DOCSLIB.ORG

Light, Nutrients, and Food-Chain Length Constrain Planktonic Energy Transfer Efficiency Across Multiple Trophic Levels

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Light, Nutrients, and Food-Chain Length Constrain Planktonic Energy Transfer Efficiency Across Multiple Trophic Levels Light, nutrients, and food-chain length constrain planktonic energy transfer efficiency across multiple trophic levels Elizabeth M. Dickman1, Jennifer M. Newell1, María J. Gonza´ lez, and Michael J. Vanni2 Department of Zoology, Miami University, Oxford, OH 45056 Edited by David M. Karl, University of Hawaii, Honolulu, HI, and approved October 3, 2008 (received for review June 8, 2008) The efficiency of energy transfer through food chains [food chain carnivores, because their tissue chemical composition is similar efficiency (FCE)] is an important ecosystem function. It has been to that of their prey (8). Thus, it has been proposed that the hypothesized that FCE across multiple trophic levels is constrained carbon/nutrient stoichiometry of primary producers constrains by the efficiency at which herbivores use plant energy, which energy transfer across multiple trophic levels, i.e., from primary depends on plant nutritional quality. Furthermore, the number of producers to carnivores (8, 9), but this hypothesis has not been trophic levels may also constrain FCE, because herbivores are less explicitly tested. efficient in using plant production when they are constrained by The stoichiometry of aquatic primary producers (algae) often carnivores. These hypotheses have not been tested experimentally reflects the supply of nutrients and light (8, 10, 11). Algal cell in food chains with 3 or more trophic levels. In a field experiment carbon/nitrogen (C/N) and carbon/phosphorus (C/P) ratios de- manipulating light, nutrients, and food-chain length, we show that crease with increasing nutrients and decreasing light intensity FCE is constrained by algal food quality and food-chain length. FCE (10, 12, 13), and the ecological efficiency of aquatic herbivores across 3 trophic levels (phytoplankton to carnivorous fish) was is often higher under low light and/or high nutrient conditions, highest under low light and high nutrients, where algal quality was when algal C/P is relatively low (14–16). However, other aspects best as indicated by taxonomic composition and nutrient stoichi- of algal food quality may also covary with stoichiometry, such as ometry. In 3-level systems, FCE was constrained by the efficiency at morphological features (e.g., size, shape, presence of gelatinous which both herbivores and carnivores converted food into pro- sheaths) and biochemicals (e.g., essential fatty acid and sterol duction; a strong nutrient effect on carnivore efficiency suggests a concentrations) (11, 17). Because algal species differ in these carryover effect of algal quality across 3 trophic levels. Energy characteristics, phytoplankton taxonomic identity may be a transfer efficiency from algae to herbivores was also higher in surrogate of food quality. 2-level systems (without carnivores) than in 3-level systems. Our We explored the general hypothesis that FCE depends on light results support the hypothesis that FCE is strongly constrained by intensity, nutrient supply, and food-chain length. This field study light, nutrients, and food-chain length and suggest that carryover explicitly quantifies how light and nutrients interactively regulate effects across multiple trophic levels are important. Because many FCE in systems with 3 trophic levels. In a field experiment using environmental perturbations affect light, nutrients, and food- aquatic mesocosms, we tested 3 specific hypotheses: (i) FCE (in chain length, and many ecological services are mediated by FCE, it food chains of equal length) is highest under low light/high will be important to apply these findings to various ecosystem nutrient conditions and lowest at high light/low nutrients; (ii) types. herbivore ecological efficiency is higher in food chains with just 2 trophic levels than with 3 trophic levels, because herbivores are ͉ ͉ ͉ ͉ ecological efficiency ecological stoichiometry fish zooplankton unconstrained by predation in 2-level systems; and (iii) because phytoplankton herbivores are more constrained than carnivores by food quality, FCE across 3 trophic levels is constrained by herbivore ecological lucidating the constraints on the efficiency of energy transfer efficiency (ratio of herbivore production to primary production). Ethrough food chains is necessary for understanding many ecological processes (1–6). Food chain efficiency (FCE), defined Results as the proportion of energy fixed by primary producers that is In treatments with 3 trophic levels, FCE increased with decreas- transferred to the top trophic level, depends on the ecological ing light (P Ͻ 0.0001) and increasing nutrients (P ϭ 0.0004), and efficiencies at each trophic coupling (1). FCE can regulate was highest in the low light/high nutrient treatment, as predicted attributes