bioenergetics models

How raising snails allows to better understand the dynamics of a parasite?by Thomas Boyer and Thibault Dindart

Published by Charlotte Recapet the July 19, 2021 on 9:38 AM

The analysis of epidemiological dynamics depends on host and parasite interactions. But these interactions fluctuate because hosts and parasites are heterogeneous entities that exist in dynamic environments. Resource availability is a powerful environmental constraint of intra-host infection dynamics (temporal patterns of growth, reproduction, parasite production and survival).

In this study, researchers developed, parameterized and validated an explicit resource infection dynamics model by incorporating a parasitism module in the energy balance theory. The mechanisms explained are the multivariate dynamic responses of the human parasite Schistosoma mansoni and its intermediate host snail to resource variation and host density. This parasite is widespread in Africa and inter-tropical America. Worldwide, more than 200 million people are infected with it, 9 million suffering from its symptoms, which cause more than 200,000 deaths every year. It tends to have erratic localizations (liver, spleen) and the accumulation in these organs of lost eggs makes the severity of the infection.
The most common symptoms are diarrhea and even dysentery. Complications can appear such as rectal prolapse, fistulas, occlusion, appendicitis.

To do this, they have parameterized the model using an experiment that manipulates food resources and follows the growth, reproduction, parasite production and survival of snail hosts. The model is then validated by simulating the dynamics of infection for host individuals undergoing different levels of intraspecific competition and comparing these predictions with the results of another experiment that manipulated host and resource density, and hence the intensity of resource competition

This bioenergy perspective suggests that variation in resource availability and competition could explain the infective dynamics of this parasite. To begin with, total cercaria production could be low when snail densities are low (because there are few infected snails) or when snail densities are high (if competition limits per capita parasite production). This potential relationship between snail density and the risk of human exposure could be the reason for the success or failure of current and proposed methods of schistosome control, which depend on reducing the density of snail vectors by molluscicides or predators. If resource competition in natural snail populations is strong enough, then snail reduction programmes could backfire, as reducing intermediate host densities could free the remaining hosts from resource competition, thereby increasing parasite production rates per host.

To conclude this bio-energetic model seems to indicate that the fight against the parasite Schistosoma mansoni begins with the regulation of snail populations (first host of the cycle). In order to better regulate this infectious dynamics, measures must be taken collectively between the countries concerned and must be directed towards a total reduction of snails or a limitation by intra-species competition. This model would establish priority levels of parasite infection for certain areas and would be followed by measures to control the parasite. A first measure would be the installation of pipelines to prevent the release of infected faeces into watercourses.

Read the full study: Civitello, D.J., Fatima, H., Johnson, L.R., Nisbet, R.M. and Rohr, J.R. (2018), Bioenergetic theory predicts infection dynamics of human schistosomes in intermediate host snails across ecological gradients. Ecol Lett, 21: 692-701. https://doi.org/10.1111/ele.12937

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Does food quality increases moult organism vulnerability to pollutant impacts? by Charlotte Couedel, Axel Rochaud and Stellia Sebihi

Published by Charlotte Recapet the April 27, 2021 on 9:06 AM

Today's ecotoxicology

For a long time, ecotoxicology focused on the lethal effects of pollutants, with increased individual mortality translating into smaller population size or population extinction. There has been a shift from the study of lethal doses to the impact of smaller doses on more specific processes such as physiology and behaviour (Rand and Petrocelli, 1985; Døving, 1991). The article deals with the effect of pollutants on moulting.

Possible use of ecotoxicology (the case of the article)

Pollutants are an environmental factor causing stress in individuals. Lack of resources is another factor. For this reason, the study attempts to demonstrate and quantify the impact of food quality on the resistance to pollutants of moulting organisms.

Hypotheses of the effect of the diet on the assimilation and detoxification of pollutants

When a pollutant is assimilated by an organism, the body sets up the detoxification system, but it requires energy. Food allows the assimilation of energy by organisms. Good quality of food makes an individual capable of accumulating the energy necessary to ensure vital functions. An organism with energy from good quality food, should be able to activate an effective detoxification. Thanks to this detoxification, the body should be less impacted by pollutants. The study seeks to demonstrate whether this is true.

Hypotheses illustration

The interest of the biological model

Gammaridae are macro-invertebrates that are mainly detritus feeders. They feed on detritus, corpses, living or decaying plants. Moreover, they are at the base of the human food chain as they are often industrially bred as fish food. Gammaridae are used to determine the biological quality of watercourses. They are rather pollution tolerant organisms but are nevertheless affected by pollution. Could the physiological changes noticed in Gammaridae be noticed in humans?

A picture of two Gammaridae

Way to understand the effects

The experiment is designed to evaluate single and combined effects of leaf litter stoichiometric quality and Cd exposure on G. fossarum survival and growth. Phosphorus (P) is used as the nutrient in leaf litter. Cadmium (Cd) is used as the pollutant. Phosphorus (P) is a nutrient naturally present in the Gammaridae's food, in this case, leaf litter. Also, industrial activities are often sources of cadmium released into aquatic environments. The main route of exposure to cadmium (Cd) is through the ingestion of contaminated water and food, so Gammaridae is particularly exposed to this type of pollutant.

