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Fish response to environmental stressors in the Lake Victoria Basin ecoregion
Abstract
Freshwater organisms face multiple threats associated with habitat degradation, pollution, and eutrophication, in addition to overharvesting and species invasions. Furthermore, there is mounting evidence that freshwaters are highly sensitive to climate change. This chapter provides an overview of contemporary environmental changes in inland waters of the Lake Victoria Basin (LVB) ecoregion of East Africa with a focus on climate change, eutrophication, and land use. Case studies of fishes in the Lake Victoria basin and swamp-river systems of Western Uganda are used to explore potential effects of these stressors on morpho-physiological, performance, and fitness-related traits. Overall, fishes in the LVB ecoregion show acclimation capacity in upper thermal tolerance and aerobic performance, and adaptive plasticity in traits related to hypoxia tolerance (e.g., gill size). However, a trait-based climate change vulnerability assessment revealed that over 70% of LVB ecoregion fishes are vulnerable to climate change; and fish kills associated with turnover events or anoxic upwellings highlight the danger of rapid change in dissolved oxygen for some species. Plasticity may allow some fishes to persist in the face of multiple stressors in the LVB ecoregion. However, there may be consequences for fitness-related traits such as body size that could affect demographic stability and contributions of fish to food security.
Marine and freshwater fishes are currently facing complex and extensive challenges, from warming waters and habitat degradation to direct human-wildlife interactions. Within this context, Conservation Physiology has made some important contributions to advance our understanding of the underlying mechanisms leading to these problems, as well as in offering practical solutions. However, there remains much space for the field to grow and significantly expand its impact on real-world conservation of fish biodiversity. As the planet continues to change, so do the problems that fish encounter, and so must the field. Importantly, systemic changes must occur to better represent the diversity of peoples and knowledges who have been historically systemically excluded in this field (and others). In this chapter, we discuss some of the remaining key challenges that conservation physiologists need to overcome to protect and effectively manage fish species around the globe in the coming decades, including those related to diversity, equity, inclusivity and justice. We make suggestions for overcoming these challenges, highlighting examples of how physiological knowledge has been used to conserve fishes and other taxa and providing key resources for understanding and addressing inequities in the field, which may serve as guidance for scientists and practitioners seeking to advance these goals. We finish with a list of priority actions needed to ensure that the field of Conservation Physiology remains relevant and successful in its quest to promote long-term sustainable and equitable conservation solutions.
Understanding the influence of land use/land cover (LULC) on water quality is pertinent to sustainable water management. This study aimed at assessing the spatio-seasonal variation of water quality in relation to land use types in Lake Muhazi, Rwanda. The National Sanitation Foundation Water Quality Index (NSF-WQI) was used to evaluate the anthropogenically-induced water quality changes. In addition to Principal Components Analysis (PCA), a Cluster Analysis (CA) was applied on 12-clustered sampling sites and the obtained NSF-WQI. Lastly, the Partial Least Squares Path Modelling (PLS-PM) was used to estimate the nexus between LULC, water quality parameters, and the obtained NSF-WQI. The results revealed a poor water quality status at the Mugorore and Butimba sites in the rainy season, then at Mugorore and Bwimiyange sites in the dry season. Furthermore, PCA displayed a sample dispersion based on seasonality while NSF-WQI’s CA hierarchy grouped the samples corresponding to LULC types. Finally, the PLS-PM returned a strong positive correlation (+ 0.831) between LULCs and water quality parameters in the rainy season but a negative correlation coefficient (− 0.542) in the dry season, with great influences of cropland on the water quality parameters. Overall, this study concludes that the lake is seasonally influenced by anthropogenic activities, suggesting sustainable land-use management decisions, such as the establishment and safeguarding protection belts in the lake vicinity.
Pesticides are widely used to protect food production and meet global food demand but are also ubiquitous environmental pollutants, causing adverse effects on water quality, biodiversity and human health. Here we use a global database of pesticide applications and a spatially explicit environmental model to estimate the world geography of environmental pollution risk caused by 92 active ingredients in 168 countries. We considered a region to be at risk of pollution if pesticide residues in the environment exceeded the no-effect concentrations, and to be at high risk if residues exceeded this by three orders of magnitude. We find that 64% of global agricultural land (approximately 24.5 million km2) is at risk of pesticide pollution by more than one active ingredient, and 31% is at high risk. Among the high-risk areas, about 34% are in high-biodiversity regions, 5% in water-scarce areas and 19% in low- and lower-middle-income nations. We identify watersheds in South Africa, China, India, Australia and Argentina as high-concern regions because they have high pesticide pollution risk, bear high biodiversity and suffer from water scarcity. Our study expands earlier pesticide risk assessments as it accounts for multiple active ingredients and integrates risks in different environmental compartments at a global scale. Pesticide pollution is a risk for two-thirds of agriculture land. A third of high-risk areas are in high-biodiversity regions and a fifth are in low- and lower-middle-income areas, according to environmental modelling combined with pesticide application data.
Agriculture is expanding in tropical mountainous areas, yet its climatic effect is poorly understood. Here, we investigate how elevation regulates the biophysical climate impacts of deforestation over tropical mountainous areas by integrating satellite-observed forest cover changes into a high-resolution land–atmosphere coupled model. We show that recent forest conversion between 2000 and 2014 increased the regional warming by 0.022 ± 0.002 °C in the Southeast Asian Massif, 0.010 ± 0.007 °C in the Barisan Mountains (Maritime Southeast Asia), 0.042 ± 0.010 °C in the Serra da Espinhaço (South America) and 0.047 ± 0.008 °C in the Albertine Rift mountains (Africa) during the local dry season. The deforestation-driven local temperature anomaly can reach up to 2 °C where forest conversion is extensive. The warming from mountain deforestation depends on elevation, through the intertwined and opposing effects of increased albedo causing cooling and decreased evapotranspiration causing warming. As the elevation increases, the albedo effect increases in importance and the warming effect decreases, analogous to previously highlighted decreases of deforestation-induced warming with increasing latitude. As most new croplands are encroaching lands at low to moderate elevations, deforestation produces higher warming from suppressed evapotranspiration. Impacts of this additional warming on crop yields, land degradation and biodiversity of nearby intact ecosystems should be incorporated into future assessments.
