Invasion of exotic plants in nutrient‐enriched urban bushland

Abstract Invasion of exotic plants into vegetation communities on low‐nutrient soils in Sydney is often attributed to increased phosphorus in the soil. However, the composition of native vegetation in nearby national parks has been found to be more closely correlated to combinations of soil nutrients and other environmental variables than phosphorus alone. This study examined whether phosphorus or a range of variables better matched patterns of native and exotic plants in urban bushland. Sites in urban bushland and national parks were sampled, vegetation frequency recorded and soil samples collected. Soil samples were analysed for a range of chemical and physical attributes. A significantly greater number of exotic species were found at the urban sites, and significantly fewer native species than in the national parks. All measured soil nutrients were found to be of a significantly increased concentration in urban soil. Using regression analysis, an index of all measured soil nutrients was found to explain more of the variation in the percentage of exotic species at a site than phosphorus alone. Multivariate analysis showed a gradient of sites from minimal exotic invasion to heavy invasion. However, this gradient also corresponded to changes in native species. The gradient was found to match that of increasing soil nutrient levels. A combination of soil nutrients was found to correlate better with the multivariate species composition than was phosphorus alone. The results suggest that it is the increase of many soil nutrients, not phosphorus alone, that is contributing to the invasion of urban bushland by exotic plants and the alteration of the suite of native species.     

Urbanization and exotic plants in northern Sydney streams

The vegetation and sediment of urban and non‐urban streams in the northern Sydney region were compared to examine the possible effects of urbanization on within‐stream vegetation. Many sediment characteristics were significantly different in urban streams. At least one exotic plant species was found in each urban stream sampled, but none were found in the non‐urban streams. The presence of exotic species led to the overall number and abundance of plant species being significantly higher in urban streams. Interestingly, the number and abundance of native species at the urban sites were the same as non‐urban sites, but a different suite of species was usually present. This suggests that urban streams favour exotic plants and certain native plants that are adapted to the modified conditions. The differences between the plant communities in the urban and non‐urban streams appeared to be associated with the increased level of nutrients in the urban stream sediment. Several multivariate techniques were used to assess the relative importance of individual nutrients, but no nutrients were directly associated with the observed differences. In particular, total phosphorus levels were less important in explaining the vegetation patterns than a combination of nutrients. It is therefore likely that the general increase of nutrients in stream sediment has enhanced exotic invasion and altered stream plant communities in Sydney streams.     

Fire‐related cues and germination from the soil seed bank of senescent remnants of mallee vegetation on Eastern Kangaroo Island

Plant communities dominated by narrow‐leaved mallee (Eucalyptus cneorifolia) are almost entirely confined to north‐eastern Kangaroo Island, South Australia, an area which has been extensively cleared for agriculture. Consequently, surviving examples consist mostly of small remnants which are thought to be senescent due to the exclusion of fire. This senescence is associated with the loss of many native understory species. Prescribed burns have been suggested as a management tool to stimulate the restoration of native plants from the soil seed bank; however, no seed bank studies have previously been conducted on Kangaroo Island and the seed bank literature usually focuses on particular species rather than on plant communities. We conducted an experiment to investigate the effects of the fire‐related cues heat and smoke on the germination of plants from the seed bank in soil sampled from 10 long‐ungrazed narrow‐leaved mallee sites on Kangaroo Island. Eighty trays of soil were monitored in a controlled glasshouse for five months after being subjected to heat and/or smoke treatments. The overall number of native, but not exotic, plant species germinating from the soil seed bank was significantly increased by all three fire‐related treatments (heat, smoke and heat plus smoke) compared with the control (no fire‐related treatment). Different plant life forms exhibited varying responses to heat and smoke treatments. The results of this study illustrate that the application of fire‐related treatments to soil seed banks in controlled glasshouse conditions can stimulate the recruitment of native species, including several species of conservation concern. These findings also indicate the potential of using these treatments for the ex situ germination of fire dependent species for revegetation purposes and indicate aspects of prescribed burns that may be important for restoring different components of native vegetation.     

