Effects of climate on size structure and functioning of aquatic food webs

In aquatic food webs, the role of body size is notoriously strong. It is also well known that temperature has an effect on body size. For instance, Bergmann’s rule states that body size increases from warm to cold climates. This thesis addresses the question how climate shapes the size structure of fish and zooplankton communities, and how this affects the strength of the trophic cascade from fish to plankton. I combine three different approaches: a space-for-time substitution study of data from the 83 shallow lakes distributed along a latitudinal gradient in South America, simple mathematical models to explore climate effects on the dynamics of trophic interactions, and an experimental analysis of trophic interactions using outdoor mesocosms

Trophic model of the coastal fisheries ecosystem of the west coast of peninsular Malaysia

A preliminary mass-balance trophic model was constructed for the coastal fisheries ecosystem of the West Coast of Peninsular Malaysia (0 - 120 m depth). The ecosystem was partitioned into 15 trophic groups, and biomasses for selected groups were obtained from research (trawl) surveys conducted in the area in 1987 and 1991. Trophic interactions of the groups are presented. The network analysis indicates that fishing fleets for demersal fishes and prawns have a major direct or indirect impact on most high-trophic level groups in the ecosystem.Fishery resources, Demersal fisheries, Fishery surveys, Biomass, Population density, Shrimp fisheries, Catch/effort, Trawling, Mathematical models, Coastal fisheries, Marine fisheries, Ecosystems, Trophic structure, ISEW, Malaysia, Malaya,

The impact of climate change on the structure of Pleistocene mammoth steppe food webs

Species interactions shape predator-prey networks, impacting community structure and, potentially, ecological dynamics. It is likely that global climatic perturbations that occur over long periods of time have a significant impact on species interactions patterns. However, observations of how these patterns change over time are typically limited to extant communities, which is particularly problematic for communities with long-lived species. Here we integrate stable isotope analysis and network theory to reconstruct patterns of trophic interactions for six independent mammalian communities that inhabited mammoth steppe environments spanning western Europe to eastern Alaska during the Pleistocene. We use a Bayesian mixing model to quantify the proportional contribution of prey to the diets of local predators, and assess how the structure of trophic interactions changed across space and the Last Glacial Maximum (LGM), a global climatic event that severely impacted mammoth steppe communities. We find that large felids had diets that were more constrained than other co-occurring predators, and largely influenced by an increase in {\it Rangifer} abundance after the LGM. Moreover, the structural organization of Beringian and European communities strongly differed: compared to Europe, species interactions in Beringian communities before the LGM were highly compartmentalized, or modular. This modularity was lost during the LGM, and partially recovered after the glacial retreat, and we suggest that changes in modularity among predators and prey may have been driven by geographic insularity

Community trait overdispersion due to trophic interactions: concerns for assembly process inference

The expected link between competitive exclusion and community trait overdispersion has been used to infer competition in local communities, and trait clustering has been interpreted as habitat filtering. Such community assembly process inference has received criticism for ignoring trophic interactions, as competition and trophic interactions might create similar trait patterns. While other theoretical studies have generally demonstrated the importance of predation for coexistence, ours provides the first quantitative demonstration of such effects on assembly process inference, using a trait-based ecological model to simulate the assembly of a competitive primary consumer community with and without the influence of trophic interactions. We quantified and contrasted trait dispersion/clustering of the competitive communities with the absence and presence of secondary consumers. Trophic interactions most often decreased trait clustering (i.e. increased dispersion) in the competitive communities due to evenly distributed invasions of secondary consumers and subsequent competitor extinctions over trait space. Furthermore, effects of trophic interactions were somewhat dependent on model parameters and clustering metric. These effects create considerable problems for process inference from trait distributions; one potential solution is to use more process-based and inclusive models in inference

Data reliability in complex directed networks

The availability of data from many different sources and fields of science has made it possible to map out an increasing number of networks of contacts and interactions. However, quantifying how reliable these data are remains an open problem. From Biology to Sociology and Economy, the identification of false and missing positives has become a problem that calls for a solution. In this work we extend one of newest, best performing models -due to Guimera and Sales-Pardo in 2009- to directed networks. The new methodology is able to identify missing and spurious directed interactions, which renders it particularly useful to analyze data reliability in systems like trophic webs, gene regulatory networks, communication patterns and social systems. We also show, using real-world networks, how the method can be employed to help searching for new interactions in an efficient way.Comment: Submitted for publicatio

The ecology of chalk-stream invertebrates studied in a recirculating stream

To study and qualify the factors influencing interactions between various trophic levels in natural hard-water streams, a recirculating artificial stream channel was constructed. This structure has enabled patterns of population change of stream fauna to be observed under partially controlled physical and chemical conditions. Initial colonization of the substratum by invertebrates and subsequent succession was studied along with depth distribution and growth and production studies of invertebrates

Towards the integration of niche and network theories

The quest for understanding how species interactions modulate diversity has progressed by theoretical and empirical advances following niche and network theories. Yet, niche studies have been limited to describe coexistence within tropic levels despite incorporating information about multi-trophic interactions. Network approaches could address this limitation, but they have ignored the structure of species interactions within trophic levels. Here we call for the integration of niche and network theories to reach new frontiers of knowledge exploring how interactions within and across trophic levels promote species coexistence. This integration is possible due to the strong parallelisms in the historical development, ecological concepts, and associated mathematical tools of both theories. We provide a guideline to integrate this framework with observational and experimental studies

The shape of ecological networks

We study the statistics of ecosystems with a variable number of co-evolving species. The species interact in two ways: by prey-predator relationships and by direct competition with similar kinds. The interaction coefficients change slowly through successful adaptations and speciations. We treat them as quenched random variables. These interactions determine long-term topological features of the species network, which are found to agree with those of biological systems.Comment: 4 pages, 2 figure

Propagation of Cascades in Complex Networks: From Supply Chains to Food Webs

A general theory of top-down cascades in complex networks is described which explains two similar types of perturbation amplifications in the complex networks of business supply chains (the `bullwhip effect') and ecological food webs (trophic cascades). The dependence of the strength of the effects on the interaction strength and covariance in the dynamics as well as the graph structure allows both explanation and prediction of widely recognized effects in each type of system.Comment: 16 pages, 3 figure