Christian Mulder

Mesocosm Experiments as a Tool for Ecological Climate-Change Research

Abstract Predicting the ecological causes and consequences of global climate change requires a variety of approaches, including the use of experiments, models, and surveys. Among experiments, mesocosms have become increasingly popular because they provide an important bridge between smaller, more tightly controlled, microcosm experiments (which can suffer from limited realism) and the greater biological complexity of natural systems (in which mechanistic relationships often cannot be identified). A new evaluation of the contribution of the mesocosm approach, its potential for future research, as well as its limitations, is timely. As part of this review, we constructed a new database of over 250 post-1990 studies that have explored different components of climate change across a range of organisational levels, scales, and habitats. Issues related to realism, reproducibility and control are assessed in marine, freshwater, and terrestrial systems. Some general patterns emerged, particularly at the ecosystem level, such as consistent and predictable effects on whole-system respiration rates. There are, however, also many seemingly idiosyncratic, contingent responses, especially at the community level, both within and among habitat types. These similarities and differences in both the drivers and responses highlight the need for caution before making generalisations. Finally, we assess future directions and prospects for new methodological advances and the need for greater international coordination and interdisciplinarity.

Three allometric relations of population density to body mass: theoretical integration and empirical tests in 149 food webs

Predicting species population density-body mass scaling in community food webs (henceforth webs) is important for conservation and to understand community structure. Very different types of scaling have been studied, based on either individuals or species. The individual size distribution (ISD) describes the distribution of individual-organism body masses regardless of taxonomy, and contains the same information as the abundance spectrum. Focusing instead on species, the local size-density relationship (LSDR) plots population densities vs. mean body masses of species. The distribution of species mean body masses (the species-mean-size distribution, SMSD) is also important but previously little studied in webs. We here combine and formalize theory of several authors to predict: how these three descriptions are related; the forms of the LSDR and ISD; and variation in scaling among webs. We describe empirically the SMSDs of two pelagic, one estuarine, and 146 soil webs by power laws and generalizations. We test theory and find it broadly validated.

A belowground perspective on dutch agroecosystems: How soil organisms interact to support ecosystem services

Summary 1. New patterns and trends in land use are becoming increasingly evident in Europes heavily modified landscape and else whereas sustainable agriculture and nature restoration are developed as viable long-term alternatives to intensively farmed arable land. The success of these changes depends on how soil biodiversity and processes respond to changes in management. To improve our understanding of the community structure and ecosystem functioning of the soil biota, we analyzed abiotic variables across 200 sites, and biological variables across 170 sites in The Netherlands, one of the most intensively farmed countries. The data were derived from the Dutch Soil Quality Network (DSQN), a long-term monitoring framework designed to obtain ecological insight into soil types ( STs ) and ecosystem types ( ETs ). 2. At the outset we describe ST s and biota, and we estimate the contribution of various groups to the provision of ecosystem services. We focused on interactive effects of soil properties on community patterns and ecosystem functioning using food web models. Ecologists analyze soil food webs by means of mechanistic and statistical modelling, linking network structure to energy flow and elemental dynamics commonly based on allometric scaling. 3. We also explored how predatory and metabolic processes are constrained by body size, diet and metabolic type, and how these constraints govern the interactions within and between trophic groups. In particular, we focused on how elemental fluxes determine the strengths of ecological interactions, and the resulting ecosystem services, in terms of sustenance of soil fertility. 4. We discuss data mining, food web visualizations, and an appropriate categorical way to capture subtle interrelationships within the DSQN dataset. Sampled metazoans were used to provide an overview of belowground processes and influences of land use. Unlike most studies to date we used data from the entire size spectrum, across 15 orders of magnitude, using body size as a continuous trait crucial for understanding ecological services. 5. Multimodality in the frequency distributions of body size represents a performance filter that acts as a buffer to environmental change. Large differences in the body-size distributions across ET s and ST s were evident. Most observed trends support the hypothesis that the direct influence of ecological stoichiometry on the soil biota as an independent predictor (e.g. in the form of nutrient to carbon ratios), and consequently on the allometric scaling, is more dominant than either ET or ST . This provides opportunities to develop a mechanistic and physiologically oriented model for the distribution of species’ body sizes, where responses of invertebrates can be predicted. 6. Our results highlight the different roles that organisms play in a number of key ecosystem services. Such a trait-based research has unique strengths in its rigorous formulation of fundamental scaling rules, as well as in its verifiability by empirical data. Nonetheless, it still has weaknesses that remain to be addressed, like the consequences of intraspecific size variation, the high degree of omnivory, and a possibly inaccurate assignment to trophic groups. 7. Studying the extent to which nutrient levels influence multitrophic interactions and how different land-use regimes affect soil biodiversity is clearly a fruitful area for future research to develop predictive models for soil ecosystem services under different management regimes. No similar efforts have been attempted previously for soil food webs, and our dataset has the potential to test and further verify its usefulness at an unprecedented space scale.