Phenotypes of Two Generations of Sporobolus airoides Seedlings Derived from Acroptilon repens‐invaded and Non‐invaded Grass Populations

Although the ecological impacts of invasive species are well known, the evolutionary impacts on recipient native grass communities are not. We suggest that remnant native plants may provide desirable seed sources for restoration and native plant production. Native populations exposed to the selective pressures associated with exotic invasion may retain traits that increase their ability to coexist with invasive species. Two generations of Sporobolus airoides Torr. (Alkali sacaton) plants derived from lineages collected from within long‐term invaded areas of Acroptilon repens (L.) DC (Russian knapweed) and from adjacent non‐invaded areas were propagated in a greenhouse to evaluate generational changes in phenotypic traits from the production environment. Given the difference in invasion history of the two populations, we hypothesized that invaded and non‐invaded subpopulations would differ phenotypically. Phenotypic measurements revealed that invaded subpopulations had greater vegetative growth, whereas non‐invaded subpopulations had increased sexual reproduction. Phenotypic expression changed from the first to the second generation, predominantly in the invaded subpopulation. Generational phenotypic shifts are disadvantageous for native seed production which requires a standard product to sell commercially. However, phenotypic variation may improve field seed survival. This research demonstrates the potential value of targeting post‐invasion remnant grass populations for restoration.     

The effects of development,vegetation‐type conversion,and fire on low‐elevation Southern California spider assemblages

California sage scrub (CSS), a native ecosystem type of low‐elevation areas of Southern California, is increasingly threatened by urban development, altered fire regimes, and vegetation‐type conversion to non‐native grasslands. Using pitfall traps, we examined how suburbanization, type conversion, and fire influence ground‐dwelling spider assemblages in eastern Los Angeles County, CA, by surveying spiders in three habitats (CSS, non‐native grasslands, and suburban areas) before and after a fire that occurred in a small portion of our study site. Spider assemblages in the suburban habitat differed from those in CSS and non‐native grassland habitats, but CSS and grassland assemblages did not significantly differ. This suggests that the urban development, but not vegetation‐type conversion to non‐native grasslands, has significant effects on ground‐dwelling spider assemblages. Fire had no observable effect on assemblages. Because ground‐dwelling spiders were not impacted by fire and type conversion, increased fire frequencies, which often result in the establishment of non‐native grasses, may not deleteriously influence this animal group, a differing pattern from other taxonomic groups. However, the rapid urban development occurring in low‐elevation areas of Southern California means that species requiring non‐suburban sites for their survival (15 species, 24.1%) may be threatened and require conservation assessment.     

Inhibitory effects of Eucalyptus globulus on understorey plant growth and species richness are greater in non‐native regions

Aim

We studied the novel weapons hypothesis in the context of the broadly distributed tree species Eucalyptus globulus. We evaluated the hypothesis that this Australian species would produce stronger inhibitory effects on species from its non‐native range than on species from its native range.

Location

We worked in four countries where this species is exotic (U.S.A., Chile, India, Portugal) and one country where it is native (Australia).

Time period

2009–2012.

Major taxa studied

Plants.

Methods

We compared species composition, richness and height of plant communities in 20 paired plots underneath E. globulus individuals and open areas in two sites within its native range and each non‐native region. We also compared effects of litter leachates of E. globulus on root growth of seedlings in species from Australia, Chile, the U.S.A. and India.

Results

In all sites and countries, the plant community under E. globulus canopies had lower species richness than did the plant community in open areas. However, the reduction was much greater in the non‐native ranges: species richness declined by an average of 51% in the eight non‐native sites versus 8% in the two native Australian sites. The root growth of 15 out of 21 species from the non‐native range were highly suppressed by E. globulus litter leachates, whereas the effect of litter leachate varied from facilitation to suppression for six species native to Australia. The mean reduction in root growth for Australian plants was significantly lower than for plants from the U.S.A., Chile and India.

Main conclusions

Our results show biogeographical differences in the impact of an exotic species on understorey plant communities. Consistent with the novel weapons hypothesis, our findings suggest that different adaptations of species from the native and non‐native ranges to biochemical compounds produced by an exotic species may play a role in these biogeographical differences.     

