Processes of community assembly in an environmentally heterogeneous,high biodiversity region

Despite decades of study, the relative importance of niche‐based versus neutral processes in community assembly remains largely ambiguous. Recent work suggests niche‐based processes are more easily detectable at coarser spatial scales, while neutrality dominates at finer scales. Analyses of functional traits with multi‐year multi‐site biodiversity inventories may provide deeper insights into assembly processes and the effects of spatial scale. We examined associations between community composition, species functional traits, and environmental conditions for plant communities in the Kouga‐Baviaanskloof region, an area within South Africa's Cape Floristic Region (CFR) containing high α and β diversity. This region contains strong climatic gradients and topographic heterogeneity, and is comprised of distinct vegetation classes with varying fire histories, making it an ideal location to assess the role of niche‐based environmental filtering on community composition by examining how traits vary with environment. We combined functional trait measurements for over 300 species with observations from vegetation surveys carried out in 1991/1992 and repeated in 2011/2012. We applied redundancy analysis, quantile regression, and null model tests to examine trends in species turnover and functional traits along environmental gradients in space and through time. Functional trait values were weakly associated with most spatial environmental gradients and only showed trends with respect to vegetation class and time since fire. However, survey plots showed greater compositional and functional stability through time than expected based on null models. Taken together, we found clear evidence for functional distinctions between vegetation classes, suggesting strong environmental filtering at this scale, most likely driven by fire dynamics. In contrast, there was little evidence of filtering effects along environmental gradients within vegetation classes, suggesting that assembly processes are largely neutral at this scale, likely the result of very high functional redundancy among species in the regional species pool.     

A dynamic multi‐scale occupancy model to estimate temporal dynamics and hierarchical habitat use for nomadic species

Distribution models are increasingly being used to understand how landscape and climatic changes are affecting the processes driving spatial and temporal distributions of plants and animals. However, many modeling efforts ignore the dynamic processes that drive distributional patterns at different scales, which may result in misleading inference about the factors influencing species distributions. Current occupancy models allow estimation of occupancy at different scales and, separately, estimation of immigration and emigration. However, joint estimation of local extinction, colonization, and occupancy within a multi‐scale model is currently unpublished. We extended multi‐scale models to account for the dynamic processes governing species distributions, while concurrently modeling local‐scale availability. We fit the model to data for lark buntings and chestnut‐collared longspurs in the Great Plains, USA, collected under the Integrated Monitoring in Bird Conservation Regions program. We investigate how the amount of grassland and shrubland and annual vegetation conditions affect bird occupancy dynamics and local vegetation structure affects fine‐scale occupancy. Buntings were prevalent and longspurs rare in our study area, but both species were locally prevalent when present. Buntings colonized sites with preferred habitat configurations, longspurs colonized a wider range of landscape conditions, and site persistence of both was higher at sites with greener vegetation. Turnover rates were high for both species, quantifying the nomadic behavior of the species. Our model allows researchers to jointly investigate temporal dynamics of species distributions and hierarchical habitat use. Our results indicate that grassland birds respond to different covariates at landscape and local scales suggesting different conservation goals at each scale. High turnover rates of these species highlight the need to account for the dynamics of nomadic species, and our model can help inform how to coordinate management efforts to provide appropriate habitat configurations at the landscape scale and provide habitat targets for local managers.     

Grazing effects on the species‐area relationship: Variation along a climatic gradient in NE Spain

Questions: Does grazing have the same effect on plant species richness at different spatial scales? Does the effect of spatial scale vary under different climatic conditions and vegetation types? Does the slope of the species‐area curve change with grazing intensity similarly under different climatic conditions and vegetation types? Location: Pastures along a climatic gradient in northeastern Spain. Methods: In zones under different regimes of sheep grazing (high‐, low‐pressure, abandonment), plant species richness was measured in different plot sizes (from 0.01 to 100 m2) and the slope of the species‐area curves was calculated. The study was replicated in five different locations along a climatic gradient from lowland semi‐arid rangelands to upland moist grasslands. Results: Species richness tended to increase with grazing intensity at all spatial scales in the moist upland locations. On the contrary, in the most arid locations, richness tended to decrease, or remain unchanged, with grazing due to increased bare soil. Grazing differentially affected the slope (z) of the species‐area curve (power function S=c A^z) in different climatic conditions: z tended to increase with grazing in arid areas and decrease in moist‐upland ones. ß‐diversity followed similar pattern as z. Conclusions: Results confirm that the impact of grazing on plant species richness are spatial‐scale dependent. However, the effects on the species‐area relationship vary under different climatic conditions. This offers a novel insight on the patterns behind the different effects of grazing on diversity in moist vs. arid conditions reported in the literature. It is argued that the effect of spatial scale varies because of the different interaction between grazing and the intrinsic spatial structure of the vegetation. Variations in species‐area curves with grazing along moisture gradients suggest also a different balance of spatial components of diversity (i.e. a‐ and ß‐diversity).     

