Seasonal variability in resilience of a coral reef fish to marine heatwaves and hypoxia

Climate change projections indicate more frequent and severe tropical marine heatwaves (MHWs) and accompanying hypoxia year-round. However, most studies have focused on peak summer conditions under the assumption that annual maximum temperatures will induce the greatest physiological consequences. This study challenges this idea by characterizing seasonal MHWs (i.e., mean, maximum, and cumulative intensities, durations, heating rates, and mean annual occurrence) and comparing metabolic traits (i.e., standard metabolic rate (SMR), Q10 of SMR, maximum metabolic rate (MMR), aerobic scope, and critical oxygen tension (P_crit)) of winter- and summer-acclimatized convict tang (Acanthurus triostegus) to the combined effects of MHWs and hypoxia. Fish were exposed to one of six MHW treatments with seasonally varying maximum intensities (winter: 24.5, 26.5, 28.5°C; summer: 28.5, 30.5, 32.5°C), representing past and future MHWs under IPCC projections (i.e., +0, +2, +4°C). Surprisingly, MHW characteristics did not significantly differ between seasons, yet SMR was more sensitive to winter MHWs (mean Q10 = 2.92) than summer MHWs (mean Q10 = 1.81), despite higher absolute summer temperatures. Concurrently, MMR increased similarly among winter +2 and +4°C treatments (i.e., 26.5, 28.5°C) and all summer MHW treatments, suggesting a ceiling for maximal MMR increase. Aerobic scope did not significantly differ between seasons nor among MHW treatments. While mean P_crit did not significantly vary between seasons, warming of +4°C during winter (i.e., 28.5°C) significantly increased P_crit relative to the winter control group. Contrary to the idea of increased sensitivity to MHWs during the warmest time of year, our results reveal heightened sensitivity to the deleterious effects of winter MHWs, and that seasonal acclimatization to warmer summer conditions may bolster metabolic resilience to warming and hypoxia. Consequently, physiological sensitivity to MHWs and hypoxia may extend across larger parts of the year than previously expected, emphasizing the importance of evaluating climate change impacts during cooler seasons when essential fitness-related traits such as reproduction occur in many species.     

Hot or not? Comparative behavioral thermoregulation,critical temperature regimes,and thermal tolerances of the invasive lionfish <Emphasis Type="Italic">Pterois</Emphasis> sp. versus native western North Atlantic reef fishes

Temperature influences the geographic range, physiology, and behavior of many ectothermic species, including the invasive lionfish Pterois sp. Thermal parameters were experimentally determined for wild-caught lionfish at different acclimation temperatures (13, 20, 25 and 32 °C). Preferences and avoidance were evaluated using a videographic shuttlebox system, while critical thermal methodology evaluated tolerance. The lionfish thermal niche was compared experimentally to two co-occurring reef fishes (graysby Cephalopholis cruentata and schoolmaster Lutjanus apodus) also acclimated to 25 °C. The physiologically optimal temperature for lionfish is likely 28.7 ± 1 °C. Lionfish behavioral thermoregulation was generally linked to acclimation history; tolerance and avoidance increased significantly at higher acclimation temperatures, but final preference did not. The tolerance polygon of lionfish shows a strong correlation between thermal limits and acclimation temperature, with the highest CT_max at 39.5 °C and the lowest CT_min at 9.5 °C. The tolerance range of invasive lionfish (24.61 °C) is narrower than those of native graysby (25.25 °C) and schoolmaster (26.87 °C), mostly because of lower thermal maxima in the former. Results show that lionfish display “acquired” thermal tolerance at higher and lower acclimation temperatures, but are no more eurythermal than other tropical fishes. Collectively, these results suggest that while lionfish range expansion in the western Atlantic is likely over the next century from rising winter sea temperatures due to climate change, the magnitude of poleward radiation of this invasive species is limited and will likely be equivalent to native tropical and subtropical fishes with similar thermal minima.     