such as food-chain length and biomass (7) and eco- by our first hypothesis (Fig. 1A). Across all treatments, herbivore system services such as fisheries production (2, 3), export of efficiency was affected by the main and interactive (P ϭ 0.0010) carbon from ecosystems (4), and concentrations of contaminants effects of light and fish and was greater under low light condi- in organisms (5). tions than under high light (P ϭ 0.0003). In support of our second Although FCE may regulate the number of trophic levels, the hypothesis, herbivore efficiency was much higher in the absence reverse may also be true: the number of trophic levels may of fish than in their presence (P Ͻ 0.0001; Fig. 1 B and C). determine FCE (1). With 3 trophic levels (plants, herbivores, and carnivores), herbivores may be held in check by carnivores and thus inefficiently consume plant biomass. However, with 2 (or 4) Author contributions: E.M.D., J.M.N., M.J.G., and M.J.V. designed research; E.M.D. and levels, herbivore biomass is unconstrained (or less constrained) J.M.N. performed research; E.M.D., J.M.N., M.J.G., and M.J.V. analyzed data; and E.M.D., by predation, possibly leading to higher herbivore production J.M.N., M.J.G., and M.J.V. wrote the paper. relative to primary production (1). The authors declare no conflict of interest. Ecological efficiencies often depend on food-quality attributes This article is a PNAS Direct Submission. such as edibility and nutritional quality. The ecological efficiency 1E.M.D. and J.M.N. contributed equally to this work. of herbivores often depends on plant nutrient stoichiometry 2To whom correspondence should be addressed. E-mail: [email protected]. (carbon/nutrient ratio) relative to the respiratory demands, This article contains supporting information online at www.pnas.org/cgi/content/full/ nutrient demands, and assimilation efficiency of the herbivore 0805566105/DCSupplemental. (6, 7). Stoichiometric constraints may be less important for © 2008 by The National Academy of Sciences of the USA 18408–18412 ͉ PNAS ͉ November 25, 2008 ͉ vol. 105 ͉ no. 47 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0805566105 Downloaded by guest on September 29, 2021 Fish Present Fish Absent A Food Chain Efficiency (Phytoplankton to Fish) 0.07 A 300 B 350 a a 0.06 250 300 a 250 0.05 200 b 0.04 200 b b 150 c 0.03 150 d c Seston C:P 100 0.02 100 c Fish production / c 0.01 c 50 50 phytoplankton production 0.00 High Low High Low 0 0 Nutrients Nutrients Nutrients Nutrients High Light Low Light 14 14 C D a 12 12 Herbivore Efficiency (Phytoplankton to Zooplankton) a a 0.20 2.0 10 a a 10 b Fish Present Fish Absent b b B C 8 8 0.15 1.5 a a 6 6 b Seston C:N 0.10 1.0 4 4 b b 2 2 0.05 0.5 b 0 0 b b Zooplankton production / phytoplankton production 0.00 0.0 High Low High Low High Low High Low E 1.5 F 1.5 Nutrients Nutrients Nutrients Nutrients Nutrients Nutrients Nutrients Nutrients a High Light Low Light High Light Low Light a 1.0 1.0 Carnivore Efficiency (Zooplankton to Fish) ab D 1.5 b b b D b food quality 0.5 0.5 1.2 b a 0.9 a Phytoplankton compositional 0.0 0.0 0.6 High Low High Low High Low High Low Nutrients Nutrients Nutrients Nutrients Nutrients Nutrients Nutrients Nutrients ab Fish production / 0.3 High Light Low Light High Light Low Light zooplankton production b 0.0 High Low High Low Fig. 2. Quality of phytoplankton as a food resource based on cell stoichi- Nutrients Nutrients Nutrients Nutrients High Light Low Light ometry (A–D) and taxonomic composition (E and F). Phytoplankton responses were measured in each mesocosm throughout the study, and each point Fig. 1. Light, nutrient, and fish effects on FCE (2-way ANOVA, n ϭ 12, P ϭ represents a mesocosm mean, averaged over the experiment. Horizontal lines 0.0009) (A), herbivore efficiency (3-way ANOVA, n ϭ 23, P ϭ 0.0003) (B and C), represent treatment means, and letters indicate treatments that are signifi- and carnivore efficiency (2-way ANOVA, n ϭ 12, P ϭ 0.0138) (D). Each point cantly different from each other (Tukey post hoc test). Treatments with fish represents the efficiency for an individual mesocosm, obtained by using present and absent were analyzed separately. production rates averaged over the experiment for each trophic level. Hori- zontal lines represent treatment means, and letters indicate treatments that are significantly different from each other (Tukey post hoc test). For herbivore Nutrients decreased seston C/P at low light (as expected), but efficiency, all 8 treatments (fish absent and fish present) were analyzed increased C/P at high light (Fig. 2 A and B and Table S1). It is together, although they are graphically depicted separately. Note that the unclear why C/P increased in response to nutrients in the high scale differs in fish present vs. fish absent treatments for herbivore efficiency. In 2 mesocosms, carnivore efficiency exceeded 1, possibly because toward the light treatments; perhaps the marked increase in phytoplankton end of the experiment fish consumed some benthic algae, although zooplank- biomass (Fig.
