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    • Pyramids, built by the Egyptians and reversed by sharksby Pierre Labourgade, Valentin Santanbien and Morgan Schler

      Published by Charlotte Recapet the October 5, 2020 on 8:28 AM

      The case of a extreme inverted trophic pyramid of reef sharks supported by spawning groupers in Fakarava, French Polynesia

      Predators play a key role in the structure and functioning of ecosystems (Paine 1966; Begon et al. 2006). Through food webs, the relationship between preys and predators is crucial in order to maintain a balance, including in marine ecosystems (Woodson et al. 2018). A trophic pyramid is a graphic representation designed to show the biomass at each level of the food chain. The lowest level starts with decomposers and the pyramid ends with top predators. This is called a pyramid because generally, the biomass in the lower levels turns out to be much higher than in the upper levels (Figure 1 A). However, in the marine environment, and in some remote and almost unoccupied areas, predators may dominate in terms of biomass, generating an inverted pyramid (Figure 1 B).


      Figure 1 Diagram of a normal (A) and inverted (B) trophic pyramid

      Aggregations of grey reef sharks, Carcharhinus amblyrhynchos are observed on some reefs in the Indo-Pacific (Robbins 2006) (Figure 2). The southern pass of Fakarava atoll in French Polynesia has a population of around 600 individuals of this species (Mourier et al. 2016) (Figure 3). This makes it one of the few places to present such a large grouping. With such a large population on a reef channel of just over 1 kilometer, the area has up to three times the biomass per hectare documented for any other reef shark aggregation (Nadon et al. 2012). The biomass of predators is then much greater than preys, thus generating an inverted trophic pyramid. During this study, scientists tried to understand how those large group of sharks can survive when prey biomass is insufficient.


      Figure 2. Aggregation of grey reef sharks


      Figure 3. Panoramic view of Fakarava atoll

      During the study period, video-assisted underwater visual surveys conducted across the pass allow the researchers to find that sharks population can represent up to 700 individuals. Then, scientists use bioenergetic models based on known value of parameters that influence energetics needs of shark-like “asymptotic length”, “growth rate” or “proportion of fish in the diet” to determine prey biomass needed for all the individuals. According to bioenergetic models, the food requirements to maintain that large population is approximately 90 tons of fish per year, which is not provided by the environment as it is. However, the pass is used as a breeding ground for many fish species, thereby reducing the prey-shark ratio. This means that the prey biomass will be much higher than that of sharks during these reproduction periods (Mourier et al. 2016), leading to frenetic predation behavior in the shark that will allow it to meet its energy needs (Robbins and Renaud 2016; Weideli, Mourier, and Planes 2015). Furthermore, the continuous presence of prey aggregation is ensured by the successive migration of different species to this site, in order to meet the metabolic demands of the shark population present (Craig 1998). With simulation-based on researcher bioenergetic model, sharks would not have enough energetic income after 75 days if other prey species didn’t migrate to the pass. There is, therefore, an idea of metapopulation where the exchange of individuals between populations in normal and inverted trophic pyramids ensures that the energy needs of each individual are met (Figure 4). This exchange of individuals between populations will allow the long-term maintenance of the species and, in the case presented here, of the shark.


      Figure 3. Diagram of the transfer of potential prey for the shark between two normal pyramids and one inverted trophic pyramid via migratory flows

      The temporal aspect in the movement of individuals between populations is therefore important to be considered during the development of management and conservation measures. Indeed, if we want to ensure the sustainability of the grey reef shark in this pass, we must not only protect the habitat on-site, but also the original habitat of different species that come to reproduce in the pass. These species are indeed essential for the survival of sharks since they represent the only source of energy available during certain periods of the year.

      Read the full study: Mourier, J., Maynard, J., Parravicini, V., Ballesta, L., Clua, E., Domeier, M.L., and Planes, S. (2016). Extreme inverted trophic pyramid of reef sharks supported by spawning groupers. Current Biology 26 (15): 2011–2016.

      Other cited articles:

      Begon, Michael, Colin R. Townsend, et John L. Harper. 2006. Ecology: from individuals to ecosystems. Sirsi i9781405111171.

      Craig, P. C. (1998). Temporal spawning patterns of several surgeonfishes and wrasses in American Samoa. Pacific Science, 52(1), 35-39.

