Large-Scale Variation in Combined Impacts of Canopy Loss and Disturbance on Community Structure and Ecosystem Functioning

Ecosystems are under pressure from multiple human disturbances whose impact may vary depending on environmental context. We experimentally evaluated variation in the separate and combined effects of the loss of a key functional group (canopy algae) and physical disturbance on rocky shore ecosystems at nine locations across Europe. Multivariate community structure was initially affected (during the first three to six months) at six locations but after 18 months, effects were apparent at only three. Loss of canopy caused increases in cover of non-canopy algae in the three locations in southern Europe and decreases in some northern locations. Measures of ecosystem functioning (community respiration, gross primary productivity, net primary productivity) were affected by loss of canopy at five of the six locations for which data were available. Short-term effects on community respiration were widespread, but effects were rare after 18 months. Functional changes corresponded with changes in community structure and/or species richness at most locations and times sampled, but no single aspect of biodiversity was an effective predictor of longer-term functional changes. Most ecosystems studied were able to compensate in functional terms for impacts caused by indiscriminate physical disturbance. The only consistent effect of disturbance was to increase cover of non-canopy species. Loss of canopy algae temporarily reduced community resistance to disturbance at only two locations and at two locations actually increased resistance. Resistance to disturbance-induced changes in gross primary productivity was reduced by loss of canopy algae at four locations. Location-specific variation in the effects of the same stressors argues for flexible frameworks for the management of marine environments. These results also highlight the need to analyse how species loss and other stressors combine and interact in different environmental contexts.     

Effects of nonnative species on the stability of riverine fish communities

Despite the increasing ubiquity of biological invasions worldwide, little is known about the scale-dependent effects of nonnative species on real-world ecological dynamics. Here, using an extensive time series dataset of riverine fish communities across different biogeographic regions of the world, we assessed the effects of nonnative species on the temporal variability and synchrony in abundance at different organizational levels (population, metapopulation, community and metacommunity) and spatial scales (stream reach and river basin). At the reach scale, we found that populations of nonnative species were more variable over time than native species, and that this effect scaled up to the community level – significantly destabilizing the dynamics of riverine fish communities. Nonnative species not only contributed to reduced community stability, but also increased variability of native populations. By contrast, we found no effect of nonnative species dominance on local interspecific synchrony among native species. At the basin scale, nonnative metapopulations were again more variable than the native ones. However, neither native metapopulations nor metacommunities showed differences in temporal variability or synchrony as nonnative species dominance increased basin-wide. This suggests a ‘dilution effect’ where the contribution to regional stability of local native populations from sites displaying low levels of invasion reduced the destabilizing effects of nonnative species. Overall, our results indicate that accounting for the destabilizing effect of nonnative species is critical to understanding native species persistence and community stability.     

Changes in woody vegetation abundance and diversity after natural disturbances causing different levels of mortality

Questions: How does woody vegetation abundance and diversity differ after natural disturbances causing different levels of mortality? Location: Abies balsamea–Betula papyrifera boreal mixed‐wood stands of southeast Quebec, Canada. Methods: Woody vegetation abundance and diversity were quantified and compared among three disturbance‐caused mortality classes, canopy gap, moderate‐severity disturbances, and catastrophic fire, using redundancy analysis, a constrained linear ordination technique, and diversity indices. Results: Substantial changes in canopy tree species abundance and diversity only occurred after catastrophic fire. Shade‐tolerant, late‐successional conifer species remained dominant after canopy gap and moderate‐severity disturbances, whereas shade‐intolerant, early‐successional colonizers dominated canopy tree regeneration after catastrophic fire. Density and diversity of mid‐tolerant and shade‐intolerant understory tree and shrub species increased as the impact of disturbance increased. Highest species richness estimates were observed after catastrophic fire, with several species establishing exclusively under these conditions. Relative abundance of canopy tree regeneration was most similar after canopy gap and moderate‐severity disturbances. For the sub‐canopy tree and shrub community, relative species abundances were most similar after moderate‐severity disturbances and catastrophic fire. Vegetation responses to moderate‐severity disturbances thus had commonalities with both extremes of the disturbance‐caused mortality gradient, but for different regeneration layers. Conclusions: Current spatio‐temporal parameters of natural disturbances causing varying degrees of mortality promote the development of a complex, multi‐cohort forest condition throughout the landscape. The projected increase in time intervals between catastrophic fires may lead to reduced diversity within the system.     

