Sublethal effects on reproduction in native fauna: are females more vulnerable to biological invasion?

Although invasive species are a major threat to survivorship of native species, we know little about their sublethal effects. In soft-sediment marine systems, mat-forming invasive species often have positive effects, facilitating recruitment and enhancing the diversity and abundance of native invertebrates. However, because mat-forming invasive species change the habitat in which they invade, and benthic invertebrates are sensitive to environmental disturbance, important sublethal effects on native species may exist. Using a model marine system we show that the widespread mat-forming invasive alga Caulerpa taxifolia (Vahl) C. Agardh has strong negative effects on the reproductive traits of a native bivalve Anadara trapezia (Deshayes, 1840) (e.g. timing of reproductive development and spawning, and follicle and gamete production) even though the invader has positive effects on recruitment. Moreover, gender specific responses occurred and indicated that females were more susceptible to invasion than males. Our results indicate that sublethal effects of an invasive species on reproductive traits will have severe consequences for fitness of the native species.     

Do settlement dynamics influence competitive interactions between an alien tunicate and its native congener?

Variation in density of early stages, that is, larvae and juveniles, is a major determinant of the distribution and abundance of the adult population of most marine invertebrates. These early stages thus play a key role in competitive interactions, and, more specifically, in invasion dynamics when biologically similar native and non‐native species (NNS) come into contact in the same habitat. We examined the settlement dynamics and settlement rate of two important members of the fouling community that are common on human‐made infrastructures around the world: Ciona robusta (formerly known as Ciona intestinalis type A) and C. intestinalis (formerly known as C. intestinalis type B). In the western English Channel, the two species live in close syntopy following the recent introduction of C. robusta in the native European range of C. intestinalis. Using settlement panels replaced monthly over 2 years in four marinas (including one studied over 4 years) and species‐diagnostic molecular markers to distinguish between juveniles of both species (N = 1,650), we documented similar settlement dynamics of both species, with two settlement periods within a calendar year. With one exception, settlement times were highly similar in the congeners. Although the NNS showed lower settlement density than that of the native congener, its juvenile recruitment was high during the second settlement period that occurs after the warm season, a pattern also observed in adult populations. Altogether, our results suggest that species’ settlement dynamics do not lead to the dominance of one species over the other through space monopolization. In addition, we showed that changes over time are more pronounced in the NNS than in the native species. This is possibly due to a higher sensitivity of the NNS to changes of environmental factors such as temperature and salinity. Environmental changes may thus eventually modify the strength of competitive interactions between the two species as well as species dominance.     

Geographical variation in germination traits of the salt-marsh cordgrass Spartina alterniflora in its invasive and native ranges

入侵地和原产地盐沼植物互花米草种子萌发性状的地理变异 种子萌发是植物早期生活史中最重要的阶段,决定了植物的生态位和地理分布范围,对外来植物的入侵潜力有重要影响。盐沼植物互花米草(Spartina alterniflora)在中国沿海滩涂的入侵范围最大,并已入侵到比原产地更低的纬度范围,这为我们研究互花米草在不同地理区域之间以及沿纬度梯度的萌发性状差异和适应提供了契机。在控温培养箱中淡水培养条件下,我们比较研究了来自入侵地(19°–40° N)10 个地点和原产地(27°–43° N)16个地点不同纬度互花米草种群的种子萌发性状差异,以及这种差异与各种 群来源地潮差和气候因素的相关性。原产地互花米草种群种子的萌发率和萌发指数比入侵地种群分别高10%和20%,但入侵地互花米草种群的萌发速度比原产地快3 d。入侵地互花米草种群的萌发率和萌发 指数随着纬度升高呈现线性递增的变化趋势,而原产地呈现线性递减的变化趋势。入侵地和原产地互花米草种群的平均萌发时间都与纬度呈现线性负相关。入侵地互花米草种群的萌发率和萌发指数与年日均温、年日最低均温、和年日最高均温呈现负相关,而在原产地呈现相反的相关关系。入侵地和原产地互花米草种群的平均萌发时间分别与年日均温、年日最低均温和年日最高均温呈现正相关关系。我们的研究结果表明,入侵地和原产地互花米草种群的萌发率和萌发指数已沿纬度进化出不同的渐变群格局,但平均萌发时间进化出与原产地一致的纬度渐变群格局,即在生物入侵过程中沿纬度梯度种子萌发策略会随着入侵时间和过程而发生变化。     

