Interspecific transfer of genetic information through polyploid bridges.

Hybridization blurs species boundaries and leads to intertwined lineages resulting in reticulate evolution. Polyploidy, the outcome of whole genome duplication (WGD), has more recently been implicated in promoting and facilitating hybridization between polyploid species, potentially leading to adaptive introgression. However, because polyploid lineages are usually ephemeral states in the evolutionary history of life it is unclear whether WGD-potentiated hybridization has any appreciable effect on their diploid counterparts. Here, we develop a model of cytotype dynamics within mixed-ploidy populations to demonstrate that polyploidy can in fact serve as a bridge for gene flow between diploid lineages, where introgression is fully or partially hampered by the species barrier. Polyploid bridges emerge in the presence of triploid organisms, which despite critically low levels of fitness, can still allow the transfer of alleles between diploid states of independently evolving mixed-ploidy species. Notably, while marked genetic divergence prevents polyploid-mediated interspecific gene flow, we show that increased recombination rates can offset these evolutionary constraints, allowing a more efficient sorting of alleles at higher-ploidy levels before introgression into diploid gene pools. Additionally, we derive an analytical approximation for the rate of gene flow at the tetraploid level necessary to supersede introgression between diploids with nonzero introgression rates, which is especially relevant for plant species complexes, where interspecific gene flow is ubiquitous. Altogether, our results illustrate the potential impact of polyploid bridges on the (re)distribution of genetic material across ecological communities during evolution, representing a potential force behind reticulation.

The duplication of genomes and genetic networks and its potential for evolutionary adaptation and survival during environmental turmoil.

The importance of whole-genome duplication (WGD) for evolution is controversial. Whereas some view WGD mainly as detrimental and an evolutionary dead end, there is growing evidence that polyploidization can help overcome environmental change, stressful conditions, or periods of extinction. However, despite much research, the mechanistic underpinnings of why and how polyploids might be able to outcompete or outlive nonpolyploids at times of environmental upheaval remain elusive, especially for autopolyploids, in which heterosis effects are limited. On the longer term, WGD might increase both mutational and environmental robustness due to redundancy and increased genetic variation, but on the short-or even immediate-term, selective advantages of WGDs are harder to explain. Here, by duplicating artificially generated Gene Regulatory Networks (GRNs), we show that duplicated GRNs-and thus duplicated genomes-show higher signal output variation than nonduplicated GRNs. This increased variation leads to niche expansion and can provide polyploid populations with substantial advantages to survive environmental turmoil. In contrast, under stable environments, GRNs might be maladaptive to changes, a phenomenon that is exacerbated in duplicated GRNs. We believe that these results provide insights into how genome duplication and (auto)polyploidy might help organisms to adapt quickly to novel conditions and to survive ecological uproar or even cataclysmic events.

Body-size shifts in aquatic and terrestrial urban communities.

Body size is intrinsically linked to metabolic rate and life-history traits, and is a crucial determinant of food webs and community dynamics1,2. The increased temperatures associated with the urban-heat-island effect result in increased metabolic costs and are expected to drive shifts to smaller body sizes 3 . Urban environments are, however, also characterized by substantial habitat fragmentation 4 , which favours mobile species. Here, using a replicated, spatially nested sampling design across ten animal taxonomic groups, we show that urban communities generally consist of smaller species. In addition, although we show urban warming for three habitat types and associated reduced community-weighted mean body sizes for four taxa, three taxa display a shift to larger species along the urbanization gradients. Our results show that the general trend towards smaller-sized species is overruled by filtering for larger species when there is positive covariation between size and dispersal, a process that can mitigate the low connectivity of ecological resources in urban settings 5 . We thus demonstrate that the urban-heat-island effect and urban habitat fragmentation are associated with contrasting community-level shifts in body size that critically depend on the association between body size and dispersal. Because body size determines the structure and dynamics of ecological networks 1 , such shifts may affect urban ecosystem function.

Neutral processes underlying the macro eco-evolutionary dynamics of mixed-ploidy systems.

