Effects of invasive species on plant communities: an example using submersed aquatic plants at the regional scale

Submersed aquatic plants have a key role in maintaining functioning aquatic ecosystems through their effects on the hydrological regime, sedimentation, nutrient cycling and habitat of associated fauna. Modifications of aquatic plant communities, for example through the introduction of invasive species, can alter these functions. In the Sacramento-San Joaquin River Delta, California, a major invasive submersed plant, Brazilian waterweed Egeria densa, has become widespread and greatly affected the functionality of the submersed aquatic plant community. Rapid assessments of the distribution and abundance of this species are therefore crucial to direct management actions early in the season. Given the E. densa bimodal growth pattern (late spring and fall growth peaks), summer assessments of this species may indicate which and where other submersed species may occur and fall assessments may indicate where this and other species may occur in the following spring, primarily because the Delta’s winter water temperatures are usually insufficient to kill submersed aquatic plant species. We assessed community composition and distribution in the fall of 2007 and summer of 2008 using geostatistical analysis; and measured summer biomass, temperature, pH, salinity, and turbidity. In the fall of 2007, submersed aquatic plants covered a much higher proportion of the waterways (60.7%) than in the summer of 2008 (37.4%), with a significant overlap between the seasonal distribution of native and non-native species. Most patches were monospecific, and multispecies patches had significantly higher dominance by E. densa, co-occurring especially with Ceratophyllum demersum. As species richness of non-natives increased there was a significant decrease in richness of natives, and of native biomass. Sustained E. densa summer biomass negatively affected the likelihood of presence of Myriophyllum spicatum, Potamogeton crispus, and Elodea canadensis but not their biomass within patches. Depth, temperature and salinity were associated with biomass; however, the direction of the effect was species specific. Our results suggest that despite native and invasive non-native submersed plant species sharing available niches in the Delta, E. densa affects aquatic plant community structure and composition by facilitating persistence of some species and reducing the likelihood of establishment of other species. Successful management of this species may therefore facilitate shifts in existing non-native or native plant species.     

Behavioural plasticity in nest-site selection of a colonial seabird in response to an invasive carnivore

Non-native invasive species are one of the most serious threats to biodiversity and are considered the leading cause of extinction of several bird taxa, including seabirds. Introduced American mink (Neovison vison) have caused devastating effects on island populations of several colonial seabird species. In this study, we investigated the direct and indirect effects of mink on population dynamics and reproductive success of European shags (Phalacrocorax aristotelis) breeding at Illas Cíes, one of the most important colonies of the species in Southern Europe. A severe episode of mink predation on adult shags occurred in the year when mink arrived in the breeding colonies, though the number of shags killed dropped abruptly in subsequent years. We found that, after the arrival of mink, shags moved to nest-sites that afforded greater protection from carnivores. This shift caused a substantial reduction in mortality by predation, but probably entailed a cost in terms of their reproductive success because sites with lower levels of predation risk showed a higher risk of egg loss by nest flooding due to poorer drainage. Our study highlights that behavioural plasticity may allow shags to cope with invasive predators.     

Sampling error in non-invasive genetic analyses of an endangered social carnivore

Modern non-invasive genetic technologies are useful in studies of rare and difficult-to-observe species. An examination of endangered African wild dog (Lycaon pictus) faecal DNA revealed that 11.4% of samples were assigned incorrectly to an individual. Sampling mistakes in the field are not normally considered in non-invasive genetic assessments, but can be a significant source of error. To ensure meticulous data interpretation, non-invasive genetic studies should track and report sampling inaccuracies.     

