Toward a mechanistic understanding and prediction of biotic homogenization

The widespread replacement of native species with cosmopolitan, nonnative species is homogenizing the global fauna and flora. While the empirical study of biotic homogenization is substantial and growing, theoretical aspects have yet to be explored. Consequently, the breadth of possible ecological mechanisms that can shape current and future patterns and rates of homogenization remain largely unknown. Here, we develop a conceptual model that describes 14 potential scenarios by which species invasions and/or extinctions can lead to various trajectories of biotic homogenization (increased community similarity) or differentiation (decreased community similarity); we then use a simulation approach to explore the model's predictions. We found changes in community similarity to vary with the type and number of nonnative and native species, the historical degree of similarity among the communities, and, to a lesser degree, the richness of the recipient communities. Homogenization is greatest when similar species invade communities, causing either no extinction or differential extinction of native species. The model predictions are consistent with current empirical data for fish, bird, and plant communities and therefore may represent the dominant mechanisms of contemporary homogenization. We present a unifying model illustrating how the balance between invading and extinct species dictates the outcome of biotic homogenization. We conclude by discussing a number of critical but largely unrecognized issues that bear on the empirical study of biotic homogenization, including the importance of spatial scale, temporal scale, and data resolution. We argue that the study of biotic homogenization needs to be placed in a more mechanistic and predictive framework in order for studies to provide adequate guidance in conservation efforts to maintain regional distinctness of the global biota.     

A new framework for investigating biotic homogenization and exploring future trajectories: oceanic island plant and bird assemblages as a case study

Studies of biotic homogenization have focused primarily on characterizing changes that have occurred between some past baseline and the present day. In order to understand how homogenization may change in the future, it is important to contextualize the processes driving these changes. Here, we examine empirical patterns of change in taxonomic similarity among oceanic island plant and bird assemblages. We use these empirical cases to unpack dynamic properties of biotic homogenization, thereby elucidating two important factors that have received little attention: 1) initial similarity and 2) the influence of six classes of introduction and extinction events. We use Jaccard's Index to explore the interplay among these factors in determining the changes in similarity that have occurred between human settlement and the present. Specifically, we develop general formulas for changes in similarity resulting from each of the six types of introductions and extinctions, so that the effect of each event type is formulated in terms of initial similarity and species richness. We then apply these insights to project how similarity levels would change in the future if the present patterns of introductions and extinctions continue. We show that the six event types, along with initial similarity, can show dramatically different behavior in different systems, leading to widely variable influences on similarity. Plant and bird biotas have homogenized only slightly to date, but their trajectories of change are highly divergent. Although existing patterns of colonization and extinction might not continue unchanged, if they were to do so then plant assemblages would show little additional change, whereas bird assemblages would become much more strongly homogenized. Our results suggest that moderate changes in similarity observed to date mask the potential for more dramatic changes in the future, and that the interaction among initial similarity and differential introduction and extinction regimes drives these dynamics.     

The varying role of population abundance in structuring indices of biotic homogenization

Aim An important component of human‐induced global change is the decrease or increase in community distinctiveness (taxonomic homogenization or differentiation, respectively) that follows the loss of native species and gain of non‐native species. We use simulation approaches to assess the extent to which conclusions about the outcome of the homogenization process depend on whether or not abundance data are incorporated. Location Data were produced through computer simulation. Methods The frequency with which occurrence‐based similarity indices and abundance‐based similarity indices give different views of changes in community similarity, and the conditions under which such differences occurred were assessed using both deterministic and stochastic modelling approaches to simulate species assemblage states. Results Occurrence‐based and abundance‐based indices were positively correlated across the set of simulations for both the deterministic and stochastic models. However, in both situations approximately one quarter (25%) of models resulted in contrasting outcomes for the two approaches of calculating changes in compositional similarity; that is, one data type showed a positive value (homogenization), whereas the other showed a negative value (differentiation). Main conclusions In the majority of cases, species abundances will not change drastically enough after perturbation to produce large differences between homogenization scores measured using occurrence versus abundance information. However, in cases where these changes are large, it is important to recognize that the choice of metric to analyse homogenization trends will influence the qualitative and quantitative conclusions drawn. Studies of real assemblages are therefore necessary to evaluate the role of species abundance in defining the magnitude and direction of changes in community composition across space, and the implications of these changes for native biodiversity.     

