For instance, Rhizoprionodon lalandii has a high recovery potential (Stevens et al., 2000; Lessa et al., 2009). In lower diversity systems, the exclusion of a single species has a higher chance of causing significant changes because there is a smaller set of species capable of replacing the lost species. This finding highlights the importance of large sharks in controlling the overall structure of the system. This result suggests that, in this system, elasmobranch mesopredators will increase in structural importance if the populations of large sharks decline. Five large sharks (G. cuvier, C. oscurus, C. taurus, S. lewini and S. zygaena) showed the largest values of the Ki; being considered as keystone species in the network leads to powerful top-down effects (Table 2). Additional data concerning elasmobranchs, teleostean fishes, reptiles, birds and invertebrates were obtained through the grey literature and published articles from 1980–2012. Perturbations and indirect effects in complex food webs. trophic cascades and apparent competition). 2010). On the other hand, after elimination of top predators from the system, the topological importance list was drastically changed. Failure to recognize the importance of direct and indirect effects can lead to serious gaps in our understanding of the causes and consequences of disturbances in foodwebs (Montoya et al., 2005). These data were used to construct a quadratic matrix of presence/absence (“who eats whom”) in predator diets, in which rows represent predator species and columns represent prey species. The “mesopredator release” hypothesis for elasmobranchs (see Myers et al., 2007; Ritchie and Johnson, 2009) may be applicable in a structural context off the coast of Brazil, in contrast with the tropical system along the Pacific coast (Navia et al., 2010). 2007; Ferretti et al. We tested the hypotheses that some elasmobranchs can be considered key elements within the foodweb, and that large predators have topological importance (act as keystones), so that, when large predators are excluded, mesopredator elasmobranchs occupy higher topological positions. We thank Lida Teneva and James Nienow for English reviewing, as well as the two anonymous reviewers. According to CCi, only two elasmobranchs were among the 20 largest values: S. guggenheim and Z. brevirostris in eighth and 13th position, respectively. Z. brevirostris, C. obscurus, S. zygaena) may act as key elements with high structural importance within the ecosystem, serving as important links between other compartments of the foodweb. When this information was not available, genera, families or orders were used. Stevens et al. To answer the two central questions of this study we assessed the structural importance of the sharks considered to be the top predators in the trophic network under study by computing all indices with and without these species. Nodes in light grey represent top predators, black for mesopredators, medium grey for teleosts, white for invertebrates, and dark grey for phytoplankton, zooplankton and detritus. This high diversity leads to high structural complexity of foodwebs and high topological redundancy, suggesting that trophic relationships could easily be readjusted if one or more of the components were lost (e.g. apparent competition, exclusion competition, behaviour effects). They also had the largest top-down values. However, they did not have the highest BCi index values in the system. For general enquiries about the Shark Control Program, call us on 13 25 23 or email scp@daf.qld.gov.au. The size of nodes is proportional to the structural importance of each node. This is observed in foodwebs with very low values of C (≈0.03) (Dunne et al., 2002b). Carcharhinus obscurus, S. zygaena and Zapteryx brevirostris were found to be the elasmobranchs with largest values of centrality, and can, therefore, be considered key elements in the topological structure. Only the first 20 components are presented by decreasing rank in Ki index. An example of this is a trophic cascade, in which the reduction of a predator population changes the relative abundance of prey populations as they are freed from direct (predation) and indirect effects (e.g. The top 20 compartments of the foodweb off the coast of southern Brazil in terms of the two centrality indexes (Betweenness and Closeness); the node degree for each compartment is also given. As a result, the hungry rays ate all the bay scallops, forcing the fishery to close. Stevens et al., 2000; Myers et al., 2007). The results of the present study indicate that reductions in shark populations may lead to changes in other species through indirect effects on the foodweb (see values in Table 2), as recorded in tropical (Navia et al., 2010; Navia, 2013) and temperate environments (Coll et al., 2013). “sharks” or “rays”), as is common in mass balance studies (e.g. Due to overexploitation and lack of proper management, many shark species are under considerable risk of unrecoverable decline with some species having declined to near extinction in recent years. When dietary information was unavailable for some species in the study zone, information from similar ecosystems was used to complete foodweb. trophic cascades vs. apparent competition). A study in North Carolina showed that the loss of the great sharks increased the ray populations below them. Kbu = bottom-up importance index, Ktd = top-down importance index, Kdir = direct effect importance index, Kind = indirect effect importance index, and Ki = topological importance index. It also characterizes positional importance, separating direct from indirect effects, as well as bottom-up from top-down effects in the trophic network (Jordán, 2001). However, the BC analysis in the present study shows that some species of sharks and rays (e.g. Sharks’ control over species below them in the food chain indirectly affects