Vincent Pellissier

Multi-scale assessment of pollination of Lotus corniculatus (L.) in a peri-urban fringe

Background: The plant pollination service provided by pollinators is crucial for ecosystem maintenance, especially in highly fragmented landscapes, such as urban areas. Aims: We assessed at multiple spatial scales the influence of the degree of urbanisation on pollinator communities and pollination success. Methods: Artificial populations of Lotus corniculatus (Fabaceae) were established along an urbanisation gradient in gardens where pollinator communities were surveyed. We described the plant and pollinator composition of gardens (local scale) and measured landscape structure in nested buffers (small and large landscape scales). Results: At the local scale, species richness and abundance of pollinators increased with the number of spontaneous plants and the size of semi-natural areas, while at the large landscape scale pollinators were more diverse and abundant in diversified landscapes with extensive green space and low cover of impervious surfaces. At the small landscape scale, the pollination success was favoured by agricultural areas, and disfavoured by housing density and fragmentation. It was also favoured by green space cover at the large landscape scale. Conclusions: The variables that affect pollination differ across spatial scales. Local and landscape variables have different effects according to spatial scale, suggesting different underlying causes, such as the presence of flower resources or of suitable habitats in the surroundings. Our results emphasise the importance of accounting for all spatial scales when managing pollination services.

Relative importance of the land‐use composition and intensity for the bird community composition in anthropogenic landscapes

Abstract Humans are changing the biosphere by exerting pressure on land via different land uses with variable intensities. Quantifying the relative importance of the land‐use composition and intensity for communities may provide valuable insights for understanding community dynamics in human‐dominated landscapes. Here, we evaluate the relative importance of the land‐use composition versus land‐use intensity on the bird community structure in the highly human‐dominated region surrounding Paris, France. The land‐use composition was calculated from a land cover map, whereas the land‐use intensity (reverse intensity) was represented by the primary productivity remaining after human appropriation (NPP remaining), which was estimated using remote sensing imagery. We used variance partitioning to evaluate the relative importance of the land‐use composition versus intensity for explaining bird community species richness, total abundance, trophic levels, and habitat specialization in urban, farmland, and woodland habitats. The land‐use composition and intensity affected specialization and richness more than trophic levels and abundance. The importance of the land‐use intensity was slightly higher than that of the composition for richness, specialization, and trophic levels in farmland and urban areas, while the land‐use composition was a stronger predictor of abundance. The intensity contributed more to the community indices in anthropogenic habitats (farmland and urban areas) than to those in woodlands. Richness, trophic levels, and specialization in woodlands tended to increase with the NPP remaining value. The heterogeneity of land uses and intensity levels in the landscape consistently promoted species richness but reduced habitat specialization and trophic levels. This study demonstrates the complementarity of NPP remaining to the land‐use composition for understanding community structure in anthropogenic landscapes. Our results show, for the first time, that the productivity remaining after human appropriation is a determinant driver of animal community patterns, independent of the type of land use.

A Holistic Landscape Description Reveals That Landscape Configuration Changes More over Time than Composition: Implications for Landscape Ecology Studies

Background Space-for-time substitution—that is, the assumption that spatial variations of a system can explain and predict the effect of temporal variations—is widely used in ecology. However, it is questionable whether it can validly be used to explain changes in biodiversity over time in response to land-cover changes. Hypothesis Here, we hypothesize that different temporal vs spatial trajectories of landscape composition and configuration may limit space-for-time substitution in landscape ecology. Land-cover conversion changes not just the surface areas given over to particular types of land cover, but also affects isolation, patch size and heterogeneity. This means that a small change in land cover over time may have only minor repercussions on landscape composition but potentially major consequences for landscape configuration. Methods Using land-cover maps of the Paris region for 1982 and 2003, we made a holistic description of the landscape disentangling landscape composition from configuration. After controlling for spatial variations, we analyzed and compared the amplitudes of changes in landscape composition and configuration over time. Results For comparable spatial variations, landscape configuration varied more than twice as much as composition over time. Temporal changes in composition and configuration were not always spatially matched. Significance The fact that landscape composition and configuration do not vary equally in space and time calls into question the use of space-for-time substitution in landscape ecology studies. The instability of landscapes over time appears to be attributable to configurational changes in the main. This may go some way to explaining why the landscape variables that account for changes over time in biodiversity are not the same ones that account for the spatial distribution of biodiversity.

Computing the Expected Value and Variance of Geometric Measures

Let P be a point set in ℝd, and let M be a function that maps any subset of P to a positive real. We examine the problem of computing the mean and variance of M when a subset in P is selected according to a random distribution. We consider two distributions; in the first distribution (the Bernoulli distribution), each point p in P is included in the random subset independently, with probability π(p). In the second distribution (the fixed-size distribution), exactly s points are selected uniformly at random among all possible subsets of s points in P. We present efficient algorithms for computing the mean and variance of several geometric measures when point sets are selected under one of the described random distributions. We also implemented four of those algorithms: an algorithm that computes the mean 2D bounding box volume in the Bernoulli distribution, an algorithm for the mean 2D convex hull area in the fixed-size distribution, an algorithm that computes the exact mean and variance of the mean pairwise distance (MPD) for d-dimensional point sets in the fixed-size distribution, and an (1 − ε)-approximation algorithm for the same measure. We conducted experiments where we compared the performance of our implementations with a standard heuristic approach, and we show that our implementations are very efficient. We also compared the implementation of our exact MPD algorithm and the (1 − ε)-approximation algorithm; the approximation method performs faster on real-world datasets for point sets of up to 13 dimensions, and provides high-precision approximations.

Trophic interactions among vertebrate guilds and plants shape global patterns in species diversity

Trophic interactions play critical roles in structuring biotic communities. Understanding variation in trophic interactions among systems provides important insights into biodiversity maintenance and conservation. However, the relative importance of bottom-up versus top-down trophic processes for broad-scale patterns in biodiversity is poorly understood. Here, we used global datasets on species richness of vascular plants, mammals and breeding birds to evaluate the role of trophic interactions in shaping large-scale diversity patterns. Specifically, we used non-recursive structural equation models to test for top-down and bottom-up forcing of global species diversity patterns among plants and trophic guilds of mammals and birds (carnivores, invertivores and herbivores), while accounting for extrinsic environmental drivers. The results show that trophic linkages emerged as being more important to explaining species richness than extrinsic environmental drivers. In particular, there were strong, positive top-down interactions between mammal herbivores and plants, and moderate to strong bottom-up and/or top-down interactions between herbivores/invertivores and carnivores. Estimated trophic interactions for separate biogeographical regions were consistent with global patterns. Our findings demonstrate that, after accounting for environmental drivers, large-scale species richness patterns in plant and vertebrate taxa consistently support trophic interactions playing a major role in shaping global patterns in biodiversity. Furthermore, these results suggest that top-down forces often play strong complementary roles relative to bottom-up drivers in structuring biodiversity patterns across trophic levels. These findings underscore the importance of integrating trophic forcing mechanisms into studies of biodiversity patterns.