Food webs of Mediterranean coastal wetlands

En aquesta tesi doctoral s'han estudiat els efectes directes i indirectes de dos tipus d'espècies claus de la comunitat aquàtica dels aiguamolls de l'Empordà (aiguamolls costaners mediterranis amb una xarxa tròfica senzilla). S'han realitzat experiments al camp utilitzant microcosmos i mesocosmos i els resultats han estat analitzats mitjançant tres aproximacions: la taxonòmica, la funcional i la de mides. S'ha comprovat que en situacions amb absència de predadors i dominància d'una única espècie en el zooplàncton (en aquest cas Calanipeda aquaedulcis i Daphnia magna), la segregació del recurs entre els diferents estadis de desenvolupament de la mateixa espècie zooplanctònica és una estratègia per evitar la competència intraespecífica en condicions de limitació de recurs. Per altra banda, la presència de diferents top-predators a la comunitat aquàtica (en aquest cas la medusa Odessia maeotica i el peix Aphanius iberus) desencadena una cascada tròfica en el plàncton però amb efectes top-down diferents segons el top-predator.

Terrestrial detritus supports the food webs in lowland intermittent streams of south-eastern Australia : a stable isotope study

1. Large amounts of terrestrial detritus enter many low-order forested streams, and this organic
material is often the major basal resource in the metazoan food webs of such systems. However,
despite their apparently low biomass, algae are the dominant food of organisms in a number of
aquatic communities which conventionally would have been presumed to be dependent on
allochthonous detritus, particularly those in the tropics and also in lowland intermittent streams
in arid Australia.
2. The dual stable isotope signatures (d13C and d15N) of potential primary food sources were
compared with the isotopic signatures of common aquatic animals in lowland intermittent
streams in south-eastern Australia, in both spring and summer, to determine whether
allochthonous detritus was an important nutritional resource in these systems. The isotopic
signatures of the major potential allochthonous plant food sources (Eucalyptus, Phalaris and
Juncus) overlapped, but were distinct from algae and the dominant macrophytes growing in the
study reaches. The isotopic signatures of biofilm were more spatially and temporally variable
than those of the other basal resources.
3. Despite allochthonous detritus having relatively high C : N ratios compared to other
potential basal resources, results from ISOSOURCE mixing model calculations demonstrated
that this detritus, and the associated biofilm, were the major energy sources assimilated by
macroinvertebrate primary consumers in both spring and summer. The importance of these
energy sources was also reflected in animals higher in the food web, including predatory
macroinvertebrates and fish. These resources were supplemented by autochthonous sources of
higher nutritional value (i.e. filamentous algae and macrophytes, which had relatively low
C : N ratios) when they became more prolific as the streams dried to disconnected pools in
summer.
4. The results highlight the importance of allochthonous detritus (particularly from Eucalyptus)
as a dependable energy source for benthic macroinvertebrates and fish in lowland intermittent
streams of south-eastern Australia. This contrasts with previous stable isotope studies
conducted in lowland intermittent streams in arid Australia, which have reported that the fauna
are primarily dependent on autochthonous algae.

Eco-evolutionary Dynamics of Individual-Based Food Webs

The past decade has seen the rise of high resolution datasets. One of the main surprises of analysing such data has been the discovery of a large genetic, phenotypic and behavioural variation and heterogeneous metabolic rates among individuals within natural populations. A parallel discovery from theory and experiments has shown a strong temporal convergence between evolutionary and ecological dynamics, but a general framework to analyse from individual-level processes the convergence between ecological and evolutionary dynamics and its implications for patterns of biodiversity in food webs has been particularly lacking. Here, as a first approximation to take into account intraspecific variability and the convergence between the ecological and evolutionary dynamics in large food webs, we develop a model from population genomics and microevolutionary processes that uses sexual reproduction, genetic-distance-based speciation and trophic interactions. We confront the model with the prey consumption per individual predator, species-level connectance and prey–predator diversity in several environmental situations using a large food web with approximately 25,000 sampled prey and predator individuals. We show higher than expected diversity of abundant species in heterogeneous environmental conditions and strong deviations from the observed distribution of individual prey consumption (i.e. individual connectivity per predator) in all the environmental conditions. The observed large variance in individual prey consumption regardless of the environmental variability collapsed species-level connectance after small increases in sampling effort. These results suggest (1) intraspecific variance in prey–predator interactions has a strong effect on the macroscopic properties of food webs and (2) intraspecific variance is a potential driver regulating the speed of the convergence between ecological and evolutionary dynamics in species-rich food webs. These results also suggest that genetic–ecological drift driven by sexual reproduction, equal feeding rate among predator individuals, mutations and genetic-distance-based speciation can be used as a neutral food web dynamics test to detect the ecological and microevolutionary processes underlying the observed patterns of individual and species-based food webs at local and macroecological scales.

