Habitat connectivity and dispersal ability drive the assembly mechanisms of macroinvertebrate communities in river networks

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Compositional changes over space and time along an occurrence–abundance continuum: anthropogenic homogenization of the North American avifauna

Aim Changes in community attributes due to the influence of anthropogenic activities have been examined primarily using occurrence data with little consideration of associated changes in abundance. To determine how this influences our perception of biotic homogenization, we examined compositional patterns for avian assemblages over space and time along an occurrence–abundance continuum. Location The contiguous United States and southern Canada. Methods We examined avian assemblages at 951 Breeding Bird Survey (BBS) routes from 1970 to 2005 that contained a total of 443 species. We used five dissimilarity indices to estimate compositional patterns along an occurrence–abundance continuum of assemblage structure (from species occurrence to transformed abundance to raw abundance) for 396,925 unique combinations of BBS route pairs. We examined annual plots of dissimilarity by distance between BBS routes pairs to estimate spatial and temporal patterns for each index. Results Dissimilarity declined with increasing distance between route pairs for occurrence and transformed abundance, reaching an asymptote at approximately 2500 km. For raw abundance, dissimilarity peaked at intermediate distances (1000–2500 km) with no evidence of an asymptote. Avian assemblages became more similar over time at all points along the continuum. Occurrence and transformed abundance presented the weakest temporal trends, which were uniform or poorly delineated as a function of distance between routes. Raw abundance presented the strongest temporal trends, which declined in strength with increasing distance between routes. Main conclusions With the addition of abundance, there was a substantial and consistent pattern of degradation of β‐diversity for North American avifauna that differed considerably from that observed from occurrence data alone. The geographical expansion of a few species, which recently benefited from the direct and indirect consequences of anthropogenic activities, probably played a prominent role in these patterns. When broad‐scale expansions in occupancy are evident, minor gains in similarity based on species occurrence can mask more substantial gains in similarity based on local abundance. When abundance information is unavailable, its role can be estimated by how occupancy has responded geographically to anthropogenic activities and the expectations of the abundance–occupancy relationship. Our findings support previous work indicating that widespread and locally abundant species will tend to benefit more from anthropogenic activities, creating a possible synergism that enhances biotic homogenization.     

Whole‐landscape modelling of compositional turnover in aquatic invertebrates informs conservation gap analysis: An example from south‐western Australia

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BIODIVERSITY RESEARCH: Conserving macroinvertebrate diversity in headwater streams: the importance of knowing the relative contributions of α and β diversity

Aim We investigated partitioning of aquatic macroinvertebrate diversity in eight headwater streams to determine the relative contributions of α and β diversity to γ diversity, and the scale dependence of α and β components. Location Great Dividing Range, Victoria, Australia. Methods We used the method of Jost (Ecology, 2007, 88, 2427–2439) to partition γ diversity into its α and β components. We undertook the analyses at both reach and catchment scales to explore whether inferences depended on scale of observation. Results We hypothesized that β diversity would make a large contribution to the γ diversity of macroinvertebrates in our dendritic riverine landscape, particularly at the larger spatial scale (among catchments) because of limited dispersal among sites and especially among catchments. However, reaches each had relatively high taxon richness and high α diversity, while β diversity made only a small contribution to γ diversity at both the reach and catchment scales. Main conclusions Dendritic riverine landscapes have been thought to generate high β diversity as a consequence of limited dispersal and high heterogeneity among individual streams, but this may not hold for all headwater stream systems. Here, α diversity was high and β diversity low, with individual headwater stream reaches each containing a large portion of γ diversity. Thus, each stream could be considered to have low irreplaceability since losing the option to use one of these sites in a representative reserve network does not greatly diminish the options available for completing the reserve network. Where limited information on individual taxonomic distributions is available, or time and money for modelling approaches are limited, diversity partitioning may provide a useful ‘first‐cut’ for obtaining information about the irreplaceability of individual streams or subcatchments when establishing representative freshwater reserves.     

Niche expansion after competitor extinction? A comparative assessment of habitat generalists and specialists in the tree floras of south‐eastern North America and south‐eastern Europe

