“The biogeographical patterns of species richness and abundance distribution in stream diatoms are driven by climate and water chemistry”
Sophia I. Passy, Chad A. Larson, Aurélien Jamoneau, William Budnick, Jani Heino, Thibault Leboucher, Juliette Tison-Rosebery, and Janne Soininen (Nov 2018)
Stream diatom richness and abundance distribution at subcontinental scales are driven by climate and water chemistry
The declines in biodiversity at higher latitudes and elevations are among the oldest studied, yet still not fully explained ecological patterns. Related to biodiversity is the balance between common and rare species, reflected in the shape of the species abundance distribution (SAD). Higher biodiversity is linked to greater community functionality and improved services to humans, such as increased water quality. The degree of species commonness and rarity has implications for ecosystem functions and conservation. While there are many, predominantly climate-based theories and hypotheses about the spatial variability in biodiversity, little is known about the patterns and causes of variability in the SAD. To address this deficiency, an international team of scientists, led by Dr. Sophia Passy from the University of Texas at Arlington, has explored the biodiversity and SAD of diatoms, an important group of producers in stream ecosystems. They demonstrated that in both the US and Finland, diatom richness and the SAD exhibited distinct spatial patterns (i.e., primarily longitudinal in the US, but latitudinal in Finland), deviating from prior observations and thus inconsistent with existing climate-based concepts. These patterns were instead described with climate-water chemistry models, showing that more diverse communities with a more even distribution of common and rare species occur in streams of higher temperature seasonality and total phosphorus levels. Given that temperature seasonality is projected to decrease with global warming because the cold months are becoming warmer, diatom biodiversity and abundance equality may decline and lead to diminished community services. The operation of both climate and water chemistry mechanisms in structuring diatom communities underscores their complex response to the environment and the necessity for novel predictive frameworks.
In this inter-continental study of stream diatoms, we asked three important but still unresolved ecological questions: 1) What factors drive the biogeography of species richness and species abundance distribution (SAD); 2) Are climate-related hypotheses, which have dominated the research on the latitudinal and altitudinal diversity gradients, adequate in explaining spatial biotic variability; and 3) Is the SAD response to the environment independent of richness? We tested a number of climatic theories and hypotheses (i.e., the species-energy theory, the metabolic theory, the energy variability hypothesis, and the climatic tolerance hypothesis) but found no support for any of these concepts as the relationships of richness with explanatory variables were non-existent, weak or unexpected. Instead, we demonstrated that diatom richness and SAD evenness generally increased with temperature seasonality and at mid- to high total phosphorus concentrations. The spatial patterns of diatom richness and the SAD—mainly longitudinal in the US, but latitudinal in Finland—were defined primarily by the covariance of climate and water chemistry with space. The SAD was not entirely controlled by richness, emphasizing its utility for ecological research. Thus, we found support for the operation of both climate and water chemistry mechanisms in structuring diatom communities, which underscores their complex response to the environment and the necessity for novel predictive frameworks.