Species traits and species diversity affect community stability in a multiple stressor framework

Transcription

1 The following supplement accompanies the article Species traits and species diversity affect community stability in a multiple stressor framework Sabine Flöder 1, *, Helmut Hillebrand 2 1 Aquatic Ecology, Botanical Institute, University of Cologne, Gyrhofstr. 15, Cologne, Germany 2 Present address: Plankton Ecology, Institute for Chemistry and Biology of the Marine Environment (ICBM), Carl-von-Ossietzky University Oldenburg, Schleusenstrasse 1, Wilhelmshaven, Germany * sfloeder@yahoo.com Aquatic Biology 17: (2012) Supplement 1. Species used in the experiments, culture sources, information on taxonomy, form of organisation and biovolume Table S1. Strains in the species pool: CCAC code, taxon, form of organisation and biovolume. Except for Scenedesmus obliquus, the strains were provided by the culture collection of algae at the University of Cologne (CCAC); S. obliquus originates from the culture collection at the Max Planck Institute in Plön Species CCAC code Taxonomic group Form of organization Biovolume [µm 3 cell 1 ] Cylindrospermum sp. M 1160 Cyanobacterium Filaments 36 Cyclotella sp. M 2259 Diatom single cells short filaments 187 Melosira sp. M 1934 Diatom Filaments 752 Fragilaria capucina M 1767 Diatom Colonies 432 Cryptomonas pyrenoidifera M 1983 Cryptomonad single cells, flagellate 1588 Tetraselmis sp. M 1365 Chlorophyte single cells, flagellate Chlamydomonas tericola M 1259 Chlorophyte single cells, flagellate 406 Pediastrum sp. M 2625 Chlorophyte Colonies 350 Oocystis sp. M 1782 Chlorophyte four cells in mother cell wall 496 Tetraedron minimum M 1793 Chlorophyte single cells 298 Scenedesmus obliquus Chlorophyte single cells, coenobia 174

2 Supplement 2. Results of the monoculture experiment Species traits concerning competitive ability, and resistance and resilience in response to the stressors ph reduction and grazing differed among the species in the pool. Initial growth rates (Days 0 to 7) ranged from to d 1 (Table S2) and were significantly affected by species identity (F-ratio = 400.8, p > ). The lowest initial growth rate in the experiment, which differed significantly from all other initial growth rates (Tukey s HSD), was determined for the large green flagellate Tetraselmis sp., and the highest one for the small coccale green alga Scenedesmus obliquus. This high initial growth rate differed significantly (Tukey s HSD) from the growth rates of the other species in the pool, except for Fragilaria capucina and Cylindrospermum sp. The initial growth rates of 6 species (Ped, Tee, Cyc, Mel, Cyl and Fra, see Table S2 for abbreviations) were within the narrow range of 0.53 to 0.57 d 1 (insignificant differences within this group, Tukey s HSD), while the remaining 3 species (Ooc, Cry and Chl) initially grew with intermediate rates of 0.30 to 0.50 d 1 (excluding Chl and Ped, species in this group differed significantly from the other species, Tukey s HSD). Except for the colonial chlorophyte Pediastrum sp. and the slow-growing Tetraselmis sp. and Oocystis sp., the species in the pool either reached carrying capacity before the experimental stressor was applied or were in transition from exponential growth to carrying capacity. Species identity affected the remaining SRP concentration in the control treatments signifycantly (F-ratio = 3.49, p = 0.007). Four species reduced the phosphorus concentration of the medium to low levels, indicating high competitive abilities (Mel: 2.5 µg l 1, Chl: 2.6 µg l 1, Cyl: 2.9 µg l 1, Sce: 3.3 µg l 1 ). Species (F-ratio = 3.27, p < 0.004) and stressor identity (F-ratio = 5.26, p < 0.03) affected percent biovolume loss in the monoculture experiment (significant 2-way interaction, F-ratio = 2.53, p < 0.02). On average, more biovolume was lost due to grazing (30.54%) than to ph reduction (24.20%) (significant difference, Tukey s HSD). Losses due to ph reduction ranged from 1.52% (Ped) to 45.8% (Cry), and losses due to grazing from 18.1% (Fra) to 49.5% (Tes) (Table S2). Of the fast-growing species in the pool, Fragilaria capucina displayed low levels of biovolume loss due both stressors, but recovered slowly from ph reduction. Melosira sp. lost only 12% of its biovolume due to ph reduction and 27% due to grazing, but it did not recover from grazing. Based on its high initial growth rate, the comparably low biovolume loss due to ph reduction (17.0%) and grazing (23.5%) and the capability to recover within 5 d (recovery rate ph: 0.038, gra: 0.053), we identified Scenedesmus obliquus as the best-performing species in our species pool. For that reason, this species was chosen to be present in each richness level of species combination A (S. obliquus present). 2

