Species from distant regions are being transported through the agency of humans and are increasingly entering native biotic communities as non-native species. The effect of these species can range from relatively benign to strongly negative (i.e., invasive species). One of the consequences of the arrival and establishment of non-native species is a mixing of earth’s biota to the extent that today composition, structure, and function of biotic communities are altered almost everywhere. Comparing natural, old plant communities with these novel, fused communities provides a unique opportunity to understand the way communities form and function. By comparatively investigating species interactions within old (sympatric) and novel (allopatric) plant species combinations, we tested the hypothesis that evolutionary processes in populations and even communities are interrupted whenever new species arrive. If so, we predict that old communities will be more stable and borders between sympatric species pairs will be more distinct compared to novel communities. Focusing on urban disturbed sites, community level responses between sympatric and allopatric species are investigated (1) by describing and monitoring the temporal dynamics of border zones between them by using a belt transect method and (2) by characterizing vegetation assemblies of disturbed sites in terms of their biogeographical composition.
Results/Conclusions
Assessing border zones between species pairs show that novel (allopatric) pairs have significantly larger overlap zones compared to old sympatric species pairs. This indicates that old neighbors are more effective at delineating interaction zones and are better in partitioning resources, thereby reducing local competition. When following the dynamics of these border zones over nine years, similar pattern emerges as old, sympatric neighbor zones tend to be more static than novel ones. The biogeographical composition of vegetation assemblies at disturbed sites shows that non-native species from similar regions often associate with each other, indicating non-random assembly based on species origin. This seems to indicate that species pairs that originate in similar geographic regions can assemble into “expat communities” which are predominately found in urban areas and less frequently in rural disturbed areas. These patterns are in agreement with the hypothesis that plant communities consisting of formerly sympatric species differ from novel assemblies. Further investigations of origin-specific species interaction will lead to a better understanding of the structure and function of plant communities and will aid in predicting future dynamics of increasingly prevalent novel communities.