Biotic homogenization destabilizes ecosystem functioning by decreasing spatial asynchrony

Understanding how biodiversity (B) affects ecosystem functioning (EF) is essential for assessing the consequences of ongoing biodiversity changes. An increasing number of studies, however, show that environmental conditions affect the shape of BEF relationships. Here, we first use a game-theoretic community model to reveal that a unimodal response of the BEF slope can be expected along environmental stress gradients, but also how the ecological mechanisms underlying this response may vary depending on how stress affects species interactions. Next, we analysed a global dataset of 44 experiments that crossed biodiversity with environmental conditions. Confirming our main model prediction, the effect of biodiversity on ecosystem functioning tends to be greater at intermediate levels of environmental stress, but varies among studies corresponding to differences in stress-effects on species interactions. Together, these results suggest that increases in stress from ongoing global environmental changes may amplify the consequences of biodiversity changes.

Selective logging is one of the most prevalent land uses of forests worldwide, affecting biodiversity and ecosystem functioning. However, the effect of selective logging on the dual nature of temporal stability, and the scale dependence of this effect, remain to be elucidated. By conducting several decade‐long experiments in temperate forest ecosystems, we tested the effects of selective logging on aggregate and compositional stability at multiple spatial scales. As expected, forest ecosystem stability at larger spatial scales was enhanced both by the stability of local scales (i.e. α stability) and asynchronous dynamics among local communities (i.e. spatial asynchrony). We found that the negative effects of selective logging on both facets of forest stability propagated from local to larger spatial scales due to reduced α stability and the biological insurance effects of α diversity. However, both spatial aggregate and compositional asynchrony were not affected by selective logging. Interestingly, despite the selective logging, α diversity still provided biological insurance effects for maintaining aggregate and compositional stability. Our results imply that selective logging may destabilize the aggregate ecosystem functioning and species composition of forest ecosystems at local and larger spatial scales. To our knowledge, this study provides the first evidence of the scale dependence of aggregate and compositional stability of forest ecosystems in response to selective logging. Our findings suggest that forest management should avoid excessive selective logging and strive to protect forest diversity to safeguard the sustainability of the functioning and composition of natural forest ecosystems at multiple spatial scales.Keywords: aggregate stability, compositional stability, diversity, ecosystem functioning, spatial asynchrony, stability

Summary 1. Biotic homogenization (BH), a dominant process shaping the response of natural communities to human disturbance, reflects both the expansion of exotic species at large scales and other mechanisms that often operate at smaller scales. 2. Here, we examined the relationship between BH in plant communities and spatio-temporal landscape disturbance (habitat fragmentation and surrounding habitat conversion) at a local scale (1 km²), using data from a standardized monitoring programme in France. We quantified BH using both a spatial partitioning of taxonomic diversity and the average habitat specialization of communities, which informs on functional BH. 3. We observed a positive relationship between local taxonomic diversity and landscape fragmentation or instability. This increase in local taxonomic diversity was, however, paralleled by a decrease in average community specialization in more fragmented landscapes and in more unstable landscapes around forest sites. The decrease in average community specialization suggests that landscape disturbance causes functional BH, but there was limited evidence for concurrent taxonomic BH. 4. Synthesis. Our results show that landscape disturbance is partly responsible for functional BH at small scales via the extirpation of specialist species, with possible consequences for ecosystem functioning. However, this change in community composition is not systematically associated with taxonomic BH. This has direct relevance in designing biodiversity indicators: metrics incorporating species sensitivity to disturbance (such as species specialization to habitat) appear much more reliable than taxonomic diversity for documenting the response of communities to disturbance.