Inland standing waters are particularly vulnerable to increasing water temperature. Here, using a high-resolution numerical model, we find that the velocity of climate change in the surface of inland standing waters globally was 3.5±2.3 km decade-1 from 1861-2005, which is similar to, or lower than, rates of active dispersal of some motile species. However, from 2006-2099, the velocity of climate change will increase to 8.7±5.5 km decade-1 under a low emission pathway (RCP 2.6) and 57.0±17.0 km decade-1 under a high emission pathway (RCP 8.5), meaning that thermal habitat in inland standing waters will move faster than the ability of some species to disperse to cooler areas. The fragmented distribution of standing waters in a landscape will restrict redistribution, even for species with a high dispersal ability, so that the negative consequences of rapid warming for freshwater species are likely to be much greater than for terrestrial and marine realms.
Climate change is one of the most severe threats to global lake ecosystems. Lake surface conditions, such as ice cover, surface temperature, evaporation and water level, respond dramatically to this threat, as observed in recent decades. In this Review, we discuss physical lake variables and their responses to climate change. Decreases in winter ice cover and increases in lake surface temperature modify lake mixing regimes and accelerate lake evaporation. Where not balanced by increased mean precipitation or inflow, higher evaporation rates will favour a decrease in lake level and surface water extent. Together with increases in extreme-precipitation events, these lake responses will impact lake ecosystems, changing water quantity and quality, food provisioning, recreational opportunities and transportation. Future research opportunities, including enhanced observation of lake variables from space (particularly for small water bodies), improved in situ lake monitoring and the development of advanced modelling techniques to predict lake processes, will improve our global understanding of lake responses to a changing climate.
Accelerating deforestation rates in Earth's tropical rainforests have dramatic impacts on local public health, agricultural productivity, and global climate change. We used satellite observations to quantify the local temperature changes in deforested patches of rainforests across the tropics and found local warming larger than that predicted from more than a century of climate change under a worst-case emissions scenario. We show that the most extreme warming is typically found in large patches of deforestation; the combined effects of deforestation and climate change on tropical temperatures present a uniquely difficult challenge to the long term public health, occupational safety, and economic security of tropical populations.
Water temperature is critical for the ecology of lakes. However, the ability to predict its spatial and seasonal variation is constrained by the lack of a thermal classification system. Here we define lake thermal regions using objective analysis of seasonal surface temperature dynamics from satellite observations. Nine lake thermal regions are identified that mapped robustly and largely contiguously globally, even for small lakes. The regions differed from other global patterns, and so provide unique information. Using a lake model forced by 21st century climate projections, we found that 12%, 27% and 66% of lakes will change to a lower latitude thermal region by 2080–2099 for low, medium and high greenhouse gas concentration trajectories (Representative Concentration Pathways 2.6, 6.0 and 8.5) respectively. Under the worst-case scenario, a 79% reduction in the number of lakes in the northernmost thermal region is projected. This thermal region framework can facilitate the global scaling of lake-research. Water temperature is a critical variable for lakes, but its spatial and temporal patterns are not well characterised globally. Here, the authors use surface temperature dynamics to define lake thermal regions that group lakes with similar patterns, and show how these regions shift under climate change.
The role of inland fisheries in livelihoods, food security and sustainable development is often overshadowed by the higher profile interest in ocean issues. Whilst inland fisheries' catch and contribution to global nutrition, food security and the economy, are less than that of marine fisheries, global‐level comparisons of fish production obscure considerable livelihood impacts in certain countries and sub‐national areas. To highlight these contributions, this paper synthesizes recent data and innovative approaches for assessing such livelihood contributions and their importance in countries with limited access to ocean resources and aquaculture. Inland fisheries are crucial for many socially, economically and nutritionally vulnerable groups of people around the world, but the challenges in monitoring inland fisheries preclude a complete understanding of the magnitude of their contributions. This situation is rapidly improving with increasing recognition of inland fisheries in development discourses, which has also encouraged research to enhance knowledge on the importance of inland fisheries. We review this work, including collated information published in a recent Food and Agriculture Organization report, to provide an up to date characterization of the state of knowledge on the role of inland fisheries.
The recorded catches of most of the larger commercial fish species in Africa, such as large breams (Cichlidae), carps (Cyprinidae) and perches (Perciformes), which have been the focus of fisheries management, have not changed greatly over the past three decades. In contrast, the landings of small species of herring (Clupeidae), carp, bream and characin species – mostly zooplankton feeders, predominantly living in open waters of African lakes and reservoirs – in short, “small pelagic fish”– have steadily increased. These fisheries have developed in addition to the fishing of, and sometimes as a reaction to, decreased catch rates of larger species, introductions and the creation of large water bodies such as reservoirs. They now represent nearly three quarters of the total inland fish catch of the African continent, although a large proportion of the inland fishery catch statistics are acknowledged to be incomplete and unreliable. Stock assessments and estimates of exploitation levels are also largely absent. The expansion, technical development and marketing of these fisheries have nearly all been achieved by a multitude of local stakeholders with very limited scientific monitoring or management. Even though small pelagic fish species, and small fish in general, have always been part of the catch of subsistence fisheries in the large water bodies of Africa, they have conventionally been regarded by fisheries managers as resources with “low economic value” and consequently have been afforded low priority with respect to research and monitoring. As a result, there are still major gaps in our biological knowledge and understanding of the full potential of many species. Common to all, however, is their small size and corresponding high turnover rate, with most species being able to reproduce their own biomass around five times or more per year, which is at least twice the rate of the larger commercial species. This unparalleled level of production, together with the relatively simple technologies used for their capture, the reduced availability of bigger species because of heavy exploitation and an increased demand for fish, are the main reasons for the considerable increase in fishing effort on smaller species that has been observed in African inland fisheries over the past three decades.
Nevertheless, due to the small size of these species and the corresponding necessity of using fishing gear with small mesh sizes, many of the fisheries are operating within the constraints of the current fisheries legislation, which is largely aimed at protecting juveniles of the larger species. Many of the capture techniques are therefore illegal and this can cause conflict between fishers and managers. The theoretical foundation for the conventional single species legislation is increasingly challenged and there is an urgent need to examine and evaluate the fishing patterns from an ecosystem perspective and revise the legislation where necessary. The fishing pressure on most of the small species is only a fraction of the pressure on large fish species, and there is huge potential for increased production and more balanced exploitation if the overall fishing pressure was directed away from the large fish towards the small. In fact, this is what is already happening in many African fisheries, as evidenced by the huge increase in their catches, but it is taking place without comprehensive scientific evaluation of pressures, ecosystem effects or governance.
