No abstract

Understanding how communities of living organisms assemble has been a central question in ecology since the early days of the discipline. Disentangling the different processes involved in community assembly is not only interesting in itself but also crucial for an understanding of how communities will behave under future environmental scenarios. The traditional concept of assembly rules reflects the notion that species do not co-occur randomly but are restricted in their co-occurrence by interspecific competition. This concept can be redefined in a more general framework where the co-occurrence of species is a product of chance, historical patterns of speciation and migration, dispersal, abiotic environmental factors, and biotic interactions, with none of these processes being mutually exclusive. Here we present a survey and meta-analyses of 59 papers that compare observed patterns in plant communities with null models simulating random patterns of species assembly. According to the type of data under study and the different methods that are applied to detect community assembly, we distinguish four main types of approach in the published literature: species co-occurrence, niche limitation, guild proportionality and limiting similarity. Results from our meta-analyses suggest that non-random co-occurrence of plant species is not a widespread phenomenon. However, whether this finding reflects the individualistic nature of plant communities or is caused by methodological shortcomings associated with the studies considered cannot be discerned from the available metadata. We advocate that more thorough surveys be conducted using a set of standardized methods to test for the existence of assembly rules in data sets spanning larger biological and geographical scales than have been considered until now. We underpin this general advice with guidelines that should be considered in future assembly rules research. This will enable us to draw more accurate and general conclusions about the non-random aspect of assembly in plant communities.

Intensification or abandonment of agricultural land use has led to a severe decline of semi-natural habitats across Europe. This can cause immediate loss of species but also time-delayed extinctions, known as the extinction debt. In a pan-European study of 147 fragmented grassland remnants, we found differences in the extinction debt of species from different trophic levels. Present-day species richness of long-lived vascular plant specialists was better explained by past than current landscape patterns, indicating an extinction debt. In contrast, short-lived butterfly specialists showed no evidence for an extinction debt at a time scale of c. 40 years. Our results indicate that management strategies maintaining the status quo of fragmented habitats are insufficient, as time-delayed extinctions and associated co-extinctions will lead to further biodiversity loss in the future.

Transformation of landscapes is considered to be one of the main drivers behind species loss, regionally and globally. Theory and empirical studies suggest that landscape structure influences species diversity in many habitats. These effects may be manifested at different spatial scales depending on species response to landscape heterogeneity. A similar, but often neglected, scaling issue concerns the temporal scale of species response to landscape change. In this study, we found time lags of 50–100 years in the response of plant species diversity to changing configuration of habitats in the landscape. When analyzing remnants of traditionally managed seminatural grasslands in Sweden, we found that species diversity was not related to present‐day connectivity of the investigated sites, irrespective of spatial scale (3.1–12.5 km2). However, when using maps depicting landscapes 50 and 100 years ago, respectively, strong positive effects of habitat connectivity appeared, at increasing spatial scale for the older landscapes. Thus, analyses of how species diversity relates to present‐day landscapes may be misleading, and future species loss may be expected even if the present landscape is maintained.