Theoretical studies of the relationship between ecosystem complexity and stability usually conclude that systems with more species, more interspecific interactions per species (connectance), or stronger interactions are not as likely to be stable as systems with fewer of these attributes (Gardner and Ashby 1970; May 1972, 1974, 1979; DeAngelis 1975; Gilpin 1975; Pimm 1979a, 1979b, 1980). Yet, in one of the few empirical investigations of this problem, McNaughton (1977) concluded that increased complexity stabilized certain ecosystem properties; more precisely, that a large mammalian grazer changed total green plant biomass less in more diverse than in less diverse grassland plots. We try to resolve this apparent contradiction between theory and empiricism by investigating, in model grazing systems, the relationship between complexity and the lack of change in plant biomass (which we call biomass stability) following the removal of an herbivore. We established a set of structured food web models composed of one herbivore and n plant species. The models were based on the familiar Lotka-Volterra equations, and we selected their parameters over intervals designed to be biologically sensible and also to reflect the pattern of interactions in the food web. Only those models with a locally stable equilibrium involving species with positive biomasses were retained for further analysis. From each model of this subset, the herbivore was removed and the resultant change in total plant biomass followed until a new stable equilibrium was achieved. Relative biomass stability was calculated from the relative change in the total plant biomasses of the two equilibria. Clearly, a ratio near unity indicates biomass stability, while a large ratio indicates that biomass has increased considerably, following removal of the herbivore. We modeled webs of varying complexity as measured by: (1) the number of species; (2) the number of competitive interactions between plant species; and (3) species diversity (a measure combining the number of species and their relative abundances), and related these features to biomass stability. Our conclusion is that increased complexity can enhance biomass stability, even

Biodiversity contributes to stabilizing ecosystem productivity across spatial scales as much as environmental heterogeneity in a large temperate forest region

AimLand use change reorganizes local communities, resulting in complex changes in biodiversity at larger scales. The biotic homogenization hypothesis predicts that the replacement of sensitive loser species with widespread winner species will lead to loss of beta diversity and ultimately loss of regional diversity at multiple levels of ecological organization. We ask if land use is associated with biotic homogenization patterns in bee communities at two large spatial scales, using both species and phylogenetic dissimilarity indices.LocationNorth‐eastern USA (New Jersey, New York and Pennsylvania).Time period2013–2015.Major taxa studiedSuperfamily Apoidea (bees).MethodsWe sampled bee communities from replicated sites in forest, agriculture and urban land use types within a large spatial extent spanning four distinct ecoregions. We compared pairwise compositional dissimilarity within and between ecoregions, using both species and phylogenetic dissimilarity indices. We also investigated how compositional difference is related to geographic distance between sites.ResultsForested, agricultural and urban landscapes did not differ detectably in either mean pairwise species dissimilarity or slope of distance‐decay. Dissimilarity among both agricultural and urban bee communities increased with geographic distance. However, urban landscapes had significantly lower phylogenetic pairwise dissimilarity, indicating strong phylogenetic homogenization at within‐ and between‐ecoregion scales.Main conclusionsWe did not detect bee species homogenization in agricultural or urban land use types. However, urban land use was associated with phylogenetic homogenization across a large regional extent. Urban bee communities are dominated by closely related species that maintain beta diversity at the species level, but contribute to low phylogenetic beta diversity relative to forest and agricultural bee communities. We observed similar levels of homogenization at landscape and regional spatial extents, despite inter‐site distances differing by an order of magnitude between these scales. Further urbanization could result in loss of bee biodiversity and evolutionary history at multiple spatial scales.