Ecological filtering of exotic plants in an Australian sub‐alpine environment

We investigated some of the factors influencing exotic invasion of native sub‐alpine plant communities at a site in southeast Australia. Structure, floristic composition and invasibility of the plant communities and attributes of the invasive species were studied. To determine the plant characteristics correlated with invasiveness, we distinguished between roadside invaders, native community invaders and non‐invasive exotic species, and compared these groups across a range of traits including functional group, taxonomic affinity, life history, mating system and morphology. Poa grasslands and Eucalyptus‐Poa woodlands contained the largest number of exotic species, although all communities studied appeared resilient to invasion by most species. Most community invaders were broad‐leaved herbs while roadside invaders contained both herbs and a range of grass species. Over the entire study area the richness and cover of native and exotic herbaceous species were positively related, but exotic herbs were more negatively related to cover of specific functional groups (e.g. trees) than native herbs. Compared with the overall pool of exotic species, those capable of invading native plant communities were disproportionately polycarpic, Asteracean and cross‐pollinating. Our data support the hypothesis that strong ecological filtering of exotic species generates an exotic assemblage containing few dominant species and which functionally converges on the native assemblage. These findings contrast with those observed in the majority of invaded natural systems. We conclude that the invasion of closed sub‐alpine communities must be viewed in terms of the unique attributes of the invading species, the structure and composition of the invaded communities and the strong extrinsic physical and climatic factors typical of the sub‐alpine environment.     

Native jewelweed,but not other native species,displays post‐invasion trait divergence

Invasive exotic plants reduce the diversity of native communities by displacing native species. According to the coexistence theory, native plants are able to coexist with invaders only when their fitness is not significantly smaller than that of the exotics or when they occupy a different niche. It has therefore been hypothesized that the survival of some native species at invaded sites is due to post‐invasion evolutionary changes in fitness and/or niche traits. In common garden experiments, we tested whether plants from invaded sites of two native species, Impatiens noli‐tangere and Galeopsis speciosa, outperform conspecifics from non‐invaded sites when grown in competition with the invader (Impatiens parviflora). We further examined whether the expected superior performance of the plants from the invaded sites is due to changes in the plant size (fitness proxy) and/or changes in the germination phenology and phenotypic plasticity (niche proxies). Invasion history did not influence the performance of any native species when grown with the exotic competitor. In I. noli‐tangere, however, we found significant trait divergence with regard to plant size, germination phenology and phenotypic plasticity. In the absence of a competitor, plants of I. noli‐tangere from invaded sites were larger than plants from non‐invaded sites. The former plants germinated earlier than inexperienced conspecifics or an exotic congener. Invasion experience was also associated with increased phenotypic plasticity and an improved shade‐avoidance syndrome. Although these changes indicate fitness and niche differentiation of I. noli‐tangere at invaded sites, future research should examine more closely the adaptive value of these changes and their genetic basis.     

Periphyton density is similar on native and non‐native plant species

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Non‐native plant species benefit from disturbance: a meta‐analysis

Disturbances, such as fire and grazing, are often claimed to facilitate plant species richness and plant invasions in particular, although empirical evidence is contradictory. We conducted a meta‐analysis to synthesize the literature on how non‐native plant species are affected by disturbances. We explored whether the observed impact of disturbance on non‐native plant communities is related to its type and frequency, to habitat type, study approach (observational or experimental), and to the temporal and spatial scales of the study. To put the results in a broader context, we also conducted a set of parallel analyses on a data set involving native plant species. The diversity and abundance of non‐native plant species were significantly higher at disturbed sites than at undisturbed sites, while the diversity and abundance of native plant species did not differ between the two types of sites. The effect of disturbance on non‐native plant species depended on the measure used to evaluate the impact (species diversity or abundance) and on disturbance type, with grazing and anthropogenic disturbances leading to higher diversity and abundance of non‐native plant species than other disturbance types examined. The impact of disturbance on non‐natives was also associated with study approach, habitat type and temporal scale, but these factors covaried with disturbance type, complicating the interpretation of the results. Overall, our results indicate that disturbance has a positive impact particularly on non‐native plant species (at least when they are already present in the community), and that the strength of this impact depends primarily on the disturbance type. Synthesis Empirical evidence of the effect of disturbances on plant species richness is contradictory. Here we use a meta‐analysis to synthesize the published literature on how different types of disturbances influence the diversity and abundance of plant species, focusing in particular on non‐native plants. Our study supports the hypothesis that disturbances generally facilitate the diversity and abundance of non‐native plant species, although the strength of this facilitation depends primarily on the disturbance type.     

