James J. Elser

Nutritional constraints in terrestrial and freshwater food webs

Biological and environmental contrasts between aquatic and terrestrial systems have hindered analyses of community and ecosystem structure across Earths diverse habitats. Ecological stoichiometry provides an integrative approach for such analyses, as all organisms are composed of the same major elements (C, N, P) whose balance affects production, nutrient cycling, and food-web dynamics. Here we show both similarities and differences in the C:N:P ratios of primary producers (autotrophs) and invertebrate primary consumers (herbivores) across habitats. Terrestrial food webs are built on an extremely nutrient-poor autotroph base with C:P and C:N ratios higher than in lake particulate matter, although the N:P ratios are nearly identical. Terrestrial herbivores (insects) and their freshwater counterparts (zooplankton) are nutrient-rich and indistinguishable in C:N:P stoichiometry. In both lakes and terrestrial systems, herbivores should have low growth efficiencies (10–30%) when consuming autotrophs with typical carbon-to-nutrient ratios. These stoichiometric constraints on herbivore growth appear to be qualitatively similar and widespread in both environments.

Regulation of lake primary productivity by food web structure

We performed whole-lake manipulations of fish populations to test the hypothesis that higher trophic levels regulate zooplankton and phytoplankton community structure, biomass, and primary productivity. The study involved three lakes and spanned 2 yr. Results demonstrated hierarchical control of primary production by abiotic factors and a trophic cascade involving fish predation. In Paul Lake, the reference lake, productivity varied from year to year, illustrating the effects of climatic factors and the natural dynamics of unmanipulated food web interactions. In Tuesday Lake, piscivore addition and planktivore reduction caused an increase in zooplankton biomass, a compositional shift from a copepod/rotifer assemblage to a cladoceran assemblage, a reduction in algal biomass, and a continuous reduction in primary productivity. In Peter Lake, piscivore reduction and planktivore addition decreased zooplanktivory, because potential planktivores remained in littoral refugia to escape from remaining piscivores. Both zooplankton biomass and the dominance of large cladocerans increased. Algal biomass and primary production increased because of increased concentrations of gelatinous colonial green algae. Food web effects and abiotic factors were equally potent regulators of primary production in these experiments. Some of the unexplained variance in primary productivity of the worlds lakes may be attributed to variability in fish populations and its effects on lower trophic levels.

Organism size, life history and N: P stoichiometry

E cosystem science and evolutionary biology have long been infrequent and uncomfortable bedfellows (Hagen 1992, Holt 1995, Mclntosh 1985). However, the convergence of a global decline in biodiversity and global alterations in biogeochemical cycles provides motivation to overcome past inhibitions. Currently, attempts are being made {Jones and Lawton 1995) to understand relationships between the foci of evolutionary biology (the individual in its species population) and ecosystem science (energy and material flow and storage). Analysis of relationships between species and ecosystems requires a framework appropriate for moving between levels

The stoichiometry of consumer-driven nutrient recycling : theory, observations, and consequences

Ecologists are increasingly recognizing the importance of consumers in reg- ulating ecosystem processes such as nutrient cycling. Ecologists have recently made con- siderable progress in understanding nutrient cycling and trophic interactions in pelagic systems by application of a new concept, ecological stoichiometry, to consumer-driven processes. In this paper we synthesize these conceptual advances within pelagic ecology and attempt to illustrate how they may be usefully applied in other ecosystems. Stoichi- ometric theory shows that both grazer and algal elemental composition are critical param- eters influencing rates and ratios of nutrient release. Thus, the stoichiometry of nutrient recycling is a feedback mechanism linking grazer dynamics and algal nutritional status. Incorporation of such effects into a fully dynamic stoichiometric model generates profound changes in the predicted dynamics of algae and grazers, suggesting that adoption of a stoichiometric view may substantively alter our view of the interaction between trophic dynamics and nutrient cycling. The basic predictions of stoichiometric models of nutrient release are generally supported by experimental data showing that N:P release ratios are primarily a function of algal N:P ratio and secondarily a function of grazer N:P ratio, and that rates of P release by grazers are also related to food P:C. Furthermore, evidence for effects of nutrient release stoichiometry on phytoplankton communities and pelagic eco- system function is accumulating, including data showing consistent alterations in algal physiological status and ecosystem-scale changes in N fixation in response to altered grazer community structure and elemental composition. As the general features of the stoichi- ometry of algae-zooplankton interactions reflect fundamental biological processes linked to plant and animal mineral nutrition, the stoichiometric view of consumer-driven nutrient recycling can easily be applied to other ecosystems, including terrestrial and benthic food webs. A suite of potential applications of stoichiometric thinking to benthic and terrestrial habitats is suggested.

Shifts in Lake N:P Stoichiometry and Nutrient Limitation Driven by Atmospheric Nitrogen Deposition

Nitrogen Overload The cycling of essential nutrients in terrestrial ecosystems has been altered by human activities. Elser et al. (p. 835) report a comparative analysis of lakes in Norway, Sweden, and in the United States that suggests that this is also true in aquatic ecosystems such as lakes. Deposition of anthropogenically derived atmospheric nitrogen controls whether N or P is growth-limiting for phytoplankton. Under elevated conditions of atmospheric N inputs, lake phytoplankton become consistently P-limited because the N:P ratio is strongly distorted. This is in contrast to conditions of low N deposition when lake phytoplankton are N-limited. These effects are even observed in remote lakes, demonstrating the indirect yet wide-ranging effects of humans on global food webs. Deposition of anthropogenically derived nitrogen can cause phosphorus to become the limiting nutrient of lake phytoplankton. Human activities have more than doubled the amount of nitrogen (N) circulating in the biosphere. One major pathway of this anthropogenic N input into ecosystems has been increased regional deposition from the atmosphere. Here we show that atmospheric N deposition increased the stoichiometric ratio of N and phosphorus (P) in lakes in Norway, Sweden, and Colorado, United States, and, as a result, patterns of ecological nutrient limitation were shifted. Under low N deposition, phytoplankton growth is generally N-limited; however, in high–N deposition lakes, phytoplankton growth is consistently P-limited. Continued anthropogenic amplification of the global N cycle will further alter ecological processes, such as biogeochemical cycling, trophic dynamics, and biological diversity, in the world’s lakes, even in lakes far from direct human disturbance.

