DOI: 10.5061/dryad.xpnvx0kbf
title: The effects of temperature and dispersal on species diversity in natural microbial metacommunities

description: 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.
description: We sampled <i>S. purpurea</i> 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.