Non-invasive Exotic Species Research Articles

Comparative litter decomposability traits of selected native and exotic woody species from an urban environment of north-western Siwalik region, India

Exotic plants can potentially modify ecosystem functions like cycling of nutrients by adjusting their decomposition rates. However, these effects are largely unknown for urban ecosystems, though they act as reservoirs of exotic plants. The present study evaluated the decomposition rates of five native and five exotic (three invasive and two non-invasive) species by conducting the litter bag experiment. Our study, however, did not find any significant differences in overall decomposition rates of native and exotic species but decomposition rates were strongly correlated with initial chemical quality of the litter. Further, litter carbon, lignin to nitrogen ratio and carbon to nitrogen ratio seemed to be good predictors for decomposition rates in this study. Interestingly, invasive exotic species had higher decomposition rate while non-invasive exotic species showed a slower rate as compared to the native species. In conclusion, our study indicates that invasive exotic plants try to maintain a higher chemical quality of litter than native and non-invasive exotic species which promotes their rapid decomposition. Thus, the better chemical quality of litter may facilitate the naturalisation and invasion of exotic plants irrespective of their origin.

Effects of invasive Rhus typhina L. on bacterial diversity and community composition in soil

ABSTRACT Rhus typhina L., an exotic plant that is widely distributed in China, is harmful to native ecosystems; however, little is known about its effects on soil microbial communities. Therefore, we investigated the effects of R. typhina on soil bacterial communities by 16S rRNA gene sequencing and compared the effects of R. typhina and native (Rhus chinensis Mill.) and non-invasive exotic species (Amorpha fruticosa L. and Robinia pseudoacacia L.) on soil bacterial communities via a greenhouse experiment. R. typhina decreased the diversity of the soil bacterial community compared with R. chinensis, and A. fruticosa. R. typhina soil was characterized by more Actinobacteria and lesser Proteobacteria and Acidobacteria than the other soils. Among Actinobacteria members, there was a difference in the relative abundance of Nocardioides and Streptomyces, and this could be advantageous for the growth of R. typhina. However, there was no significant difference in the soil bacterial community structure associated with R. typhina and other species. Moreover, the concentrations of total carbon, potassium, and nitrate were found to be the major soil factors influencing the relative abundance of soil bacteria.

Competitive ability, not tolerance, may explain success of invasive plants over natives

When entering a new community, introduced species leave behind members of their native community while simultaneously forming novel biotic interactions. Escape from enemies during the process of introduction has long been hypothesized to drive the increased performance of invasive species. However, recent studies and quantitative syntheses find that invaders often receive similar, or even more, damage from enemies than do native species. Therefore, invasives may be those more tolerant to enemy damage, or those able to maintain competitive ability in light of enemy damage. Here, we investigate whether tolerance and competitive ability could contribute to invasive plant success. We determined whether invasive plants were more competitive than native or noninvasive exotic species in both the presence and absence of simulated herbivory. We found competition and herbivory additively reduced individual performance, and affected the performance of native, invasive, and noninvasive exotic species’ to the same degree. However, invasives exerted stronger competitive effects on an abundant native species (Elymus canadensis) in both the presence and absence of herbivory. Therefore, while invasive species responded similarly to competition and simulated herbivory, their competitive effects on natives may contribute to their success in their introduced range.

No release for the wicked: enemy release is dynamic and not associated with invasiveness.

The enemy release hypothesis predicts that invasive species will receive less damage from enemies, compared to co-occurring native and noninvasive exotic species in their introduced range. However, release operating early in invasion could be lost over time and with increased range size as introduced species acquire new enemies. We used three years of data, from 61 plant species planted into common gardens, to determine whether (1) invasive, noninvasive exotic, and native species experience differential damage from insect herbivores. and mammalian browsers, and (2) enemy release is lost with increased residence time and geographic spread in the introduced range. We find no evidence suggesting enemy release is a general mechanism contributing to invasiveness in this region. Invasive species received the most insect herbivory, and damage increased with longer residence times and larger range sizes at three spatial scales. Our results show that invasive and exotic species fail to escape enemies, particularly over longer temporal and larger spatial scales.