Adaptation to an invasive host is driving the loss of a native ecotype

Locally adapted populations are often used as model systems for the early stages of ecological speciation, but most of these young divergent populations will never become complete species. The maintenance of local adaptation relies on the strength of natural selection overwhelming the homogenizing effects of gene flow; however, this balance may be readily upset in changing environments. Here I show that soapberry bugs (Jadera haematoloma) have lost adaptations to their native host plant (Cardiospermum corindum) and are regionally specializing on an invasive host (Koelreuteria elegans), collapsing a classic and well‐documented example of local adaptation. All populations that were adapted to the native host—including those still found on that host today—are now better adapted to the invasive host in multiple phenotypes. Weak differentiation remains in two traits, suggesting that homogenization across the region is incomplete. This study highlights the potential for adaptation to invasive species to disrupt native communities by swamping adaptation to native conditions through maladaptive gene flow.     

The density and spatial arrangement of the invasive oyster Crassostrea gigas determines its impact on settlement of native oyster larvae

Understanding how the density and spatial arrangement of invaders is critical to developing management strategies of pest species. The Pacific oyster, Crassostrea gigas, has been translocated around the world for aquaculture and in many instances has established wild populations. Relative to other species of bivalve, it displays rapid suspension feeding, which may cause mortality of pelagic invertebrate larvae. We compared the effect on settlement of Sydney rock oyster, Saccostrea glomerata, larvae of manipulating the spatial arrangement and density of native S. glomerata, and non‐native C. gigas. We hypothesized that while manipulations of dead oysters would reveal the same positive relationship between attachment surface area and S. glomerata settlement between the two species, manipulations of live oysters would reveal differing density‐dependent effects between the native and non‐native oyster. In the field, whether oysters were live or dead, more larvae settled on C. gigas than S. glomerata when substrate was arranged in monospecific clumps. When, however, the two species were interspersed, there were no differences in larval settlement between them. By contrast, in aquaria simulating a higher effective oyster density, more larvae settled on live S. glomerata than C. gigas. When C. gigas was prevented from suspension feeding, settlement of larvae on C. gigas was enhanced. By contrast, settlement was similar between the two species when dead. While the presently low densities of the invasive oyster C. gigas may enhance S. glomerata larval settlement in east Australian estuaries, future increases in densities could produce negative impacts on native oyster settlement. Synthesis and applications: Our study has shown that both the spatial arrangement and density of invaders can influence their impact. Hence, management strategies aimed at preventing invasive populations reaching damaging sizes should not only consider the threshold density at which impacts exceed some acceptable limit, but also how patch formation modifies this.     

Ocean currents influence the genetic structure of an intertidal mollusc in southeastern Australia – implications for predicting the movement of passive dispersers across a marine biogeographic barrier

Major disjunctions among marine communities in southeastern Australia have been well documented, although explanations for biogeographic structuring remain uncertain. Converging ocean currents, environmental gradients, and habitat discontinuities have been hypothesized as likely drivers of structuring in many species, although the extent to which species are affected appears largely dependent on specific life histories and ecologies. Understanding these relationships is critical to the management of native and invasive species, and the preservation of evolutionary processes that shape biodiversity in this region. In this study we test the direct influence of ocean currents on the genetic structure of a passive disperser across a major biogeographic barrier. Donax deltoides (Veneroida: Donacidae) is an intertidal, soft‐sediment mollusc and an ideal surrogate for testing this relationship, given its lack of habitat constraints in this region, and its immense dispersal potential driven by year‐long spawning and long‐lived planktonic larvae. We assessed allele frequencies at 10 polymorphic microsatellite loci across 11 sample locations spanning the barrier region and identified genetic structure consistent with the major ocean currents of southeastern Australia. Analysis of mitochondrial DNA sequence data indicated no evidence of genetic structuring, but signatures of a species range expansion corresponding with historical inundations of the Bassian Isthmus. Our results indicate that ocean currents are likely to be the most influential factor affecting the genetic structure of D. deltoides and a likely physical barrier for passive dispersing marine fauna generally in southeastern Australia.     