Dominance by a canopy forming seaweed modifies resource and consumer control of bloom-forming macroalgae

Degradation of ecological resources by large-scale disturbances highlights the need to demonstrate biological properties that increase resistance to change and promote the resilience of ecosystem regimes. Coastal eutrophication is a global-scale disturbance that drives ecosystem change by increasing primary production and favouring ephemeral and bloom-forming life-forms. Recent synthesis indicates that consumption processes increase the resistance of coastal communities to nutrient loading by controlling the responses of ephemeral macroalgae. Here we suggest a similar ecological function for canopy cover by demonstrating that the presence of a canopy species modifies both resource and consumer control of bloom-forming algae associated with nutrient enrichment. We tested effects of canopy presence on the interaction between consumer and resource control, by field-manipulations of a dominant canopy forming seaweed ( Fucus vesiculosus ), grazer presence (dominated by the gastropod Littorina littorea ) and nutrient enrichment (common agricultural NPK fertilizer). Canopy cover and grazers jointly controlled strong increases of ephemeral bloom-forming algae (dominated by Ulva spp) from nutrient enrichment; nutrients increased ephemeral recruitment almost 10-fold, but only in the absence of both grazers and canopy cover. Recruitment success of the canopy-forming seaweed itself decreased additively with 56.1, 71.3 and 50.5% from independent effects of canopy cover, grazers and nutrient enrichment, respectively. A meta-analysis of nine nutrient enrichment experiments including seaweed, seagrass and stream communities, showed that in the presence of canopies average nutrient effects were reduced by more than 90% compared to without canopies. This corroborates the generality of our finding that dominating canopy species are important for aquatic ecosystems by increasing community resistance to the propagation of nutrient effects.     

Synchrony in small mammal community dynamics across a forested landscape

Long‐term studies at local scales indicate that fluctuations in abundance among trophically similar species are often temporally synchronized. Complementary studies on synchrony across larger spatial extents are less common, as are studies that investigate the subsequent impacts on community dynamics across the landscape. We investigate the impact of species population fluctuations on concordance in community dynamics for the small mammal fauna of the White Mountain National Forest, USA. Hierarchical open population models, which account for imperfect detection, were used to model abundance of the most common species at 108 sites over a three year period. Most species displayed individualistic responses of abundance to forest type and physiographic characteristics. However, among species, we found marked synchrony in population fluctuations across years, regardless of landscape affinities or trophic level. Across the region, this population synchrony led to high within‐year concordance of community composition and aggregate properties (e.g. richness and diversity) independent of forest type and low among‐year similarity in communities, even for years with similar species richness. Results suggest that extrinsic factors primarily drive abundance fluctuations and subsequently community dynamics, although local community assembly may be modified by species dispersal abilities and biotic interactions. Concordant community dynamics across space and over time may impact the stability of regional food webs and ecosystem functions.     

Dendrochronological analysis of the canopy history of two Ohio old-growth forests

This study examined the temporal patterns of establishment, suppression, and release of major tree species in two old-growth Ohio forest remnants as a means to determine the past disturbance history of these forests. Increment cores were taken from a total of 154 trees from two well-drained, upland plots and two poorly-drained, bottomland plots in each of the two forested areas. Acer saccharum and Fagus grandifolia exhibited multiple episodes of suppression and release prior to becoming canopy trees, and could tolerate suppressions as long as 84 years. In contrast, Quercus macrocarpa, Q. muehlenbergii, Prunus serotina, and Acer saccharinum rerely exhibited any tolerance to suppression and appeared to have entered the canopy after single disturbances had opened large areas of canopy. There was clear synchrony in the temporal pattern of establishment and final release from suppression among trees from bottomland plots scattered throughout the stands, indicating that relatively large disturbances were important in these poorly-drained areas. In contrast, there was little synchrony among trees from well-drained upland plots, except in a single instance where selective cutting of Quercus trees opened the canopy. Thus, the canopy of upland site was likely subjected only to small disturbances resulting from the death of one or a few trees. At the whole of forest level, there was evidence of episodic recruitment of canopy trees in both forests. Establishment of Fraxinus spp. and Quercus spp. were particularly episodic, and few Fraxinus or Quercus trees alive today established during the last century. These data suggest that large disturbances have affected canopy dynamics of both upland and bottomland areas prior to 1900 and in bottomland forests through this century. In contrast, disturbances in upland areas during this century have been restricted to small, treefall-generated canopy gaps.     

