Population Differentiation In Daphnia Alters Community Assembly In Experimental Ponds

Most studies of community assembly ignore how genetic differentiation within species affects their colonization and extinction. However, genetic differentiation in ecologically relevant traits may be substantial enough to alter the colonization and extinction processes that drive community assembly. We measured significant molecular genetic and quantitative trait differentiation among three Daphnia pulex X pulicaria populations in southwestern Michigan ponds and investigated whether this differentiation could alter the assembly of pond zooplankton communities in experimental mesocosms. In this study, we monitored the invasion success of different D. pulex x pulicaria populations after their introduction into an established zooplankton community. We also monitored the invasion success of a diverse array of zooplankton species into different D. pulex x pulicaria populations. Zooplankton community composition depended on the D. pulex X pulicaria source population. Daphnia pulex X pulicaria from one population failed to invade zooplankton communities, while those from other populations successfully invaded similar communities. If population differentiation in other species plays a role in community assembly similar to that demonstrated in our study, assembly may be more sensitive to evolutionary processes than has been previously generally considered.Integrative Biolog

Coexistence of the Niche and Neutral Perspectives in Community Ecology

The neutral theory for community structure and biodiversity is dependent on the assumption that species are equivalent to each other in all important ecological respects. We explore what this concept of equivalence means in ecological communities, how such species may arise evolutionarily, and how the possibility of ecological equivalents relates to previous ideas about niche differentiation. We also show that the co-occurrence of ecologically similar or equivalent species is not incompatible with niche theory as has been supposed, because niche relations can sometimes favor coexistence of similar species. We argue that both evolutionary and ecological processes operate to promote the introduction and to sustain the persistence of ecologically similar and in many cases nearly equivalent species embedded in highly structured food webs. Future work should focus on synthesizing niche and neutral perspectives rather than dichotomously debating whether neutral or niche models provide better explanations for community structure and biodiversity

The niche concept in metacommunities

La ciencia de la ecología estudia las interacciones de los sistemas biológicos con el medio ambiente. El concepto de nicho ecológico es la base central de este estudio ya que describe cómo organismos en diferentes niveles de organización interactúan con su medio ambiente a distintas escalas espacio-temporales. Se ha creado cierta confusión acerca de este concepto porque han existido diferentes definiciones y diferentes perspectivas sobre la forma de encararlo. En este artículo, hacemos una revisión de perspectivas previas, proponemos un concepto sintético del nicho y consideramos cómo las relaciones de nicho influyen en las interacciones entre las especies dentro del marco de las metacomunidades. Asimismo, contrastamos algunos de los puntos de vista resultantes con estudios previos enfocados en comunidades locales y discutimos la posible relevancia del mecanismo de deriva a nivel de especies – mecanismo análogo a la deriva génica – sobre patrones observados cuando la diferenciación de nichos entre especies es escasa o inexistente.The science of ecology studies how biological systems interact with their environment. The niche concept lies at the heart of this study because it describes how organisms at different levels of organization interact with their environment at different spatio-temporal scales. However, because different definitions and different perspectives about this concept have existed, confusion has emerged. In this paper we review how such perspectives might be synthesized and we consider how niche relations influence species interactions in metacommunities. We contrast some of the resulting insights with previous work focused on local communities and we use this synthesis to discuss the possible importance of drift mechanisms that exist in the absence of niche differences among species

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

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

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

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

Recasting spatial food web ecology as an ecosystem science

Background/questions/methods

Food webs are complex systems in which organisms interact with each other and with the abiotic aspects of their environment, thus acting as the conduit for transfers of energy and nutrients through ecosystems. Classical approaches to food webs focus strongly on patterns and processes occurring at the community level rather than at the broader ecosystem scale. Recent developments in community ecology suggest that spatial processes may be important in affecting food web dynamics and affect ecosystems as well, thus leading to the idea of meta-ecosystems. Here, we make a synthesis on how the links between food web dynamics and spatial ecosystem dynamics may be studied through (i) identifying differences between metacommunity and landscape ecology approaches when dealing with food webs, (ii) arguing that a tighter synthesis of the two approaches is needed for a good understanding of how diversity, ecosystem process and trait distributions in landscapes are related, and (iii) laying out how this gap can be efficiently bridged under the framework of meta-ecosystems.

Results/conclusions

We identify two possible sets of processes that drive spatial food webs and the ecosystems they occur in: trait-dependent processes and material-dependent processes. Both of these have been shown to be important in affecting various aspects of food web ecology and we ask how they may compare to each other and how they may interact. We argue that interactions between them, while complex, are likely and depend strongly on the size of the meta-ecosystem and its connectivity. A more integrative framework to the study of spatial food webs, which takes into account both approaches, might be key in better understanding the links between ecosystem and community dynamics at large spatial scales.
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A comparison of taxon co-occurrence patterns for macro- and microorganisms

We examine co-occurrence patterns of microorganisms to evaluate community assembly “rules.” We use methods previously applied to macroorganisms, both to evaluate their applicability to microorganisms and to allow comparison of co-occurrence patterns observed in microorganisms to those found in macroorganisms. We use a null model analysis of 124 incidence matrices from microbial communities, including bacteria, archaea, fungi, and algae, and we compare these results to previously published findings from a meta-analysis of almost 100 macroorganism data sets. We show that assemblages of microorganisms demonstrate nonrandom patterns of co-occurrence that are broadly similar to those found in assemblages of macroorganisms. These results suggest that some taxon co-occurrence patterns may be general characteristics of communities of organisms from all domains of life. We also find that co-occurrence in microbial communities does not vary among taxonomic groups or habitat types. However, we find that the degree of co-occurrence does vary among studies that use different methods to survey microbial communities. Finally, we discuss the potential effects of the undersampling of microbial communities on our results, as well as processes that may contribute to nonrandom patterns of co-occurrence in both macrobial and microbial communities such as competition, habitat filtering, historical effects, and neutral processes

Novel community data in ecology -- properties and prospects

New technologies for acquiring biological information such as eDNA, acoustic or optical sensors, make it possible to generate spatial community observations at unprecedented scales. The potential of these novel community data to standardize community observations at high spatial, temporal, and taxonomic resolution and at large spatial scale ('many rows and many columns') has been widely discussed, but so far, there has been little integration of these data with ecological models and theory. Here, we review these developments and highlight emerging solutions, focusing on statistical methods for analyzing novel community data, in particular joint species distribution models; the new ecological questions that can be answered with these data; and the potential implications of these developments for policy and conservation