3 Table S2. Species traits regarding growth, competitive ability, resistance and resilience. Initial growth rate (Days 0 to 7), final concentration of soluble reactive phosphorus (SRP) in control treatments, growth phase (exp: exponential; cc: carrying capacity; exp cc: transition from exp to cc) when stress was applied, biovolume loss (as percentage loss in relation to the averaged biovolume of the controls), recovery rate and recovery time of monocultures in response to the stressors ph reduction (ph) and grazing (gra). Cyl: Cylindrospermum sp.; Cyc: Cyclotella sp.; Mel: Melosira sp.; Fra: Fragilaria capucina; Cry: Cryptomonas pyrenoidifera; Tes: Tetraselmis sp.; Chl: Chlamydomonas tericola; Ped: Pediastrum sp.; Ooc: Oocystis sp.; Tee: Tetraedron minimum; Sce: Scenedesmus obliquus Species Initial growth rate (d 1 ) ± SE SRP (µg l 1 ) ± SE Growth phase % Biovolume loss ± SE Recovery rate (d 1 ) Recovery time (d) ph Gra ph Gra ph Gra Cyl ± ± 0.77 cc ± ± Cyc ± ± 3.27 cc ± ± Mel ± ± 1.12 exp cc ± ± Fra ± ± 0.22 cc ± ± Cry ± ± 0.18 cc ± ± Tes ± ± 1.38 exp ± ± Chl ± ± 1.14 cc ± ± Ped ± ± 1.41 exp 1.52 ± ± Ooc ± ± 0.76 exp ± ± Tee ± ± 0.73 exp cc ± ± Sce ± ± 0.55 exp cc ± ±

4 Supplement 3. Calculation of exact recovery dates To derive exact recovery dates, we used growth rates calculated for periods between postdisturbance sampling dates, and the corresponding B tot values determined for the last sampling date before recovery was reached. Growth rates were calculated according to: where µ denotes the specific growth rate per day, t 1 and t 2 are 2 points in time during the course of the experiment and B 1 and B 2 denote the total community biovolume at t 1 and t 2 respectively. Assuming communities grew at the same rate during periods between sampling dates, we replaced B 2 in Eq. (S1) by the B tot value that marked recovery and solved the equation for t 2. Recovery rate was then calculated for the time span between Day 14 (t 1 ) and recovery rate recovery date (t 2 ) using Eq. (S1). If a community did not recover within the course of the experiment, or the community growth rate was negative but growing species were present, community development was projected beyond the end of the experiment based on the growth of surviving species. The exponential regression B tot = ae bt was fitted to the projected community development, where B tot denotes total biovolume and t the time. Recovery time was calculated by inserting the recovery biovolume into the above formula (Eq. S2) and by solving the exponential function for t. In case a community did not recover and no growing species were present, the negative community growth rate determined for the end of the experiment was used reflecting a negative recovery rate. (S1) (S2) 4

