Metabolic theories in ecology interpret ecological patterns at different levels through the lens of metabolism, typically applying allometric power scaling laws to describe rates of energy use. This requires a sound theory for metabolism at the individual level. Commonly used mechanistic growth models lack some potentially important aspects and fail to accurately capture a growth pattern often observed in insects. Recently, a new model (MGM–the Maintenance-Growth Model) was developed for ontogenetic and post-mature growth, based on an energy balance that expresses growth as the net result of assimilation and metabolic costs for maintenance and feeding. The most important contributions of MGM are: 1) the division of maintenance costs into a non-negotiable and a negotiable part, potentially resulting in maintenance costs that increase faster than linearly with mass and are regulated in response to food restriction; 2) differentiated energy allocation strategies between sexes and 3) explicit description of costs for finding and processing food. MGM may also account for effects of body composition and type of growth at the cellular level. The model was here calibrated and evaluated using empirical data from an experiment on house crickets growing under ad libitum conditions. The procedure involved parameter estimations from the literature and collected data, using statistical models to account for individual variation in parameter values. It was found that ingestion rate cannot be generally described by a simple allometry, here requiring a more complex description after maturity. Neither could feeding costs be related to ingestion rate in a simplistic manner. By the unusual feature of maintenance costs increasing faster than linearly with body mass, MGM could well capture the differentiated growth patterns of male and female crickets. Some other mechanistic growth models have been able to provide good predictions of insect growth during early ontogeny, but MGM may accurately describe the trajectory until terminated growth.

A growing animal ingests food from the environment and distributes the assimilated energy between chemical energy stored in synthesized biomass and energy spent on metabolic processes, including food processing, maintenance, activity and overhead costs for growth. Under food restriction, the growth rate is usually decreased. However, the extent of this reduction may be influenced by a potential trade -off with maintenance metabolism. The latter seems to be down-regulated under food restriction in some animals and up-regulated in others. Recently, the Maintenance-Growth Model (MGM) was developed for ontogenetic and post-mature growth, including several aspects not considered by common mechanistic growth models, most importantly the division of maintenance costs into non-negotiable and negotiable parts, where the latter can be up- or downregulated under food restriction. Using empirical data, MGM has been calibrated and successfully applied to an insect growing under ad libitum conditions. Here, the model is further calibrated to newly collected individual data for the same species growing under two different regimes of food restriction, complemented with previously collected data for food-limited cohorts. We find that two alternative model scenarios of MGM are able to generate rather good predictions of observed growth under food restriction, assuming either upregulated maintenance or decreased effective assimilation (assimilation minus energy spent on processing and searching food). We find the latter scenario least plausible, implying that the current study provides the first indication for the occurrence of upregulated maintenance in an insect species when food is scarce, an unexpected result that requires further investigation. The inclusion of maintenance regulation in MGM enables the new growth model to be used in the modelling of life-history dependent trade-offs between maintenance, growth and maturation for various other species.

Metabolic theories in ecology interpret ecological patterns at different levels through the lens of metabolism, typically applying allometric scaling to describe energy use. This requires a sound theory for individual metabolism. Common mechanistic growth models, such as 'von Bertalanffy', 'dynamic energy budgets' and the 'ontogenetic growth model' lack some potentially important aspects, especially regarding regulation of somatic maintenance. We develop a model for ontogenetic growth of animals, applicable to ad libitum and food limited conditions, based on an energy balance that expresses growth as the net result of assimilation and metabolic costs for maintenance, feeding and food processing. The most important contribution is the division of maintenance into a 'non-negotiable' and a 'negotiable' part, potentially resulting in hyperallometric scaling of maintenance and downregulated maintenance under food restriction. The model can also account for effects of body composition and type of growth at the cellular level. Common mechanistic growth models often fail to fully capture growth of insects. However, our model was able to capture empirical growth patterns observed in house crickets.

1.Many fish stocks have declined, because of overharvesting, habitat destruction and introduced species. Despite efforts to rehabilitate some of these stocks, not all are responding or are recovering only slowly.

2.In freshwater systems, introduced crayfish are often problematic, and it has been suggested that their egg predation could reduce recruitment in depleted stocks of native fish.

3.Here, we report the results of a field experiment, using experimental cages, on the extent of predation on eggs of great Arctic charr (Salvelinus umbla) in Lake Vättern, Europe's fifth largest lake. Here, the great Arctic charr has declined dramatically and is listed as critically endangered.

4.We were able to partition the total loss rate of eggs into background mortality, predation by introduced signal crayfish (Pacifastacus leniusculus) and predation by native fish. The mortality rate of charr eggs because of crayfish was estimated at more than five times that because of native fish. Of the total loss of eggs, 80% is believed to be caused by crayfish and 14% by fish, with 6% being natural background mortality.

5.In a worst case scenario, our data infer that only 25% of the original number of eggs would survive, compared with 75% in the absence of crayfish. This could impair recovery of the stock of the endangered great Arctic charr in Lake Vättern. 6.Contrary to earlier claims that crayfish predation on eggs of great Arctic charr is insignificant, our results indicate that crayfish predation may exceed fish predation and suggest that the abundance of signal crayfish on the spawning sites of great Arctic charr should be managed.

