Segregation,nestedness and homogenisation in plant communities dominated by native and alien species

ABSTRACT

Background: Highly modified landscapes offer the opportunity to assess how environmental factors influence the integration of alien plant species into native vegetation communities and determine the vulnerability of different communities to invasion.

Aims: To examine the importance of biotic and abiotic drivers in determining whether alien plant species segregate spatially from native plant communities or become integrated and lead to biotic homogenisation.

Methods: Ordination and classification of a floristic survey of over 1200 systematically located 6 m × 6 m plots were used to examine how plant community segregation, nestedness and homogenisation varied in relation to climate, environmental and human-related factors across Banks Peninsula, New Zealand.

Results: The analyses of community structure indicated that native and alien plant communities were spatially and ecologically segregated due to different responses primarily to an anthropogenic impact gradient and secondly to environmental factors along an elevation gradient. Human-land use appeared most strongly linked to the distribution of alien species and was associated with increased vegetation homogenisation. However, despite spatial segregation of alien and native plant communities, biotic homogenisation not only occurred in highly managed grasslands but also in relatively less managed shrublands and forest.

Conclusions: The role played by anthropogenic factors in shaping alien and native plant species community structure should not be ignored and, even along a marked environmental gradient, if the recipient sites have a long history of human-related disturbance, biotic homogenisation is often strong.     

Flowering phenology of invasive alien plant species compared with native species in three Mediterranean-type ecosystems

Background and Aims

Flowering phenology is a potentially important component of success of alien species, since elevated fecundity may enhance invasiveness. The flowering patterns of invasive alien plant species and related natives were studied in three regions with Mediterranean-type climate: California, Spain and South Africa''s Cape region.

Methods

A total of 227 invasive–native pairs were compared for seven character types across the regions, with each pair selected on the basis that they shared the same habitat type within a region, had a common growth form and pollination type, and belonged to the same family or genus.

Key Results

Invasive alien plant species have different patterns of flowering phenology from native species in the three regions. Whether the alien species flower earlier, later or at the same time as natives depends on the climatic regime in the native range of the aliens and the proportion of species in the invasive floras originating from different regions. Species invading at least two of the regions displayed the same flowering pattern, showing that flowering phenology is a conservative trait. Invasive species with native ranges in temperate climates flower earlier than natives, those from Mediterranean-type climates at the same time, and species from tropical climates flower later. In California, where the proportion of invaders from the Mediterranean Basin is high, the flowering pattern did not differ between invasive and native species, whereas in Spain the high proportion of tropical species results in a later flowering than natives, and in the Cape region early flowering than natives was the result of a high proportion of temperate invaders.

Conclusions

Observed patterns are due to the human-induced sympatry of species with different evolutionary histories whose flowering phenology evolved under different climatic regimes. The severity of the main abiotic filters imposed by the invaded regions (e.g. summer drought) has not been strong enough (yet) to shift the flowering pattern of invasive species to correspond with that of native relatives. It does, however, determine the length of the flowering season and the type of habitat invaded by summer-flowering aliens. Results suggest different implications for impacts at evolutionary time scales among the three regions.Key words: Biological invasions, flowering phenology, genetic inertia, Cape Floristic Region, California, Spain, Mediterranean-type ecosystems, water availability, climatic origin     

Invasions of alien gammarid species and retreat of natives in the Vistula Lagoon (Baltic Sea, Poland)

During the last decades of the twentieth century, the alien gammarid species Gammarus tigrinus, Dikerogammarus haemobaphes, Pontogammarus robustoides and Obesogammarus crassus invaded the lower Vistula River and its deltaic, partly brackish regions. In brackish waters of the Vistula Lagoon the native Atlantic-boreal species Gammarus zaddachi and Gammarus duebeni have been replaced or at least outnumbered by the aliens. As compared to our earlier studies, through the years 1998–2004 we could observe nearly total decline of the native gammarid populations along the coasts of the Lagoon, and overdomination of the North-American G. tigrinus in most places. Possible reasons for the observed phenomena are e.g. increasing pollution and eutrophication of the Lagoon accompanied by competition between the native and the alien species.     

