Employing dynamic model to study food web stability and possible regime shifts in Somme Bay

Food webs

Generalist Predators, Interaction Strength and Food-web Stability

Relation between complexity and stability in food webs with adaptive behavior

Since the early twentieth century, different general laws have been investigated to understand mechanisms driving stability in natural ecosystems, but until today the mechanisms are still generally unexplored. The main goal for ecology is to understand mechanisms driving food web dynamics, to counteract the hazard of global species loss. The studies presented in this thesis investigate the general scaling of different strucural food web (e.g. diversity, connectance, vulnerabilty) and species properties (e.g. body mass, trophic level), and how these properties influence secondary extinctions in food webs. The backbone of this thesis is a database of food webs , including information about predator–prey interactions, the metabolic type, and the species’ body mass. The relationship between diversity and topology is widely discussed, especially the hypothesis that a constant number of species per link leads to a decreasing connectance with increasing number of species. The alternative to this idea has been the ’constant connectance hypothesis’, where connectance is constant with increasing number of species. As part of my thesis (Chapter 2), I analysed the scaling of topological properties based on my compiled database and found power–law scaling relationships with diversity and complexity for most properties. Also, connectance tends to decrease with increasing number of species. The results illustrate the lack of universal constants in food web ecology as a function of diversity and complexity. Furthermore, common measures of bio–complexity (e.g. the fractions of top, intermediate and basal species, and the average trophic level) have been reinvestigated, as scale–dependent on diversity and connectance to. Interestingly, the scale dependence is partly significantly different between ecosystem types. A lot of species’ characteristics depend on body mass (eg. predator–prey interactions, metabolism, mobility) thus nominating body mass as the most important species attribute. Chapter 3 illustrates the distribution of mean population body masses in communities for different ecosystem types. The body masses are often roughly log–normally (terrestrial and stream ecosystems) or multi–modally (lake and marine ecosystems) distributed, and most networks exhibit exponential cumulative degree distributions. An exception are stream networks which most often possess uniform degree distributions. Furthermore, with increasing body mass vulnerability decreases in 70% of the food webs and generality increases in 80% of the food webs. Facing paradigms developed by Elton, I analysed the relationship of predator mass to prey mass and trophic level and the relationship between predator–prey body–mass ratio (hereafter: mass ratio) and trophic level (Chaper 4). In 1927, Elton suggested that (i) the mean prey mass increases with predator mass, (ii) the predators become larger in size with increasing trophic level, and (iii) the mass ratio is constant across trophic levels. After analysing the data base, the result supports the paradigms (i) and (ii). However, consistant with theoretical derivations, I found a systematic decrease in mass ratios with the trophic level of the predator. This result indicates the general pattern that on average predators at the top of the food webs are more similar in size to their prey than those closer to the base. Food–web stability is critically dependent on species loss. In two subsequent projects (Chapter 5, 6), I applied a bioenergetic model approach to simulate species loss in a set of (Chapter 5) 1000 model food webs and (Chapter 6) 30 empirical food webs randomly chosen from the food web data base . I analysed the stability of model food webs in respect of effects of topological, size–based, and dynamical properties. Stabiltiy has been messured as the number of secondary extinctions after removing one species from the network. The results show that food–web robustness is affected by factors from all three groups. However, the most striking effect was related to the body mass–abundance relationship which points to the importance of body mass relationships for food web stability. Additionally to the network–related properties (e.g. diversity, connectance), I analysed species related properties (e.g. body mass, trophic level). Overall, ecosystem-types (lake, stream, marine, and terrestrial ecosystems) react in the same way to species loss. I found food webs with high diversity and a low standard deviation of vulnerability were less affected by secondary extinctions. At the species level, consistent with classical conservation biology findings, I found that the loss of large–bodied top predators increases the extinction–risk for all others species in the ecosystem. The work presented here contributes to the understanding of underlying mechanisms and dynamics between interacting species in ecosystems. It illustrates differences between ecosystem types, where ”streams tend to be different than other ecosystems”. Overall, the studies show how energy fluxes can contribute to the stability of natural communities, how topological properties influence the interplay between animal populations and how complex communities react to species loss.

Stability and reactivity of food webs with trophic interactions.

