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

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

Benthic trophic interactions in an Antarctic shallow water ecosystem affected by recent glacier retreat

The western Antarctic Peninsula is experiencing strong environmental changes as a consequence of ongoing regional warming. Glaciers in the area are retreating rapidly and increased sediment-laden meltwater runoff threatens the benthic biodiversity at shallow depths. We identified three sites with a distinct glacier-retreat related history and different levels of glacial influence in the inner part of Potter Cove (King George Island, South Shetland Islands), a fjord-like embayment impacted since the 1950s by a tidewater glacier retreat. We compared the soft sediment meio- and macrofauna isotopic niche widths (delta C-13 and delta N-15 stable isotope analysis) at the three sites to investigate possible glacier retreat-related influences on benthic trophic interactions. The isotopic niches were locally shaped by the different degrees of glacier retreat-related disturbance within the Cove. Wider isotopic niche widths were found at the site that has become ice-free most recently, and narrower niches at the older ice-free sites. At an intermediate state of glacier retreat-related disturbance (e.g. via ice-growler scouring) species with different strategies could settle. The site at the earliest stage of post-retreat development was characterized by an assemblage with lower trophic redundancy. Generally, the isotopic niche widths increased with increasing size spectra of organisms within the community, excepting the youngest assemblage, where the pioneer colonizer meiofauna size class displayed the highest isotopic niche width. Meiofauna at all sites generally occupied positions in the isotopic space that suggested a detrital-pool food source and/or the presence of predatory taxa. In general ice scour and glacial impact appeared to play a two-fold role within the Cove: i) either stimulating trophic diversity by allowing continuous re-colonization of meiofaunal species or, ii) over time driving the benthic assemblages into a more compact trophic structure with increased connectedness and resource recycling

Spectral Measures of Bipartivity in Complex Networks

We introduce a quantitative measure of network bipartivity as a proportion of even to total number of closed walks in the network. Spectral graph theory is used to quantify how close to bipartite a network is and the extent to which individual nodes and edges contribute to the global network bipartivity. It is shown that the bipartivity characterizes the network structure and can be related to the efficiency of semantic or communication networks, trophic interactions in food webs, construction principles in metabolic networks, or communities in social networks.Comment: 16 pages, 1 figure, 1 tabl

Plankton food for benthic fish: de visu evidence of trophic interaction between rainbow wrasse (Coris julis) and pelagic tunicates (Pegea confoederata)

Salps (pelagic tunicates) are rarely observed in nature and trophic interactions involving them mainly rely on the inspection of stomach contents of their potential predators. Moreover, salps have soft bodies that are hardly identified in potential consumers. We involved recreational SCUBA-divers and photographers in collecting de visu evidence of i) massive occurrence of salps and ii) trophic interactions involving salps as preys and benthic animals as consumers. Direct evidence of trophic interactions between salps and benthic fish was documented by photographic frames. We detected a long colony of the salp Pegea confaederata being transported by currents close to the substrate on top of Banco di Santa Croce, an underwater rocky outcrop in the Gulf of Naples (Italy). An individual of the rainbow wrasse Coris julis attacked the above-mentioned salp colony by selectively detaching individuals and biting their stomach. Our report of a trophic interaction between labrids and salps is the second in fifty years and the previous one was only indirect. In this study, citizen science allowed detecting both neglected marine animals like salps and trophic interactions involving them. Visual, direct evidence of predation on salps by benthic fish adds further knowledge about patterns of living-matter fluxes between plankton and benthos, opening new questions on the potential of global change in modifying the efficient circulation of organic matter in marine systems

Dispersal-mediated trophic interactions can generate apparent patterns of dispersal limitation in aquatic metacommunities

Dispersal is a major organising force in metacommunities, which may facilitate compositional responses of local communities to environmental change and affect ecosystem function. Organism groups differ widely in their dispersal abilities and their communities are therefore expected to have different adaptive abilities. In mesocosms, we studied the simultaneous compositional response of three plankton communities (zoo-, phyto- and bacterioplankton) to a primary productivity gradient and evaluated how this response was mediated by dispersal intensity. Dispersal enhanced responses in all three planktonic groups, which also affected ecosystem functioning. Yet, variation partitioning analyses indicated that responses in phytoplankton and bacterial communities were not only controlled by dispersal directly but also indirectly through complex trophic interactions. Our results indicate that metacommunity patterns emerging from dispersal can cascade through the food web and generate patterns of apparent dispersal limitation in organisms at other trophic levels.

Predator water balance alters intraguild predation in a streamsidefood web

Previous work suggests that animal water balance can influence trophic interactions, with predators increasing their consumption of water-laden prey to meet water demands.But it is unclear how the need for water interacts with the need for energy to drive trophic interactions under shifting conditions. Using manipulative field experiments, we show that water balance influences the effects of top predators on prey with contrasting ratios of water and energy, altering the frequency of intraguild predation. Water-stressed top predators (large spiders) negatively affect water-laden basal prey (crickets), especially male prey with higher water content, whereas alleviation of water limitation causes top predators to switch to negatively affecting energy-rich midlevel predators (small spiders). Thus, the relative water and energy content of multiple prey, combined with the water demand of the top predator, influences trophic interactions in ways that can alter the strength of intraguild predation. These findings underscore the need for integration of multi resource approaches for understanding implications of global change for food webs

Predator diversity and abundance provide little support for the enemies hypothesis in forests of high tree diversity

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