Contrasted Effects of Diversity and Immigration on Ecological Insurance in Marine Bacterioplankton Communities

The ecological insurance hypothesis predicts a positive effect of species richness on ecosystem functioning in a variable environment. This effect stems from temporal and spatial complementarity among species within metacommunities coupled with optimal levels of dispersal. Despite its importance in the context of global change by human activities, empirical evidence for ecological insurance remains scarce and controversial. Here we use natural aquatic bacterial communities to explore some of the predictions of the spatial and temporal aspects of the ecological insurance hypothesis. Addressing ecological insurance with bacterioplankton is of strong relevance given their central role in fundamental ecosystem processes. Our experimental set up consisted of water and bacterioplankton communities from two contrasting coastal lagoons. In order to mimic environmental fluctuations, the bacterioplankton community from one lagoon was successively transferred between tanks containing water from each of the two lagoons. We manipulated initial bacterial diversity for experimental communities and immigration during the experiment. We found that the abundance and production of bacterioplankton communities was higher and more stable (lower temporal variance) for treatments with high initial bacterial diversity. Immigration was only marginally beneficial to bacterial communities, probably because microbial communities operate at different time scales compared to the frequency of perturbation selected in this study, and of their intrinsic high physiologic plasticity. Such local “physiological insurance” may have a strong significance for the maintenance of bacterial abundance and production in the face of environmental perturbations

Species Richness and Trophic Diversity Increase Decomposition in a Co-Evolved Food Web

Ecological communities show great variation in species richness, composition and food web structure across similar and diverse ecosystems. Knowledge of how this biodiversity relates to ecosystem functioning is important for understanding the maintenance of diversity and the potential effects of species losses and gains on ecosystems. While research often focuses on how variation in species richness influences ecosystem processes, assessing species richness in a food web context can provide further insight into the relationship between diversity and ecosystem functioning and elucidate potential mechanisms underpinning this relationship. Here, we assessed how species richness and trophic diversity affect decomposition rates in a complete aquatic food web: the five trophic level web that occurs within water-filled leaves of the northern pitcher plant, Sarracenia purpurea. We identified a trophic cascade in which top-predators — larvae of the pitcher-plant mosquito — indirectly increased bacterial decomposition by preying on bactivorous protozoa. Our data also revealed a facultative relationship in which larvae of the pitcher-plant midge increased bacterial decomposition by shredding detritus. These important interactions occur only in food webs with high trophic diversity, which in turn only occur in food webs with high species richness. We show that species richness and trophic diversity underlie strong linkages between food web structure and dynamics that influence ecosystem functioning. The importance of trophic diversity and species interactions in determining how biodiversity relates to ecosystem functioning suggests that simply focusing on species richness does not give a complete picture as to how ecosystems may change with the loss or gain of species

Propagule Pressure: A Null Model for Biological Invasions

null model, propagule pressure Invasion ecology has been criticised for its lack of general principles. To explore this criticism, we con-ducted a meta-analysis that examined characteristics of invasiveness (i.e. the ability of species to establish in, spread to, or become abundant in novel communities) and invasibility (i.e. the susceptibility of habitats to the establishment or proliferation of invaders). There were few consistencies among invasiveness char-acteristics (3 of 13): established and abundant invaders generally occupy similar habitats as native species, while abundant species tend to be less affected by enemies; germination success and reproductive output were significantly positively associated with invasiveness when results from both stages (establishment/ spread and abundance/impact) were combined. Two of six invasibility characteristics were also significant: communities experiencing more disturbance and with higher resource availability sustained greater establishment and proliferation of invaders. We also found that even though ‘propagule pressure ’ was considered in only 29 % of studies, it was a significant predictor of both invasiveness and invasibility (55 of 64 total cases). Given that nonindigenous species are likely introduced non-randomly, we contend that ‘propagule biases ’ may confound current paradigms in invasion ecology. Examples of patterns that could be confounded by propagule biases include characteristics of good invaders and susceptible habitats, release from enemies, evolution of ‘invasiveness’, and invasional meltdown. We conclude that propagule pressure should serve as the basis of a null model for studies of biological invasions when inferring process from patterns of invasion

CCDC 224893: Experimental Crystal Structure Determination

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures

Structure and Bonding of KSiH₃ and Its 18-Crown-6 Derivatives: Unusual Ambidentate Behavior of the SiH₃^– Anion

Density functional theory (DFT) calculations of [K(18-crown-6)SiH₃] (<b>1</b>) and KSiH₃ (<b>2</b>) have shown that both the classical <i>tet</i> and non-classical <i>inv</i> coordination modes of the [SiH₃]⁻ anion to the K⁺ ion are energetically accessible. Single-crystal X-ray structures of the <i>tet</i> and <i>inv</i> derivatives [K(18-crown-6)SiH₃·THF] (<b>1a</b>) and [K(18-crown-6)SiH₃·HSiPh₃] (<b>1b</b>) confirm this conclusion, showing that small changes in the coordination sphere of the metal are sufficient to alter the orientation of the anion. A topological analysis of the calculated electron densities for <b>1</b> and <b>2</b> reveals that the K···Si interaction in the <i>tet</i> conformer of <b>2</b> possesses a significant amount of covalent character. In contrast, the <i>inv</i> form of <b>2</b> displays primarily electrostatic character for the K···Si and K···H interactions. Incorporation of the 18-crown-6 ligand in <b>1</b> reduces the polarizing power of the K⁺ cation, hardening the cation–anion interaction in both conformers. The experimental structures of <b>1a</b> and <b>1b</b> bear out these conclusions, with the strongly bound tetrahydrofuran (THF) ligand softening the K⁺ ion in <b>1a</b> and favoring the <i>tet</i> conformer, while the weakly interacting HSiPh₃ ligand in <b>1b</b> has minimal effect on the K⁺ center, resulting in an <i>inv</i> orientation