Marco Ortiz

Trophic models of four benthic communities in Tongoy Bay (Chile): comparative analysis and preliminary assessment of management strategies

Steady-state trophic flow models of four benthic communities (seagrass, sand–gravel, sand and mud habitats) were constructed for a subtidal area in Tongoy Bay (Chile). Information of biomass, catches, food spectrum and dynamics of the commercial and non-commercial populations was used and the ECOPATH II software of Christensen and Pauly [Ecol. Modell. 61 (1992a) 169] was applied. The sea star Meyenaster gelatinosus and the crabs Cancer polyodon, C. porteri and Paraxanthus barbiger were found to be the most prominent predators in the benthic system. The scallop Argopecten purpuratus as well as other bivalves represented the principal secondary producers in the seagrass, sand–gravel and sand habitats, while the Infauna dominated the mud habitat. The highest total biomass and system throughput (33579.3 t/km2/year) was calculated for the sand–gravel habitat. The sand habitat had a negative net system production due to the amount of primary production imported from deeper waters to satisfy the food requirements of the large beach clam (Mulinia sp.) populations. The mean trophic level of the fishery varied between 2.06 (sand–gravel) and 3.92 (sand) reflecting the fact that the fishery concentrates on primary producers (i.e. algae and filter feeding), and on top predators (i.e. snails and crabs). Fishery is strongest in sand–gravel habitat, where annual catches amount to 122.05 g/m2. Low values of the relative Ascendency (A/C) (from 27.4 to 32.7%) suggest that the systems analysed are immature and highly resistant to external perturbations. Manipulations of the input data for the exploited species suggest that seagrass and sand–gravel habitats have a potential for a ∼3 times higher than the present production of scallops and the red algae Chondrocanthus chamissoi. Preliminary results of Mixed Trophic Impacts (MTI) analysis suggest that any management policy aimed at a man-made increase in the standing stocks of A. purpuratus and Ch. chamissoi in seagrass and sand–gravel habitats, and a removal of the seastar M. gelatinosus in the seagrass habitat appears justified.

Dynamical simulation of mass-balance trophic models for benthic communities of north-central Chile: assessment of resilience time under alternative management scenarios

Abstract Within Tongoy Bay (north-central Chile), four different habitats (seagrass, sand-gravel, sand and mud) were identified and subsequently considered as subsystems for the construction of trophic mass-balance models. By using the ecosim software package of Walters et al. (Ecosystems, 2 (1997) 539), different management scenarios were simulated for these subsystems and for an overall integrated system (combining all habitats) in order to explore strategies of sustainable resource use. System Recovery Time (SRT) as well as single species responses differed with the scenario simulated. The most important findings were: (1) an increase in the harvest of the scallop Argopecten purpuratus would not drastically affect the global properties of the systems; (2) a depletion of its principal predator, the sea star Meyenaster gelatinosus, greatly improved the availability of scallops in the subsystem seagrass, sand-gravel as well as in the integrated model; (3) a biomass reduction of the snail Xantochorus cassidiformis would produce only a small negative impact on the SRT, while a simulated harvest on the clam Mulinia sp. drastically changed the system properties. This clam may thus be considered as a keystone species; (4) predictions of the maximum sustainable yield (MSY) of the scallop were lower than those obtained by single species approaches. Our simulations allow us to conclude that: (1) a strong fishery of the scallop predator, the sea star M. gelatinosus, would greatly increase the yield of the scallop and the other populations or resources of the systems; (2) trophic mass balance models and their simulated scenarios offer great possibilities for the planning of interventions or manipulations of natural systems.