Polyploidy, i.e. the occurrence of multiple sets of chromosomes, is regarded as an important phenomenon in plant ecology and evolution, with all flowering plants likely having a polyploid ancestry. Owing to genome shock, minority cytotype exclusion and reduced fertility, polyploids emerging in diploid populations are expected to face significant challenges to successful establishment. Their establishment and persistence are often explained by possible fitness or niche differences that would relieve the competitive pressure with diploid progenitors. Experimental evidence for such advantages is, however, not unambiguous, and considerable niche overlap exists among most polyploid species and their diploid counterparts. Here, we develop a neutral spatially explicit eco-evolutionary model to understand whether neutral processes can explain the eco-evolutionary patterns of polyploids. We present a general mechanism for polyploid establishment by showing that sexually reproducing organisms assemble in space in an iterative manner, reducing frequency-dependent mating disadvantages and overcoming potential reduced fertility issues. Moreover, we construct a mechanistic theoretical framework that allows us to understand the long-term evolution of mixed-ploidy populations and show that our model is remarkably consistent with recent phylogenomic estimates of species extinctions in the Brassicaceae family.

Egg provisioning explains the penetrance of symbiont-mediated sex allocation distortion in haplodiploids.

Maternally transmitted symbionts such as Wolbachia can alter sex allocation in haplodiploid arthropods. By biasing population sex ratios towards females, these changes in sex allocation may facilitate the spread of symbionts. In contrast to symbiont-induced cytoplasmic incompatibility (CI), the mechanisms that underpin sex allocation distortion remain poorly understood. Using a nuclear genotype reference panel of the haplodiploid mite Tetranychus urticae and a single Wolbachia variant that is able to simultaneously induce sex allocation distortion and CI, we unraveled the mechanistic basis of Wolbachia-mediated sex allocation distortion. Host genotype was an important determinant for the strength of sex allocation distortion. We further show that sex allocation distortion by Wolbachia in haplodiploid mites is driven by increasing egg size, hereby promoting egg fertilization. This change in reproductive physiology was also coupled to increased male and female adult size. Our results echo previous work on Cardinium symbionts, suggesting that sex allocation distortion by regulating host investment in egg size is a common strategy among symbionts that infect haplodiploids. To better understand the relevance that sex allocation distortion may have for the spread of Wolbachia in natural haplodiploid populations, we parametrized a model based on generated phenotypic data. Our simulations show that empirically derived levels of sex allocation distortion can be sufficient to remove invasion thresholds, allowing CI to drive the spread of Wolbachia independently of the initial infection frequency. Our findings help elucidate the mechanisms that underlie the widespread occurrence of symbionts in haplodiploid arthropods and the evolution of sex allocation.

Arthropod food webs predicted from body size ratios are improved by incorporating prey defensive properties.

Trophic interactions are often deduced from body size differences, assuming that predators prefer prey smaller than themselves because larger prey are more difficult to subdue. This has mainly been confirmed in aquatic ecosystems, but rarely in terrestrial ecosystems, especially in arthropods. Our goal was to validate whether body size ratios can predict trophic interactions in a terrestrial, plant-associated arthropod community and whether predator hunting strategy and prey taxonomy could explain additional variation. We conducted feeding trials with arthropods from marram grass in coastal dunes to test whether two individuals, of the same or different species, would predate each other. From the trial results, we constructed one of the most complete, empirically derived food webs for terrestrial arthropods associated with a single plant species. We contrasted this empirical food web with a theoretical web based on body size ratios, activity period, microhabitat, and expert knowledge. In our feeding trials, predator-prey interactions were indeed largely size-based. Moreover, the theoretical and empirically based food webs converged well for both predator and prey species. However, predator hunting strategy, and especially prey taxonomy improved predictions of predation. Well-defended taxa, such as hard-bodied beetles, were less frequently consumed than expected based on their body size. For instance, a beetle of average size (measuring 4 mm) is 38% less vulnerable than another average arthropod with the same length. Body size ratios predict trophic interactions among plant-associated arthropods fairly well. However, traits such as hunting strategy and anti-predator defences can explain why certain trophic interactions do not adhere to size-based rules. Feeding trials can generate insights into multiple traits underlying real-life trophic interactions among arthropods.

Experimental evolution of dispersal: Unifying theory, experiments and natural systems.

