Interaction of species traits and environmental disturbance predicts invasion success of aquatic microorganisms

Abstract

Factors such as increased mobility of humans, global trade and climate change are affecting the range of many species, and cause large-scale translocations of species beyond their native range. Many introduced species have a strong negative influence on the new local environment and lead to high economic costs. There is a strong interest to understand why some species are successful in invading new environments and others not. Most of our understanding and generalizations thereof, however, are based on studies of plants and animals, and little is known on invasion processes of microorganisms. We conducted a microcosm experiment to understand factors promoting the success of biological invasions of aquatic microorganisms. In a controlled lab experiment, protist and rotifer species originally isolated in North America invaded into a natural, field-collected community of microorganisms of European origin. To identify the importance of environmental disturbances on invasion success, we either repeatedly disturbed the local patches, or kept them as undisturbed controls. We measured both short-term establishment and long-term invasion success, and correlated it with species-specific life-history traits. We found that environmental disturbances significantly affected invasion success. Depending on the invading species' identity, disturbances were either promoting or decreasing invasion success. The interaction between habitat disturbance and species identity was especially pronounced for long-term invasion success. Growth rate was the most important trait promoting invasion success, especially when the species invaded into a disturbed local community. We conclude that neither species traits nor environmental factors alone conclusively predict invasion success, but an integration of both of them is necessary.

Figure 1. Invasion success of the nine invading protist and rotifer species in response to disturbance of the local environment.

Invasion success is given as the proportion of communities in which the invading species was present, irrespective of the invading species’ density. A) initial and B) final measurement. Full species names of the abbreviations are given in the Material and Method section.

Figure 2. Correlation between invasion success and traits of the invading species.

Traits concern size (A, B), growth rate (C, D) and carrying capacity (E, F). Left hand panels are the results of the initial measurement and right hand panels are the results of the final measurement. Invasion success is given as the proportion of communities in which the invading species was present, irrespective of the invading species’ density. For significant correlations, we added lines of the values predicted by the GLM. Dashed lines are for undisturbed patches, solid lines for disturbed patches.

Figure 3. Invasion success in response to the invading species’ trophy.

Boxplots are given separately for predators vs. bacteriotrophs (A, B) and autotrophs vs. heterotrophs (C, D). Invasion success is given as the proportion of communities in which the invading species was present, irrespective of the invading species’ density. Grey boxes stand for disturbed patches, white boxes stand for undisturbed patches. Left hand panels show results of the initial measurement, and right hand panels show results of the final measurement.

Figure 4. Density and median size of all microorganisms (protists and rotifers) in the individual communities.

Density (A) and size distribution (B) was measured with a CASY particle counter, and includes all individuals between 3.2 µm and 120 µm. Mean±SE values are given separately for replicates with different invading species and disturbance levels. For full species names see Material and Method section.

Figure 5. Community diversity (Shannon-index) in response to disturbance of the local environment and the invading species.

Boxplots are given for the control communities without invasions, and the communities invaded either by Cephalodella sp. or Tetrahymena sp.