Irradiance acclimation,capture ability,and efficiency in invasive and non-invasive alien plant species

We tested the hypothesis that invasive (IN) species could capture resources more rapidly and efficiently than noninvasive (NIN) species. Two IN alien species, Ageratina adenophora and Chromolaena odorata, and one NIN alien species, Gynura sp. were compared at five irradiances. Photon-saturated photosynthetic rate (P _max), leaf mass (LMA) and nitrogen content (N_A) per unit area, and photosynthetic nitrogen utilization efficiency (PNUE) increased significantly with irradiance. LMA, N_A, and PNUE all contributed to the increased P _max, indicating that both morphological and physiological acclimation were important for the three alien species. Under stronger irradiance, PNUE was improved through changes in N allocation. With the increase of irradiance, the amount of N converted into carboxylation and bioenergetics increased, whereas that allocated to light-harvesting components decreased. The three alien species could adequately acclimate to high irradiance by increasing the ability to utilize and dissipate photon energy and decreasing the efficiency of photon capture. The two IN species survived at 4.5 % irradiance while the NIN species Gynura died, representing their different invasiveness. Ageratina generally exhibited higher respiration rate (R _D) and N_A. However, distinctly higher P _max, PNUE, P _max/R _D, or P _max/LMA were not detected in the two invasive species, nor was lower LMA. Hence the abilities to capture and utilize resources were not always associated with invasiveness of the alien species.     

Positive interactions, discontinuous transitions and species coexistence in plant communities

The population and community level consequences of positive interactions between plants remain poorly explored. In this study we incorporate positive resource-mediated interactions in classic resource competition theory and investigate the main consequences for plant population dynamics and species coexistence. We focus on plant communities for which water infiltration rates exhibit positive dependency on plant biomass and where plant responses can be improved by shading, particularly under water limiting conditions. We show that the effects of these two resource-mediated positive interactions are similar and additive. We predict that positive interactions shift the transition points between different species compositions along environmental gradients and that realized niche widths will expand or shrink. Furthermore, continuous transitions between different community compositions can become discontinuous and bistability or tristability can occur. Moreover, increased infiltration rates may give rise to a new potential coexistence mechanism that we call controlled facilitation.     

Livestock reduces juvenile tree growth of alien invasive species with a minimal effect on natives: a field experiment using exclosures

Many alien invasive tree species were originally introduced to their non-native ranges for use in forestry and as urban trees. These alien species were selected for their fast growth and not necessarily for possessing mechanisms which deter browsing. Instead, many tree species native to semiarid areas of the world evolved mechanisms which deter browsing, presumably at the cost of slower growth. In a semiarid rangeland we observed that livestock exclusion greatly promoted the growth of juveniles of several alien species but not of native species, and we hypothesized that this increase in growth of aliens was due to livestock preference for alien and not native trees. With the objective of quantifying our observations and understanding the mechanism underlying the increased growth rates of alien juvenile trees under livestock exclusion, we assessed growth and browsing levels in juveniles of two alien invasive and four abundant native tree species within three parcels where livestock was excluded and three parcels with livestock at 0.20 cattle equivalents.ha^?1. Alien species grew around four-fold faster under livestock exclusion than with livestock and, as predicted, received five times more browsing than natives. Instead, native species did not significantly increase their growth rate with livestock exclusion. The results support our hypothesis and the implications for management would be that stocking paddocks with livestock to browse existing alien juveniles and re-growth of felled adults should be effective in delaying invasions of trees used for forestry without significantly affecting the growth of the most abundant native trees.     

Ecological effects of invasive alien species on native communities,with particular emphasis on the interactions between aphids and ladybirds

The ecological effects of introduced species on native organisms can sometimes, but not always be significant. The risks associated with invasive alien pests are difficult to quantify. This paper concentrates on the ecological effects of invasive insect predators that feed on pest insects, because the former may potentially affect the biological control of the latter. The literature indicates that invasive predatory insects generally are resistant to changes in environmental conditions, long-lived and voracious with a high reproductive rate, high dispersal ability, able to spread very rapidly across landscapes and exhibit phenotypic plasticity. Their colonization of patches of prey may induce native predators to leave, but the evidence that invaders negatively affect the abundance of the native species is scarce and not persuasive. Insect predators do not substantially affect the abundance of their prey, if the ratio of generation time of the predator to that of the prey is large (the generation time ratio hypothesis), therefore the effect of an invasion by long-lived alien predators on systems consisting of long-lived native predators and short-lived prey on the abundance of the prey is hard to detect.     