Genetically modified herbicide-tolerant (GMHT) oilseed rape (OSR; Brassica napus L.) was approved for commercial cultivation in Canada in 1995 and currently represents over 95% of the OSR grown in western Canada. After a decade of widespread cultivation, GMHT volunteers represent an increasing management problem in cultivated fields and are ubiquitous in adjacent ruderal habitats, where they contribute to the spread of transgenes. However, few studies have considered escaped GMHT OSR populations in North America, and even fewer have been conducted at large spatial scales (i.e. landscape scales). In particular, the contribution of landscape structure and large-scale anthropogenic dispersal processes to the persistence and spread of escaped GMHT OSR remains poorly understood. We conducted a multi-year survey of the landscape-scale distribution of escaped OSR plants adjacent to roads and cultivated fields. Our objective was to examine the long-term dynamics of escaped OSR at large spatial scales and to assess the relative importance of landscape and localised factors to the persistence and spread of these plants outside of cultivation. From 2005 to 2007, we surveyed escaped OSR plants along roadsides and field edges at 12 locations in three agricultural landscapes in southern Manitoba where GMHT OSR is widely grown. Data were analysed to examine temporal changes at large spatial scales and to determine factors affecting the distribution of escaped OSR plants in roadside and field edge habitats within agricultural landscapes. Additionally, we assessed the potential for seed dispersal between escaped populations by comparing the relative spatial distribution of roadside and field edge OSR. Densities of escaped OSR fluctuated over space and time in both roadside and field edge habitats, though the proportion of GMHT plants was high (93-100%). Escaped OSR was positively affected by agricultural landscape (indicative of cropping intensity) and by the presence of an adjacent field planted to OSR. Within roadside habitats, escaped OSR was also strongly associated with large-scale variables, including road surface (indicative of traffic intensity) and distance to the nearest grain elevator. Conversely, within field edges, OSR density was affected by localised crop management practices such as mowing, soil disturbance and herbicide application. Despite the proximity of roadsides and field edges, there was little evidence of spatial aggregation among escaped OSR populations in these two habitats, especially at very fine spatial scales (i.e. <100 m), suggesting that natural propagule exchange is infrequent. Escaped OSR populations were persistent at large spatial and temporal scales, and low density in a given landscape or year was not indicative of overall extinction. As a result of ongoing cultivation and transport of OSR crops, escaped GMHT traits will likely remain predominant in agricultural landscapes. While escaped OSR in field edge habitats generally results from local seeding and management activities occurring at the field-scale, distribution patterns within roadside habitats are determined in large part by seed transport occurring at the landscape scale and at even larger regional scales. Our findings suggest that these large-scale anthropogenic dispersal processes are sufficient to enable persistence despite limited natural seed dispersal. This widespread dispersal is likely to undermine field-scale management practices aimed at eliminating escaped and in-field GMHT OSR populations. Agricultural transport and landscape-scale cropping patterns are important determinants of the distribution of escaped GM crops. At the regional level, these factors ensure ongoing establishment and spread of escaped GMHT OSR despite limited local seed dispersal. Escaped populations thus play an important role in the spread of transgenes and have substantial implications for the coexistence of GM and non-GM production systems. Given the large-scale factors driving the spread of escaped transgenes, localised co-existence measures may be impracticable where they are not commensurate with regional dispersal mechanisms. To be effective, strategies aimed at reducing contamination from GM crops should be multi-scale in approach and be developed and implemented at both farm and landscape levels of organisation. Multiple stakeholders should thus be consulted, including both GM and non-GM farmers, as well as seed developers, processors, transporters and suppliers. Decisions to adopt GM crops require thoughtful and inclusive consideration of the risks and responsibilities inherent in this new technology.

Biodiversity positively affects vegetation productivity and the ability of ecosystems to withstand disturbance events and invasions of alien species (ecosystem stability). However, the relationship between biodiversity and ecosystem functioning remains understudied at the landscape or whole ecosystem scales.In particular, biodiversity can not be easily monitored at large spatial scales or frequent intervals. Therefore, confronting measurements of biodiversity and ecosystem functioning at the same spatial and temporal scales remains challenging. In this work, we present new methods to systematically bridge these scale gaps. We focus on collecting ecosystem data and developing metrics able to decypher the effect of plant biodiversity on ecosystem functioning. Based on eddy covariance fluxes from 78 NEON and ICOS sites, we compute key ecosystem functional properties related to ecosystem productivity and stability. Moreover, we calculate biodiversity indices from field surveys of species abundances, functional traits, and structural properties at these sites. Finally, we compute remote sensing metrics of biodiversity based on Sentinel 2 measurements. These metrics exploit the fine scale multispectral information from different and complementary perspectives, and are adapted to match the footprint of typical eddy covariance sites.We investigate the relationship between ground- and satellite-based biodiversity metrics to understand the capability of remote sensing to contribute to biodiveristy-ecosystem function analyses that may one day be scaled globally. Despite dominant environmental and climatic constraints, we hypothesize that ecosystem functional properties covary with biodiversity metrics. To elucidate this point, we analyze the multivariate relationship between the different biodiversity estimates, ecosystem functional properties related to water, carbon, and energy fluxes, structural variables of the vegetation, and climate.Assessing whether biodiversity effects apply to the functioning and stability of ecosystems is pivotal to understanding ecosystem processes and developing appropriate forecast models and climate change mitigation strategies.