Chapter Six – Individual Trait Variation and Diversity in Food Webs

In recent years, there has been a renewed interest in the ecological consequences of individual trait variation within populations. Given that individual variability arises from evolutionary dynamics, to fully understand eco-evolutionary feedback loops, we need to pay special attention to how standing trait variability affects ecological dynamics. There is mounting empirical evidence that intra-specific phenotypic variation can exceed species-level means, but theoretical models of multi-trophic species coexistence typically neglect individual-level trait variability. What is needed are multispecies datasets that are resolved at the individual level that can be used to discriminate among alternative models of resource selection and species coexistence in food webs. Here, using one the largest individual-based datasets of a food web compiled to date, along with an individual trait-based stochastic model that incorporates Approximate Bayesian computation methods, we document intra-population variation in the strength of prey selection by different classes or predator phenotypes which could potentially alter the diversity and coexistence patterns of food webs. In particular, we found that strongly connected individual predators preferentially consumed common prey, whereas weakly connected predators preferentially selected rare prey. Such patterns suggest that food web diversity may be governed by the distribution of predator connectivity and individual trait variation in prey selection. We discuss the consequences of intra-specific variation in prey selection to assess fitness differences among predator classes (or phenotypes) and track longer term food web patterns of coexistence accounting for several phenotypes within each prey and predator species.

Dispersal dynamics in food webs

Studies of food webs suggest that limited nonrandom dispersal can play an important role in structuring food webs. It is not clear, however, whether density-dependent dispersal fits empirical patterns of food webs better than density-independent dispersal. Here, we study a spatially distributed food web, using a series of population-dispersal models that contrast density-independent and density-dependent dispersal in landscapes where sampled sites are either homogeneously or heterogeneously distributed. These models are fitted to empirical data, allowing us to infer mechanisms that are consistent with the data. Our results show that models with density-dependent dispersal fit the α, β, and γ tritrophic richness observed in empirical data best. Our results also show that density-dependent dispersal leads to a critical distance threshold beyond which site similarity (i.e., β tritrophic richness) starts to decrease much faster. Such a threshold can also be detected in the empirical data. In contrast, models with density-independent dispersal do not predict such a threshold. Moreover, preferential dispersal from more centrally located sites to peripheral sites does not provide a better fit to empirical data when compared with symmetric dispersal between sites. Our results suggest that nonrandom dispersal in heterogeneous landscapes is an important driver that shapes local and regional richness (i.e., α and γ tritrophic richness, respectively) as well as the distance-decay relationship (i.e., β tritrophic richness) in food webs.

Data to investigate the role of Pomatoschistus microps in intertidal food webs, sampled off the island of Sylt

Ecological network analysis (ENA) was used to study the effects of Pomatoschistus microps on energy transport through the food web, its impact on other compartments and its possible role as a keystone species in the trophic webs of an Arenicola tidal flat ecosystem and a sparse Zostera noltii bed ecosystem. Three ENA models were constructed: (a) model 1 contains data of the original food web from prior research in the investigated area by Baird et al. (2007), (b) an updated model 2 which included biomass and diet data of P. microps from recent sampling, and (c) model 3 simulating a food web without P. microps. A comparison of energy transport between the different models revealed that more energy is transported from lower trophic levels up the food chain, in the presence of P. microps (models 1 and 2) than in its absence (model 3). Calculations of the keystone index (KSi) revealed the high overall impact (measured as eps_i) of this fish species on food webs. In model 1, P. microps was assigned a low KSi in the Arenicola flat and in the sparse Z. noltii bed. Calculations in model 2 ranked P. microps first for keystoneness and eps_i in both communities, the Arenicola flat and the sparse Z. noltii bed. Taken together, our results give insight into the role of P. microps when considering a whole food web and reveal direct and indirect trophic interactions of this small-sized fish species. These results might illustrate the impact and importance of abundant, widespread species in food webs and facilitate further investigations.