5 Supplement 4. Details on the index of stressor-induced community co-tolerance The co-tolerance index is inspired by the conceptual model of species co-tolerances published in Vinebrooke et al. (2004) (Oikos 104: ), according to which species tolerance to multiple stressors can be positively or negatively correlated, or randomly distributed. A positive correlation would result in higher diversity, because species that survived the impact of Stressor A are likely to tolerate Stressor B (stressor-induced community tolerance). Conversely, a negative correlation between species tolerances to multiple stressors would lead to a strong decrease in biodiversity (stressor-induced community sensitivity). Vinebrooke et al. s model uses species loss as a measure to analyse community sensitivity to single and multiple stressors. Due to the considerable effort necessary to screen entire affected areas, however, assessing species loss as a result of disturbance is difficult in most habitats. In microalgal communities it is impossible, because of the small size of the organisms. A small number of surviving organisms (or in case of microalgal communities a single cell) can initiate the recovery of a population that may or may not be tolerant to another stressor. In contrast, a population may not recover from a disturbance, although a large number of organisms displaying no obvious signs of damage are still present after a disturbance. Parameters measuring population losses are not suitable as a substitute for species loss, since they provide no information about the post-disturbance performance of a species. Our co-tolerance index is based on post-disturbance growth rates and allows for population recovery from the stressors (before the effect of the last stressor). As a consequence, threshold levels for accepting stressor-induced community tolerance and stressor-induced community sensitivity are high compared to those in Vinebrooke et al. s conceptual model. Our simple co-tolerance index is calculated according to (S growd2 S growd1 ) S tot, where S growd1 is the number of species growing after the first disturbance, S growd2 is the number of species growing after the second disturbance and S tot is the total species number. The index is applicable if 90% S tot S grow 10% S tot, and if S grow = 0. Allowing for 2 stressors of similar strength, stressor-induced community tolerance is accepted if S growd2 S growd1 (Fig. S1, lines A,B). If S growd2 S growd1 < S tot 2, the stressor-induced community sensitivity (Fig. S1, line D) is assumed. Fig. S1. Schematic representation of the number of growing species after disturbance by multiple stressors in communities displaying (A, B) stressor-induced community tolerance, (C) randomly distributed species co-tolerances and (D) stressor-induced community sensitivity. Arrows denote the impact of a disturbance 5

6 The resulting co-tolerance index ranges from 1 to +1. Scores of 0 or higher indicate stressor-induced community tolerance. Scores between 0 and 0.5 indicate randomly distributed species co-tolerances and scores below 0.5 point towards stressor-induced community sensibility (Fig. S2). Examples I and VIII (Fig. S2) illustrate the extremes of the co-tolerance index. In Example I none of a total number of 10 species grow immediately after the first disturbance, but all species continue to grow after the second disturbance. The cotolerance index for this example is high (+1), because all populations recover from the stressor that causes the first disturbance, and they are resistant to the stressor that causes the second disturbance. In Example VIII, all populations continue to grow after the first disturbance, but cease to grow after the second disturbance. Since all species survive the impact of the first disturbance, but are sensitive to the stressor that caused the second disturbance, the cotolerance index is low ( 1). Post-stress growth rates determined for the diversity experiment are shown in Fig. S3 (for co-tolerance index, see Fig. 3 in the main document). Fig. S2. Examples of the co-tolerance index: (A, B) stressor-induced community tolerance, (C) randomly distributed species co-tolerances and (D) stressor-induced community sensitivity Fig. S3. Number of growing species after stressor application. The resulting co-tolerance index is published in the main document (Fig. 3). (A) Species combination A (incl. best-performing species), (B) species combination B (excl.). Lines serve to provide orientation only 6