Small fish are processed, sold and eaten whole. Most of the catch is simply sundried which is the most environmentally friendly and energy-efficient processing technology available, requiring limited investments to obtain potentially high quality products, although rainy seasons limit year-round preservation, and spoilage through overheating and rainfall remain serious issues. In addition, small whole fish are among the most vital suppliers of micronutrients, such as vitamins, iodine, iron, zinc and calcium, which all play a critical role in cerebral development, immune system support and general health. Thus, the unique combination of high-quality protein and important micronutrients in small fish plays a significant role in combating the triple burden of hunger, micronutrient deficiency and noncommunicable diseases. Malnutrition, or so-called “hidden hunger”, is responsible for about a third of premature deaths in sub-Saharan Africa, but national food policies virtually overlook the essential link between the production, distribution and consumption of small sun-dried fish and human health. In fact, the qualities of fish are hardly recognized in the global food security discourse, and fish is strikingly missing from current strategies to combat nutrient deficiency among disadvantaged groups.
The lack of recognition of the importance of small pelagic fish for nutrition, food security, livelihoods and public health has also prevented the necessary investments for improving the quality, shelf life and public awareness of this vitally important resource. Most of the processing and packaging is done under basic, open conditions on the landing beaches, with unhygienic facilities and little protection from contaminants, insect infestations and moisture. Quality control in the whole value chain is virtually absent: there are significant post-harvest losses in the processing and trade of what are essentially low-quality, contaminated products, some of which are even infested with human pathogens. These factors all contribute to a vicious cycle that maintains the image of a “low-value” commodity, prevents the dissemination of knowledge and awareness of the huge potential that small pelagic fish have, and which could be greatly improved with proper policy attention as well as public and private investments.
In summary, catching small pelagic fish, which are simply sun-dried, affordably purchased in local, often remote markets and consumed whole, is the most high yielding, eco-friendly, low carbon dioxide (CO2)-emission and nourishing way of utilizing the high productive potential of African inland waters. However, a range of social, technical, economic, legal and policy barriers inhibit the full potential of utilizing small fish to improve nutrition in low-income populations. These include lack of enabling fisheries management legislation and food safety challenges in processing and marketing. In addition, their local use as fishmeal in animal feeds, including for aquaculture, is increasingly competing for these resources.
Equatorial fishes, and the critically important fisheries based on them, are thought to be at-risk from climate warming because the fishes have evolved in a relatively aseasonal environment and possess narrow thermal tolerance windows that are close to upper thermal limits. We assessed survival, growth, aerobic performance and critical thermal maxima (CTmax) following acute and 21 d exposures to temperatures up to 4°C higher than current maxima for six species of freshwater fishes indigenous to tropical countries and of importance for human consumption. All six species showed 1.3-1.7°C increases in CTmax with a 4°C rise in acclimation temperature, values which match up well with fishes from other climatic regions, and five species had survival >87% at all temperatures over the treatment period. Specific growth rates varied among and within each species in response to temperature treatments. For all species, the response of resting metabolic rate (RMR) was consistently more dynamic than for maximum metabolic rate, but in general both acute temperature exposure and thermal acclimation had only modest effects on aerobic scope (AS). However, RMR increased after warm acclimation in 5 of 6 species, suggesting incomplete metabolic compensation. Taken in total, our results show that each species had some ability to perform at temperatures up to 4°C above current maxima, yet also displayed certain areas of concern for their long-term welfare. We therefore suggest caution against the overly broad generalization that all tropical freshwater fish species will face severe challenges from warming temperatures in the coming decades and that future vulnerability assessments should integrate multiple performance metrics as opposed to relying on a single response metric. † These authors contributed equally to the work.
Even though the idea that modes of speciation other than allopatric speciation are possible in nature is now widespread, compelling examples of ecological speciation in sympatry remain rare. We studied an undescribed radiation of haplochromine cichlids in a young crater lake in western Uganda, and in the small river that is nearby but has currently no known surface connection to the lake. We describe two different modes of speciation that occurred in this cichlid lineage within the past 1,500–10,000 years. Not constrained by gene flow, allopatric divergence between river and lake cichlids affects many different morphological traits as well as nuptial colouration—muted in the river, but intensified and polymorphic in lake cichlids—and neutral genetic differentiation. More surprisingly, we demonstrate a case for sympatric speciation within the small lake that is associated with dramatic differences in male breeding colouration (yellow with bright red-chest versus bright blue) and subtle differences in microhabitat, feeding regime and morphology. Reproductive isolation by assortative mating is suggested by significant differentiation between yellow and blue males in neutral markers of gene flow despite complete sympatry. We hypothesize speciation is mediated by divergent selection on sexual signalling between microhabitats.
Fishes faced with novel thermal conditions often modify physiological functioning to compensate for elevated temperatures. This physiological plasticity (thermal acclimation) has been shown to improve metabolic performance and extend thermal limits in many species. Adjustments in cardiorespiratory function are often invoked as mechanisms underlying thermal plasticity because limitations in oxygen supply have been predicted to define thermal optima in fishes, however few studies have explicitly linked cardiorespiratory plasticity to metabolic compensation. Here we quantify thermal acclimation capacity in the commercially harvested Nile perch (Lates niloticus) of East Africa, and investigate mechanisms underlying observed changes. We reared juvenile Nile perch for 3 months under two temperature regimes, and then measured a series of metabolic traits (e.g., aerobic scope, AS) and critical thermal maximum (CTmax) upon acute exposure to a range of experimental temperatures. We also measured morphological traits of heart ventricles, gills, and brains to identify potential mechanisms for compensation. We found that long-term (3-months) exposure to elevated temperature induced compensation in upper thermal tolerance (CTmax) and metabolic performance (SMR, MMR and AS), and induced cardiac remodeling in Nile perch. Furthermore, variation in heart morphology influenced variations in metabolic function and thermal tolerance. These results indicate that plastic changes enacted over longer exposures lead to differences in metabolic flexibility when acutely exposed to temperature variation. Furthermore, we established functional links between cardiac plasticity, metabolic performance, and thermal tolerance, providing evidence that plasticity in cardiac capacity may be one mechanism for coping with climate change.