Dispersal is a central life history trait that affects the ecological and evolutionary dynamics of populations and communities. The recent use of experimental evolution for the study of dispersal is a promising avenue for demonstrating valuable proofs of concept, bringing insight into alternative dispersal strategies and trade-offs, and testing the repeatability of evolutionary outcomes. Practical constraints restrict experimental evolution studies of dispersal to a set of typically small, short-lived organisms reared in artificial laboratory conditions. Here, we argue that despite these restrictions, inferences from these studies can reinforce links between theoretical predictions and empirical observations and advance our understanding of the eco-evolutionary consequences of dispersal. We illustrate how applying an integrative framework of theory, experimental evolution and natural systems can improve our understanding of dispersal evolution under more complex and realistic biological scenarios, such as the role of biotic interactions and complex dispersal syndromes.

Using environmental DNA metabarcoding to monitor fish communities in small rivers and large brooks: Insights on the spatial scale of information.

Monitoring fish communities is central to the evaluation of ecological health of rivers. Both presence/absence of fish species and their relative quantity in local fish assemblages are crucial parameters to measure. Fish communities in lotic systems are traditionally monitored via electrofishing, characterized by a known limited efficiency and high survey costs. Analysis of environmental DNA could serve as a non-destructive alternative for detection and quantification of lotic fish communities, but this approach still requires further insights in practical sampling schemes incorporating transport and dilution of the eDNA particles; optimization of predictive power and quality assurance of the molecular detection method. Via a controlled cage experiment, we aim to extend the knowledge on streamreach of eDNA in small rivers and large brooks, as laid out in the European Water Framework Directive's water typology. Using a high and low source biomass in two river transects of a species-poor river characterized by contrasting river discharge rates, we found strong and significant correlations between the eDNA relative species abundances and the relative biomass per species in the cage community. Despite a decreasing correlation over distance, the underlying community composition remained stable from 25 to 300 m, or up to 1 km downstream of the eDNA source, depending on the river discharge rate. Such decrease in similarity between relative source biomass and the corresponding eDNA-based community profile with increasing distance downstream from the source, might be attributed to variation in species-specific eDNA persistence. Our findings offer crucial insights on eDNA behaviour and characterization of riverine fish communities. We conclude that water sampled from a relatively small river offers an adequate eDNA snapshot of the total fish community in the 300-1000 m upstream transect. The potential application for other river systems is further discussed.

Dispersal syndromes in challenging environments: A cross-species experiment.

Dispersal is a central biological process tightly integrated into life-histories, morphology, physiology and behaviour. Such associations, or syndromes, are anticipated to impact the eco-evolutionary dynamics of spatially structured populations, and cascade into ecosystem processes. As for dispersal on its own, these syndromes are likely neither fixed nor random, but conditional on the experienced environment. We experimentally studied how dispersal propensity varies with individuals' phenotype and local environmental harshness using 15 species ranging from protists to vertebrates. We reveal a general phenotypic dispersal syndrome across studied species, with dispersers being larger, more active and having a marked locomotion-oriented morphology and a strengthening of the link between dispersal and some phenotypic traits with environmental harshness. Our proof-of-concept metacommunity model further reveals cascading effects of context-dependent syndromes on the local and regional organisation of functional diversity. Our study opens new avenues to advance our understanding of the functioning of spatially structured populations, communities and ecosystems.

Behavioral Strategies and the Spatial Pattern Formation of Nesting.

AbstractNesting in dense aggregations is common in central place foragers, such as group-living birds and insects. Both environmental heterogeneity and behavioral interactions are known to induce clustering of nests, but their relative importance remains unclear. We developed an individual-based model that simulated the spatial organization of nest building in a gregarious digger wasp, Bembix rostrata. This process-based model integrates environmental suitability, as derived from a microhabitat model, and relevant behavioral mechanisms related to local site fidelity and conspecific attraction. The drivers behind the nesting were determined by means of inverse modeling in which the emerging spatial and network patterns from simulations were compared with those observed in the field. Models with individual differences in behavior that include the simultaneous effect of a weak environmental cue and strong behavioral mechanisms yielded the best fit to the field data. The nest pattern formation of a central place foraging insect cannot be considered as the sum of environmental and behavioral mechanisms. We demonstrate the use of inverse modeling to understand complex processes that underlie nest aggregation in nature.