Feeding behaviour of the asteroid Meyenaster gelatinosus in response to changes in abundance of the scallop Argopecten purpuratus in northern Chile

Between winter 1996 and autumn 1997, the prey spectrum and feeding behaviour of the sea star Meyenaster gelatinosus (MEYEN, 1834) and the seasonal abundance of the dominant benthic macrofauna were analysed in: (1) a shore-fringing seagrass meadow and (2) an adjacent sand-gravel benthic system of Tongoy Bay (central-north Chile). Of 524 sea stars collected, 130 (25 %) had food in their stomachs or were feeding on the scallop Argopecten purpuratus (LAMARCK, 1819), snails Calytraea trochiformis (BORN, 1778), Tegula luctuosa (ORBIGNY, 1841), and T. eryomphala (JONES, 1844), and small epifauna constituted by the polyplacophora Chiton spp., gastropods such as Nassarius gayi (KIENER, 1835), Nucula pisum (SOWERBY, 1835), Turritela cingulata (SOWERBY, 1825) and Mitrella unifasciata (SOWERBY, 1832). The scallop A. purpuratus suffered an intensive commercial harvest during the study period which decreased significantly its abundance in both habitats. Before harvesting. A. purpuratus constituted the principal prey for M. gelatinosus in the seagrass habitat, whereas all prey types were equally distributed in the sea stars diet in the sand-gravel habitat. After harvesting, the frequency of occurrence of the scallop in the stomachs of the sea star in the seagrass habitat decreased and was substituted by small epifauna. Furthermore, in the seagrass habitat after harvesting, the breadth of the feeding-niche of the sea star increased. indicating that the sea star became more of a generalist feeder. In the sand-gravel habitat alter harvesting, the diet composition remained unaltered, but M. gelatinosus showed an inverse tendency, specialising on the scallop. Additionally, M. gelatinosus increased its feeding preferences on A. purpuratus in both habitats (especially in sand-gravel) when the abundance of the scallop declined. M. gelatinosus feeds on a wide size-range (4 to 14cm) of the scallop with a marked dominance of larger size classes. Our results confirm, in part, the observations of local fishermen of Puerto Aldea regarding the possible negative impact of M. gelatinosus on the standing stock of the commercially exploited scallop.

Additive partitioning of reef fish diversity variation: a promising marine biodiversity management tool

Additive partitioning was applied to variation in reef fish spatial diversity at Isla Isabel National Park, Nayarit state, Mexico, and to identify the environmental and spatial variables that best explains it. Analyses included expected and observed species curves, rare species analysis, additive partitioning of alpha- and beta-diversity, and canonical redundancy analysis. A total of 10,517 individuals were recorded from 75 species and 33 reef fish families, representing 85% of expected richness. Species richness beta-diversity was dependent on the site scale, while the alpha-diversity of the Shannon diversity was most significant at the transect scale. Canonical partitioning showed species richness and Shannon diversity was explained by spatially-structured environmental components. Variation in species composition and abundance was explained by a purely environmental component. Therefore, elements of habitat structure (especially corals), topographic complexity, and refuge availability determine fish species diversity. Our results suggest that greater emphasis is required to conserve sites that promote β-diversity, increasing fish spatial diversity. In Isla Isabel, these sites would be mostly those located at eastern and southern of protected sides, where coral reef patches are well represented. The results of this multi-scale analysis are valuable and useful as an addition and complement to the holistic management strategies implemented at Isla Isabel.

Mass-balanced trophic and loop models of complex benthic systems in northern Chile (SE Pacific) to improve sustainable interventions: a comparative analysis

The capacity to withstand perturbations and the propagation of direct and indirect effects of harvesting were explored in seagrass and sand–gravel subtidal subsystems of northern Chile using Ecopath II, Ecosim and Loop Analysis theoretical frameworks. The relative Ascendancy and Redundancy from Ecopath II and Ecosim models and the holistic stability measure (Fn) given by Loop Analysis, all suggest that the sand–gravel subsystem is the more resistant and its fishery more sustainable. This outcome is relevant since the theoretical frameworks used are based on different assumptions but arrive at similar conclusions. If the fishery is included in the qualitative ecological models, which were found locally stable only when the fishery is self-controlled. While this conclusion is not new, its relevance here is that it emerges from qualitative multispecies modelling (Loop Analysis). Based on our model predictions, the biomass removal of the sea star Meyenaster gelatinosus would increase the standing stock of the commercial scallop Argopecten purpuratus only in the seagrass habitat, whereas the same man-made intervention may have an opposite effect in the sand–gravel habitat. We recommend to enunciate holistic models based on the theoretical framework used in the current work, which explore the reality from different perspectives assuming different givens. Based on these holistic explorations alternative and complementary management scenarios could be designed.