Effects of mean intensity and temporal variability of disturbance on the invasion of <Emphasis Type="Italic">Caulerpa racemosa</Emphasis> var. <Emphasis Type="Italic">cylindracea</Emphasis> (Caulerpales) in rock pools

Disturbance is a key factor influencing the invasibility of habitats and assemblages. This relationship was extensively studied in terrestrial systems, but it was scarcely tested in the marine environment. We investigated experimentally the interactive effects of changes in the intensity and temporal variability of mechanical disturbance by boulders on invasion dynamics of the green alga Caulerpa racemosa var. cylindracea in littoral rock pools. We tested the hypothesis that the success of invasion of C. racemosa would be (1) greater under large than under low intensity of disturbance, (2) greater under large than under low temporal variability of disturbance and that (3) interactive effects could also occur, with variability of disturbance magnifying the effects of intensity. C. racemosa was virtually absent in pools maintained under high intensity of disturbance, independently of temporal variability. High intensity of disturbance was also associated with lower density and length of fronds and thinner diameter of the stolons of the alga. The total number of native taxa and the abundance of encrusting coralline algae increased under high intensity of disturbance. Differently, turf-forming algae were positively affected by temporal variability of disturbance, while canopy-forming algae did not respond to experimental treatments. Our results suggest a direct negative effect of the most severe experimental conditions on the spread of C. racemosa in rock pools. This likely overwhelmed likely concomitant positive and negative effects mediated by resident organisms. The results of this study help anticipating invasion dynamics of C. racemosa in rock pools under climate change scenarios, in which both intensity and temporal variability of extreme meteorological events are predicted to increase.     

Annual variation of community biomass is lower in more diverse stream fish communities

Anthropogenic influences have disproportionally affected freshwater ecosystems, and a loss of biodiversity is forecasted to greatly reduce ecosystem function and services. Loss of species may destabilize communities by limiting the stabilizing forces of compensatory dynamics and/or statistical averaging, both of which are effects that can buffer variation in aggregate community properties. Currently, support for positive diversity‐stability relationships stems from experiments with simple communities at small spatial and temporal scales, and application to natural communities is limited. Using a long‐term dataset of 35 stream fish communities matched with hydrologic data, we show that community stability (annual variation of standing biomass of fishes) was less variable in more species‐rich communities and was not associated with stream hydrology. Only the statistical averaging model of community stability was consistent with observed patterns of lower biomass variation in more species‐rich communities. Our findings suggest anthropogenically induced extirpation of vertebrate consumers may lower community biomass stability in complex ecosystems.     

Species synchrony and its drivers: neutral and nonneutral community dynamics in fluctuating environments

Independent species fluctuations are commonly used as a null hypothesis to test the role of competition and niche differences between species in community stability. This hypothesis, however, is unrealistic because it ignores the forces that contribute to synchronization of population dynamics. Here we present a mechanistic neutral model that describes the dynamics of a community of equivalent species under the joint influence of density dependence, environmental forcing, and demographic stochasticity. We also introduce a new standardized measure of species synchrony in multispecies communities. We show that the per capita population growth rates of equivalent species are strongly synchronized, especially when endogenous population dynamics are cyclic or chaotic, while their long-term fluctuations in population sizes are desynchronized by ecological drift. We then generalize our model to nonneutral dynamics by incorporating temporal and nontemporal forms of niche differentiation. Niche differentiation consistently decreases the synchrony of species per capita population growth rates, while its effects on the synchrony of population sizes are more complex. Comparing the observed synchrony of species per capita population growth rates with that predicted by the neutral model potentially provides a simple test of deterministic asynchrony in a community.     

Phylogenetic diversity stabilizes community biomass

Aims The relationship between biodiversity and ecological stability is a long-standing issue in ecology. Current diversity–stability studies, which have largely focused on species diversity, often report an increase in the stability of aggregate community properties with increasing species diversity. Few studies have examined the linkage between phylogenetic diversity, another important dimension of biodiversity, and stability. By taking species evolutionary history into account, phylogenetic diversity may better capture the diversity of traits and niches of species in a community than species diversity and better relate to temporal stability. In this study, we investigated whether phylogenetic diversity could affect temporal stability of community biomass independent of species diversity.Methods We performed an experiment in laboratory microcosms with a pool of 12 bacterivorous ciliated protist species. To eliminate the possibility of species diversity effects confounding with phylogenetic diversity effects, we assembled communities that had the same number of species but varied in the level of phylogenetic diversity. Weekly disturbance, in the form of short-term temperature shock, was imposed on each microcosm and species abundances were monitored over time. We examined the relationship between temporal stability of community biomass and phylogenetic diversity and evaluated the role of several stabilizing mechanisms for explaining the influence of phylogenetic diversity on temporal stability.Important findings Our results showed that increasing phylogenetic diversity promoted temporal stability of community biomass. Both total community biomass and summed variances showed a U-shaped relationship with phylogenetic diversity, driven by the presence of large, competitively superior species that attained large biomass and high temporal variation in their biomass in both low and high phylogenetic diversity communities. Communities without these species showed patterns consistent with the reduced strength of competition and increasingly asynchronous species responses to environmental changes under higher phylogenetic diversity, two mechanisms that can drive positive diversity–stability relationships. These results support the utility of species phylogenetic knowledge for predicting ecosystem functions and their stability.     