Non‐native fishes and climate change: predicting species responses to warming temperatures in a temperate region

1. Temperate regions with fish communities dominated by cold‐water species (physiological optima <20 °C) are vulnerable to the effects of warming temperatures caused by climate change, including displacement by non‐native cool‐water (physiological optima 20–28 °C) and warm‐water fishes (physiological optima >28 °C) that are able to establish and invade as the thermal constraints on the expression of their life history traits diminish. 2. England and Wales is a temperate region into which at least 38 freshwater fishes have been introduced, although 14 of these are no longer present. Of the remaining 24 species, some have persisted but failed to establish, some have established populations without becoming invasive and some have become invasive. The aim of the study was to predict the responses of these 24 non‐native fishes to the warming temperatures of England and Wales predicted under climate change in 2050. 3. The predictive use of climate‐matching models and an air and water temperature regression model suggested that there are six non‐native fishes currently persistent but not established in England and Wales whose establishment and subsequent invasion would benefit substantially from the predicted warming temperatures. These included the common carp Cyprinus carpio and European catfish Silurus glanis, fishes that also exert a relatively high propagule pressure through stocking to support angling and whose spatial distribution is currently increasing significantly, including in open systems. 4. The potential ecological impacts of the combined effects of warming temperatures, current spatial distribution and propagule pressure on the establishment and invasion of C. carpio and S. glanis were assessed. The ecological consequences of C. carpio invasion were assessed as potentially severe in England and Wales, with impacts likely to relate to habitat destruction, macrophyte loss and increased water turbidity. However, evidence of ecological impacts of S. glanis elsewhere in their introduced range was less clear and so their potential impacts in England and Wales remain uncertain.     

Seed germination of exotic and native winter annuals differentially responds to temperature and moisture,especially with climate change scenarios

Seeds of winter annuals require a summer after-ripening period for dormancy loss and low autumn temperatures for germination. With current and future changes in moisture and temperature, we tested the effects of warming along a relative humidity (RH) gradient on dormancy loss and effects of decreased diurnal temperature range (DTR) on germination. We further reasoned that the effects of changes in these variables would be disproportionate between the exotic and native winter annuals. Seeds of exotic species (Buglossoides arvensis, Lamium purpureum and Ranunculus parviflorus) and co-occurring native species (Galium aparine, Paysonia stonensis and Plantago virginica) were collected in middle Tennessee. After-ripening occurred over a 15–100% RH gradient at 25 and 30°C and germination was tested at 20/10 and 20/15°C. Niche breadth was calculated using Levins' B. Fresh Ranunculus seeds had high germination and those of other species did not. Germination for these species increased with after-ripening, mostly across the RH gradient irrespective of temperature. A decrease in DTR showed mixed results – the extreme being Ranunculus with no germination at 20/15°C. Most exotic species had wider germination niche breadths than native species. With climate change, we suggest that a decrease in DTR may have a larger effect on germination than increasing moisture or warming on dormancy break. Moreover, there is not a clear-cut winner with climate change when we compare exotic versus native species because the responses of our six species were species specific.     

Climate warming increases biological control agent impact on a non‐target species

Climate change may shift interactions of invasive plants, herbivorous insects and native plants, potentially affecting biological control efficacy and non‐target effects on native species. Here, we show how climate warming affects impacts of a multivoltine introduced biocontrol beetle on the non‐target native plant Alternanthera sessilis in China. In field surveys across a latitudinal gradient covering their full distributions, we found beetle damage on A. sessilis increased with rising temperature and plant life history changed from perennial to annual. Experiments showed that elevated temperature changed plant life history and increased insect overwintering, damage and impacts on seedling recruitment. These results suggest that warming can shift phenologies, increase non‐target effect magnitude and increase non‐target effect occurrence by beetle range expansion to additional areas where A. sessilis occurs. This study highlights the importance of understanding how climate change affects species interactions for future biological control of invasive species and conservation of native species.     

Range reshuffling: Climate change,invasive species,and the case of Nothofagus forests in Aotearoa New Zealand

Aim

The impact of climate change on forest biodiversity and ecosystem services will be partly determined by the relative fortunes of invasive and native forest trees under future conditions. Aotearoa New Zealand has high conservation value native forests and one of the world's worst invasive tree problems. We assess the relative effects of habitat redistribution on native Nothofagus and invasive conifer (Pinaceae) species in New Zealand as a case study on the compounding impacts of climate change and tree invasions.

