Experimental evidence that evolutionary relatedness does not affect ecosystem functioning in freshwater algal communities

Background/Question/Methods

The impact of biodiversity loss on the functioning of ecosystems has been an active area of research for two decades. We now know that declining species richness results in reduced standing stocks of biomass and lower resource use efficiency. However, the functional traits of species that impact their interaction strengths and therefore also ecosystem functioning are still largely unknown. Identifying and measuring the functional traits responsible for biodiversity-ecosystem functioning relationships has been difficult and labor-intensive. As a result, a growing number of studies have used evolutionary relatedness (or phylogenetic distance) between species as a proxy for functional differentiation and tested whether relatedness impacts ecosystem functioning.

Here we report the results of a microcosm experiment using freshwater green algae in which we manipulated the phylogenetic distance of communities containing two species, and measured resource use efficiency and rates of primary production after 46 days. The microcosms were maintained using semi-continuous culture under constant light and temperature. On the final day of the experiment we measured levels of dissolved nitrate, ammonium and phosphate, and levels of gross primary production, respiration and net primary production.

Results/Conclusions

Phylogenetic distance (PD) did not affect remaining dissolved nitrate (F_1,82  =  1.68, P = 0.20), ammonium (F_1,82  =  0.35, P = 0.56) or phosphate (F_1,82  =  0.02,  P =0.88). There was also no effect of PD on levels of respiration (log+1, F_1,81 = 1.28, P=0.26), gross primary production (log +1, F_1,81 = 0.08, P =0.78) or net primary production (F_1,81 = 0.13, 0.72). This was despite significant effects of species richness and community composition on the availability of all nutrients (P<0.05), a positive effect of species richness on net primary production (F_1,78 =  6.34, P  = 0.01), and significant effects of community composition on gross and net primary production (GPP, F_27,78 = 1.93 P = 0.01; NPP, F_27,78  = 1.80 P = 0.02). Bicultures tended to reduce nitrate and phosphate concentrations by 33% and 48% more than expected based on monocultures, respectively. On average bicultures also displayed 70% more primary production than monocultures.

These results confirm findings from previous studies showing that high diversity communities have greater nutrient use efficiency and levels of primary production than low diversity communities. However, evolutionary relatedness did not explain levels of ecosystem functioning, suggesting that the traits responsible for the strength of species' interactions and ecosystem functioning are not conserved among species of freshwater green algae.