Preadapted for invasiveness: do species traits or their plastic response to shading differ between invasive and non‐invasive plant species in their native range?

Aim Species capable of vigorous growth under a wide range of environmental conditions should have a higher chance of becoming invasive after introduction into new regions. High performance across environments can be achieved either by constitutively expressed traits that allow for high resource uptake under different environmental conditions or by adaptive plasticity of traits. Here we test whether invasive and non‐invasive species differ in presumably adaptive plasticity. Location Europe (for native species); the rest of the world and North America in particular (for alien species). Methods We selected 14 congeneric pairs of European herbaceous species that have all been introduced elsewhere. One species of each pair is highly invasive elsewhere in the world, particularly so in North America, whereas the other species has not become invasive or has spread only to a limited degree. We grew native plant material of the 28 species under shaded and non‐shaded conditions in a common garden experiment, and measured biomass production and morphological traits that are frequently related to shade tolerance and avoidance. Results Invasive species had higher shoot–root ratios, tended to have longer leaf‐blades, and produced more biomass than congeneric non‐invasive species both under shaded and non‐shaded conditions. Plants responded to shading by increasing shoot–root ratios and specific leaf area. Surprisingly, these shade‐induced responses, which are widely considered to be adaptive, did not differ between invasive and non‐invasive species. Main conclusions We conclude that high biomass production across different light environments pre‐adapts species to become invasive, and that this is not mediated by plasticities of the morphological traits that we measured.     

A test for a cost of opportunism in invasive species in the Commelinaceae

Many invasive species can respond opportunistically to favorable growing conditions. In a previous work, we found that invasive species in the family Commelinaceae were more opportunistic than their noninvasive congeners and could therefore outperform noninvasive relatives in an environment with abundant resources and no competition. Contrary to the expectation that superior performance under favorable conditions comes at the cost of reduced performance under stressful conditions, invasive species did not perform more poorly relative to noninvasive congeners under any conditions we examined. Here we expand our search for potential costs of opportunism in invasive species to additional environmental conditions in which invasive taxa have been shown or predicted to perform poorly. We grew four invasive and four noninvasive species in environments consisting of all possible combinations of high and low soil resources and presence and absence of clipping (removal of aboveground biomass). We also fed leaves of each species to a generalist herbivore to assess resistance to herbivory. We found that the advantage of invasive species is reduced but not eliminated by low soil resources and clipping. At low soil resources, invasive species produced softer leaves than noninvasive species and might therefore be less resistant to generalist herbivory than noninvasive species, although a direct comparison of resistance in a no-choice bioassay revealed no difference. The invasive species outperformed noninvasive species only under the most favorable conditions, and the noninvasive species did not outperform the invasive species in any environment.     

Do exotic pine plantations favour the spread of invasive herbivorous mammals in Patagonia?

Changes in land use patterns and vegetation can trigger ecological change in occupancy and community composition. Among the potential ecological consequences of land use change is altered susceptibility to occupancy by invasive species. We investigated the responses of three introduced mammals (red deer, Cervus elaphus; wild boar, Sus scrofa; and European hare, Lepus europaeus) to replacement of native vegetation by exotic pine plantations in the Patagonian forest‐steppe ecotone using camera‐trap surveys (8633 trap‐days). We used logistic regression models to relate species presence with habitat variables at stand and landscape scales. Red deer and wild boar used pine plantations significantly more frequently than native vegetation. In contrast, occurrence of European hares did not differ between pine plantations and native vegetation, although hares were recorded more frequently in firebreaks than in plantations or native vegetation. Presence of red deer and wild boar was positively associated with cover of pine plantations at the landscape scale, and negatively associated with mid‐storey cover and diversity at the stand scale. European hares preferred sites with low arboreal and mid‐storey cover. Our results suggest that pine plantations promote increased abundances of invasive species whose original distributions are associated with woodlands (red deer and wild boar), and could act as source or pathways for invasive species to new areas.     

