This paper is the product of an international workshop aiming to make progress in our general understanding of adaptation. We met from 5-7 February 2025 in Hannover (Germany), funded by the foundation “Volkswagen Stiftung”. For our group of theoretical and empirical biologists, social scientists, and philosophers of science we set up a program to facilitate communication and collaboration between people with diverse backgrounds and viewpoints. The overall goal that the scientific community should strive for, we think, should be to obtain concrete conceptual, analytical, and experimental tools for researchers to understand and study all the processes of adaptation, and thereby the global phenomenon of adaptation. Our workshop aimed to contribute to this overall goal. For this, we discussed the relative strengths and weaknesses of different approaches, identified areas of consensus, identified areas of disagreement, and resolved (sub)areas of disagreement. Here we briefly report on the progress we have made during the workshop. We lay out the problem, discuss terminology, present a visual framework to think about adaptation, suggest useful approaches for its study, and provide recommendations for practitioners and policymakers.

Individuals differ. While seemingly trivial, this insight has nevertheless led to paradigm shifts, as three key fields of organismal biology have seen marked changes in key concepts over the past few decades. In animal behaviour, it has become increasingly recognised that behavioural differences among individuals can be stable over time and across contexts, giving rise to the concept of animal personalities. In ecology, attention has similarly shifted towards variation in the ecological niches occupied by species, populations and individuals, giving rise to the concept of niche specialisation or individual niche variation. In evolutionary biology, where individual variation has always been central, there is a growing awareness of the complex and dynamic ways in which individuals interact with the environment to produce unique phenotypes. Additionally, recent theoretical and empirical research suggests that fitness landscapes are not only complex, with multiple fitness peaks, but might even be more accurately described as constantly shifting 'fitness seascapes', where the fitness peak that an individual can reach - whether local or global - depends on its genotype and its interaction with the environment. Moreover, the previous distinction between ecological and evolutionary timescales is being replaced by a more integrative view that recognises that evolution can occur on ecological timeframes. These shifting perspectives over the past two decades underscore the need for a more integrated conceptual framework that transcends disciplines. While in behaviour, ecology and evolution, the concept of individualisation has contributed to major scientific progress, sufficient cross-fertilisation is still lacking. Here, we propose a new conceptual unification: the individualised niche. By merging the niche concept with the fitness concept, new explanatory power for both ecological and evolutionary processes emerges.

Phenotypic responses to climate affect individual fitness, but the extent to which this translates into effects on population dynamics remains poorly understood. We assemble 213 time series on phenotypes and population sizes of wild vertebrates globally and match them with local climate data. Our meta-analysis shows that morphological traits are mostly climate insensitive. However, phenology is earlier in warmer-than-average years, which contributes positively to population growth in most species. At lower latitudes, temperature has weaker effects on phenology but stronger direct negative effects on population growth, likely because these populations are less capable of tracking climate via plasticity. Variation in the phenology-mediated effect of temperature on population growth cannot be explained by latitude, generation time, migratory mode, or diet. This suggests that simple relationships between species characteristics and population responses to warming may not occur in nature. Instead, we may need to embrace ecological complexity by considering local-scale predictors that capture intra-specific variation.

ABSTRACT Human activity has altered nearly all ecosystems on Earth, contributing to substantial biodiversity loss across taxonomic, genetic and functional dimensions, from the intraspecific to the ecosystem scale. Yet some components of biodiversity remain hidden or underappreciated, including behavioural diversity. Behaviour is often among the earliest whole‐organism responses to environmental change, and diversity in behaviour can occur within individuals, among individuals, among populations and across communities and ecosystems. This variation may influence growth, survival, reproduction, ecological interactions and responses to anthropogenic pressures, although its effects on population stability and resilience are likely context‐, scale‐ and component‐dependent. In this review, we examine behavioural diversity in fishes, defined here as variation in behavioural traits across levels of biological organisation, from individuals to ecosystems. Our review has two complementary aims. First, we provide a conceptual overview of behavioural diversity, clarify how it differs from related constructs such as personality, plasticity, and behavioural syndromes, and summarise the evolutionary, ontogenetic, ecological, stochastic and anthropogenic processes that generate or erode it. Second, we evaluate why behavioural diversity matters for fish ecology, fisheries management, aquaculture, restoration and conservation. We conclude by identifying key research needs, including improved quantification, stronger empirical links between behavioural diversity and ecological outcomes, and the development of management approaches sensitive to behavioural diversity. More explicit consideration of behavioural diversity should improve our ability to predict fish behavioural diversity responses and to design conservation and management measures that account for variation rather than only average behavioural responses.

