Spreading of alien species and diversity of communities

A new species invading a new area may cause a decrease in diversity of the community already present there. Comparison of temporal changes in species diversity of the “new” community (including alien species) with those of the “original” community (including only native species) may clarify our understanding of the effect of alien species. Using a simulation-based modelling approach we considered several scenarios describing the invasion of native communities by alien species and calculated the trends in Shannon-Wiener indices and in the numbers of species of the “original” and “new” communities during the course of the invasion. We found that despite a large increase in the population size of the invasive alien species the diversity of the original community may be little affected. Native species numbers may stay relatively constant for a long time and then suddenly collapse. The results indicate some possibly still concealed consequences of the spread of the invasive ladybird Harmonia axyridis (Pallas).