Assessing Riparian Quality Using Two Complementary Sets of Bioindicators

Biological indicators are being increasingly used to rapidly monitor changing river quality. Among these bioindicators are macroinvertebrates. A short-coming of macroinvertebrate rapid assessments is that they use higher taxa, and therefore lack taxonomic resolution and species-specific responses. One subset of invertebrate taxa is the Odonata, which as adults, are sensitive indicators of both riparian and river conditions. Yet adult Odonata are not necessarily an umbrella taxon for all other taxa. Therefore, we investigated whether the two metrics of aquatic macroinvertebrate higher taxa and adult odonate species might complement each other, and whether together they provide better clarity on river health and integrity than one subset alone. Results indicated that both metrics provide a similar portrait of large-scale, overall river conditions. At the smaller spatial scale of parts of rivers, Odonata were highly sensitive to riparian vegetation, and much more so than macroinvertebrate higher taxa. Odonate species were more sensitive to vegetation structure than they were to vegetation composition. Landscape context is also important, with the odonate assemblages at point localities being affected by the neighbouring dominant habitat type. Overall, benthic macroinvertebrates and adult Odonata species provide a highly complementary pair of metrics which together provide large spatial scale (river system) and small spatial scale (point localities) information on the impact of stressors such as riparian invasive alien trees. As adult Odonata are easy to sample and are sensitive to disturbance at both small and large spatial scales, they are valuable indicators for rapid assessment of river condition and riparian quality.     

Patterns of species richness in a limestone grassland under different treatments in relation to spatial scale

Abstract. In an experiment in a limestone grassland on the Baltic island of Öland, SE Sweden, nutrient and water supply, light intensity and grazing regime were altered in 10 combinations during four years in 10 plots of 0.25 m² with subplots of 0.01 m² and 0.0004 m². Only the combined application of fertilizer and shade led to a strong decrease in average species richness (S₁) at all scales. When comparing species numbers summed up over all 10 replicates of each treatment (S_n) at the three quadrat sizes, differences in effect of these treatments were much smaller, and were so already at the finest scale. α-diversity, measured as (S_n - S₁ was quite constant over different scales for most treatments, i.e. diversity did not increase with an increase in scale. The ‘richness ratio’S_n/S₁ decreased with increasing scale, indicating an increasing degree of homogeneity at larger scales. Treatments which only included fertilizer or shade, maintained high species richness; this high richness was also maintained in combination with grazing and could then be explained by the denser packing of vegetation. Patterns of species richness were correlated at the large scale, but not at the finer scales, indicating a high degree of spatial and temporal heterogeneity at the finer scales. With increasing quadrat size species persistence increased which explains the small effect of certain treatments. Clearly, a range of scales has to be sampled in this type of vegetation to be able to measure different patterns, which may occur under different experimental treatments. The finest scale in this study can become too small, when certain treatments result in a coarse-grained vegetation pattern. The quadrat size of 10 cm x 10 cm should be included in the range of scales. It combines accuracy in sampling with efficiency in time effort, a reasonably large number of species sampled, and a strong differentiation in the effects of the various treatments.     

Accommodating scenarios of climate change and management in modelling the distribution of the invasive tree Schinus molle in South Africa

Determining the potential range of invasive alien species under current conditions is important. However, we also need to consider future distributions under scenarios of climate change and different management interventions when formulating effective long‐term intervention strategies. This paper combines niche modelling and fine‐scale process‐based modelling to define regions at high risk of invasion and simulate likely dynamics at the landscape scale. Our study species is Schinus molle (Peruvian pepper tree; Anacardiaceae), a native of central South America, introduced to South Africa in about 1850 where it was widely planted along roads. Localities of planted and naturalized trees were mapped along 5380 km of roads – a transect that effectively samples a large part of western South Africa. Correlative modelling was used to produce profiles of present and future environmental conditions characterizing its planted and naturalized ranges. A cellular‐automata simulation model was used to estimate the dynamics of S. molle under future climates and different management scenarios. The overall potential range of S. molle in the region is predicted to shrink progressively with predicted climate change. Some of the potential range of S. molle defined based on current conditions (including areas where it is currently highly invasive) is likely to become less favourable. The species could persist where it is well established long after conditions for recruitment have deteriorated. Some areas where the species is not widely naturalized now (notably the fynbos biome) are likely to become more favourable. Our modelling approach allows for the delineation of areas likely to be invaded in future by considering a range of factors at different scales that mediate the interplay of climatic variables and other drivers that define the dimensions of human intervention such as distance from planted trees and the density of planted plants, both of which affect propagule pressure.     

