topics

Organisms require energy and chemical elements ("nutrients") to build up biomass, to maintain basal metabolism and to produce offsprings. The capacity for a given organism to survive and reproduce depends on the availability of assimilable energy and nutrients in the environment relative to requirements.

Energy and nutrient requirements vary accross species, and within species, accross individuals and along the life cycle. For instance, fast growing species tend to require more phosphorus (P) than slow growing species because fast growing cells contain more ribosomes (the organelle of biosynthesis, the most P rich organelle in cell due to its high content of ribisomial RNA, see Elser et al. 1996). In the same vein, vertebrate species generally contain more P than most invertebrates, due to their P-rich skeleton.

These examples illustrates how stoichiometric constraints scale up from molecules to life history traits.

Zooplanktonic herbivore (Zoo) influence nutrient (nitrogen N and phosphorus P)
balance in the surface layer of the water column through diverse mechanisms.
They can control the fixation of nitrogen by consuming the n-fixers (Nfix), by consuming
the n-fixer competitors (Alg), and by influencing the competitive arena through
differential recycling of N vs P. Their biomass also represents a direct sink of both N and P.
In addition, they produce pellets (sinking material) that sink away from the surface layer.

I am interested in how phenotypic strategies contribute to define nutrient requirements in species. For instance, as mentioned earlier, maximum growth rate in unicellular organisms such as algae or bacteria relies on ribosome density in cells. On the other hand, the ability to take up energy and nutrients in autotrophs mainly relies on canal proteins and chloroplasts. Since P to N ratios are much higher in nucleic acids (DNA and RNA) than in proteins (figure 1), ribosomes have higher P to N ratios than canal proteins or chloroplast. As a consequence, "opportunistic" algae that can grow fast in case of abundant resource (nutrients and light) are likely to have higher P to N ratios than "gleaner" algae with low maximum growth rate but high resource affinity (Klausmeier et al. 2004).

In the same vein, like bones antlers in male cervids are demanding structures in both calcium (Ca) and P. Since antler's size partly define access to females, hence reproductive success, selection for larger antlers in males may lead to higher P and Ca requirements relative to other nutrients.

Hence, phenotypic strategies have costs in terms of nutrient and energy requirements. These chemical constraints are worth being explored because they are likely to shape cost-benefit trade-offs in phenotypic strategies and may therefore represent a key for a better understanding of evolutive mechanisms.

Species affect the biogeochemistry of their environment through diverse mechanisms, with indirect effects on other species. For instance, the quality of litter produced by terrestrial plants affect the recycling of essential nutrients, with consequences for the nutrition of other plant species. Hence, recycling can add up to direct competitive mechanisms (e. g. nutrient consumption) to enhance or deplete coexistance of plant species competiting for the same essential nutrients (Daufresne and Hedin 2005).

Another well documented example is the way zooplanktonic herbivores can affect P versus N recycling in the water column. High P to N taxons (e. g. Cladocerans) tend to retent more P in their biomass than lower N to P taxons (e. g., Copepods). As a consequence, Cladocerans tend to enhance P-limitation in autotrophs, whereas Copepods promote N-limitation (Sterner and Elser 2002, Daufresne and Loreau 2001a).

Large terrestrial herbivores can also impact biogeochemical cycles, by increasing or decreasing recycling speed (Hobbs 1996), or by enhancing nutrient retention or nutrient leaks from ecosystems (De Mazancourt et al.1999), with positive or negative consequences for global primary production.

I currently interested in identifying and documenting mechanisms by which cervids affect temperate forest biogeochemistry, especially the cycles of P and N. I am also attempting to develop a general theoretical framework of species interactions within biogeochemical cycles.

T. Daufresne INRA-CEFS

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	Main Research Topics
		I am interested in how chemical constraints shape growth an reproduction strategies, species interactions in communities, and ecosystem functions.
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	Organisms require energy and chemical elements ("nutrients") to build up biomass, to maintain basal metabolism and to produce offsprings. The capacity for a given organism to survive and reproduce depends on the availability of assimilable energy and nutrients in the environment relative to requirements. Energy and nutrient requirements vary accross species, and within species, accross individuals and along the life cycle. For instance, fast growing species tend to require more phosphorus (P) than slow growing species because fast growing cells contain more ribosomes (the organelle of biosynthesis, the most P rich organelle in cell due to its high content of ribisomial RNA, see Elser et al. 1996). In the same vein, vertebrate species generally contain more P than most invertebrates, due to their P-rich skeleton. These examples illustrates how stoichiometric constraints scale up from molecules to life history traits.
		Diversity of N:P ratios at different scales from molecules to organisms. P:N ratios are derived from Sterner and Elser (2002).


			In turn, organisms influence the biogeochemistry of their environment through consumption, excretion and transformation of nutrients and energy, with cascading effects on food webs, community structure, and ecosystem functioning (e. g., Sterner 1990, Hessen 1997). To better understand the links between biogeochemistry and ecological mechanisms such as evolution of life history traits, competitive interactions or trophic interactions among species a new approach reffered to as "Ecological stoichiometry" was recently proposed (Sterner and Elser 2002). Ecological stoichiometry revisits ecological principles in the light of stoichiometric constraints (Hessen, 1997). It was first developed by limnologists in the 90s, and has contributed to improve our understanding of plankonic food webs, with a particular focus on herbivory. Since then, it has spread to most branches of ecology, from micro to macro scales.
	Zooplanktonic herbivore (Zoo) influence nutrient (nitrogen N and phosphorus P) balance in the surface layer of the water column through diverse mechanisms. They can control the fixation of nitrogen by consuming the n-fixers (Nfix), by consuming the n-fixer competitors (Alg), and by influencing the competitive arena through differential recycling of N vs P. Their biomass also represents a direct sink of both N and P. In addition, they produce pellets (sinking material) that sink away from the surface layer.