Dominant species,rather than diversity,regulates temporal stability of plant communities

A growing body of empirical evidence suggests that the temporal stability of communities typically increases with diversity. The counterview to this is that dominant species, rather than diversity itself, might regulate temporal stability. However, empirical studies that have explicitly examined the relative importance of diversity and dominant species in maintaining community stability have yielded few clear-cut patterns. Here, using a long-term data set, we examined the relative importance of changes in diversity components and dominance hierarchy following the removal of a dominant C4 grass, Bouteloua gracilis, in stabilizing plant communities. We also examined the relationships between the variables of diversity and dominance hierarchy and the statistical components of temporal stability. We found a significant negative relationship between temporal stability and species richness, number of rare species, and relative abundance of rare species, whereas a significant positive relationship existed between temporal stability and relative abundance of the dominant species. Variances and covariances summed over all species significantly increased with increasing species richness, whereas they significantly decreased with increasing relative abundance of dominant species. We showed that temporal stability in a shortgrass steppe plant community was controlled by dominant species rather than by diversity itself. The generality of diversity–stability relationships might be restricted by the dynamics of dominant species, especially when they have characteristics that contribute to stability in highly stochastic systems. A clear implication is that dominance hierarchies and their changes might be among the most important ecological components to consider in managing communities to maintain ecosystem functioning.     

Resource heterogeneity, soil fertility, and species diversity: effects of clonal species on plant communities

Spatial heterogeneity in soil resources is widely thought to promote plant species coexistence, and this mechanism figures prominently in resource-ratio models of competition. However, most experimental studies have found that nutrient enhancements depress diversity regardless of whether nutrients are uniformly or heterogeneously applied. This mismatch between theory and empirical pattern is potentially due to an interaction between plant size and the scale of resource heterogeneity. Clonal plants that spread vegetatively via rhizomes or stolons can grow large and may integrate across resource patches, thus reducing the positive effect of small-scale resource heterogeneity on plant species richness. Many rhizomatous clonal species respond strongly to increased soil fertility, and they have been hypothesized to drive the descending arm of the hump-shaped productivity-diversity relationship in grasslands. We tested whether clonals reduce species richness in a grassland community by manipulating nutrient heterogeneity, soil fertility, and the presence of rhizomatous clonal species in a 6-year field experiment. We found strong and consistent negative effects of clonals on species richness. These effects were greatest at high fertility and when soil resources were applied at a scale at which rhizomatous clonals could integrate across resource patches. Thus, we find support for the hypothesis that plant size and resource heterogeneity interact to determine species diversity.     

濒危植物永瓣藤所在群落物种多样性初步研究

永瓣藤(Monimopetalum chinense)是卫矛科(Celastraceae)的一个中国特有单型属植物,被列为国家二级稀有濒危保护物种。以11个样地调查资料为基础,从物种丰富度指数、物种多样性指数和均匀度指数等方面对濒危植物永瓣藤所在群落物种多样性进行了分析。结果表明:永瓣藤所在群落物种多样性较高,植物群落结构复杂,物种丰富;从平均值来看,物种丰富度指数和物种多样性指数灌木层>草本层>乔木层,物种均匀度指数草本层>灌木层>乔木层;乔木层物种贫乏,物种多样性较低,优势树种突出;永瓣藤受人类活动干扰较大,森林的乱砍滥伐和旅游的不合理开发给该物种带来严重的威胁。     

Light interception in species with different functional groups coexisting in moorland plant communities

Competition for light is one of the most essential mechanisms affecting species composition. It has been suggested that similar light acquisition efficiency (Φ_mass, absorbed photon flux per unit aboveground mass) may contribute to species coexistence in multi-species communities. On the other hand, it is known that traits related with light acquisition vary among functional groups. We studied whether Φ_mass was similar among species with different functional groups coexisting in moorland communities. We conducted stratified clipping in midsummer when the stand biomass reached a maximum. Light partitioning among species was estimated using a model accounting for both direct and diffuse light. Evergreen species were found to have a significantly lower Φ_mass than deciduous species, which resulted from their lower absorbed photon flux per unit leaf area and lower specific leaf area. Shrubs had a smaller leaf mass fraction, but their Φ_mass was not lower than that of herbs because they had a higher leaf position due to the presence of wintering stems. Species with vertical leaves had a higher Φ_mass than those with horizontal leaves despite vertical leaves being a decided disadvantage in terms of light absorption. This higher Φ_mass was achieved by a greater leaf height in species with vertical leaves. Our results clearly demonstrate that light acquisition efficiency was different among the functional groups. However, the trend observed is not necessarily the same as that expected based on prior knowledge, suggesting that disadvantages in some traits for light acquisition efficiency are partly compensated for by other traits.     

Fluazifop, a grass-selective herbicide which inhibits acetyl-CoA carboxylase in sensitive plant species.