Location

Aotearoa New Zealand.

Methods

We use species distribution models (SDMs) to predict the current and future distribution of habitat for five native Nothofagus species and 13 invasive conifer species under two 2070 climate scenarios. We calculate habitat loss/gain for all species and examine overlap between the invasive and native species now and in future.

Results

Most species will lose habitat overall. The native species saw large changes in the distribution of habitat with extensive losses in North Island and gains mostly in South Island. Concerningly, we found that most new habitat for Nothofagus was also suitable for at least one invasive species. However, there were refugia for the native species in the wetter parts of the climate space.

Main Conclusion

If the predicted changes in habitat distribution translate to shifts in forest distribution, it would cause widespread ecological disruption. We discuss how acclimation, adaptation and biotic interactions may prevent/delay some changes. But we also highlight that the poor establishment capacity of Nothofagus, and the contrasting ability of the conifers to invade, will present persistent conservation challenges in areas of both new habitat and forest retreat. Pinaceae are problematic invaders globally, and our results highlight that control of invasions and active native forest restoration will likely be key to managing forest biodiversity under future climates.     

Thermal plasticity of photosynthesis: the role of acclimation in forest responses to a warming climate

The increasing air temperatures central to climate change predictions have the potential to alter forest ecosystem function and structure by exceeding temperatures optimal for carbon gain. Such changes are projected to threaten survival of sensitive species, leading to local extinctions, range migrations, and altered forest composition. This study investigated photosynthetic sensitivity to temperature and the potential for acclimation in relation to the climatic provenance of five species of deciduous trees, Liquidambar styraciflua, Quercus rubra, Quercus falcata, Betula alleghaniensis, and Populus grandidentata. Open‐top chambers supplied three levels of warming (+0, +2, and +4 °C above ambient) over 3 years, tracking natural temperature variability. Optimal temperature for CO₂ assimilation was strongly correlated with daytime temperature in all treatments, but assimilation rates at those optima were comparable. Adjustment of thermal optima was confirmed in all species, whether temperatures varied with season or treatment, and regardless of climate in the species' range or provenance of the plant material. Temperature optima from 17° to 34° were observed. Across species, acclimation potentials varied from 0.55 °C to 1.07 °C per degree change in daytime temperature. Responses to the temperature manipulation were not different from the seasonal acclimation observed in mature indigenous trees, suggesting that photosynthetic responses should not be modeled using static temperature functions, but should incorporate an adjustment to account for acclimation. The high degree of homeostasis observed indicates that direct impacts of climatic warming on forest productivity, species survival, and range limits may be less than predicted by existing models.     

Winter cold tolerance and the geographic range separation of Bromus tectorum and Bromus rubens,two severe invasive species in North America

The invasive grasses Bromus rubens and Bromus tectorum are responsible for widespread damage to semiarid biomes of western North America. Bromus. tectorum dominates higher and more northern landscapes than its sister species B. rubens, which is a severe invader in the Mojave desert region of the American Southwest. To assess climate thresholds controlling their distinct geographic ranges, we evaluated the winter cold tolerance of B. tectorum and B. rubens. Freezing tolerance thresholds were determined using electrolyte leakage and whole‐plant mortality. The responses of the two species to winter cold and artificial freezing treatments were similar in 2007–2008 and 2009–2010. When grown at minimum temperatures of 10 °C, plants of both species had cold tolerance thresholds near ?10 °C, while plants acclimated to a daily minimum of ?10 to ?30 °C survived temperatures down to ?31 °C. In the winter of 2010–2011, a sudden severe cold event on December 9, 2010 killed all B. rubens populations, while B. tectorum was not harmed; all tested plants were 7–8 weeks old. Controlled acclimation experiments demonstrated that 8‐week‐old plants of B. rubens had a slower acclimation rate to subzero temperatures than B. tectorum and could not survive a rapid temperature drop from 1 to ?14 °C. Four‐month‐old B. rubens populations were as cold tolerant as B. tectorum. Our results show that severe and sudden freeze events in late autumn can kill young plants of B. rubens but not B. tectorum. Such events could exclude B. rubens from the relatively cold, Intermountain steppe biome of western North America where B. tectorum predominates.     