Invading from the garden? A comparison of leaf herbivory for exotic and native plants in natural and ornamental settings

Abstract The enemies release hypothesis proposes that exotic species can become invasive by escaping from predators and parasites in their novel environment. Agrawal et al. (Enemy release? An experiment with congeneric plant pairs and diverse above‐ and below‐ground enemies. Ecology, 86, 2979–2989) proposed that areas or times in which damage to introduced species is low provide opportunities for the invasion of native habitat. We tested whether ornamental settings may provide areas with low levels of herbivory for trees and shrubs, potentially facilitating invasion success. First, we compared levels of leaf herbivory among native and exotic species in ornamental and natural settings in Cincinnati, Ohio, United States. In the second study, we compared levels of herbivory for invasive and noninvasive exotic species between natural and ornamental settings. We found lower levels of leaf damage for exotic species than for native species; however, we found no differences in the amount of leaf damage suffered in ornamental or natural settings. Our results do not provide any evidence that ornamental settings afford additional release from herbivory for exotic plant species.

Increasing use of exotic forestry tree species as refuges from nest predation by the Critically Endangered Mauritius fody Foudia rubra

The population of the Critically Endangered Mauritius fody Foudia rubra fell by 55% over 1975–1993 because of habitat destruction and predation. The species was believed to be dependent on a small grove of introduced, non-invasive Cryptomeria japonica trees that offered protection from nest predation. We investigated the current population size and distribution of the fody and compared nesting success in forestry plantations to that of a released population on an offshore island. The population size on the mainland has remained stable over the past 10 years, with increases in pine Pinus spp. plantations, but continues to decline in areas of predominantly native vegetation. Only 16% of pairs found were estimated to nest in native tree species. Up to 81% of nest failures on the mainland were attributed to predation but nesting success in C. japonica and pine trees was similar to that of a released population on a predator-free offshore island. The mainland population is increasingly dependent on plantations for survival and we predict this will continue. Management and protection of non-invasive exotic species, together with creation of native habitat refuges on the mainland and offshore islands can be used to increase numbers of threatened birds in areas where predator control is not feasible.

Comparison of damage to native and exotic tallgrass prairie plants by natural enemies

We surveyed the prevalence and amount of leaf damage related to herbivory and pathogens on 12 pairs of exotic (invasive and noninvasive) and ecologically similar native plant species in tallgrass prairie to examine whether patterns of damage match predictions from the enemy release hypothesis. We also assessed whether natural enemy impacts differed in response to key environmental factors in tallgrass prairie by surveying the prevalence of rust on the dominant C4 grass, Andropogon gerardii, and its congeneric invasive exotic C4 grass, A. bladhii, in response to fire and nitrogen fertilization treatments. Overall, we found that the native species sustain 56.4% more overall leaf damage and 83.6% more herbivore-related leaf damage when compared to the exotic species. Moreover, we found that the invasive exotic species sustained less damage from enemies relative to their corresponding native species than the noninvasive exotic species. Finally, we found that burning and nitrogen fertilization both significantly increased the prevalence of rust fungi in the native grass, while rust fungi rarely occurred on the exotic grass. These results indicate that reduced damage from enemies may in part explain the successful naturalization of exotic species and the spread of invasive exotic species in tallgrass prairie.