Fluazifop is a grass-selective herbicide that appears to act by inhibiting fatty acid synthesis de novo in sensitive species. Results from four different types of experiment show that this inhibition is due to an action of fluazifop on acetyl-CoA carboxylase and not on fatty acid synthetase. The acetyl-CoA carboxylase from sensitive barley (Hordeum vulgare), but not from resistant pea (Pisum sativum), is inhibited by the R stereoisomer, a finding that agrees with the herbicidal specificity of fluazifop.     

Impact of plant functional group, plant species, and sampling time on the composition of nirK-type denitrifier communities in soil

We studied the influence of eight nonleguminous grassland plant species belonging to two functional groups (grasses and forbs) on the composition of soil denitrifier communities in experimental microcosms over two consecutive years. Denitrifier community composition was analyzed by terminal restriction fragment length polymorphism (T-RFLP) of PCR-amplified nirK gene fragments coding for the copper-containing nitrite reductase. The impact of experimental factors (plant functional group, plant species, sampling time, and interactions between them) on the structure of soil denitrifier communities (i.e., T-RFLP patterns) was analyzed by canonical correspondence analysis. While the functional group of a plant did not affect nirK-type denitrifier communities, plant species identity did influence their composition. This effect changed with sampling time, indicating community changes due to seasonal conditions and a development of the plants in the microcosms. Differences in total soil nitrogen and carbon, soil pH, and root biomass were observed at the end of the experiment. However, statistical analysis revealed that the plants affected the nirK-type denitrifier community composition directly, e.g., through root exudates. Assignment of abundant T-RFs to cloned nirK sequences from the soil and subsequent phylogenetic analysis indicated a dominance of yet-unknown nirK genotypes and of genes related to nirK from denitrifiers of the order Rhizobiales. In conclusion, individual species of nonleguminous plants directly influenced the composition of denitrifier communities in soil, but environmental conditions had additional significant effects.     

Resource availability, species composition and sown density effects on productivity of experimental plant communities

Productivity of artificial grassland communities was investigated in a 6-year field experiment on the Qinghai-Tibetan Plateau. In the experiment, assemblages varying in seven species compositions and four density gradients were grown in fertilized and non-fertilized subplots. We measured biomass of sown species as an indicator of community productivity. In general, 6-years of experiments indicated that: (i) species composition had a significant influence on community productivity. During the initial phase of the experiment, sown density significantly affected community productivity, but the effects disappear with the increase of grown years. This productivity increased with biodiversity increase and fertilization, while the biodiversity effects disappeared when the influence of composition was removed. (ii) The increase of community productivity with biodiversity was resulted from joint effects of selection and complementarity. (iii) With an increase of growth time, the selection effects become weaker while complementarities become enhanced. Influence of density on both effects was significantly different in early stages, but ultimately this all became insignificant. Fertilization dramatically increased the complementarity effects in all experiment processes, but had different influences on selection effects during different experimental period.     

Defining critical habitat for plant species with poor occurrence knowledge and identification of critical habitat networks

We applied spatial distribution and reserve selection modeling to identify critical habitat network for 50 critically endangered plant species of Uzbekistan. We created maps of relative habitat suitability, converted them into maps of species critical habitat, and the latter served as input to the spatial conservation prioritization program Zonation. Although all studied species were extremely rare, the extent of their predicted critical habitat ranged from 2.6 to 15,508 km² and did not correlate with the number of occurrence records. It appears that the extent of the threatened species predicted suitable area is an indicator of whether the species rarity is inherent due to vary narrow ecological niche or caused solely by anthropogenic activity. This has important conservation implications. Imperiled species from the first category are less amenable (if at all) to such conservation actions as trans- or relocation (i.e. there is no alternative to protecting the last remaining populations). Species from the second group can potentially grow in many more locations than they currently occupy, and therefore, for these species, translocation can be the most appropriate strategy when their last remaining locations are degraded or can not be protected for whatever reason. Based on results of nature reserve design that utilized SDM-predicted maps for 50 imperiled species, we recommend adding new areas to the existing network of protected areas to cover critical habitats of highly threatened plant species. The highest priority for adding have the territories harboring critical habitat of disproportionally high number of imperiled species, and we found that they all are concentrated in the eastern part of Uzbekistan.