Interspecific association between an ungulate and a carnivore or a primate

In the Brazilian Pantanal, we observed collared peccaries (Pecari tajacu) associating with South American coatis (Nasua nasua) 13 times and three times with black howler monkeys (Alouatta caraya). The scansorial coati drop fruit whilst feeding that both collared peccaries and other coatis consume. Young coatis were also observed chewing on the pulp left over from seeds spat out by collared peccaries who had removed the hard exocarp. Both species react to each other’s alarm calls. Peccaries also benefited from the fruits dropped by black howler monkeys. Ecologically, coatis are omnivores; but taxonomically, they are carnivores. To our knowledge, the collared peccary/coati association is the first report of an interspecific association between an ungulate and a carnivore.     

Evidence of rapid change in genetic structure and diversity during range expansion in a recovering large terrestrial carnivore

Recovery of natural populations occurs often with simultaneous or subsequent range expansions. According to population genetic theory, genetic structuring emerges at the expansion front together with decreasing genetic diversity, owing to multiple founder events. Thereupon, as the expansion proceeds and connectivity among populations is established, homogenization and a resurgence of genetic diversity are to be expected. Few studies have used a fine temporal scale combined with genetic sampling to track range expansions as they proceed in wild animal populations. As a natural experiment, the historical eradication of large terrestrial carnivores followed by their recovery and recolonization may facilitate empirical tests of these ideas. Here, using brown bear (Ursus arctos) as model species, we tested predictions from genetic theory of range expansion. Individuals from all over Finland were genotyped for every year between 1996 and 2010 using 12 validated autosomal microsatellite markers. A latitudinal shift of about 110 km was observed in the distribution and delineation of genetic clusters during this period. As the range expansion proceeded, we found, as theory predicts, that the degree of genetic structure decreased, and that both genetic variation and admixture increased. The genetic consequences of range expansions may first be detected after multiple generations, but we found major changes in genetic composition after just 1.5 generations, accompanied by population growth and increased migration. These rapid genetic changes suggest an ongoing concerted action of geographical and demographic expansion combined with substantial immigration of bears from Russia during the recovery of brown bears within the large ecosystem of northern Europe.     

Genetic differentiation and gene flow between wild and cultivated Prunus avium: An analysis of molecular genetic evidence at a regional scale

The presence of conspecific wild-type and cultivar populations has been a common landscape feature for centuries. As orchards generally continue to expand towards the natural forest, two important issues are raised: the potential reduction of cultivar genetic diversity compared to wild populations and the extent of gene flow between the two population types. These questions were addressed in a study of Prunus avium in northern Greece using nine simple sequence repeat loci to analyse genetic variation in 93 wild-type individuals and 21 cultivars representing the local cultivated germplasm. Results showed a significant reduction of genetic diversity parameters in the cultivated germplasm compared to natural populations. Bayesian, frequency-based and Markov chain – Monte Carlo analyses have revealed that the wild and cultivar groups are genetically divergent and that realized between-group gene flow is almost completely absent. This result was further verified by a principal component analysis showing a clear separation of the two groups in low multivariate space after a principal coordinate analysis. The significant disjunction in flowering time and a considerable geographic distance between the two groups could primarily account for the absence of substantial gene flow. These findings indicate that local wild cherry can provide a source of genetic variation for future breeding in the genetically restricted cultivar group.     

Monitoring an invasive perennial at the landscape scale with remote sensing

Summary   Worldwide, invasive weeds threaten agricultural, natural and urban ecosystems. In Australia's agricultural and grazing regions, invasive species often establish across extensive areas where weed management is hampered by an inability to detect the location and timing of an outbreak. In these vast landscapes, an effective detection and monitoring system is required to delineate the extent of the invasion and identify spatial and temporal factors associated with weed establishment and thickening. In this study, we utilize a time series of remote sensing imagery to detect the spatial and temporal patterns of Prickly Acacia ( Acacia nilotica ) invasion in the Mitchell grass plains of North Queensland. We develop a spectral index from Landsat images which is applied to images from 1989 to 2004, in combination with a classification mask, to identify locations and monitor changes in Prickly Acacia density across 29 000 km² of Mitchell grass plains. The approach identified spectral and temporal signatures consistent with Prickly Acacia infestation on 1.9% of this landscape. Field checking of results confirmed presence of the weed in previously unrecorded locations. The approach may be used to evaluate future spread, or outcomes of management strategies for Prickly Acacia in this landscape and could be employed to detect and monitor invasions in other extensive landscapes.     

