The impact of a biological invasion on native communities is expected to be uneven across invaded landscapes due to differences in local abiotic conditions, invader abundance, and traits and composition of the native community. One way to improve predictive ability about the impact of an invasive species given variable conditions is to exploit known mechanisms driving invasive species' success. Invasive plants frequently exhibit allelopathic traits, which can be directly toxic to plants or indirectly impact them via disruption of root symbionts, including mycorrhizal fungi. The indirect mechanism – mutualism disruption – is predicted to impact plants that rely on mycorrhizas but not affect non‐mycorrhizal plant species. To assess whether invader‐driven mutualism disruption explains observed changes in native plant communities, we analyzed long‐term (1998–2018) plant cover data from forest plots across the state of Illinois. We evaluated native plant communities experiencing a range of abundance of invasive allelopathic garlic mustard
Invasive forest pests can affect the composition and physical structure of forest canopies that may facilitate invasion by non‐native plants. However, it remains unclear whether this process is generalizable across invasive plant species at broad spatial scales, and how other landscape characteristics may simultaneously facilitate non‐native plant invasion. Here, we assembled a dataset of over 3000 repeatedly measured forest plots and quantified the impact of emerald ash borer (EAB,
- Award ID(s):
- 1638702
- NSF-PAR ID:
- 10480800
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Ecosphere
- Volume:
- 14
- Issue:
- 12
- ISSN:
- 2150-8925
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Alliaria petiolata and varying environmental conditions. Consistent with the mutualism disruption hypothesis, we showed that as garlic mustard abundance increased over time in 0.25 m2sampling quadrats, the abundance of mycorrhizal plant species decreased, but non‐mycorrhizal plant species did not. Over space and time, garlic mustard abundance predicted plant abundances and diversity at the quadrat level, but this relationship was not present at a larger scale when quadrats were aggregated within sites. Garlic mustard's impact on the plant community was highly localized, yet it was as important as abiotic variables for predicting local plant diversity. We showed that garlic mustard abundance was a key predictor of patterns of plant diversity across invasion intensity and environmental heterogeneity in a way that is consistent with mutualism disruption. Our work indicates that the mutualism disruption hypothesis can provide generalizable predictions of the impacts of allelopathic invasive plants that are evident at a broad spatial scale. -
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Abstract The mechanisms causing invasive species impact are rarely empirically tested, limiting our ability to understand and predict subsequent changes in invaded plant communities. Invader disruption of native mutualistic interactions is a mechanism expected to have negative effects on native plant species. Specifically, disruption of native plant‐fungal mutualisms may provide non‐mycorrhizal plant invaders an advantage over mycorrhizal native plants. Invasive
Alliaria petiolata (garlic mustard) produces secondary chemicals toxic to soil microorganisms including mycorrhizal fungi, and is known to induce physiological stress and reduce population growth rates of native forest understory plant species. Here, we report on a 11‐yr manipulative field experiment in replicated forest plots testing if the effects of removal of garlic mustard on the plant community support the mutualism disruption hypothesis within the entire understory herbaceous community. We compare community responses for two functional groups: the mycorrhizal vs. the non‐mycorrhizal plant communities. Our results show that garlic mustard weeding alters the community composition, decreases community evenness, and increases the abundance of understory herbs that associate with mycorrhizal fungi. Conversely, garlic mustard has no significant effects on the non‐mycorrhizal plant community. Consistent with the mutualism disruption hypothesis, our results demonstrate that allelochemical producing invaders modify the plant community by disproportionately impacting mycorrhizal plant species. We also demonstrate the importance of incorporating causal mechanisms of biological invasion to elucidate patterns and predict community‐level responses. -
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Abstract Questions A recently introduced non‐native annual grass,
Ventenata dubia , is challenging previous conceptions of community resistance in forest mosaic communities in the Inland Northwest. However, little is known of the drivers and potential ecological impacts of this rapidly expanding species. Here we (1) identify abiotic and biotic habitat characteristics associated with theV .dubia invasion and examine how these differ betweenV .dubia and other problematic non‐native annual grasses,Bromus tectorum andTaeniatherum caput‐medusae ; and (2) determine how burning influences relationships betweenV .dubia and plant community composition and structure to address potential impacts on Inland Northwest forest mosaic communities.Location Blue Mountains of the Inland Northwest, USA.
