Diversity of Organismal Stoichiometry

All organisms contain the same major elements (e.g., C, N, P, Ca, hydrogen (H), oxygen (O), S (sulfur)), but existing information indicates that the concentrations of these elements in organisms can differ substantially among and within taxonomic groups. Patterns of elemental abundance are best described for C, N, and P (Table 1), which have received most of the attention in ecological stoichiometry because of their importance in biological structures and because N and P commonly limit production in nature.

First, let us examine the relative amounts of C, N, and P in plants and other autotrophic organisms. The C:N:P composition of individual autotrophs is generally quite variable because molecules containing these elements can be stored in large quantities in vacuoles or in the cytoplasm. Despite this variability, there are still clear differences in C:N:P composition among species. One way to categorize this variation is by making comparisons among major habitats, such as oceanic, freshwater, and terrestrial systems. In oceans, particulate matter (which provides a measure of phytoplankton biomass) tends to be rich in N and P relative to autotrophic biomass in freshwater and terrestrial systems. Seminal work in the mid-1900s by Alfred Redfield showed that the relative amount of C, N, and P in marine particulate matter was 106:16:1 (molar ratio). This description is referred to as the Redfield ratio. More recent findings have shown that although the average C:N:P composition of marine parti-culate matter tends to be relatively homogeneous across sampling locations, there is more than threefold variation in C:P and N:P ratios across different phytoplankton phyla and superfamilies. In lakes, particulate matter tends to contain much less P than it does in oceans. On average, C:P ratios in freshwater particulates are about 300, and N:P ratios are about 30; these values are

Table 1 Approximate ranges for C, N, and P concentrations across species in select taxa

Taxon

% C

% N

%P

Terrestrial plants (leaves)

36-64a

0.25-6.4b

0.02-1.0b

Benthic invertebrates

35-57c

6-12.0c

<0.2-1.8c

Zooplankton

7-12.5a

0.5-2.5a

Terrestrial insects

36-61a

6-12.0d

0.35-1.5e

Freshwater fish

~40-50'

8-12.0'

1.5-4.5'

Birds and mammals

0.7-3.79

aElser JJ, Fagan WF, Denno R F, et al. (2000) Nutritional constraints in terrestrial and freshwater food webs. Nature 408: 578-580. bGüsewell S (2004) N:P ratios in terrestrial plants: Variation and functional significance. New Phytologist 164: 243-266.

cCross WF, Benstead JP, Rosemond AD, and Wallace JB (2003) Consumer-resource stoichiometry in detritus-based streams. Ecology Letters 6:721-732. dFagan WF, Siemann E, Mittler C, et al. (2002) Nitrogen in insects: Implications for trophic complexity and species diversification. American Naturalist 160: 784-802.

eWoods HA, Fagan WF, Elser JJ, Harrison JF (2004) Allometric and phylogenetic variation in insect phosphorus content. Functional Ecology 18:103-109. fSterner RW and George NB (2000) Carbon, nitrogen and phosphorus stoichiometry of cyprinid fishes. Ecology 81: 127-140.

gGillooly JF, Allen AP, Brown JH, etal. (2005) The metabolic basis of whole-organism RNA and phosphorus content. Proceedings of the National Academy of Sciences of the United State of America 102:11923-11927.

Element concentrations are reported as % dry mass. Data are for whole organisms unless otherwise noted.

aElser JJ, Fagan WF, Denno R F, et al. (2000) Nutritional constraints in terrestrial and freshwater food webs. Nature 408: 578-580. bGüsewell S (2004) N:P ratios in terrestrial plants: Variation and functional significance. New Phytologist 164: 243-266.

cCross WF, Benstead JP, Rosemond AD, and Wallace JB (2003) Consumer-resource stoichiometry in detritus-based streams. Ecology Letters 6:721-732. dFagan WF, Siemann E, Mittler C, et al. (2002) Nitrogen in insects: Implications for trophic complexity and species diversification. American Naturalist 160: 784-802.

eWoods HA, Fagan WF, Elser JJ, Harrison JF (2004) Allometric and phylogenetic variation in insect phosphorus content. Functional Ecology 18:103-109. fSterner RW and George NB (2000) Carbon, nitrogen and phosphorus stoichiometry of cyprinid fishes. Ecology 81: 127-140.

gGillooly JF, Allen AP, Brown JH, etal. (2005) The metabolic basis of whole-organism RNA and phosphorus content. Proceedings of the National Academy of Sciences of the United State of America 102:11923-11927.

Element concentrations are reported as % dry mass. Data are for whole organisms unless otherwise noted.

substantially higher than the Redfield values of 106 and 16. Phosphorus content in lake particulates is also highly variable: N:P ratios in one survey ranged from 6.5 to 125 across sites. Mean C:N ratio in lake particulates is about 10, similar to values for ocean particulate matter. Terrestrial autotrophs tend to have much higher C:N and C:P ratios than oceanic and freshwater autotrophs: the mean C:N ratio in leaves is around 36, and the mean C:P ratio is near 1000. C:nutrient ratios in whole plants are likely even higher than ratios in leaves because wood, bark, and other secondary growth generally contain low concentrations of N and P. Concentrations of N and P in leaves also differ substantially among plant species (Table 1), showing 25- and 50-fold ranges of variation, respectively.

Next, consider the elemental composition of animals. Because animals have limited capacity for nutrient storage and possess mechanisms for selectively acquiring and retaining particular substances, individual animals tend to maintain their elemental composition within limited bounds. However, there are often significant differences in whole-body stoichiometry among animal species.

Patterns of elemental abundances in invertebrates have been most thoroughly cataloged for two groups: zooplankton and insects. Zooplankton species, which consist mostly of rotifers, cladocerans, copepods, and other small crustaceans, vary considerably in P content, but are less variable in terms of N and C (Table 1). For example, P concentration (% dry mass) in zooplankton varies fivefold across species, whereas zooplankton N content varies less than twofold. Terrestrial insect species also vary considerably in P concentration, which can range from ~0.35% to ~1.5% dry mass in adults (Table 1). As in zooplankton, N concentration in insect species is more tightly constrained, varying only about twofold (~6—12% dry mass) among adults across species. Zooplankton and insects contain similar concentrations of C, N, and P, but both groups contain considerably more N and P than autotrophs (Table 1 ). The compositional disparity between invertebrate animals and autotrophs is particularly striking in terrestrial systems.

Less information is available about whole-body C:N:P stoichiometry for vertebrates, although descriptions are available for some fish, birds, and mammals (Table 1). In fish, P content can range from 1.5% to 4.5%. The N content in fish is again less variable (8-12%) than P content, and mean values are similar to levels found in invertebrates. P content appears also to be high and variable in terrestrial vertebrates. For example, reported levels for P content range from 0.67% in pigs to 3.67% in humans. P concentrations in vertebrates are considerably higher than in invertebrates due in large part to the high P content of bone.

These comparisons illustrate that there are clear compositional differences among major taxonomic groups: the low N and P concentrations in terrestrial plants and the high P content in vertebrates are particularly distinctive. Perhaps more importantly, there is also considerable variation in C:N:P stoichiometry within groups such as zooplankton, insects, and fish, whose members have many morphological and ecological similarities. It is thus likely that the adaptations distinguishing species are reflected in the elemental composition of organisms. If so, the chemical composition of organisms may be a key factor guiding the evolutionary dynamics of populations. To get a better understanding of the evolutionary relevance of stoichiometric variation, we must first explore how elemental composition is connected to functional capabilities that ultimately affect an organism's reproductive success. Useful information for making this connection comes from a multilevel analysis of organismal biochemistry.

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