Climate and the distribution of Fallopia japonica: use of an introduced species to test the predictive capacity of response surfaces

Abstract. The relationship between present climate and the distribution in Europe of the aggressively invasive exotic Fallopia japonica is described by fitting a response surface based on three bioclimatic variables: mean temperature of the coldest month, the annual temperature sum > 5 °C, and the ratio of actual to potential evapotranspiration. The close fit between the observed and simulated distributions suggests that the species' European distribution is climatically determined. The response surface also provides a simulation of the extent of the area of native distribution of F. japonica in Southeast Asia that is generally accurate, confirming the robustness of the static correlative model upon which it is based. Simulations of the potential distribution of F. japonica under two alternative 2 x CO₂ climate change scenarios indicate the likelihood of considerable spread into higher latitudes and possible eventual exclusion of the species from central Europe. However, despite the robustness of the response surface with present-day climate, the reliability of these simulations as forecasts is likely to be limited because no account is taken of the direct effects of CO₂ and their interaction with the species' physiological responses to climate. Similarly, no account is taken of the potential impact of interactions with ‘new’ species as ecosystems change in composition in response to climate change. Nevertheless, the simulations indicate both the possible magnitude of the impacts of forecast climate changes and the regions that may be susceptible to invasion by F. japonica.     

Photosynthesis of temperate Eucalyptus globulus trees outside their native range has limited adjustment to elevated CO2 and climate warming

Eucalyptus species are grown widely outside of their native ranges in plantations on all vegetated continents of the world. We predicted that such a plantation species would show high potential for acclimation of photosynthetic traits across a wide range of growth conditions, including elevated [CO₂] and climate warming. To test this prediction, we planted temperate Eucalyptus globulus Labill. seedlings in climate‐controlled chambers in the field located >700 km closer to the equator than the nearest natural occurrence of this species. Trees were grown in a complete factorial combination of elevated CO₂ concentration (eC; ambient [CO₂] +240 ppm) and air warming treatments (eT; ambient +3 °C) for 15 months until they reached ca. 10 m height. There was little acclimation of photosynthetic capacity to eC and hence the CO₂‐induced photosynthetic enhancement was large (ca. 50%) in this treatment during summer. The warming treatment significantly increased rates of both carboxylation capacity (V_cmax) and electron transport (J_max) (measured at a common temperature of 25 °C) during winter, but decreased them significantly by 20–30% in summer. The photosynthetic CO₂ compensation point in the absence of dark respiration (Γ*) was relatively less sensitive to temperature in this temperate eucalypt species than for warm‐season tobacco. The temperature optima for photosynthesis and J_max significantly changed by about 6 °C between winter and summer, but without further adjustment from early to late summer. These results suggest that there is an upper limit for the photosynthetic capacity of E. globulus ssp. globulus outside its native range to acclimate to growth temperatures above 25 °C. Limitations to temperature acclimation of photosynthesis in summer may be one factor that defines climate zones where E. globulus plantation productivity can be sustained under anticipated global environmental change.     

Testing the hypothesis of greater eurythermality in invasive than in native ladybird species: from physiological performance to life‐history strategies

1. Global warming and biological invasions are important threats to biodiversity. Nonetheless, there is little information on how these factors influence performance or life‐history traits of invasive and native species. 2. The effects of temperature on physiological and fitness traits of two invasive alien species (Harmonia axyridis and Hippodamia variegata) and one native species (Eriopis chilensis) of coccinellid were evaluated, testing a model of eurythermality. Eggs of all species were exposed to four temperature treatments (20, 24, 30 and 33 °C). In adult F₂ we measured fecundity, locomotor performance, development time (total and per life stage), survival, and preferred body temperature in a thermal gradient. 3. It was found that H. axyridis had comparatively better performance at low temperatures (i.e. 20 °C), while the performance of H. variegata and E. chilensis did not change with temperature or was better at higher temperatures (30 °C). The standardised Levins index showed that all species are eurythermic. E. chilensis had a high niche overlap with the invasive alien ladybird species, rejecting the hypothesis of greater eurythermality of invasive species than native species. 4. Although there were differences in the temperature preferences and in the response of some physiological and life‐history traits of ladybirds to temperature, both the native and invasive alien species are eurythermic, contrary to the prediction. The better performance of H. axyridis at lower temperatures may result in displacement of its current distribution, and thus not all invasive species will respond favourably to global warming.     

