Breaking down barriers and expanding understanding
Some of the main tenants of The No Ivy League mission are to promote research and seek relevant societal change. To those ends we have created this dynamic space: The Know Ivy League. Here we will provide you with resources to challenge your conventions of invasion biology and ivy. It is necessary to expand our understanding outside of the comfort zone in order to create a stronger scientific community.
Please use the comment feature to fuel discussion around these issues.
Introduction of Organisms
- Plant Dispersal: The Role of Man
- Horticulture as a Pathway of Invasive Plant Introductions in the United States
- Evolution of invasiveness in plants following hybridization
- The more you introduce the more you get: the role of colonization pressure and propagule pressure in invasion ecology
Impact and Competition with Existing Organisms
- Invasion resistance and persistence: established plants win, even with disturbance and high propagule pressure
- Ecosystem-level consequences of invasions by native species as a way to investigate relationships between evenness and ecosystem function
- Predicting the spread of an invasive plant: combining experiments and ecological niche model
Novel Ecosystems and Management
- Novel ecosystems: theoretical and management aspects of the new ecological world order
- Management on the basis of the best scientific data or integration of ecological research within management? Lessons learned from the Northern spotted owl saga on the connection between research and management in conservation biology
Rethinking Invasion Biology
- How wide is the "knowing-doing" gap in invasion biology?
- Subjectivity and flexibility in invasion terminology: too much of a good thing?
Pacific Northwest Plants as Invaders
Ivy in Eurasia
- Phylogeny and Biogeography of Ivies (Hedera spp., Araliaceae), a Polyploid Complex of Woody Vines
- Ivy (Hedera helix L.) dynamics in riverine forests: Effects of river regulation and forest disturbance
- Phylogeography of the common ivy (Hedera sp.) in Europe: genetic differentiation through space and time
Ivy in the Pacific Northwest
- Prevalence of different horticultural taxa of ivy (Hedera spp., Araliaceae) in invading populations
- Understory community changes associated with English ivy invasions in Seattle's urban parks
Authors: Sarah Hayden Reichard and Peter White
Description: An in-depth investigation of past and present vectors for invasion through horticultural practices in the United States.
Source: BioScience, Vol. 51, No. 2 (February 2001), pp. 103-113
Authors: Midori M. Clarke, Sarah H. Reichard & Clement W. Hamilton
Abstract: 'English' ivy (Hedera spp.) is a complex of invasive plant pests that are separated into several distinct taxa. To better understand the invasion by ivy of Pacific Northwest native forests, we investigated the taxonomic identity of 58 selected invasive populations in the Pacific Northwest. Random amplified polymorphic DNA (RAPD) markers revealed that 83% of the 119 samples from invading populations were derived from H. hibernica (Kirchner), which has been frequently sold as English ivy, although this apparently is an incorrect common name. It is used widely in urban landscapes in the Northwest. The remaining 20 samples were either H. helix 'California,' 'Pittsburgh,' 'Star,' other cultivars not investigated in the study, or possible hybrids.
Source: Biological Invasions (2006) 8: 149-157
Author: M. Ethelwyn Millner
Description: An old article examining the tendency of Hedera helix to fuse to itself.
