Effects of vegetation removal on native understory recovery in an exotic-rich urban forest1

We used multiscale plots to sample vascular plant diversity and soil characteristics in and adjacent to 26 long-term grazing exclosure sites in Colorado, Wyoming, Montana, and South Dakota, USA. The exclosures were 7–60 yr old (31.2 ± 2.5 yr, mean ± 1 se). Plots were also randomly placed in the broader landscape in open rangeland in the same vegetation type at each site to assess spatial variation in grazed landscapes. Consistent sampling in the nine National Parks, Wildlife Refuges, and other management units yielded data from 78 1000-m2 plots and 780 1-m2 subplots. We hypothesized that native species richness would be lower in the exclosures than in grazed sites, due to competitive exclusion in the absence of grazing. We also hypothesized that grazed sites would have higher native and exotic species richness compared to ungrazed areas, due to disturbance (i.e., the intermediate-disturbance hypothesis) and the conventional wisdom that grazing may accelerate weed invasion. Both hypotheses were soundly rejected. Although native species richness in 1-m2 subplots was significantly higher (P < 0.05) in grazed sites, we found nearly identical native or exotic species richness in 1000-m2 plots in exclosures (31.5 ± 2.5 native and 3.1 ± 0.5 exotic species), adjacent grazed plots (32.6 ± 2.8 native and 3.2 ± 0.6 exotic species), and randomly selected grazed plots (31.6 ± 2.9 native and 3.2 ± 0.6 exotic species). We found no significant differences in species diversity (Hill’s diversity indices, N1 and N2), evenness (Hill’s ratio of evenness, E5), cover of various life-forms (grasses, forbs, and shrubs), soil texture, or soil percentage of N and C between grazed and ungrazed sites at the 1000-m2 plot scale. The species lists of the long-ungrazed and adjacent grazed plots overlapped just 57.9 ± 2.8%. This difference in species composition is commonly attributed solely to the difference in grazing regimes. However, the species lists between pairs of grazed plots (adjacent and distant 1000-m2 plots) in the same vegetation type overlapped just 48.6 ± 3.6%, and the ungrazed plots and distant grazed plots overlapped 49.4 ± 3.6%. Differences in vegetation and soils between grazed and ungrazed sites were minimal in most cases, but soil characteristics and elevation were strongly correlated with native and exotic plant diversity in the study region. For the 78 1000-m2 plots, 59.4% of the variance in total species richness was explained by percentage of silt (coefficient = 0.647, t = 5.107, P < 0.001), elevation (coefficient = 0.012, t = 5.084, P < 0.001), and total foliar cover (coefficient = 0.110, t = 2.104, P < 0.039). Only 12.8% of the variance in exotic species cover (log10cover) was explained by percentage of clay (coefficient = −0.011, t = −2.878, P < 0.005), native species richness (coefficient = −0.011, t = −2.156, P < 0.034), and log10N (coefficient = 2.827, t = 1.860, P < 0.067). Native species cover and exotic species richness and frequency were also significantly positively correlated with percentage of soil N at the 1000-m2 plot scale. Our research led to five broad generalizations about current levels of grazing in these Rocky Mountain grasslands: (1) grazing probably has little effect on native species richness at landscape scales; (2) grazing probably has little effect on the accelerated spread of most exotic plant species at landscape scales; (3) grazing affects local plant species and life-form composition and cover, but spatial variation is considerable; (4) soil characteristics, climate, anddisturbances may have a greater effect on plant species diversity than do current levels of grazing; and (5) few plant species show consistent, directional responses to grazing or cessation of grazing.

