Thierry Backeljau

Bio: Thierry Backeljau is an academic researcher from Royal Belgian Institute of Natural Sciences. The author has contributed to research in topics: Population & Monophyly. The author has an hindex of 38, co-authored 253 publications receiving 5133 citations. Previous affiliations of Thierry Backeljau include University of Antwerp & Royal Museum for Central Africa.

Molecular and morphological data reveal cryptic taxonomic diversity in the terrestrial slug complex Arion subfuscus/fuscus (Mollusca, Pulmonata, Arionidae) in continental north‐west Europe

Abstract: The importance and abundance of cryptic species among invertebrate taxa is well documented. Nowadays, taxonomic, phylogenetic and conservation biological studies frequently use molecular markers to delineate cryptic taxa. Such studies, however, often face the problem of the differential resolution of the molecular markers and techniques involved. This issue is explored in the present study of cryptic taxa within the terrestrial slug complex Arion subfuscus/fuscus in continental north-west Europe. To this end, morphological, allozyme and mitochondrial 16S rDNA sequence data have been jointly evaluated. Using allozyme data and gonad type, two distinct groups were consistently delineated, even under sympatric conditions. The 16S rDNA data strongly supported both those groups and even suggested the presence of three distinct taxa within one of them. However, in view of: (1) the allopatric distribution of three OTUs, (2) the lack of allozyme or morphological differentiation, and (3) the extremely high degree of intraspecific mtDNA variation reported in pulmonate gastropods, they are, for the time being, not regarded as valid species under the biological species concept. By means of 16S rDNA and allozyme data, the position of type and topotype material of A. subfuscus s.s. and A. fuscus relative to the newly defined OTUs was determined, thus clarifying the nomenclature of this species complex. Additionally, gonad type proved to be a useful character for distinguishing the two species in north-west Europe.

Dispersal and gene flow in free-living marine nematodes

Abstract: Dispersal and gene flow determine connectivity among populations, and can be studied through population genetics and phylogeography. We here review the results of such a framework for free-living marine nematodes. Although field experiments have illustrated substantial dispersal in nematodes at ecological time scales, analysis of the genetic diversity illustrated the importance of priority effects, founder effects and genetic bottlenecks for population structuring between patches <1 km apart. In contrast, only little genetic structuring was observed within an estuary (<50 km), indicating that these small scale fluctuations in genetic differentiation are stabilized over deeper time scales through extensive gene flow. Interestingly, nematode species with contrasting life histories (extreme colonizers vs persisters) or with different habitat preferences (algae vs sediment) show similar, low genetic structuring. Finally, historical events have shaped the genetic pattern of marine nematodes and show that gene flow is restricted at large geographical scales. We also discuss the presence of substantial cryptic diversity in marine nematodes, and end with highlighting future important steps to further unravel nematode evolution and diversity.

Urbanization drives cross-taxon declines in abundance and diversity at multiple spatial scales.

Abstract: The increasing urbanization process is hypothesized to drastically alter (semi-)natural environments with a concomitant major decline in species abundance and diversity. Yet, studies on this effect of urbanization, and the spatial scale at which it acts, are at present inconclusive due to the large heterogeneity in taxonomic groups and spatial scales at which this relationship has been investigated among studies. Comprehensive studies analysing this relationship across multiple animal groups and at multiple spatial scales are rare, hampering the assessment of how biodiversity generally responds to urbanization. We studied aquatic (cladocerans), limno-terrestrial (bdelloid rotifers) and terrestrial (butterflies, ground beetles, ground- and web spiders, macro-moths, orthopterans and snails) invertebrate groups using a hierarchical spatial design, wherein three local-scale (200 m × 200 m) urbanization levels were repeatedly sampled across three landscape-scale (3 km × 3 km) urbanization levels. We tested for local and landscape urbanization effects on abundance and species richness of each group, whereby total richness was partitioned into the average richness of local communities and the richness due to variation among local communities. Abundances of the terrestrial active dispersers declined in response to local urbanization, with reductions up to 85% for butterflies, while passive dispersers did not show any clear trend. Species richness also declined with increasing levels of urbanization, but responses were highly heterogeneous among the different groups with respect to the richness component and the spatial scale at which urbanization impacts richness. Depending on the group, species richness declined due to biotic homogenization and/or local species loss. This resulted in an overall decrease in total richness across groups in urban areas. These results provide strong support to the general negative impact of urbanization on abundance and species richness within habitat patches and highlight the importance of considering multiple spatial scales and taxa to assess the impacts of urbanization on biodiversity.