Plant invasions in treeless vegetation of the Australian Alps

A total of 128 invasive plant species have been recorded in treeless vegetation in the Australian Alps. Most of these are forbs and most are uncommon. Cover of invasive species is generally minimal unless there has been gross disturbance to natural vegetation and soils. Although there is a significantly positive correlation between invasive and native species diversity, suggesting that conditions that allow numerous native species to co-exist also permit more plant invasions, altitude is the most important determinant of invasive species diversity. Only 22 of the 128 species have been recorded above 1800 m. Some plant communities (e.g. those with high pH or relatively nutrient-rich soils), however, seem to be vulnerable to invasions regardless of altitude. Most invasive species appear to have been introduced unintentionally (e.g. as seed attached to vehicles, animals and humans) but a few were introduced to assist with revegetation of disturbed soils and for amenity plantings in ski resorts, and have subsequently established in native vegetation. Treeless communities in the Australian Alps are likely to face increasing pressure from invasive species as a result of global warming and continued introduction of non-native plants to ski resort gardens. Whilst it may be difficult to prevent invasive species of low elevations migrating to higher elevations as temperatures rise, the risk of invasion from garden plants could be minimised through regulation. Non-native plants in ski resort gardens pose a far greater risk than most invasive species currently present in the Alps because they have been chosen for their capacity to survive at high altitudes.     

Ecological impacts of invasive African olive (Olea europaea ssp. cuspidata) in Cumberland Plain Woodland,Sydney, Australia

African olive (Olea europaea ssp. cuspidata) is a small evergreen tree which has become highly invasive at a landscape scale in the western Sydney and Hunter Valley regions of New South Wales, Australia. African olive invasion results in the formation of a dense and permanent mid‐canopy in grassy woodland vegetation. We investigated the relationship between African olive and native species establishment, abundance and diversity, using field surveys and a manipulative shading experiment. There were 78% fewer native species beneath African olive canopy in the field compared to uninvaded woodland sites. The shading experiment showed that simulated dense African olive shade levels produced the lowest dry weight for the three native species studied, with simulated canopy edge light providing optimal conditions for the native shrub Bursaria spinosa and African olive. Dense African olive shade levels produced the highest mortality rate for native species; however, African olive was able to maintain an 88% survival rate under dense canopy shade. This study confirms the adaptability of African olive and its capacity to decrease native plant diversity and substantially modify native vegetation at the community level.     

Vulnerability of Mediterranean Basin ecosystems to climate change and invasion by exotic plant species

Aim To assess at a broad scale the vulnerability of Mediterranean vegetation to alien plant invasion under different climatic and disturbance scenarios. Location We simulated the vegetation biogeography and dynamics on five of the main islands of the Mediterranean Basin: Mallorca, Corsica, Sardinia, Crete and Lesvos. Methods We used LPJ‐GUESS, a generalized ecosystem model based on dynamic processes describing establishment, competition, mortality and ecosystem biogeochemistry. We simulated the vegetation distribution and dynamics using a set of plant functional types (PFTs) based on bioclimatic and physiological parameters, which included tree and shrub PFTs defined especially for the Mediterranean. Additionally, two invasive PFTs, an invasive tree type and an invasive herb type, were defined and used to estimate the vulnerability to invasion of a range of different ecosystems. The model was used to simulate climate changes and associated changes in atmospheric [CO₂] to 2050 according to two SpecialReport on Emissions Scenarios climate scenarios (A1Fi and B1) combined with mean disturbance intervals of 3 and 40 years. Results The simulations and scenarios showed that the effect of climate change alone is likely to be negligible in many of the simulated ecosystems, although not all. The simulated progression of an invasion was highly dependent on the initial ecosystem composition and local environmental conditions, with a particular contrast between drier and wetter parts of the Mediterranean, and between mountain and coastal areas. The rate of ecosystem disturbance was the main factor controlling susceptibility to invasion, strongly influencing vegetation development on the shorter time scale. Main conclusions Further invasion into Mediterranean island ecosystems is likely to be an increasing problem: our simulations predict that, in the longer term, almost all the ecosystems will be dominated by exotic plants irrespective of disturbance rates.     