Biomass allocation, morphology and photosynthesis of invasive and noninvasive exotic species grown at four irradiance levels

Wetestedthehypothesesthatinvasivespecieshadhigherirradianceplasticity,captureability and efficiency than noninvasive species using two invasive aliens ‐ Ageratina adenophora and Chromolaena odorata, and one noninvasive alien ‐ Gynura sp. The three aliens were grown at 4.5%,12.5%,36%,50%and100%irradiancesfor64daysbeforeharvesting.Theplasticresponse of specific leaf area (SLA) contributed to improved light interception at low irradiance, carbon gain and water balance at high irradiance. It was a good predictor for intraspecific irradiance responses of leaf area ratio (LAR), leaf area:root mass ratio, maximum photosynthetic rate (Pmax) and net assimilation rate (NAR). Biomass allocation-related traits were species specific and their plasticity to irradiance was low. The high root mass fraction, leaf mass fraction and LAR distinguished the two invaders from Gynura. However, other resource capture-related

A Comparison of Plastic Responses to Competition by Invasive and Non-invasive Congeners in the Commelinaceae

Evidence supporting an association between phenotypic plasticity and invasiveness across a range of plant taxa is based primarily on comparisons between invasive species and native species whose potential invasiveness is typically unknown. Comparison of invasive and non-invasive exotic species would provide a better test of whether plasticity promotes invasion. Such comparisons should distinguish between adaptive and non-adaptive plasticity because they have different consequences for invasiveness. Adaptive plasticity is expected to promote the invasion of multiple habitats, but non-adaptive plasticity may reflect specialization for invading more favorable habitats only. We grew four invasive and four non-invasive species of the Commelinaceae with and without competitors and compared their putatively adaptive plasticity of three traits related to competitive ability and non-adaptive plasticity in performance. The invasive species grew significantly more than the non-invasive species only in the non-competitive environment. The invasive species had greater plasticity of performance (total biomass) in response to competition than non-invasives, but there was no consistent difference in the plasticities of the traits related to competitive ability. These results are consistent with specialization of these invasive taxa for invading the more productive non-competitive environment rather than a superior ability to invade both competitive and non-competitive environments. A comprehensive understanding of the relationship between plasticity and invasiveness will require many more comparisons of the plasticity of invasive and non-invasive taxa in a range of traits in response to a variety of environments.

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 broadleaved 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.

Invasion success of exotic plants in natural ecosystems: the role of disturbance, plant attributes and freedom from herbivores

Invasion of natural ecosystems by exotic species is a major threat to biodiversity globally. We assessed two alternative (but not exclusive) hypotheses to explain the success of exotic species in urban bushland on low fertility sandstone-derived soils in Sydney, Australia. These were that success of exotic species is promoted by: (1) plant attributes in particular disturbance types; and (2) freedom from herbivores. We tested these at sites subject to different types of disturbance: nutrient and water enrichment (below stormwater outlets), nutrient enrichment (riparian zones of creeks with an urban catchment) and physical disturbance (tracks), and control sites. At each site we estimated percentage cover of all species and surveyed leaves for damage by herbivores. Species were classified as native, non-invasive exotic or invasive exotic. We found that sites without any disturbance did not support exotic plants. Physically disturbed sites on low fertility soils supported only one exotic species, suggesting that nutrient enrichment is a critical prerequisite for exotic species invasion on low fertility soils. Exotic species cover was highest and native species richness most reduced in areas of highest nutrient enrichment. Both invasive exotic and non-invasive exotic species had significantly lower levels of leaf herbivory than native species, implying that release from pests alone cannot account for the success of invasive species. Specific leaf area of invasive exotic species was consistently higher than specific leaf area of non-invasive exotic and native species, regardless of disturbance type. In physically disturbed sites of higher soil fertility, exotic species were small herbs and grasses of long flowering duration and with small unassisted or wind-dispersed seeds. In sites subject to nutrient-enrichment, exotic species were more likely to be climbers, able to propagate vegetatively, and with seeds dispersed by vertebrates. Thus different plant attributes contribute to exotic species success under different disturbance types. The clearest consistent difference we found between invasive exotic and non-invasive exotic species was in specific leaf area, suggesting that large specific leaf area facilitates invasiveness.