Low‐head dams induce biotic homogenization/differentiation of fish assemblages in subtropical streams

Extensive distribution of widespread species and the loss of native species driven by anthropogenic disturbances modify community similarity, resulting in a decrease or increase in community distinctiveness. Data from four basins in the Wannan Mountains, China, were used to evaluate the effects of low‐head dams on patterns of fish faunal homogenization and differentiation based on abundance data. We aimed to examine the spatial changes in taxonomic and functional similarities of fish assemblages driven by low‐head dams and to examine whether the changes in the similarity of fish assemblages differed between taxonomic and functional components. We found that low‐head dams significantly decreased the mean taxonomic similarity but increased the mean functional similarity of fish assemblages in impoundments using abundance‐based approaches, suggesting that taxonomic differentiation accompanied functional homogenization in stream fish assemblages. These results show the importance of population abundance in structuring fish faunal homogenization and differentiation at small scales, especially when the major differences among assemblages are in species abundance ranks rather than species identities. Additionally, we also found only a weak positive correlation between changes in mean taxonomic and functional similarities, and partial pairs exhibited considerable variation in patterns of fish faunal homogenization and differentiation for taxonomic and functional components. In conclusion, this study highlighted that the observed taxonomic differentiation of current fish assemblages (short‐term phenomenon) is probably an early warning sign of further homogenization in regions where native species are completely predominated and that changes in taxonomic similarity cannot be used to predict changes in functional similarity.     

Bumblebee community homogenization after uphill shifts in montane areas of northern Spain

Widespread alterations in species distribution and abundance as a result of global environmental change include upwards and polewards shifts driven by local extinctions in the south or at lower elevations and colonizations of newly available habitat elements in the north or at higher elevations. Although cumulative changes on patterns of community composition are also expected, studies following a community-level approach are still scarce. Here, we estimate changes in abundance and distribution of bumblebee (Bombus spp.) species over two decades along an elevational gradient to test whether these changes entailed concomitant alterations on patterns of community composition. Bumblebee species showed an overall trend to shift uphill their upper- or lower-elevational boundaries, resulting in narrower elevational ranges from one period to another, coincident with a regional warming of ca. 0.9 °C. Changes in elevational ranges were, however, mainly related to retractions of the lower limit of species distribution, rather than to variations in their upper elevational limit. Species turnover was associated with colonization and extinction events and also with variability in the relative abundance of short-, medium- and long-tongued species along the elevational gradient. Extinctions were especially relevant at medium elevations, while only communities at higher elevations had a positive net outcome between colonization and extinction events. The combination of these effects resulted in the homogenization of bumblebee assemblages, especially between medium and upper elevations. The changes reported in our study strongly match with predictions of global change driving elevational shifts in species distribution and provide the first evidence of elevational changes in bumblebees at both species and community level.     

Pattern and process of biotic homogenization in the New Pangaea

Human activities have reorganized the earth''s biota resulting in spatially disparate locales becoming more or less similar in species composition over time through the processes of biotic homogenization and biotic differentiation, respectively. Despite mounting evidence suggesting that this process may be widespread in both aquatic and terrestrial systems, past studies have predominantly focused on single taxonomic groups at a single spatial scale. Furthermore, change in pairwise similarity is itself dependent on two distinct processes, spatial turnover in species composition and changes in gradients of species richness. Most past research has failed to disentangle the effect of these two mechanisms on homogenization patterns. Here, we use recent statistical advances and collate a global database of homogenization studies (20 studies, 50 datasets) to provide the first global investigation of the homogenization process across major faunal and floral groups and elucidate the relative role of changes in species richness and turnover. We found evidence of homogenization (change in similarity ranging from −0.02 to 0.09) across nearly all taxonomic groups, spatial extent and grain sizes. Partitioning of change in pairwise similarity shows that overall change in community similarity is driven by changes in species richness. Our results show that biotic homogenization is truly a global phenomenon and put into question many of the ecological mechanisms invoked in previous studies to explain patterns of homogenization.     