Recent experiments, mainly in terrestrial environments, have provided evidence of the functional importance of biodiversity to ecosystem processes and properties. Compared to terrestrial systems, aquatic ecosystems are characterised by greater propagule and material exchange, often steeper physical and chemical gradients, more rapid biological processes and, in marine systems, higher metazoan phylogenetic diversity. These characteristics limit the potential to transfer conclusions derived from terrestrial experiments to aquatic ecosystems whilst at the same time provide opportunities for testing the general validity of hypotheses about effects of biodiversity on ecosystem functioning. Here, we focus on a number of unique features of aquatic experimental systems, propose an expansion to the scope of diversity facets to be considered when assessing the functional consequences of changes in biodiversity and outline a hierarchical classification scheme of ecosystem functions and their corresponding response variables. We then briefly highlight some recent controversial and newly emerging issues relating to biodiversity‐ecosystem functioning relationships. Based on lessons learnt from previous experimental and theoretical work, we finally present four novel experimental designs to address largely unresolved questions about biodiversity‐ecosystem functioning relationships. These include (1) investigating the effects of non‐random species loss through the manipulation of the order and magnitude of such loss using dilution experiments; (2) combining factorial manipulation of diversity in interconnected habitat patches to test the additivity of ecosystem functioning between habitats; (3) disentangling the impact of local processes from the effect of ecosystem openness via factorial manipulation of the rate of recruitment and biodiversity within patches and within an available propagule pool; and (4) addressing how non‐random species extinction following sequential exposure to different stressors may affect ecosystem functioning. Implementing these kinds of experimental designs in a variety of systems will, we believe, shift the focus of investigations from a species richness‐centred approach to a broader consideration of the multifarious aspects of biodiversity that may well be critical to understanding effects of biodiversity changes on overall ecosystem functioning and to identifying some of the potential underlying mechanisms involved.