Qualitative modelling of the kelp forest of lessonia nigrescens bory (Laminariales: Phaeophyta) in eulittoral marine ecosystems of the south-east Pacific: an approach to management plan assessment

This contribution aims at the assessment of the sustainability of a management plan for Lessonia nigrescens in Chilean coastal ecosystems, specially when the harvest pressure could be increased due to that this macroalgae is considered as a food source for the abalon (Haliotis discus hannoi) farming along the Chilean coast. Three qualitative conceptual models were developed and compared; the first and second ones assuming homogeneous and heterogeneous environmental conditions, respectively, and the third expanded one considering eco-social components under the assumption of environmental heterogeneity. For all these models, the factors described by Vasquez [Bot. Mar. 38 (1995) 251] were included in the management plan for the harvesting of brown macroalgae. Based on Levinss Loop Analysis, the system will have a just weakly unstable moving equilibrium if the harvesting activities do not negatively affect the macrophytas reproduction and recruitment processes. Therefore, coincidence of reproduction and/or recruitment with harvesting must be avoided in a putative management plan. Regarding the qualitative predictions of the systems behaviour after negatively affecting reproduction and recruitment, only an ecological model assuming a heterogeneous environment shows realistic responses in comparison to experimental studies on the same species. This contribution is the first approach considering the dynamical behaviour of L. nigrescens and its surroundings. Nevertheless, further models, based on the same modelling theory, may integrate more variables from social and economical fields in order to improve the scope of this complex system. Further comparative studies should focus on the assessment of the significance of the structural functions of L. nigrescens and other macroalgae for the conservation of this benthic system.

Self-feedbacks determine the sustainability of human interventions in eco-social complex systems: Impacts on biodiversity and ecosystem health

Several administrative polices have been implemented in order to reduce the negative impacts of fishing on natural ecosystems. Four eco-social models with different levels of complexity were constructed, which represent the seaweed harvest in central-northern Chile under two different regimes, Management and Exploitation Areas for Benthic Resources (MAEBRs) and Open Access Areas (OAAs). The dynamics of both regimes were analyzed using the following theoretical frameworks: (1) Loop Analysis, which allows the local stability or sustainability of the models and scenarios to be assessed; and (2) Hessian´s optimization procedure of a global fishery function (GFF) that represents each dynamics of each harvest. The results suggest that the current fishing dynamics in MAEBRs are not sustainable unless the market demand presents some type of control (i.e. taxes). Further, the results indicated that if the demand changes to a self-negative feedback (self-control) in MAEBRs, the stability is increased and, simultaneously, a relative maximum for the GFF is reached. Contrarily, the sustainability of the model/system representing the harvest (principally by cutting plants) in OAAs is not reached. The implementation of an “ecological” tax for intensive artisanal fisheries with low operational cost is proposed. The network analysis developed here is proposed as a general strategy for studying the effects of human interventions in marine coastal ecosystems under transient (short-term) dynamics.