Null model of biotic homogenization: a test with the European freshwater fish fauna

In recent years, there has been growing concern about how species invasions and extinctions could change the distinctiveness of formerly disparate fauna and flora, a process called biotic homogenization. In the present study, a null model of biotic of homogenization was developed and applied to the European freshwater fish fauna. We found that non-native fish species led to the greatest homogenization in south-western Europe and greatest differentiation in north-eastern Europe. Comparing these observed patterns to those expected by our null model empirically demonstrated that biotic homogenization is a non-random ecological pattern, providing evidence for previous assumptions. The place of origin of non-native species was also considered by distinguishing between exotic (originating from outside Europe) and translocated species (originating from within Europe). We showed that exotic and translocated species generated distinct geographical patterns of biotic homogenization across Europe because of their contrasting effects on the changes in community similarity among river basins. Translocated species promoted homogenization among basins, whereas exotic species tended to decrease their compositional similarity. Quantifying the individual effect of exotic and translocated species is therefore an absolute prerequisite to accurately assess the spatial dynamics of biotic homogenization.     

Mesocosms Reveal Ecological Surprises from Climate Change

Understanding, predicting, and mitigating the impacts of climate change on biodiversity poses one of the most crucial challenges this century. Currently, we know more about how future climates are likely to shift across the globe than about how species will respond to these changes. Two recent studies show how mesocosm experiments can hasten understanding of the ecological consequences of climate change on species’ extinction risk, community structure, and ecosystem functions. Using a large-scale terrestrial warming experiment, Bestion et al. provide the first direct evidence that future global warming can increase extinction risk for temperate ectotherms. Using aquatic mesocosms, Yvon-Durocher et al. show that human-induced climate change could, in some cases, actually enhance the diversity of local communities, increasing productivity. Blending these theoretical and empirical results with computational models will improve forecasts of biodiversity loss and altered ecosystem processes due to climate change.     

Ecological filtering or random extinction? Beta‐diversity patterns and the importance of niche‐based and neutral processes following habitat loss

Although both niche‐based and neutral processes are involved in community assembly, most models on the effects of habitat loss are stochastic, assuming neutral communities mainly affected by ecological drift and random extinction. Given that habitat loss is considered the most important driver of the current biodiversity crisis, unraveling the processes underlying the effects of habitat loss is critical from both a theoretical and an applied perspective. Here we unveil the importance of niche‐based and neutral processes to species extinction and community assembly across a gradient of habitat loss, challenging the predictions of neutral models. We draw on a large dataset containing the distribution of 3653 individuals of 42 species, representing 35% of the small mammal species of the Atlantic Forest hotspot, obtained in 68 sites across three continuously‐forested landscapes and three adjacent 10 000‐ha fragmented landscapes differing in the amount of remaining forest (50%, 30% and 10%). By applying a null‐model approach, we investigated β‐diversity patterns by detecting deviations of observed community similarity from the similarity between randomly assembled communities. Species extinction following habitat loss was decidedly non‐random, in contrast to the notion that fragmented communities are mainly driven by ecological drift. Instead, habitat loss led to a strong biotic homogenization. Moreover, species composition changed abruptly at the same level of landscape‐scale habitat loss that has already been associated with a drastic decline in species richness. Habitat loss, as other anthropogenic disturbances, can thus be seen as a strong ecological filter that increases (rather than decreases) the importance of deterministic processes in community assembly. As such, critical advances for the development of conservation science lie on the incorporation of the relevant niche traits associated with extinction proneness into models of habitat loss. The results also underscore the fundamental importance of pro‐active measures to prevent human‐modified landscapes surpassing critical ecological thresholds.     