The experiment design

144 microcosms were performed for each of the 3 levels of Cd concentrations (0 ; 0.35 ; 0.7). For each group, 72 microcosms were realised with Sycamore discs and 72 with Alder discs. It allows to observe the effect in different conditions. Then, among these 72 microcosms, three batches of 24 have been realized. The first batch is a control batch where the composition of the litter was not modified. The second batch was a P- batch, where the litter was deficient in phosphorus and therefore in nutritional value (and which does not allow individuals to extract a lot of energy). Finally, the third lot was P+, it was enriched in phosphorus, the nutritional value is very good.

Several metrics were measured to validate the initial hypotheses. The metrics were chosen for their relevance to evaluate organisms sensitivity to resources quality (leaf species and P content) and pollutant (Cd concentration in water): Cd bioaccumulation and survival rate. But also for their ecological importance: time-to-death, mass growth, time to moult and feeding rate.

Results to remember

• The Gammaridae's moult frequency and growth is amplified by a nutrient-rich diet (P+).
• A presence of pollutants (cadmium) in the Gammaridae’s life site reduces their growth and raise their probability of death.
• A nutrient-rich diet amplified effects of cadmium.
• If we make the connection: The higher quality of food ressources, the more moulting there is and the greater the effect of cadmium. So moulting makes Gammaridae vulnerable to pollutants.
• Species sensitivity to pollutants might be underestimated in ecosystems facing both nutrient constraint and pollutant.

Schematization of the main results

What to infer from this experiment.

The presence of pollutants in the water causes problems in the survival of Gammaridae. Ecotoxicologists are well aware of the bioaccumulation of pollutants in the food chain. As a result, a predator will be more contaminated by the pollutants than is prey. Indeed, predators will keep in them the majority of the pollutants present in their prey. Thus, humans present in the upper part of the trophic chain will be much more contaminated than the Gammare.

So why discharge pollutants into the water? Let's drink it directly!

Read the full study: Arce-Funck, J., Crenier, C., Danger, M., Billoir, E., Usseglio-Polatera, P., and Felten, V. (2018) High stoichiometric food quality increases moulting organism vulnerability to pollutant impacts: An experimental test with Gammarus fossarum (Crustacea: Amphipoda), Science of The Total Environment, 645, 1484-1495, https://doi.org/10.1016/j.scitotenv.2018.07.227.

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Spatial response of plaice and sole to climate changeby Arnaud Dupond and Alix Pascal

Published by Charlotte Recapet the August 3, 2020 on 7:54 AM

Scientists admit that climate change is one of the main driving forces which change species distribution and abundance in many ecosystems.

In this case, a modification of abiotic variables can affect the “niche concept” and can also change species geographical distributions. For the marine environment, this notion of geographical dependence is important. Indeed, marine organisms show several distinct stages during their life and each of these stages evolve in a specific habitat.

The objective of this study was to use different models based on physiological aspects and environmental variables in order to estimate habitat occupation by plaice and sole under different climate change scenarios in the North Sea.

How to reach this objective?

To predict new habitats, researchers considered environmental variables and determined their effect on the food web, but also the effect of the food web on the water chemistry.  To do that, the ecosystem model used functional groups of taxa. Their taxa are phytoplanktonic, planktonic and macrobenthic organisms. Some of them have a direct effect on the water chemistry and are regulated by other taxa. The food web is also used to quantify the availability of the habitat’s resources. Thanks, of this two types of model results fit more precisely reality of the distribution.

The sample strategy is built like this, data are collected daily on surface of 10x10km sin the North Sea.

Figure 1: Schematic simplification of the models used in the studies

Results of the study

The model of environmental variables shows the predictions for temperature and food conditions between 1989 and 2002. This data showed benthic production is concentrated along the southern coast during the year 1989, whereas in 2002 is concentrated in the Southern bight (figure 2).

Figure 2: Comparison of the benthic production between 1989 and 2002 in the North Sea

An important fact is that the temperature rate inside of which the growth is positive will change with the size of the fish and according to abundance of food (the more food is abundant the higher rate of temperature is). Indeed, bigger fish need higher temperature to grow optimally. Figure 3 defines the areas of maximum potential daily growth of each class size of plaices in 1989 at the left, and in 2002 at the right.

Figure 3: Comparison between the three size ranges of plaices and soles, of the speed growth in regard of two environmental parameters, the food availability and the temperature

The results for maximum potential growth per day seem to give the same result as the estimate of average abundance. (Figure 4).

Figure 4: Comparison of the plaice and sole abundance distribution in the North Sea

Conclusion

For the plaice, migrations during different stages of life maximize their physiological performance during the summer season, in the winter, the adult’s distribution is determined by the best spawning habitat and shows maximisation of their fitness. Sole differs in their physiological traits and have a higher optimal growth temperature which explains the difference in life habitat. As for plaice, the area indicated high quality habitat for the different size class.