     

Gap pattern and colonization opportunities in plant communities: effects of species richness, mortality, and spatial aggregation

We investigate how perturbations that induce mortality transform original spatial patterns in plant communities into binary spatial patterns of survivors and perished individuals. By means of computer simulation, we analyse effects of average mortality, interspecific variation of mortality around the mean, spatial distribution of the species (clumping degree), and species richness. Gap spatial pattern is quantified by four spatial indices or landscape metrics (gap area, density, shape and coherence). In single‐species communities, the emerging gap patterns are subject to critical phenomena: opportunities for colonizers to establish increase with mortality, but more rapidly at specific mortality thresholds. In multi‐species communities, neither species richness nor interspecific variation of mortality influences gap spatial pattern when community assembly is random. Colonization opportunities would therefore not be affected by local species extinction in such a system, nor by the presence of species with divergent sensitivities to perturbation. In a community that is highly spatially aggregated, increases in interspecific mortality variation shift the pattern towards fewer gaps that are larger and more isodiametric, which suggests increased establishment chances for colonizers. Similar changes are induced in communities characterized by large interspecific mortality differences if clumping degree is increased. Loss of species richness only modifies gap spatial pattern to a substantial extent if mortality variation is high: in this case, depauperate communities exhibit a wider variety of colonization opportunities (more gaps which are on average smaller, but the largest gap is larger) than species‐richer ones. These findings may explain the contrast between the negative diversity‐invasibility relationship often found in small‐scale experimental studies and the positive diversity‐invasibility relationship found in observational studies at larger scale. They also demonstrate that the pre‐disturbance spatial structure of a community significantly affects colonization opportunities for alien species, and is therefore a likely determinant of the trajectory of secondary succession following perturbation.     

Relationships between leaf morphological traits, nutrient concentrations and isotopic signatures for Mediterranean woody plant species and communities

Background and aims

Soil factors are driving forces that influence spatial distribution and functional traits of plant species. We test whether two anchor morphological traits—leaf mass per area (LMA) and leaf dry matter content (LDMC)—are significantly related to a broad range of leaf nutrient concentrations in Mediterranean woody plant species. We also explore the main environmental filters (light availability, soil moisture and soil nutrients) that determine the patterns of these functional traits in a forest stand.

Methods

Four morphological and 19 chemical leaf traits (macronutrients and trace elements and δ¹³C and δ¹⁵N signatures) were analysed in 17 woody plant species. Community-weighted leaf traits were calculated for 57 plots within the forest. Links between LMA, LDMC and other leaf traits were analysed at the species and the community level using standardised major axis (SMA) regressions

Results

LMA and LDMC were significantly related to many leaf nutrient concentrations, but only when using abundance-weighted values at community level. Among-traits links were much weaker for the cross-species analysis. Nitrogen isotopic signatures were useful to understand different resource-use strategies. Community-weighted LMA and LDMC were negatively related to light availability, contrary to what was expected.

Conclusion

Community leaf traits have parallel shifts along the environmental factors that determine the community assembly, even though they are weakly related across individual taxa. Light availability is the main environmental factor determining this convergence of the community leaf traits.     

The Goldilocks effect: intermittent streams sustain more plant species than those with perennial or ephemeral flow

1. Drylands worldwide are typified by extreme variability in hydrologic processes, which structures riparian communities at various temporal and spatial scales. One key question is how underlying differences in hydrology over the length of interrupted perennial rivers influence spatial and temporal patterns in species richness and species composition. 2. We examined effects of differences in dry season hydrology on species richness, composition and cover of herbaceous plant communities in the streamside zone (the zone influenced directly by low flows in the channel). Data were collected at ephemeral, intermittent and perennial flow reaches on three rivers of the desert Southwest (Arizona, U.S.A.): Lower Cienega Creek, Hassayampa River and Lower San Pedro River. 3. Patterns of species richness varied with temporal scale of analysis, that is between single‐year and multi‐year time frames. At the annual timescale, quadrat species richness (m^?2) and herbaceous cover were higher at sites with perennial flow than at either intermittent or ephemeral sites. In contrast to this single‐year pattern, the highest long‐term richness occurred at intermittent sites. 4. Quadrat species richness, total species richness at a site (per 18 1‐m² plots) and cover were more variable year to year at non‐perennial sites than at perennial flow sites. On two of the three rivers, ephemeral sites had the highest inter‐annual compositional variance, while the perennial sites had the lowest. 5. Compositional differences between the hydrologic site types were dominated by species turnover, not nestedness. The perennial sites had more wetland and perennial species than the other two site types. The intermittent sites had more annual species than did the other two types. 6. High long‐term species richness and distinct species composition of intermittent sites are probably sustained by pronounced temporal variability in environmental conditions (i.e. frequent and persistent flow events, and dry periods). Plants at these sites take advantage of greater moisture than those at ephemeral sites and also experience less competition from resident species than those at perennial sites. 7. Conservation of desert riparian diversity depends upon the protection of consistently wet conditions at perennial flow sites, as well as the maintenance of the processes that cause fluctuations in environmental conditions at non‐perennial sites.