Freshwater biodiversity is declining faster than marine or terrestrial diversity, yet its drivers are much less known. Although dams were shown to negatively affect river habitats, fragmentation by bridges has received less attention and is not as well understood. We tested whether and how bridges present barriers to aquatic insects by studying mass swarmings of Palingenia longicauda mayflies on river Tisza (NE-Hungary). Behavioural observations showed that upon approaching the bridge, upstream-flying mayflies typically turned back and 86% of them never crossed the bridge. Lack of physical contact showed that the bridge was an optical, rather than a mechanical barrier for the polarotactic mayflies. Imaging polarimetry showed that the bridge disrupted the horizontally polarizing channel guiding the flight of mayflies above the river. Energy loss, demonstrated by calorimetry, and time constraints forced females to lay eggs only downstream from the bridge. Counts of larval skins shed by swarming individuals showed nearly 2 to 1 female per male downstream from the bridge, while sex ratio above the bridge was slightly male-biased. We suggest that the surplus of parthenogenetic females, that produce only female larvae, downstream from the bridge may have led to the observed sex-ratio bias since the construction of the bridge (1942). Our results demonstrate that bridges can be optical barriers for aquatic insects and can cause population-level impacts, such as biased sex ratios, in natural populations. Sex ratio biases due to bridges may decrease effective population size and genetic variability, which may have contributed to the recent extinction of this species from most of Europe.

Freshwater biodiversity has declined dramatically in Europe in recent decades. Because of massive habitat pollution and morphological degradation of water bodies, many once widespread species persist in small fractions of their original range. These range contractions are generally believed to be accompanied by loss of intraspecific genetic diversity, due to the reduction of effective population sizes and the extinction of regional genetic lineages. We aimed to assess the loss of genetic diversity and its significance for future potential reintroduction of the long-tailed mayfly Palingenia longicauda (Olivier), which experienced approximately 98% range loss during the past century. Analysis of 936 bp of mitochondrial DNA of 245 extant specimens across the current range revealed a surprisingly large number of haplotypes (87), and a high level of haplotype diversity (). In contrast, historic specimens (6) from the lost range (Rhine catchment) were not differentiated from the extant Rába population (, ), despite considerable geographic distance separating the two rivers. These observations can be explained by an overlap of the current with the historic (Pleistocene) refugia of the species. Most likely, the massive recent range loss mainly affected the range which was occupied by rapid post-glacial dispersal. We conclude that massive range losses do not necessarily coincide with genetic impoverishment and that a species' history must be considered when estimating loss of genetic diversity. The assessment of spatial genetic structures and prior phylogeographic information seems essential to conserve once widespread species.

No abstract

Ecosystem restoration implies focusing on multiple trophic levels and ecosystem functioning, yet higher trophic levels, that is, animals, are less frequently targeted by restoration than plants. Habitat diversity, the spatial heterogeneity between and within habitat patches in a landscape, is a well-known driver of species diversity, and offers possible ways to increase species diversity at multiple trophic levels. We argue that habitat diversity is central in whole-ecosystem restoration as we review its importance, provide a practical definition for its components, and propose ways to target it in restoration. Restoration targeting habitat diversity is used commonly in aquatic ecosystems, mostly to increase the physical diversity of habitats, meant to provide more niches available to a higher number of animal species. To facilitate the uptake of habitat diversity in terrestrial ecosystem restoration, we distinguish between compositional and structural habitat diversity, because different animal groups will respond to different aspects of habitat diversity. We also propose four methods to increase habitat diversity: varying the starting conditions to obtain divergent successional pathways, emulating natural disturbances, establishing keystone structures, and applying ecosystem engineer species. We provide two case studies to illustrate how these components and methods can be incorporated in restoration. We conclude that targeting habitat diversity is a promising way to restore habitats for a multitude of species of animals and plants, and that it should become mainstream in restoration ecology and practice. We encourage the restoration community to consider compositional and structural habitat diversity and to specifically target habitat diversity in ecosystem restoration.