The eighteenth-century Malthusian prediction of population growth outstripping food production has not yet come to bear. Unprecedented agricultural land expansions since 1700, and technological innovations that began in the 1930s, have enabled more calorie production per capita than was ever available before in history. This remarkable success, however, has come at a great cost. Agriculture is a major cause of global environmental degradation. Malnutrition persists among large sections of the population, and a new epidemic of obesity is on the rise. We review both the successes and failures of the global food system, addressing ongoing debates on pathways to environmental health and food security. To deal with these challenges, a new coordinated research program blending modern breeding with agroecological methods is needed. We call on plant biologists to lead this effort and help steer humanity toward a safe operating space for agriculture. Expected final online publication date for the Annual Review of Plant Biology Volume 69 is April 29, 2018. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
The Commentary by Pörtner, Bock and Mark (Pörtner et al., 2017) elaborates on the oxygen- and capacity-limited thermal tolerance (OCLTT) hypothesis. Journal of Experimental Biology Commentaries allow for personal and controversial views, yet the journal also mandates that ‘opinion and fact must be clearly distinguishable’ (http://jeb.biologists.org/content/article- types#comms). We contend that Pörtner et al. (2017) do not meet this requirement, and that they present a biased account of the OCLTT hypothesis. We raise two main points: (1) Pörtner et al. (2017) do not do justice to the growing number of empirical studies that failed to support the OCLTT hypothesis when specifically testing its predictions, and (2) in response to these studies, and without new empirical evidence to support OCLTT, Pörtner and colleagues have gradually redefined the core assumptions of the hypothesis so that it is increasingly difficult to test and has lost predictive power.
Human activities often replace native forests with warmer, modified habitats that represent novel thermal environments for biodiversity. Reducing biodiversity loss hinges upon identifying which species are most sensitive to the environmental conditions that result from habitat modification. Drawing on case studies and a meta-analysis, we examined whether observed and modelled thermal traits, including heat tolerances, variation in body temperatures, and evaporative water loss, explained variation in sensitivity of ectotherms to habitat modification. Low heat tolerances of lizards and amphibians and high evaporative water loss of amphibians were associated with increased sensitivity to habitat modification, often explaining more variation than non-thermal traits. Heat tolerances alone explained 24–66% (mean = 38%) of the variation in species responses, and these trends were largely consistent across geographic locations and spatial scales. As habitat modification alters local microclimates, the thermal biology of species will likely play a key role in the reassembly of terrestrial communities.
Accurate estimates of historical forest extent and associated deforestation rates are crucial for quantifying tropical carbon cycles and formulating conservation policy. In Africa, data-driven estimates of historical closed-canopy forest extent and deforestation at the continental scale are lacking, and existing modelled estimates diverge substantially. Here, we synthesize available palaeo-proxies and historical maps to reconstruct forest extent in tropical Africa around 1900, when European colonization accelerated markedly, and compare these historical estimates with modern forest extent to estimate deforestation. We find that forests were less extensive in 1900 than bioclimatic models predict. Resultantly, across tropical Africa, ~ 21.7% of forests have been deforested, yielding substantially slower deforestation than previous estimates (35–55%). However, deforestation was heterogeneous: West and East African forests have undergone almost complete decline (~ 83.3 and 93.0%, respectively), while Central African forests have expanded at the expense of savannahs (~ 1.4% net forest expansion, with ~ 135,270 km2 of savannahs encroached). These results suggest that climate alone does not determine savannah and forest distributions and that many savannahs hitherto considered to be degraded forests are instead relatively old. These data-driven reconstructions of historical biome distributions will inform tropical carbon cycle estimates, carbon mitigation initiatives and conservation planning in both forest and savannah systems.
Climate change is a mounting threat to biological diversity, compromising ecosystem structure and function, and undermining the delivery of essential services worldwide. As the magnitude and speed of climate change accelerates, greater understanding of the taxonomy and geography of climatic vulnerability is critical to guide effective conservation action. However, many uncertainties remain regarding the degree and variability of climatic risk within entire clades and across vast ecosystem boundaries. Here we integrate physiological estimates of thermal sensitivity for 2,960 ray-finned fishes with future climatic exposure, and demonstrate that global patterns of vulnerability differ substantially between freshwater and marine realms. Our results suggest that climatic vulnerability for freshwater faunas will be predominantly determined by elevated levels of climatic exposure predicted for the Northern Hemisphere, whereas marine faunas in the tropics will be the most at risk, reflecting their higher intrinsic sensitivity. Spatial overlap between areas of high physiological risk and high human impacts, together with evidence of low past rates of evolution in upper thermal tolerance, highlights the urgency of global conservation actions and policy initiatives if harmful climate effects on the world's fishes are to be mitigated in the future. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
Increasing water temperatures due to anthropogenic climate change are predicted to negatively impact the aerobic metabolic performance of aquatic ectotherms. Specifically, it has been hypothesized that thermal increases result in reductions in aerobic scope (AS), which lead to decreases in energy available for essential fitness and performance functions. Consequences of warming are anticipated to be especially severe for warm-adapted tropical species as they are thought to have narrow thermal windows and limited plasticity for coping with elevated temperatures. In this study we test how predicted warming may affect the aerobic performance of Nile perch (Lates niloticus), a commercially-harvested fish species in the Lake Victoria basin of East Africa. We measured critical thermal maxima (CTmax) and key metabolic variables such as AS and excess post-exercise oxygen consumption (EPOC) across a range of temperatures, and compared responses between acute (3-day) exposures and 3-week acclimations. CTmax increased with acclimation temperature, however 3-week acclimated fish had higher overall CTmax than acutely-exposed individuals. Nile perch also showed the capacity to increase or maintain high AS even at temperatures well beyond their current range, however acclimated Nile perch had lower AS compared to acutely-exposed fish. These changes were accompanied by lower EPOC, suggesting that drops in AS may reflect improved energy utilization after acclimation, a finding that is supported by improvements in growth at high temperatures over the acclimation period. Overall, the results challenge predictions that tropical species have limited thermal plasticity, and that high temperatures will be detrimental due to limitations in AS.