Nutrient co-limitation of primary producer communities.

Synergistic interactions between multiple limiting resources are common, highlighting the importance of co-limitation as a constraint on primary production. Our concept of resource limitation has shifted over the past two decades from an earlier paradigm of single-resource limitation towards concepts of co-limitation by multiple resources, which are predicted by various theories. Herein, we summarise multiple-resource limitation responses in plant communities using a dataset of 641 studies that applied factorial addition of nitrogen (N) and phosphorus (P) in freshwater, marine and terrestrial systems. We found that more than half of the studies displayed some type of synergistic response to N and P addition. We found support for strict definitions of co-limitation in 28% of the studies: i.e. community biomass responded to only combined N and P addition, or to both N and P when added separately. Our results highlight the importance of interactions between N and P in regulating primary producer community biomass and point to the need for future studies that address the multiple mechanisms that could lead to different types of co-limitation.

The Light: Nutrient Ratio in Lakes: The Balance of Energy and Materials Affects Ecosystem Structure and Process

The amounts of solar energy and materials are two of the chief factors determining ecosystem structure and process. Here, we examine the relative balance of light and phosphorus in a set of freshwater pelagic ecosystems. We calculated a ratio of light: phosphorus by putting mixed‐layer mean light in the numerator and total P concentration in the denominator. This light: phosphorus ratio was a good predictor of the C:P ratio of particulate matter (seston), with a positive correlation demonstrated between these two ratios. We argue that the balance between light and nutrients controls “nutrient use efficiency” at the base of the food web in lakes. Thus, when light energy is high relative to nutrient availability, the base of the food web is carbon rich and phosphorus poor. In the opposite case, where light is relatively less available compared to nutrients, the base of the food web is relatively P rich. The significance of this relationship lies in the fact that the composition of sestonic material is known to influence a large number of ecosystem processes such as secondary production, nutrient cycling, and (we hypothesize) the relative strength of microbial versus grazing processes. Using the central result of increased C:P ratio with an increased light: phosphorus ratio, we make specific predictions of how ecosystem structure and process should vary with light and nutrient balance. Among these predictions, we suggest that lake ecosystems with low light: phosphorus ratios should have several trophic levels simultaneously carbon or energy limited, while ecosystems with high light: phosphorus ratios should have several trophic levels simultaneously limited by phosphorus. Our results provide an alternative perspective to the question of what determines nutrient use efficiency in ecosystems.

Phosphorus cycle: A broken biogeochemical cycle

Excess phosphorus is polluting our environment while, ironically, mineable resources of this essential nutrient are limited. James Elser and Elena Bennett argue that recycling programmes are urgently needed.

Nitrogen in Insects: Implications for Trophic Complexity and Species Diversification

Disparities in nutrient content (nitrogen and phosphorus) between herbivores and their plant resources have lately proven to have major consequences for herbivore success, consumer‐driven nutrient cycling, and the fate of primary production in ecosystems. Here we extend these findings by examining patterns of nutrient content between animals at higher trophic levels, specifically between insect herbivores and predators. Using a recently compiled database on insect nutrient content, we found that predators exhibit on average 15% greater nitrogen content than herbivores. This difference persists after accounting for variation from phylogeny and allometry. Among herbivorous insects, we also found evidence that recently derived lineages (e.g., herbivorous Diptera and Lepidoptera) have, on a relative basis, 15%–25% less body nitrogen than more ancient herbivore lineages (e.g., herbivorous Orthoptera and Hemiptera). We elaborate several testable hypotheses for the origin of differences in nitrogen content between trophic levels and among phylogenetic lineages. For example, interspecific variation in insect nitrogen content may be directly traceable to differences in dietary nitrogen (including dilution by gut contents), selected for directly in response to the differential scarcity of dietary nitrogen, or an indirect consequence of adaptation to different feeding habits. From some functional perspectives, the magnitude rather than the source of the interspecific differences in nitrogen content may be most critical. We conclude by discussing the implications of the observed patterns for both the trophic complexity of food webs and the evolutionary radiation of herbivorous insects.

CARBON SEQUESTRATION IN ECOSYSTEMS: THE ROLE OF STOICHIOMETRY

The fate of carbon (C) in organisms, food webs, and ecosystems is to a major extent regulated by mass-balance principles and the availability of other key nutrient elements. In relative terms, nutrient limitation implies excess C, yet the fate of this C may be quite different in autotrophs and heterotrophs. For autotrophs nutrient limitation means less fixation of inorganic C or excretion of organic C, while for heterotrophs nutrient limitation means that more of ingested C will “go to waste” in the form of egestion or respiration. There is in general a mismatch between autotrophs and decomposers that have flexible but generally high C:element ratios, and consumers that have lower C:element ratios and tighter stoichiometric regulation. Thus, C-use efficiency in food webs may be governed by the element ratios in autotroph biomass and tend to increase when C:element ratios in food approach those of consumers. This tendency has a strong bearing on the sequestration of C in ecosystems, since more C will be di...