Global biodiversity is rapidly declining, resulting in far-reaching impacts on the functioning of ecosystems and human wellbeing. In recent decades, anthropogenic land use has been identified as a major driver of biodiversity loss, especially through the expansion and intensification of agricultural systems. While the drivers of biodiversity loss have been relatively clearly established, variability in the way that whole ecosystems respond to these drivers is still poorly understood. This is, in part, because we still lack a clear understanding of how species interactions govern the way that complex communities respond to environmental stressors, as well as their role in mediating ecosystem functioning. &#13;\nSpecies interactions can moderate community responses to land-use change via trophic cascades, whereby extinctions at the top or bottom of a food chain produce cascading effects through the rest of the food web due to the disruption of resource availability or predatory control of consumers. Additionally, species interactions are fundamental for ecosystem functioning as they are almost always directly linked to processes such as decomposition, herbivory, predation, pollination, and seed dispersal. Therefore, an approach to studying biodiversity and ecosystem functioning of naturally complex communities that incorporates multiple trophic levels and their interactions is crucial for predicting future global-change scenarios. Despite the conceptual advantage of a multitrophic approach, this has been rarely applied in the context of biodiversity and ecosystem functioning of ecosystems undergoing land-use change. In addition, while there has been considerable evidence established for the role of biodiversity in maintaining ecosystem functioning in local-scale experiments, there is still very limited knowledge of how this relationship scales up to landscapes in real-world ecosystems. In this thesis, I aimed to achieve a conceptual advance in biodiversity-ecosystem functioning (BEF) research within the context of global environmental change by investigating responses of complex multitrophic communities to land-use change and the resulting consequences for ecosystem functioning. &#13;\nFirstly, in Chapter 2, I combined data from a wide taxonomic range of trophic groups to test how communities of interacting species respond to tropical land-use intensification in Sumatra, Indonesia. I employed structural equation modelling to test if land-use intensification directly impacted all trophic groups or, alternatively, if it affected only lower trophic levels, resulting in bottom-up trophic cascades. Results from this model suggested that direct land-use impacts were generally much stronger than bottom-up trophic effects. Interestingly though, the number of direct effects from land-use intensification decreased considerably from plants to predators, whereas the number of bottom-up trophic effects increased dramatically with increasing trophic level. These findings suggest that the underlying mechanisms of land-use intensification that alter communities highly depend on the trophic level in question, indicating the need for trophic level-specific conservation management strategies. &#13;\nThe results from Chapter 2 provided strong evidence for the importance of species interactions in moderating community responses to land use, leading to the question of how ecological processes carried out by multitrophic communities are resultantly affected. One major challenge of BEF research has been to fully incorporate species interactions across multiple trophic levels to quantify a trophically broad measure of ecosystem functioning. In Chapter 3, I overcame this challenge by developing a measure of ecosystem functioning that integrates food web and metabolic theory to calculate community energy flux across multiple trophic levels. By calculating energy flux of multitrophic macroinvertebrate communities, I demonstrated that declining species diversity with increasing land-use intensity led to concomitantly strong declines in community energy flux. Furthermore, I found that the relationship between species richness and energy flux was steeper in the most intense land-use system, oil palm, but this result did not hold when trophic guilds were analysed independently. Thus, these findings suggest that if trophic groups are omitted, it is possible that BEF relationships could be misinterpreted in response to anthropogenic land use.&#13;\nIn order to extend the previous chapter’s findings beyond the provisioning of ecosystem functioning of multitrophic communities, in Chapter 4, I investigated the functional stability and resilience of the macroinvertebrate communities to future perturbations. Using a trait-based approach, I determined how communities were assembled among different land-use types. I then calculated functional stability and community resilience by measuring the number of functionally redundant species within functional effect groups (based on traits that determine species’ influence on ecosystem processes) and the dispersion of traits within functional response groups (based on traits that determine species’ responses to disturbances). In doing so, I found that litter invertebrate communities in oil palm plantations were more randomly assembled, as well as having significantly fewer functionally redundant species. However, the jungle rubber agroforest system harboured communities with considerably higher functional redundancy than in oil palm. These results indicate that communities in high-intensity land-use systems are more susceptible to functional collapse given future perturbations, but low-intensity agroforests could help to maintain higher functional stability in anthropogenic landscapes.&#13;\nFinally, in Chapter 5, I investigated how ecosystem functioning varies across spatial and environmental gradients and the mechanisms that give rise to spatial turnover in ecosystem functioning. To test this, I used data on litter macroinvertebrate communities from landscapes in Indonesia and Germany and applied the energy flux calculations developed in Chapter 3 as a measure of multitrophic ecosystem functioning. I then employed structural equation modelling based on distance matrices to establish how environmental and geographic distance drive turnover in species composition, species richness, functional trait dispersion and community biomass, and how these factors consequentially drive spatial turnover in community energy flux in a tropical and temperate region. Environmental distance appeared to be more important in the Indonesian compared with the German region for driving species turnover. However, the mechanisms that determined spatial turnover in ecosystem functioning were remarkably similar between the tropical and temperate regions, such that species richness and community biomass were the most important variables explaining spatial variability in energy flux. These results suggest that mechanisms such as species identity and niche complementarity may become redundant for predicting ecosystem functioning at the landscape scale. Instead, species richness and biomass should be sufficient for predicting multitrophic ecosystem functioning at large spatial scales.&#13;\nOverall, in this thesis I demonstrate that species interactions are important for mediating responses of multitrophic communities to land-use intensification and that the loss of species across trophic levels has drastic consequences for the provisioning of multitrophic ecosystem functioning. Furthermore, this species loss reduces the stability of ecosystem functioning in intensified agricultural landscapes. Finally, I demonstrate that species richness and community biomass are the key components for developing a framework aimed at predicting likely scenarios of functional losses in intensified land-use systems at the landscape scale. Ultimately, by incorporating real-world complexity into studies that integrate across multiple ecological concepts, this thesis presents a significant advance toward understanding how ecosystems respond to anthropogenic land-use change, thus highlighting important areas for future exploration.