Temperature is a core component of a species' fundamental niche. At the fine scale over which most organisms experience climate (mm to ha), temperature depends upon the amount of radiation reaching the Earth's surface, which is principally governed by vegetation. Tropical regions have undergone widespread and extreme changes to vegetation, particularly through the degradation and conversion of rainforests. As most terrestrial biodiversity is in the tropics, and many of these species possess narrow thermal limits, it is important to identify local thermal impacts of rainforest degradation and conversion. We collected pantropical, site-level (<1 ha) temperature data from the literature to quantify impacts of land-use change on local temperatures, and to examine whether this relationship differed aboveground relative to belowground and between wet and dry seasons. We found that local temperature in our sample sites was higher than primary forest in all human-impacted land-use types (N = 113,894 daytime temperature measurements from 25 studies). Warming was pronounced following conversion of forest to agricultural land (minimum +1.6°C, maximum +13.6°C), but minimal and nonsignificant when compared to forest degradation (e.g., by selective logging; minimum +1°C, maximum +1.1°C). The effect was buffered belowground (minimum buffering 0°C, maximum buffering 11.4°C), whereas seasonality had minimal impact (maximum buffering 1.9°C). We conclude that forest-dependent species that persist following conversion of rainforest have experienced substantial local warming. Deforestation pushes these species closer to their thermal limits, making it more likely that compounding effects of future perturbations, such as severe droughts and global warming, will exceed species' tolerances. By contrast, degraded forests and belowground habitats may provide important refugia for thermally restricted species in landscapes dominated by agricultural land.
Worldwide, urbanization leads to tremendous anthropogenic environmental alterations, causing strong selection pressures on populations of animals and plants. Although a key feature of urban areas is their higher temperature ("urban heat islands"), adaptive thermal evolution in organisms inhabiting urban areas has rarely been studied. We tested for evolution of a higher heat tolerance (CTMAX ) in urban populations of the water flea Daphnia magna, a keystone grazer in freshwater ecosystems, by carrying out a common garden experiment at two temperatures (20 °C and 24 °C) with genotypes of 13 natural populations ordered along a well-defined urbanization gradient. We also assessed body size and haemoglobin concentration to identify underlying physiological drivers of responses in CTMAX . We found a higher CTMAX in animals isolated from urban compared to rural habitats and in animals reared at higher temperatures. We also observed substantial genetic variation in thermal tolerance within populations. Overall, smaller animals were more heat tolerant. While urban animals mature at smaller size, the effect of urbanization on thermal tolerance is only in part caused by reductions in body size. Although urban Daphnia contained higher concentrations of haemoglobin, this did not contribute to their higher CTMAX . Our results provide evidence of adaptive thermal evolution to urbanization in the water flea Daphnia. In addition, our results show both evolutionary potential and adaptive plasticity in rural as well as urban Daphnia populations, facilitating responses to warming. Given their important ecological role in ponds and lakes, these adaptive responses in Daphnia likely impact food web dynamics, top-down control of algae, water quality, and the socio-economic value of urban ponds. This article is protected by copyright. All rights reserved.
Phenotypes reflect a complex interplay between the direct and indirect effects of multiple selective pressures; adaptive responses in individual traits may therefore be constrained by architectonic considerations and correlations with other traits. We explored whether and how three suites of neighbouring structures (respiratory, trophic and cranial) have changed over time in Rastrineobola argentea, a small pelagic cyprinid fish that is endemic to the Lake Victoria Basin in East Africa. We compared museum specimens from 1966 to modern (2010) specimens, spanning a period of almost 50 years of strong anthropogenic change in the lake, including increased frequency and extent of hypoxia and anoxia, and a dramatic restructuring of the lacustrine fish community and food-web. We found that modern R. argentea had significantly larger gills, wider heads, and shorter and more tightly packed gill rakers than their historical counterparts. Larger gill size probably represents a direct, adaptive response to more frequent bouts of hypoxia over time; the increase in head width and decrease in raker spacing may reflect correlated (and potentially maladaptive) responses to the increase in gill size. Integrative studies such as this are key to unravelling complex interactions between phenotypes and the environment.
For fishes, the availability of dissolved oxygen (DO) can affect performance and fitness traits and influence distribution patterns. Hypoxia occurs naturally in habitats characterized by low mixing and/or light limitation such as dense wetlands and profundal zones of deep lakes. In addition, human activities are increasing the frequency and extent of aquatic hypoxia through eutrophication and pollution. Thus, it has become increasingly important to understand consequences of hypoxia for fishes and mechanisms that facilitate persistence in low-DO habitats. With strong specialization in some cichlid species and high levels of intraspecific variation in others, cichlids have been a key group for exploring strategies for dealing with hypoxia. These include behavioral responses (e.g. aquatic surface respiration), evolution of mechanisms to maximize oxygen uptake and delivery, metabolic depression, use of anaerobic metabolism, and air breathing in a few species. Despite the diversity of strategies that have permitted some cichlids to persist under extreme hypoxia, low DO can incur potential costs such as smaller body size. Such costs may be offset by benefits of hypoxic habitats such as reduced predation risk. This review details the mechanisms used by cichlids for tolerating hypoxia and the costs and benefits of hypoxia tolerance.
Recreational fisheries contribute substantially to the sociocultural and economic well-being of coastal and riparian regions worldwide, but climate change threatens their sustainability. Fishery managers require information on how climate change will impact key recreational species; however, the absence of a global assessment hinders both directed and widespread conservation efforts. In this study, we present the first global climate change vulnerability assessment of recreationally targeted fish species from marine and freshwater environments (including diadromous fishes). We use climate change projections and data on species’ physiological and ecological traits to quantify and map global climate vulnerability and analyze these patterns alongside the indices of socioeconomic value and conservation effort to determine where efforts are sufficient and where they might fall short. We found that over 20% of recreationally targeted fishes are vulnerable to climate change under a high emission scenario. Overall, marine fishes had the highest number of vulnerable species, concentrated in regions with sensitive habitat types (e.g., coral reefs). However, freshwater fishes had higher proportions of species at risk from climate change, with concentrations in northern Europe, Australia, and southern Africa. Mismatches in conservation effort and vulnerability were found within all regions and life-history groups. A key pattern was that current conservation effort focused primarily on marine fishes of high socioeconomic value rather than on the freshwater and diadromous fishes that were predicted to be proportionately more vulnerable. While several marine regions were notably lacking in protection (e.g., Caribbean Sea, Banda Sea), only 19% of vulnerable marine species were without conservation effort. By contrast, 72% of freshwater fishes and 33% of diadromous fishes had no measures in place, despite their high vulnerability and cultural value. The spatial and taxonomic analyses presented here provide guidance for the future conservation and management of recreational fisheries as climate change progresses.