Sex chromosome evolution and gene regulation are closely linked but remain understudied in many taxa. Young neo-sex chromosomes offer unique insights into these processes. We examine dosage compensation and sex-biased gene expression in Vandiemenella viatica grasshoppers by comparing the ancestral X chromosome in the P24X0 race with derived neo-sex chromosomes in the P24XY race. The P24XY neo-XY arose via X-autosome fusion: the XL arm represents the ancestral X and the XR arm a former autosome (chromosome 1 in P24X0) now part of the neo-X and homologous to the neo-Y. We first assess dosage compensation via male and female gene expression. In somatic tissues, male P24X0 X-linked and P24XY XL-linked genes are upregulated to match both female expression and autosomal levels, indicating near-complete dosage compensation. In testes, expression of X-linked and the XL-linked genes is reduced nearly 4-fold reflecting absent dosage compensation and the presence of meiotic X chromosome inactivation. We then analyze sex-biased gene expression across tissues and chromosomes. Gonads show stronger sex-biased gene expression than somatic tissues. Female-biased genes are concentrated on the P24X0 X and P24XY XL, whereas male-biased genes are enriched on autosomes and the XR arm of the neo-X. Overall, the ancestral X in P24X0 and the XL arm of the P24XY neo-X are hypertranscribed, while the XR arm retains autosomal expression, male-biased enrichment, and lacks dosage compensation. These patterns show that dosage compensation is regulated at levels of chromosome arms and illustrate how chromosome structure, gene regulation, and reproduction interact, shedding light on sex chromosome evolution in V. viatica.

Some prey species have evolved background matching, that is they resemble their surrounding environment in terms of colour and/or brightness. When prey populations inhabit patchy environments, they may even have evolved specialised phenotypes: each phenotype matching a specific subset of patches. To benefit from the match between their phenotype and this subset of patches, individuals should preferentially select patches within that subset, a process known as matching habitat choice. Matching habitat choice is particularly beneficial to colour polymorphic prey populations, as it reduces individuals and population-level predation risk. We tested for matching habitat choice in green-brown polymorphic grasshoppers using experimental arenas lined with green-brown checkerboards. Because previous work suggested that grasshoppers may distinguish green and brown solely achromatically, individuals were tested on green-brown checkerboards that were either achromatically-mismatched (different luminances and hues) or achromatically-matched (same luminance, different hues). Grasshoppers selected coloured microhabitats independently of their colour morph. They preferred green patches on achromatically-mismatched checkerboards and tended to prefer brown patches on achromatically-matched checkerboards. We conclude that green-brown polymorphic grasshoppers do not engage in matching habitat choice for hue, even though they likely distinguish green and brown chromatically. We finally investigated the potential of the preferred patches to provide increased concealment from their natural predators. Both morphs were better concealed achromatically on the preferred patches. Green-brown polymorphic grasshoppers may thus perform matching habitat choice, though through brightness matching instead of hue matching. Such brightness-based habitat choice could reduce predation risk at long distance and under low-light conditions, highlighting the importance of considering both hue and brightness in studies of habitat choice for colour purposes.

Repeatability, more generally known as intraclass correlation, represents an important quantity of interest in many scientific fields. It represents a metric for summarizing variance decomposition to identify sources of variation in an outcome of interest (e.g. organismal traits). The estimation of variance components is often achieved through linear mixed-effects models or their extension, generalized linear mixed-effects models. Here, we review variants of calculating repeatabilities from mixed-effects models for a variety of conditions and applications. We also recommend which variant might be appropriate under what conditions, focusing on behavioural biology/ecology examples. However, the decision is ultimately with the researcher, since it depends upon their research question, and there is no one-size-fits-all solution. We also highlight the importance of the scope of inference, which affects how repeatabilities are used and interpreted. We recommend transparent reporting of statistical results, including all variance components, which are the building blocks of repeatability. This review aims to assist empiricists in choosing an appropriate repeatability variant and interpretation concerning their questions and the scope of inference.