Load more

Recommended publications
  • Backyard Food
    Suggested Grades: 2nd - 5th BACKYARD FOOD WEB Wildlife Champions at Home Science Experiment 2-LS4-1: Make observations of plants and animals to compare the diversity of life in different habitats. What is a food web? All living things on earth are either producers, consumers or decomposers. Producers are organisms that create their own food through the process of photosynthesis. Photosynthesis is when a living thing uses sunlight, water and nutrients from the soil to create its food. Most plants are producers. Consumers get their energy by eating other living things. Consumers can be either herbivores (eat only plants – like deer), carnivores (eat only meat – like wolves) or omnivores (eat both plants and meat - like humans!) Decomposers are organisms that get their energy by eating dead plants or animals. After a living thing dies, decomposers will break down the body and turn it into nutritious soil for plants to use. Mushrooms, worms and bacteria are all examples of decomposers. A food web is a picture that shows how energy (food) passes through an ecosystem. The easiest way to build a food web is by starting with the producers. Every ecosystem has plants that make their own food through photosynthesis. These plants are eaten by herbivorous consumers. These herbivores are then hunted by carnivorous consumers. Eventually, these carnivores die of illness or old age and become food for decomposers. As decomposers break down the carnivore’s body, they create delicious nutrients in the soil which plants will use to live and grow! When drawing a food web, it is important to show the flow of energy (food) using arrows.
    [Show full text]
  • Ecosystem Structure and Function. Dr
    TOPIC: - ECOSYSTEM STRUCTURE AND FUNCTION. DR. ABHAY KRISHNA SINGH PAPER NAME: - ENVIRONMENTAL GEOGRAPHY SUBJECT: - GEOGRAPHY SEMESTER: - M.A. –IV PAPER CODE: - (GEOG. 403) UNIVERSITY DEPARTMENT OF GEOGRAPHY, DR. SHYMA PRASAD MUKHERJEE UNIVERSITY, RANCHI. Environmental Sciences INTRODUCTION: - All organisms need energy to perform the essential functions such as maintenance, growth, repair, movement, locomotion and reproduction; all of these processes require energy expenditure. The ultimate source of energy for all ecological systems is Sun. The solar energy is captured by the green plants (primary producers or autotrophs) and transformed into chemical energy and bound in glucose as potential energy during the process of photosynthesis. In this stored form, other organisms take the energy and pass it on further to other organisms. During this process, a reasonable proportion of energy is lost out of the living system. The whole process is called flow of energy in the ecosystem. It is the amount of energy that is received and transferred from organism to organism in an ecosystem that modulates the ecosystem structure. Without autotrophs, there would be no energy available to all other organisms that lack the capability of fixing light energy. A fraction i.e. about 1/50 millionth of the total solar radiation reaches the earth’s atmosphere. About 34% of the sunlight reaching the earth’s atmosphere is reflected back into the atmosphere, 10% is held by ozone layer, water vapors and other atmospheric gases. The remaining 56% sunlight reaches the earth’s surface. Only a fraction of this energy reaching the earth’s surface (1 to 5%) is used by green plants for photosynthesis and the rest is absorbed as heat by ground vegetation or water.