Aim The aim of this study was to test R.H. MacArthur’s hypothesis that realized niche breadth is constrained by species pool size – the greater the number of species in a region, the more competition restricts the distribution of each species with respect to environmental tolerances and habitat characteristics. Location The northern Balkan region in south‐eastern Europe (Illyrian Floristic Province) and the southern Appalachian region of the USA. Methods We compared co‐occurrence‐based distributions of habitat specialization of tree species in two geographic regions that are ecologically similar but differ in species pool size. We applied two methods. First, we used a rank‐ordering of species along a gradient of estimated niche breadth that is based solely on species co‐occurrence information derived from vegetation databases from each region. To compare niche‐breadth distributions of different datasets we developed a procedure that standardizes expected values of species co‐occurrences independently of the size of the species pool. Second, we calculated species turnover along an elevational gradient for both regions, estimated as the rate of decay of compositional similarity with elevation distance. Results Despite a twofold larger species pool, and in contrast to our hypothesis, there was no greater specialization trend in the tree species of the southern Appalachian region, regardless of phylogenetic subgroupings or whether rare species were included. After correcting for differences in species pools, the similarity decay with elevation distance was marginally stronger in the southern Appalachian region. Main conclusions MacArthur’s hypothesis was not supported by our analysis. While the compositional distance decay with elevation revealed only a slight trend towards narrower realized niches in the tree flora of the southern Appalachian region, the co‐occurrence approach suggested the opposite. Our results indicate that species distributions are largely constrained by environmental tolerances, and that biotic pressure in the form of competition from ecologically similar species plays a relatively minor role in the ability of species to establish mature individuals in different habitat types.     

Rapid and unpredictable changes of the G‐matrix in a natural bird population over 25 years

Knowledge of the genetic variances and covariances of traits (the G ‐matrix) is fundamental for the understanding of evolutionary dynamics of populations. Despite its essential importance in evolutionary studies, empirical tests of the temporal stability of the G ‐matrix in natural populations are few. We used a 25‐year‐long individual‐based field study on almost 7000 breeding attempts of the collared flycatcher (Ficedula albicollis) to estimate the stability of the G‐matrix over time. Using animal models to estimate G for several time periods, we show that the structure of the time‐specific G‐matrices changed significantly over time. The temporal changes in the G‐matrix were unpredictable, and the structure at one time period was not indicative of the structure at the next time period. Moreover, we show that the changes in the time‐specific G‐matrices were not related to changes in mean trait values or due to genetic drift. Selection, differences in acquisition/allocation patterns or environment‐dependent allelic effects are therefore likely explanations for the patterns observed, probably in combination. Our result cautions against assuming constancy of the G ‐matrix and indicates that even short‐term evolutionary predictions in natural populations can be very challenging.     

Distance decay of similarity among European urban floras: the impact of anthropogenic activities on β diversity

Aim We examine how two categories of non‐native species (archaeophyte and neophyte, introduced before and after ad 1500, respectively) have had different impacts on β diversity across European urban floras. Our goal is to use the unique biological perspective provided by urban areas, and the contrasting historical and geographical perspectives provided by archaeophytes and neophytes, to infer how non‐native species will impact upon β diversity in the future. Location Twenty‐two urban areas located in seven European countries. Methods We used the β‐sim dissimilarity index to estimate the level of β diversity for 231 unique pair‐wise combinations of 22 urban floras. We examined bivariate plots of dissimilarity by geographical separation of city centres to evaluate distance decay of similarity for native species, archaeophytes and neophytes. Results Based on average percentages, 52.8% (SD = 8.2%) of species in the urban floras were identified as non‐native with 28.3% (SD = 6.9%) classified as neophytes and 24.5% (SD = 4.9%) as archaeophytes. Relative to native species, across urban floras, archaeophytes were associated with higher compositional similarity and weaker distance decay patterns, whereas neophytes were associated with lower compositional similarity and stronger distance decay patterns. Main conclusions Across European urban floras, archaeophytes and neophytes occurred in similar numbers but archaeophytes were consistently associated with lower β diversity and neophytes with higher β diversity. Thus, the impact of non‐native species on β diversity can be determined, at least in part, through their historical and geographical associations with anthropogenic activities. If archaeophytes represent the long‐term biogeographical outcome for human commensal species, neophytes could develop similar patterns. The consequences, however, are likely to be more substantial ecologically and geographically due to the increasing numbers of neophytes and their global anthropogenic associations. Nevertheless, at present, our findings suggest that, based on occurrence information, neophytes have not achieved this state with European urban floras retaining regionally distinct assemblages of neophytes.     

Parental and hybrid Daphnia from the D. longispina complex: long‐term dynamics in genetic structure and significance of overwintering modes