The relationship between egg size and the duration of the facultative feeding period in marine invertebrate larvae

Feeding larvae of marine invertebrates fuel development from both endogenous egg energy and exogenous energy obtained from the planktonic environment. Although both sources of energy likely influence certain larval stages, only the effects of exogenous food have been well studied. Despite the lack of research on the effects of egg size on larval stages, investigators have hypothesized that egg size influences the duration of the facultative feeding stage—the stage in which larvae can feed but do not have to because development is still being fueled by egg energy. To test this hypothesis, we investigated six species of sand dollars with different sized eggs and quantified the duration of the larval facultative feeding period of each species by comparing when fed and starved larvae diverged in size. Regardless of whether phylogeny was taken into account, the duration of the facultative feeding period was positively correlated with egg size. We further determined that our conclusions were not sensitive to either our estimation of the duration of the facultative feeding period, or the branch lengths of the phylogeny we used. This relationship is likely a result of larger eggs being provisioned with more energy, and may affect how well larvae can cope with natural variability in food concentrations. Furthermore, our results support an assumption of a theoretical model developed to understand the evolution of different life-history strategies in marine invertebrate larvae, which suggests that this relationship has important evolutionary consequences.     

Competition during early ontogeny: Effects of native and invasive planktivores on the growth,survival, and habitat use of bluegill

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Molecular species identification of morphologically similar mussel larvae reveals unexpected discrepancy between relative abundance of adults and settlers

Understanding the relative importance of larval supply vs. post-settlement mortality underlies studies of marine invertebrate recruitment, yet is often hampered by researchers' inability to identify species among morphologically similar larvae or early juveniles. In New Zealand, two species of co-occurring intertidal mytilid mussels have morphologically indistinguishable settlers: the blue mussel Mytilus galloprovincialis, which is often numerically dominant in the mid-zone of the rocky intertidal, and the ribbed mussel Aulacomya atra maoriana which is often much less abundant. In this study, we obtained samples of newly settled mussels from 6 sample dates April-May 2005 from the rocky intertidal in Wellington Harbour, New Zealand. We used PCR-RFLP of the cytochrome c oxidase subunit I (COI) mitochondrial gene region to identify settlers to species. Of a total of 224 settlers that could be identified, 64% were identified as Mytilus galloprovincialis and 36% as Aulocomya atra maoriana. The percentage of A. atra maoriana in the samples was unexpectedly high and ranged from 22–50% among the sample dates. This study reinforces the need to quantify larval supply at the species level to understand the relative importance of pre- and post-settlement mortality, and also demonstrates the usefulness of the COI region as a species-specific marker for identifying mussel larvae and juveniles.     

Transient sensitivities of non-indigenous shrub species indicate complicated invasion dynamics

As biological invasions increasingly affect natural systems, the need for methods that can quantify the processes responsible for invasion success has increased. Further, methods should be geared to the formulation of management strategies. Demographic analyses are designed to explore the causes and properties of population change. Matrix population models, a commonly used technique for demographic analysis, have been applied to the analysis of stage-structured populations. However, most commonly, analyses have focused on long-term outcomes dynamics (ergodic dynamics). The methods available for analysis of matrix population models have recently been extended to facilitate analysis of the transient dynamics most important to invasion analysis. In this paper we analyze the transient population dynamics of three invasive shrubs and compare them to ergodic dynamics. Cytisus scoparius, Clidemia hirta, and Ardisia elliptica come from different parts of the world and are all now found in the United States of America. They also have published transition matrices that measure the probabilities that any one life-history stage will transition to another over an annual time step. These matrices have been estimated from multi-year data collected from plots in various environments. Our comparative study of transient and ergodic dynamics of invasive shrubs shows that, for all the considered shrub species, there was a clear difference between the sensitivities drawn from these two approaches. The transient sensitivities of earlier life-history transitions showed magnified importance relative to ergodic sensitivities. This was especially true of A. elliptica for which the stable population structure was most different from the starting structure analyzed in detail here. For other species, as stable population structures were heavily weighted towards early stages, the differences in the importance of early transitions transiently and ergodically were less dramatic. Late life transitions showed magnified importance in areas towards the center of the invasion or in older invasion areas. Finally, populations with shorter estimated generation times show greater transient sensitivity to early life-history stages; but the pattern was complex and varied according to species, and was also observed across other life-history transitions. Overall, the ambiguity and complexity of the results highlight the power of considering transient population dynamics for invading species, as well as the importance of specific biological and ecological knowledge of the invading species. Although there may be commonalities across invasions, important decisions on control or inference on population dynamics should treat invasions as individual, unique events.     

