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