Grazers, producer stoichiometry, and the light : nutrient hypothesis revisited.

The stoichiometric light : nutrient hypothesis (LNH) links the relative supplies of key resources with the nutrient content of tissues of producers. This resource-driven variation in producer stoichiometry, in turn, can mediate the efficiency of grazing. Typically, discussions of the LNH attribute this resource–stoichiometry link to bottom-up effects of light and phosphorus, which are mediated through producer physiology. Emphasis on bottom-up effects implies that grazers must consume food of quality solely determined by resource supply to ecosystems (i.e., they eat what they are served). Here, we expand upon this largely bottom up interpretation with evidence from pond surveys, a mesocosm experiment, and a model. Data from shallow ponds showed the ‘‘LNH pattern’’ (positive correlation of an index of light: phosphorus supply with algal carbon : phosphorus content). However, algal carbon: phosphorus content also declined as zooplankton biomass increased in the ponds. The experiment and model confirmed that this latter correlation was partially caused by the various bottom-up and top-down roles of grazers: the LNH pattern emerged only in treatments with crustacean grazers, not those without them. Furthermore, model and experiment clarified that another bottom-up factor, natural covariation of nitrogen : phosphorus ratios with light : phosphorus supply (as seen in ponds), does not likely contribute to the LNH pattern. Finally, the experiment produced correlations between shifts in species composition of algae, partially driven by grazing effects of crustaceans, and algal stoichiometry. These shifts in species composition might shape stoichiometric response of producer assemblages to resource supply and grazing, but their consequences remain largely unexplored. Thus, this study accentuated the importance of grazing for the LNH; deemphasized a potentially confounding, bottom-up factor (covarying nitrogen : phosphorus supply); and highlighted an avenue for future research for the LNH (grazer-mediated shifts in producer composition).We thank T. Darcy-Hall, A. Downing, P. Geddes, and N. Howe for help with sampling the cattle tank experiment and natural ponds. We analyzed the C:N samples in the Robertson lab at the W. K. Kellogg Biological Station (KBS) with the help of A. Corbin and T. Darcy-Hall. Thanks also go to G. Mittelbach, N. Consolatti, A. Tessier, and P. Woodruff for technical support. Finally, S. Diehl kindly provided us with data from his experiments. Primary funding came from NSF DEB 98-15799, DEB 02-35579, and funds from Indiana University. This is KBS contribution number 1277

Inedible Producers in Food Webs: Controls on Stoichiometric Food Quality and Composition of Grazers

Ecological stoichiometry and food web theories focus on distinct mechanisms that shape communities. These mechanisms, however, likely interact in ways that neither theory alone addresses. To illustrate, we show how a model that tracks flow of energy and nutrients through two producers and two grazers reveals two indirect, interrelated roles for “neutrally inedible” producers. First, inedible producers can exert controls over the nutrient content of edible producers and indirectly influence whether grazers are nutrient or energy limited. Second, through these controls, inedible producers can shape community assembly by excluding grazers that are weak competitors for nutrients contained in edible producers. A mesocosm experiment revealed patterns consistent with both predictions: high abundances of inedible algae were accompanied by low phosphorus contents of edible algae and low abundances of the grazer Daphnia. Both lines of inference suggest that interactions between stoichiometry and plant heterogeneity may shape plankton communities

Stoichiometry and planktonic grazer composition over gradients of light, nutrients, and predation risk

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 specifically, 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 stoichiometry because body composition did not correctly predict response of these other species. However, two potential explanations could maintain consistency with stoichiometric principles: species could differentially use a high-P resource (bacteria), or body composition might not always directly correlate with nutrient demands of grazers. Although our data cannot differentiate between these explanations, both suggest potential avenues for future empirical and theoretical study.We thank T. Darcy, A. Downing, P. Geddes, and N. Howe for help with sampling, and G. Dwyer, T. Wootton, J. Bergelson, D. Spiller, W. DeMott, and an anonymous reviewer for their comments on the manuscript. We analyzed the C:N 2300 SPENCER R. HALL ET AL. Ecology, Vol. 85, No. 8 samples in the Robertson laboratory at Kellogg Biological Station (KBS) with the help of A. Corbin and T. Darcy. Thanks also go to G. Mittelbach, N. Consolatti, A. Tessier, and P. Woodruff at KBS for technical support. M. Bishop of the Michigan DNR permitted us to sample ponds in Barry and Middleville SGAs. Primary funding came from NSF DEB 98-15799 to M. A. Leibold and V. H. Smith. S. R. Hall was also supported by an NSF Graduate Fellowship, a University of Chicago Harper Fellowship and Hinds Fund Award, a Department of Education GAANN training grant, and a NSF DDIG (DEB 01-05014, PI Mathew Leibold). This is KBS contribution number 1128