This study can predict the evolution of species distribution with a model of environmental changes and one of physiological changes but in our case,  we can just explain data collected not the prediction made with the model.

Read the full study: Teal, L.R., van Hal, R., van Kooten, T., Ruardij, P. and Rijnsdorp, A.D. (2012), Bio‐energetics underpins the spatial response of North Sea plaice (Pleuronectes platessa L.) and sole (Solea solea L.) to climate change. Glob Change Biol, 18, 3291-3305. https://doi.org/10.1111/j.1365-2486.2012.02795.x

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A little bit of salt and heat... a good recipe for goby metabolism?by Maxime Deau, Quentin Garreau and Dorian Raoux

Published by Charlotte Recapet the June 1, 2020 on 2:18 PM

As the literature shows, a variety of factors influence the well-being of fish populations. For example, we know that some fish may or may not be very sensitive to changes in the conditions of their living environment (water temperature or salinity). These changes can affect their metabolism (reduced fertility, growth, etc.) or even, in the worst case, lead to the death of individuals. The goby (Pomatoschistusmicrops) (Figure 1), a relatively tolerant species and an essential central link in the food web is one of the species studied in the observation of the impact of these changes on the fish fauna in the Minho estuary in Portugal (Figure 2).

In this study, the researchers were able to model the evolutionary dynamics of p.microps populations based on models that take into account different parameters of goby's life cycle like fertility, mortality, migration rate and the effect of  environmental parameters such as salinity and temperature. The aim of these models is to describe the evolution of the  different life stages of this fish by establishing the possible impacts of climate change on their metabolism. In this framework, they studied both the impacts of temperature and salinity and combined the impact of both.

It has been noted that salinity directly influences the metabolism of individuals. Indeed, it plays a particular role in the survival of many aquatic organisms but also on their growth (strong allocation of energy to osmoregulation; Rigal, F. and al.,2008). Therefore, it plays a role in the growth of the goby as well as indirectly on its prey. The latter will be less available, which implies a higher energy expenditure for predation. However, this species resists large variations in salinity (0 to 51 psu). For temperatures, the impact is more diverse. Since the goby does not thermoregulate, its metabolism is directly influenced by the temperature of the environment. In addition to its significant effect on pregnancy, it also has an impact on migration, reproduction, recruitment and mortality. (Sogard, 1997; Hurst et al., 2000; Hales and Able,2001; Hurst, 2007; Jones and Miller, 1966; Claridge et al.,1985; Wiederholm, 1987).

Regarding the goby’s responses to these parameters, the research team has implemented them in the model, running different scenarios (salinity and temperature variations). Temperature and salinity variations studied separately led to population crash, except for a salinity lower than the current state ( -5psu). However, the combination of the two variables gave scenarios showing an increase in the population when the salinity was -5 psu, with temperatures ranging from +1 to +3°C, with an optimum at +2°C (see figure 5).
For example, in extreme temperatures, the fish activity will be greatly reduced, which will imply a decrease in the search for preys or sexual partners, causing feeding and mating problems (predation of eggs by males (Magnhagen, 1992). However, a slight increase in temperature could cause a longer reproduction period, allowing for a greater number of offspring to be generated. It has also been noted that with an increase in temperature, there is a delay in the breeding period, leading to the appearance of offspring in a period that may be less favorable for their proper development (early winter/lower metabolism).

In conclusion, climate change, through its effects on water temperature and salinity, will have a significant impact  on common goby populations. Indeed, these parameters have a great influence on the metabolism of these fish (whatever  their stage of development).In many scenarios, increases in temperature and salinity can cause crash populations. But  beware, in some cases (increase in temperature and decrease in salinity) the population of Pomatoschistus microps would tend to increase. Even if this scenario seems favorable for this species, some others will suffer. in fact, a study conducted on Arctic fish species has confirmed these trends

It is therefore clear that climate change affects population dynamics by changing fish environment and impacting their metabolism. It’s therefore important to continue this kind of study to have a better idea of these repercussion on a global scale. We are largely responsible for climate change, so it is up to us to make sure that we limit our impacts. Here is a link that will teach you how to reduce your carbon footprint through 20 examples of simple everyday actions: http://www.globalstewards.org/reduce-carbon-footprint.htm

Read the full study: Souza, A.T., Ilarri, M.I., Timóteo, S., Marques, J.C., Martins, I. (2018) Assessing the effects of temperature and salinity oscillations on a key mesopredator fish from European coastal systems, Science of The Total Environment 640–641, 1332-1345, https://doi.org/10.1016/j.scitotenv.2018.05.348.