Combined disturbances and the role of their spatial and temporal properties in shaping community structure

Disturbances are characteristic for many ecosystems. However, we still lack generalizations concerning their role in shaping communities, particularly when disturbances co-occur. To study such effects, we used a novel modeling approach that is unrestricted by a priori tradeoffs among specific plant traits, except for those generated by allocation principles. Thus, trait combinations were emergent properties associated with biotic and abiotic constraints. Specifically, we asked which traits dominate under specific disturbance regimes, whether single and combined disturbance regimes promote similar trait tradeoffs and how complex disturbance regimes affect species richness and functional diversity. Overall, disturbances’ temporal properties governed the outcome of combined disturbances and were a stronger assortative force than spatial disturbance properties: low temporal predictability decreased seed-dispersability and dormancy, but increased competitive ability and disturbance tolerance. Evidence for tradeoffs between different colonization modes and between dormancy and disturbance tolerance were found, while surprisingly, the widely accepted colonization–competition tradeoff was not generated. Diversity was highest at intermediate disturbance intensity, but decreased monotonically with increasing unpredictability. In accordance with our results, future models should avoid restrictive assumptions about tradeoffs to generate robust and more general predictions about the role of disturbances for community dynamics.     

Temporal stability in forest productivity increases with tree diversity due to asynchrony in species dynamics

Theory predicts a positive relationship between biodiversity and stability in ecosystem properties, while diversity is expected to have a negative impact on stability at the species level. We used virtual experiments based on a dynamic simulation model to test for the diversity–stability relationship and its underlying mechanisms in Central European forests. First our results show that variability in productivity between stands differing in species composition decreases as species richness and functional diversity increase. Second we show temporal stability increases with increasing diversity due to compensatory dynamics across species, supporting the biodiversity insurance hypothesis. We demonstrate that this pattern is mainly driven by the asynchrony of species responses to small disturbances rather than to environmental fluctuations, and is only weakly affected by the net biodiversity effect on productivity. Furthermore, our results suggest that compensatory dynamics between species may enhance ecosystem stability through an optimisation of canopy occupancy by coexisting species.     

Trophic compensation reinforces resistance: herbivory absorbs the increasing effects of multiple disturbances

Disturbance often results in small changes in community structure, but the probability of transitioning to contrasting states increases when multiple disturbances combine. Nevertheless, we have limited insights into the mechanisms that stabilise communities, particularly how perturbations can be absorbed without restructuring (i.e. resistance). Here, we expand the concept of compensatory dynamics to include countervailing mechanisms that absorb disturbances through trophic interactions. By definition, ‘compensation’ occurs if a specific disturbance stimulates a proportional countervailing response that eliminates its otherwise unchecked effect. We show that the compounding effects of disturbances from local to global scales (i.e. local canopy‐loss, eutrophication, ocean acidification) increasingly promote the expansion of weedy species, but that this response is countered by a proportional increase in grazing. Finally, we explore the relatively unrecognised role of compensatory effects, which are likely to maintain the resistance of communities to disturbance more deeply than current thinking allows.     

Mycorrhizal suppression and phosphorus addition influence the stability of plant community composition and function in a temperate steppe

