The Cyanobacteria Prochlorococcus and Synechococcus account for a substantial fraction of marine primary production. Here, we present quantitative niche models for these lineages that assess present and future global abundances and distributions. These niche models are the result of neural network, nonparametric, and parametric analyses, and they rely on >35,000 discrete observations from all major ocean regions. The models assess cell abundance based on temperature and photosynthetically active radiation, but the individual responses to these environmental variables differ for each lineage. The models estimate global biogeographic patterns and seasonal variability of cell abundance, with maxima in the warm oligotrophic gyres of the Indian and the western Pacific Oceans and minima at higher latitudes. The annual mean global abundances of Prochlorococcus and Synechococcus are 2.9 ± 0.1 × 10 27 and 7.0 ± 0.3 × 10 26 cells, respectively. Using projections of sea surface temperature as a result of increased concentration of greenhouse gases at the end of the 21st century, our niche models projected increases in cell numbers of 29% and 14% for Prochlorococcus and Synechococcus, respectively. The changes are geographically uneven but include an increase in area. Thus, our global niche models suggest that oceanic microbial communities will experience complex changes as a result of projected future climate conditions. Because of the high abundances and contributions to primary production of Prochlorococcus and Synechococcus, these changes may have large impacts on ocean ecosystems and biogeochemical cycles.climate change | marine biogeochemistry | microbial biogeography M arine phytoplankton contribute roughly one-half of the global net primary production and play a key role in regulating global biogeochemical cycles (1). Marine phytoplankton are very diverse (2), including phylogenetic, biochemical, metabolic, and ecological variability (3-6). Thus, understanding the contribution of different phytoplankton groups to ecosystem functioning is central to predicting the biogeochemical impact of future environmental changes (7). However, our limited quantitative understanding of the global distribution and abundance of most phytoplankton groups constrains our ability to incorporate phytoplankton diversity into ocean biogeochemical models.The marine Cyanobacteria Prochlorococcus and Synechococcus are abundant in many ocean regions. The known geographical distributions of the two lineages are based primarily on individual cruises or time series observations and secondarily on macroecological descriptions, indicating central tendencies and boundary constraints related to light, temperature, nutrients, and chlorophyll a concentration (8-10). Prochlorococcus is present from the surface to a depth of ∼150 m in the open ocean between 40°N and 40°S. The population size declines beyond these latitudinal limits, and Prochlorococcus is thought to be absent at temperatures below 15°C (11). Furthermore, the lineage is believed to be out...

Summary 1.Hundreds of experiments have now manipulated species richness (SR) of various groups of organisms and examined how this aspect of biological diversity influences ecosystem functioning. Ecologists have recently expanded this field to look at whether phylogenetic diversity (PD) among species, often quantified as the sum of branch lengths on a molecular phylogeny leading to all species in a community, also predicts ecological function. Some have hypothesized that phylogenetic divergence should be a superior predictor of ecological function than SR because evolutionary relatedness represents the degree of ecological and functional differentiation among species. But studies to date have provided mixed support for this hypothesis. 2. Here, we reanalyse data from 16 experiments that have manipulated plant SR in grassland ecosystems and examined the impact on above-ground biomass production over multiple time points. Using a new molecular phylogeny of the plant species used in these experiments, we quantified how the PD of plants impacts average community biomass production as well as the stability of community biomass production through time. 3. Using four complementary analyses, we show that, after statistically controlling for variation in SR, PD (the sum of branches in a molecular phylogenetic tree connecting all species in a community) is neither related to mean community biomass nor to the temporal stability of biomass. These results run counter to past claims. However, after controlling for SR, PD was positively related to variation in community biomass over time due to an increase in the variances of individual species, but this relationship was not strong enough to influence community stability. 4. In contrast to the non-significant relationships between PD, biomass and stability, our analyses show that SR per se tends to increase the mean biomass production of plant communities, after controlling for PD. The relationship between SR and temporal variation in community biomass was either positive, non-significant or negative depending on which analysis was used. However, the increases in community biomass with SR, independently of PD, always led to increased stability. These results suggest that PD is no better as a predictor of ecosystem functioning than SR. 5. Synthesis. Our study on grasslands offers a cautionary tale when trying to relate PD to ecosystem functioning suggesting that there may be ecologically important trait and functional variation among species that is not explained by phylogenetic relatedness. Our results fail to support the hypothesis that the conservation of evolutionarily distinct species would be more effective than the conservation of SR as a way to maintain productive and stable communities under changing environmental conditions.