Other cited studies:

Claridge, P.N., Hardisty,M.W., Potter, I.C., Williams, C.V., 1985. Abundance, life history and ligulosis in the Gobies (Teleostei) of the inner Severn Estuary. J.Mar. Biol. Assoc. U. K. 65, 951–968.
Hales, L.S., Able, K.W., 2001.Winter mortality, growth, and behavior of young-of-the-year of four coastal fishes in New Jersey (USA) waters. Mar. Biol. 139, 45–54.
Hurst, T., 2007. Causes and consequences of winter mortality in fishes. J. Fish Biol. 71, 315–345.
Hurst, T.P., Schultz, E.T., Conover, D.O., 2000. Seasonal energy dynamics of young of the year Hudson River striped bass. Trans. Am. Fish. Soc. Taylor & Francis 129, 145–157.
Jones, D., Miller, P.J., 1966. Seasonal migrations of the common Goby, Pomatoschistus microps (Kroyer), in Morecambe Bay and elsewhere. Hydrobiologia 27, 515–528.
Magnhagen, C., 1992. Alternative reproductive behaviour in the common goby, Pomatoschistus microps: an ontogenetic gradient? Anim. Behav. 44, 182–184.
Rigal, F., Chevalier, T., Lorin-Nebel, C., Charmantier, G., Tomasini, J.-A., Aujoulat, F., Berrebi, P., 2008. Osmoregulation as a potential factor for the differential distribution of two cryptic gobiid species, Pomatoschistus microps and P. marmoratus in French Mediterranean lagoons. Sci. Mar. 72, 469–476.
Sogard, S.M., 1997. Size-selective mortality in the juvenile stage of teleost fishes: a review. Bull. Mar. Sci. 60, 1129–1157.
Wiederholm, A.-M., 1987. Distribution of Pomatoschistus minutus and P. microps (Gobiidae, Pisces) in the Bothnian Sea: importance of salinity and temperature.Memoranda Societatis pro fauna et flora Fennica 63, 56–62.

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Why should we think about cougars when planning our cities?by Amaïa Lamarins and Gautier Magné

Published by Charlotte Recapet the February 3, 2020 on 2:02 PM

A puma family above the nighttime lights of San Jose - National Geographic - (photo courtesy of Chris Fust)

Humans have modified 75% of earth land surface which has important consequences on wildlife. In fact, human presence and activities are perceived as a threat by animals which adapt their behaviors to avoid it. Gaynor and his collaborators’ meta-analysis showed that many species are modifying their daily activities and identified 117 diurnal mammals becoming more and more active at night. Consequently, these animals face constrained access to resources and are susceptible to shifting their diet to nocturnal prey. Thus, anthropic activities influence growth, breeding, survival and community interactions of wild animals.

 
Shift in rhythmic activity of diurnal species due to human disturbance - Ana Benítez-López.

In southern California, the habitat of cougars, an apex nocturnal predator, is reduced by the expansion of cities. No, we’re not talking about the rampant nightclub predators (whose habitats remain undisturbed), we’re talking about mountain lions! You’ve probably already heard about pumas roaming across big cities like Santa Cruz, California. They likely are not curious tourists hoping to take in the sites, but are rather disturbed by human activities, which cause their nighttime activity to be higher in developed areas than in natural ones. This shift increases their daily energy expenditure: because of humans, pumas need to eat around 160-190 kg of additional meat per year (for females and males, respectively)! Are there sufficient deer populations to meet these needs? Unfortunately, it seems not, since a significant number of puma attacks on cattle have been recorded.

These results, showing human-induced behavioral change for pumas, come from a recent study published by members of the Santa Cruz puma project. By wide-scale monitoring of 22 wild pumas, they were able to link their behavior with their subsequent energetic expenditures: pumas’ behavior and movement were measured through spatial GPS location data, recorded every 15min, and energetic cost of movement was estimated considering their weight and travel velocity. An interesting methodological point to note: in order to avoid underestimating the energy expenditures via GPS tracking, scientists calibrated their estimations using accelerometers. Thanks to these methods they figured out the effect of housing densities on pumas’ activity and energetic costs, taking into consideration the time of day and sex of the animal.

Indeed, they were right in taking into account these factors because, according to their findings, response to human activities differs between day and night and between males and females. During the day pumas are more likely to stay inactive, especially near urban areas. At night, being close to houses increases time spent active by 8.8% and 5.8%, respectively, for males and females. Consequently, estimated daily caloric expenditure increases by 11.6% for males and 10.1% for females in high housing density areas. Below you will find an outline summarizing these results:

Urban development negatively affects pumas by increasing nighttime activity and energy expenditure.

Such studies underline the role of bioenergetics to estimate the costs of human-induced behavioral changes but do not provide insight on global energetic allocation. Further work is needed to understand the consequences of energetic balance disturbances and identify which individual functions are affected (growth, maintenance, maturation or reproduction). Besides, human impact could be underestimated because such tracking doesn’t allow us to know if pumas get all available energy from their prey near humans; some observations reported they often have to leave their prey because they fear humans. This partial feeding would constrain pumas to hunt more prey!