Tough Adults,Frail Babies: An Analysis of Stress Sensitivity across Early Life-History Stages of Widely Introduced Marine Invertebrates

All ontogenetic stages of a life cycle are exposed to environmental conditions so that population persistence depends on the performance of both adults and offspring. Most studies analysing the influence of abiotic conditions on species performance have focussed on adults, while studies covering early life-history stages remain rare. We investigated the responses of early stages of two widely introduced ascidians, Styela plicata and Microcosmus squamiger, to different abiotic conditions. Stressors mimicked conditions in the habitats where both species can be found in their distributional ranges and responses were related to the selection potential of their populations by analysing their genetic diversity. Four developmental stages (egg fertilisation, larval development, settlement, metamorphosis) were studied after exposure to high temperature (30°C), low salinities (26 and 22‰) and high copper concentrations (25, 50 and 100 µg/L). Although most stressors effectively led to failure of complete development (fertilisation through metamorphosis), fertilisation and larval development were the most sensitive stages. All the studied stressors affected the development of both species, though responses differed with stage and stressor. S. plicata was overall more resistant to copper, and some stages of M. squamiger to low salinities. No relationship was found between parental genetic composition and responses to stressors. We conclude that successful development can be prevented at several life-history stages, and therefore, it is essential to consider multiple stages when assessing species'' abilities to tolerate stress. Moreover, we found that early development of these species cannot be completed under conditions prevailing where adults live. These populations must therefore recruit from elsewhere or reproduce during temporal windows of more benign conditions. Alternatively, novel strategies or behaviours that increase overall reproductive success might be responsible for ensuring population survival.     

Soil conditioning effects of Phragmites australis on native wetland plant seedling survival

Interactions between introduced plants and soils they colonize are central to invasive species success in many systems. Belowground biotic and abiotic changes can influence the success of introduced species as well as their native competitors. All plants alter soil properties after colonization but, in the case of many invasive plant species, it is unclear whether the strength and direction of these soil conditioning effects are due to plant traits, plant origin, or local population characteristics and site conditions in the invaded range. Phragmites australis in North America exists as a mix of populations of different evolutionary origin. Populations of endemic native Phragmites australis americanus are declining, while introduced European populations are important wetland invaders. We assessed soil conditioning effects of native and non‐native P. australis populations on early and late seedling survival of native and introduced wetland plants. We further used a soil biocide treatment to assess the role of soil fungi on seedling survival. Survival of seedlings in soils colonized by P. australis was either unaffected or negatively affected; no species showed improved survival in P. australis‐conditioned soils. Population of P. australis was a significant factor explaining the response of seedlings, but origin (native or non‐native) was not a significant factor. Synthesis: Our results highlight the importance of phylogenetic control when assessing impacts of invasive species to avoid conflating general plant traits with mechanisms of invasive success. Both native (noninvasive) and non‐native (invasive) P. australis populations reduced seedling survival of competing plant species. Because soil legacy effects of native and non‐native P. australis are similar, this study suggests that the close phylogenetic relationship between the two populations, and not the invasive status of introduced P. australis, is more relevant to their soil‐mediated impact on other plant species.     

Role of habitat in mediating mortality during the post-settlement transition phase of temperate marine fishes

The transition phase describes a distinct post-settlement stage associated with the recruitment to benthic habitats by pelagic life stages. The habitat shift is often accompanied by feeding shifts and metamorphosis from larval to juvenile phases. Density-dependent settlement, growth and mortality are often the major factors controlling recruitment success of this phase. Habitat use also becomes more pronounced after settlement. The role of habitat-mediated post-settlement mortality is elucidated by focusing on the early life history of Atlantic cod ( Gadus morhua ) and cunner ( Tautogolabrus adspersus ) in the north-west Atlantic. In these species, settlement can occur over all bottom types, but habitat-specific differences in post-settlement mortality rates combined with size and priority at settlement effects on growth and survival determine recruitment and eventual year-class strength. These results and those from other temperate marine fish species along with work on tropical reef species emphasize the generality of habitat-based density-dependent mortality during the transition phase and its potential for population regulation. These results have implications for fisheries management and can be used to outline a procedure to assist managers in identifying and managing essential transitional habitats including the potential role of marine protected areas in habitat conservation.     