Effects of extreme temperatures on the growth and photosynthesis of invasive Bidens alba and its native congener B. biternata

Temperatures are expected to fluctuate widely under climate change but little is known about how extreme temperatures might affect the physiology and performance of invasive compared to native plant species. In this study, we evaluated the effects of high (40/35°C) and low (10/5°C) temperature regimes on the growth and photosynthesis of the invasive Asteraceae species Bidens alba and its native congener B. biternata using a growth chamber experiment. Results showed that invasive B. alba had significantly greater total biomass and relative growth rate, accompanied by higher net photosynthetic rate (Pn), than native B. biternata at both low and high temperature extremes. The reduction in Pn for B. alba was mainly caused by stomatal limitations, but for B. biternata it was caused by non‐stomatal factors, indicating that greater damage to physiological processes may occur in native B. biternata under both low and high temperature stress. Higher cyclic electron flow around photosystem I in invasive B. alba than in native B. biternata under extreme temperatures might alleviate the negative effect of temperature extremes to photosynthetic and thus promote its photosynthetic efficiency. To conclude, the invasive B. alba has both greater cold and heat tolerance than its native congener B. biternata, suggesting that the invader may outperform native species under future extreme temperature conditions.     

Thermal shock and germination in North‐West European Genisteae: implications for heathland management and invasive weed control using fire

Question: Is the stimulation of germination by thermal shock (resulting from the passage of fire) commonly observed for Mediterranean‐climate Fabaceae also apparent for NW European Genisteae (Fabaceae) species? Location: Southern England and NW France. Methods: The germination of Cytisus scopiarius, Genista anglica, Ulex europaeus, Ulex gallii and Ulex minor was examined following exposure to a range of temperatures (50°C, 65°C, 80°C, 95°C and 110°C) applied to seeds for 5 min. A sixth Mediterranean‐origin species (Spartium junceum) was also included since it is a common invasive in NW Europe and North America. Results: All five native NW European species displayed increased germination following thermal shock, even when seeds were heated to 110°C. However, there was some variation depending on provenance: in contrast to seeds collected from southern England, germination of French C. scopiarius seeds was unaffected by temperature. Spartium junceum germinated most at 95°C, but was the only species to show reduced germination when seeds were heated to 110°C. Conclusions: The NW European Genisteae appear to be pre‐adapted to the high temperatures associated with fire; a response attributable to their evolutionary origins in the fire‐prone ecosystems of the Mediterranean Basin. Consequently, projected increases in fire frequency linked to climate change may stimulate their regeneration in NW European heathlands, potentially altering the species composition of these ecosystems. Additionally, a clearer understanding of the interaction between thermal shock and germination may explain why fire has so frequently been ineffective in controlling invasive Genisteae populations worldwide.     

Thermal physiology of native cool-climate,and non-native warm-climate Pumpkinseed sunfish raised in a common environment

Contemporary evolution of thermal physiology has the potential to help limit the physiological stress associated with rapidly changing thermal environments; however it is unclear if wild populations can respond quickly enough for such changes to be effective. We used native Canadian Pumpkinseed (Lepomis gibbosus) sunfish, and non-native Pumpkinseed introduced into the milder climate of Spain ~100 years ago, to assess genetic differences in thermal physiology in response to the warmer non-native climate. We compared temperature performance reaction norms of two Canadian and two Spanish Pumpkinseed populations born and raised within a common environment. We found that Canadian Pumpkinseed had higher routine metabolic rates when measured at seasonally high temperatures (15 °C in winter, 30 °C in summer), and that Spanish Pumpkinseed had higher critical thermal maxima when acclimated to 30 °C in the summer. Growth rates were not significantly different among populations, however Canadian Pumpkinseed tended to have faster growth at the warmest temperatures measured (32 °C). The observed differences in physiology among Canadian and Spanish populations at the warmest acclimation temperatures are consistent with the introduced populations being better suited to the warmer non-native climate than native populations. The observed differences could be the result of either founder effects, genetic drift, and/or contemporary adaptive evolution in the warmer non-native climate.     