The Demographic Consequences of Adaptation: Evidence from Experimental Evolution.

AbstractThe process of adaptation toward novel environments is directly connected to the acquisition of higher fitness relative to others. Such increased fitness is obtained by changes in life history traits that may directly impact population dynamics. From a functional perspective, increased fitness can be achieved through higher resource use or more efficient resource use, each potentially having its own impact on population dynamics. In the first case, adaptation is expected to directly translate into higher population growth. In the second case, adaptation requires less energy and hence may lead to higher carrying capacity. Adaptation may thus lead to changes in ecological dynamics and vice versa. Here, by using a combination of evolutionary experiments with spider mites and a population dynamic model, we investigate how an increase in fecundity (a validated proxy for adaptation) affects a population's ecological dynamics. Our results show that adaptation can positively affect population growth rate and either positively or negatively affect carrying capacity, depending on the ecological condition leading to variation in adaptation. These findings show the importance of evolution for population dynamics in changing environments, which may ultimately affect the stability and resilience of populations.

Dissecting the costs of a facultative symbiosis in an isopod living with ants.

The balance between costs and benefits is expected to drive associations between species. While these balances are well understood for strict associations, we have no insights to which extent they determine facultative associations between species. Here, we quantified the costs of living in a facultative association, by studying the effects of red wood ants on the facultatively associated isopod Porcellio scaber. Porcellio scaber frequently occurred in and near hostile red wood ant nests and might outnumber obligate nest associates. The facultative association involved different costs for the isopod. We found that the density of the isopod decreases near the nest with higher ant traffic. Individuals in and near the nest were smaller than individuals further away from the nest. Smaller individuals were also found at sites with higher ant traffic. A higher proportion of wounded individuals was found closer to the nest and with higher ant traffic. We recorded pregnant females and juveniles in the nest suggesting that the life cycle can be completed inside the nests. Lab experiments showed that females died sooner and invested less in reproduction in presence of red wood ants. Porcellio scaber rarely provoked an aggression response, but large numbers were carried as prey to the nest. These preyed isopods were mainly dried out corpses. Our results showed that the ant association incurred several costs for a facultative associate. Consequently, red wood ant nests and their surrounding territory act as an alternative habitat where demographic costs are offset by a stable resource provisioning and protection.

Exploring the faecal microbiome of the Eurasian nuthatch (Sitta europaea).

Gastrointestinal microbiota fulfill pivotal roles in providing a host with nutrition and protection from pathogenic microorganisms. Up to date, most microbiota research has focused on humans and other mammals, whereas birds and especially wild birds lag behind. Within the field of the avian gut microbiome, research is heavily biased towards poultry. In this study, we analyzed the gut microbiome of the Eurasian nuthatch (Sitta europaea), using faecal samples of eight nestlings originating from three nuthatch nests in the south of Ghent (Belgium), using Illumina sequencing of the 16S rRNA gene. Relative frequency analysis showed a dominance of Firmicutes and Actinobacteria and to a lesser extent Proteobacteria. Bacteroidetes and other phyla were relatively rare. At higher taxonomic levels, a high degree of inter-individual variation in terms of overall microbiota community structure as well as dominance of certain bacteria was observed, but with a higher similarity for the nestlings sharing the same nest. When comparing the nuthatch faecal microbiome to that of great tit nestlings that were sampled during the same breeding season and in the same forest fragment, differences in the microbial community structure were observed, revealing distinct dissimilarities in the relative abundancy of taxa between the two bird species. This study is the first report on the nuthatch microbiome and serves as a reference study for nuthatch bacterial diversity and can be used for targeted screening of the composition and general functions of the avian gut microbiome.

Species-specific effects of thermal stress on the expression of genetic variation across a diverse group of plant and animal taxa under experimental conditions.