Source: New Phytologist, Vol. 31, No. 1 (Feb. 15, 1932), pp. 2-25
Title: Phylogeography of the common ivy (Hedera sp.) in Europe: genetic differentiation through space and time
Authors: D. Grivet and R. J. Petit
Abstract: We studied the phylogeography of ivy (Hedera sp.), a liana widespread in Europe, throughout its natural range. The populations sampled belong to four closely related species differing by ploidy levels and morphological characters. Chloroplast (cp) markers were used and 13 haplotypes were detected, usually shared across species, contrary to ribosomal internal transcribed spacer (ITS) variants. We demonstrated the existence of a strong overall cpDNA phylogeographical structure. Several methods of data analysis were conducted to describe how this structure and the genetic diversity change through space and time. Southern populations, especially those from Spain, are the most divergent. Pairwise estimates of differentiation point to isolation by distance, and the existence of a latitudinal gradient of divergence was demonstrated using a regression procedure. Similarly, latitudinal differences in haplotype richness and diversity exist, as shown by population permutations ('differentiation through space'). Finally, we measured differentiation by taking into account successive levels of divergence between haplotypes ('differentiation through time'). Genetic differentiation turns out to be much greater when differences between closely related haplotypes are not considered. Further, these results indicate that the phylogeographical structure is essentially due to the relative distribution of the most similar haplotypes. Diversity decreases from south to north, whereas haplotype frequencies change longitudinally. It appears that Hedera survived in Spanish and Balkan refugia during the last ice age. A third refugium must have been present in the Alps or in Italy. During the northward expansion, the decrease in overall diversity was attenuated by some mixing of lineages at intermediate latitudes, resulting in comparatively higher levels of differentiation in the south.
Source: Molecular Ecology (2002) 11, 1351-1362
Authors: Dunmail J. Hodkinson and Ken Thompson
Abstract: 1.) Human activity is an increasingly important mechanism of plant dispersal, particularly in densely populated countries such as England. We investigated which species are commonly dispersed by the following vectors: (i) soil carried on motor vehicles, (ii) topsoil, (iii) sugar factory topsoil, (iv) horticultural stock, and (v) garden throw-outs. 2.) We compared the ecological traits of the species associated with each vector with those of a representative sample of the regional flora. Traits examined were life history, canopy height, lateral spread, flowering start, flowering period, seed persistence in the soil, vegetative reproduction, wind dispersal, log seed weight and specific leaf area. 3.) We identified two major anthropogenic dispersal pathways, each associated with a clearly defined group of species. Species associated with topsoil, cars and horticulture depend essentially on soil movement, and are often small and fast-growing, but their most consistent unifying characteristic is the production of numerous, small, persistent seeds. In contrast, garden throw-outs, which are themselves functionally similar to increasing garden escapes, tend to be tall, spreading perennials with transient seed banks, attributes which are almost the exact opposite of the soil-borne group. 4.) Some recent studies of the British flora have failed to find any dispersal-related differences between those species with increasing or decreasing ranges, or between natives and invasive aliens. Others have found contradictory attributes of aliens; they were more likely to have bigger seeds than natives, but also more likely to have a persistent seed bank. These findings are consistent with the suggestion that there exist two contrasting groups of successful alien invaders: tall, spreading competitors and small, short-lived, fast-growing species with high reproductive outputs. The parallel with the two groups of species identified here is remarkable, and is further evidence of the probable importance of anthropogenic dispersal in the modern landscape.
Source: Journal of Applied Ecology, Vol. 34, No. 6 (Dec. 1997), pp. 1484-1496
Authors: Richard J. Hobbs, et al.
Abstract: We explore the issues relevant to those types of ecosystems containing new combinations of species that arise through human action, environmental change, and the impacts of the deliberate and inadvertent introduction of species from other regions. Novel ecosystems (also termed 'emerging ecosystems') result when species occur in combinations and relative abundances that have not occurred previously within a given biome. Key characteristics are novelty, in the form of new species combinations and the potential for changes in ecosystem functioning, and human agency, in that these ecosystems are the result of deliberate or inadvertent human action. As more of the Earth becomes transformed by human actions, novel ecosystems increase in importance, but are relatively little studied. Either the degradation or invasion of native or 'wild' ecosystems or the abandonment of intensively managed systems can result in the formation of these novel systems. Important considerations are whether these new systems are persistent and what values they may have. It is likely that it may be very difficult or costly to return such systems to their previous state, and hence consideration needs to be given to developing appropriate management goals and approaches.
