Adaptation and competition in deteriorating environments

Abstract

Evolution might rescue populations from extinction in changing environments. Using experimental evolution with microalgae, we investigated if competition influences adaptation to an abiotic stressor, and vice versa, if adaptation to abiotic change influences competition. In a first set of experiments, we propagated monocultures of five species with and without increasing salt stress for approximately 180 generations. When assayed in monoculture, two of the five species showed signatures of adaptation, that is, lines with a history of salt stress had higher population growth rates at high salt than lines without prior exposure to salt. When assayed in mixtures of species, however, only one of these two species had increased population size at high salt, indicating that competition can alter how adaptation to abiotic change influences population dynamics. In a second experiment, we cultivated two species in monocultures and in pairs, with and without increasing salt. While we found no effect of competition on adaptation to salt, our experiment revealed that evolutionary responses to salt can influence competition. Specifically, one of the two species had reduced competitive ability in the no-salt environment after long-term exposure to salt stress. Collectively, our results highlight the complex interplay of adaptation to abiotic change and competitive interactions.

Keywords: abiotic stressor; adaptation; competition; eco-evolutionary dynamics; environmental change; experimental evolution.

Figure 1.

Experimental design. In experiment 1, we propagated monocultures of six species of microalgae with or without increasing salt for approximately 185 generations, and then quantified growth rate in monoculture and population size in mixtures of species with and without salt. White boxes are flasks without salt, grey boxes are flasks with salt. In experiment 2, we propagated two species in monoculture and in pairs, with and without salt. After approximately 230 generations, we reisolated the species on agar plates and quantified the chlorophyll a concentration (as a proxy for biomass) of all lines when competing either with a competitor of the same salt and competition history (coevolved competitor) or with a reference line (naive competitor; salt history: no salt, competition history: monoculture). Bold species abbreviations denote lines cultivated in pairs during the selection experiment.

Figure 2.

Effects of salt history and assay environment on the monoculture growth rates of five species of microalgae (sorted by salt tolerance). Control lines (blue/light grey) have a salt history of 37 transfers at 0 g of NaCl l⁻¹, selection lines (red/dark grey) have a salt history of 37 transfers at increasing salt stress with a final concentration of 6 g of NaCl l⁻¹. Salt concentrations in the two assay environments were the same as in the final selection environments, i.e. 0 and 6 g of NaCl l⁻¹, respectively. Insets give the p-values for assay environment (A), salt history (H) and their interaction (AxH). Bold font denotes p-values < 0.05. For F-statistics, see the electronic supplementary material, table S2. Values are means ± s.e., n = 3. (Online version in colour.)

Figure 3.

Effects of salt history and assay environment on species abundances in the mixture assay. Mixtures of six species were assembled either from control lines (blue/light grey) or from selection lines (red/dark grey). Species abundances were averaged over four transfers in assay environments with and without salt, respectively. Insets give the p-values for assay environment (A), salt history (H) and their interaction (AxH). For F-statistics, see the electronic supplementary material, table S3. Abundances were ln-transformed prior to analyses except for Pseudokirchneriella. For consistency with figure 2, we do not show Anabaena, which was also part of the communities. For time-resolved data of all six species, see the electronic supplementary material, figure S3. Values are means ± s.e. (s.e. too small to be visible), n = 3. (Online version in colour.)

Figure 4.

Effects of competition history, salt history and assay environment on competitive abilities of Anabaena and Scenedesmus. Relative chlorophyll a of (a) Anabaena in competition with a reference line of Scenedesmus and (b) of Scenedesmus in competition with a reference line of Anabaena. Values are means ± s.e., n = 3. (Online version in colour.)