Bumble bees selectively use native and exotic species to maintain nutritional intake across highly variable and invaded local floral resource pools

1. Changes to plant community composition after invasion are well documented but how these shifts directly affect higher trophic levels is still poorly understood. One potentially important factor is the change in nutritional availability after an invasion. Shifts in nutrient availability could affect the nutrient intake of organisms that live in invaded habitats, causing reduced fecundity and survival. 2. The effects of the interaction among nutrient availability, selection, and diet on nutrient intake of a native bumble bee were examined. No nutritional differences were found between exotic and native pollen or collected and non‐collected pollen in protein or amino acid content, suggesting that differences in nutrient intake from random are based on selection. 3. Nutrient intake was simulated when pollen was selected randomly across all available plant species and when selection was restricted to native plants only or exotic plants only using a permutation model and compared with observed collection. The results suggest that pollen collection is non‐random and that selecting only native or exotic plants cannot provide the protein or amino acid intake observed. 4. These results may help to explain why the responses of native bees to exotic plants are so variable. If the exotic plants in a community can supply the necessary nutrients, bees may readily incorporate them into their diets, but if not, exotic plants may be avoided.     

Community‐level plant–soil feedbacks explain landscape distribution of native and non‐native plants

Plant–soil feedbacks (PSFs) have gained attention for their potential role in explaining plant growth and invasion. While promising, most PSF research has measured plant monoculture growth on different soils in short‐term, greenhouse experiments. Here, five soil types were conditioned by growing one native species, three non‐native species, or a mixed plant community in different plots in a common‐garden experiment. After 4 years, plants were removed and one native and one non‐native plant community were planted into replicate plots of each soil type. After three additional years, the percentage cover of each of the three target species in each community was measured. These data were used to parameterize a plant community growth model. Model predictions were compared to native and non‐native abundance on the landscape. Native community cover was lowest on soil conditioned by the dominant non‐native, Centaurea diffusa, and non‐native community cover was lowest on soil cultivated by the dominant native, Pseudoroegneria spicata. Consistent with plant growth on the landscape, the plant growth model predicted that the positive PSFs observed in the common‐garden experiment would result in two distinct communities on the landscape: a native plant community on native soils and a non‐native plant community on non‐native soils. In contrast, when PSF effects were removed, the model predicted that non‐native plants would dominate all soils, which was not consistent with plant growth on the landscape. Results provide an example where PSF effects were large enough to change the rank‐order abundance of native and non‐native plant communities and to explain plant distributions on the landscape. The positive PSFs that contributed to this effect reflected the ability of the two dominant plant species to suppress each other's growth. Results suggest that plant dominance, at least in this system, reflects the ability of a species to suppress the growth of dominant competitors through soil‐mediated effects.     

Postwildfire seeding to restore native vegetation and limit exotic annuals: an evaluation in juniper‐dominated sagebrush steppe

Reestablishment of perennial vegetation is often needed after wildfires to limit exotic species and restore ecosystem services. However, there is a growing body of evidence that questions if seeding after wildfires increases perennial vegetation and reduces exotic plants. The concern that seeding may not meet restoration goals is even more prevalent when native perennial vegetation is seeded after fire. We evaluated vegetation cover and density responses to broadcast seeding native perennial grasses and mountain big sagebrush (Artemisia tridentata Nutt. spp. vaseyana [Rydb.] Beetle) after wildfires in the western United States in six juniper (Juniperus occidentalis ssp. occidentalis Hook)‐dominated mountain big sagebrush communities for 3 years postfire. Seeding native perennial species compared to not seeding increased perennial grass and sagebrush cover and density. Perennial grass cover was 4.3 times greater in seeded compared to nonseeded areas. Sagebrush cover averaged 24 and less than 0.1% in seeded and nonseeded areas at the conclusion of the study, respectively. Seeding perennial species reduced exotic annual grass and annual forb cover and density. Exotic annual grass cover was 8.6 times greater in nonseeded compared to seeded areas 3 years postfire. Exotic annual grass cover increased over time in nonseeded areas but decreased in seeded areas by the third‐year postfire. Seeded areas were perennial‐dominated and nonseeded areas were annual‐dominated at the end of the study. Establishing perennial vegetation may be critical after wildfires in juniper‐dominated sagebrush steppe to prevent the development of annual‐dominated communities. Postwildfire seeding increased perennial vegetation and reduced exotic plants and justifies its use.     