We used multiscale plots to sample vascular plant diversity and soil characteristics in and adjacent to 26 long-term grazing exclosure sites in Colorado, Wyoming, Montana, and South Dakota, USA. The exclosures were 7–60 yr old (31.2 ± 2.5 yr, mean ± 1 se). Plots were also randomly placed in the broader landscape in open rangeland in the same vegetation type at each site to assess spatial variation in grazed landscapes. Consistent sampling in the nine National Parks, Wildlife Refuges, and other management units yielded data from 78 1000-m2 plots and 780 1-m2 subplots. We hypothesized that native species richness would be lower in the exclosures than in grazed sites, due to competitive exclusion in the absence of grazing. We also hypothesized that grazed sites would have higher native and exotic species richness compared to ungrazed areas, due to disturbance (i.e., the intermediate-disturbance hypothesis) and the conventional wisdom that grazing may accelerate weed invasion. Both hypotheses were soundly rejected. Although native species richness in 1-m2 subplots was significantly higher (P < 0.05) in grazed sites, we found nearly identical native or exotic species richness in 1000-m2 plots in exclosures (31.5 ± 2.5 native and 3.1 ± 0.5 exotic species), adjacent grazed plots (32.6 ± 2.8 native and 3.2 ± 0.6 exotic species), and randomly selected grazed plots (31.6 ± 2.9 native and 3.2 ± 0.6 exotic species). We found no significant differences in species diversity (Hill’s diversity indices, N1 and N2), evenness (Hill’s ratio of evenness, E5), cover of various life-forms (grasses, forbs, and shrubs), soil texture, or soil percentage of N and C between grazed and ungrazed sites at the 1000-m2 plot scale. The species lists of the long-ungrazed and adjacent grazed plots overlapped just 57.9 ± 2.8%. This difference in species composition is commonly attributed solely to the difference in grazing regimes. However, the species lists between pairs of grazed plots (adjacent and distant 1000-m2 plots) in the same vegetation type overlapped just 48.6 ± 3.6%, and the ungrazed plots and distant grazed plots overlapped 49.4 ± 3.6%. Differences in vegetation and soils between grazed and ungrazed sites were minimal in most cases, but soil characteristics and elevation were strongly correlated with native and exotic plant diversity in the study region. For the 78 1000-m2 plots, 59.4% of the variance in total species richness was explained by percentage of silt (coefficient = 0.647, t = 5.107, P < 0.001), elevation (coefficient = 0.012, t = 5.084, P < 0.001), and total foliar cover (coefficient = 0.110, t = 2.104, P < 0.039). Only 12.8% of the variance in exotic species cover (log10cover) was explained by percentage of clay (coefficient = −0.011, t = −2.878, P < 0.005), native species richness (coefficient = −0.011, t = −2.156, P < 0.034), and log10N (coefficient = 2.827, t = 1.860, P < 0.067). Native species cover and exotic species richness and frequency were also significantly positively correlated with percentage of soil N at the 1000-m2 plot scale. Our research led to five broad generalizations about current levels of grazing in these Rocky Mountain grasslands: (1) grazing probably has little effect on native species richness at landscape scales; (2) grazing probably has little effect on the accelerated spread of most exotic plant species at landscape scales; (3) grazing affects local plant species and life-form composition and cover, but spatial variation is considerable; (4) soil characteristics, climate, anddisturbances may have a greater effect on plant species diversity than do current levels of grazing; and (5) few plant species show consistent, directional responses to grazing or cessation of grazing.

Exotic species research has generated several paradigms about the effects of invasion on native ecosystems and the site characteristics that promote invasibility. We are interested in translating these theoretical paradigms into management recommendations. Using vegetation surveys of urban riparian forests in central North Carolina, we tested the competition and resource availability paradigms. We assessed the association between exotic and native species and identified potential resources that promote invasion. Exotic and native species richness was negatively correlated (r = −0.66, p = 0.0009), conforming to the predictions of the competition paradigm. In particular, native woody species were negatively associated with several exotic growth forms. Two of the most common exotic species, Hedera helix (English ivy) and Microstegium vimineum (Japanese stilt grass), did not co-occur with several native woody plants, suggesting that they may preclude the establishment and regeneration of native woody...

AimA better understanding of plant invasions on islands can be gained from comparing patterns of exotic and native species richness. We asked four questions: (1) Is exotic species richness on islands related to native species richness? (2) If they are related, does this result from similar responses of native and exotic species to specific island characteristics? (3) Is residual variation in native‐exotic richness relationships associated with distinctive island characteristics? (4) Are relationships between species richness and island characteristics scale‐dependent, and do they differ between native and exotic species?LocationNorthern New Zealand.TaxonVascular plants.MethodsWe conducted field surveys and augmented our field data with previously published surveys to quantify the number of native and exotic plant species on 264 islands. We then explored the relationship of species richness and several island characteristics (e.g. area and isolation) using multiple and iterative regression techniques.ResultsSeventy‐two percent of among‐island variation in exotic species richness was positively related to native species richness. Both native and exotic richness increased with island area and declined with isolation and exposure to ocean‐borne disturbances (a proxy for salt spray, wave action, etc). However, exotic species responded more strongly to these three variables. Exotic richness also decreased with latitude and the distance from the nearest urban area, but native species did not. Island area was a better predictor of species richness on larger islands, whereas isolation and exposure were better predictors on smaller islands. Scale‐dependent relationships between species richness and island characteristics were stronger for exotic species.Main ConclusionsInsular distribution patterns of native and exotic plant species richness are governed by similar biogeographic principles. However, in New Zealand, exotic species exhibited subtle, yet distinctive, invasion patterns preferring larger, less isolated, less exposed islands that were located at higher latitudes and closer to urban areas.