Unfortunately, this is not the only human-induced threat affecting pumas. In the region of Santa Cruz and southern California, they are targeted by ranchers, resulting in political tension about their conservation. In fact, cougars have been protected since 1990. However, 98 pumas are killed each year due to depredation hunting permits. It appears necessary to ensure coexistence between urban development, human activities, puma populations and their prey. In a recent study, development strategies are suggested, such as rural residence development, to ensure landscape connectivity and conservation of parcels where pumas have been geo-located. Nowadays, no cities are expanding regarding puma, deer or other wild animals’ living areas (to our modest knowledge!). The only measures taken when pumas are too close to urban zones consist in doing nothing or frightening or relocating it, and in the worst case killing it. And if designing our lives and activities regarding nature and wildlife was the challenge of tomorrow, would you be ready?

Ideal residential development maintaining pumas landscape connectivity. Graphical abstract of the paper of Smith and al 2019

Cited study: Wang, Y., Smith, J.A., Wilmers, C.C. (2017) Residential development alters behavior, movement, and energetics in an apex predator, the puma. PLoS ONE 12(10): e0184687. https://doi.org/10.1371/journal.pone.0184687

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What does the future has in store for red salmon in a context of global climate change?by Camille Sestac and Amandine Tauzin

Published by Charlotte Recapet the November 1, 2019 on 1:18 PM

Pacific salmon have extremely complex life histories and may be threatened by global climate change, as Peter S. Rand and colleagues investigate in their recent study.

Life cycle of Sockeye Salmon

Among all species, fishes must adapt to face disruptions caused by global climate change. Sockeye salmon (Oncorhyncus nerka), an anadromous species of salmon found in the Northern Pacific Ocean and rivers discharging into it, has a complex life cycle. As a migratory species, their energetic demands are high during spawning migration. Climate change might have important impacts on populations and their migration via variation of river discharge, increase of water temperature and decline of growth conditions. Aiming to better understand the impacts of these disruptions on the migratory performance of this species of salmon, Peter S. Rand from Wild Salmon Center teamed up with researchers from British Columbia. Their goal is to evaluate the effects of past and future trends in river discharge and temperature on the migratory performance of Sockeye Salmon in the Fraser River.

In a context of global climate change, it is crucial to understand the effects of disruptions on ecosystems and the populations living in them. Indeed, it is important to know the impacts of these disruptions on every stage of their life cycle (the juvenile freshwater period, the estuarine period, and the subadult marine period) so that we can maintain the populations stock. It’s especially important for fishery management because the fishing quota has greatly increased over the last decades and has threatened populations of Pacific salmon, particularly during their spawning migration. That’s why with three main objectives, these scientists used analysis to improve the understanding of how changes in river conditions can affect the energy use and the mortality rate in Sockeye salmon population. To do so, they used several models: one to search a link between energetic conditions of individuals and en route mortality, one to simulate the energy use during spawning migration and one to hindcast and forecast energy use by simulating fish’s behaviour and migration conditions (for more information, a tip, read the article!).

Long-range forecasts of lower Fraser river temperature during the summer of 2018

Using these friendly models, Rand and his colleagues proved that energy reserves and energy depletion of early Stuart Sockeye salmon are major factors that can affect their ability to reach their spawning grounds. They also stated that energy depletion is a function of both river temperature and discharge. Therefore, this population is structured by condition-dependant mortality. Nevertheless, this group of researchers brought to light a mechanism that allows fishes to cope with some environmental variability, providing a certain degree of resilience over time. Therefore, even if energetic demands and migration mortality increase as a result of exposure to warmer temperatures, it will be compensated by reduced time travel to the spawning ground as the river flow will be lower.

However, increase of temperature means increase of diseases appearing and developing and that stress added may be a direct cause of increased mortality during migration. Finally, as if it wasn’t already bad enough for our salmons, ocean productivity can be affected by climate change and thus affect their river migration success. In fact, this can lead to a decrease of body size and body energy content. It implies that individuals will start their migration with lower energy densities and will be more likely to exhaust their energy stock before even reaching the spawning grounds.

Salmon jumping over a weirAccording to the US-Canada Commission, a 21° C temperature spike was measured on the Fraser River in 2009. However, sockeye salmon show signs of physiological stress and migratory difficulties above 19°C and from 20°C, the first signs of illness and death appear. But migration of Sockeye salmon is not only threatened by climate change. In fact, migration of salmon specially is impacted by humans or natural obstacles. Dams and weirs form large obstacles for this migratory species and can be very difficult to cross. Many studies have already proved that this kind of obstacles, even when equipped with crossing devices, delay their migration and thus jeopardize their reproduction. This can lead to a decline of the population and in some cases to its extinction, as it happened in Belgium.

So, whilst some questions have been answered, it seems that more studies need to be carried out to improve our knowledge about the impact of global change which seems to be another sword of Damocles hanging over the head of Sockeye salmon.