    [Show full text]
  • Response of Marine Food Webs to Climate-Induced Changes in Temperature and Inflow of Allochthonous Organic Matter
    Response of marine food webs to climate-induced changes in temperature and inflow of allochthonous organic matter Rickard Degerman Department of Ecology and Environmental Science 901 87 Umeå Umeå 2015 1 Copyright©Rickard Degerman ISBN: 978-91-7601-266-6 Front cover illustration by Mats Minnhagen Printed by: KBC Service Center, Umeå University Umeå, Sweden 2015 2 Tillägnad Maria, Emma och Isak 3 Table of Contents Abstract 5 List of papers 6 Introduction 7 Aquatic food webs – different pathways Food web efficiency – a measure of ecosystem function Top predators cause cascade effects on lower trophic levels The Baltic Sea – a semi-enclosed sea exposed to multiple stressors Varying food web structures Climate-induced changes in the marine ecosystem Food web responses to increased temperature Responses to inputs of allochthonous organic matter Objectives 14 Material and Methods 14 Paper I Paper II and III Paper IV Results and Discussion 18 Effect of temperature and nutrient availability on heterotrophic bacteria Influence of food web length and labile DOC on pelagic productivity and FWE Consequences of changes in inputs of ADOM and temperature for pelagic productivity and FWE Control of pelagic productivity, FWE and ecosystem trophic balance by colored DOC Conclusion and future perspectives 21 Author contributions 23 Acknowledgements 23 Thanks 24 References 25 4 Abstract Global records of temperature show a warming trend both in the atmosphere and in the oceans. Current climate change scenarios indicate that global temperature will continue to increase in the future. The effects will however be very different in different geographic regions. In northern Europe precipitation is projected to increase along with temperature.
    [Show full text]
  • 7.014 Handout PRODUCTIVITY: the “METABOLISM” of ECOSYSTEMS
    7.014 Handout PRODUCTIVITY: THE “METABOLISM” OF ECOSYSTEMS Ecologists use the term “productivity” to refer to the process through which an assemblage of organisms (e.g. a trophic level or ecosystem assimilates carbon. Primary producers (autotrophs) do this through photosynthesis; Secondary producers (heterotrophs) do it through the assimilation of the organic carbon in their food. Remember that all organic carbon in the food web is ultimately derived from primary production. DEFINITIONS Primary Productivity: Rate of conversion of CO2 to organic carbon (photosynthesis) per unit surface area of the earth, expressed either in terns of weight of carbon, or the equivalent calories e.g., g C m-2 year-1 Kcal m-2 year-1 Primary Production: Same as primary productivity, but usually expressed for a whole ecosystem e.g., tons year-1 for a lake, cornfield, forest, etc. NET vs. GROSS: For plants: Some of the organic carbon generated in plants through photosynthesis (using solar energy) is oxidized back to CO2 (releasing energy) through the respiration of the plants – RA. Gross Primary Production: (GPP) = Total amount of CO2 reduced to organic carbon by the plants per unit time Autotrophic Respiration: (RA) = Total amount of organic carbon that is respired (oxidized to CO2) by plants per unit time Net Primary Production (NPP) = GPP – RA The amount of organic carbon produced by plants that is not consumed by their own respiration. It is the increase in the plant biomass in the absence of herbivores. For an entire ecosystem: Some of the NPP of the plants is consumed (and respired) by herbivores and decomposers and oxidized back to CO2 (RH).