Nutrient enrichment can reduce ecosystem stability, typically measured as temporal stability of a single function, e.g. plant productivity. Moreover, nutrient enrichment can alter plant–soil interactions (e.g. mycorrhizal symbiosis) that determine plant community composition and productivity. Thus, it is likely that nutrient enrichment and interactions between plants and their soil communities co-determine the stability in plant community composition and productivity. Yet our understanding as to how nutrient enrichment affects multiple facets of ecosystem stability, such as functional and compositional stability, and the role of above–belowground interactions are still lacking. We tested how mycorrhizal suppression and phosphorus (P) addition influenced multiple facets of ecosystem stability in a three-year field study in a temperate steppe. Here we focused on the functional and compositional stability of plant community; functional stability is the temporal community variance in primary productivity; compositional stability is represented by compositional resistance, turnover, species extinction and invasion. Community variance was partitioned into population variance defined as community productivity weighted average of the species temporal variance in performance, and species synchrony defined as the degree of temporal positive covariation among species. Compared to treatments with mycorrhizal suppression, the intact AM fungal communities reduced community variance in primary productivity by reducing species synchrony at high levels of P addition. Species synchrony and population variance were linearly associated with community variance with the intact AM fungal communities, while these relationships were decoupled or weakened by mycorrhizal suppression. The intact AM fungal communities promoted the compositional resistance of plant communities by reducing compositional turnover, but this effect was suppressed by P addition. P addition increased the number of species extinctions and thus promoted compositional turnover. Our study shows P addition and AM fungal communities can jointly and independently modify the various components of ecosystem stability in terms of plant community productivity and composition.     

Stability and synchrony across ecological hierarchies in heterogeneous metacommunities: linking theory to data

Understanding stability across ecological hierarchies is critical for landscape management in a changing world. Recent studies showed that synchrony among lower‐level components is key to scaling temporal stability across two hierarchical levels, whether spatial or organizational. But an extended framework that integrates both spatial scale and organizational level simultaneously is required to clarify the sources of ecosystem stability at large scales. However, such an extension is far from trivial when taking into account the spatial heterogeneities in real‐world ecosystems. In this paper, we develop a partitioning framework that bridges variability and synchrony measures across spatial scales and organizational levels in heterogeneous metacommunities. In this framework, metacommunity variability is expressed as the product of local‐scale population variability and two synchrony indices that capture the temporal coherence across species and space, respectively. We develop an R function ‘var.partition’ and apply it to five types of desert plant communities to illustrate our framework and test how diversity shapes synchrony and variability at different hierarchical levels. As the observation scale increased from local populations to metacommunities, the temporal variability of plant productivity was reduced mainly by factors that decreased species synchrony. Species synchrony decreased from local to regional scales, and spatial synchrony decreased from species to community levels. Local and regional species diversity were key factors that reduced species synchrony at the two scales. Moreover, beta diversity contributed to decreasing spatial synchrony among communities. We conclude that our new framework offers a valuable toolbox for future empirical studies to disentangle the mechanisms and pathways by which ecological factors influence stability at large scales.     

Using Light-Use and Production Efficiency Models to Predict Photosynthesis and Net Carbon Exchange During Forest Canopy Disturbance

Vegetation growth models are used with remotely sensed and meteorological data to monitor terrestrial carbon dynamics at a range of spatial and temporal scales. Many of these models are based on a light-use efficiency equation and two-component model of whole-plant growth and maintenance respiration that have been parameterized for distinct vegetation types and biomes. This study was designed to assess the robustness of these parameters for predicting interannual plant growth and carbon exchange, and more specifically to address inconsistencies that may arise during forest disturbances and the loss of canopy foliage. A model based on the MODIS MOD17 algorithm was parameterized for a mature upland hardwood forest by inverting CO₂ flux tower observations during years when the canopy was not disturbed. This model was used to make predictions during a year when the canopy was 37% defoliated by forest tent caterpillars. Predictions improved after algorithms were modified to scale for the effects of diffuse radiation and loss of leaf area. Photosynthesis and respiration model parameters were found to be robust at daily and annual time scales regardless of canopy disturbance, and differences between modeled net ecosystem production and tower net ecosystem exchange were only approximately 2 g C m⁻² d⁻¹ and less than 23 g C m⁻² y⁻¹. Canopy disturbance events such as insect defoliations are common in temperate forests of North America, and failure to account for cyclical outbreaks of forest tent caterpillars in this stand could add an uncertainty of approximately 4–13% in long-term predictions of carbon sequestration.     

Effects of kelp canopies on bleaching and photosynthetic activity of encrusting coralline algae

Canopy-forming algae often coexist with an understorey of encrusting coralline algae that bleach following the loss of canopies. We tested the hypothesis that canopy loss causes a reduction in photosynthetic activity of encrusting coralline algae concomitant with their bleaching. When canopies were experimentally removed, corallines bleached and their photosynthetic activity was rapidly reduced to half their activity observed under canopies. This result prompted us to test, and subsequently accept, the hypothesis that exposure of understorey corallines to enhanced light intensity per se (simulation of canopy loss) acts as a mechanism that causes bleaching and reduced photosynthetic activity. Despite bleaching, encrusting corallines maintained reduced levels of photosynthetic activity, and this may explain why, under certain conditions, bleached corallines can persist in the absence of canopy-forming algae. Nevertheless, our data provide evidence that the positive association between canopy-forming algae and encrusting coralline algae is maintained because of shade provided by the canopy.     