Little evidence for release from herbivores as a driver of plant invasiveness from a multi‐species herbivore‐removal experiment

Enemy release is frequently posed as a main driver of invasiveness of alien species. However, an experimental multi‐species test examining performance and herbivory of invasive alien, non‐invasive alien and native plant species in the presence and absence of natural enemies is lacking. In a common garden experiment in Switzerland, we manipulated exposure of seven alien invasive, eight alien non‐invasive and fourteen native species from six taxonomic groups to natural enemies (invertebrate herbivores), by applying a pesticide treatment under two different nutrient levels. We assessed biomass production, herbivore damage and the major herbivore taxa on plants. Across all species, plants gained significantly greater biomass under pesticide treatment. However, invasive, non‐invasive and native species did not differ in their biomass response to pesticide treatment at either nutrient level. The proportion of leaves damaged on invasive species was significantly lower compared to native species, but not when compared to non‐invasive species. However, the difference was lost when plant size was accounted for. There were no differences between invasive, non‐invasive and native species in herbivore abundance. Our study offers little support for invertebrate herbivore release as a driver of plant invasiveness, but suggests that future enemy release studies should account for differences in plant size among species.     

Not all non‐natives are equally unequal: reductions in herbivore β‐diversity depend on phylogenetic similarity to native plant community

Effects of host plant α‐ and β‐diversity often confound studies of herbivore β‐diversity, hindering our ability to predict the full impact of non‐native plants on herbivores. Here, while controlling host plant diversity, we examined variation in herbivore communities between native and non‐native plants, focusing on how plant relatedness and spatial scale alter the result. We found lower absolute magnitudes of β‐diversity among tree species and among sites on non‐natives in all comparisons. However, lower relative β‐diversity only occurred for immature herbivores on phylogenetically distinct non‐natives vs. natives. Locally in that comparison, non‐native gardens had lower host specificity; while among sites, the herbivores supported were a redundant subset of species on natives. Therefore, when phylogenetically distinct non‐natives replace native plants, the community of immature herbivores is likely to be homogenised across landscapes. Differences in communities on closely related non‐natives were subtler, but displayed community shifts and increased generalisation on non‐natives within certain feeding guilds.     

Non‐native plants have greater impacts because of differing per‐capita effects and nonlinear abundance–impact curves

Invasive, non‐native species can have tremendous impacts on biotic communities, where they reduce the abundance and diversity of local species. However, it remains unclear whether impacts of non‐native species arise from their high abundance or whether each non‐native individual has a disproportionate impact – that is, a higher per‐capita effect – on co‐occurring species compared to impacts by native species. Using a long‐term study of wetlands, we asked how temporal variation in dominant native and non‐native plants impacted the abundance and richness of other plants in the recipient community. Non‐native plants reached higher abundances than natives and had greater per‐capita effects. The abundance–impact relationship between plant abundance and richness was nonlinear. Compared with increasing native abundance, increasing non‐native abundance was associated with steeper declines in richness because of greater per‐capita effects and nonlinearities in the abundance–impact relationship. Our study supports eco‐evolutionary novelty of non‐natives as a driver of their outsized impacts on communities.     

