Abstract
The size structure of autotroph communities – the relative abundance of small vs. large individuals – shapes the functioning of ecosystems. Whether common mechanisms underpin the size structure of unicellular and multicellular autotrophs is, however, unknown. Using a global data compilation, we show that individual body masses in tree and phytoplankton communities follow power-law distributions and that the average exponents of these individual size distributions (ISD) differ. Phytoplankton communities are characterized by an average ISD exponent consistent with three-quarter-power scaling of metabolism with body mass and equivalence in energy use among mass classes. Tree communities deviate from this pattern in a manner consistent with equivalence in energy use among diameter size classes. Our findings suggest that whilst universal metabolic constraints ultimately underlie the emergent size structure of autotroph communities, divergent aspects of body size (volumetric vs. linear dimensions) shape the ecological outcome of metabolic scaling in forest vs. pelagic ecosystems.
Highlights
The size structure of autotroph communities – the relative abundance of small vs. large individuals – shapes the functioning of ecosystems
The individual size distribution (ISD)—the frequency distribution of individual body sizes in a community—describes how energy and resources in an ecosystem is partitioned among individuals[8] and is one of the most extensively studied patterns in aquatic and terrestrial ecology[2,9,10,11,12,13]
Metabolic scaling theory (MST) proposes that the decline in the number of large individuals can be explained by the sub-linear scaling of metabolic rate with body mass[14,15], and by trade-offs between the number of individuals and the amount of resources that each individual can acquire in an ecosystem with finite resources[15,16,17]
Summary
The size structure of autotroph communities – the relative abundance of small vs. large individuals – shapes the functioning of ecosystems. Since metabolic rates tend to scale as M3/4 for large vascular plants[15,17,18,19] and eukaryotic algae[15,19,20,21,22], the theoretical expectation is that the ISD follows a power-law with an exponent approximating -3⁄4 in both tree and phytoplankton communities. This notion has received some empirical support[12,17], though many counter examples exist[10,11,23,24]. Previous tests of energetic equivalence have used a wide variety of aggregation methods[8], statistical techniques[25,26], and measures of body size[16,27] for assessing the scaling of abundance and body size in tree and phytoplankton communities, severely limiting efforts to reconcile these scaling laws across aquatic and terrestrial realms
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