The biodiversity of food webs is composed of horizontal (i.e. within trophic levels) and vertical diversity (i.e. the number of trophic levels). Understanding their joint effect on stability is a key challenge. Theory mostly considers their individual effects and focuses on small perturbations near equilibrium in hypothetical food webs. Here, we study the joint effects of horizontal and vertical diversity on the stability of hypothetical (modelled) and empirical food webs. In modelled food webs, horizontal and vertical diversity increased and decreased stability, respectively, with a stronger positive effect of producer diversity on stability at higher consumer diversity. Experiments with an empirical plankton food web, where we manipulated horizontal and vertical diversity and measured stability from species interactions and from resilience against large perturbations, confirmed these predictions. Taken together, our findings highlight the need to conserve horizontal biodiversity at different trophic levels to ensure stability.

An experimental investigation is conducted to study the effect of sample orientation on piloted ignition and opposed-wind flame spread. Two types of wood (red oak and mahogany) were used for the purpose and two orientations (horizontal and vertical) were investigated. In the horizontal mode, axisymmetric fire spread over wood samples was studied and the corresponding piloted ignition tests were conducted on smaller samples of the same wood. In the vertical mode, lateral flame spread and piloted ignition tests were conducted in a radiant panel test apparatus.The experimental data were reduced according to the thermal flame spread theory of deRis using the measured surface temperatures.It was found that as long as the temperatures are defined consistently with the thermal theory, the results are orientation independent within the measurement error. The reasons for this orientation independence are: (i) dominant re-radiative losses, and (ii) insensitivity of the flame spread rate to the induced air velocity at ambient 02 concentrations.

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A key question in ecology is what limits species richness. Coexistence theory presents the persistence of species amidst heterospecifics as a balance between niche differences and fitness differences that favour and hamper coexistence, respectively. With most applications focusing on species pairs, we know little about how niche and fitness differences respond to species richness, i.e. what constraints richness most. We present analytical proof that, in absence of higher-order interactions, the average fitness difference increases with richness, while the average niche difference stays constant. Analysis of a simple model with higher-order interactions, extensive simulations that relaxed all assumptions, and analyses of empirical data, confirmed these results. Our work thus shows that fitness differences, not niche difference, limit species richness. Our results contribute to the expansion of coexistence theory towards multi-species communities.

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