    [Show full text]
  • Model-Based Analysis of the Energy Fluxes and Trophic Structure of a Portunus Trituberculatus Polyculture Ecosystem
    Vol. 9: 479–490, 2017 AQUACULTURE ENVIRONMENT INTERACTIONS Published December 5 https://doi.org/10.3354/aei00247 Aquacult Environ Interact OPENPEN ACCESSCCESS Model-based analysis of the energy fluxes and trophic structure of a Portunus trituberculatus polyculture ecosystem Jie Feng1, Xiang-Li Tian1,*, Shuang-Lin Dong1, Rui-Peng He1, Kai Zhang1, Dong-Xu Zhang1, Qing-Qi Zhang2 1The Key Laboratory of Mariculture, Ministry of Education, Fisheries College, Ocean University of China, Qingdao 266003, PR China 2Marine Fishery Technology Guiding Office of Ganyu, Lianyungang 222100, PR China ABSTRACT: We constructed a quantitative Ecopath model of a trophic network to evaluate the energy flow and properties in a polyculture ecosystem containing 4 species (swimming crab Por- tunus trituberculatus, white shrimp Litopenaeus vannamei, short-necked clam Ruditapes philip- pinarum, and redlip mullet Liza haematochila) over a 90 d experimental period. The model con- tained 10 consumers, 4 detritus groups, and 4 primary producers. Ecotrophic efficiency values indicated that the system had high energy utilization efficiency. However, benthic bacteria con- verted the largest amount of energy back to the detritus groups, which had the lowest ecotrophic efficiency (0.01). When aggregating the network to discrete trophic levels (TLs), most of the throughput and biomass of the system were distributed on the first 2 TLs; consequently, there was high energy transfer efficiency between TL I and II (81.98%). The trophic flow of this ecosystem was dominated by energy that originated from the detritus groups (73.77%). Imported artificial food was particularly important for the trophic flow of the total ecosystem, contributing 31.02% to total system consumption.
    [Show full text]
  • Seventh Grade
    Name: _____________________ Maui Ocean Center Learning Worksheet Seventh Grade Our mission is to foster understanding, wonder and respect for Hawai‘i’s Marine Life. Based on benchmarks SC.6.3.1, SC. 7.3.1, SC. 7.3.2, SC. 7.5.4 Maui Ocean Center SEVENTH GRADE 1 Interdependent Relationships Relationships A food web (or chain) shows how each living thing gets its food. Some animals eat plants and some animals eat other animals. For example, a simple food chain links plants, cows (that eat plants), and humans (that eat cows). Each link in this chain is food for the next link. A food chain always starts with plant life and ends with an animal. Plants are called producers (they are also autotrophs) because they are able to use light energy from the sun to produce food (sugar) from carbon dioxide and water. Animals cannot make their own food so they must eat plants and/or other animals. They are called consumers (they are also heterotrophs). There are three groups of consumers. Animals that eat only plants are called herbivores. Animals that eat other animals are called carnivores. Animals and people who eat both animals and plants are called omnivores. Decomposers (bacteria and fungi) feed on decaying matter. These decomposers speed up the decaying process that releases minerals back into the food chain for absorption by plants as nutrients. Do you know why there are more herbivores than carnivores? In a food chain, energy is passed from one link to another. When a herbivore eats, only a fraction of the energy (that it gets from the plant food) becomes new body mass; the rest of the energy is lost as waste or used up (by the herbivore as it moves).
    [Show full text]
  • Stoichiometric Constraints Modulate the Effects of Temperature and Nutrients on Biomass Distribution And
    bioRxiv preprint doi: https://doi.org/10.1101/589895; this version posted January 28, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. RESEARCH ARTICLE Stoichiometric constraints modulate the effects of temperature and nutrients on biomass distribution and community stability Cite as: Sentis A, Haegeman B and Montoya J.M. (2020) Stoichiometric Arnaud Sentis1,2*, Bart Haegeman1, and José M. Montoya1 constraints modulate the effects of 1 temperature and nutrients on Centre for Biodiversity Theory and Modelling, Theoretical and Experimental biomass distribution and community Ecology Station, CNRS and Paul Sabatier University, Moulis, France. stability. bioRxiv 589895, ver. 7 peer- 2 reviewed and recommended by PCI INRAE, Aix Marseille Univ., UMR RECOVER, 3275 route Cézanne, 13182 Aix-en- Ecology. Provence, France. Posted: 28th January 2020 This article has been peer-reviewed and recommended by Recommender: Peer Community in Ecology Elisa Thébault ABSTRACT Temperature and nutrients are two of the most important drivers of global change. Reviewers: Two anonymous reviewers Both can modify the elemental composition (i.e. stoichiometry) of primary producers and consumers. Yet their combined effect on the stoichiometry, Correspondence: [email protected] dynamics, and stability of ecological communities remains largely unexplored. To fill this gap, we extended the Rosenzweig-MacArthur consumer-resource model by including thermal dependencies, nutrient dynamics, and stoichiometric constraints on both the primary producer and the consumer. We found that stoichiometric constraints dampen the paradox of enrichment and increased persistence at high nutrient levels.