Constraints on primary producer n:p stoichiometry along n:p supply ratio gradients

A current principle of ecological stoichiometry states that the nitrogen to phosphorus ratio (N:P) of primary producers should closely match that from environmental nutrient supplies. This hypothesis was tested using data from ponds in Michigan, USA, a freshwater mesocosm experiment, a synthesis of studies from diverse systems (cultures, lakes, streams, and marine and terrestrial environments), and simple dynamic models of producer growth and nutrient content. Unlike prior laboratory studies, the N:P stoichiometry of phytoplankton in Michigan ponds clustered around and below the Redfield ratio (7.2:1 by mass), despite wide variation in N:P supply ratios (2:1–63:1 by mass) and the presence of grazers. In a mesocosm experiment, the N:P stoichiometry of phytoplankton cells again deviated from a nearly 1:1 relationship with N:P supply. Phytoplankton seston exhibited lower N:P content than expected at high N:P supply ratios, and often higher N:P content than anticipated at low N:P supply ratios, regardless of herbivore presence. Similar deviations consistently occur in the N:P stoichiometry of algae and plants in the other diverse systems. The models predicted that both high loss rates (sinking, grazing) and physiological limits to nutrient storage capacity could attenuate producer stoichiometry. In the future, research should evaluate how limits to elemental plasticity of producers can influence the role of stoichiometry in structuring communities and ecosystem processes.We thank T. Darcy-Hall, A. Downing, P. Geddes, and N. Howe for help with tank and field sampling; and C. Klausmeier, G. Mittelbach, two anonymous reviewers, and Editor P. Leavitt for very helpful comments on the manuscript. We analyzed the C:N samples in the Robertson lab at Kellogg Biological Station (KBS) with the help of A. Corbin and T. Darcy-Hall. Thanks also go to G. Mittelbach, N. Consolatti, A. Tessier, and P. Woodruff at KBS for technical support. M. Bishop of the Michigan DNR permitted us to sample ponds in Barry and Middleville State Game Areas. Primary funding came from NSF DEB 98-15799 to Mathew A. Leibold and Val H. Smith. Spencer R. Hall was also supported by an NSF Graduate Fellowship, a University of Chicago Harper Fellowship and Hinds Fund Award, a Department of Education GAANN training grant, and a NSF DDIG (DEB 01-05014, P.I., Mathew Leibold). Some of the data described in this article were produced by the U.S. Environmental Protection Agency through its Environmental Monitoring and Assessment Program (EMAP). This is KBS contribution #1169

Developing a School Functioning Index for Middle Schools

Despite widespread recognition of schools\u27 role in the healthy development of youth, surprisingly little research has examined the relationships between schools\u27 overall functioning and the health‐related behavior of students. School functioning could become an important predictor of students\u27 health‐related behavior and may be amenable to intervention. This paper describes the development and testing of the School Functioning Index (SFI) as a first step in investigating this question. The index was developed for use with middle schools and conceived as a predictor of students\u27 violent behavior, with the potential for extending research applications to additional health and social behaviors. Using social cognitive theory, social ecological theory, and social disorganization theory as guides, three domains were identified to operationalize school functioning and identify candidate SFI items: 1) resources available to the school and students; 2) stability of the school population; and 3) the schools\u27 performance as a socializing agent for students. Data for candidate SFI items were collected from public archives and directly from 16 middle schools participating in a school‐based dietary intervention study. Data collection from schools, particularly concerning student aggressive behavior and disciplinary actions, presented challenges. The final SFI comprised nine items and demonstrated good internal consistency and variability. The SFI was modestly correlated in expected directions with violence and other health behaviors. This work supports the feasibility of combining multiple school‐level indicators to create a measure of overall school functioning. Further investigation of validity and more acceptable data collection methods are warranted

Perceptions of the Family Mealtime Environment and Adolescent Mealtime Behavior: Do Adults and Adolescents Agree?