Freshwater fish are restricted by their physiology to rivers and lakes, and are generally limited in their capacity to disperse across basins. As a result, there is often a close match between the evolutionary history of river basins and their natural history. Thus, the regional landscape and ecological features, such as temperature, have shaped the evolution and adaptation of local fish assemblages. Climate change is expected to affect fish diversity and increase extinction, especially in low latitudes, and it has been suggested that species that inhabit low latitude species are more susceptible since they live close to their maximum thermal limits and have low capacity for acclimation. To understand the mechanisms of variation in thermal tolerance across a broad‐scale of South American fishes is fundamental to be able to assess the vulnerability of species and habitat to global warming. Herein, we present the first attempt to analyze the vulnerability of South American freshwater fish species, based on the review of upper thermal limits of 106 species from a broad range of latitudinal habitats. Our findings show that upper thermal limits decrease with latitude, while the thermal safety margin (TSM) increase. Furthermore, the latitude has little effects on the acclimation response ratio, and the TSM decreased with rising temperatures. These data suggest that thermal phenotypic acclimation has low potential for mitigating global warming. These results indicate that South American fish species living in tropical areas are more susceptible to global warming since they are already living close to their maximum habitat temperature.
Highlights
• • South American tropical fish species are living close to their thermal limits.
• • Short‐term acclimation does not protect against environmental warming.
• • South American tropical fish species are highly vulnerable to climate change.
Logging and habitat conversion create hotter microclimates in tropical forest landscapes, representing a powerful form of localised anthropogenic climate change. It is widely believed that these emergent conditions are responsible for driving changes in communities of organisms found in modified tropical forests, although the empirical evidence base for this is lacking. Here we investigated how interactions between the physiological traits of genera and the environmental temperatures they experience lead to functional and compositional changes in communities of ants, a key organism in tropical forest ecosystems. We found that the abundance and activity of ant genera along a gradient of forest disturbance in Sabah, Malaysian Borneo, was defined by an interaction between their thermal tolerance (CTmax) and environmental temperature. In more disturbed, warmer habitats, genera with high CTmax had increased relative abundance and functional activity, and those with low CTmax had decreased relative abundance and functional activity. This interaction determined abundance changes between primary and logged forest that differed in daily maximum temperature by a modest 1.1 °C, and strengthened as the change in microclimate increased with disturbance. Between habitats that differed by 5.6 °C (primary forest to oil palm) and 4.5 °C (logged forest to oil palm), a 1 °C difference in CTmax among genera led to a 23 % and 16 % change in relative abundance, and a 22 % and 17 % difference in functional activity. CTmax was negatively correlated with body size and trophic position, with ants becoming significantly smaller and less predatory as microclimate temperatures increased. Our results provide evidence to support the widely held, but never directly tested, assumption that physiological tolerances underpin the influence of disturbance‐induced microclimate change on the abundance and function of invertebrates in tropical landscapes.
Some cope better than others
Increasingly, research is revealing how organisms may, or may not, adapt to a changing climate. Understanding the limitations placed by a species's physiology can help to determine whether it has an immediate potential to deal with rapid change. Many studies have looked at physiological tolerance to climate change in fishes, with results indicating a range of responses. Dahlke et al. conducted a meta-analysis to explore how life stage may influence a species's ability to tolerate temperature change (see the Perspective by Sunday). They found that embryos and breeding adult fishes are much more susceptible to temperature change than those in other life stages and that this factor must therefore be considered in evaluations of susceptibility.
Science , this issue p. 65 ; see also p. 35
Global environmental change has influenced lake surface temperatures, a key driver of ecosystem structure and function. Recent studies have suggested significant warming of water temperatures in individual lakes across many different regions around the world. However, the spatial and temporal coherence associated with the magnitude of these trends remains unclear. Thus, a global data set of water temperature is required to understand and synthesize global, long-term trends in surface water temperatures of inland bodies of water. We assembled a database of summer lake surface temperatures for 291 lakes collected in situ and/or by satellites for the period 1985–2009. In addition, corresponding climatic drivers (air temperatures, solar radiation, and cloud cover) and geomorphometric characteristics (latitude, longitude, elevation, lake surface area, maximum depth, mean depth, and volume) that influence lake surface temperatures were compiled for each lake. This unique dataset offers an invaluable baseline perspective on global-scale lake thermal conditions as environmental change continues.
Aim
To test if physiological acclimation can buffer species against increasing extreme heat due to climate change.
Location
Global.
Time period
1960 to 2015.
Major taxa studied
Amphibians, arthropods, brachiopods, cnidarians, echinoderms, fishes, molluscs, reptiles.
Methods
We draw together new and existing data quantifying the warm acclimation response in 319 species as the acclimation response ratio (ARR): the increase in upper thermal limit per degree increase in experimental temperature. We develop worst‐case scenario climate projections to calculate the number of years and generations gained by ARR until loss of thermal safety. We further compute a vulnerability score that integrates across variables estimating exposure to climate change and species‐specific tolerance through traits, including physiological plasticity, generation time and latitudinal range extent.
Results
ARR is highly variable, but with marked differences across taxa, habitats and latitude. Polar terrestrial arthropods show high ARRs [95% upper confidence limit (UCL95%) = 0.68], as do some polar aquatic invertebrates that were acclimated for extended durations (ARR > 0.4). While this physiological plasticity buys 100s of years until thermal safety is lost, combination with long generation times leads to decreased potential for evolutionary adaptation. Additionally, 27% of marine polar invertebrates have no capacity for acclimation and reptiles and amphibians have minimal ARR (UCL95% = 0.16). Low physiological plasticity, long generations times and restricted latitudinal ranges combine to distinguish reptiles, amphibians and polar invertebrates as being highly vulnerable amongst ectotherms.