Subalpine vegetation pattern three decades after stand‐replacing fire: effects of landscape context and topography on plant community composition,tree regeneration,and diversity

Objective: Our purpose was to characterize vegetation compositional patterns, tree regeneration, and plant diversity, and their relationships to landscape context, topography, and light availability across the margins of four stand‐replacing subalpine burns. Location: Four 1977 to 1978 burns east of the Continental Divide in Colorado: the Ouzel burn, a burn near Kenosha Pass, the Badger Mountain burn, and the Maes Creek burn. Methods: Vegetation and environmental factors were sampled in 200 0.01‐ha plots on transects crossing burn edges, and stratified by elevation. We utilized dissimilarity indices, mixed‐effects models, and randomization tests to assess relationships between vegetation and environment. Results: Three decades after wildfire, plant communities exhibited pronounced compositional shifts across burn edges. Tree regeneration decreased with increasing elevation and distance into burn interiors; concomitant increases in forbs and graminoids were linked to greater light availability. Richness was roughly doubled in high‐severity burn interiors due to the persistence of a suite of native species occurring primarily in this habitat. Richness rose with distance into burns, but declined with increasing elevation. Only three of 188 plant species were non‐native; these were widespread, naturalized species that comprised <1% total cover. Conclusions: These subalpine wildfires generated considerable, persistent increases in plant species richness at local and landscape scales, and a diversity of plant communities. The findings suggest that fire suppression in such systems must lead to reduced diversity. Concerns about post‐fire invasion by exotic plants appear unwarranted in high‐elevation wilderness settings.     

Ecological engineering by a native leaf-cutting ant increases the performance of exotic plant species

Numerous mechanisms are proposed to explain why exotic plants successfully invade natural communities. However, the positive effects of native engineers on exotic plant species have received less consideration. We tested whether the nutrient-rich soil patches created by a native ecological engineer (refuse dumps from the leaf-cutting ant Acromyrmex lobicornis) increase the performance of exotic more than native plants. In a greenhouse experiment, individuals from several native and exotic species were planted in pots with refuse dumps (RDs) and non-nest soils (NNSs). Total plant biomass and foliar nutrient content were measured at the end of the experiment. We also estimated the cover of exotic and native plant species in external RDs from 54 field ant nests and adjacent areas. Greenhouse plants showed more biomass and foliar nutrient content in RDs than in NNS pots. Nevertheless, differences in the final mean biomass among RD and NNS plants were especially great in exotics. Accordingly, the cover of exotic plants was higher in field RDs than in adjacent, non-nest soils. Our results demonstrated that plants can benefit from the enhanced nutrient content of ant RDs, and that A. lobicornis acts as an ecosystem engineer, creating a substrate that especially increases the performance of exotics. This supports the fluctuating resource hypothesis as a mechanism to promote biological invasions, and illustrates how this hypothesis may operate in nature. Since ant nests and exotic plants are more common in disturbed than in pristine environments, the role of ant nests in promoting biological invasions might be of particular interest. Proposals including the use of engineer species to restore disturbed habitats should be planned with caution because of their potential role in promoting invasions.     

Afforestation causes changes in post‐fire regeneration in native shrubland communities of northwestern Patagonia,Argentina

Question: What are the effects of fire in native shrubland communities and in pine plantations established in these shrublands? Location: Northern Patagonia, Argentina. Methods: We surveyed four sites in Chall‐Huaco valley, located in northwest Patagonia. Each site was a vegetation mosaic composed of an unburned Pinus ponderosa plantation, a plantation burned in 1996, and an unburned matorral and a matorral burned by the same fire. We recorded the cover of all vascular plant species. We also analysed species richness, total cover, proportion of exotic species, abundance of woody species and herb species, cover of exotic species, abundance of woody and herb species and differences in composition of species. For both shrubs and tree species we recorded the main strategy of regeneration (by resprouting or by seed). Results: We found that fire had different effects on native matorral and pine plantations. Five years after fire, plantations came to be dominated by herbs and exotic species, showing differences in floristic composition. In contrast, matorral communities remained very similar to unburned matorral in terms of species richness, proportion of woody species, and herb species and proportion of exotics. Also, pine plantations were primarily colonized by seedlings, while matorrals were primarily colonized by resprouting. Conclusions: Matorrals are highly fire resilient communities, and the practice of establishing plantations on matorrals produces a strong reduction in the capacity of matorral to return to its original state. The elimination of shrubs owing to the effect of plantations can hinder regeneration of native ecosystems. Burned plantations may slowly develop into ecosystems similar to the native ones, or they may produce a new ecosystem dominated by exotic herbs. This study shows that plantations of exotic conifers affect native vegetation even after they have been removed, as in this case by fire.