Genetic variation contributes to intraspecific differences in the chemical defence in many insect species, yet the underlying genetic mechanisms remain poorly understood. The horseradish flea beetle, Phyllotreta armoraciae, sequesters glucosinolates from its horseradish host plant and activates them using endogenous myrosinase enzymes. Of the three known myrosinases in P. armoraciae, PaMyr1 functions primarily in adults, whereas PaMyr2 and PaMyr3 are responsible for myrosinase activity in larvae. Here, we identify natural genetic variation at the myrosinase locus that gives rise to three distinct myrosinase haplotypes, only one of which retains a functional PaMyr3 gene. This variation affected PaMyr gene expression and myrosinase activity in larvae but not in adults. Larvae expressing both PaMyr2 and PaMyr3 showed elevated myrosinase activity toward 2-propenyl glucosinolate, the major glucosinolate in horseradish. Gene expression and biochemical analyses indicate that elevated myrosinase activity results from a subfunctionalization of PaMyr3, which confers greater catalytic efficiency rather than higher total myrosinase abundance. Importantly, PaMyr3-expressing larvae were less susceptible to a model generalist predator in laboratory assays, suggesting a selective advantage under high predation pressure. Consistent with this hypothesis, the PaMyr3-containing haplotype occurred at higher frequency in a natural population than in long-term laboratory populations lacking predators. Together, our results link structural genetic variation to intraspecific differences in insect chemical defence with potential consequences for predator-prey interactions in natural populations.

1. Animal ecologists frequently quantify variance in hierarchically structured traits in wild populations. Importantly, phenotypic plasticity within the period of measurement can modify the trait of interest in response to various unmeasured, temporally or spatially changeable, environmental conditions. Non-random sampling among units of the random effect (e.g. individuals) regarding the environment at issue may lead to estimates of the variance among (partial derivative(2)(I)) or within (partial derivative(2)(W)) such units that conflate several types of processes. This mixing of underlying biology can affect interpretations of the random effect variance. Here, we explore the conditions leading to this situation and assess potential solutions when relevant information is missing. 2. We simulated a trait's phenotypic values that depended on the environmental variable, and individuals that differed in their deviation to the mean population phenotype (random intercepts). We also simulated different types of variation in an environmental variable that was either shared or specific to each individual. We then varied the repeatability in the timing of sampling (R-IS(2)) and analysed simulated datasets using linear mixed-effect models with different fixed-and random-effect structures. 3. In the presence of unmeasured environmental factors, the estimated among-individual variance (partial derivative(2)(I)) contained a larger signature of the current environment as the strength of the temporal autocorrelation and the repeatability in the timing of sampling (R-IS(2)) increased. For low to moderate values of R-IS(2)(e.g. <60% of the total variance in our simulations) the risk of pre-study and within-study effects conflating estimates of variance components was low and could easily be corrected with a model including period or individual-period combination as random effects. Higher R(IS)(2)led to an increase in conflating effects that were difficult to correct. 4. Our study shows the importance of limiting the variance among individuals in the timing structure of sampling (R-IS(2)). We recommend researchers estimate R(IS)(2)and report it in papers. Finally, R(IS)(2)can be limited by sampling all individuals in the same period, or sensitivity analyses could be conducted by removing extreme sampling dates at the analysis stage to reduce R-IS(2).

Grasshoppers represent true outliers in genome sizes, both within insects and within animals in general. Their genomes are large and generally variable in sizes and feature a high abundance of repetitive DNA sequences. This has hampered the assembly of grasshopper genomes to the chromosome level. Here we present a chromosome-level reference genome for the club-legged grasshopper (Gomphocerus sibiricus, Acrididae: Gomphocerinae) using PacBio HiFi long-read and Hi-C sequencing technologies. In male haploid cells, the species has a chromosome set of n = 9 with an X0 sex-determination system, characterized by an absence of a Y chromosome. Our assembly spans 9.57 Gb in total, with 8.87 Gb organized into 9 chromosomes-8 autosomes and the X chromosome. The final assembly has a scaffold N50 value of 1.58 Gb, covers 96.7% single copy Insecta orthologs, and contains 42,665 predicted protein-coding genes and 43,385 mRNA transcripts. We compiled a curated, nonredundant, species-specific repeat library and used it to annotate repetitive DNA, covering 81.69% of the genome, mostly DNA transposons, long-interspersed nuclear element and long-terminal repeat retrotransposons. The genome of the club-legged grasshopper shows high degree of synteny with the locusts Schistocerca gregaria and Locusta migratoria, and the analysis strongly indicates 3 autosome-autosome centric fusions in Gomphocerinae. The genome offers a valuable resource for grasshopper genomics and for exploring the genetic basis of a transspecies color polymorphism.