Assessing the genetic adaptive potential of populations and species is essential for better understanding evolutionary processes. However, the expression of genetic variation may depend on environmental conditions, which may speed up or slow down evolutionary responses. Thus, the same selection pressure may lead to different responses. Against this background, we here investigate the effects of thermal stress on genetic variation, mainly under controlled laboratory conditions. We estimated additive genetic variance (VA), narrow-sense heritability (h2) and the coefficient of genetic variation (CVA) under both benign control and stressful thermal conditions. We included six species spanning a diverse range of plant and animal taxa, and a total of 25 morphological and life-history traits. Our results show that (1) thermal stress reduced fitness components, (2) the majority of traits showed significant genetic variation and that (3) thermal stress affected the expression of genetic variation (VA, h2 or CVA) in only one-third of the cases (25 of 75 analyses, mostly in one clonal species). Moreover, the effects were highly species-specific, with genetic variation increasing in 11 and decreasing in 14 cases under stress. Our results hence indicate that thermal stress does not generally affect the expression of genetic variation under laboratory conditions but, nevertheless, increases or decreases genetic variation in specific cases. Consequently, predicting the rate of genetic adaptation might not be generally complicated by environmental variation, but requires a careful case-by-case consideration.

Spatial connectedness imposes local- and metapopulation-level selection on life history through feedbacks on demography.

Dispersal evolution impacts the fluxes of individuals and hence, connectivity in metapopulations. Connectivity is therefore decoupled from the structural connectedness of the patches within the spatial network. Because of demographic feedbacks, local selection also drives the evolution of other life history traits. We investigated how different levels of connectedness affect trait evolution in experimental metapopulations of the two-spotted spider mite. We separated local- and metapopulation-level selection and linked trait divergence to population dynamics. With lower connectedness, an increased starvation resistance and delayed dispersal evolved. Reproductive performance evolved locally by transgenerational plasticity or epigenetic processes. Costs of dispersal, but also changes in local densities and temporal fluctuations herein are found to be putative drivers. In addition to dispersal, demographic traits are able to evolve in response to metapopulation connectedness at both the local and metapopulation level by genetic and/or non-genetic inheritance. These trait changes impact the persistence of spatially structured populations.

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

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.

Transient local adaptation and source-sink dynamics in experimental populations experiencing spatially heterogeneous environments.

Local adaptation is determined by the strength of selection and the level of gene flow within heterogeneous landscapes. The presence of benign habitat can act as an evolutionary stepping stone for local adaptation to challenging environments by providing the necessary genetic variation. At the same time, migration load from benign habitats will hinder adaptation. In a community context, interspecific competition is expected to select against maladapted migrants, hence reducing migration load and facilitating adaptation. As the interplay between competition and spatial heterogeneity on the joint ecological and evolutionary dynamics of populations is poorly understood, we performed an evolutionary experiment using the herbivore spider mite Tetranychus urticae as a model. We studied the species's demography and local adaptation in a challenging environment that consisted of an initial sink (pepper plants) and/or a more benign environment (cucumber plants). Half of the experimental populations were exposed to a competitor, the congeneric T. ludeni. We show that while spider mites only adapted to the challenging pepper environment when it was spatially interspersed with benign cucumber habitat, this adaptation was only temporary and disappeared when the populations in the benign cucumber environment were expanding and spilling-over to the challenging pepper environment. Although the focal species outcompeted the competitor after about two months, a negative effect of competition on the focal species's performance persisted in the benign environment. Adaptation to challenging habitat in heterogeneous landscapes thus highly depends on demography and source-sink dynamics, but also on competitive interactions with other species, even if they are only present for a short time span.

The distinct phenotypic signatures of dispersal and stress in an arthropod model: from physiology to life history.