Freshwater mesocosm experiment 2012 in Vancouver, Canada on effects of increased temperature and food chain length on oxygen fluxes and plankton community structure

Over broad thermal gradients, the effect of temperature on aerobic respiration and photosynthesis rates explains variation in community structure and function. Yet for local communities, temperature dependent trophic interactions may dominate effects of warming. We tested the hypothesis that food chain length modifies the temperature-dependence of ecosystem fluxes and community structure. In a multi-generation aquatic food web experiment, increasing temperature strengthened a trophic cascade, altering the effect of temperature on estimated mass-corrected ecosystem fluxes. Compared to consumer-free and 3-level food chains, grazer-algae (2-level) food chains responded most strongly to the temperature gradient. Temperature altered community structure, shifting species composition and reducing zooplankton density and body size. Still, food chain length did not alter the temperature dependence of net ecosystem fluxes. We conclude that locally, food chain length interacts with temperature to modify community structure, but only temperature, not food chain length influenced net ecosystem fluxes.

Fluctuating silicate:nitrate ratios and coastal plankton food webs

Marine diatoms require dissolved silicate to form an external shell, and their growth becomes Si-limited when the atomic ratio of silicate to dissolved inorganic nitrogen (Si:DIN) approaches 1:1, also known as the “Redfield ratio.” Fundamental changes in the diatom-to-zooplankton-to-higher trophic level food web should occur when this ratio falls below 1:1 and the proportion of diatoms in the phytoplankton community is reduced. We quantitatively substantiate these predictions by using a variety of data from the Mississippi River continental shelf, a system in which the Si:DIN loading ratio has declined from around 3:1 to 1:1 during this century because of land-use practices in the watershed. We suggest that, on this shelf, when the Si:DIN ratio in the river decreases to less than 1:1, then (i) copepod abundance changes from >75% to <30% of the total mesozooplankton, (ii) zooplankton fecal pellets become a minor component of the in situ primary production consumed, and (iii) bottom-water oxygen consumption rates become less dependent on relatively fast-sinking (diatom-rich) organic matter packaged mostly as zooplankton fecal pellets. This coastal ecosystem appears to be a pelagic food web dynamically poised to be either a food web composed of diatoms and copepods or one with potentially disruptive harmful algal blooms. The system is directed between these two ecosystem states by Mississippi River water quality, which is determined by land-use practices far inland.

Comparison of aquatic food chains using nitrogen isotopes.

Recent studies have shown the utility of delta(15)N to model trophic structure and contaminant bioaccumulation in aquatic food webs. However, cross-system comparisons in delta(15)N can be complicated by differences in delta(15)N at the base of the food chain. Such baseline variation in delta(15)N is difficult to resolve using plankton because of the large temporal variability in the delta(15)N of small organisms that have fast nitrogen turnover. Comparisons using large primary consumers, which have stable tissue isotopic signatures because of their slower nitrogen turnover, show that delta(15)N increases markedly with the human population density in the lake watershed. This shift in delta(15)N likely reflects the high delta(15)N of human sewage. Correcting for this baseline variation in delta(15)N, we report that, contrary to expectations based on previous food-web analysis, the food chains leading up to fish varied by about only one trophic level among the 40 lakes studied. Our results also suggest that the delta(15)N signatures of nitrogen at the base of the food chain will provide a useful tool in the assessment of anthropogenic nutrient inputs.

Exploring the role of large predators in marsh food webs: evidence for a behaviorally-mediated trophic cascade

The influence of large predators on lower trophic levels in oligotrophic, structurally complex, and frequently disturbed aquatic environments is generally thought to be limited. We looked for effects of large predators in two semi-permanent, spikerush-dominated marshes by excluding large fish (>12 mm body depth) and similarly sized herpetofauna from 1 m2 cages (exclosures) for 2 weeks. The exclosures allowed for colonization by intermediate (in size and trophic position) consumers, such as small fish, shrimp, and crayfish. Exclosures were compared to control cages that allowed large fish to move freely in and out. At the end of the experiment, intermediate-consumer densities were higher in exclosures than in controls at both sites. Decapod crustaceans, especially the riverine grass shrimp (Palaemonetes paludosus), accounted for the majority of the response. Effects of large fish on shrimp were generally consistent across sites, but per capita effects were sensitive to estimates of predator density. Densities of intermediate consumers in our exclosures were similar to marsh densities, while the open controls had lower densities. This suggests that these animals avoided our experimental controls because they were risky relative to the surrounding environment, while the exclosures were neither avoided nor preferred. Although illuminating about the dynamics of open-cage experiments, this finding does not influence the main results of the study. Small primary consumers (mostly small snails, amphipods, and midges) living on floating periphyton mats and in flocculent detritus (“floc”) were less abundant in the exclosures, indicative of a trophic cascade. Periphyton mat characteristics (i.e., biomass, chlorophyll a, TP) were not clearly or consistently affected by the exclosure, but TP in the floc was lower in exclosures. The collective cascading effects of large predators were consistent at both sites despite differences in drought frequency, stem density, and productivity.