Influence of Vegetation on Bat Use of Riparian Areas at Multiple Spatial Scales

ABSTRACT Research on habitat use by bats typically occurs at a single fine spatial scale, despite recent work demonstrating the importance of considering multiple spatial scales when investigating vertebrate habitat selection. We assessed bat use of 118 stream reaches located throughout the Oregon Coast Range, USA, and measured vegetation characteristics at 3 spatial scales surrounding each of these locations. We used an information-theoretic approach to determine vegetation characteristics most closely related to bat activity and a multilevel modeling approach to evaluate variation in bat activity at different spatial scales. Characteristics of vegetation at the finest spatial scale explained more variation in bat activity than did characteristics of vegetation at broader spatial scales, suggesting that fine-scale anthropogenic or natural disturbance events that alter cover of shrubs or trees in riparian areas are likely to influence bat habitat use. The influence of vegetation on activity varied by species of bat and appeared to operate more strongly through constraints imposed by vegetation architecture than through influences on abundance of insect prey. This diversity of responses to vegetation characteristics among bat species suggests that the best strategy for biodiversity conservation over broad spatial scales is maintenance or creation of a diversity of riparian vegetation conditions. We recommend that land managers planning to manipulate riparian vegetation strive to create diversity in shrub coverage, canopy coverage, and open space above the stream channel to promote foraging habitat for all species.     

Fine‐scale vascular plant species richness in different alpine vegetation types: relationships with biomass and cover

Abstract. Vascular plant species richness was related to biomass and vegetation cover in nine different alpine vegetation types on the Hardangervidda mountain plateau, western Norway. Each vegetation type was sampled within an 8m × 6m area, and the species‐richness pattern analysed. Evidence for a unimodal relationship between species richness and both biomass and cover was found at the within‐vegetation type scale. Cover was a better predictor for species richness than biomass, suggesting that light may be an important factor influencing species richness at this scale in alpine vegetation. The possibility that the results are an artefact of small grain size is also discussed, and several arguments for an ecological explanation of the humpback relationship between species richness and cover are discussed.     

Abiotic constraints and biotic resistance control the establishment success and abundance of invasive <Emphasis Type="Italic">Humulus japonicus</Emphasis> in riparian habitats

Dispersal, abiotic and biotic constraints are all involved in explaining the success of invasive plants but how these factors influence the different life stages of an invader remains poorly known. Focusing on highly invaded riparian habitats we asked: (1) how do propagule pressure, resource availability and resident vegetation influence the success of the invasive Asian vine Humulus japonicus at different stages of its life cycle (i.e. seedling, vegetative and flowering) (2) what is the influence of increasing resource availability on the performance and trait plasticity of H. japonicus compared to a functionally similar co-occurring native species? To answer the first question we performed a repeated field survey along the Gardon River (S France) with detailed measurements of distance to the riverbed, soil characteristics, light availability, and resident vegetation cover. To answer the second question we used a greenhouse experiment to compare the biomass and three functional traits of H. japonicus and Galium aparine along a gradient of increasing water and nitrogen availability. Initial germination success was only determined by abiotic constraints, while the role of biotic resistance increased for later stages with establishment success favoured by the interaction of low resident vegetation cover and high soil fertility, and final integrated success favoured by high light availability. H. japonicus performed better and showed higher plasticity in plant height than G. aparine under increased resource availability while their biomass did not differ in the lower part of the resource gradient. Our study demonstrates that by combining field and experimental studies and analysing responses at different life stages we can gain a more complete understanding of how ecological filters shape successful invasions in the course of the life cycle.     