    [Show full text]
  • Towards an Integration of Ecological Stoichiometry and the Metabolic Theory of Ecology to Better Understand Nutrient Cycling
    Ecology Letters, (2009) 12: 369–384 doi: 10.1111/j.1461-0248.2009.01302.x IDEA AND PERSPECTIVE Towards an integration of ecological stoichiometry and the metabolic theory of ecology to better understand nutrient cycling Abstract Andrew P. Allen1* and James Ecologists have long recognized that species are sustained by the flux, storage and F. Gillooly2 turnover of two biological currencies: energy, which fuels biological metabolism and 1Department of Biological materials (i.e. chemical elements), which are used to construct biomass. Ecological Sciences, Macquarie University, theories often describe the dynamics of populations, communities and ecosystems in Sydney, NSW 2109, Australia terms of either energy (e.g. population-dynamics theory) or materials (e.g. resource- 2 Department of Zoology, competition theory). These two classes of theory have been formulated using different University of Florida, 223 assumptions, and yield distinct, but often complementary predictions for the same or Bartram Hall, P.O. Box 118525, similar phenomena. For example, the energy-based equation of von Bertalanffy and the Gainesville, FL 32611, USA *Correspondence: E-mail: nutrient-based equation of Droop both describe growth. Yet, there is relatively little [email protected] theoretical understanding of how these two distinct classes of theory, and the currencies they use, are interrelated. Here, we begin to address this issue by integrating models and concepts from two rapidly developing theories, the metabolic theory of ecology and ecological stoichiometry theory. We show how combining these theories, using recently published theory and data along with new theoretical formulations, leads to novel predictions on the flux, storage and turnover of energy and materials that apply to animals, plants and unicells.
    [Show full text]
  • Plants Are Producers! Draw the Different Producers Below
    Name: ______________________________ The Unique Producer Every food chain begins with a producer. Plants are producers. They make their own food, which creates energy for them to grow, reproduce and survive. Being able to make their own food makes them unique; they are the only living things on Earth that can make their own source of food energy. Of course, they require sun, water and air to thrive. Given these three essential ingredients, you will have a healthy plant to begin the food chain. All plants are producers! Draw the different producers below. Apple Tree Rose Bushes Watermelon Grasses Plant Blueberry Flower Fern Daisy Bush List the three essential needs that every producer must have in order to live. © 2009 by Heather Motley Name: ______________________________ Producers can make their own food and energy, but consumers are different. Living things that have to hunt, gather and eat their food are called consumers. Consumers have to eat to gain energy or they will die. There are four types of consumers: omnivores, carnivores, herbivores and decomposers. Herbivores are living things that only eat plants to get the food and energy they need. Animals like whales, elephants, cows, pigs, rabbits, and horses are herbivores. Carnivores are living things that only eat meat. Animals like owls, tigers, sharks and cougars are carnivores. You would not catch a plant in these animals’ mouths. Then, we have the omnivores. Omnivores will eat both plants and animals to get energy. Whichever food source is abundant or available is what they will eat. Animals like the brown bear, dogs, turtles, raccoons and even some people are omnivores.
    [Show full text]
  • Stoichiometry and Planktonic Grazer Composition Over Gradients of Light, Nutrients, and Predation Risk
    Ecology, 85(8), 2004, pp. 2291±2301 q 2004 by the Ecological Society of America STOICHIOMETRY AND PLANKTONIC GRAZER COMPOSITION OVER GRADIENTS OF LIGHT, NUTRIENTS, AND PREDATION RISK SPENCER R. HALL,1,3 MATHEW A. LEIBOLD,1,4 DAVID A. LYTLE,1,5 AND VAL H. SMITH2 1Department of Ecology and Evolution, University of Chicago, 1101 E. 57th Street Chicago, Illinois 60637 USA 2Program in Ecology and Population Biology, Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, Kansas 66045 USA Abstract. Mechanisms that explain shifts in species composition over environmental gradients continue to intrigue ecologists. Ecological stoichiometry has recently provided a new potential mechanism linking resource (light and nutrient) supply gradients to grazer performance via elemental food-quality mechanisms. More speci®cally, it predicts that light and nutrient gradients should determine the relative dominance of P-rich taxa, such as Daphnia, in grazer assemblages. We tested this hypothesis in pond mesocosms (cattle tanks) by creating gradients of resource supply and predation risk, to which we added diverse assemblages of algal producer and zooplankton grazer species. We then characterized the end-point composition of grazer assemblages and quantity and elemental food quality of edible algae. We found that somatically P-rich Daphnia only dominated grazer assemblages in high-nutrient, no-predator treatments. In these ecosystems, P sequestered in producers exceeded a critical concentration. However, other grazers having even higher body P content did not respond similarly. These grazers were often abundant in low-nutrient environments with poorer food quality. At face value, this result is problematic for ecological stoichi- ometry because body composition did not correctly predict response of these other species.