Lemon sole Microstomus kitt in the northern North Sea: a multidisciplinary approach to the early life-history dynamics

Lemon sole Microstomus kitt is a commercially valuable flatfish species that occurs in shelf waters around the northeast Atlantic. Only the most basic life-history information is available for the North Sea. Spawning is generally assumed to occur between early May and October, with a peak between May and August. Lemon sole larvae have been found in the water column in the northern North Sea in winter during standard surveys. Larvae captured in November/December 2016 and January/February 2017 using the International Council for the Exploration of the Seas standard 2 m Midwater Ring trawls (MIK) were analysed to gain a better understanding of the pelagic early life-history stages of lemon sole, especially in relation to the timing of spawning and the dispersal of overwintering larvae. Larval age was estimated from sagittal otolith primary increment counts. The larvae caught in November/December ranged in nominal age from 4 to 45 days post-hatching which suggests that spawning continues into late October and November. Most, but not all, of the larvae caught in January/February were post metamorphosis, and the difference in age between the two sampling dates was consistent with the elapsed time between samplings. The estimated hatching dates confirm that lemon sole spawning extends into late autumn in the northern North Sea, with overwintering larvae in all developmental stages. Drift modelling of eggs and larvae released at historically documented spawning grounds in the northern North Sea suggests that these grounds are also the source for all of the larvae sampled during the 2016–2017 surveys.     

Native and invasive plant interactions in wetlands and the minimal role of invasiveness

The effects of invasive plants on plants native to areas that are being invaded can be quite variable, depending on the species of the invasive plant involved as well as the physical characteristics of the location being invaded. My study focuses on the effects of Phragmites australis Linnaeus (common reed) and Lythrum salicaria L. (purple loosestrife) on the same native plant community. Uninvaded plots dominated by native plants Typha angustifolia L. (narrowleaf cattail) and Typha latifolia L. (broadleaf cattail) served as the control. I surveyed percent cover of species during early summer and midsummer for 3 years in six Hudson River freshwater tidal wetlands (sites). Differences in species richness, composition and abundance were small, but significant among invaded and uninvaded plots and among sites. However, these differences remained significant when data for dominant species (invasive and native) were removed. Differences in native plant species abundance were attributed to invasive plant species-specific characteristics and differences in species richness and composition were attributed to physical location (zonation) in these freshwater tidal marshes. “Invasive” status of a dominant plant species was less important in invasive plant–native plant interactions than species-specific characteristics and zonation. Further research into the effects of site and land-use on invasive plant impacts is recommended.     

Ecosystem functioning impacts of the invasive seaweed Sargassum muticum (Fucales,Phaeophyceae)

Ongoing changes in natural diversity due to anthropogenic activities can alter ecosystem functioning. Particular attention has been given to research on biodiversity loss and how those changes can affect the functioning of ecosystems, and, by extension, human welfare. Few studies, however, have addressed how increased diversity due to establishment of nonindigenous species (NIS) may affect ecosystem function in the recipient communities. Marine algae have a highly important role in sustaining nearshore marine ecosystems and are considered a significant component of marine bioinvasions. Here, we examined the patterns of respiration and light‐use efficiency across macroalgal assemblages with different levels of species richness and evenness. Additionally, we compared our results between native and invaded macroalgal assemblages, using the invasive brown macroalga Sargassum muticum (Yendo) Fensholt as a model species. Results showed that the presence of the invader increased the rates of respiration and production, most likely as a result of the high biomass of the invader. This effect disappeared when S. muticum lost most of its biomass after senescence. Moreover, predictability–diversity relationships of macroalgal assemblages varied between native and invaded assemblages. Hence, the introduction of high‐impact invasive species may trigger major changes in ecosystem functioning. The impact of S. muticum may be related to its greater biomass in the invaded assemblages, although species interactions and seasonality influenced the magnitude of the impact.     

Lower thermal limits to larval development do not predict poleward range limits of the introduced tropical barnacle Megabalanus coccopoma

As the earth's climate has warmed, many tropical species have expanded their ranges poleward and encountered high‐latitude seasonal temperature regimes, in which further permanent expansion is limited by physiological vulnerability to cold temperatures. The barnacle Megabalanus coccopoma is native to shorelines from Baja California to Peru and has been introduced to many locations worldwide, including the southeastern USA. The ability of larvae to develop successfully at local temperatures can be an important factor limiting the spread of invasive species. To determine if cold temperatures limited larval success near the northern range limit of M. coccopoma along the Atlantic southeastern USA coast, we measured lower temperature limits to larval development, examined the effects of temperature on larval growth and energy accumulation, and calculated a larval energy budget to estimate the extent of potential larval dispersal in this region. Larvae were able to develop through metamorphosis at 16°C, which is much colder than sea surface temperatures during the spawning season in their invasive range, making it unlikely northern range limits are set by a lower temperature limit to larval development. Energy budgets suggest that for larvae produced at the northern end of the invasive range, long distance dispersal to sites far poleward of the current range limit is possible. Similar to the findings of the handful of other studies on cold tolerances of tropical marine invertebrate larvae, larvae should be successful far poleward of current adult distributions.     