Main conclusions
In some taxa the combined effects of acclimation capacity and generation time can provide 100s of years and generations before thermal safety is lost. The accuracy of assessments of vulnerability to climate change will be improved by considering multiple aspects of species’ biology that, in combination may increase persistence under extreme heat events, and increase the probability for evolutionary rescue.
Hypoxia and climate warming are pervasive stressors in aquatic systems that may have interactive effects on fishes because both affect aerobic metabolism. We explored independent and interactive effects of dissolved oxygen (DO) and temperature on thermal tolerance, behavior, and fitness-related traits of juvenile F1 offspring of the African cichlid Pseudocrenilabrus multicolor. Fish were reared in a split brood design with four treatments (low or high DO; cool or hot temperature); thermal tolerance, growth, and condition were measured after 1 month in the rearing treatments, following which behavioral traits were measured over 3.6 months. Critical thermal maximum was higher in fish reared under hot conditions, but was not affected by hypoxia. There was an interactive effect of DO and temperature on agitation temperature (temperature at which fish show behavioral signs of thermal stress) and the thermal agitation window (oC between the onset of agitation and final loss of equilibrium). Fish reared and tested under hot, normoxic conditions showed a higher agitation temperature; while fish reared and tested under hot, hypoxic conditions showed the largest thermal agitation window. Fish grew more quickly in length under hot than under cool conditions, and more quickly under normoxic than hypoxic conditions. Fish reared under cool, normoxic conditions were characterized by higher condition than other groups. Both cool and hypoxic rearing conditions reduced activity and aggression. These results highlight the importance of integrating physiological tolerance measures with sub-lethal behavioral effects of hypoxia and high temperature to gain a fuller understanding of species responses to multiple stressors.
Climatic changes influence the thermal and oxygen dynamics of a lake and thus its ecological functioning. The impacts of climatic changes on tropical lakes are so far poorly studied and the extent of the effects is therefore uncertain, most investigations describing only potential effects. In this study, we applied the one-dimensional lake ecosystem model GOTM-ERGOM to quantify the effects of climate change on thermal stratification, oxygen dynamics, and primary production in meso-oligotrophic Lake Volta. GOTM-ERGOM was calibrated and validated using two years of observed data. The validated model was used to evaluate a series of future climate change scenarios. The model simulations showed good agreement with observed water temperature, dissolved oxygen and chlorophyll-a and indicated intensified stratification and reduced oxygen levels in the productive water layers of the lake. However, the longer-lasting stratification (prolonged stability) did not translate into permanent stratification. A relatively small (1 m) upward shift of thermocline depth resulted in an 8%–12% volume loss of the oxygen-rich upper mixed layer, which may be significant for the fisheries of the lake as it diminishes the size of suitable fish habitats. Light limitation of primary production renders the lake somewhat resilient to intensive algae blooms, as traceable in both the present and in the future climate scenarios. In the long term, the ongoing climate change may affect riparian communities that depend on the lake's fisheries for their livelihood. In consequence, future lake management strategies for implementation need to account for the impacts of future climate change.
In the 12 years since Dudgeon et al. (2006) reviewed major pressures on freshwater ecosystems, the biodiversity crisis in the world's lakes, reservoirs, rivers, streams and wetlands has deepened. While lakes, reservoirs and rivers cover only 2.3% of the Earth's surface, these ecosystems host at least 9.5% of the Earth's described animal species. Furthermore, using the World Wide Fund for Nature's Living Planet Index, freshwater population declines (83% between 1970 and 2014) continue to outpace contemporaneous declines in marine or terrestrial systems. The Anthropocene has brought multiple new and varied threats that disproportionately impact freshwater systems. We document 12 emerging threats to freshwater biodiversity that are either entirely new since 2006 or have since intensified: (i) changing climates; (ii) e‐commerce and invasions; (iii) infectious diseases; (iv) harmful algal blooms; (v) expanding hydropower; (vi) emerging contaminants; (vii) engineered nanomaterials; (viii) microplastic pollution; (ix) light and noise; (x) freshwater salinisation; (xi) declining calcium; and (xii) cumulative stressors. Effects are evidenced for amphibians, fishes, invertebrates, microbes, plants, turtles and waterbirds, with potential for ecosystem‐level changes through bottom‐up and top‐down processes. In our highly uncertain future, the net effects of these threats raise serious concerns for freshwater ecosystems. However, we also highlight opportunities for conservation gains as a result of novel management tools (e.g. environmental flows, environmental DNA) and specific conservation‐oriented actions (e.g. dam removal, habitat protection policies, managed relocation of species) that have been met with varying levels of success. Moving forward, we advocate hybrid approaches that manage fresh waters as crucial ecosystems for human life support as well as essential hotspots of biodiversity and ecological function. Efforts to reverse global trends in freshwater degradation now depend on bridging an immense gap between the aspirations of conservation biologists and the accelerating rate of species endangerment.
Pcrit - generally defined as the PO2 below which the animal can no longer maintain a stable rate of O2 consumption (ṀO2 ), such that ṀO2 becomes dependent upon PO2 - provides a single number into which a vast amount of experimental effort has been invested. Here, with specific reference to water-breathers, I argue that this focus on the Pcrit is not useful for six reasons: (1) calculation of Pcrit usually involves selective data editing; (2) the value of Pcrit depends greatly on the way it is determined; (3) there is no good theoretical justification for the concept; (4) Pcrit is not the transition point from aerobic to anaerobic metabolism, and it disguises what is really going on; (5) Pcrit is not a reliable index of hypoxia tolerance; and (6) Pcrit carries minimal information content. Preferable alternatives are loss of equilibrium (LOE) tests for hypoxia tolerance, and experimental description of full ṀO2 versus PO2 profiles accompanied by measurements of ventilation, lactate appearance and metabolic rate by calorimetry. If the goal is to assess the ability of the animal to regulate ṀO2 from this profile in a mathematical fashion, promising, more informative alternatives to Pcrit are the regulation index and Michaelis-Menten or sigmoidal allosteric analyses.