Source: Global Ecology and Biogeography, (2006) 15, 1-7
Authors: McGlone, CM; Sieg, CH; Kolb, TE
Abstract: Disturbances and propagule pressure are key mechanisms in plant community resistance to invasion, as well as persistence of invasions. Few studies, however, have experimentally tested the interaction of these two mechanisms. We initiated a study in a southwestern ponderosa pine (Pinus ponderosa Laws.)/bunch grass system to determine the susceptibility of remnant native plant communities to cheatgrass (Bromus tectorum L.) invasion, and persistence of cheatgrass in invaded areas. We used a 2 x 2 factorial design consisting of two levels of aboveground biomass removal and two levels of reciprocal seeding. We seeded cheatgrass seeds in native plots and a native seed mixture in cheatgrass plots. Two biomass removal disturbances and sowing seeds over 3 years did not reverse cheatgrass dominance in invaded plots or native grass dominance in non-invaded native plots. Our results suggest that two factors dictated the persistence of the resident communities. First, bottlebrush squirreltail (Elymus elymoides (Raf.) Swezey) was the dominant native herbaceous species on the study site. This species is typically a poor competitor with cheatgrass as a seedling, but is a strong competitor when mature. Second, differences in pretreatment levels of plant-available soil nitrogen and phosphorus may have favored the dominant species in each community. Annual species typically require higher levels of plant-available soil nutrients than perennial plants. This trend was observed in the annual cheatgrass community and perennial native community. Our study shows that established plants and soil properties can buffer the influences of disturbance and elevated propagule pressure on cheatgrass invasion.
Source: Biological Invasions, Volume: 13, Issue: 2, Pages: 291-304, DOI: 10.1007/s10530-010-9806-8, Published: Feb 2011
Authors: Esler, KJ; Prozesky, H; Sharma, GP; McGeoch, M
Abstract: Invasion biology is a growing discipline with clear ecological, social and economic implications. A wide range of research effort is thus required to address the invasion problem, and literature on the topic is extensive. However, the extent to which the invasion biology research is addressing the challenges associated with management and mitigation of the impacts of invasions has been questioned. Using bibliometric analysis, we investigated the extent to which the literature on the subject contributes to implementation of knowledge generated, by addressing aspects of management, policy, and/or implementation; the impact of these papers as indicated by the number of citations they attract; and the geopolitical scale of focus of invasion ecology papers, particularly those that attempt to bridge the knowing-doing gap. We then compared these findings with the information needs of conservation practitioners. We first looked globally at popular search engines and then narrowed our focus to South Africa - one of three regions outside USA where researchers producing highly cited papers in invasion ecology are well represented. At this level, we conducted a content analysis of invasion ecology-related papers, of which at least one author was affiliated to a South African institution. The knowledge base in the field of invasion biology is comprised largely of research oriented towards "knowing," while research aimed at strategically applying or implementing that knowledge is poorly represented in the scientific literature, and the scale of its emphasis is not local. Conservation practitioners clearly indicate a need for basic knowledge. However, invasion science must develop channels for effective engagement to ensure that the research is contextualised, and will deal with the complex ecological, social and economic challenges posed by invasions.
Source: Biological Invasions Volume: 12 Issue: 12 Special Issue: SI Pages: 4065-4075 DOI: 10.1007/s10530-010-9812-x Published: Dec 2010
Authors: Ahern, RG; Landis, DA; Reznicek, AA; Schemske, DW
Abstract: We investigated the relative contribution of minimum residence time, growth habit, and history of invasiveness to the spread of exotic plants in Michigan and California. Our data include minimum residence time as estimated by earliest herbarium collection records, growth habit, and history of invasiveness for over 2000 records from two herbaria (MI = 943, CA = 1131). Our data support the hypothesis that minimum residence time is highly associated with landscape spread, explaining 39-44% of variation in the number of counties invaded. In contrast, growth habit and history of invasiveness explained a small fraction of variation in spread in California but not Michigan. Over the past 30 years exotic plant species frequently became established in Michigan and California (a parts per thousand yen50 species per decade), suggesting that many more species will become invasive over time. There is an urgent need to develop effective policies for exotic plant management. In both states we found significant positive correlations between minimum residence time and species occurrence on state invasive plant lists. Further, we found historical information on the pest status of a plant species introduced into a similar environment to be relevant in determining landscape spread of exotic plants. We conclude that efforts to predict exotic species spread based on biological characteristics may have limited success, and instead endorse pest risk analysis for proposed new imports coupled with rapid detection and early response for unintended and unwanted introductions.