Patterns of bryophyte diversity and rarity at a regional scale

Information on bryophyte diversity and rarity were combined withinformation on soil conditions and land use for Walloon Brabant (centralBelgium, 1091 km²) in order to investigate whichlandscape features sustain the most rare and diverse species assemblages.Presence–absence of 325 bryophyte species was recorded in 87 grid-squaresof 4 × 4 km. Species diversity was significantly correlatedwith forest cover (r = 0.71, P <0.001), sandy soils (r = 0.61, P <0.001), loamy soils, (r = –0.68,P < 0.001), and agricultural fields(r = –0.49, P < 0.001). Themost diverse grid-squares possessed up to 182 species and were characterized byat least 10% forest cover and the presence of unique micro-habitats.Grid-squares with forest cover reaching at least 10% but lacking uniquemicro-habitats contained between 90 and 130 species. Below 10% forest cover,diversity ranged between 55 and 110 species per grid-square. However, even theleast diverse cultivated areas included a significant amount of the regionallyrare species. A number of the latter are characteristic in other areas forspecific primary habitats lacking in Walloon Brabant but display an unexpectedability to disperse throughout hostile areas and colonize secondary habitats.The tendency of such species to occur in man-made habitats decreased our abilityto predict species richness and rarity from landscape features and soilconditions.     

Use of cross-taxon congruence for hotspot identification at a regional scale

One of the most debated problems in conservation biology is the use of indicator (surrogate) taxa to predict spatial patterns in other taxa. Cross-taxon congruence in species richness patterns is of paramount importance at regional scales to disclose areas of high conservation value that are significant in a broader biogeographical context but yet placed in the finer, more practical, political context of decision making. We analysed spatial patterns of diversity in six arthropod taxa from the Turkish fauna as a regional case study relevant to global conservation of the Mediterranean basin. Although we found high congruence in cross-taxon comparisons of species richness (0.241相似文献   

The effects of environmental variation on bryophytes at a regional scale

The distribution of bryophytes in central Belgium was investigated using species grid‐mapping superimposed on a series of maps which included information on soil conditions and land use. Our objectives were to assess the influence of environmental variation on the bryoflora at a regional scale, to examine how bryophytes respond to environmental variation, and to assess the extent to which species ecological and life‐history traits determine the accuracy of the predictability of species occurrence in order to provide comprehensive lists of species based on environmental conditions. The first two axes of a correspondence analysis (CA) of the floristic data explained 14.6% of the total χ². CA1 was significantly correlated with loamy‐sandy soils on a sand layer (r=?0.74, p<0.001), forest cover (r=?0.80, p<0.001), loamy soils (r=0.79, p<0.001), and agricultural fields cover (r=0.61, p<0.001). CA2 had a low but significant correlation coefficient with pebbly soils cover (r=0.38, p<0.001). The probability of occurrence of 59% of the investigated species could be significantly predicted by logistic regression from the sets of environmental variables. About 55% of the species exhibited an increasing probability of occurrence with increasing forest cover and loamy‐sandy soils cover, 1% with agricultural fields and loamy soils cover, and 3% with pebbly soils cover. The predictability of species occurrence varied as a function of four life‐history traits (minimum spore size, life expectancy, type of gametophyte and papillose leaf cell walls) and three ecological traits (indicator values of light, temperature and soil acidity). The most predictable species, including a number of leafy liverworts, were characteristic for acidic, fresh and shaded conditions and displayed a strong preference for forest habitats. Taxa with limited predictability included epiphytes and mosses characteristic of pebbly soils due to the ability of these species to efficiently disperse and adapt to various ecological conditions. Species for which the distribution range could not successfully be predicted were either ubiquitous, characteristic for ephemeral habitats, or highly successful in a very common habitat.     