Distinguishing between turnover and nestedness in the quantification of biotic homogenization

Compositional changes through local extinction and colonization are inherent to natural communities, but human activities are increasingly influencing the rate and nature of the species being lost and gained. Biotic homogenization refers to the process by which the compositional similarity of communities increases over time through a non-random reshuffling of species. Despite the extensive conceptual development of the homogenization framework, approaches to quantify patterns of homogenization are scarcely developed. Most studies have used classical dissimilarity indices that actually quantify two components of compositional variation: turnover and nestedness. Here we demonstrate that a method that partitions those two components reveals patterns of homogenization that are otherwise obscured using traditional techniques. The forest understorey vegetation of an unmanaged reserve was recorded in permanent plots in 1979 and 2009. In only thirty years, the local species richness significantly decreased and the variation in the species composition from site to site shifted towards a structure with reduced true species turnover and increased dissimilarity due to nestedness. A classic analysis masked those patterns. In summary, we illustrated the need to move beyond the simple quantification of homogenization using classical indices and advocate integration of the multitude of ways to quantify community similarity into the homogenization framework.     

Comparative analysis of some models of gene regulation in mixed-substrate microbial growth

Mixed-substrate microbial growth is of fundamental interest in microbiology and bioengineering. Several mathematical models have been developed to account for the genetic regulation of such systems, especially those resulting in diauxic growth. In this work, we compare the dynamics of three such models (Narang, 1998a. The dynamical analogy between microbial growth on mixtures of substrates and population growth of competing species. Biotechnol. Bioeng. 59, 116-121; Thattai and Shraiman, 2003. Metabolic switching in the sugar phosphotransferase system of Escherichia coli. Biophys. J. 85(2), 744-754; Brandt et al., 2004. Modelling microbial adaptation to changing availability of substrates. Water Res. 38, 1004-1013). We show that these models are dynamically similar--the initial motion of the inducible enzymes in all the models is described by the Lotka-Volterra equations for competing species. In particular, the prediction of diauxic growth corresponds to "extinction" of one of the enzymes during the first few hours of growth. The dynamic similarity occurs because in all the models, the inducible enzymes possess properties characteristic of competing species: they are required for their own synthesis, and they inhibit each other. Despite this dynamic similarity, the models vary with respect to the range of dynamics captured. The Brandt et al. model always predicts the diauxic growth pattern, whereas the remaining two models exhibit both diauxic and non-diauxic growth patterns. The models also differ with respect to the mechanisms that generate the mutual inhibition between the enzymes. In the Narang model, mutual inhibition occurs because the enzymes for each substrate enhance the dilution of the enzymes for the other substrate. The Brandt et al. model superimposes upon this dilution effect an additional mechanism of mutual inhibition. In the Thattai and Shraiman model, the mutual inhibition is entirely due to competition for the phosphoryl groups. For quantitative agreement with the data, all models must be modified to account for specific mechanisms of mutual inhibition, such as inducer exclusion.     

Species abundance distribution and dynamics in two locally coupled communities

This study considered a model for species abundance dynamics in two local community (or islands) connected to a regional metacommunity. The model was analyzed using continuous probabilistic technique that employs Kolmogorov-Fokker-Planck forward equation to derive the probability density of the species abundance in the two local communities. Using this technique, we proposed a classification for the species abundance dynamics in the local communities. This classification was made based on such characteristics as immigration intensity, species representation in the metacommunity and the size of local communities. We further distinguished several different scenarios for species abundance dynamics using different ecological characteristics such as species persistence, extinction and monodominance in one or both local communities. The similarity of the species abundance distributions between the two local communities was studied using the correlation coefficient between species abundances in two local communities. The correlation is a function of migration rates between local communities and between local and metacommunity. Immigration between local communities drives the homogenization of the local communities, while immigration from the metacommunity will differentiate them. This community subdivision model provides useful insights for studying the effect of landscape fragmentation on species diversity.     