Source: Biological Invasions Volume: 12 Issue: 9 Pages: 3157-3169 DOI: 10.1007/s10530-010-9707-x Published: Sep 2010
Authors: Lockwood, JL; Cassey, P; Blackburn, TM
Aim: We argue that 'propagule pressure,' a key term in invasion biology, has been attributed at least three distinct definitions (with usage of a related term causing additional confusion). All of the definitions refer to fundamental concepts within the invasion process, with the result that the distinct importance of these different concepts has been at best diluted, and at worst lost.
Methods: We reviewed pertinent literature on propagule pressure to resolve confusion about different uses of the term 'propagule pressure' and we introduced a new term for one variant, colonization pressure. We conducted a computer simulation whereby the introduction of species is represented as a simple sampling process to elucidate the relationship between propagule and colonization pressure.
Results: We defined colonization pressure as the number of species introduced or released to a single location, some of which will go on to establish a self-sustaining population and some of which will not. We subsequently argued that colonization pressure should serve as a null hypothesis for understanding temporal or spatial differences in exotic species richness, as the more species that are introduced, the more we should expect to establish. Finally, using a simple simulation, we showed that propagule pressure is related to colonization pressure, but in a non-linear manner.
Main conclusion: We suggest that the nature of the relationship between propagule pressure and colonization pressure, as well as the efficacy of various proxy measures of each, require more detailed exploration if invasion ecology is to continue to develop into a more predictive science.
Source: Diversity and Distributions Volume: 15 Issue: 5 Pages: 904-910 DOI: 10.1111/j.1472-4642.2009.00594.x Published: Sep 2009
Authors: Colautti, RI; Richardson, DM
Abstract: Invasions biologists have frequently debated whether the definition of invasive should include ecological and economic impacts. More recent criticisms posit that objective definitions are impossible in any absolute sense, while subjectivity is desirable for its flexibility and motivational qualities. We argue that such criticisms underestimate the extent of subjectivity already present in invasion biology. Ecological questions may be methodological if they relate directly to other ecological theories and models, or motivational if they focus on issues important to society as a whole. Motivational questions are important for engaging scientists, improving public understanding of science, and often have applied benefits. In contrast, methodological questions are constrained by established scientific theories, and are therefore more efficient for the development of scientific knowledge. Contrary to recent critiques, we suggest that greater objectivity is both achievable and desirable for the discipline of invasion biology and ecology generally.
Source: Biological Invasions Volume: 11 Issue: 6 Pages: 1225-1229 DOI: 10.1007/s10530-008-9333-z Published: Jun 2009
Author: Ellstrand, NC
Abstract: Despite the fact that invasion biology started as a science grounded almost exclusively in ecology, it rapidly embraced evolutionary research in the last decade. As the contents of this Special Issue testify, the subfield of evolution of invasiveness by hybridization has received increasing attention. Hybrid derived invasives have proved to be an excellent opportunity for the study of the evolution of invasiveness.