The family mealtime environment has great potential to affect the eating behaviors of youth in the family. It is difficult to determine the important elements of a healthy mealtime environment because a valid assessment of the family environment is so difficult to obtain. The objective of this study is to examine the level of agreement between adult and adolescent perceptions of the family mealtime environment and adolescent mealtime behavior. A telephone survey was used to query adult and adolescent family members about how they perceive the family mealtime environment and the adolescent\u27s mealtime behavior. A convenience sample of 282 adult/adolescent pairs from four schools in the Minneapolis/St. Paul area completed the telephone surveys. Frequencies of responses and the associations between the adult and adolescent responses are presented. Pearson correlations and regression were used to examine the level of association between adult and adolescent responses. Mixed-model regression was used for the continuous variables, and mixed-model logistic regression was used for the dichotomous variables. This study showed very little concordance between adolescent and adult responses. Only one question regarding arguments about eating during mealtime showed concordance. Adults and adolescents living in the same household seem to have different perceptions of the family mealtime environment and adolescent eating patterns. Researchers need to be aware of and concerned with the validity of the use of self-report for descriptions of family mealtime. They also need to be aware of the difference in adult and adolescent perceptions and consider these differences when designing messages for the family

Quantifying invertebrate resistance to floods: a global-scale meta-analysis

Floods are a key component of the ecology and management of riverine ecosystems around the globe, but it is not clear whether floods have predictable effects on organisms that can allow us to generalize across regions and continents. To address this, we conducted a global-scale meta-analysis to investigate effects of natural and managed floods on invertebrate resistance, the ability of invertebrates to survive flood events. We considered 994 studies for inclusion in the analysis, and after evaluation based on a priori criteria, narrowed our analysis to 41 studies spanning six of the seven continents. We used the natural-log-ratio of invertebrate abundance before and within 10 days after flood events because this measure of effect size can be directly converted to estimates of percent survival. We conducted categorical and continuous analyses that examined the contribution of environmental and study design variables to effect size heterogeneity, and examined differences in effect size among taxonomic groups. We found that invertebrate abundance was lowered by at least one-half after flood events. While natural vs. managed floods were similar in their effect, effect size differed among habitat and substrate types, with pools, sand, and boulders experiencing the strongest effect. Although sample sizes were not sufficient to examine all taxonomic groups, floods had a significant, negative effect on densities of Coleoptera, Eumalacostraca, Annelida, Ephemeroptera, Diptera, Plecoptera, and Trichoptera. Results from this study provide guidance for river flow regime prescriptions that will be applicable across continents and climate types, as well as baseline expectations for future empirical studies of freshwater disturbance.Keywords: Quantitative synthesis, Environmental flows, River management, Disturbance ecolog

Are Differences in Exposure to a Multicomponent School-Based Intervention Associated With Varying Dietary Outcomes in Adolescents?

Multicomponent interventions are recommended for health behavior change among adolescents. However, it is difficult to disentangle the effects of multiple intervention components. This article reports outcomes associated with varying levels of exposure to a school-based nutrition intervention, Teens Eating for Energy and Nutrition at School (TEENS). Four incremental exposureswere possible: (1) control group, (2) school environment interventions only, (3) classroom plus environment interventions, and (4) peer leaders plus classroom plus environment interventions. Patterns suggesting dose response were observed, with peer leaders reporting the largest increases in fruit, vegetable, and lower fat food consumption. Students exposed to classroom plus environment interventions also improved, whereas students exposed only to school environment interventions showed trends toward choosing lower fat foods and declining fruit intake and no change in vegetable intake. Control students’ choices remained stable. Future studies may investigate mechanisms for peer leaders’ changes, maximizing curriculum effectiveness, and improving environmental interventions