The green-brown polymorphism in Orthoptera is a prominent example of the coexistence of multiple colour variants, especially since this polymorphism is shared by many species. The processes that maintain phenotypic polymorphisms depend on the underlying genetic and developmental regulation of body colouration, but these are not well understood for Orthoptera. Here we report on the inheritance of the green-brown polymorphism in the meadow grasshopper Pseudochorthippus parallelus, a species with four discrete colour morphs that differ in the distribution of green colouration across the body. We provide the most detailed analysis of the green-brown polymorphism to date using half-sib full-sib breeding and phenotyping of 4,300 offspring. The data strongly support a simple Mendelian control of the presence/absence of green colour in different regions of the body, involving four autosomal loci, two of which are genetically linked. However, estimation of population allele and haplotype frequencies using probabilistic simulations shows weak linkage disequilibrium in the population. The contrast between pedigree and population linkage suggests the presence of long-standing allelic variation and thus corroborates that long-term balancing selection is acting. Our study confirms and extends our understanding of inheritance patterns within the Chorthippus clade, providing unprecedented insights into the number and linkage of loci involved. The results have implications for the maintenance of polymorphisms and suggest that fluctuations in the phenotypic composition of populations can be generated by the segregation of genetic variants even in the absence of fluctuating selection.

Recent declines in arthropod diversity, abundance and biomass are central to the global biodiversity crisis. Yet, we lack a mechanistic understanding of the respective contributions of species richness, species identity and abundance to overall biomass change, and how the environment filters these processes. Synthesizing 11 years of data from a biodiversity experiment and from farmed grasslands in central Europe across a gradient of plant species richness and land-use intensity, we show that local arthropod biomass declines were predominantly (>90%) linked to species richness losses. Abundance declines among persisting species accounted for only 5-8% of lost biomass. The role of species identity depended on the environment and diminished over time: especially under high plant diversity and low land-use intensity, arthropod species with both below-average total biomass and above-average individual biomass (large, rare species) contributed disproportionately to species turnover-but this was only detectable in early years when the communities were still relatively abundant. We conclude that arthropod communities are currently homogenizing towards few common species of similar biomass, probably reducing their adaptability to future environmental change. Increasing the diversity and reducing the land-use intensity of grasslands may mitigate ongoing community simplification and loss of arthropod diversity and functioning.

Many biological features are expressed as 'time-to-event' traits, such as time to first reproduction or time to first response to some stimulus. The analysis of these traits frequently produces right-censored data in cases where no event has occurred within a certain time frame. The Cox proportional hazards (CPH) model, a type of survival analysis, accounts for censored data by estimating the hazard of an event occurring at each time point. While random effect variances can be estimated in CPH models, it is currently not possible to estimate within-cluster variance. Consequently, we lack a general method for calculating ecologically and evolutionary relevant variances and metrics like repeatability from time-toevent data. We here present a solution to this issue. We first describe the characteristics of CPH models and introduce repeatability as an intraclass correlation coefficient (ICC). We demonstrate how CPH models with discrete time intervals are comparable to binomial generalized linear mixed-effects models (GLMMs) with the complementary log-log link. Through this equivalence, we show how to estimate an ICC using the estimates of the random effects variance component(s) resulting from CPH models and the distribution-specific variance (within-cluster variance) from the binomial GLMM. We provide a case study and online materials to demonstrate how our new method for ICC for time-to-event data can be implemented and used. We conclude that the proposed method will not only generate a standard way to quantify consistent individual differences (ICC) from time-to-event data, but also broaden the use of survival analysis outside of the typical implementation for survivorship studies. (c) 2025 The Authors. Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour. This is an open access article under the CC BY license (http://creativecommons.org/licenses/ by/4.0/).