Due to the complex interactions between species in food webs, the extinction of one species could lead to a cascade of further extinctions and hence cause dramatic changes in species composition and ecosystem processes. We found that the risk of additional species extinction, following the loss of one species in model food webs, decreases with the number of species per functional group. For a given number of species per functional group, the risk of further extinctions is highest when an autotroph is removed and lowest when a top predator is removed. In addition, stability decreases when the distribution of interaction strengths in the webs is changed from equal to skew (few strong and many weak links). We also found that omnivory appears to stabilize model food webs. Our results indicate that high biodiversity may serve as an insurance against radical ecosystem changes.

Many ecologically relevant life-history traits of organisms, (such as generation time and ingestion rate) are significantly correlated to body size. Since these individual and species characteristics can affect the interactions between the species in a community, it is possible that the distribution of body sizes (in communities) can affect different properties of foodwebs as well. To explore this possibility, this thesis analyzes the effect of real body size distributions on various aspects of food web organization. To begin with, data on the body sizes of the organisms in a collection of documented food webs was estimated (resulting in data on almost 700 animal consumers in 52 food webs).

Part of this data is used to analyze the "cascade model" assumption of a constant predation probability for every potential predator-prey pair in a number of these web. The original assumption is compared to four heterogeneous alternatives termed predator-dominance (i),prey-dominance (ii), distance-dominance (iii) and size-ratio dominance (iv). In these alternatives the predation probabilities are non-constant and determined either by the identity of the predator (i) or the prey (ii), or by the difference in rank (iii) or size (iv) between thepredator and its prey. In the restricted set of webs for which adequate body size data was available (16 webs) the null-hypothesis (equal predation probabilities) was rejected in favor of any of these alternatives in 7 out of 16 cases (and specifically, in favor of the size-ratio alternative in 4 out of 16 cases) at a significance level of P=0.06.

Furthermore, the relationship between prey size, predator size and the trophic position of consumers have been analyzed and compared with the predictions of the cascade model. It is shown that the relative size difference between a consumer and its average prey tends to decrease with the trophic position of the consumer in these webs. Furthermore, the trophic links do not seem to be distributed randomly between consumers and their potential prey, as assumed by the cascade model. Instead large predators tend to eat prey of larger size than the cascade model predicts.

The implications of these findings, for the stability of Lotka-Volterra food chains, is analyzed. It is suggested that the trophic interaction coefficients of the Lotka-Volterra model should be affected by the relative size difference between a predator and its prey. It is shown that assuming that the prey interaction strength is positively correlated to the predator-prey body mass ratio, (which can motivated by energetical arguments), in combination with decreasing size ratios between consumers and their prey, with trophic position of the consumer, affects the stability properties of a food chain. The probabilities of local stability and resilience are both increased, when compared to chains where the size ratio does not change with trophic position.

The effect of the generation time of the primary producer, relative to that of the primary consumer, on food chain length is analyzed. In phytoplankton based systems the turnover rates of the primary producers exceed that of the primary consumers while the opposite is true in many terrestrial (tree based) systems. Furthermore, pelagic food webs appear to have longer food chains on average than terrestrial webs. In particular, phytoplankton chains seem to be longer than for example tree based food chains. We show that one reason for this could be that in model Lotka-Volterra food chains (with linear functional responses), resilience increases with the growth rate of the primary producer.

Finally, the capacity of Lotka-Volterra food chain models to produce feasible food chains where the abundance pattern is pyramidal ("the pyramid of numbers") is analyzed. It is shown that density-dependent consumer mortality as well as the distribution of body sizes may affect this capability. Without density-dependent consumer mortality it is difficult (and under some assumptions impossible) for a model Lotka-Volterra food chain to be pyramidal for more than one chain length. With density-dependent consumer mortality this is not a problem. Furthermore, if some of the parameters (i.e. the interaction strengths and death rates) are scaled to body size in a realistic way, it may be possible for model food chains of adjacent lengths to be pyramidal to some extent, even without density-dependent consumer mortality.

The main theme of this thesis is that considering realistic distributions of body sizes in communities can give a better description of how trophic links are distributed and an understanding of how dynamical instability or unrealistic patterns of abundance can be avoided in real communities.

The majority of species in the ecosystems of the world are rare. Because the contributions to community biomass and productivity of many of these species are small it has been suggested that loss of rare species should have relatively small ecological consequences. However, the extent to which rare species affect the structure and stability of ecosystems is largely unknown. Using a theoretical approach, based on analytical methods, we here   investigate how perturbations to rare as well as common species affect the structure (distribution of equilibrium abundances of species) and resilience (recovery rate) of complex ecological communities. We show that, contrary to expectation, resilience and structure of ecological communities are generally more sensitive to perturbations to rare than to common species. We find the explanation for this to lie in the cause of rarity: rare species tend to interact strongly, on a per capita basis, with other species. Our results suggest that many rare species are likely to fill important ecological roles in ecosystems.