Shrubs, granivores and annual plant community stability in an arid ecosystem

Compensatory population dynamics among species stabilise aggregate community variables. Inter-specific competition is thought to be stabilising as it promotes asynchrony among populations. However, we know little about other inter-specific interactions, such as facilitation and granivory. Such interactions are also likely to influence population synchrony and community stability, especially in harsh environments where they are thought to have relatively strong effects in plant communities. We use a manipulative experiment to test the effects of granivores (harvester ants) and nurse plants (dwarf shrubs) on annual plant community dynamics in the Negev desert, Israel. We present evidence for weak and inconsistent effects of harvester ants on plant abundance and on population and community stability. By contrast, we show that annual communities under shrubs were more species rich, had higher plant density and were temporally less variable than communities in the inter-shrub matrix. Species richness and plant abundance were also more resistant to drought in the shrub under-storey compared with the inter-shrub matrix, although population dynamics in both patch types were synchronised. Hence, we show that inter-specific interactions other than competition affect community stability, and that hypothesised mechanisms linking compensatory dynamics and community stability may not operate to the same extent in arid plant communities.     

Population synchrony and stability in environmentally forced metacommunities

A general prediction from simple metapopulation models is that spatially synchronized forcing can spatially synchronize population dynamics and destabilize metapopulations. In contrast, spatially asynchronous forcing is predicted to decrease population synchrony and promote temporal stability and population persistence, especially in the presence of dispersal. Only recently have studies begun to experimentally address these predictions. Moreover, few studies have experimentally examined how such processes operate in the context of competition communities. Stabilizing processes may continue to operate when placed within a metacommunity context with multiple competing consumers but only at low to intermediate levels of dispersal. High dispersal rates can reverse these predictions and lead to destabilization. We tested this under controlled conditions using an experimental aquatic system composed of three competing species of zooplankton. Metacommunities experienced different levels of dispersal and environmental forcing in the form of spatially synchronous or asynchronous pH perturbations. We found support that dispersal can have contrasting effects on population stability depending on the degree to which population dynamics were synchronized in space. Dispersal under synchronous forcing or no forcing had either neutral of positive effects on spatial population synchrony of all three zooplankton species. In these treatments, dispersal reduced population stability at the local and metapopulation levels for two of three species. In contrast, asynchronously varying environments reduced population synchrony relative to unforced systems, regardless of dispersal level. In these treatments, dispersal enhanced temporal stability and persistence of populations not by reducing population synchrony but by enhancing population minima and spatial averaging of abundances. High dispersal rates under asynchronous forcing reduced the abundance of one species, consistent with increasing regional competition and general metacommunity theory. However, no effects on its stability or persistence were observed. Our work highlights the context‐dependent effects of dispersal on population dynamics in varying environments.     

Establishment of the introduced kelp Undaria pinnatifida in Tasmania depends on disturbance to native algal assemblages

Despite recent rapid increases in the occurrence of nonindigenous marine organisms in the marine environment, few studies have critically examined the invasion process for a marine species. Here we use manipulative experiments to examine processes of invasion for the Asian kelp Undaria pinnatifida (Harvey) Suringar at two sites on the east coast of Tasmania. Disturbance to reduce cover of the native algal canopy was found to be critical in the establishment of U. pinnatifida, while the presence of a stable native algal canopy inhibited invasion. In the first sporophyte growth season following disturbance of the canopy, U. pinnatifida recruited in high densities (up to 19 plants m⁻²) while remaining rare or absent in un-manipulated plots. The timing of disturbance was also important. U. pinnatifida recruited in higher densities in plots where the native canopy was removed immediately prior to the sporophyte growth season (winter 2000), compared with plots where the canopy was removed 6 months earlier during the period of spore release (spring 1999). Removal of the native canopy also resulted in a significant increase in cover of sediment on the substratum. In the second year following canopy removal, U. pinnatifida abundance declined significantly, associated with a substantial recovery of native canopy-forming species. A feature of the recovery of the native algal canopy was a significant shift in species composition. Species dominant prior to canopy removal showed little if any signs of recovery. The recovery was instead dominated by canopy-forming species that were either rare or absent in the study areas prior to manipulation of the canopy.