Methods We measured environmental and plant community characteristics in 110 recently burned and nearby unburned plots. Plots were stratified to capture a range of
V .dubia cover, elevations, biophysical classes, and fire severities. We investigated relationships betweenV .dubia , wildfire, environmental, and plant community characteristics using non‐metric multidimensional scaling and linear regressions.Results Ventenata dubia was most abundant in sparsely vegetated, basalt‐derived rocky scablands interspersed throughout the forested landscape. Plant communities most heavily invaded byV .dubia were largely uninvaded by other non‐native annual grasses.Ventenata dubia was abundant in both unburned and burned areas, but negative relationships betweenV .dubia cover and community diversity were stronger in burned plots, where keystone sagebrush species were largely absent after fire.Conclusions Ventenata dubia is expanding the overall invasion footprint into previously uninvaded communities. Burning may exacerbate negative relationships betweenV .dubia and species richness, evenness, and functional diversity, including in communities that historically rarely burned. Understanding the drivers and impacts of theV .dubia invasion and recognizing how these differ from other annual grass invasions may provide insight into mechanisms of community invasibility, grass‐fire feedbacks, and aid the development of species‐specific management plans. -
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Abstract Whether wild herbivores confer biotic resistance to invasion by exotic plants remains a key question in ecology. There is evidence that wild herbivores can impede invasion by exotic plants, but it is unclear whether and how this generalises across ecosystems with varying wild herbivore diversity and functional groups of plants, particularly over long‐term (decadal) time frames.
Using data from three long‐term (13‐ to 26‐year) exclosure experiments in central Kenya, we tested the effects of wild herbivores on the density of exotic invasive cacti,
Opuntia stricta andO. ficus‐indica (collectively,Opuntia ), which are among the worst invasive species globally. We also examined relationships between wild herbivore richness and elephant occurrence probability with the probability ofO. stricta presence at the landscape level (6150 km2).Opuntia densities were 74% to 99% lower in almost all plots accessible to wild herbivores compared to exclosure plots.Opuntia densities also increased more rapidly across time in plots excluding wild herbivores. These effects were largely driven by megaherbivores (≥1000 kg), particularly elephants.At the landscape level, modelled
Opuntia stricta occurrence probability was negatively correlated with estimated species richness of wild herbivores and elephant occurrence probability. On average,O. stricta occurrence probability fell from ~0.56 to ~0.45 as wild herbivore richness increased from 6 to 10 species and fell from ~0.57 to ~0.40 as elephant occurrence probability increased from ~0.41 to ~0.84. These multi‐scale results suggest that any facilitative effects ofOpuntia by wild herbivores (e.g. seed/vegetative dispersal) are overridden by suppression (e.g. consumption, uprooting, trampling).Synthesis . Our experimental and observational findings that wild herbivores confer resistance to invasion by exotic cacti add to evidence that conserving and restoring native herbivore assemblages (particularly megaherbivores) can increase community resistance to plant invasions. -
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Abstract Phylogenetic and functional diversity are theorised to increase invasion resistance. Experimentally testing whether plant communities higher in these components of diversity are less invasible is an important step for guiding restoration designs.
To investigate how phylogenetic and functional diversity of vegetation affect invasion resistance in a restoration setting, we used experimental prairie restoration plots. The experiment crossed three levels of phylogenetic diversity with two levels of functional diversity while species richness was held constant. We allowed invaders to colonise plots; these included native species from neighbouring plots and non‐native invasive species from a surrounding old field. We tested if invader biomass was influenced by phylogenetic and functional diversity, and phylogenetic and hierarchical trait distances between invaders and planted species. We binned each invader into three categories: native species from neighbouring experimental plots (
site‐specific invaders ), native species not part of the experimental species pool (native invaders ) or non‐native species (non‐native invaders ).Counter to expectation, both non‐native and native invaders became more abundant in more phylogenetically diverse plots. However, plots with higher abundance of planted Asteraceae, a dominant family of the tallgrass prairie, had lower invader biomass for both native and non‐native invaders.
We also found that hierarchical trait differences shaped invasion. The species that became most abundant were non‐native invaders that were taller, and native invaders with low specific leaf area relative to planted species. Site‐specific invaders were not influenced by any plot‐level diversity metrics tested.
Synthesis and application : Our results suggest that greater phylogenetic diversity may lower resistance to invasion. This effect may be due to more even but sparser niche packing in high‐diversity plots, associated with greater availability of unsaturated niche space for colonisation. However, trait composition fostered invasion resistance in two ways in our study. First, establishment of native species with strongly dominant traits may confer invasion resistance. Second, species mixes that optimise trait differences between planted vegetation and likely invaders may enhance invasion‐resistance.
