Effects of connectivity and recurrent local disturbances on community structure and population density in experimental metacommunities

Abstract

Metacommunity theory poses that the occurrence and abundance of species is a product of local factors, including disturbance, and regional factors, like dispersal among patches. While metacommunity ideas have been broadly tested there is relatively little work on metacommunities subject to disturbance. We focused on how localized disturbance and dispersal interact to determine species composition in metacommunities. Experiments conducted in simple two-patch habitats containing eight protozoa and rotifer species tested how dispersal altered community composition in both communities that were disturbed and communities that connected to refuge communities not subject to disturbance. While disturbance lowered population densities, in disturbed patches connected to undisturbed patches this was ameliorated by immigration. Furthermore, species with high dispersal abilities or growth rates showed the fastest post-disturbance recovery in presence of immigration. Connectivity helped to counteract the negative effect of disturbances on local populations, allowing mass-effect-driven dispersal of individuals from undisturbed to disturbed patches. In undisturbed patches, however, local population sizes were not significantly reduced by emigration. The absence of a cost of dispersal for undisturbed source populations is consistent with a lack of complex demography in our system, such as age- or sex-specific emigration. Our approach provides an improved way to separate components of population growth from organisms' movement in post-disturbance recovery of (meta)communities. Further studies are required in a variety of ecosystems to investigate the transient dynamics resulting from disturbance and dispersal.

Figure 1. Set-up of the experimental microcosms.

We had three different types of metacommunities (A, B and C), each consisting of two patches and all eightfold replicated. Metacommunities of type A consisted of two isolated patches, one of which (A2) was regularly disturbed, while the other (A1) was undisturbed. Metacommunities of type B consisted of two connected patches, one of which (B2) was regularly disturbed, while the other (B1) was undisturbed. Metacommunities of type C consisted of two connected patches (C1 and C2), which were both undisturbed.

Figure 2. Mean species richness at the first sampling (32 days).

Mean species richness (±se) within single communities in undisturbed and disturbed patches (white and grey bars respectively). A1 and A2 were isolated patches, while B1, B2 and C1 were connected patches (see also Fig. 1 and 2).

Figure 3. Mean species richness at the end of the experiment.

Mean species richness (±se) within single communities in undisturbed and disturbed patches (white and grey bars respectively). A1 and A2 were isolated patches, while B1, B2 and C1 were connected patches (see also Fig. 1).

Figure 4. Population densities after 32 and 43 days.

Species-specific linear correlations between the density in the first and second sampling, after 32 and 43 days respectively (density data log₁₀-transformed; for statistics see also table 1). In all but one species (Chi), density at the second sampling was strongly correlated with density at the first sampling. Abbreviations of the species: Chil.  =  Chilomonas sp., Colp. Colpidium sp., Eupl.  =  Euplotes aediculatus, P. aur.  =  Paramecium aurelia, P. bur.  =  Paramecium bursaria, Rot.  =  rotifer, and Spir.  =  Spirostomum sp.

Figure 5. Population densities of all protozoa and rotifer species.

Density (log₁₀) of all eight species within single communities in undisturbed and disturbed patches (white and grey boxplots respectively) after 32 days (first sampling; A–H) and at the end of the experiment after 43 days (I–Q). A1 and A2 were isolated patches, while B1, B2 and C1 were connected patches (see also Fig. 1). Boxplots give median (bold line), first and third quartile (box). Whiskers give either the range of the data or 1.5 times the interquartile range, whichever is smaller.

Figure 6. Relationship between population size and growth rate for Chilomonas sp. (A) and the rotifer species (B).

Black points show growth rates from 5 replicates each at high nutrients microcosms as generally used in our study. Lines show the best fit estimates derived for r and K. Although we show all points, r and K were determined separately for each microcosm and then averaged to generate the best fit line.

Figure 7. Relationship between species traits and species-specific differences in density between communities experiencing different treatments.

Each point stands for a different species. Treatments follow Fig. 1. For species that went extinct in one treatment, values could not be calculated. The predicted rate of spread (D, H, J) is calculated from data on growth rate and velocity. For relationships with p<0.1, the least-square line is given.