Summer warming and increased winter snow cover affect Sphagnum fuscum growth, structure and production in a sub-arctic bog

Sphagnum mosses form a major component of northern peatlands, which are expected to experience substantially higher increases in temperature and winter precipitation than the global average. Sphagnum may play an important role in the responses of the global carbon cycle to climate change. We investigated the responses of summer length growth, carpet structure and production in Sphagnum fuscum to experimentally induced changes in climate in a sub‐arctic bog. Thereto, we used open‐top chambers (OTCs) to create six climate scenarios including changes in summer temperatures, and changes in winter snow cover and spring temperatures. In winter, the OTCs doubled the snow thickness, resulting in 0.5–2.8°C higher average air temperatures. Spring air temperatures in OTCs increased by 1.0°C. Summer warming had a maximum effect of 0.9°C, while vapor pressure deficit was not affected. The climate manipulations had strong effects on S. fuscum. Summer warming enhanced the length increment by 42–62%, whereas bulk density decreased. This resulted in a trend (P<0.10) of enhanced biomass production. Winter snow addition enhanced dry matter production by 33%, despite the fact that the length growth and bulk density did not change significantly. The addition of spring warming to snow addition alone did not significantly enhance this effect, but we may have missed part of the early spring growth. There were no interactions between the manipulations in summer and those in winter/spring, indicating that the effects were additive. Summer warming may in the long term negatively affect productivity through the adverse effects of changes in Sphagnum structure on moisture holding and transporting capacity. Moreover, the strong length growth enhancement may affect interactions with other mosses and vascular plants. Because winter snow addition enhanced the production of S. fuscum without affecting its structure, it may increase the carbon balance of northern peatlands.     

Physiological and growth responses of switchgrass (Panicum virgatum L.) in native stands under passive air temperature manipulation

In the central Great Plains of North America, climate change predictions include increases in mean annual temperature of 1.5–5.5 °C by 2100. Ecosystem responses to increased temperatures are likely to be regulated by dominant plant species, such as the potential biofuel species Panicum virgatum (switchgrass) in the tallgrass prairie. To describe the potential physiological and whole‐plant responses of this species to future changes in air temperatures, we used louvered open‐sided chambers (louvered OSC; 1 × 1 m, adjustable height) to passively alter canopy temperature in native stands of P. virgatum growing in tallgrass prairie at varying topographic positions (upland/lowland). The altered temperature treatment decreased daily mean temperatures by 1 °C and maximum temperatures by 4 °C in May and June, lowered daytime stomatal conductance and transpiration, decreased tiller density, increased specific leaf area, and delayed flowering. Among topographic contrasts, aboveground biomass, flowering tiller density, and tiller weight were greater in lowland sites compared to upland sites, with no temperature treatment interactions. Differences in biomass production responded more to topography than the altered temperature treatment, as soil water status varied considerably between topographic positions. These results indicate that while water availability as a function of topography was a strong driver of plant biomass, many leaf‐level physiological processes were responsive to the small decreases in daily mean and maximum temperature, irrespective of landscape position. The varying responses of leaf‐level gas exchange and whole‐plant growth of P. virgatum in native stands to altered air temperature or topographic position illustrate that accurately forecasting yields for P. virgatum in mixed communities will require greater integration of physiological responses to simulated climate change (increased temperature) and resource availability over natural environmental gradients (soil moisture).     

