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 vegetationenvironment 5 feedbacks that are crucial for critical transitions in ecosystems. Here, we reveal 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 largescale climate. We call for linking local ecosystem feedbacks with large-scale land10 atmosphere feedbacks in global and regional climate models in order to yield climate predictions that we are more confident about

Severity of seabed spatial competition decreases towards the poles

For more than a century ecologists have considered that competitive interactions between species are more intense at low latitudes 1 and 2. This is frequently invoked as either an explanation or a consequence of higher species richness in the tropics, also suggesting that competition shifts from intra- to inter-specific towards the tropics [1]. Another common assumption is that within a community, intraspecific competition needs to be relatively strong, compared to inter-specific competition, in order to enable stable coexistence of species [3]. However, many analyses have found no consistent large scale geographic patterns in the intensity of intra- or interspecific competition [4]. Here, we show a clear latitudinal trend in contest competition for space in nearshore marine environments, for bryozoans (sessile, colonial, suspension feeding animals). Bryozoans form species-rich assemblages with other encrusting fauna and flora (corraline algae), and are highly abundant across the globe [5]. We find that whilst the intensity of competition (percentage of bryozoan colonies involved in direct physical spatial interactions with bryozoan or other encrusters) differed little with latitude, its severity (percentage of bryozoan colonies involved in contests with a win/loss outcome, leading to death of the loser) was three times lower at the poles than in the tropics. The cause of this change in severity was a strong shift in taxonomic relatedness of competitors, from interactions between species of different families dominating at lower latitudes, to mainly intraspecific competition at the poles

Stability in real food webs: weak links in long loops

Increasing evidence that the strengths of interactions among populations in biological communities form patterns that are crucial for system stability requires clarification of the precise form of these patterns, how they come about, and why they influence stability. We show that in real food webs, interaction strengths are organized in trophic loops in such a way that long loops contain relatively many weak links. We show and explain mathematically that this patterning enhances stability, because it reduces maximum "loop weight" and thus reduces the amount of intraspecific interaction needed for matrix stability. The patterns are brought about by biomass pyramids, a feature common to most ecosystems. Incorporation of biomass pyramids in 104 food-web descriptions reveals that the low weight of the long loops stabilizes complex food webs. Loop-weight analysis could be a useful tool for exploring the structure and organization of complex communities