Effects of angiotensin II on pressor responses to norepinephrine in humans

In previous studies in the conscious rabbit and in isolated artery preparations, low doses of angiotensin II synergistically amplified the pressor effects of the sympathetic neurotransmitter, norepinephrine (NE). To determine whether these observations could be replicated in humans, 9 normal adult male volunteers (mean age: 34) each were given 3 i.v. doses of NE (25, 50 and 100 micrograms.kg-1.min-1) during consecutive 10 min infusion periods. On a second study day, the procedure was repeated during infusion of angiotensin II in a subpressor dose (1.25 ng.kg-1.min-1). The angiotensin II didn't alter the BP responses to NE, but it attenuated the heart rate (HR) decreases associated with the NE infusions by 80% (P less than 0.05), 42% (P less than 0.05) and 42% (P less than 0.1). The two study days were then repeated following 2 weeks of oral treatment with the angiotensin converting enzyme inhibitor captopril (which, despite significantly decreasing baseline BP, also tended to decrease HR). In the presence of captopril, the pressor responses to the NE challenges were reduced by 50% (P less than 0.05), 33% (P less than 0.05) and 13% (P less than 0.1) compared with the pre-captopril responses. Thus, angiotensin II in subpressor doses appears to enhance NE pressor effects by attenuating the compensatory HR responses, whereas inhibition of endogenous angiotensin II mechanisms weakens the BP-raising actions of NE. These findings in humans are consistent with earlier observations that the renin-angiotensin system can directly amplify sympathetic pressor effects in two separate ways: by modifying baroreceptor sensitivity and by enhancing the actions of norepinephrine on vascular smooth muscle.     

Interaction strengths in balanced carbon cycles and the absence of a relation between ecosystem complexity and stability

The strength of interactions is crucial to the stability of ecological networks. However, the patterns of interaction strengths in mathematical models of ecosystems have not yet been based upon independent observations of balanced material fluxes. Here we analyse two Antarctic ecosystems for which the interaction strengths are obtained: (1) directly, from independently measured material fluxes, (2) for the complete ecosystem and (3) with a close match between species and ‘trophic groups’. We analyse the role of recycling, predation and competition and find that ecosystem stability can be estimated by the strengths of the shortest positive and negative predator‐prey feedbacks in the network. We show the generality of our explanation with another 21 observed food webs, comparing random‐type parameterisations of interaction strengths with empirical ones. Our results show how functional relationships dominate over average‐network topology. They make clear that the classic complexity‐instability paradox is essentially an artificial interaction‐strength result.     

Local ecosystem feedbacks and critical transitions in the climate

Global and regional climate models, such as those used in IPCC assessments, are the best tools available for climate predictions. Such models typically account for large-scale land-atmosphere feedbacks. However, these models omit local vegetation-environment feedbacks that may be crucial for critical transitions in ecosystems at larger scales. In this viewpoint paper, we propose the hypothesis that, if the balance of feedbacks is positive at all scales, local vegetation-environment feedbacks may trigger a cascade of amplifying effects, propagating from local to large scale, possibly leading to critical transitions in the large-scale climate. We call for linking local ecosystem feedbacks with large-scale land-atmosphere feedbacks in global and regional climate models in order to improve climate predictions.     

Modelling food webs and nutrient cycling in agro-ecosystems

Agricultural practices affect the spatial patterns and dynamics of the decomposition of soil organic matter and the availability of plant-limiting nutrients. The biological processes underlying these patterns and dynamics are the trophic interactions among the organisms in the soil community food web. Food web models simulate nutrient flow rates close to observed rates and clarify the role of the various groups of organisms in the cycling of nutrients. Several large interdisciplinary programs are currently focusing on these interactions, with a view to developing and managing sustainable forms of agriculture.     

Beyond connectedness: why pairwise metrics cannot capture community stability

The connectedness of species in a trophic web has long been a key structural characteristic for both theoreticians and empiricists in their understanding of community stability. In the past decades, there has been a shift from focussing on determining the number of interactions to taking into account their relative strengths. The question is: How do the strengths of the interactions determine the stability of a community? Recently, a metric has been proposed which compares the stability of observed communities in terms of the strength of three‐ and two‐link feedback loops (cycles of interaction strengths). However, it has also been suggested that we do not need to go beyond the pairwise structure of interactions to capture stability. Here, we directly compare the performance of the feedback and pairwise metrics. Using observed food‐web structures, we show that the pairwise metric does not work as a comparator of stability and is many orders of magnitude away from the actual stability values. We argue that metrics based on pairwise‐strength information cannot capture the complex organization of strong and weak links in a community, which is essential for system stability.     

Feedback spectra of soil food webs across a complexity gradient, and the importance of three-species loops to stability

It has been shown that in real food webs, the strongest omnivorous feedback, a three-link positive feedback, is a good indicator of system stability, suggesting that the strongest positive feedback in a food web could be the Achilles heel of stability. However, the complete spectrum of feedbacks in observed food webs has never been analyzed. Here, we have quantified all the feedbacks in 32 soil food webs along a complexity gradient, including trophic feedbacks and feedbacks resulting from recycling of organic matter. We found that, although the maximum omnivorous feedback was rarely the strongest positive feedback in a system, it stood out over longer and stronger feedbacks as the indicator of stability. The results emphasize the importance of small substructures in complex networks.     

Linking saturation,stability and sustainability in food webs with observed equilibrium structure

Stability of a dynamic equilibrium in a predator-prey system depends both on the type of functional response and on the point of equilibrium on the response curve. Saturation effects from Holling type II responses are known to destabilise prey populations, while a type III (sigmoid) response curve has been shown to provide stability at lower levels of saturation. These effects have also been shown in multi-trophic model systems. However, stability analyses of observed equilibria in real complex ecosystems have as yet not assumed non-linear functional responses. Here, we evaluate the implications of saturation in observed balanced material-flow structures, for system stability and sustainability. We first make the effects of the non-linear functional responses on the interaction strengths in a food web transparent by expressing the elements of Jacobian ‘community’ matrices for type II and III systems as simple functions of their linear (type I) counterparts. We then determine the stability of the systems and distinguish two critical saturation levels: (1) a level where the system is just as stable as a type I system and (2) a level above which the system cannot be stable unless it is subsidised, separating a stable materially sustainable regime from an unsustainable one. We explain the stabilising and destabilising effects in terms of the feedbacks in the systems. The results shed light on the robustness of observed patterns of interaction strengths in complex food webs and suggest the implausibility of saturation playing a significant role in the equilibrium dynamics of sustainable ecosystems.