贵州下寒武统牛蹄塘生物群中海绵新材料

描述了贵州下寒武统牛蹄塘生物群中海绵化石1新属(Zunyispongiagen.nov.),2新种(Zunyispongiatriangulariagen.etsp.nov.,Choiafanensis.sp.nov.),通过对其形态功能的分析和讨论证实了寒武纪早期海绵动物的骨骼是由细小骨针向粗大骨针演变,骨架结构从不稳定型向稳定型发展。     

Understanding ecosystem robustness

Understanding the properties of ecosystems that make them either vulnerable or resistant to species extinctions is vital in the context of rapidly increasing habitat modification and climatic warming. New research by Sinha et al. incorporating 'small-world' topologies and population dynamics into ecosystem network models provides new explanations for species link distributions, extinction events and the maintenance of biodiversity. Their work reveals that previously unconnected ecosystems that come into contact with each other by anthropogenic or natural means are vulnerable to large losses in species.     

Sinoatrial node pacemaker cells:cardiomyocyte-or neuron-like cells?

The sinoatrial node(SAN)is the headquarter of heartbeat throughout our lifetime(Lakatta et al.,2010;Cingolani et al.,2018;Peters et al.,2020).Every beat of the heart is triggered by a bioelectric pulse spontaneously released by SAN pacemaker cells(SANPCs)(Yaniv et al.,2014;Yavari et al.,2017).In adult human heart,the SAN is a crescent-shaped structure of 1-2 cm long and 0.5 cm wide,which is located at the junction of the superior vena cava and the right atrium and lies along the sulcus terminalis(John et al.,2016).However,the nature of SANPCs remains incompletely known.In general,SANPCs have long been considered as specialized cardiomyocytes(Van Eif et al.,2018;Linscheid et al.,2019;Galang et al.,2020;).However,SANPCs do not have myofibril and T-tube,thus not sharing the contractility property of cardiomyocytes(Satoh,2003;Protze et al.,2017).Interestingly,SANPCs share some electrophysiolog-ical characteristics with neurons:excitability and conductiv-ity.In addition,SANPCs have their intrinsic autonomic rhythm,while neurons also possess the intrinsic ability to generate spontaneous electrical impulses(Lisman et al.,2018).Whether SANPCs are neuron-like cells that reside in the heart remains enigmatic in the field.     

Does functional homogenization accompany taxonomic homogenization of British birds and how do biotic factors and climate affect these processes?

Environmental change has reshuffled communities often causing taxonomic homogenization rather than differentiation. Some studies suggest that this increasing similarity of species composition between communities is accompanied by an increase in similarity of trait composition—functional homogenization—although different methodologies have failed to come to any consistent conclusions. Functional homogenization could have a large effect on ecosystem functioning and stability. Here, we use the general definition of homogenization as “reduced spatial turnover over time” to compare changes in Simpson's beta diversity (taxonomic turnover) with changes in Rao's quadratic entropy beta diversity (functional turnover) in British breeding birds at three spatial scales. Using biotic and climatic variables, we identify which factors may predispose a site to homogenization. The change in turnover measures between two time periods, 20 years apart, was calculated. A null model approach was taken to identify occurrences of functional homogenization and differentiation independent of changes in taxonomic turnover. We used conditional autoregressive models fitted using integrated nested Laplace approximations to determine how environmental drivers and factors relating to species distributions affect changes in spatial turnover of species and functional diversity. The measurement of functional homogenization affects the chance of rejection of the null models, with many sites showing taxonomic homogenization unaccompanied by functional homogenization, although occurrence varies with spatial scale. At the smallest scale, while temperature‐related variables drive changes in taxonomic turnover, changes in functional turnover are associated with variation in growing degree days; however, changes in functional turnover become more difficult to predict at larger spatial scales. Our results highlight the multifactorial processes underlying taxonomic and functional homogenization and that redundancy in species traits may allow ecosystem functioning to be maintained in some areas despite changes in species composition.     