Species pools and the "hump-back" model of plant species diversity: an empirical analysis at a relevant spatial scale

We investigated the relative roles of productivity, the species pool, and spatial habitat structure in determining local species richness (alpha diversity) of plant communities within a single, well-defined landscape unit, at spatial and ecological scales where the relationship between community productivity and species diversity often assumes a unimodal or "hump-back" form. At high levels of productivity, the decrease-phase of the unimodal model of the diversity-productivity relationship is typically explained as the dynamic outcome of increased competitive exclusion, but it may also be the passive consequence of a small pool of species possessing attributes necessary to competitively survive in high-fertility environments. We conducted statistical analyses of previously collected data to determine whether variations in local richness in the herbaceous vegetation of a Slovakian mountain valley were best explained by habitat productivity itself (which presumably leads to more intense competition) or by the sizes of the relevant community species pools. We also used measures of spatial habitat structure to investigate the extent to which habitat patchiness influenced patterns of species diversity. In the study system, both community biomass and size of the species pools contributed significantly to local species richness, but the positive effect of the species pools was about twice as important as the negative effect of biomass. The combined area of related associations (alliance area), association perimeter, and habitat patch geometry were all closely related to species pool size.     

Spatial and temporal heterogeneity in population dynamics of citrus aphids at a regional scale

A quick method of evaluating aphid infestations on young clementine trees is developed. It uses visual abundance classes. This method is used to compare the population dynamics of two aphid species in 11 orchards in Corsica (France). The curves describing the succession of the abundance indices on each sampling tree between April and July are compared using principal component analyses. The first three factorial axes illustrate the intensity, precocity and flatness of the infestation curves. For each species, a significant between-orchard variability is demonstrated by comparing the mean factorial coordinates of the sampling trees of the different orchards. The validity of such methods for analyzing the demographic strategies of phytophagous and entomophagous insects at a large scale is discussed.     

Global resource acquisition patterns of invasive and native plant species do not hold at the regional scale in Mediterranean type ecosystems

Invasive species may outperform native species by acquiring more resources or by efficiently using limited resources. Studies comparing leaf traits as a metric of carbon capture strategies in native and invasive species have come to different conclusions. Some studies suggest that invasive species are better at acquiring resources, but that native and invasive species use resources similarly. Other studies have found that native and invasive species differ in resource use efficiency, which implies different biochemical or physiological mechanisms of carbon capture. To resolve this debate, we examined relationships among four leaf traits (photosynthetic rate, specific leaf area, foliar nitrogen, foliar phosphorus) in co-occurring native and invasive species from eight plant communities across five Mediterranean-climate ecosystems. We performed standardized major axis regression for all trait combinations within and across sites, testing for slope homogeneity and shifts in elevation (y-intercept) or along a common slope between species groups. Across the global dataset, native and invasive species had similar carbon capture strategies (i.e., similar slopes), with invasive species occupying a position of greater resource acquisition. However, these patterns did not hold when regions were analyzed individually. Regional differences may be driven by differences in life form between native and invasive species, and variation in soil resource availability among regions. Our context-dependent results reveal not only that management of invasive species will differ across regions but also that global comparisons of invasive and native species can be misleading.     

Stipa tenacissima aerial biomass estimated at regional scale in an Algerian steppe, using geostatistical tools

Data from systematic sampling may be independent or autocorrelated. In the latter case geostatistical tools are used to identify the spatial patterns within the universe sampled. Special formulas have been derived by Russo & Bresler (1982) to estimate the variance of a value averaged over several transect samples. We applied these formulas to the green biomass of the dominant perennial steppe grass, Stipa tenacissima or alfa, in a 400 km² region in North-West Algeria; thirty years ago, this region was considered one of the best sites for alfa. A two-level sampling design was implemented with stratification of the region and systematic sampling within each stratum; globally the study included fifteen transects, representing 713 1 m² quadrats. Autocorrelation up to 500 meters was detected in five semi-variograms or correlograms, which were fitted to linear models with a sill. Biomass averaged only 165 (±55) kg ha^-1. We discuss the processes that have lead to the rapid degradation of alfa steppes in northern Algeria and the variation in spatial patterns of alfa stands. Ignoring autocorrelation in systematic sampling leads to biased estimates of variances and standard errors.     