Cited paper: Rand, P.S. et al. (2011) Effects of River Discharge, Temperature, and Future Climates on Energetics and Mortality of Adult Migrating Fraser River Sockeye Salmon. Trans. Am. Fish. Soc. 135(3), 655-667. https://doi.org/10.1577/T05-023.1

Featured images: Life cycle of Sockeye salmon by Camille Sestac, graph from https://www.pac.dfo-mpo.gc.ca/science/habitat/frw-rfo/index-eng.html , Sockeye Salmon from www.ryanvolberg.com

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A damned energy loss for migratory fishes: dams!by Manon Salerno

Published by Charlotte Recapet the June 10, 2019 on 9:42 AM

Many species of fish grow in the sea and breed in rivers. These migratory fish are called anadromous. When a migratory fish is ready to breed, it leaves the sea and up a river to lay watershed upstream. It will find the optimum conditions to reproduce and allow the development of its offspring. But to do so, they spend a lot of energy on the upstream and sometimes, obstacles like dams in their path does not make it easy for them. This is the case of American Shad in the Connecticut River in the United States. Since the 1970s, 4 hydroelectric dams have been built in the river. Even if they are equipped with fish ladders, these obstacles require the Shad more energy to cross them than if they were not present. We know energy availability can be a limiting factor in migration. Thus, in 1999, scientists wanted to understand energy management in these fish, especially when it is modified by the presence of such.

Any organism needs energy to perform the movements / migrations necessary for its life cycle. When they are heading into a period that will not allow them to feed (overwintering, migration), some species store energy, such as the bear before hibernating. For American Shad, this stock has to be created before migration because it will not feed during this move. First, scientists have found these are subcutaneous lipid reserves and skin constitute a special tissue for energy storage, which is rather unusual. Salmon, for example, usually mobilizes lipids from muscles and viscera. In contrast, for migration, somatic tissues (red and white muscles and skin) provide about 90% of the energy required in shad.

According to this study, crossing dams is expensive in energy, especially for females. In fact, American Shad is a species able to reproduce itself several times in its life, but if migration requires too much energy, it will only happen once. It is therefore easy to understand a multitude of dams can have an influence on the reproduction of these fish and therefore on population size, even if they are equipped with systems allowing fish to pass. Not to mention some fish do not even find the fish ladder. These are more likely to be stressed, eaten by predators such as birds, or competing with other fish and unlikely to breed.

Although fish ladders are quite efficient at the upstream for the American Shad, it is sometimes not suitable for other species. In addition, the outmigration can also present risks of mortality (water retention, drop height etc ...). It is therefore essential to remove the dams for which their function is not provided anymore. But in the United States, the erasure of small dams often meets opposition from local communities. Even though many dams have been removed, they represent a strong historical or landscape value for the inhabitants, creating tensions between the supporters of the restoration and the local communities. This situation reminds the context existing in France, where the aesthetic and historical arguments are very powerful. Many dams are attached to mills and water plants of olden times are therefore seen as a "living historical landscape" very characteristic of their landscape. Because of the local character of each operation, an opposition not necessarily collective but influential and well directed, is enough to block some sites.

Cited study: J. B. K. Leonard and S. D. McCormick (1999) Effects of migration distance on whole-body and tissue-specific energy use in American shad (Alosa sapidissima). Canadian Journal of Fisheries and Aquatic Sciences 56(7), 1159-1171

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A new approach of modeling dissolved organic matter release by phytoplankton. Is it an improvement?by Bastien Mourguiart and Thomas Panarotto

Published by Charlotte Recapet the April 12, 2019 on 6:53 PM

Phytoplankton is playing key roles in marine ecosystems. These microscopic plants are known, in particular, to be a part of the “Biological Pump”. Using photosynthesis as metabolism, it fixes carbon of the atmosphere to produce energy. This process reduces atmospheric concentration of CO2 and limits the greenhouse effect. It also produces oxygen indispensable to the life of many organisms.

Phytoplankton forms the basis of the marine food chain. Autotrophic organisms, they convert sunlight energy into chemical energy (food). This food constituted by molecules with carbon (organic matter) can pass directly along the food chain when zooplankton eats the phytoplankton and in turn are consumed by larger animals such as fishes, whales, squids, shellfishes and birds. Organic matter (OM) can also be released by phytoplankton in a dissolved form named dissolved organic matter (DOM). Organic matter can then be absorbed by bacteria and enter the main food chain when bacteria are eaten by zooplankton.

Marine food web

Heterotrophic prokaryotes (all animals) use carbon contained in DOM as a major source of energy.  So, products excreted by phytoplankton are really important in the functioning of marine ecosystems and understand how DOM is released in the environment is essential.

Livanou et al. present in their article “A DEB-based approach of modeling dissolved organic matter release by phytoplankton” a new model to calculate DOM release by phytoplankton. They apply Dynamic Energy Budget (DEB) theory on phytoplankton cells for that. In this study, the metabolism theory leads to describe DOM fluxes, based on assumptions about energy uptake, storage, and utilization of N and C. The authors are mainly interested in how DOM is excreted by phytoplankton under different nitrate concentrations.

They calibrate and test the goodness of fit of the model using past laboratory data. In this previous experiment, others scientists (Flynn et al. 2008), measured DOM released by one species of phytoplankton with two phase of nutrient concentration: one with enough nitrate for all the individuals and one with nitrate in limitation. The results of DEB-Model fit well to experimental data according to Livanou et al. even it does not explain all the information: in the figure below, lines (the model) do not fit exactly the points (experimental data).