Contingently faded heart rate biofeedback: Attempted replication of large-magnitude decreases

McKinney et al. (1980) reported large-magnitude reductions in heart rate (HR) from resting baseline levels, employing shaping and fading techniques and a reinforcement program in which a secondary reinforcer was awarded both contingently and immediately during training. The four male subjects in this group showed significantly greater HR decreases than a group of four males receiving beat-by-beat analogue HR feedback. The present study compared decreases in HR in 20 male subjects receiving the contingently faded biofeedback procedure to those shown by 10 male subjects for whom reinforcement was contingent on vigilant observation of a visual display, and independent of HR. The former group showed significantly greater decreases in HR that could not be attributed to elevated baseline levels. However, the decreases in HR were not as large as those reported by McKinney et al. (1980). It is argued that future research should assess variables contributing to individual differences in performance.This research was supported by Ontario Heart Foundation Research Grant 15–37 to R. Pavloski.     

Response of ecosystems to realistic extinction sequences

Recent research suggests that effects of species loss on the structure and functioning of ecosystems will critically depend on the order with which species go extinct. However, there are few studies of the response of natural ecosystems to realistic extinction sequences. Using an extinction scenario based on the International Union for Conservation of Nature (IUCN) Red List, de Visser et al. sequentially deleted species from a topological model of the Serengeti food web. Under this scenario, large-bodied species like top predators and mega-herbivores go extinct first. The resulting changes in the trophic structure of the food web might affect the robustness of the ecosystem to future disturbances.     

Changes in the composition of British butterfly assemblages over two decades

Changes in the abundance and distribution of individual species have been widely documented in Britain and other countries in recent decades, but little has been done to determine changes in community composition over broad geographic areas. Here, we studied species turnover in 51 butterfly assemblages in Britain since 1976, examining extinction and colonisation events together with variation in the abundances of the species. We showed that the species turnover that occurred over 20 years in Britain was associated with colonisation and extinction events and also with variability in the abundance of the species. These changes in community composition differed according to the habitat requirements of the species and their previous distributions, being more evident for habitat specialists and for southerly distributed species. Colonising species often became abundant components of the communities they joined, although this was more evident for generalist than for specialist species. The abundance of species following their arrival, increased with time since colonisation. Species turnover associated with southerly species expanding northwards is consistent with being a response to climate change. The results suggest that climate- and habitat-driven changes in the identity and abundance of species within communities are widespread, and probably ubiquitous. Similar changes are likely to be occurring in other groups of organisms that are similarly undertaking major range shifts associated with climate change.     

A stochastic model for integrating changes in species richness and community similarity across spatial scales

Human activities have elevated the extinction of natural populations as well as the invasion of new areas by non-native species. These dual processes of invasion and extinction may change the richness and similarity of communities, but the form these changes take is likely to depend on the manner in which invasions and extinctions occur and the spatial scale at which the changes are measured. Here, we explore the influence of differing patterns of extinction and invasion on the similarity and richness of a meta-community. In particular, we model simple stochastic processes analogous to realistic modes of human-mediated introduction of non-native species and range expansion by native species. We show that different modes of invasion and extinction can produce very different changes in diversity, and that the relative magnitude of these changes depends both on where in the meta-community diversity is measured and the degree of initial species aggregation. At any spatial scale of measurement, changes in the richness and similarity of communities following invasion and extinction are not necessarily strongly coupled: relatively large increases in richness may or may not also be associated with relatively large increases in similarity among communities. Thus, in real systems, the influence of human-induced invasions and extinctions on diversity will depend on both the precise mode of these processes (especially invasion), and how species populations are distributed across space.