The climate is changing and temperatures are predicted to further increase in the future. Species respond to these changes by either adapting to the local warmer conditions and/or range shifting to higher latitudes. Some of these successful range shifting plants can become invasive in their new range. Therefore, there is a conceptual analogy of successful range shifts and biological invasions originating from other continents. Intra-continental plant species shift their ranges within the same contiguous land mass from which they originate. Inter-continental species originate from other continents from where they have been introduced before expanding in their new range. The aim of this thesis is to gain a better understanding of the plant-insect interactions that may contribute to the success of exotic plants that have expanded their ranges due to climate warming. More specifically I aimed to clarify whether climate warming-induced range expanding exotic plants are less suitable than native congener plants and whether these plants suffer less from aboveground enemies than native congener plants. In addition, I investigated if inter –and intra-continental exotic plant species differed in their suitability and if they responded differently to potential aboveground enemies. In the first greenhouse experiment, I tested the hypothesis that inter- and intra-continental exotic plants and phylogenetically related native plants from the same habitat do not respond differently to two aboveground polyphagous herbivores. Further I tested if intra- and inter-continental exotic plants experience less negative soil feedback than related native plants. I grew fifteen plant species with and without naive polyphagous locusts (Schistocerca gregaria) and cosmopolitan aphids (Myzus persicae) and exposed all plants to soils from their invaded range in order to test the feedback from the soil community to plant biomass production. My results show that that both inter –and intra-continental exotic plants on average were better defended against aboveground and belowground enemies than related native plant species. This suggests that successful range expanding plants may include species with invasive properties. Exotic plants have been shown to have neutral to positive soil feedbacks, while native plants experience negative effects from their soil biota. Belowground interactions can influence aboveground interactions and may change the relationships between exotic plants and their enemies. I examined how the performance of the two aboveground polyphagous herbivores S. gregaria and M. persicae species was influenced by feedback interactions between the plants and their soil biota and compared these responses in intra- and inter-continental exotic and related native plants. Locust mass was negatively affected by the plant specific soil community and larger on native than on exotic plants. Locust survival was also higher on native plants, but not affected by soil type. There were no differences between inter –and intra-continental plants. Aphid population size was not affected by soil type, but was highest on the intra-continental range expander. The body size of M. persicae was larger on control than on soils with specific plant communities and not affected by plant origin. One way of measuring the release of exotic plants from natural enemies is by comparing their herbivore loads with related plants that are native in the invaded range. These loads can be influenced by top down control of insect predators and parasitoids. In the field, I examined herbivore loads and predator pressure on two exotic (inter-continental and intra-continental) and two related native plant species. I found smaller herbivore loads on the exotic plant species than on the related native plants. Moreover, the herbivores on the exotic plants had a higher predator pressure than herbivores on the phylogenetically related native plants. These results imply that both types of exotic plants have a double advantage: enhanced bottom-up and top-down control of herbivores. Finally, I set up a field experiment to test the effect of herbivory on communities of exotic and native plants. I created ten communities with six exotic plant species and their phylogenetically related native species that co-occur in the same riverine habitat. Half of the communities were exposed to herbivory and the other half was grown in a herbivory-free environment. This study was done in order to test if exotic plants may dominate invaded plant communities exposed to aboveground herbivory and if this advantage of the exotic plants under herbivory would disappear when all plants were free of herbivores. Herbivory reduced aboveground plant biomass by almost half. However, exotic plants did not become the exclusive dominants in these communities, as some native species were well protected against aboveground herbivory as well. Plant species varied considerably in their responses to herbivory resulting in changes in community ranking. Interestingly, the proportional biomass contributions to the community were similar for exotic and native plant species and also not different between inter –and intra-continental plants. I conclude that release from aboveground enemies is not the only factor explaining the invasive success of intra- and inter-continental exotic plant species. In conclusion, climate warming-induced range expanding plant species originating from the same continent may possess invasive properties comparable to introduced inter-continental exotic plants. In the greenhouse and in the field, both inter- and intra-continental exotic plant species were more resistant against aboveground herbivores than native plants. In the greenhouse, the exotic plants suffered less from herbivory than related natives, although this did not result in their absolute dominance in the field when exposed to herbivory. Therefore, aboveground enemy exposure is not the only factor predicting the invasive success of intra- and inter-continental exotic plant species.