The stability of ecological systems has been a long-standing focus of ecology. Recently, tools from random matrix theory have identified the main drivers of stability in ecological communities whose network structure is random. However, empirical food webs differ greatly from random graphs. For example, their degree distribution is broader, they contain few trophic cycles, and they are almost interval. Here we derive an approximation for the stability of food webs whose structure is generated by the cascade model, in which ‘larger' species consume ‘smaller' ones. We predict the stability of these food webs with great accuracy, and our approximation also works well for food webs whose structure is determined empirically or by the niche model. We find that intervality and broad degree distributions tend to stabilize food webs, and that average interaction strength has little influence on stability, compared with the effect of variance and correlation.

Complex dynamics of a four-species food web with two preys, one middle predator, and one top predator are investigated. Via the method of Jacobian matrix, the stability of coexisting equilibrium for all populations is determined. Based on this equilibrium, three bifurcations, i.e., Hopf bifurcation, Hopf-Hopf bifurcation, and period-doubling bifurcation, are analyzed by center manifold theorem, bifurcation theorem, and numerical simulations. We reveal that, influenced by the three bifurcations, the food web can exhibit very complex dynamical behaviors, including limit cycles, quasiperiodic behaviors, chaotic attractors, route to chaos, period-doubling cascade in orbits of period 2, 4, and 8 and period 3, 6, and 12, periodic windows, intermittent period, and chaos crisis. However, the complex dynamics may disappear with the extinction of one of the four populations, which may also lead to collapse of the food web. It suggests that the dynamical complexity and food web stability are determined by the food web structure and existing populations.

Parasites are widespread in nature. Nevertheless, they have only recently been incorporated into food web studies and community ecology. Earlier studies revealed the large effects of parasites on food web network structures, suggesting that parasites affect food web dynamics and their stability. However, our understanding of the role of parasites in food web dynamics is limited to a few theoretical studies, which only assume parasite-induced mortality or virulence as a typical characteristic of parasites, without any large difference in terms of predation effects. Here, I present a food web model with parasites in which parasites change the mortality and interaction strengths of hosts by affecting host activity. The infected food web shows that virulence and infection rate have virtually no effect on food web stability without any difference in interaction strengths between susceptible and infected individuals. However, if predation rates are weakened through a restriction of the activity of infected individuals, virulence and infection rate can greatly influence stability: diseases with lower virulence and higher transmission rate tend to increase stability. The stabilization is stronger in cascade than random food webs. The present results suggest that parasites can greatly influence food web stability if parasite-induced diseases prevent host foraging activity. Parasite-induced infectious disease, by weaking species interactions, may play a key role in maintaining food webs.

Allometric structure and topology of food webs: Energetic constraints conserve food-web structure across ecosystems and space

Debate exists about the effects of hydrological variation on food web dynamics and the relative importance of different sources of organic carbon fuelling food webs in floodplain rivers. Stable carbon isotope analyses and ecological stoichiometry were used to determine the basal sources in dry season macroinvertebrate food webs in two floodplain river systems of Australia’s wet–dry tropics that have contrasting flow regimes. Algae, associated with phytoplankton and biofilm, were the primary food source, potentially contributing >55% organic carbon to the biomass of a wide range of primary and secondary consumers. However, many consumers assimilated other sources in addition to algae, e.g. detritus from local C3 riparian vegetation. Food webs were characterised by substantial flexibility in the number and types of sources identified as important, which was indicative of generalist feeding strategies. These findings suggest ‘dynamic stability’ in the food webs, which imparts resilience against natural disturbances like flow regime seasonality and variation in hydrological connectivity. This adaptation may be characteristic of macroinvertebrate assemblages in highly seasonal river systems or in those with high levels of flow variability.

The structure of food webs is frequently described using phenomenological stochastic models. A prominent example, the niche model, was found to produce artificial food webs resembling real food webs according to a range of summary statistics. However, the size structure of food webs generated by the niche model and real food webs has not yet been rigorously compared. To fill this void, I use a body mass based version of the niche model and compare prey-predator body mass allometry and predator-prey body mass ratios predicted by the model to empirical data. The results show that the model predicts weaker size structure than observed in many real food webs. I introduce a modified version of the niche model which allows to control the strength of size-dependence of predator-prey links. In this model, optimal prey body mass depends allometrically on predator body mass and on a second trait, such as foraging mode. These empirically motivated extensions of the model allow to represent size structure of real food webs realistically and can be used to generate artificial food webs varying in several aspects of size structure in a controlled way. Hence, by explicitly including the role of species traits, this model provides new opportunities for simulating the consequences of size structure for food web dynamics and stability.