Phylogeography of the Rhabditis (Pellioditis) marina species complex: evidence for long‐distance dispersal, and for range expansions and restricted gene flow in the northeast Atlantic

Abstract: Pinpointing processes that structure the geographical distribution of genetic diversity of marine species and lead to speciation is challenging because of the lack of obvious dispersal barriers and the likelihood of substantial (passive) dispersal in oceans. In addition, cryptic radiations with sympatric distributions abound in marine species, challenging the allopatric speciation mechanism. Here, we present a phylogeographical study of the marine nematode species complex Rhabditis (Pellioditis) marina to investigate processes shaping genetic structure and speciation. Rhabditis (P.) marina lives on decaying macroalgae in the intertidal, and may therefore disperse over considerable distances. Rhabditis (P.) marina consists of several cryptic species sympatrically distributed at a local scale. Genetic variation in the COI gene was screened in 1362 specimens from 45 locations around the world. Two nuclear DNA genes (ITS and D2D3) were sequenced to infer phylogenetic species. We found evidence for ten sympatrically distributed cryptic species, seven of which show a strong genetic structuring. A historical signature showed evidence for restricted gene flow with occasional long-distance dispersal and range expansions pre-dating the last glacial maximum. Our data also point to a genetic break around the British Isles and a contact zone in the Southern Bight of the North Sea. We provide evidence for the transoceanic distribution of at least one cryptic species (PmIII) and discuss the dispersal capacity of marine nematodes. The allopatric distribution of some intraspecific phylogroups and of closely related cryptic species points to the potential for allopatric speciation in R. (P.) marina.

The Theory of Island Biogeography

Abstract: Preface to the Princeton Landmarks in Biology Edition vii Preface xi Symbols Used xiii 1. The Importance of Islands 3 2. Area and Number of Speicies 8 3. Further Explanations of the Area-Diversity Pattern 19 4. The Strategy of Colonization 68 5. Invasibility and the Variable Niche 94 6. Stepping Stones and Biotic Exchange 123 7. Evolutionary Changes Following Colonization 145 8. Prospect 181 Glossary 185 References 193 Index 201

Confounding factors in the detection of species responses to habitat fragmentation

Abstract: Habitat loss has pervasive and disruptive impacts on biodiversity in habitat remnants. The magnitude of the ecological impacts of habitat loss can be exacerbated by the spatial arrangement -- or fragmentation -- of remaining habitat. Fragmentation per se is a landscape-level phenomenon in which species that survive in habitat remnants are confronted with a modified environment of reduced area, increased isolation and novel ecological boundaries. The implications of this for individual organisms are many and varied, because species with differing life history strategies are differentially affected by habitat fragmentation. Here, we review the extensive literature on species responses to habitat fragmentation, and detail the numerous ways in which confounding factors have either masked the detection, or prevented the manifestation, of predicted fragmentation effects. Large numbers of empirical studies continue to document changes in species richness with decreasing habitat area, with positive, negative and no relationships regularly reported. The debate surrounding such widely contrasting results is beginning to be resolved by findings that the expected positive species-area relationship can be masked by matrix-derived spatial subsidies of resources to fragment-dwelling species and by the invasion of matrix-dwelling species into habitat edges. Significant advances have been made recently in our understanding of how species interactions are altered at habitat edges as a result of these changes. Interestingly, changes in biotic and abiotic parameters at edges also make ecological processes more variable than in habitat interiors. Individuals are more likely to encounter habitat edges in fragments with convoluted shapes, leading to increased turnover and variability in population size than in fragments that are compact in shape. Habitat isolation in both space and time disrupts species distribution patterns, with consequent effects on metapopulation dynamics and the genetic structure of fragment-dwelling populations. Again, the matrix habitat is a strong determinant of fragmentation effects within remnants because of its role in regulating dispersal and dispersal-related mortality, the provision of spatial subsidies and the potential mediation of edge-related microclimatic gradients. We show that confounding factors can mask many fragmentation effects. For instance, there are multiple ways in which species traits like trophic level, dispersal ability and degree of habitat specialisation influence species-level responses. The temporal scale of investigation may have a strong influence on the results of a study, with short-term crowding effects eventually giving way to long-term extinction debts. Moreover, many fragmentation effects like changes in genetic, morphological or behavioural traits of species require time to appear. By contrast, synergistic interactions of fragmentation with climate change, human-altered disturbance regimes, species interactions and other drivers of population decline may magnify the impacts of fragmentation. To conclude, we emphasise that anthropogenic fragmentation is a recent phenomenon in evolutionary time and suggest that the final, long-term impacts of habitat fragmentation may not yet have shown themselves.