Taphonomy and palaeobiology of early Middle Triassic coprolites from the Luoping biota, southwest China: implications for reconstruction of fossil food webs

Abundant, exceptionally preserved coprolites are documented from the Luoping biota (Anisian, Middle Triassic) of Yunnan Province, southwest China. These coprolites can be categorized into fourmorphological types: A) bead to ribbon-shaped, B) short to long cylindrical-shaped, C) flattened, disk-like, and D) segmented faeces. Detailed multi-disciplinary studies reveal that coprolite type A was likely produced by invertebrate animals,while coprolite types B to D could be faeces generated by carnivorous fishes or marine reptiles, perhaps from different taxonomicgroups. When compared with coprolites reported from the Lower Triassic, the Luoping forms indicate more complicated predation-prey food web networks. These evidences, combined with body fossil discoveries fromLuoping, suggest the emergence of complex trophic ecosystems in the Anisian,marking the full biotic recovery following the Permian–Triassic Mass Extinction.

Diversity and Stability in Food Webs : Impacts of Non-Equilibrium Dynamics, Topology and Variation

With progressive climate change, the preservation of biodiversity is becoming increasingly important. Only if the gene pool is large enough and requirements of species are diverse, there will be species that can adapt to the changing circumstances. To maintain biodiversity, we must understand the consequences of the various strategies. Mathematical models of population dynamics could provide prognoses. However, a model that would reproduce and explain the mechanisms behind the diversity of species that we observe experimentally and in nature is still needed. A combination of theoretical models with detailed experiments is needed to test biological processes in models and compare predictions with outcomes in reality. In this thesis, several food webs are modeled and analyzed. Among others, models are formulated of laboratory experiments performed in the Zoological Institute of the University of Cologne. Numerical data of the simulations is in good agreement with the real experimental results. Via numerical simulations it can be demonstrated that few assumptions are necessary to reproduce in a model the sustained oscillations of the population size that experiments show. However, analysis indicates that species "thrown together by chance" are not very likely to survive together over long periods. Even larger food nets do not show significantly different outcomes and prove how extraordinary and complicated natural diversity is. In order to produce such a coexistence of randomly selected species—as the experiment does—models require additional information about biological processes or restrictions on the assumptions. Another explanation for the observed coexistence is a slow extinction that takes longer than the observation time. Simulated species survive a comparable period of time before they die out eventually. Interestingly, it can be stated that the same models allow the survival of several species in equilibrium and thus do not follow the so-called competitive exclusion principle. This state of equilibrium is more fragile, however, to changes in nutrient supply than the oscillating coexistence. Overall, the studies show, that having a diverse system means that population numbers are probably oscillating, and on the other hand oscillating population numbers stabilize a food web both against demographic noise as well as against changes of the habitat. Model predictions can certainly not be converted at their face value into policies for real ecosystems. But the stabilizing character of fluctuations should be considered in the regulations of animal populations.

Resource allocation in two species systems: Is it worth acknowledging species interactions?

It is becoming increasingly popular to consider species interactions when managing ecological foodwebs. Such an approach is useful in determining how management can affect multiple species, with either beneficial or detrimental consequences. Identifying such actions is particularly valuable in the context of conservation decision making as funding is severely limited. This paper outlines a new approach that simplifies the resource allocation problem in a two species system for a range of species interactions: independent, mutualism, predator-prey, and competitive exclusion. We assume that both species are endangered and we do not account for decisions over time. We find that optimal funding allocation is to the conservation of the species with the highest marginal gain in expected probability of survival and that, across all except mutualist interaction types, optimal conservation funding allocation differs between species. Loss in efficiency from ignoring species interactions was most severe in predator-prey systems. The funding problem we address, where an ecosystem includes multiple threatened species, will only become more commonplace as increasing numbers of species worldwide become threatened. © 2011 Elsevier B.V.