Biological invasion is a leading threat to the ecological integrity of forest ecosystems around the world. The objective of our study was to examine the relationship between the abundance of non-native species in the vegetation and the soil seed bank to abiotic factors and distance from the nearest forest edge. We identified and estimated the abundance of plant species in sixty-two 0.4 ha permanent plots in natural areas across the Shawnee National Forest in southern Illinois in late spring and summer 2002. The distance of each plot to the closest forest edge was determined using GIS. A 550 cm3 soil core was extracted from five random points within each of the plots in August 2002 to allow measurement of soil nutrient levels and estimates of the abundance of exotic and native species in the soil seed bank. Soils from these cores were placed in trays in a greenhouse on 10 January 2003. Seedlings emerging from the soil cores were identified and tallied every two weeks until no further seed germination was observed. The plots were located deep in the forest interior (up to 730 m), yet forest edge effects predominate. Distance from forest edges was negatively related to density of both native and non-native species in the seed bank. The largest numbers of non-natives in the seed bank were present in those plots closest to the forest edge. None of the nine important abiotic factors (K, Ca, P, Mg, pH, CEC, and OM, % silt, canopy opening) measured in this study was related to the seed bank density of the native and non-native species. Potassium was negatively and calcium was positively related to richness of understory native species and distance from forest edge. Phosphorus, Mg, pH, CEC, % silt, OM, and canopy opening were not related to the richness of native species and distance. Commelina communis, Lonicera japonica, Microstegium vimineum, and Rosa multiflora were non-native species present in the understory vegetation while Barbarea vulgaris, Cardamine hirsuta, Lactuca serriola, Lespedeza cuneata, M. vimineum, Mollugo verticillata, and Stellaria media were non-native species present in the seed bank. The discordance between non-native species richness in the seed bank and vegetation community indicates that management of invasive species must extend beyond eradication of non-natives in the aboveground vegetation. Land and forest managers should eradicate seedlings of non-native species before they reach seed production stage to decrease their accumulation in the seed bank even at sites located at a great distance from the forest edge.

Land managers require landscape-scale information on where exotic plant species have successfully established, to better guide research, control, and restoration efforts. We evaluated the vulnerability of various habitats to invasion by exotic plant species in a 100,000 ha area in the southeast corner of Grand Staircase-Escalante National Monument, Utah. For the 97 0.1-ha plots in 11 vegetation types, exotic species richness (log10) was strongly negatively correlated to the cover of cryptobiotic soil crusts (r =− 0.47, P< 0.001), and positively correlated to native species richness (r = 0.22, P< 0.03), native species cover (r = 0.23, P< 0.05), and total nitrogen in the soil (r = 0.40, P< 0.001). Exotic species cover was strongly positively correlated to exotic species richness (r = 0.68, P< 0.001). Only 6 of 97 plots did not contain at least one exotic species. Exotic species richness was particularly high in locally rare, mesic vegetation types and nitrogen rich soils. Dry, upland plots (n = 51) had less than half of the exotic species richness and cover compared to plots (n = 45) in washes and lowland depressions that collect water intermittently. Plots dominated by trees had significantly greater native and exotic species richness compared to plots dominated by shrubs. For the 97 plots combined, 33% of the variance in exotic species richness could be explained by a positive relationship with total plant cover, and negative relationships with the cover of cryptobiotic crusts and bare ground. There are several reasons for concern: (1) Exotic plant species are invading hot spots of native plant diversity and rare/unique habitats. (2) The foliar cover of exotic species was greatest in habitats that had been invaded by several exotic species. (3) Continued disturbance of fragile cryptobiotic crusts by livestock, people, and vehicles may facilitate the further invasion of exotic plant species.