Connecting the green and brown worlds : allometric and stoichiometric predictability of above- and below-ground networks

Abstract We examine the potential of trait-based parameters of taxa for linking above- and below-ground ecological networks (hereafter ‘green’ and ‘brown’ worlds) to understand and predict community dynamics. This synthesis considers carbon, nitrogen and phosphorus-related traits, the abundance of component species and their size distribution across trophic levels under different forms of management. We have analysed existing and novel databases on plants, microbes and invertebrates that combine physico-chemical and biological information from (agro)ecosystems spanning the globe. We found (1) evidence that traits from above- and below-ground systems may be integrated in the same model and (2) a much greater than expected stoichiometric plasticity of plants and microbes which has implications for the entire food-web mass–abundance scaling. Nitrogen and phosphorus are primary basal resources (hence, drivers) and more retranslocation of P than of N from leaves will lead to higher N:P in the litter and soil organic matter. Thus, under nutrient-rich conditions, higher foliar concentrations of N and P are reflected by lower N:P in the brown litter, suggesting less P retranslocated than N. This apparent stoichiometric dichotomy between green and brown could result in shifts in threshold elemental ratios critical for ecosystem functioning. It has important implications for a general food-web model, given that resource C:N:P ratios are generally assumed to reflect environmental C:N:P ratios. We also provide the first evidence for large-scale allometric changes according to the stoichiometry of agroecosystems. Finally, we discuss insights that can be gained from integrating carbon and nitrogen isotope data into trait-based approaches, and address the origin of changes in Δ 13 C and Δ 15 N fractionation values in relation to consumer–resource body-mass ratios.

Scaling of offspring number and mass to plant and animal size: model and meta-analysis

The scaling of reproductive parameters to body size is important for understanding ecological and evolutionary patterns. Here, we derived allometric relationships for the number and mass of seeds, eggs and neonates from an existing model on population production. In a separate meta-analysis, we collected 79 empirical regressions on offspring mass and number covering different taxa and various habitats. The literature review served as a validation of the model, whereas, vice versa, consistency of isolated regressions with each other and related ecological quantities was checked with the model. The total offspring mass delivered in a reproductive event scaled to adult size with slopes in the range of about 3/4 to 1. Exponents for individual seed, egg and neonate mass varied around 1/2 for most heterotherms and between 3/4 and 1 for most homeotherms. The scaling of the progeny number released in a sowing, clutch or litter was opposite to that of their size. The linear regressions fitted into a triangular envelope where maximum offspring mass is limited by the size of the adult. Minimum seed and egg size scaled with weight exponents of approximately 0 up to 1/4. These patterns can be explained by the influence of parents on the fate of their offspring, covering the continuum of r-strategists (pelagic–aquatic, arial, most invertebrates, heterotherms) and K-strategists (littoral–terrestrial, some invertebrates, homeotherms).

Driving forces from soil invertebrates to ecosystem functioning: the allometric perspective.