Food webs show the architecture of trophic relationships, revealing the biodiversity and species interactions in an ecosystem. Understanding which factors modulate the structure of food webs offers us the ability to predict how they will change when influential factors are altered. To date, most of the research about food webs has focused on species interactions whereas the influences of surrounding environments have been overlooked. Here, using network analysis, we identified how the structure of aquatic food webs varied across a range of geophysical conditions within a whole stream system. Within a headwater basin in the Cascade Mountain Range, Oregon, USA, macroinvertebrate and vertebrate composition was investigated at 18 sites. Predator–prey interactions were compiled based on existing literature and dietary analysis. Several structural network metrics were calculated for each food web. We show that the structure of food webs was predictable based on geophysical features at both local (i.e., slope) and broader (i.e., basin size) spatial extents. Increased omnivory, greater connectance, shorter path lengths, and ultimately greater complexity and resilience existed downstream compared to upstream in the stream network. Surprisingly, the variation in food web structure was not associated with geographic proximity. Structural metric values and abundance of omnivory suggest high levels of stability for these food webs. There is a predictable variation in the structure of food webs across the network that is influenced by both longitudinal position within streams and patchy discontinuities in habitat. Hence, findings illustrate that the slightly differing perspectives from the River Continuum Concept, Discontinuity Patch Dynamics, and Process Domains can be integrated and unified using food web networks. Our analyses extend ecologists’ understanding of the stability of food webs and are a vital step toward predicting how webs and communities may respond to both natural disturbances and current global environmental change.

The mechanisms of species coexistence make ecologists fascinated, although theoretical work shows that omnivory can promote coexistence of species and food web stability, it is still a lack of the general mechanisms for species coexistence in the real food webs, and is unknown how omnivory affects the interactions between competitor and predator. In this work, we first establish an omnivorous food web model with a competitor based on two natural ecosystems (the plankton community and fig–fig wasp system). We analyze the changes of both food web structure and stability under the different resource levels and predation preference of the generalist/top predator. The results of model analyses show that weak predation strength can promote stable coexistence of predators and prey. Moreover, the evolutionary trend of food web structure changes with the relative predation strength is more diverse than the relative competition strength, and an integration of both omnivory, increased competition, top-down control and bottom-up control can promote species diversity and food web stability. Our theoretical predictions are consistent with empirical data in the plankton community: the lower concentration of nutrient results in a more stable population dynamics. Our theoretical work could enrich the general omnivorous theory on species coexistence and system stability in the real food webs.

Incorporating demographic diversity into food web models: Effects on community structure and dynamics

Nonpoint source pollution entering rivers will pollute water quality, degrading the health of aquatic ecosystems. However, owing to the lack of quantitative research on the effects of nonpoint source pollution on the structure of aquatic food webs, there is a lack of quantitative basis for river management. Nonpoint source pollution is not only difficult to control effectively, but also the success rate of water ecological restoration projects is low. With the increasing proportion of nonpoint source pollution in water environmental problems, it is urgent to quantitatively assess and predict the impact of nonpoint source pollution on the structure of food webs. Therefore, this thesis presents a method for quantitatively assessing and predicting the impact of nonpoint source pollution on the structure of food webs through using fuzzy clustering to screen the typical points of the impact of nonpoint source pollution, then using canonical correspondence analysis (CCA) and partial least squares regression analysis to comprehensively filtrate the driving factors affect food web that results in nonpoint source pollution, and then determining the impact of each driving factor on the structure of food webs. Finally, the change trend of food web structure is predicted. The results show that (1) the driving factors that the nonpoint source pollution that affects the food web structure is NH3‐N and chemical oxygen demand (COD). The increase in NH3‐N and COD promotes the growth of phytoplankton, causing the change of the primary productivity of the ecosystem, and ultimately changes the entire food web structure; (2) NH3‐N and COD affect the stability, maturity, connectivity and complexity of the aquatic food web structure. The increase of NH3‐N increases the connectivity and maturity of the food web structure but reduces complexity and stability; the increase of COD increases the connection of the food web structure, while reducing the other three indicators; (3) in some areas with good water quality, aquatic species diversity is high, the relationship of interspecies dietary is complex, food web structure level index is high and the structure of food web is stable. The food web structure in the rainy season will be better than that in the dry season. In some areas with severe water pollution and poor food web structure, the ability of the food web to resist external interference is weak. The food web structure in the rainy season will be worse than that in the dry season owing to rainfall into the river. The methods and conclusions in this treatise can provide a reliable and quantitative scientific basis for river ecosystem management and ecosystem restoration and can improve the success rate of ecological restoration projects.