Spatial and temporal trends in different dimensions of macrophyte biodiversity in boreal lakes

To comprehensively understand the impact of anthropogenic activities on biodiversity, we must understand how biodiversity has changed over time and its underlying processes. Regardless of a recent increase in scientific interest towards changes in community composition, i.e. beta diversity, these changes have not been studied comprehensively in lake environments in a spatio-temporal framework. In addition, although biotic homogenisation has gained much attention in recent decades, it is still unclear how this process acts at different levels of biodiversity through time. The main aim of this thesis is to study temporal and spatial biodiversity patterns of vascular aquatic macrophyte communities in small boreal lakes during a period of 70 years. The focus is on beta diversity-environment relationships and different dimensions of biodiversity, with special attention to functional features. This thesis is based on three separate case studies that all have utilised temporal presence-absence data of vascular aquatic macrophytes from 27 to 28 lakes from the 1940s to the 2010s. Vascular aquatic macrophyte communities showed only moderately different spatial beta diversity patterns in relation to human impact across decades. The patterns of different dimensions of spatial beta diversity diverged only slightly from each other. The temporal change in aquatic macrophyte communities at the lake level has beenmodest since the 1940s. Nevertheless, it seems that even relatively modest changes in the environment affect temporal gains and losses of species at the lake level. There were no signs of either biotic homogenisation or biotic differentiation (taxonomic, phylogenetic or functional), but the changes in the environment have affected functional community composition and changes in functional richness to some extent. By using the spatial and temporal beta diversity perspective, this thesis highlights the fact that even though biotic homogenisation is a pervasive problem globally, it is not an unambiguous process acting similarly at all spatial and temporal scales or in different environments and different organism groups. There are likely five partly interdependent reasons why no signs of biotic homogenisation were detected in the study area during the 70-year study period: the modest changes in the environment from the 1940s to the 2010s, high ecological resilience of the lakes, information on species presence and absence was used instead of abundance data, biotic interactions and complex community-environment relationships together with stochastic processes and climate change. The results highlight that relying on only one or two survey points in time can result in limited knowledge of the ecological phenomenon under study, and an exceptional year in terms of weather conditions can hinder detecting overall long-term trends in compositional changes, especially under ongoing climate change. The patterns detected in macrophyte beta diversity are likely to represent situations in the extensive boreal and glaciated areas of Eurasia and North America, with largely similar species pools in many regions. Therefore, lakes across the boreal region and areas that have faced glaciation and postglacial processes might be resistant against moderate levels of human pressure.

Linking biotic homogenisation with large-scale changes of species associations

AimThe impact of global change on biodiversity is commonly assessed in terms of changes in species distributions, species richness and species composition across communities. Whether and how much interactions between species are also changing is much less documented and mostly limited to local studies of ecological networks. Moreover, we largely ignore how biotic homogenisation (i.e. the replacement of a set of diverse and mainly specialist species by a few generalists) is affecting or being affected by changes in the structure of species interactions. Here, we approximate species interactions with species associations based on the correlation in species spatial co-occurrence to understand the spatio-temporal changes of species interactions and their relationship to biotic homogenisation.LocationFrance.Time period2001-2017.Major taxa studiedCommon breeding birds.MethodsWe use network approaches to build three community-aggregated indices to characterise species associations and we compare them to changes in species composition in communities. We evaluate the spatial distribution and temporal dynamics of these indices in a dataset of bird co-abundances of more than 100 species monitored for 17 years (2001-2017) from 1,969 sites across France. We finally test whether spatial and temporal changes of species associations are related to species homogenisation estimated as the spatio-temporal dynamics of β-diversity.ResultsWe document a non-random spatial distribution of both structure and temporal changes in species association networks. We also report a directional change in species associations linked to β-diversity modifications in space and time, suggesting that biotic homogenisation affects not only species composition but also species associations.Main ConclusionsOur study highlights the importance of evaluating changes of species association networks, in addition to species turnover when studying biodiversity responses to global change.

Consistent marine biogeographic boundaries across the tree of life despite centuries of human impacts

Over millennia, ecological and evolutionary mechanisms have shaped macroecological distributions across the tree of life. Research describing patterns of regional and global biogeography has traditionally focussed on the study of conspicuous species. Consequently, there is limited understanding of cross-phyla biogeographic structuring, and an escalating need to understand the macroecology of both microscopic and macroscopic organisms. Here we used environmental DNA (eDNA) metabarcoding to explore the biodiversity of marine metazoans, micro-eukaryotes and prokaryotes along an extensive and highly heterogeneous coastline. Our results showed remarkably consistent biogeographic structure across the kingdoms of life, which were underpinned by environmental and anthropogenic influence. Additionally, metazoan communities displayed biographic patterns that suggest regional biotic homogenisation of conspicuous species. Against the backdrop of global pervasive anthropogenic environmental change, our work highlights the importance of considering multiple domains of life to understand the maintenance and drivers of marine biodiversity across broad taxonomic, ecological and geographical scales.

The Homogocene: a research prospectus for the study of biotic homogenisation

In an era of global change, the process of biotic homogenisation by which regional biotas become more similar through time has attracted considerable attention from ecologists. Here, a retrospective look at the literature is taken and the question asked how comprehensive is the understanding of this global phenomenon? The goal is to identify potential areas for additional and future enquiries to advance this research frontier and best ensure the long-term preservation of biological diversity across the world. Six propositions are presented here to; (1) broaden our geographic and taxonomic understanding, (2) diversify the spatial and temporal scales of inquiry, (3) reconcile past and embrace new approaches to quantification, (4) improve our knowledge of the underlying drivers, (5) reveal the conservation implications and (6) forecast future homogenisation. It is argued that significant progress in the understanding of the causes, consequences and conservation implication of biotic homogenisation will come by integrating concepts and approaches from ecology, evolution and conservation across a hierarchy of spatial and temporal scales.

Scale dependence of biotic homogenisation by urbanisation: a comparison of urban bird communities between central Argentina and northern Finland

Recent studies showed contrasting results about the homogenising force of urbanisation on bird community composition at large and regional scales. We studied whether urbanisation promotes the homogenisation of wintering bird communities and if this varies when comparing towns located within a specific region and towns located in two different biomes of two countries. We used both similarity indices based on the presence/absence data and the abundance data in comparing communities. Processes governing bird community dissimilarity between urbanisation levels were examined with the partitioning of Sörensen index in species turnover and nestedness. We made bird surveys in town centres and suburban habitats of three cities located in the Pampean region of Argentina and in the boreal region of Finland using a single-visit study plot method. Rarefacted species richness did not differ amongst the town centres between the countries, but it was higher in the suburban areas of Argentina than in Finland. At the country-level comparison, we found a higher similarity amongst the town centres than amongst the suburban areas; whereas at the regional comparison, similarity between town centres was comparable to the similarity between suburban areas. The use of an abundance-based index produced a higher similarity between town centre communities of both countries than when using a presence-based index. The dissimilarity between habitats in Argentina was related to nestedness and to species turnover in Finland. Our results indicate that urban-based biotic homogenisation of bird communities is dependent on the scale used, being more evident when comparing cities of different biomes where the same and abundant bird species, such as sparrows and doves, dominate. At the regional scale, quite a high beta-diversity can still be found within urban habitats. Processes of community dissimilarity between urban habitats may differ according to the regional pool of species, being more related to nestedness toward the tropics.