      Nadon, M. O., Baum, J.K., Williams, I.D., McPherson, J.M., Zgliczynski, B.J., Richards, B.L., Schroeder, R.E., and Brainard, R.E. (2012). Re-creating missing population baselines for Pacific reef sharks. Conservation Biology 26 (3): 493–503.

      Paine, R.T. (1966). Food web complexity and species diversity. The American Naturalist 100 (910): 65–75.

      Robbins, W. D., and Renaud, P. (2016). Foraging mode of the grey reef shark, Carcharhinus amblyrhynchos, under two different scenarios. Coral Reefs 35 (1): 253–260.

      Robbins, W.D. (2006). Abundance, demography and population structure of the grey reef shark (Carcharhinus amblyrhynchos) and the white tip reef shark (Triaenodon obesus)(Fam. Charcharhinidae). PhD Thesis, James Cook University.

      Weideli, O. C., Mourier, J., and Planes, S. (2015). A massive surgeonfish aggregation creates a unique opportunity for reef sharks. Coral Reefs 34 (3): 835–835.

      Woodson, C. B., Schramski, J.R., and Joye, S.B. (2018). A unifying theory for top-heavy ecosystem structure in the ocean. Nature communications 9 (1): 1–8.

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    • Albatrosses: the laziest ocean birds?by Manon Yerle

      Published by Charlotte Recapet the July 8, 2019 on 9:51 AM

      Movement ecology is a new scientific discipline that studies the movement paradigm in the animal kingdom. The fact is: How foragers choose their hunting technics? Foragers are animals that need searching in wild food resources. Since 1960s foraging theories are studied over the world. It’s known that free-handing animals must maximize their energy gain in order to spend as little as energy possible to find and catch their preys. To better understand how this works, some scientists conducted a study about the foraging strategy of the wandering albatross (Diomedea exulans). In this study they integrate instantaneous energy budgets within the movement ecology.


      Wandering albatross (Diomedea exulans) in sit-and-wait (SAW) foraging strategy. Credit: John Harrison

      Maybe, you have already heard “albatrosses are always sitting on water”, “albatrosses are lazy birds” …In fact when you see one, most of the time, it’s resting on surface of the water and does NOTHING! Why is it doing that whereas it could be attacked by sharks? Moreover, water is very cold! To answer at this primordial question, the foraging strategy of the wandering albatross was studied and well characterised in the Southern Indian Ocean. Four strategies are known: foraging-in-flight (FII), area-restricted-search (ARS), sit-and-wait (SAW) and resting (RES). Strategies depend on external conditions like weather features or wind, etc… In the study, between 2002 and 2005, during brooding periods, 45 birds were tracked but prey data capture were available only for 18 foraging trips. Over 18 birds, only 5 were studied because they were complete for all data.


      Albatross in FII foraging strategy. Credit: John Harrison

      Authors assumed that net energy gain equal to energy gain minus energy expenditure. Energy gain is estimated by prey capture data (stomach temperature and digestion time) and conversion factors corresponding to the diet of the wandering albatross. Energy expenditure is estimated with continuous measures of heart rates values during trips. Finally, total trip net energy is estimated by cumulating instantaneous net energy gain along the trip. Assuming external factors (wind speed and angle between flight and wind) affects foraging, they implement the flying cost model to provide energy expenditure estimates.

      The most used foraging strategy is sit-and-wait because this strategy is better for the brooding period; they obtained higher net energetic gain when foraging trips are short. So, albatrosses aren’t lazy, but they are searching for food. Are their results available regarding fewer numbers of individuals? In statistical analyses it is assumed that results are available if the number of individuals is higher than 30, which is not the case here.  Moreover, optimal models don’t work in wild life because there is always external factors that prevent it.

      Whereas previous studies (Weimerskirch et al. 2005) identified FII strategy as the most optimal for long trips, our study identified another strategy for shorts trips, SAW. In fact, birds need to provision chicks frequently and that requires more energy than during incubation. Breeding stage defines the foraging strategy used. Another study should be managed during the incubation period implementing an instantaneous energy-budget model. Moreover, they should implement internal factors as thermoregulation that is more important in SAW for example. Now, we know how wandering albatrosses choose their foraging strategies. We can ask if SAW really is a good foraging strategy because albatrosses are more vulnerable against predators like tiger sharks.

