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Aim There has been a wide interest in the effect of biotic interactions on species' occurrences and abundances at large spatial scales, coupled with a vast development of the statistical methods to study them. Still, evidence for whether the effects of within-trophic-level biotic interactions (e.g. competition and heterospecific attraction) are discernible beyond local scales remains inconsistent. Here, we present a novel hypothesis-testing framework based on joint dynamic species distribution models and functional trait similarity to dissect between environmental filtering and biotic interactions. Location France and Finland. Taxon Birds. Methods We estimated species-to-species associations within a trophic level, independent of the main environmental variables (mean temperature and total precipitation) for common species at large spatial scale with joint dynamic species distribution (a multivariate spatiotemporal delta model) models. We created hypotheses based on species' functionality (morphological and/or diet dissimilarity) and habitat preferences about the sign and strength of the pairwise spatiotemporal associations to estimate the extent to which they result from biotic interactions (competition, heterospecific attraction) and/or environmental filtering. Results Spatiotemporal associations were mostly positive (80%), followed by random (15%), and only 5% were negative. Where detected, negative spatiotemporal associations in different communities were due to a few species. The relationship between spatiotemporal association and functional dissimilarity among species was negative, which fulfils the predictions of both environmental filtering and heterospecific attraction. Main conclusions We showed that processes leading to species aggregation (mixture between environmental filtering and heterospecific attraction) seem to dominate assembly rules, and we did not find evidence for competition. Altogether, our hypothesis-testing framework based on joint dynamic species distribution models and functional trait similarity is beneficial in ecological interpretation of species-to-species associations from data covering several decades and biogeographical regions. peerReviewed

1. To prevent local species extinction and to counteract population declines, we must ensure species have access to resources they require for life. This can be done through ecological restoration where previously depleted resources are reintroduced. If the restoration is conducted as a one-off action in a large area, it resembles a natural resource pulse, which should lead to increased abundance of individuals, accompanied possibly by increased species richness. Species–energy relationship and underlying theory enable predictions about how different features of resource pulses affect species richness. 2. We conducted a large-scale, controlled, randomized and replicated field experiment to study the effect of a resource addition on polypore species richness in a previously managed boreal forest landscape in Finland. We manipulated the amount and distribution of dead wood and studied the effects on polypore assemblages on added and natural dead wood during nine years after manipulation (2004–2012). 3. By adding dead wood, species richness grew, mainly through increasing abundances: a large amount of dead wood resulted in higher abundance, higher number and faster accumulation of species than a small amount of dead wood. 4. For a given abundance, dead wood addition contained fewer species than natural dead wood. This is most probably because added dead wood was of low diversity and provided habitat only for a limited number of species. 5. Species richness on natural dead wood increased substantially during the study period, and this increase was not related to the resource manipulation. Thus, habitat improvement through natural succession can occur within a relatively short time period irrespective of human intervention. 6. Synthesis and applications. We demonstrate how the introduction of dead wood additions can strengthen polypore populations. The species taking advance of the introduced resource were primarily common species, instead of rare or red-listed species. Thus, we recommend ensuring the natural formation of dead wood while the populations of the common species supporting ecosystem functions can be increased by adding dead wood in the landscape. peerReviewed

Aim Understanding the relative contribution of different species interactions in shaping community assembly has been a pivotal aim in community ecology. Biotic interactions are acknowledged to be important at local scales, although their signal is assumed to weaken over longer distances. We examine the relationship between positive, neutral and negative pairwise bird abundance distributions and the phylogenetic and functional distance between these pairs after first controlling for habitat associations. Location France and Finland. Time period 1984 to 2011 (Finland), 2001 to 2012 (France). Major Taxa studied Birds. Methods We used results from French and Finnish land bird monitoring programmes, from which we created three independent datasets (French forests, French farmlands and Finnish forests). Separately for the three datasets, we fitted linear mixed-effects models for pairwise abundance values across years per point count station to infer the association between all common species pairs, while controlling for geographical distribution and habitat associations, and saved pairwise regression coefficients for further analyses. We used a null model approach to infer whether the observed associations (effect sizes) differ from random. Finally, using quantile regression, we analysed the relationships between functional dissimilarity/phylogenetic distance and effect sizes. Results Our results show both negative and positive species interactions, although negative interactions were twice as common as positive interactions. Closely related species were more likely to show strong associations, both negative and positive, than more distant species across broad spatial scales. For functional dissimilarity, the results varied across datasets. Main conclusions Our results emphasize the potential of functional and phylogenetic proximity in generating both negative and positive species associations, which can produce pervasive patterns from local to geographical scales. Future assembly studies should refrain from strict dichotomies, such as compensatory dynamics versus environmental forcing, and instead consider the possibility of positive interactions.

Aim An area’s ability to support species may be dependent not only on the total amount of available energy it contains but also on energy density (i.e. available energy per unit area). Acknowledging these two aspects of energy availability may increase mechanistic understanding of how increased energy availability results in increased species richness. We studied the relationship between energy density, its variation in space and boreal forest bird species richness and investigated two possible mechanisms: (1) metabolic constraints of organisms, and (2) increased resource availability for specialists. Location Protected areas in Finland’s boreal forest. Methods We tested whether bird species richness was best determined by total energy availability in an area or by energy density and its variation within the area, before and after including bird abundance in the models. We evaluated two main explanatory variables: tree growth reflecting the rate of energy production and tree volume as a measure of biomass. In addition, we modelled individual species’ responses to energy density and its variation, and evaluated the prediction of the metabolic constraints hypothesis that small species are limited by energy density whereas large species are limited by total energy availability in the area. Results Energy density and its variation were good predictors of species richness: together with abundance they explained 84% of variation in species richness (compared with 74% for abundance alone). Pure metabolic constraints were unlikely to explain this relationship. Instead, the mechanism probably involved increased habitat heterogeneity benefiting specialist species. Total energy availability was also an important factor determining species richness but its effect was indirect via abundance. Main conclusions Our results corroborate the importance of energy availability as a driver of species richness in forest bird communities, and they indicate that energy density and its variation in the landscape strongly influence species richness even after accounting for abundance.