Evolutionary ecology of microsporidia associated with the invasive ladybird Harmonia axyridis

Invasive species are characterized by the rapid growth and spread of their populations after establishing a foothold in new habitats, and there are now many examples of such species negatively affecting biodiversity and the economy. It is unclear why some species can become successful invaders, whereas most (even if closely related) remain noninvasive. We previously proposed a hypothesis that parasites associated with invading species can promote their invasive success if they are harmless toward the invaders but harmful to their competitors and/or predators in the newly colonized habitat. Here we discuss whether microsporidia that have recently been discovered in the invasive ladybird Harmonia axyridis contribute to its invasive success. We show that all H. axyridis beetles sourced from diverse collection sites all over the world carry abundant microsporidia. This suggests that both native and invasive H. axyridis populations are associated with these tolerated parasites, which were likely to have existed in native populations before expansion rather than being acquired in newly colonized areas. We describe the pathogenesis of the microsporidia during different developmental stages of H. axyridis and we address the possibility that the predation of its infected eggs and larvae by competing native ladybird species may lead to their infection and ultimately to their decline. Finally, we discuss our initial hypothesis: microsporidia that are tolerated by an invasive vector insect can be active against susceptible native competitors and/or predator species.     

The impact of an invasive mud crab on brood success of nest-building fish in the Northern Baltic Sea

Native fauna in species poor communities, such as those of the Baltic Sea, may be particularly vulnerable to the effects of species invasions. However, the interspecific interactions that result in the negative impacts on native species tend to be poorly understood. One contributing factor to this knowledge gap may be that the vulnerability of native species can vary between different life-history stages. For example, the parental care phase is often risky both for the developing offspring and care-giving parents. Accordingly, we investigated the interactions between invasive mud crabs, Rhithropanopeus harrissii, and native nest-building littoral fish, with a special focus on the sand goby, Pomatoschistus minutus. Firstly, our field survey demonstrated that while the recently established mud crab and native nest-building fish have different habitat and depth optima, their distributions also have a considerable overlap, with a high potential for interactions between them. Secondly, our laboratory experiments indicate that the crabs are likely to impact the fish negatively, especially by taking over occupied nests, as well as pre-occupying nesting resources that are of very limited supply in the studied population. We did not find evidence for successful predation on eggs guarded by male sand gobies naive to the mud crabs. Collectively, the results suggest that the invasive crabs have high potential to negatively impact native fish. Furthermore, the results support the scenario that the parental phase can be a particularly vulnerable life-history stage in face of novel selection pressures, such as species invasions.     

Summer and Winter Marine Heatwaves Favor an Invasive Over Native Seaweeds

Marine heatwaves (MHWs) are emerging as forceful agents of ecosystem change and are increasing in frequency, duration, and intensity with climate change. During MHWs, physiological thresholds of native species may be exceeded while the performance of invasive species with warm affinities may be enhanced. As a consequence, MHWs could significantly alter an ecosystem's invasive dynamics, but such interactions are poorly understood. Following a 10-d acclimation period, we investigated the physiological resistance and resilience of an intertidal rock pool assemblage invaded by the seaweed Sargassum muticum to realistic 14-d marine heatwave scenarios (+1.5°C, +2.0°C, +3.5°C) followed by a 14-d recovery period. We conducted mesocosm experiments in both summer and winter to investigate temporal variability of MHWs. MHW treatments had clear negative impacts on native seaweeds (Fucus serratus and Chondrus crispus) while enhancing the performance of S. muticum. This pattern was consistent across season indicating that acclimation to cooler ambient temperatures results in winter MHWs having significant impacts on native species. As climate warming advances, this may ultimately lead to changes in competitive interactions and potentially exclusion of native species, while invasive species may proliferate and become more conspicuous within temperate rocky shore environments.