Impact of tillage and residues from rotation crops on the nematode community in soil and surface mulch during the following sugarcane crop.

The impact of three cropping histories (sugarcane, maize and soybean) and two tillage practices (conventional tillage and direct drill) on plant-parasitic and free-living nematodes in the following sugarcane crop was examined in a field trial at Bundaberg. Soybean reduced populations of lesion nematode (Pratylenchus zeae) and root-knot nematode (Meloidogyne javanica) in comparison to previous crops of sugarcane or maize but increased populations of spiral nematode (Helicotylenchus dihystera) and maintained populations of dagger nematode (Xiphinema elongatum). However the effect of soybean on P zeae and M. javanica was no longer apparent 15 weeks after planting sugarcane, while later in the season, populations of these nematodes following soybean were as high as or higher than maize or sugarcane. Populations of P zeae were initially reduced by cultivation but due to strong resurgence tended to be higher in conventionally tilled than direct drill plots at the end of the plant crop. Even greater tillage effects were observed with M. javanica and X. elongatum, as nematode populations were significantly higher in conventionally tilled than direct drill plots late in the season. Populations of free-living nematodes in the upper 10 cm of soil were initially highest following soybean, but after 15, 35 and 59 weeks were lower than after sugarcane and contained fewer omnivorous and predatory nematodes. Conventional tillage increased populations of free-living nematodes in soil in comparison to direct drill and was also detrimental to omnivorous and predatory nematodes. These results suggest that crop rotation and tillage not only affect plant-parasitic nematodes directly, but also have indirect effects by impacting on natural enemies that regulate nematode populations. More than 2 million nematodes/m(2) were often present in crop residues on the surface of direct drill plots. Bacterial-feeding nematodes were predominant in residues early in the decomposition process but fungal-feeding nematodes predominated after 15 weeks. This indicates that fungi become an increasingly important component of the detritus food web as decomposition proceeds, and that that the rate of nutrient cycling decreases with time. Correlations between total numbers of free-living nematodes and mineral N concentrations in crop residues and surface soil suggested that the free-living nematode community may provide an indication of the rate of mineralisation of N from organic matter.

A systematic-ecological approach to Baltic Sea ice studies of algae and protists

The seasonal occurrence of sea ice that annually covers almost half the Baltic Sea area provides a unique habitat for halo- and cold temperature-tolerant extremophiles. Baltic Sea ice biology has more than 100 years of tradition that began with the floristic observation of species by the early pioneers using light microscopic techniques that were the only thing available at the time. Since the discovery of life within sea ice, more technologies have become available for taxonomy. Electron microscopy and genetic evidence have been used to identify sea ice biota revealing increased numbers of taxa. Meanwhile ecologists have used light microscopic cell enumeration in addition to the chemical and physical properties of sea ice in attempts to explain the food web structure of sea ice and its functions. Thus, during the Baltic winter, the sea ice hosts more abundant and diverse microbial communities than the water column beneath it. These communities are typically dominated by autotrophic diatoms together with a diverse assortment of dinoflagellates, auto- and heterotrophic flagellates, ciliates, metazoan rotifers and bacteria, which are mostly responsible for the recycling of nutrients. This thesis comprises ecological and systematic studies. In addition to the results of the previous studies carried out on landfast ice, the data presented here provide new insight into the spatial distribution of pelagial sea ice, which has remained largely unexplored. The studies reveal spatial heterogeneity in the pelagial sea ice of the Gulf of Bothnia. There were mismatches in chlorophyll-a concentrations and in photosynthetic efficiencies of the communities studied. The temporal succession was followed and experimental studies performed investigating the community responses towards increased or decreased light in landfast ice in the Gulf of Finland. The systematic studies carried out with established dinoflagellate cultures revealed a new resting cyst belonging to common sea ice dinoflagellate, Scrippsiella hangoei (Schiller) Larsen 1995. The cyst can be used to explain the overwintering of this species during prolonged periods of darkness. The dissimilarities and similarities in the material isolated from the sea ice called for description of a new subspecies Heterocapsa arctica ssp. frigida. The cells obtained in the cultured material were unlike those of the previously described species, necessitating description of ssp. frigida. As a result of its own unique habitus, the subspecies had been noted by Finnish taxonomists during the past three decades and thus its annual occurrence and geographical distribution in the Baltic Sea. This illustrates how combining ecology and systematics increases our understanding of organisms.