The functional trophic role of whale shark (Rhincodon typus) in the northern Mexican Caribbean: network analysis and ecosystem development

The whale shark (Rhincodon typus) has a high value to tourism, fishing, and pharmaceutical industry, but this species is considered as vulnerable and with potential extinction. The largest aggregation of whale sharks occurs in an upwelling system (Holbox) at northeastern Mexican Caribbean, where we analyzed the trophic role of whale shark. A mass-balanced trophic model was built to: (1) analyze the ecosystem emergent or macroscopic properties; (2) estimate the system recovery time (resilience); (3) assess the fishery effects on different target species; and (4) evaluate the whale shark systemic role in this system. The results showed that the macroscopic properties of the Holbox ecosystem were similar to other upwelling systems with high rates of primary productivity and accumulated biomass in lower trophic levels. Even so, the food web was more like the structure of coral reefs. On the other hand, Holbox system had a lower maturity and development level than coral reefs, suggesting an increased resistance to disturbances. The mixed trophic impacts and short-term dynamic simulation showed that species like snook, sharks, groupers, benthic autotrophs, small benthic epifauna, and phytoplankton had the largest direct and indirect effects. The most significant systemic role of whale sharks is to provide the resilience of the system.

Influence of the coral reef assemblages on the spatial distribution of echinoderms in a gradient of human impacts along the tropical Mexican Pacific

Fourteen species of echinoderms and their relationships to the benthic structure of the coral reefs were assessed at 27 sites—with different levels of human disturbances—along the coast of the Mexican Central Pacific. Diadema mexicanum and Phataria unifascialis were the most abundant species. The spatial variation of the echinoderm assemblages showed that D. mexicanum, Eucidaris thouarsii, P. unifascialis, Centrostephanus coronatus, Toxopneustes roseus, Holothuria fuscocinerea, Cucumaria flamma, and Echinometra vanbrunti accounted for the dissimilarities among the sites. The spatial variation among the sites was mainly explained by the cover of the hard corals (Porites, Pocillopora, Pavona, Psammocora), different macroalgae species (turf, encrusting calcareous algae, articulated calcareous algae, fleshy macroalgae), sponges, bryozoans, rocky, coral rubble, sand, soft corals (hydrocorals and octocorals), Tubastrea coccinea coral, Balanus spp., and water depth. The coverage of Porites, Pavona, and Pocillopora corals, soft coral, rock, and Balanos shows a positive relationship with the sampling sites included within the natural protected area with low human disturbances. Contrary, fleshy macroalgae, sponges, and soft coral show a positive relationship with higher disturbance sites. The results presented here show the importance of protecting the structural heterogeneity of coral reef habitats because it is a significant factor for the distribution of echinoderm species and can contribute to the design of conservation programs for the coral reef ecosystem.

Control Strategy Scenarios for the Alien Lionfish Pterois volitans in Chinchorro Bank (Mexican Caribbean): Based on Semi-Quantitative Loop Analysis.

Ecological and eco-social network models were constructed with different levels of complexity in order to represent and evaluate management strategies for controlling the alien species Pterois volitans in Chinchorro bank (Mexican Caribbean). Levins´s loop analysis was used as a methodological framework for assessing the local stability (considered as a component of sustainability) of the modeled management interventions represented by various scenarios. The results provided by models of different complexity (models 1 through 4) showed that a reduction of coral species cover would drive the system to unstable states. In the absence of the alien lionfish, the simultaneous fishing of large benthic epifaunal species, adult herbivorous fish and adult carnivorous fish could be sustainable only if the coral species present high levels of cover (models 2 and 3). Once the lionfish is added to the simulations (models 4 and 5), the analysis suggests that although the exploitation or removal of lionfish from shallow waters may be locally stable, it remains necessary to implement additional and concurrent human interventions that increase the holistic sustainability of the control strategy. The supplementary interventions would require the implementation of programs for: (1) the restoration of corals for increasing their cover, (2) the exploitation or removal of lionfish from deeper waters (decreasing the chance of source/sink meta-population dynamics) and (3) the implementation of bans and re-stocking programs for carnivorous fishes (such as grouper) that increase the predation and competition pressure on lionfish (i.e. biological control). An effective control management for the alien lionfish at Chinchorro bank should not be optimized for a single action plan: instead, we should investigate the concurrent implementation of multiple strategies.