Temporal stability of ecosystem functioning increases the predictability and reliability of ecosystem services, and understanding the drivers of stability across spatial scales is important for land management and policy decisions. We used species‐level abundance data from 62 plant communities across five continents to assess mechanisms of temporal stability across spatial scales. We assessed how asynchrony (i.e. different units responding dissimilarly through time) of species and local communities stabilised metacommunity ecosystem function. Asynchrony of species increased stability of local communities, and asynchrony among local communities enhanced metacommunity stability by a wide range of magnitudes (1–315%); this range was positively correlated with the size of the metacommunity. Additionally, asynchronous responses among local communities were linked with species’ populations fluctuating asynchronously across space, perhaps stemming from physical and/or competitive differences among local communities. Accordingly, we suggest spatial heterogeneity should be a major focus for maintaining the stability of ecosystem services at larger spatial scales.

Linking biodiversity indicators, ecosystem functioning, provision of services and human well-being in estuarine systems: Application of a conceptual framework

Assessing the effects of landscape structure on the relationship between species diversity and functional diversity

The global biodiversity loss is causing abrupt shifts in the structure and functioning of ecosystems with severe ecological and socio‐economic consequences. Therefore, improving our understanding of ecosystem dynamics and regime shifts, as well as the stabilizing role of biodiversity across multiple scales is needed. Here we investigate the temporal dynamics and stability of marine ecosystems using high‐resolution monitoring data on fish species composition, abundances and traits throughout European Seas. More specifically, we quantify and compare the direction and magnitude of community change at multiple spatial scales and levels of biological organization. Our results show less variability in community trajectories at larger spatial scales and higher levels of biological organization. The main underlying processes providing stability are statistical averaging arising from a larger pool of species, while at smaller spatial scales stability also emerge from functional complementarity channeled through the distribution of species traits within functional groups.

Summary Assessing the consequences of biodiversity changes for ecosystem functioning requires separating the net effect of biodiversity from potential confounding effects such as the identity of the gained or lost species. Additive partitioning methods allow factoring out these species identify effects by comparing species’ functional contributions against the predictions of a null model under which functional contributions are independent of biodiversity. Classic additive partitioning methods quantify biodiversity effects based on a linear relationship between species deviations from the null model and their functional traits. However, based on ecological theory, nonlinear relationships are also possible. Here, we demonstrate how additive‐partitioning methods can be extended to describe such nonlinear relationships, and explain how nonlinear biodiversity effects can be interpreted. We apply both linear and nonlinear partitioning methods to the Cedar Creek Biodiversity II experiment. Nonlinear relationships were detected in the majority of plots, and increased with diversity. Nonlinear partitioning thereby identified a convex relationship between species functional traits and their deviations from the null model, driven by strong positive effects of both species with low and high functional trait values trait values on ecosystem functioning. The presented nonlinear extension of additive partitioning methods is therefore essential for revealing more complex biodiversity effects on ecosystem functioning, that are likely to occur in biodiversity experiments.