To predict the ecological consequences of biodiversity loss, researchers have spent much time and effort quantifying how biological variation affects the magnitude and stability of ecological processes that underlie the functioning of ecosystems. Here we add to this work by looking at how biodiversity jointly impacts two aspects of ecosystem functioning at once: (1) the production of biomass at any single point in time (biomass/area or biomass/ volume), and (2) the stability of biomass production through time (the CV of changes in total community biomass through time). While it is often assumed that biodiversity simultaneously enhances both of these aspects of ecosystem functioning, the joint distribution of data describing how species richness regulates productivity and stability has yet to be quantified. Furthermore, analyses have yet to examine how diversity effects on production covary with diversity effects on stability. To overcome these two gaps, we reanalyzed the data from 34 experiments that have manipulated the richness of terrestrial plants or aquatic algae and measured how this aspect of biodiversity affects community biomass at multiple time points. Our reanalysis confirms that biodiversity does indeed simultaneously enhance both the production and stability of biomass in experimental systems, and this is broadly true for terrestrial and aquatic primary producers. However, the strength of diversity effects on biomass production is independent of diversity effects on temporal stability. The independence of effect sizes leads to two important conclusions. First, while it may be generally true that biodiversity enhances both productivity and stability, it is also true that the highest levels of productivity in a diverse community are not associated with the highest levels of stability. Thus, on average, diversity does not maximize the various aspects of ecosystem functioning we might wish to achieve in conservation and management. Second, knowing how biodiversity affects productivity gives no information about how diversity affects stability (or vice versa). Therefore, to predict the ecological changes that occur in ecosystems after extinction, we will need to develop separate mechanistic models for each independent aspect of ecosystem functioning.

Current and expected changes in biodiversity have motivated major experiments, which reported a positive relationship between plant species diversity and primary production. As a first step in addressing this relationship, these manipulative experiments controlled as many potential confounding covariables as possible and assembled artificial ecosystems for the purpose of the experiments. As a new step in this endeavor, we asked how plant species richness relates to productivity in a natural ecosystem. Here, we report on an experiment conducted in a natural ecosystem in the Patagonian steppe, in which we assessed the biodiversity effect on primary production. Using a plant species diversity gradient generated by removing species while maintaining constant biomass, we found that aboveground net primary production increased with the number of plant species. We also found that the biodiversity effect was larger in natural than in artificial ecosystems. This result supports previous findings and also suggests that the effect of biodiversity in natural ecosystems may be much larger than currently thought.biodiversity ͉ carbon cycle ͉ ecosystem functioning ͉ Patagonian steppe ͉ resource partitioning H uman activities largely impact the natural rate of change in biodiversity by influencing species invasion, displacement, and extinction rates (1, 2). For this reason, it is crucial to understand the effects of biodiversity change on the functioning of ecosystems and their capacity for providing goods and services (1, 3). The first logical attempt to address the question of the effects of biodiversity on ecosystem functioning with an experimental approach was to create gradients of plant species richness by sowing different numbers of species into homogenized soils (4-6). These experiments with artificial ecosystems showed a positive relationship between plant species richness and productivity (4-8). A further step in our endeavor to assess the effects of biodiversity change on ecosystem functioning requires tackling this issue in natural ecosystems. Observations in natural ecosystems showed inconclusive evidence of the effect of plant species richness on productivity (4, 9-11). Manipulative experiments performed in naturally assembled communities can complement results from synthetic assemblages, which represent early successional stages (12). Here, we report an experiment designed to assess the magnitude of the plant species richness effect on aboveground net primary production (ANPP) in a natural ecosystem in the Patagonian steppe.Our hypotheses were that increased plant species diversity would result in increased ANPP (5,13,14) and that the effect of biodiversity on primary production would be higher in natural than in artificial ecosystems (15). Natural ecosystems should show higher niche partitioning and stronger positive biological interactions among organisms, because species coexisted for longer periods of time and because natural ecosystems have lower frequencies of disturbance (15). Niche partitioning is the use...