Global genetic variation at OAS1 provides evidence of archaic admixture in Melanesian populations

Recent analysis of DNA extracted from two Eurasian forms of archaic human shows that more genetic variants are shared with humans currently living in Eurasia than with anatomically modern humans in sub-Saharan Africa. Although these genome-wide average measures of genetic similarity are consistent with the hypothesis of archaic admixture in Eurasia, analyses of individual loci exhibiting the signal of archaic introgression are needed to test alternative hypotheses and investigate the admixture process. Here, we provide a detailed sequence analysis of the innate immune gene OAS1, a locus with a divergent Melanesian haplotype that is very similar to the Denisova sequence from the Altai region of Siberia. We resequenced a 7-kb region encompassing the OAS1 gene in 88 individuals from six Old World populations (San, Biaka, Mandenka, French Basque, Han Chinese, and Papua New Guineans) and discovered previously unknown and ancient genetic variation. The 5' region of this gene has unusual patterns of diversity, including 1) higher levels of nucleotide diversity in Papuans than in sub-Saharan Africans, 2) very deep ancestry with an estimated time to the most recent common ancestor of >3 myr, and 3) a basal branching pattern with Papuan individuals on either side of the rooted network. A global geographic survey of >1,500 individuals showed that the divergent Papuan haplotype is nearly restricted to populations from eastern Indonesia and Melanesia. Polymorphic sites within this haplotype are shared with the draft Denisova genome over a span of ～90 kb and are associated with an extended block of linkage disequilibrium, supporting the hypothesis that this haplotype introgressed from an archaic source that likely lived in Eurasia.     

Evidence for the buffer effect operating in multiple species at a national scale

A long-standing aim of ecologists is to understand the processes involved in regulating populations. One such mechanism is the buffer effect, where lower quality habitats are increasingly used as a species reaches higher population densities, with a resultant average reduction in fecundity and survival limiting population growth. Although the buffer effect has been demonstrated in populations of a number of species, a test of its importance in influencing population growth rates of multiple species across large spatial scales is lacking. Here, we use habitat-specific population trends for 85 bird species from long-term national monitoring data (the UK Breeding Bird Survey) to examine its generality. We find that both patterns of population change and changes in habitat preference are consistent with the predictions of the buffer effect, providing support for its widespread operation.     

Dietary differentiation and the evolution of population genetic structure in a highly mobile carnivore

Recent studies on highly mobile carnivores revealed cryptic population genetic structures correlated to transitions in habitat types and prey species composition. This led to the hypothesis that natal-habitat-biased dispersal may be responsible for generating population genetic structure. However, direct evidence for the concordant ecological and genetic differentiation between populations of highly mobile mammals is rare. To address this we analyzed stable isotope profiles (δ(13)C and δ(15)N values) for Eastern European wolves (Canis lupus) as a quantifiable proxy measure of diet for individuals that had been genotyped in an earlier study (showing cryptic genetic structure), to provide a quantitative assessment of the relationship between individual foraging behavior and genotype. We found a significant correlation between genetic distances and dietary differentiation (explaining 46% of the variation) in both the marginal test and crucially, when geographic distance was accounted for as a co-variable. These results, interpreted in the context of other possible mechanisms such as allopatry and isolation by distance, reinforce earlier studies suggesting that diet and associated habitat choice are influencing the structuring of populations in highly mobile carnivores.