the economy. Indices quantifying the positional importance of nodes of the network with and without top predators in the southern Brazil ecosystem. bivalves, seabirds, macroalgae, T. lepturus) (see Table 2). The present study represents the first attempt to identify the structural relationships of elasmobranchs in a subtropical ecosystem using a topological approach. Hugo Bornatowski, Andrés Felipe Navia, Raul Rennó Braga, Vinícius Abilhoa, Marco Fábio Maia Corrêa, Ecological importance of sharks and rays in a structural foodweb analysis in southern Brazil, ICES Journal of Marine Science, Volume 71, Issue 7, September/October 2014, Pages 1586–1592, https://doi.org/10.1093/icesjms/fsu025. Comprehending these trophic interactions and the position of species within a foodweb is a crucial step in clarifying the dynamics of marine communities and the impacts individual components (i.e. Why are sharks important? Supplementary data are available at ICES Journal of Marine Science online. (i) Are sharks and rays important components in a foodweb to the extent that reductions in their populations could result in the disruption of connections within the web? However, to reduce bias caused by different levels of prey quantification or inconsistency in the use of numerical indices among all studies examined, we considered only the presence/absence of prey species in predator diets. Changes to the environment, caused by pollution or global warming can lead to certain animals becoming extinct, affecting lots of food chains. Elasmobranch names in bold. Squatina guggenheim, another shark that increases in topological importance with the exclusion of large sharks, lives up to 12 years, with first maturity at four years (Vooren and Klippel, 2005). Five other elasmobranch species in the mesopredator group were among the 20 species with the highest values of Ki (see Table 2). To construct this matrix, we gathered information on 820 stomach contents of 15 elasmobranchs (see Appendix A) from artisanal fishery landings along the central coast of the state of Paraná, southern Brazil (Figure 1), from April 2010 to March 2012. Dunne et al., 2002a, 2004; Frank et al., 2007; Navia et al., 2012; Navia, 2013). These issues need to be considered by conservation and fishery management groups since it appears that ecosystem integrity may be compromised by reductions in the populations of large predators. Understanding the trophic interactions and the position of species within a foodweb is crucial if we want to understand the dynamics of marine communities and the impact individual components of the community have on trophic network compartments. Changes to marine trophic networks caused by fishing. According to the CCi index, six elasmobranchs were ranked among the top 20 positions (see Table 1). trophic cascade). 2000; Myers et al. To construct the foodweb, the information on stomach contents was compiled to the species taxonomic level when possible. Species with large values of CCi are considered to be components whose removal will affect the majority of other groups. When we excluded top predators from the analysis, only two elasmobranch species remained in the top 20 positions, and the highest ranked species dropped to eighth place. However, increases in mesopredator populations have primarily been documented in cold and temperate waters or in other low diversity environments. The density of the network was D = 0.18, with a connectivity of C = 0.043. The node degree (Di) takes into account the number of nodes connected directly to node i. In this study, we evaluated the ecological importance of sharks and rays in a subtropical ecosystem off the coast of southern Brazil by using topological analyses. Five additional elasmobranchs were also among the 20 components with the largest values for the index: S. guggenheim, C. taurus, Z. brevirostris, S. zygaena and M. schmitti appeared at positions 12, 15, 17, 18 and 19, respectively (Table 1). This result contrasts with the findings of Navia (2013), which suggests that mesopredator release does not occur in tropical ecosystems because of their high species richness. The degree of node i (Di) is the sum of its prey (in-degree, Din,i) and its predators (out-degree, Douti): Di= Din,i+ Dout,i. We then used the network analysis software NetDraw 2.075 (Borgatti, 2002) to visualize the matrix as a trophic network, and calculated density (D = S/L) and connectance (C = L/(S)2), where S is the number of species, and L is the number of links (Martinez, 1992); these measures are used to characterize foodwebs. However, because of the long generation times and low growth and reproductive rates, the size of the population of this species declined by 87% between 1980 and 2000 (Vooren and Klippel, 2005). Mesh sizes used by the gillnet fishery include 7-, 9-, 11-, 16-, 18- and 45-cm stretch mesh. Size and sex compositions, length–weight relationship, and occurrence of the Brazilian sharpnose shark, Cascading effects of the loss of apex predatory sharks from a coastal ocean, Función ecológica de tiburones y rayas en un ecosistema costero tropical del Pacífico colombiano. For full access to this pdf, sign in to an existing account, or purchase an annual subscription. Zapteryx brevirostris showed the largest value of BCi, followed by Paralichthys sp., Polychaeta and Portunidae (Table 1). We need non-lethal shark control measures to avoid shark attacks, and better public education in order to live in peace with sharks and other marine life. Comprehending these trophic interactions and the position of species within a foodweb is a crucial step in clarifying the dynamics of marine communities and the impacts individual components (i.e. In addition to the two elasmobranch mesopredators (seabirds—Sula leucogaster and Fregata magnificens),T. lepturus, Paralichthys sp.
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