Dispersing individuals are expected to encounter costs during transfer and in the novel environment, and may also have experienced stress in their natal patch. Given this, a non-random subset of the population should engage in dispersal and show divergent stress-related responses. This includes physiological shifts as expressed in the metabolome, which form a major part of responses to stress. We analyzed how metabolic profiles and life-history traits varied between dispersers and residents of the model two-spotted spider mite Tetranychus urticae, and whether and how these syndromes varied with exposure to a stressful new host plant (tomato). Regardless of the effect of host plant, we found a physiological dispersal syndrome where, relative to residents, dispersers were characterized by lower leaf consumption and a lower concentration of several amino acids, indicating a potential dispersal-foraging trade-off. As a possible consequence of this lower food intake, dispersers also laid smaller eggs. Responses to tomato were consistent with this plant being a stressor for T.urticae, including reduced fecundity and reduced feeding. Tomato-exposed mites laid larger eggs, which we interpret as a plastic response to food stress, increasing survival to maturity. Contrary to what was expected from the costs of dispersal and from previous meta-population level studies, there was no interaction between dispersal status and host plant for any of the examined traits, meaning stress impacts were equally incurred by residents and dispersers. We thus provide novel insights into the processes shaping dispersal and the feedbacks on ecological dynamics in spatially structured populations.

The importance and adaptive value of life-history evolution for metapopulation dynamics.

The spatial configuration and size of patches influence metapopulation dynamics by altering colonisation-extinction dynamics and local density dependency. This spatial forcing as determined by the metapopulation typology then imposes strong selection pressures on life-history traits, which will in turn feed back on the ecological metapopulation dynamics. Given the relevance of metapopulation persistence for biological conservation, and the potential rescuing role of evolution, a firm understanding of the relevance of these eco-evolutionary processes is essential. We here follow a systems' modelling approach to quantify the importance of spatial forcing and experimentally observed life-history evolution for metapopulation demography as quantified by (meta)population size and variability. We therefore developed an individual-based model matching an earlier experimental evolution with spider mites to perform virtual translocation and invasion experiments that would have been otherwise impossible to conduct. We show that (a) metapopulation demography is more affected by spatial forcing than by life-history evolution, but that life-history evolution contributes substantially to changes in local- and especially metapopulation-level population sizes, (b) extinction rates are minimised by evolution in classical metapopulations, and (c) evolution is optimising individual performance in metapopulations when considering the importance of more cryptic stress resistance evolution. Ecological systems' modelling opens up a promising avenue to quantify the importance of eco-evolutionary feedbacks in spatially structured populations. Metapopulation sizes are especially impacted by evolution, but its variability is mainly determined by the spatial forcing. Eco-evolutionary dynamics can increase the persistence of classical metapopulations. Conservation of genetic variation and, hence, adaptive potential is thus not only essential in the face of environmental change; it also generates putative rescuing feedbacks that impact metapopulation persistence.

Urbanization-driven changes in web building and body size in an orb web spider.

In animals, behavioural responses may play an important role in determining population persistence in the face of environmental changes. Body size is a key trait central to many life-history traits and behaviours. Correlations with body size may constrain behavioural variation in response to environmental changes, especially when size itself is influenced by environmental conditions. Urbanization is an important human-induced rapid environmental change that imposes multiple selection pressures on both body size and (size-constrained) behaviour. How these combine to shape behavioural responses of urban-dwelling species is unclear. Using web building, an easily quantifiable behaviour linked to body size and the garden spider Araneus diadematus as a model, we evaluated direct behavioural responses to urbanization and body size constraints across a network of 63 selected populations differing in urbanization intensity. We additionally studied urbanization at two spatial scales to account for some environmental pressures varying across scales and to obtain first qualitative insights about the role of plasticity and genetic selection. Spiders were smaller in highly urbanized sites (local scale only), in line with expectations based on reduced prey biomass availability and the Urban Heat Island effect. Web surface and mesh width decreased with urbanization at the local scale, while web surface also increased with urbanization at the landscape scale. The latter two responses are expected to compensate, at least in part, for reduced prey biomass availability in cities. The use of multivariate mixed modelling reveals that although web traits and body size are correlated within populations, behavioural responses to urbanization do not appear to be constrained by size: there is no evidence of size-web correlations among populations or among landscapes, and web traits appear independent from each other. Our results demonstrate that responses in size-dependent behaviours may be decoupled from size changes, thereby allowing fitness maximization in novel environments. The spatial scale at which traits respond suggests contributions of both genetic adaptation (for web investment) and plasticity (for mesh width). Although fecundity decreased with local-scale urbanization, A. diadematus abundances were similar across urbanization gradients; behavioural responses thus appear overall successful at the population level.