    [Show full text]
  • 3.2 Energy Flows Through Ecosystems [Notes/Highlighting]
    Printed Page 60 3.2 Energy flows through ecosystems [Notes/Highlighting] To understand how ecosystems function and how to best protect and manage them, ecosystem ecologists study not only the biotic and abiotic components that define an ecosystem, but also the processes that move energy and matter within it. Plants absorb energy directly from the Sun. That energy is then spread throughout an ecosystem as herbivores (animals that eat plants) feed on plants and carnivores (animals that eat other animals) feed on herbivores. Consider the Serengeti Plain in East Africa, shown in FIGURE 3.3. There are millions of herbivores, such as zebras and wildebeests, in the Serengeti ecosystem, but far fewer carnivores, such as lions (Panthera leo) and cheetahs (Acinonyx jubatus), that feed on those herbivores. In accordance with the second law of thermodynamics, when one organism consumes another, not all of the energy in the consumed organism is transferred to the consumer. Some of that energy is lost as heat. Therefore, all the carnivores in an area contain less energy than all the herbivores in the same area because all the energy going to the carnivores must come from the animals they eat. To better understand these energy relationships, let’s trace this energy flow in more detail. Figure 3.3 Serengeti Plain of Africa. The Serengeti ecosystem has more plants than herbivores, and more herbivores than carnivores. Previous Section | Next Section 3.2.1 Photosynthesis and Respiration Printed Page 60 [Notes/Highlighting] Nearly all of the energy that powers ecosystems comes from the Sun as solar energy, which is a form of kinetic energy.
    [Show full text]
  • Herbivore Physiological Response to Predation Risk and Implications for Ecosystem Nutrient Dynamics
    Herbivore physiological response to predation risk and implications for ecosystem nutrient dynamics Dror Hawlena and Oswald J. Schmitz1 School of Forestry and Environmental Studies, Yale University, New Haven, CT 06511 Communicated by Thomas W. Schoener, University of California, Davis, CA, June 29, 2010 (received for review January 4, 2010) The process of nutrient transfer throughan ecosystem is an important lower the quantity of energy that can be allocated to production determinant of production, food-chain length, and species diversity. (20–23). Consequently, stress-induced constraints on herbivore The general view is that the rate and efficiency of nutrient transfer up production should lower the demand for N-rich proteins (24). the food chain is constrained by herbivore-specific capacity to secure Herbivores also have low capacity to store excess nutrients (24), N-rich compounds for survival and production. Using feeding trials and hence should seek plants with high digestible carbohydrate with artificial food, we show, however, that physiological stress- content to minimize the costs of ingesting and excreting excess N. response of grasshopper herbivores to spider predation risk alters the Such stress-induced shift in nutrient demand may be especially nature of the nutrient constraint. Grasshoppers facing predation risk important in terrestrial systems in which digestible carbohydrate had higher metabolic rates than control grasshoppers. Elevated represents a small fraction of total plant carbohydrate-C, and may metabolism accordingly increased requirements for dietary digestible be limiting even under risk-free conditions (25). Moreover, stress carbohydrate-C to fuel-heightened energy demands. Moreover, di- responses include break down of body proteins to produce glucose gestible carbohydrate-C comprises a small fraction of total plant (i.e., gluconeogenesis) (14), which requires excretion of N-rich tissue-C content, so nutrient transfer between plants and herbivores waste compounds (ammonia or primary amines) (26).
    [Show full text]