparative studies of alien floras across multiple habitat types indicate consistent variation in the level of plant invasions across habitats, with some habitats invaded rarely and by few alien species while others are invaded often and heavily. Several processes, both in the source area and in the invaded area, can generate these patterns.
Source-area processes generate habitat-specific species pools of alien species: (1) Some habitats tend to occur more closely to human settlements and transport corridors, therefore their species are more likely to be introduced to a new area. (2) Some habitats are richer in species, therefore there is a higher probability that some of these species will be introduced to a new area and become established. (3) Some habitats have a highly dynamic ecological regime, which selects for traits that facilitate colonization and establishment in new or disturbed areas. Such species are more likely to spread outside their native ranges.
Invaded-area processes act as filters on alien species pools influencing the establishment of species from these pools across different recipient habitats: (1) Some habitats tend to occur closer to human settlements and transport corridors, therefore they are exposed to more incoming propagules of alien species and more likely to be more invaded (“propagule pressure” or “colonization pressure” explanation). (2) Some habitats are poorer in species, therefore they contain empty niches, which are colonized by incoming alien species (“biotic resistance” explanation). (3) Some habitats have a highly dynamic ecological regime, which supports the establishment of new species, of which some can be alien (“fluctuating resources” explanation). (4) If none of these filters are acting, habitats can still differ in the degree to which they are invaded due to different sizes of their alien species pools (“alien species-pool” explanation).
In our research into habitat invasion patterns and processes conducted over the last 15 years, we applied both the source-area approach in studies of intercontinental exchange of alien plants between European habitats and habitats on other continents, as well as the invaded-area approach in fine-scale studies of European habitats. These studies provided support for most of the above-mentioned mechanisms except those involving species richness.
In this talk, I will summarize evidence for each of these mechanisms based on our own work and published literature, and I place the source-area and invaded-area perspectives of habitat invasions into a single framework.