Source: Biological Invasions Volume: 11 Issue: 5 Pages: 1089-1091 DOI: 10.1007/s10530-008-9389-9 Published: May 2009
Authors: Valery, L; Fritz, H; Lefeuvre, JC; Simberloff, D
Abstract: Biodiversity is currently undermined worldwide principally as a result of human activities. The irreversibility of species extinction has encouraged the research community to investigate the potential effect of declining species or functional group diversity and/or composition on ecosystem function since the beginning of the 1990s. However, while changes in relative abundance among species (i.e., evenness) are more frequent than extinction of species and are able to cause important changes in ecosystem function, most studies have curiously not examined thoroughly the potential role of that diversity component. The few small-scale experimental manipulations that have so far examined the relationship between evenness and ecosystem function have produced ambiguous results, sometimes indicating an effect on selected functions, and sometimes not. Because one reason for the inconsistency of the previous results may be scale-dependency issues, we propose here an alternative approach, investigation of this relationship directly at the system-level through the opportunity offered by field studies of ecosystem-level consequences of invasions by native species. Indeed, the specificities of changes in ecosystem structure induced by native invaders compared to exotic ones could constitute a useful tool to improve our understanding of the relationship between evenness and ecosystem function as well as to evaluate the importance of the spatial arrangement of species in the stability of ecosystems.
Source: Biological Invasions Volume: 11 Issue: 3 Pages: 609-617 DOI: 10.1007/s10530-008-9275-5 Published: Mar 2009
Title: Management on the basis of the best scientific data or integration of ecological research within management? Lessons learned from the Northern spotted owl saga on the connection between research and management in conservation biology
Author: Gosselin, F
Abstract: The case of the Northern spotted owl (Strix occidentalis caurina) has now become a classic case study in conservation biology, characterized by a harsh social battle but also by the quantity and quality of the research performed. Based on this example, I study the way the research-management interface was organized. The main lessons I have learned were: 1. laws that involve science in management are crucial but should be more precise; 2. scientific ad-hoc groups are useful reviewers of management plans and interpreters of the best scientific data available, even if more transparent scientific argumentation is needed on some points; 3. in such applied cases, even science that has not been strongly integrated with management can produce results that are useful for management; 4. stronger links between science and management appear necessary, but difficult to implement. This last point makes me wonder whether environmental laws should not more frequently target the incorporation of science into the management process itself rather than "only" basing management on the best scientific data available. On a more ecological level, perhaps the habitat issue has been underrated during the last few years compared to other emerging threats such as the invasion of the spotted owl range by barred owls.
Source: Biodiversity and Conservation Volume: 18 Issue: 4 Pages: 777-793 DOI: 10.1007/s10531-008-9449-6 Published: Apr 2009
Authors: Boman, S; Grapputo, A; Lindstrom, L; Lyytinen, A; Mappes, J
Abstract: Predicting the spread of invasive species is a challenge for modern ecology. Although many invasive species undergo genetic bottlenecks during introduction to new areas resulting in a loss of genetic diversity, successful invaders manage to flourish in novel environments either because of pre-adaptations or because important traits contain adaptive variation enabling rapid adaptation to changing conditions. To predict and understand invasion success, it is crucial to analyse these features. We assessed the potential of a well-known invader, the Colorado potato beetle (Leptinotarsa decemlineata), to expand north of its current range in Europe. A short growing season and harsh overwintering conditions are apparent limiting factors for this species' range. By rearing full-sib families from four geographically distinct populations (Russia, Estonia, Poland, Italy) at two fluctuating temperature regimes, we investigated (a) possible differences in survival, development time, and body size among populations and (b) the amount of adaptive variation within populations in these traits. All populations were able to complete their development in cooler conditions than in their current range. A significant genotype-environment interaction for development time and body size suggests the presence of adaptive genetic variation, indicating potential to adapt to cooler conditions. The northernmost population had the highest survival rates and fastest development times on both temperature regimes, suggesting pre-adaptation to cooler temperatures. Other populations had minor differences in development times. Interestingly, this species lacks the classical trade-off between body size and development time which could have contributed to its invasion potential. This study demonstrates the importance of considering both ecological and evolutionary aspects when assessing invasion risk.