The European soil policy is being focussed towards a more conscious and sustainable use of the soil, taking into account ecological, economical and societal dimensions. Living soil organisms are reliable bioindicators, as they provide the best reflection of the soil system, ecological services and ecosystem functioning therein. These most complex (bio)physical systems indicate, among others, the energy flow. Such processes can be described by rather simple power law relationships. In fact, the average body mass (dry weight) can be seen as an inherent species property, while population density is a much more flexible parameter reflecting ecosystem state. In this study, I review the interactions between these items in relation to feedbacks and conjectured relationships which can be seen as ecological networks. From this novel perspective, allometry can be used as an integrated measure for the anthropogenic influence on landscapes and related food webs. Allometry is, therefore, a perfect surrogate for land use intensity in modelling of field effects for restoration ecology and conservation biology. Robust correlations will be addressed between the density dependence of invertebrates and the ability of soil systems themselves to recover after disturbance. Quantitative indicators of soil community composition and related ecological services are proposed and their application for ecological risk assessment is illustrated.

Distributional (in) congruence of Biodiversity-Ecosystem Functioning

The majority of research on biodiversity–ecosystem functioning in laboratories has concentrated on a few traits, but there is increasing evidence from the field that functional diversity controls ecosystem functioning more often than does species number. Given the importance of traits as predictors of niche complementarity and community structures, we (1) examine how the diversity sensu lato of forest trees, freshwater fishes and soil invertebrates might support ecosystem functioning and (2) discuss the relevance of productive biota for monophyletic assemblages (taxocenes). In terrestrial ecosystems, correlating traits to abiotic factors is complicated by the appropriate choice of body-size distributions. Angiosperm and gymnosperm trees, for example, show metabolic incongruences in their respiration rates despite their pronounced macroecological scaling. Scaling heterotrophic organisms within their monophyletic assemblages seems more difficult than scaling autotrophs: in contrast to the generally observed decline of mass-specific metabolic rates with body mass within metazoans, soil organisms such as protozoans show opposite mass-specific trends. At the community level, the resource demand of metazoans shapes multitrophic interactions. Hence, population densities and their food web relationships reflect functional diversity, but the influence of biodiversity on stability and ecosystem functioning remains less clear. We focused on fishes in 18 riverine food webs, where the ratio of primary versus secondary extinctions (hereafter, ‘extinction partitioning’) summarizes the responses of fish communities to primary species loss (deletions) and its consequences. Based on extinction partitioning, our high-diversity food webs were just as (or even more) vulnerable to extinctions as low-diversity food webs. Our analysis allows us to assess consequences of the relocation or removal of fish species and to help with decision-making in sustainable river management. The study highlights that the topology of food webs (and not simply taxonomic diversity) plays a greater role in stabilizing the food web and enhancing ecological services than is currently acknowledged.

