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The effects of temperature and dispersal on species diversity in natural microbial metacommunities

Citation

Parain, Elodie C.; Gray, Sarah M.; Bersier, Louis-Félix (2020), The effects of temperature and dispersal on species diversity in natural microbial metacommunities, Dryad, Dataset, https://doi.org/10.5061/dryad.xpnvx0kbf

Abstract

Dispersal is key for maintaining biodiversity at local- and regional scales in metacommunities. However, little is known about the combined effects of dispersal and climate change on biodiversity. Theory predicts that alpha-diversity is maximized at intermediate dispersal rates, resulting in a hump-shaped diversity-dispersal relationship. This relationship is predicted to flatten when competition increases. We anticipate that this same flattening will occur with increased temperature because, in the rising part of the temperature performance curve, interspecific competition is predicted to increase. We explored this question using aquatic communities of Sarracenia purpurea from early- and late-successional stages, in which we simulated four levels of dispersal and four temperature scenarios. With increased dispersal, the hump shape was observed consistently in late successional communities, but only in higher temperature treatments in early succession. Increased temperature did not flatten the hump-shape relationship, but decreased the level of alpha- and gamma-diversity. Interestingly, higher temperatures negatively impacted small-bodied species. These metacommunity-level extinctions likely relaxed interspecific competition, which could explain the absence of flattening of the diversity-dispersal relationship. Our findings suggest that climate change will cause extinctions both at local- and global- scales and emphasize the importance of intermediate levels of dispersal as an insurance for local diversity.

Methods

We sampled S. purpurea inquiline communities from the site ‘Champ Buet’ in Switzerland, which is situated at 500 m above sea level (CB, 46°36’50’’N, 6°34’50’’E). Eighty leaves that were nearly open were marked at the beginning of June 2014, and 80 additional nearly opened leaves were marked two weeks later. Thus, the inquiline communities within the leaves were allowed to develop for four weeks (‘late-succession’) and two weeks (‘early-succession’), respectively. All 160 leaves were sampled at the same time. The sampled water from each successional stage was place into two separate sterilized 1L Nalgene bottles. The bottles were brought back to the laboratory and chilled at 4°C to temporarily slow community dynamics until the set-up of the experiment the following day.

The overall density of the protists was measured for the pooled early- and pooled late- succession communities. The following procedure was then applied to both stages. The water was diluted in order to reach a density of 10’000 individuals of protists per mL and eighty 50 mL macrocentrifuge tubes were filled with a 10 mL aliquot of these dilutions. As a basal food resource, we added 500 µL of an autoclaved Tetramin fish food solution (concentration of 2 mg of solid fish food in 1 mL of DI water) into each tube. This resource is consumed by the bacteria in the system, which are then consumed by the protists and rotifers.

The 4 x 4 x 2 factorial design included four dispersal levels (No-, Low-, Medium-, and High-dispersal) and four temperature treatments (Local, -2.5°C below the local average temperature, +2.5°C and +5°C above the local average temperature) and two levels of community succession (Early succession and Late succession). Our temperature treatments were based on the natural June temperatures of the field site (minimum: 10°C, average: 15.5°C, maximum: 20.9°C) according to 30 years of data acquired by WorldClim (www.worldclim.org, accessed January 2017). Each treatment was composed of five tubes forming a metacommunity, totaling 160 tubes, which were placed in Panasonic MIR-154 incubators for the experiment, equipped with new lightbulbs and with light and temperature data loggers to exclude possible unwanted variability in our experiment; all tubes were placed in a randomized design within each incubator and this design was changed after every dispersal event. Dispersal was manipulated twice a week by transferring different numbers of individuals between the tubes of a treatment. These dispersal events were done separately for every treatment: the individuals of a treatment were only allowed to disperse within their specific metacommunity. Within each treatment, an aliquot of 100 µl was removed from each of the five tubes and combined into a 15 mL sterile macrocentrifuge tube. This mixture was then diluted with autoclaved DI water according to the dispersal level of that treatment. This dilution was necessary to maintain the same volume of water across all treatments, while allowing different numbers of individuals to disperse according to treatment. For the ‘high dispersal’ treatment, 100 µL of this mixture was returned into each of the 5 tubes without dilution. For the ‘medium dispersal’ treatment, the mixture was diluted ten times and added to each of the 5 tubes, and 100 times for the ‘low dispersal’ treatment. For the no-dispersal treatment, 100 µl aliquots were also removed and re-pipetted into the same tube. The experiment lasted for 7 weeks, after an initial incubation of 8 days in the “Local Temperature” incubator. Communities were fed once a week with 500 µL of fish food at the same concentration as described above. Every week, we sampled 100 µL of water in each tube after gently mixing, and estimated the density and composition of protist- and rotifer- species. Individuals were identified according to their morphology and categorized into 18 morphospecies. We used an inverted microscope at 100x magnification and a Thoma cell microscope plate to count the protists and rotifers. Densities of common species were estimated on two grids of the Thoma cell (number of individuals per 0.2 µL). Densities of rare species (observed only outside of the Thoma grid) were set to 0.1.

Funding

Swiss National Science Foundation, Award: 31003A_138489

Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung, Award: Grant 31003A_138489