Source: Biological Invasions Volume: 10 Issue: 7 Pages: 1135-1145 DOI: 10.1007/s10530-007-9191-0 Published: Oct 2008
Authors: Ebeling, SK; Welk, E; Auge, H; Bruelheide, H
Abstract: Rapid evolutionary adjustments to novel environments may contribute to the successful spread of invasive species, and can lead to niche shifts making range dynamics unpredictable. These effects might be intensified by artificial selection in the course of breeding efforts, since many successful plant invaders were deliberately introduced and cultivated as ornamentals. We hypothesized that the invasion success of Buddleja davidii, the ornamental butterfly bush, is facilitated by local adaptation to minimum temperatures and thus, exhibits unpredictable range dynamics. To assess the potential effects of adaptive evolution and artificial selection on the spread of B. davidii, we combined a common garden experiment investigating local adaptation to frost, with ecological niche modelling of the species' native and invasive ranges. We expected that populations naturalized in sub-continental climate are less susceptible to frost than populations from oceanic climate, and that the invasive range does not match predictions based on climatic data from the native range. Indeed, we revealed significant variation among invasive B. davidii populations in frost resistance. However, frost hardiness was not related to geographic location or climatic variables of the populations' home site, suggesting that invasive B. davidii populations are not locally adapted to minimum temperatures. This is in line with results of our ecological niche model that did not detect a niche shift between the species' native range in China, and its invasive range in Europe and North America. Furthermore, our niche model showed that the potential invasive range of B. davidii is still not completely occupied. Together with the frost resistance data obtained in our experiment, the results indicate that climatic conditions are currently not limiting the further spread of the species in Europe and North America.
Source: Ecography Volume: 31 Issue: 6 Pages: 709-719 DOI: 10.1111/j.1600-0587.2008.05470.x Published: Dec 2008
Authors: Ross, CA; Faust, D; Auge, H
Abstract: Rapid evolutionary adaptations and phenotypic plasticity have been suggested to be two important, but not mutually exclusive, mechanisms contributing to the spread of invasive species. Adaptive evolution in invasive plants has been shown to occur at large spatial scales to different climatic regions, but local adaptation at a smaller scale, e.g., to different habitats within a region, has rarely been studied. Therefore, we performed a case study on invasive Mahonia populations to investigate whether local adaptation may have contributed to their spread. We hypothesized that the invasion success of these populations is promoted by adaptive differentiation in response to local environmental conditions, in particular to the different soils in these habitats. To test this hypothesis, we carried out a reciprocal transplantation experiment in the field using seedlings from five Mahonia populations in Germany that are representative for the range of habitats invaded, and a greenhouse experiment that specifically compared the responses to the different soils of these habitats. We found no evidence for local adaptation of invasive Mahonia populations because seedlings from all populations responded similarly to different habitats and soils. In a second greenhouse experiment we examined genetic variation within populations, but seedlings from different maternal families did not vary in their responses to soil conditions. We therefore suggest that local adaptation of seedlings does not play a major role for the invasion success of Mahonia populations and that phenotypic plasticity, instead, could be an important trait in this stage of the life cycle.
Source: Biological Invasions Volume: 11 Issue: 2 Pages: 441-452 DOI: 10.1007/s10530-008-9261-y Published: Feb 2009
Authors: Annik Schnitzler , Patricia Heuze´
Abstract: Ivy (Hedera helix L.) favours moist nutrient-rich substrates of the ﬂoodplains of Western Europe. In this study we investigated ivy dynamics in two forest reserves of the upper Rhine. The aim was to determine the inﬂuence of ﬂooding on ivy development following the cessation of cuttings at two forest sites of contrasting ﬂooding regimes: the Rhinau Reserve is currently ﬂooded while the Erstein site has not been ﬂooded for 30 years. We also examined the impacts of severe storms in 1993 and 1999 and a long-lasting ﬂood that occurred in 1999. Our results show that the population of Rhinau was smaller and younger because ivy is severely limited by long periods of anoxia, even though it is favoured by regular short-period ﬂoods. Indeed, juvenile growth was more rapid at Rhinau where nutrient and moist conditions are more favourable than in Erstein, but mortality was higher because of the long-lasting ﬂood of 1999. At Erstein, the ivy showed a tendency to clump around several big trees, especially oaks and ashes, which may make the host tree vulnerable to windfall. Uprooted or broken ivies were found to survive better than the fallen host tree but could not climb to another trunk highlighting a strong dependence of ivy on its hosting tree. # 2006 Elsevier B.V. All rights reserved.