	In the context ecological stoichiometry I am currently exploring three main topics: 1-What is the connection between phenotypic strategy and nutrient requirements in species? I am interested in how phenotypic strategies contribute to define nutrient requirements in species. For instance, as mentioned earlier, maximum growth rate in unicellular organisms such as algae or bacteria relies on ribosome density in cells. On the other hand, the ability to take up energy and nutrients in autotrophs mainly relies on canal proteins and chloroplasts. Since P to N ratios are much higher in nucleic acids (DNA and RNA) than in proteins (figure 1), ribosomes have higher P to N ratios than canal proteins or chloroplast. As a consequence, "opportunistic" algae that can grow fast in case of abundant resource (nutrients and light) are likely to have higher P to N ratios than "gleaner" algae with low maximum growth rate but high resource affinity (Klausmeier et al. 2004). In the same vein, like bones antlers in male cervids are demanding structures in both calcium (Ca) and P. Since antler's size partly define access to females, hence reproductive success, selection for larger antlers in males may lead to higher P and Ca requirements relative to other nutrients. Hence, phenotypic strategies have costs in terms of nutrient and energy requirements. These chemical constraints are worth being explored because they are likely to shape cost-benefit trade-offs in phenotypic strategies and may therefore represent a key for a better understanding of evolutive mechanisms.


	Large (left) and small (right) antlers from two roe deer bucks.

	2-How does ecosystem biogeochemistry influence individual traits and species demography?
	For a given species, energy and nutrient requirements of individuals may vary depending on the sex, the age, the current activity etc... Thus, demographic responses to changes in ecosystem biogeochemistry are not trivial. For instance, some change occuring in the biogeochemistry of a given nutrient may affect nutrient availability, with cascading effects on some specific age classes, or individuals of one sex but not the other. Cascading effects will have consequences on demographic traits such as the age structure of the population, the sex ratio, etc.... I am intersted in deriving population models that intergate the effect of all the main nutrients and energy on each age and sex class, and activities along the life cycle, in order to study and predict the effect of ecosystem biogeochemistry on species demography. Target species at the moment are wild ungulates such as roe dee, red deer and montane ungulates such as chamois, ibex and mouflon. In each species, I am interested in comparing populations living in contrated habitats in terms of biogeochemistry.
			Leg length measurement on live roe deer from a free population near Aurignac, South-Western France.

	3-How do species influence ecosystem biogeochemistry, hence, indirectly affect other species?
	Species affect the biogeochemistry of their environment through diverse mechanisms, with indirect effects on other species. For instance, the quality of litter produced by terrestrial plants affect the recycling of essential nutrients, with consequences for the nutrition of other plant species. Hence, recycling can add up to direct competitive mechanisms (e. g. nutrient consumption) to enhance or deplete coexistance of plant species competiting for the same essential nutrients (Daufresne and Hedin 2005). Another well documented example is the way zooplanktonic herbivores can affect P versus N recycling in the water column. High P to N taxons (e. g. Cladocerans) tend to retent more P in their biomass than lower N to P taxons (e. g., Copepods). As a consequence, Cladocerans tend to enhance P-limitation in autotrophs, whereas Copepods promote N-limitation (Sterner and Elser 2002, Daufresne and Loreau 2001a). Large terrestrial herbivores can also impact biogeochemical cycles, by increasing or decreasing recycling speed (Hobbs 1996), or by enhancing nutrient retention or nutrient leaks from ecosystems (De Mazancourt et al.1999), with positive or negative consequences for global primary production. I currently interested in identifying and documenting mechanisms by which cervids affect temperate forest biogeochemistry, especially the cycles of P and N. I am also attempting to develop a general theoretical framework of species interactions within biogeochemical cycles.

	A Cladoceran, Daphnia magna, feeding on unicellular algae. (Photo: Tatiana Voza)	Black-tailed deer (Odocoileus hemionus) in temperate rain forest (Haida Gwaii, BC, Canada). The fence on the left prevents deer browsing . (The photo on the right is by J. Merlet)

	References cited: Daufresne, T., and M. Loreau (2001a) Plant-herbivore interactions and ecological stoichiometry: when do herbivores determine plant nutrient limitation? Ecology Letters 4 : 196-206. Daufresne, T. and L. O. Hedin (2005) Plant coexistence depends on ecosystem nutrient cycles: Extension of the resource-ratio theory. PNAS 102 (26): 9212-9217. De Mazancourt, C., M. Loreau, and L. Abbadie (1999) Grazing optimization and nutrient cycling: potential impact of large herbivores in a savana system. Ecological Applications 9(3): 784-797. Elser J.J., D. Dobberfuhl, N. A. MacKay, and J. H. Schampel (1996) Organism size, life history, and N:P stoichiometry: towards a unified view of cellular and ecosystem processes. BioScience 46: 674-684. Hessen, D. O. (1997) Stoichiometry in food-webs. Lotka revisited. Oikos 79 (1): 195-200. Hobbs, N. T. (1996) Modification of ecosystems by ungulates. Journal of Wildlife Management 60(4): 695-713. Klausmeier, C., E. Litchman, T. Daufresne, and S. Levin. (2004) Optimal Nitrogen to Phosphorus Stoichiometry of Phytoplankton. Nature 529 : 171-174. Sterner, R. W. (1990) The ratio of nitrogen to phosphorus resupplied by herbivores: zooplankton and the algal competitive arena. The American Naturalist 136(2): 209-229. Sterner, R. W., and J. J. Elser (2002) Ecological Stoichiometry. Princeton, Princeton University Press, Princeton, NJ. Tilman, D. (1982) Resource competition and community structure. Princeton University Press, Princeton, NJ.

	Last updated 01/01/2009