Diatoms can be an important exception to temperature-size rules at species and community levels of organization

Abstract

Climate warming has been linked to an apparent general decrease in body sizes of ectotherms, both across and within taxa, especially in aquatic systems. Smaller body size in warmer geographical regions has also been widely observed. Since body size is a fundamental determinant of many biological attributes, climate-warming-related changes in size could ripple across multiple levels of ecological organization. Some recent studies have questioned the ubiquity of temperature-size rules, however, and certain widespread and abundant taxa, such as diatoms, may be important exceptions. We tested the hypothesis that diatoms are smaller at warmer temperatures using a system of geothermally heated streams. There was no consistent relationship between size and temperature at either the population or community level. These field data provide important counterexamples to both James' and Bergmann's temperature-size rules, respectively, undermining the widely held assumption that warming favours the small. This study provides compelling new evidence that diatoms are an important exception to temperature-size rules for three reasons: (i) we use many more species than prior work; (ii) we examine both community and species levels of organization simultaneously; (iii) we work in a natural system with a wide temperature gradient but minimal variation in other factors, to achieve robust tests of hypotheses without relying on laboratory setups, which have limited realism. In addition, we show that interspecific effects were a bigger contributor to whole-community size differences, and are probably more ecologically important than more commonly studied intraspecific effects. These findings highlight the need for multispecies approaches in future studies of climate warming and body size.

Keywords: Bergmann's rule; James’ rule; climate change; community size structure; diatoms; global warming; phytobenthos; phytoplankton; temperature-size relationships; temperature-size rule.

Figure 1

Plots showing conceptual differences among possible causes of overall community size changes with warming, for a two-species community. Compared to the reference distribution (a), community mean size could decrease from either the mean size of individual species decreasing (b), the relative abundance of small species increasing (c), species turnover effects (d) or a combination. The dashed line shows the mean body size in the reference distribution (a), and is for reference. Changes towards larger sizes could occur as well via the same three distinct routes or a combination. The abbreviation ‘sp’ is for ‘species’.

Figure 2

Map of the Hengill system within Iceland. Streams used are labelled using the numbering system outlined in Methods. The table shows mean August 2008 temperatures for the streams. The arrow is water flow direction.

Figure 3

Mean body sizes of diatoms in the fourteen sampled streams, with 95% confidence intervals of the means, plotted against temperature.

Figure 4

Relative importance of intraspecific and interspecific shifts in producing overall community size shifts in pairwise comparisons among streams, represented using vs. for 91 pairwise comparisons. Recall that . A 1:1 line divides the plot; points above the line represent pairwise comparisons where interspecific effects were the greater contributor to community size differences, and points below the line represent pairwise comparisons where intraspecific effects were the greater contributor. Triangles represent pairwise comparisons where intraspecific and interspecific effects were acting in opposite directions, and circles where they were acting in the same direction. Solid points represent pairwise comparisons where was positive, so body size increased with temperature between the two streams, and open points where was negative.