Source: Forest Ecology and Management 236 (2006) 12–17 doi:10.1016/j.foreco.2006.05.060
Authors: Adam F. Green, Tara S. Ramsey and Justin Ramsey
Abstract: Ivy ( Hedera spp., Araliaceae) is a polyploid complex of woody vines. Native to Eurasia and northern Africa, ivy is cultivated
worldwide and has become an aggressive invader of North American forests. Despite its ecological impacts and economic significance to the horticultural industry, the taxonomy of Hedera is controversial and historical relationships are poorly defined. Here we characterize the phylogeny of Hedera based on the low-copy nuclear locus Granule-bound starch synthase I ( GBSSI ) and twelve non-coding cpDNA regions. Maximum parsimony and Bayesian analyses of both data sets identified Hedera as monophyletic. For GBSSI , we isolated eighteen haplotypes that were widely shared across species. There was no evidence of fixed heterozygosity or haplotype additivity in polyploids, suggesting possible autopolyploid origins. For cpDNA, we isolated sixteen haplotypes that were highly structured by geography. Haplotype diversity and phylogenetic structure were greatest in northern Africa and southern Europe. Thus, while most members of the Araliaceae reside in tropical and subtropical Asia, the early diversification of Hedera probably occurred in the Mediterranean Basin. Geographically-structured clades included diploid and polyploid species, suggesting that genome duplication has occurred repeatedly in the genus. Closely-related ivies often differed in leaf size and trichome morphology, indicating evolutionary lability of traits traditionally used for classification. Nonetheless, we recovered similar or identical DNA sequences within morphologically-defined species. Notable exceptions included southern populations of H. helix ( H. helix subsp. caucasigena and H. helix subsp. rhizomatifera ) that had cpDNA haplotypes distinct from those of central and northern Europe ( H. helix subsp. helix ).
Source: Systematic Botany (2011), 36(4): pp. 1114–1127 DOI 10.1600/036364411X605100
Authors: Katrina M. Dlugosch
Abstract: English ivy has become a common invader in Seattle’s urban parks and in forests throughout the Paciﬁc Northwest. Despite a great deal of concern over the potential impacts of this species, no studies have investigated ivy’s effects on native vegetation in this region. In this study, paired comparisons between ivy-invaded and adjacent non-invaded plots in three Seattle parks were used to quantify changes associated with ivy invasion in the forest understory. Species diversity, percent cover, and tree regeneration were surveyed. Differences in species diversity, calculated as both richness and evenness, were not signiﬁcantly different between invaded and non-invaded plots. Ivy-invaded plots did have signiﬁcantly higher total cover, and signiﬁcantly lower non-ivy cover, than non-invaded plots. The reduction of percent cover in invaded plots was primarily due to the loss of native shrubs. A plot where ivy had been removed over ﬁve years was also surveyed, and percent cover in this plot showed intermediate values relative to invaded and non-invaded plots, for both total cover and cover of native shrubs. The number of trees regenerating in the
understory was higher in invaded plots, though this difference was not signiﬁcant. These results suggest that English ivy invasions have substantial impacts on understory cover, and may inﬂuence the species composition and diversity of forest communities over the long-term by increasing vegetative cover and suppressing dominant native shrubs.
Source: Northwest Science, Vol. 79, No. 1, 2005