7 Supplement 5. Population dynamics in the diversity experiment: initial growth rates before and after stressor application, and number of growing species Table S3. Individual growth rates before (pre, Days 0 to 7) and immediately after Stressor 1 (post 1, Days 8 to 13) and Stressor 2 (post 2, Days 14 to 17), and number of growing species for the treatments (ph ph, gra gra, ph gra, gra ph). For controls, individual growth rates and number of growing species were determined for Days 0 to 7, 8 to 17 and 8 to 31; 2, 4 and 8 species denote levels of species richness; I and II denote replicates; growing species (n) denotes the number of growing species. Species abbreviations, see Table S2 Species combination A Richness level 2 species 4 species Growing species (n) Replicate I II I II 2 sp. 4 sp. ph ph gra gra ph gra gra ph Control Species Sce Ooc Sce Ooc Sce Ooc Tee Chl Sce Ooc Tee Chl I II I II Pre Post Post Pre Post Post Pre Post Post Pre Post Post D D D

8 Species combination A Richness level 8 species Growing species (n) Replicate I II 8 species Species Sce Ooc Tee Chl Cry Mel Tes Ped Sce Ooc Tee Chl Cry Mel Tes Ped I II ph ph Pre Post Post gra gra Pre Post Post ph gra Pre Post Post gra ph Pre Post Post Control D D D

9 Species combination B Richness level 2 species 4 species Growing species (n) Replicate I II I II 2 sp. 4 sp. Species Cry Mel Cry Mel Cry Mel Tes Cyc Cry Mel Tes Cyc I II I II ph ph Pre Post Post gra gra Pre Post Post ph gra Pre Post Post gra ph Pre Post Post Control D D D

10 Species combination B Richness level 8 species Growing species (n) Replicate I II 8 species Species Cyl Cyc Tee Fra Tes Cry Mel Ped Cyl Cyc Tee Fra Tes Cry Mel Ped I II ph ph Pre Post Post gra gra Pre Post Post ph gra Pre Post Post gra ph Pre Post Post Control D D D

11 Supplement 6. Fig. S4. Time course of aggregate biovolume development in the diversity experiment. See Table S1 for full species names 11

12 Supplement 7. Dominance structure of the experimental communities Table S4. Species combination A: 2 species: Sce and Ooc, 4 species: Sce, Ooc, Tee and Chl, 8 species: Sce, Ooc, Tee, Chl, Cry, Mel, Tes and Ped. Species combination B: 2 species: Cry and Mel, 4 species: Cry, Mel, Tes and Cyc, 8 species: Cyl, Cyc, Tee, Fra, Tes, Cry, Mel and Ped. Biovolume-based dominance index and species identity (ID) before (Pre) and after (Post) stress application, and at the end (End) of the experiment. max B i : biovolume of dominant species i, B tot : total biovolume. I and II denote the replicates. Pre-stress data are given as averages ± SE (n = 8); 2, 4 and 8 species denote levels of species richness. Species abbreviations, see Table S2 Species Combination: Stress Time Dominance 2 species I II A 4 species I II 8 species I II 2 species I II B 4 species I II 8 species I II gra ph ph gra gra gra ph ph All Pre Post End Post End Post End Post End D B 0.86 ± ± ± ± ± ±.034 ID Sce Sce Sce Mel Mel Fra D B ID Sce Sce Sce Sce Sce Sce Mel Mel Mel Mel Cyl Cyl D B ID Sce Sce Sce Sce Sce Sce Mel Mel Mel Mel Fra Fra D B ID Sce Sce Sce Sce Sce Sce Mel Mel Mel Mel Cyl Cyl D B ID Sce Sce Sce Sce Sce Sce Cry Cry Cyc Cyc Cyl Cyl D B ID Sce Sce Sce Sce Sce Sce Mel Mel Mel Mel Cyl Fra D B ID Sce Sce Sce Sce Sce Sce Cry Cry Mel Mel Cyl Cyl D B ID Sce Sce Sce Sce Sce Sce Mel Mel Mel Mel Cyl Tee D B ID Sce Sce Sce Sce Sce Sce Mel Mel Mel Mel Fra Fra 12