Figure 2 in Livanou et al.

To conclude, they explain quickly that their model permit to describe how DOM is released. In no N-limitation condition, passive mode is used and DOM excreted is more accessible for bacteria. For N-limitation condition, DOM released cannot be used by bacteria and it tends to accumulate.

This study is maybe a step forward in comprehension of phytoplankton physiological mechanisms. However, in our opinion, it is not really useful to improve our understanding of energetic flows in the oceans. Moreover, the model was calibrated for only one of the thousand species of phytoplankton existing in nature. It should be calibrated for others species to catch up more processes which can change between species. The model can be more accurate catching up all the processes in this particular species too: the fitting test shows some differences from the experimental data (Figure 2). There is also limiting by the fact that only one nutrient is used as limiting nutrient: in reality, there can be more (Moore et al. 2013). To summarize, it needs very lot of work on this model to employ it in real ecosystems and be an improvement.

Cited study: Livanou E. et al. (2019). A DEB-based approach of modeling dissolved organic matter release by phytoplankton. Journal of Sea Research 143, 140-151.

Other references:

Flynn, K. J., Clark, D. R., and Xue, Y., (2008). Modeling the release of dissolved organic matter by phytoplankton, J. Phycol., 44, 1171–1187, https://doi.org/10.1111/j.1529- 8817.2008.00562.x

Moore, C. M. et al. (2013). Processes and patterns of oceanic nutrient limitation. Nature geoscience, 6(9), 701.

Image source: Maggy Wassilieff, 'Plankton - Animal plankton', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/diagram/5137/marine-food-chain (accessed 8 February 2019)

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Time and concentration dependency in the potentially affected fraction of species: The case of hydrogen peroxide treatment of ballast waterby Marie-Adèle Dutertre and Maud Hautier

Published by Charlotte Recapet the December 10, 2018 on 4:32 PM

Globalization and international trade made natural gates easier to cross for species. As a consequence, few species were able to travel long distance and settled in new habitats where they become invasive species.

More than 80% of industrialized goods in the world are transported by the oceans in container ships. In many cases, container ships is discharged in the destination port and go back empty. Whereas, the structure of this kind of ship does not allow them to travel empty and with stability. This is for a problem of stability  that ballast exists. Since the 19^th century, ballast with rocks was substitute with water. Before ships leave the port , water is loading in tank and at the destination port tanks are discharged.

http://www.seos-project.eu/modules/marinepollution/marinepollution-c04-p05.fr.html

Ballast water transport contribute to invasive species spreading. In order to fight against exotics species, waters ballast are treated with Hydrogen Peroxide (H₂O₂). But there is  a question : how to be sure that ballast water is effective and is not toxic for the marine environment ? In order to evaluate the environmental impact of the treatment, a study has been conducted. Three taxa has been chosen, among them, two crustacean, two algae and one rotifera : C. volutator, A. salina, E. costatum, D. teriolecta,  B. plicatilis. The authors of the study consider three dimensions : Hydrogen Peroxyde concentration, the effect of the Peroxide Hydrogen on organism and Hydrogen Peroxide exposure time. In the experiment, they made the tree dimensions varied and they considered as the final aim, the mortality, the immobility and the inactivation of the organism. The results are used in a mechanistic model which is based on the description of  Dynamic Energy Budget theory. The DEB theory consists of a simple set of rules that specifies how organisms acquire energy and building blocks from their environment to fuel their life cycle. It is used to rely the observed effects and the hydrogen peroxide concentration in the experiments. The DEB-tox model allows to determine ECx — Effect Concentration — : the concentration which induces a response of x% between the baseline and maximum after a specified exposure time ; and the HCx — Hazardous Concentration — : the concentration which is dangerous for x% of the population. Thanks to this values, it is possible to determine the PAF — Potentially Affected Population— with means the part of an ecosystem potentially affected by a drug concentration. The results show an interspecific response variability with means different interspecific H₂O₂ sensibility. Sensibility is a combination between time exposure and the concentration. The conclusion of the study is that the hydrogen peroxide is effective for treating ballast water.

Concentrations, effects and time exposure were studied there. The choice of the five species is a wise choice as a result of the representativeness of a wide selection of sensibility which allows to extrapolate this results to other species and then estimate the effect of hydrogen peroxide treatment on other species present in water ballast. Whereas the aim of the study was to assess environmental risks of hydrogen peroxide treatment, and the obtained results here cannot be used to conclude regarding as the environmental risks.

To assess more precisely the risk, it is important to consider the hydrogen peroxide degradation and its potential impact on marine ecosystem. The H₂O₂ is oxygenated water which would rapidly be decomposed : 2H₂O₂ => 2H₂O + O₂. In this case, the hydrogen peroxide would not impact the environment.