      Cited study: Louzao, M., Wiegand, T., Bartumeus, F. et al. (2014) Coupling instantaneous energy-budget models and behavioural mode analysis to estimate optimal foraging strategy: an example with wandering albatrosses. Mov Ecol 2, 8. doi:10.1186/2051-3933-2-8

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    • Effects of an exotic prey species on a native specialist: Example of the snail kiteby Mathieu Finkler and Hyppolyte Terrones

      Published by Charlotte Recapet the September 11, 2018 on 12:34 PM

      Exotic species have largely been studied over the years, their effects on native populations, their consequences... Most of the studies aim to see the competition between a native species and an exotic one. Here the study focus on the effect of an exotic species on a native predator.

      Florida snail kite (Rostrhamus sociabilis plumbeus) are endangered, their populations are drastically declining in recent years. It is important to study them and to determine why their numbers are falling to implement an adapted conservation strategy.

      The purpose of the study is to assess the effects of the recently introduced island apple snail (Pomacea insularum) on snail kite behavior and energetics comparing with the native prey (Pomacea paludosa).


      Juvenile snail kite - Cláudio Dias Timm - CC BY-NC-SA 2.0

      The authors determined different parameters such as the proportion of snail dropped, the searching and handling time, the consumption rate and proportion of time in flight. Caloric intake of both species has been determined by a model (Sykes 1987) and so is the daily energetic expenditure. Caloric balance seems to be perfectly suitable in this case because the difference in intake calories could affect all the life history traits and be the cause of the fast decline of the kites.

      Foraging on exotic snails led to a greater proportion of snails dropped, a lower consumption rate, a longer handling time and a lower energy balance (figure below). These conclusions are particularly true and worrying for juveniles. This results indicates that feeding on exotic snails will decrease their energy and so less energy will be available for others activities (like reproduction, growth, defence against predators...). Finally, lakes where only exotic species are present (Tohopekaliga) could form an ecological trap.


      From Cattau et al. 2010

      Even after this study, it will be hard to conclude on an optimal foraging theory because both snail species were never found together in a lake. Therefore it could be interesting to make the same study in a lake were both species are present. Furthermore, this study has been conducted during the breeding period. During breeding period, species will need more energy to feed their offspring, to protect them, potentially leading to a greater difference in energetic balance.

      Further studies may focus on the fact that kites feed on larger exotic preys (compared to native preys). Are the smallest individuals not available for kites or do kites choose to feed on larger exotic preys to compensate for their lower energetic content ?

      This method could be used in others studies of trophic relationships and not only on native-exotic conflict. For example if human overfish a species, the predator of this species will have to change preys. So it will be important to calculate the energetic balance with the new prey.

      Cited study: Cattau, C. E., Martin, J., & Kitchens, W. M. (2010). Effects of an exotic prey species on a native specialist: example of the snail kite. Biological Conservation, 143(2), 513-520.

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    • Temperature-dependent body size effects determine population responses to climate warmingby Alison Arraud and Laura Duran

      Published by Charlotte Recapet the July 5, 2018 on 1:55 PM

      Up to now, neither the size nor the stage of the individual were considered to studying the population responses to climate warming. On 2014, a scientific group proposed another way to understand the temperature effects on fish populations. They improved the interaction effects of temperature-dependence with the size and the stage of fish on their energetic thresholds responses. Energetic thresholds themselves act on the dynamic of stade-structured population (e.g. parr, smolt, adult).


      Flathead mullet (Mugil cephalus) - Roberto Pillon - CC BY 3.0 Unported

      Finally, this study found that increasing temperature could redistribute biomass across life stages and modify the regulation of the population by reworking the intra-specific competition. Other studies have shown that high temperature during ontogenesis can accelerate the development and growth of individuals or, give individuals of smaller sizes early maturation.

      This study points out the importance of taking into account the interactions between temperature and size-specific (maturing, reproduction, etc.) that will lead to a set of behavioral responses that have consequences on the structuring of a population. This is all the more important in the context of global warming.

      Cited study: Lindmark, M., et al. (2018). Temperature-dependent body size effects determine population responses to climate warming. Ecology Letters 21(2), 181-189.

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