Mapping global deforestation patterns
Forest loss is being driven by various factors, including commodity production, forestry, agriculture, wildfire, and urbanization. Curtis et al. used high-resolution Google Earth imagery to map and classify global forest loss since 2001. Just over a quarter of global forest loss is due to deforestation through permanent land use change for the production of commodities, including beef, soy, palm oil, and wood fiber. Despite regional differences and efforts by governments, conservationists, and corporations to stem the losses, the overall rate of commodity-driven deforestation has not declined since 2001.
Science , this issue p. 1108
The question of parallel evolution-what causes it, and how common it is-has long captured the interest of evolutionary biologists. Widespread urban development over the last century has driven rapid evolutionary responses on contemporary time scales, presenting a unique opportunity to test the predictability and parallelism of evolutionary change. Here we examine urban evolution in an acorn-dwelling ant species, focusing on the urban heat island signal and the ant's tolerance of these altered urban temperature regimes. Using a common-garden experimental design with acorn ant colonies collected from urban and rural populations in three cities and reared under five temperature treatments in the laboratory, we assessed plastic and evolutionary shifts in the heat and cold tolerance of F1 offspring worker ants. In two of three cities, we found evolved losses of cold tolerance, and compression of thermal tolerance breadth. Results for heat tolerance were more complex: in one city, we found evidence of simple evolved shifts in heat tolerance in urban populations, though in another, the difference in urban and rural population heat tolerance depended on laboratory rearing temperature, and only became weakly apparent at the warmest rearing temperatures. The shifts in tolerance appeared to be adaptive, as our analysis of the fitness consequences of warming revealed that while urban populations produced more sexual reproductives under warmer laboratory rearing temperatures, rural populations produced fewer. Patterns of natural selection on thermal tolerances supported our findings of fitness trade-offs and local adaptation across urban and rural acorn ant populations, as selection on thermal tolerance acted in opposite directions between the warmest and coldest rearing temperatures. Our study provides mixed support for parallel evolution of thermal tolerance under urban temperature rise, and, importantly, suggests the promising use of cities to examine parallel and non-parallel evolution on contemporary time scales.
In ectotherms, anthropogenic warming often increases energy requirements for metabolism, which can either impair growth (when resources are limiting) or lead to higher predator feeding rates and possibly stronger top-down trophic interactions. However, the relative importance of these effects in nature remains unclear because: 1) thermal adaptation or acclimation could lower metabolic costs; 2) greater prey production at warmer temperatures could compensate for higher predator feeding rates; and/or 3) temperature effects on trophic interactions via altered biological rates could be small relative to other, temperature-unrelated human impacts on food webs. 2.Here, we examined effects of deforestation-associated warming on the minnow Enteromius neumayeri, occurring in both forested (cool) and deforested (warm) streams located inside or nearby an afrotropical rainforest. Combining approaches from physiological and community ecology, we quantified impacts of anthropogenic warming on the metabolism, growth, and trophic interactions of this tropical ectotherm. We then compared these effects with impacts of land use unrelated to temperature. 3.In a long-term laboratory acclimation experiment quantifying the temperature-dependence of growth and metabolism in E. neumayeri, warming increased metabolic rates and decreased growth (at a limited ration). We found no evidence of local (thermal) adaptation, with warming affecting farm and forest populations similarly. 4.Then, using mark-recapture methods to quantify impacts of warming on performance in situ, we found similar growth rates in fish from deforested and forested streams despite their distinct thermal environments. This suggests higher prey consumption at deforested sites to compensate for greater metabolic costs, which could strengthen fish-invertebrate interactions. 5.Finally, we developed a bioenergetics model to estimate fish-invertebrate interaction strength and quantify temperature-related and unrelated impacts of land use on this interaction. We found that although warming increased fish consumption, it apparently increased invertebrate production even more and thus had a net weakening effect on estimated interaction strength. Most importantly, variation in both fish and invertebrate density not directly related to temperature had a much stronger influence on estimated interaction strength than temperature effects on predator consumption and prey growth. 6.We conclude that ectotherms can sometimes offset the metabolic costs of warming with a small increase in consumption that hardly effects food web interactions compared to non-metabolic impacts of anthropogenic disturbances. Future research should assess whether this is a common feature of heavily-impacted ecosystems facing multiple stressors. This article is protected by copyright. All rights reserved.
Observations of climate impacts on ecosystems highlight the need for an understanding of organismal thermal ranges and their implications at the ecosystem level. Where changes in aquatic animal populations have been observed, the integrative concept of oxygen- And capacitylimited thermal tolerance (OCLTT) has successfully characterised the onset of thermal limits to performance and field abundance. The OCLTT concept addresses the molecular to whole-animal mechanisms that define thermal constraints on the capacity for oxygen supply to the organism in relation to oxygen demand. The resulting 'total excess aerobic power budget' supports an animal's performance (e.g. comprising motor activity, reproduction and growth) within an individual's thermal range. The aerobic power budget is often approximated through measurements of aerobic scope for activity (i.e. the maximumdifference between resting and the highest exerciseinduced rate of oxygen consumption), whereas most animals in the field rely on lower (i.e. routine) modes of activity. At thermal limits, OCLTT also integrates protective mechanisms that extend time-limited tolerance to temperature extremes - mechanisms such as chaperones, anaerobic metabolism and antioxidative defence. Here, we briefly summarise the OCLTT concept and update it by addressing the role of routine metabolism.We highlight potential pitfalls in applying the concept and discuss the variables measured that led to the development ofOCLTT.We propose that OCLTTexplains why thermal vulnerability is highest at the whole-animal level and lowest at the molecular level. We also discuss how OCLTT captures the thermal constraints on the evolution of aquatic animal life and supports an understanding of the benefits of transitioning from water to land.
Ongoing increases in air temperature and changing precipitation patterns are altering water temperatures and flow regimes in lotic freshwater systems, and these changes are expected to continue in the coming century. Freshwater taxa are responding to these changes at all levels of biological organization. The generation of appropriate hydrologic and water temperature projections is critical to accurately predict the impacts of climate change on freshwater systems in the coming decade. The goal of this review is to provide an overview of how changes in climate affect hydrologic processes and how climate-induced changes in freshwater habitat can impact the life histories and traits of individuals, and the distributions of freshwater populations and biodiversity. Projections of biological responses during the coming century will depend on accurately representing the spatially varying sensitivity of physical systems to changes in climate, as well as acknowledging the spatially varying sensitivity of freshwater taxa to changes in environmental conditions.