Orthoptera provide a well-documented case of transspecies colour polymorphism, with green and brown morphs coexisting in many species. This colour polymorphism is likely under long-term balancing selection, but the genetic and molecular mechanisms underlying the variation remain poorly understood. Here, we used transcriptome data alongside a novel chromosome-level assembly to perform differential gene expression analysis in the club-legged grasshopper Gomphocerus sibiricus (Caelifera: Acrididae: Gomphocerinae), aiming to identify the specific genes involved in the differentiation between green and brown morphs. Since differential expression analyses are prone to false positives, we replicated the analysis using an independent sample of individuals of the same species. We found six genes consistently upregulated in green individuals across both datasets, all annotated as beta-carotene-binding proteins (βCBPs). βCBPs are known to play a key role in the colour regulation in both the migratory locust Locusta migratoria and the desert locust Schistocerca gregaria , although their exact role may differ in the club-legged grasshopper. The gene tree and chromosomal positions of βCBP copies in G. sibiricus , L. migratoria and S. gregaria indicate both ancestral (pre-speciation) and lineage-specific duplications. Our screening of publicly available orthopteran genomes revealed that homologues of the βCBP genes are largely absent from non-Caelifera species when using conservative homology thresholds. This restricted distribution suggests that βCBP-mediated pigmentation may represent a Caelifera-specific mechanism that is involved in the production of green body coloration, while other orthopteran lineages likely rely on distinct genetic pathways. Together, our findings provide new insights and lay the groundwork for understanding the evolutionary diversification of pigmentation mechanisms in Orthoptera.

The reproducibility of studies involving insect species is an underexplored area in the broader discussion about poor reproducibility in science. Our study addresses this gap by conducting a systematic multi-laboratory investigation into the reproducibility of ecological studies on insect behaviour. We implemented a 3x3 experimental design, incorporating three study sites and three independent experiments on three insect species from different orders: the turnip sawfly (Athalia rosae, Hymenoptera), the meadow grasshopper (Pseudochorthippus parallelus, Orthoptera) and the red flour beetle (Tribolium castaneum, Coleoptera). Using random-effect meta-analysis, we compared the consistency and accuracy of treatment effects on insect behavioural traits across replicate experiments. We successfully reproduced the overall statistical treatment effect in 83% of the replicate experiments, but overall effect size replication was achieved in only 66% of the replicates. Thus, though demonstrating sufficient reproducibility in some measures, this study also provides first experimental evidence for cases of poor reproducibility in insect experiments. Our findings further show that reasons causing poor reproducibility established in rodent research also hold for other study organisms and research questions. We believe that a rethinking of current best practices is required to face reproducibility issues in insect studies, but also across disciplines. Specifically, we advocate for adopting open research practices and the implementation of methodological strategies that reduce bias and problems arising from over-standardization. With respect to the latter, the introduction of systematic variation through multi-laboratory or heterogenized designs may contribute to improved reproducibility in studies involving any living organisms. ### Competing Interest Statement The authors have declared no competing interest.

Animal behaviour changes as individuals mature and these changes may affect not only average behaviour, but also behavioural variability within and between individuals. This could apply to escape behaviour, as behavioural variability might be particularly advantageous in predator-prey interactions, and prey individuals can adhere to different escape strategies. It is not yet known, however, if behavioural variability (unpredictability) is marked early in life, if it arises with time and how stable it is throughout ontogeny. We analysed the escape behaviour of the steppe grasshopper, Chorthippus dorsatus, using burst experiments repeated multiple times across four life stages. We quantified three aspects of escape behaviour and analysed the data using a multivariate double-hierarchical generalized linear model. Interindividual differences in average behaviour increased across life stages, resulting in an increase in population level variability. Unpredictability, however, remained relatively stable both within and across individuals. About 1.7-4.6% of the total phenotypic variance was explained by individual differences in unpredictability. Further, 2-6.5% was explained by differences between bursts of observations, such that interindividual differences in unpredictability explained a maximum of 11% of the total variance in escape behaviour at a given time. Our results indicate a stability of the unpredictability level in grasshopper escape behaviour, and that intraindividual variance in unpredictability is low, but that the overall magnitude of individual differences in escape behaviour increases with age. (c) 2024 The Author(s). Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour. This is an open access article under the CC BY license (http://creativecommons.org/licenses/ by/4.0/).

Colour polymorphisms correspond to the co-occurrence of several distinct colour morphs that vary in hue and/or brightness, independently of sex, age or any other state-dependent modifiers. Colour morphs could represent different life-history strategies, maximising their fitness locally in the trait space through trade-offs between all their biological functions. This mechanism could play a role in the maintenance of the green-brown polymorphism in Orthoptera. Grasshoppers are characterised by a widespread green-brown polymorphism and continuous variability in brightness within colour morphs. It has previously been found that brown individuals are warmer in the field than green conspecifics, but it is unclear if these differences are related to thermal physiology and/or thermal preferences. Therefore, we experimentally tested the thermal physiology and thermal preferences of three green-brown polymorphic species of acridid grasshoppers. We found no differences between green and brown grasshoppers, either in heat-up and equilibrium temperature patterns or in temperature preferences. Nor did we find support that the brightness variation is involved in the thermal physiology of these species. Instead, we show that body mass shapes the thermal physiology, with heavier individuals heating more slowly, and that males heated up faster and reached higher equilibrium temperatures than females. As females are heavier than males, the sex differences might be largely explained by size differences. Our results suggest that neither the thermal physiology nor the thermal preferences explain temperature differences in the field. However, green and brown individuals might still select different microhabitats in the field, which may indirectly lead to differences in body temperature. The persistence of the green-brown polymorphism may result from other mechanisms such as niche partitioning via microhabitat choice, mating preferences or frequency-dependent apostatic selection.