In recent decades, hybridization has become a focus of attention because of its role in evolutionary processes. However, little is known about changes in genetic structure within and between parental species and hybrids over time. Here, we studied processes of genetic change in parental species and hybrids from the Daphnia longispina complex (Crustacea, Cladocera) over a period of six years across ten habitats. These cyclical parthenogens respond to fluctuating environments by switching from asexual to sexual reproduction. Importantly, sexually produced diapausing eggs, which resist extreme conditions such as low temperatures and serve as dispersal stages, are produced to a lower extent by hybrids. Long‐term microsatellite data revealed clear differences between hybrids and parental species. In hybrids, clonal diversity values were lower, whereas heterozygosity and linkage disequilibrium values were higher compared to parental species. Clonal diversity of hybrids responded to the strength of the winter, with cold winters resulting in few genotypes in the following spring. In time windows when only asexual hybrid females survive, priority effects will favour the establishment of the hybrid offspring before hatchlings from parental diapause eggs can enter the community. The constant high levels of heterozygosity maintained by clonal reproduction in hybrids might lead to their successful establishment over time, when they are able to escape competition from both parental species. Although we found evidence that hybrids diversity depends on fluctuating environments, a direct link between hybrid abundance and the strength of winter was missing. Because of reduced adaptability in clonally reproducing hybrids, multiple factors must contribute to promoting their long‐term success in fluctuating environments.     

The control of community composition by distance,environment and history: a regional‐scale study of the mountain grasslands of southern New Zealand

Aim To examine the roles of physical and biotic environment, distance, direction and dispersal in determining the composition of plant communities at a regional scale. Location Grassland and shrubland at 266 sites in the mountains of southern South Island, New Zealand. Methods Species abundances of vascular plants, bryophytes and lichens were measured by height‐frequency. Predictors were soils, climate, mammal grazing, physical distance and direction between plots, and geological history represented by occurrence on the same or different mountain ranges. Relationships were evaluated with quadratic multiple regression and Mantel tests. Results Spatial autocorrelation was present for both vascular and lower plants. However, distance explained only a minor part of the variation. Distance relationships were anisotropic, with the vegetation differences being greatest in the direction of the prevailing south‐westerly wind. Even using all the environmental information, much of the species composition remained unexplained. For species presence, distance alone explained 7% of the variation, environment alone 25% and both together 28%. The abundance of species was even less predictable: distance 2%, environment 11% and both 12%. With vascular species, climate‐related factors contributed the most to prediction, especially altitude. Surprisingly, soil chemical factors were more important for lower plants that are not rooted in the substrate. Species likely to disperse further showed a tendency towards weaker distance differentiation. Vegetation composition seemed unrelated to contemporary grazing, although historical grazing/burning may have been causal. There was little evidence for a relationship of species composition with biogeographic/geological history. Main conclusions Difference in community composition in these grasslands is only weakly related to distance, and only half of the distance effect can be explained by the environmental factors measured. Dispersal and geological history do not seem to explain the discrepancy. Explicability of community differences was comparable to that in some previous case studies, although weaker than that in others, but all of the previous studies examined only part of the flora, such as ‘abundant trees’ or palms – none examined even all the vascular species. The species complement of a site seems more conservative than the abundance of the species, much of which was not explicable from current information and may reflect past management history.     

Fluorescence characterization of Trp 21 in rat glutathione S-transferase 1–1: Microconformational changes induced by S-hexyl glutathione

The glutathione S-transferase (GST) isoenzyme A1–1 from rat contains a single tryptophan, Trp 21, which is expected to lie within α-helix 1 based on comparison with the X-ray crystal structures of the pi- and mu-class enzymes. Steady-state and multifrequency phase/modulation fluorescence studies have been performed in order to characterize the fluorescence parameters of this tryptophan and to document ligand-induced conformational changes in this region of the protein. Addition of S-hexyl glutathione to GST isoenzyme A1–1 causes an increase in the steady-state fluorescence intensity, whereas addition of the substrate glutathione has no effect. Frequency-domain excited-state lifetime measurements indicate that Trp 21 exhibits three exponential decays in substrate-free GST. In the presence of S-hexyl glutathione, the data are also best described by the sum of three exponential decays, but the recovered lifetime values change. For the substrate-free protein, the short lifetime component contributes 9–16% of the total intensity at four wavelengths spanning the emission. The fractional intensity of this lifetime component is decreased to less than 3% in the presence of S-hexyl glutathione. Steady-state quenching experiments indicate that Trp 21 is insensitive to quenching by iodide, but it is readily quenched by acrylamide. Acrylamide-quenching experiments at several emission wavelengths indicate that the long-wavelength components become quenched more easily in the presence of S-hexyl glutathione. Differential fluorescence polarization measurements also have been performed, and the data describe the sum of two anisotropy decay rates. The recovered rotational correlation times for this model are 26 ns and 0.81 ns, which can be attributed to global motion of the protein dimer, and fast local motion of the tryptophan side chain. These results demonstrate that regions of GST that are not in direct contact with bound substrates are mobile and undergo microconformational rearrangement when the “H-site” is occupied.     