As urbanization expands into rural areas, an increase in the number of non-native plant species at the urban-rural interface is expected due in large part to the increased availability of propagules from ornamental plantings. A study investigating the distribution of non-native plants in the understories of riparian forests across an urban-to-rural gradient north of Columbus, GA was initiated in 2003. A significantly greater number of non-native plant species occurred at the urban sites and at one site at the urban-rural interface, where 20 to 33% of the species encountered were non-native. In contrast, at the more rural sites non-native species comprised 4–14% of the total number of species. However, the importance values of non-native species as a whole did not change significantly across the land use gradient due to the high frequency and abundance of three non-native species (Ligustrum sinense, Lonicera japonica, and Microstegium vimineum) in the majority of the watersheds. Reductions in species richness and overstory reproduction associated with these non-natives could impact long-term forest structure and ecosystem function.

Grazing and an invasive grass confound spatial pattern of exotic and native grassland plant species richness

We used data from a 15-year experiment in a C4-dominated grassland to address the effects of community structure (i.e., plant species richness, dominance) and disturbance on invasibility, as measured by abundance and richness of exotic species. Our specific objectives were to assess the temporal and spatial patterns of exotic plant species in a native grassland in Kansas (USA) and to determine the factors that control exotic species abundance and richness (i.e., invasibility). Exotic species (90% C3 plants) comprised approximately 10% of the flora, and their turnover was relatively high (30%) over the 15-year period. We found that disturbances significantly affected the abundance and richness of exotic species. In particular, long-term annually burned watersheds had lower cover of exotic species than unburned watersheds, and fire reduced exotic species richness by 80-90%. Exotic and native species richness were positively correlated across sites subjected to different fire (r = 0.72) and grazing (r = 0.67) treatments, and the number of exotic species was lowest on sites with the highest productivity of C4 grasses (i.e., high dominance). These results provide strong evidence for the role of community structure, as affected by disturbance, in determining invasibility of this grassland. Moreover, a significant positive relationship between exotic and native species richness was observed within a disturbance regime (annually burned sites, r = 0.51; unburned sites, r = 0.59). Thus, invasibility of this C4-dominated grassland can also be directly related to community structure independent of disturbance.

Quantitative integration of factors that potentially affect exotic species richness and abundance at multiple spatial scales is relatively scarce in the literature. Our aim was to address this gap by evaluating the relative importance of the biotic community, abiotic factors, and landscape characteristics on the establishment and spread of native and exotic plant species. We assessed the effect of these factors on exotic and native species richness and abundance, and used regression tree and variation partitioning analyses to evaluate how these predictors interact to favor or limit exotic and/or native species. We found that landscape filters were especially important for the arrival of both native and exotic species, whereas biotic factors seemed to regulate the abundance of plant species once they were present within the system. However, the combined effects of different types of predictors explained the largest fraction of total variation in all models regarding exotic species. Furthermore, significant predictor variables had opposite effects on native versus exotic species at both local and landscape scales, which suggests that some ecosystem properties affect native and exotic species differently. Exotic species richness and abundance were increased by low values of native species cover and diversity, high landscape heterogeneity and edge density, human disturbances (e.g., mowing and soil disruption), land use activities (e.g., developed and agricultural areas), and proximity to transportation systems, especially highways. However, exotic species were less common in areas with low anthropogenic disturbance, where natural disturbances seemed to favor native plant species.

Exotic plant species are increasingly becoming the focus of research and have been identified as a component of human-induced global change. Successful invaders may alter soil conditions, but the effect of exotic species on soil microbial communities has not been studied. We studied two exotic understory plant species (Japanese barberry [Berberis thunbergii] and Japanese stilt grass [Microstegium vimineum]) in hardwood forests in northern New Jersey, USA. We sampled bulk and rhizosphere soils under the two exotic species, as well as under a co-occurring native species (blueberry [Vaccinium spp.]). We indexed the structure (by measuring phospholipid fatty acid [PLFA] profiles) and function (by measuring enzyme activities and substrate-induced respiration [SIR] profiles) of microbial communities in the sampled soils. Soils under the three species differed in microbial community structure and function. These differences were observed in both the rhizosphere and bulk soil samples. Differences in the structural variables were correlated to differences in the functional variables as demonstrated by canonical correlation analysis. These results indicate that successful exotic invasive species can have profound effects on the microbial community of the soil.