Chapter 1 Allometry of Body Size and Abundance in 166 Food Webs

Summary The relationship between average body masses ( M ) of individuals within species and densities ( N ) of populations of different species and the mechanisms and consequences of this relationship have been extensively studied. Most published work has focused on collections of data for populations of species from a single broad taxon or trophic level (such as birds or herbivorous mammals), rather than on the populations of all species occurring together in a local food web, a very different ecological context. We here provide a systematic analysis of relationships between M and N in community food webs (hereafter simply webs ), using newly collected, taxonomically detailed data from 166 European and North American pelagic, soil, riparian, benthic, inquiline, and estuarine webs. We investigated three topics. First, we compared log( N )‐versus‐log( M ) scatter plots for webs and the slope b 1 of the ordinary‐least‐squares (OLS) regression line log( N ) = b 1 log( M ) + a 1 to the predictions of two theories (Section V.A). The energetic equivalence hypothesis (EEH) was not originally intended for populations within webs and is used here as a null‐model. The second theory, which extends the EEH to webs by recognizing the inefficiency of the transfer of energy from resources to consumers (a trophic transfer correction, or TTC), was originally proposed for webs aggregated to trophic levels. The EEH predicts approximate linearity of the log( N )‐versus‐log( M ) relationship, with slope −3/4 for all webs. The relationship was approximately linear for most but not all webs studied here. However, for webs that were approximately linear, the slope was not typically −3/4, as slopes varied widely from web to web. Predictions of the EEH with TTC were also largely falsified by our data. The EEH with TTC again predicts linearity with b 1 b 1 >−3/4, indicating that populations of larger taxa absorb more energy than populations of smaller ones. Slopes b 1 > −3/4 can occur without violating the conservation of energy, even in webs that are energetically isolated above trophic level 0 (discussed later). Second, for each web, we compared log–log scatter plots of the M and N values of the populations of each taxon with three alternate linear statistical models (Section V.B). Trophic relationships determined which taxa entered the analysis but played no further role except for the Tuesday Lake and Ythan Estuary webs. The assumptions of the model log( N ) = b 1 log( M ) + a 1 + ɛ 1 (including linearity of the expectation) were widely but not universally supported by our data. We tested and confirmed a hypothesis of Cohen and Carpenter (2005) that the model log( N ) = b 1 log( M ) + a 1 + ɛ 1 describes web scatter plots better, in general, than the model log( M ) = b 2 log( N ) + a 2 + ɛ 2 . The former model is also better than the model of symmetric linear regression. Third, since not all of our log–log scatter plots formed approximately linear patterns, we explored causes of nonlinearity and examined alternative models (Section V.C). We showed that uneven lumping of species to web nodes can cause log( N )‐versus‐log( M ) scatter plots to appear nonlinear. Attributes of the association between N and M depended on the type of ecosystem from which data were gathered. For instance, webs from the soil of organic farms were much less likely to exhibit linear log( N )‐versus‐log( M ) relationships than webs from other systems. Webs with a larger range of measured log( M ) values were more likely to appear linear. Our data rejected the hypothesis that data occupy a polygonal region in log( N )‐versus‐log( M ) space.

Occurrence of pollen and spores in relation to present‐day vegetation in a Dutch heathland area

. Pollen, spores and fungal remains in moss cushions along a transect, traversing a Betula-Quercus forest and a surrounding heathland, were analysed in order to study the relation between present-day vegetation and recent pollen deposition. Pollen and spores are divided into local types from plant species encountered along the transect and regional types, not present along the transect. Relative percentages and absolute concentration of the palynomorphs were estimated. Radiocaesium activity in the moss was measured to assess the minimal duration of palynomorph accumulation (i.e. the age of the moss samples). The absence of simultaneous trends in the regional pollen types indicates that the samples do not have major differences in age. The moss represents the later stages in the heath mosaic cycle. This is supported by the radiocaesium content of the moss. Thus, in this case the concentration values of regional pollen approach deposition values. The values of the local pollen types calculated on such a rather constant regional pollen flux is considered to represent real differences in the local deposition. The local pollen types can be arranged topographically into five groups characteristic of the local vegetation types and their ecotones. The arrangement of the curves of types from phyco-, zoo-, and myco-coenoses suggests their possible origin, either locally from the heath area or from the Betula-Quercus forest, or regionally from elsewhere. Analyses and topographical arrangement of the curves of unknown types contribute in this way to their identification and their possible source of origin.

Networking Agroecology: Integrating the Diversity of Agroecosystem Interactions

Worldwide demand for food will increase dramatically in the future as global human population grows. Increasing efficiency of crop production is unlikely to be sufficient to meet the demand, presenting a long-term threat to humanity’s ‘well-being’. Knowledge of the system-level behaviour of agroecosystems, however, remains surprisingly limited, reflecting the agricultural focus on particular species. This is starting to change towards an ecosystem and network-based approach, following the recent revolution in thinking about resource use and sustainability in our other global food production industry: commercial fisheries. Agroecosystems appear to retain plasticity of ecological processes that might be manipulated for productivity and sustainability. Network structure and dynamics have substantial impacts on ecosystem performance, but evidence from agroecosystems lags behind network theory. Here, we provide an introduction to network theory and application in agroecosystems, identify network metrics for management and environmental change, and, finally, we highlight gaps in our current knowledge and key research themes. These themes include: is the structure of agroecological networks affected by sampling; how do ecosystem services ‘emerge’ empirically from ecological organization, function and network properties; how do spatial and temporal scale and resolution influence system performance; and, can network agroecology be used to design systems that maximize ecosystem services?