Trait variations of ground flora species disentangle the effects of global change and altered land‐use in Swedish forests during 20 years

Northern forest ecosystems are exposed to a range of anthropogenic processes including global warming, atmospheric deposition, and changing land‐use. The vegetation of northern forests is composed of species with several functional traits related to these processes, whose effects may be difficult to disentangle. Here, we combined analyses of spatio‐temporal dynamics and functional traits of ground flora species, including morphological characteristics, responses to macro‐ and microclimate, soil conditions, and disturbance. Based on data from the Swedish National Forest Inventory, we compared changes in occurrence of a large number of ground flora species during a 20‐year period (1994–2013) in boreal and temperate Sweden respectively. Our results show that a majority of the common ground flora species have changed their overall frequency. Comparisons of functional traits between increasing and declining species, and of trends in mean trait values of sample plots, indicate that current floristic changes are caused by combined effects of climate warming, nitrogen deposition and changing land‐use. Changes and their relations with plant traits were generally larger in temperate southern Sweden. Nutrient‐demanding species with mesotrophic morphology were favored by ongoing eutrophication due to nitrogen deposition in the temperate zone, while dwarf shrubs with low demands on nitrogen decreased in frequency. An increase of species with less northern and less eastern distribution limits was also restricted to temperate Sweden, and indicates effects of a moister and milder macroclimate. A trend toward dense plantation forests is mirrored by a decrease of light‐demanding species in both vegetation zones, and a decrease of grassland species in the temperate zone. Although denser tree canopies may buffer effects of a warmer climate and of nitrogen deposition to some extent, traits related to these processes were weakly correlated in the group of species with changing frequency. Hence, our results indicate specific effects of these often confounded anthropogenic processes.     

It is about time: genetic variation in the timing of leaf‐litter inputs influences aquatic ecosystems

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Not everything is everywhere: the distance decay of similarity in a marine host–parasite system

Aim We test the similarity–distance decay hypothesis on a marine host–parasite system, inferring the relationships from abundance data gathered at the lowest scale of parasite community organization (i.e. that of the individual host). Location Twenty‐two seasonal samples of the bogue Boops boops (Teleostei: Sparidae) were collected at seven localities along a coastal positional gradient from the northern North‐East Atlantic to the northern Mediterranean coast of Spain. Methods We used our own, taxonomically consistent, data on parasite communities. The variations in parasite composition and structure with geographical and regional distance were examined at two spatial scales, namely local parasite faunas and component communities, using both presence–absence (neighbour joining distance) and abundance (Mahalanobis distance) data. The influence of geographical and regional distance on faunal/community divergence was assessed through the permutation of distance matrices. Results Our results revealed that: (1) geographical and regional distances do not affect the species composition in the system under study at the higher scales; (2) geographical distance between localities contributes significantly to the decay of similarity estimated from parasite abundance at the lowest scale (i.e. the individual host); (3) the structured spatial patterns are consistent in time but not across seasons; and (4) a restricted clade of species (the ‘core’ species of the bogue parasite fauna) contributes substantially to the observed patterns of both community homogenization and differentiation owing to the strong relationship between local abundance and regional distribution of species. Main conclusions The main factors that tend to homogenize the composition of parasite communities of bogue at higher regional scales are related to the dispersal of parasite colonizers across host populations, which we denote as horizontal neighbourhood colonization. In contrast, the spatial structure detectable in quantitative comparisons only, is related to a vertical neighbourhood colonization associated with larval dispersal on a local level. The stronger decline with distance in the spatial synchrony of the assemblages of the ‘core’ species indicates a close‐echoing environmental synchrony that declines with distance. Our results emphasize the importance of the parasite supracommunity (i.e. parasites that exploit all hosts in the ecosystem) to the decay of similarity with distance.     

Development of modern benthic ecosystems in eutrophic coastal oceans: The foraminiferal record over the last 200 years,Osaka Bay,Japan

The ecosystem dynamics of a modern benthic community in Osaka Bay was studied by analyzing sediment cores and fossil foraminifera deposited during the past 200 years. The results suggest that the high-density/low-diversity assemblage has appeared in the early 1900s, coinciding with the eutrophication of the bay resulting from the Japanese industrial revolution. This assemblage proliferated during the period 1960 to 1970 when the eutrophication and bottom-water hypoxia were most pronounced. The development of the assemblage has been characterized by an increase in the relative and absolute abundance of eutrophication-tolerant species (Ammonia beccarii, Eggerella advena, and Trochammina hadai) and a decrease in many other foraminiferal species, such as Ammonia tepida, Elphidium, Miliolinella subrotunda, and Valvulineria hamanakoensis, that are unable to tolerate low-oxygen conditions. Approximately thirty years after the imposition of discharge restrictions in the 1970s, this assemblage continues to predominate in the inner part of the bay, and E. advena is currently found across the entire bay. These records make a significant contribution to understanding the long-term relationship between anthropogenic impact and ecosystem change.