Furthermore, sub-lethal effects are sufficient to reduce the viability of the organisms and for that, lower concentration of H₂O₂ and lower time exposure are sufficient. The purpose is to neutralize exotic species with lower environmental and economic costs. Moreover, in order to reduce again the hydrogen peroxide used quantity, other studies show the efficiency of using UV, Ozone, and ultrasound for neutralizing species. The hydrogen peroxide treatment can also be used with alkaline water which allows to obtain the same result with lower concentration and time exposure.

An other option is to establish regulated areas for discharging and to filter and to purify ballast water before discharging in the environment.

Cited study: Smit, M. G., Ebbens, E., Jak, R.G., and Huijbregtst, M.A. (2008). Time and concentration dependency in the potentially affected fraction of species: The case of hydrogen peroxide treatment of ballast water. Environmental Toxicology and Chemistry 27(3), 746-753.

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Are vulture restaurants needed to sustain the densest breeding population of the African white-backed vulture?by Mikel Cherbero and Tom Laffleur

Published by Charlotte Recapet the August 3, 2018 on 10:00 AM

As obligate scavengers, vultures are entirely dependent on carrion. These last decades, carrion abundance has decreased in many areas. The two main causes of this trend are clearly identified. Natural habitat destruction reduces wild animal carrion abundance, which is the natural resource of scavengers. At the same time, the modification of agricultural practices, essentially the generalization of carcass rendering, has reduced the availability of cattle carrion. These factors have led to a negative trend on scavenger populations. This is especially the case in Africa, where most of avian scavenger species are now endangered. African savanna ecosystems were originally rich in avian scavengers, but most of the species are actually endangered.

White-backed vultures feeding on zebra carrion - Bernard Dupont - CC BY-SA 2.0

In this study, authors model the carrion ecology of an ecosystem in Swaziland which is home to the densest breeding population of the African white-backed vulture (Gyps africanus), a critically endangered species. They also study other threatened scavenger species of Swaziland: white-headed vulture (Trigonoceps occipitalis), Nubian vulture (Torgos tracheliotos), marabou stork (Leptoptilos crumenifer), tawny eagle (Aquila rapax) and bateleur (Terathopius ecaudatus). The purpose of this work is to better understand the feeding activity of the white backed vulture and to modelize population trends for these six species (using life-history traits and modelization of carrion availability), and based on these results authors discuss if the establishment of vulture restaurants would be beneficious.

They first calculated the foraging radius (r) of the white-backed vulture, based on the Foraging radius concept theory. The foraging radius represents the radial distance from the nest in which the energy inputs are greater than the costs of feeding and needs of the vulture and its litter. This theory is adapted to this species, because vulture always comes back to the nest after feeding. They compiled available bibliography and collected data on metabolism and life-history parameters of the species. Using this data, they applied a model created with the same purpose by Ruxton & Houston in 2002 for the Ruppell’s vulture (Gyps rueppellii), which is phylogenetically and ecologically close to the white backed vulture.

The results shows that the foraging radius is 260 km in the main part of the year. This radius is large, vultures can feed in neighboring countries (South Africa, Mozambique), it implies an international cooperation in the management of these endangered populations. A positive aspect is that individuals can spread over large area, so the studied population can form or sustain other populations. On the other hand this radius is much greater than the natural reserve surface, thus vultures can be exposed to several risks, like poisoning, when they are feeding. When vulture have to feed a chick, energy needs are logically greater so the foraging radius is reduced to 40 km. Carrion availability is more problematic during this period, which should therefore be targeted if vulture restaurants are setted up.

Using novel Population Dynamics P-Systems, they show that carrion provided by wild ungulates biomass is currently enough to sustain this vulture species. According to the model, white-backed vulture population will continue increasing in Swaziland, and will pass from approximately 300 pairs to more than 500 in twenty years. The other studied avian scavenger populations will follow the same trend, but are far less abundant than white-backed vulture. The model shows also that three main species are composing vultures’ food: the Impala (Aepyceros melampus), the blue wildebeest (Connochaetes taurinus) and the plains zebra (Equus burchelli) represent 55 % of total carrion.

However, in light of the forecasted population increases, food will become a limiting factor. This is particularly true for the period from November to April, for which the model show a carrion deficit. During this period African vultures are not breeding so they can go far away to feed themselves. But the model also shows a carrion deficit during the breeding season after five to thirteen years of simulation. This lack of food resources can be considered as a natural limiting factor. According to the model, the area has probably reach its maximum carrying capacity after twenty years.

To conclude, authors suggest that setting up supplementary feeding stations in Swaziland should be seriously considered, especially during the breeding season. Good managed restaurants would have several advantages : secure the viability of the population and thus increase its capacity to act as a source population, avoid poisoning risks and create the opportunity to capture and tag vultures. This last point would allow to improve knowledge about the avian scavenger species, necessary for a more effective conservatory management.

Cited study: Kane A., Jackson A.L., Monadjem A., Colomer M. A., Margalida A., 2015. Carrion ecology modelling for vulture conservation : are vulture restaurants needed to sustain the densest breeding population of the African white-backed vulture? Animal Conservation (18) 279-286.

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