Nutrient enrichment with phosphorus (P) and nitrogen (N) threatens biodiversity globally, particularly because it drives invasion by exotic plant species. However, the effects of nutrients on plants can interact with other ecosystem processes such as competition and grazing, and these interactions can be scale‐dependent. Furthermore, P and N are often correlated, making it difficult to separate the effects of each. After implementing a herbivore‐exclusion experiment for 3 years in grassy woodlands, SE Australia, we assessed the relationship of native and exotic plant species with soil nutrients, grazing and spatial scale. Nitrate (NO3), available P and organic carbon (C) were partitioned into two principle components and further divided into plot‐ and site‐level variation. The correlated components of NO3, P and C (principle component 1) appeared to drive a dominant competitive relationship. Two exotic annual grasses likely out‐competed native and exotic species in areas high in NO3, P and C. In contrast, responses to principle component 2 (gradient from high NO3 to high P) indicated that native and exotic species generally benefitted when NO3 was high relative to P. Plant–nutrient relationships were further modified by grazing and spatial scale. Native and exotic species benefitted from grazing but did so under specific nutrient regimes. Summed occurrence of exotic species and native richness was reduced in the absence of grazing and high values of NO3 relative to P at site scales, but not at plot scales, suggesting competition is more important at a scale of hundreds of square metres while soil properties are most important at the plot scale (25 m2). Synthesis and applications. Dividing soil nutrients into uncorrelated principal components showed that niche space for native and exotic plant species is partitioned by positive and negative covariation of nitrate (NO3), phosphorus (P) and carbon (C), and interactions of these soil properties with grazing and spatial scale. Increasing NO3 relative to P could increase species richness, but requires further experimentation to define beneficial concentrations. Native plant richness can be enhanced with adequate levels of grazing by native herbivores and, in the long term, by reducing P from areas enriched with NO3 and P. These actions will help prevent likely competitive dominance by annual exotic grasses.

Understanding the biotic and abiotic factors that influence the susceptibility of a community to invasion is beneficial for the prediction and management of invasive species and the conservation of native biodiversity. However, the relationships between factors and invasibility of a community have not been fully confirmed, and the factors most associated with the susceptibility of a community to invasion have rarely been identified. In this study, we investigated the species richness patterns in aquatic exotic and native plants and the relationships of exotic species richness with habitat and water environment factors in 262 aquatic plant communities in China. A total of 11 exotic plant species were recorded in our field survey, and we found neither a negative nor a positive relationship between aquatic exotic and native plant species richness. The aquatic exotic plant species richness is negatively correlated with the relative coverage and biomass of native plants but positively correlated with the total nitrogen (TN), total phosphorus (TP), and chemical oxygen demand (COD) concentrations in the water. The native plant species richness, native species’ relative coverage, and native species’ biomass were positively related to each other, whereas the TP, TN, and COD were also positively related to each other. The native plant species richness, native species’ relative coverage, and native species biomass were each negatively correlated with the TP, TN, and COD. In addition, biotic rather than abiotic predictors accounted for most of the variation in exotic plant richness. Our results suggest that improving the vegetation coverage and the biodiversity of native plants is the most effective approach for preventing alien plant invasions and minimizing their impacts on freshwater ecosystems.

Disturbances of oceanic origin can severely affect plant communities on islands, but it is unclear whether they promote or deter biological invasions. Here, I collected floristic data from 97 small islands subject to different levels of ocean-borne disturbances (i.e. inside and outside Wellington Harbour, New Zealand). First, I tested how relationships between the richness of native and exotic species and island characteristics (e.g. area, isolation, height, distance from nearest dwelling) changed depending on island location. Next, I assessed compositional differences on inner and outer islands for both native and exotic species, and how they vary with geographic distance between islands (i.e. distance-decay). Results show that the richness of both native and exotic plant species was similarly related to island characteristics regardless of island location. Both native and exotic species richness consistently increased with area and nearest dwelling. However, only exotics richness always declined with isolation, while natives richness alone consistently increased with height (elevation). Natives on outer, more exposed islands were floristically more homogenous, and compositional differences changed less strongly with the distance between islands than inside Wellington harbour. In contrast, exotics exhibited similar distributional patterns regardless of island location. Different levels of ocean-borne disturbances might explain distinct distributional patterns in native species. Conversely, results for exotic species might reflect a lack of coastal specialists in the species pool. Perhaps time-lags in the invasion process and non-equilibrium dynamics play a role as well. Conservation bodies should similarly manage islands sustaining different levels of ocean-borne disturbances.