Habitat Selection by the Spur-thighed Tortoise Testudo graeca in a Multisuccessional Landscape: Implications for Habitat Management

As a result of human activities, natural Mediterranean landscapes (including agro-ecosystems) are characterised by a mosaic-like structure with habitat-patches at different successional stages. These systems have high biodiversity levels and are home to a large number of species protected by European laws whose habitats should be adequately managed. In the present work, we study habitat use from an applied point of view in the spur-thighed tortoise Testudo graeca, an endangered reptile present in semi-arid Mediterranean agro-ecosystems. Results show that, at a landscape scale, the species selects simplified vegetation structures and includes in its home range re-colonisation shrubland and small non-irrigated fields. Within the home range, habitat selection patterns vary and areas with higher vegetation cover and complexity are selected. Detected patterns are discussed in terms of the ecological requirements of the species and with a hierarchical view of resources and conditions. The implications of our findings for habitat management aimed at the conservation of the species are also discussed.     

When Invasive Plants Disappear: Transformative Restoration Possibilities in the Western United States Resulting from Climate Change

Most ecologists believe that climate change poses a significant threat to the persistence of native species. However, in some areas climate change may reduce or eliminate non-native invasive species, creating opportunities for restoration. If invasive species are no longer suited to novel climate conditions, the native communities that they replaced may not be viable either. If neither invasive nor native species are climatically viable, a type of "transformative" restoration will be required, involving the translocation of novel species that can survive and reproduce under new climate conditions. Here, we illustrate one approach for restoration planning by using bioclimatic envelope modeling to identify restoration opportunities in the western United States, where the invasive plant cheatgrass ( Bromus tectorum ) is no longer climatically viable under 2100 conditions projected by the Geophysical Fluid Dynamics Laboratory (GFDL2.1) coupled atmosphere-ocean general circulation model. We then select one example of a restoration target area and identify novel plant species that could become viable at the site in the wake of climate change. We do so by identifying the closest sites that currently have climate conditions similar to those projected at the restoration target area in 2100. This approach is a first step toward identifying appropriate species for transformative restoration.     

Unusual suspects in the usual places: a phylo-climatic framework to identify potential future invasive species

A framework for identifying species that may become invasive under future climate conditions is presented, based on invader attributes and biogeography in combination with projections of future climate. We illustrate the framework using the CLIMEX niche model to identify future climate suitability for three species of Hawkweed that are currently present in the Australian Alps region and related species that are present in the neighbouring region. Potential source regions under future climate conditions are identified, and species from those emerging risk areas are identified. We use dynamically downscaled climate projections to complement global analyses and provide fine-scale projections of suitable climate for current and future (2070–2099) conditions at the regional scale. Changing climatic conditions may reduce the suitability for some invasive species and improve it for others. Invasive species with distributions strongly determined by climate, where the projected future climate is highly suitable, are those with the greatest potential to be future invasive species in the region. As the Alps region becomes warmer and drier, many more regions of the world become potential sources of invasive species, although only one additional species of Hawkweed is identified as an emerging risk. However, in the longer term, as the species in these areas respond to global climate change, the potential source areas contract again to match higher altitude regions. Knowledge of future climate suitability, based on species-specific climatic tolerances, is a useful step towards prioritising management responses such as targeted eradication and early intervention to prevent the spread of future invasive species.     

Influence of Drought and Shade on Seedling Growth of Native and Invasive Trees in the Seychelles

We studied the growth of seedlings of native and invasive tree species from secondary tropical forests on Mahé (Seychelles). We were interested in whether native or invasive species are more drought-tolerant, and therefore conducted a garden (pot) experiment comparing the growth of seedlings of five native and five invasive tree species under different light (7% and 60% transmittance) and water (natural and repeated drought stress) conditions. Differences in the responses of native and invasive species to these treatments were small. In both groups, mean relative growth rates were reduced only slightly by intermittent drought that caused wilting of leaves. However, invasive species produced clearly thinner leaves (high specific leaf area, SLA) and more root biomass than native species under high light, while these differences were small under low light. Native species performed better than invasive species under low light with low water availability. It appears that high phenotypic plasticity allows some fast-growing invasive species to cope with water stress by adjusting the relative allocation of resources to aboveground and belowground structures under high light, while this strategy is not effective when both light and water resources are limiting. We conclude that water stress may reduce the invasibility of shaded habitats by fast-growing invasive species, while water stress in unshaded habitats may have less effect on invasive species than previously recognized.