The reshuffling of genomic variation from multiple origins is an important contributor to phenotypic diversification, yet insights into the evolutionary trajectories of this combinatorial process and their interplay with genetic architecture remain scarce. We show that convergent plumage color evolution in wheatears involves a monogenic architecture with modular variation introgressed at the agouti signaling protein (ASIP) locus. Introgression of a new transposable element insertion and linked protein-coding variation underpin a transspecific throat color polymorphism, which stable isotopes suggest is associated with alternative foraging niches. Cointrogression of linked regulatory ASIP variation resulted in mantle color convergence in one species, whereas convergent color evolution at the genus level required new variation. Our results demonstrate evolutionary trajectories from introgressed variation realized within the constraints of a monogenic architecture.

The debate surrounding the role of genetic and environmental factors in shaping behaviour has a long tradition. However, their effects on complex behaviours such as unpredictability in anti-predator strategies remain poorly understood. Behaving unpredictably when escaping predators may increase the prey's chances of survival, especially when prey can rely on a complex habitat providing camouflage and shelter opportunities. We explored the effects of genetic and environmental influences on escape strategies of the steppe grasshopper Chorthippus dorsatus. Individuals from controlled breeding had been randomly assigned to one of two environmental complexity treatments during ontogeny. We then quantified escape behaviour in a large cohort through burst experiments. Using a multivariate double hierarchical animal model, we analysed the effects of pedigree and environmental complexity on both inter- and intra-individual variance in three components of escape behaviour: flight initiation distance (FID), jump distance, and jump angle. Habitat complexity affected average jump angle, but not the average FID or jump distance, nor unpredictability in any of the three traits. Pedigree relatedness accounted for 5%-6% of the total variance in average FID and average jump distance and 7% of the variance in unpredictability in jump angle. Genetic correlations suggest a behavioural syndrome structure in escape strategies that involve FID (a potential indicator of boldness). Our study demonstrates that unpredictability in escape behaviour has the potential to evolve by natural selection, as some of its components are heritable. Furthermore, we show that although habitat complexity represents a strong environmental treatment, its lasting effects during ontogeny on escape behaviour are minimal.

Arthropods make up the vast majority of terrestrial biodiversity and play essential roles in ecosystem functioning. Biodiverse grasslands support large numbers of arthropods, yet such ecosystems have faced significant declines due to land conversion and changes in management. While restoration efforts aim to conserve species-rich grasslands, the long-term effects of plant species richness on arthropod communities and associated functions (e.g. herbivory and pest-control) remain underexplored. We addressed this knowledge gap by quantifying grassland arthropods, herbivory, and predation over 13 years (2010-2022) across 80 plots with varying plant species richness. We explored (1) how arthropod communities and associated functions have changed over the study period and if any change is affected by plant species richness, (2) if effects of plant species richness on arthropods vary or even strengthen over time, and (3) if changes in arthropod communities affect associated functions. Our findings reveal that, at all levels of plant species richness, arthropod community metrics declined over time, with average losses being mainly more pronounced in species-poor plant mixtures. Plant species richness consistently had a positive effect on arthropod communities and their functions. This positive plant species richness effect strongly varied between years, but showed no consistent change over time. Predators experienced greater annual losses compared to herbivores, but herbivore declines were more influenced by changing plant species richness. Notably, temporal changes in arthropod community metrics did not predict functional changes, as the effect of plant species richness on predation strengthened over time, whereas its effect on predator richness remained stable. Overall, our study highlights the complex and dynamic interactions between plant species richness and arthropod communities over time. Maintaining high plant diversity in semi-natural grasslands might mitigate arthropod loss over time, but this stabilizing effect may take years to fully establish, emphasizing the long-term nature of conservation efforts. ### Competing Interest Statement The authors have declared no competing interest.