Legacy introductions and climatic variation explain spatiotemporal patterns of invasive hybridization in a native trout

Hybridization between invasive and native species, a significant threat to worldwide biodiversity, is predicted to increase due to climate‐induced expansions of invasive species. Long‐term research and monitoring are crucial for understanding the ecological and evolutionary processes that modulate the effects of invasive species. Using a large, multidecade genetics dataset (N = 582 sites, 12,878 individuals) with high‐resolution climate predictions and extensive stocking records, we evaluate the spatiotemporal dynamics of hybridization between native cutthroat trout and invasive rainbow trout, the world's most widely introduced invasive fish, across the Northern Rocky Mountains of the United States. Historical effects of stocking and contemporary patterns of climatic variation were strongly related to the spread of hybridization across space and time. The probability of occurrence, extent of, and temporal changes in hybridization increased at sites in close proximity to historical stocking locations with greater rainbow trout propagule pressure, warmer water temperatures, and lower spring precipitation. Although locations with warmer water temperatures were more prone to hybridization, cold sites were not protected from invasion; 58% of hybridized sites had cold mean summer water temperatures (<11°C). Despite cessation of stocking over 40 years ago, hybridization increased over time at half (50%) of the locations with long‐term data, the vast majority of which (74%) were initially nonhybridized, emphasizing the chronic, negative impacts of human‐mediated hybridization. These results show that effects of climate change on biodiversity must be analyzed in the context of historical human impacts that set ecological and evolutionary trajectories.     

Effects of thermal acclimation and photoacclimation on lipophilic pigments in an invasive and a native cyanobacterium of temperate regions

The freshwater cyanobacterium Cylindrospermopsis raciborskii spreads from tropical to temperate regions worldwide. This entails acclimation to varied light and temperature conditions. We studied the thermal and light acclimation of the photosynthetic machinery of C. raciborskii by monitoring alteration of the chlorophyll a and carotenoid content in German strains of C. raciborskii, in African and Australian strains of C. raciborskii, and in German strains of Aphanizomenon gracile, a native cyanobacterium belonging to the same order (Nostocales). Our results showed that temperate and tropical C. raciborskii strains did not differ in pigment acclimation to light and temperature. In contrast, the ratio of photoprotective carotenoids (namely the carotenoid glycoside 4-hydroxymyxol glycoside [aphanizophyll]) to chlorophyll a increased significantly more in C. raciborskii in comparison with A. gracile (1) with decreasing temperatures from 20 to 10°C and a moderate light intensity of 80?µmol photons m^?2?s^?1 and (2) with increasing light intensities at a suboptimal temperature of 15°C, compared to 20°C. We conclude that below 20°C photoinhibition is avoided by greater photoprotection in the invasive species C. raciborskii compared to the native species A. gracile.     

Biological invasions and climate change amplify each other’s effects on dryland degradation

Climate models predict that, in the coming decades, many arid regions will experience increasingly hot conditions and will be affected more frequently by drought. These regions are also experiencing rapid vegetation change, notably invasion by exotic grasses. Invasive grasses spread rapidly into native desert ecosystems due, in particular, to interannual variability in precipitation and periodic fires. The resultant destruction of non-fire-adapted native shrub and grass communities and of the inherent soil resource heterogeneity can yield invader-dominated grasslands. Moreover, recurrent droughts are expected to cause widespread physiological stress and mortality of both invasive and native plants, as well as the loss of soil resources. However, the magnitude of these effects may differ between invasive and native grasses, especially under warmer conditions, rendering the trajectory of vegetated communities uncertain. Using the Biosphere 2 facility in the Sonoran Desert, we evaluated the viability of these hypothesized relationships by simulating combinations of drought and elevated temperature (+5°C) and assessing the ecophysiological and mortality responses of both a dominant invasive grass (Pennisetum ciliare or buffelgrass) and a dominant native grass (Heteropogan contortus or tanglehead). While both grasses survived protracted drought at ambient temperatures by inducing dormancy, drought under warmed conditions exceeded the tolerance limits of the native species, resulting in greater and more rapid mortality than exhibited by the invasive. Thus, two major drivers of global environmental change, biological invasion and climate change, can be expected to synergistically accelerate ecosystem degradation unless large-scale interventions are enacted.