Modelling the cumulative spatial–temporal effects of environmental drivers and fishing in a NW Mediterranean marine ecosystem
Introduction
Biodiversity, natural resources, and ecosystem services change in response to human stressors, such as climate change and fishing activities that often have cumulative effects (Crain et al., 2008, Darling and Côté, 2008, Halpern et al., 2008). The need to consider changes in the environment as well as human activities when analysing and managing marine ecosystems highlights the necessity to perform integrated analyses (Link, 2011). Since the productivity of marine resources depends on the ecological state of communities and ecosystems, and on external drivers, the dynamics of target species in conjunction with the dynamics of non-target organisms, trophic relationships and energy flows, environmental drivers and human impacts have to be considered to manage marine resources properly (Christensen et al., 2011).
In the last decade, the scientific community has made substantial progress in the identification and quantification of multiple human threats that impact marine diversity, habitats and ecosystems (Jackson et al., 2001, Lotze et al., 2006, Halpern et al., 2008). Currently, there is an increasing knowledge on the quantification of these multiple drivers, also in the Mediterranean Sea (Coll et al., 2012, Micheli et al., 2013). However, the way these drivers may interact and combine to impact patterns of marine ecosystems is not well known (Crain et al., 2008, Darling and Côté, 2008). Multiple drivers may interact and their effects may cause impacts that accumulate in an additive way (thus the sum of their individual impacts is equal to the total impact) or may act synergistically or antagonistically (when the sum of individual impacts are greater or lower than the separated effects, respectively) at different ecological levels (Mackinson et al., 2009, Ainsworth et al., 2011). Despite their importance, a comprehensive understanding of these impacts and their interactions is mostly lacking. Therefore, there is a growing need to develop and use novel methodologies of data integration, assimilation and modelling human impacts at different scales, taking into account spatial–temporal dynamics (Coll et al., 2013a).
Species distribution models and ecosystem models are two methodologies that are developed to study the spatial and temporal dynamics of marine resources and ecosystems (Guisan and Zimmermann, 2000, Fulton, 2010). The main aim of statistical modelling is to use the information about where a species occurs and the relationship with associated environmental conditions to predict how likely the species is to occur in non-sampled locations. Different techniques to apply species distribution models (SDM) exist and have been widely applied to the marine environment (e.g., Kaschner et al., 2001, Jones et al., 2012), including the Mediterranean Sea (e.g., Morfin et al., 2012, Pennino et al., 2013, Saraux et al., 2014). Despite their popularity, it is known that environmental drivers alone may not be sufficient to account for species distributions (Navarro et al., 2015). Other ecological processes, including trophic interactions (such as competition, predation, and facilitation), behavioural parameters, and population dynamics may affect the spatial arrangement of a species, in addition to human activities.
These processes can be considered in process-based oriented modelling, like in food web models such as Ecopath with Ecosim (EwE) approach (Christensen and Walters, 2004). EwE is a freely available software and approach that allows building food web models by describing the ecosystem by means of energy flows between functional groups, each representing a species, a sub-group of a species (e.g., juveniles and adults) or a group of species that have functional and ecological similarities. The functional groups can be set to represent consumers and primary producers, as well as non-living groups (e.g., detritus).
EwE consists of three main linked routines that have been developed over the last three decades: Ecopath, Ecosim and Ecospace. Ecopath is the mass-balance routine that allows building of spatially and time-averaged models of the trophic web (Polovina, 1984, Christensen and Pauly, 1992), and which serves to parameterize the dynamic modules of EwE; Ecosim is the time dynamic routine (Walters et al., 1997, Walters et al., 2000); and Ecospace is the spatial–temporal dynamic module that allows representing temporal and spatial 2D dynamics of trophic web components (Walters et al., 1999, Walters et al., 2010). Critical overviews of EwE approach and descriptions of the recent developments are available in the literature (Christensen and Walters, 2004, Steenbeek et al., 2014, Steenbeek et al., 2016, Ainsworth and Walters, 2015, Coll et al., 2015a, Heymans et al., 2016).
Ecospace has been widely applied to quantify the spatial impact of fisheries on marine species (Christensen et al., 2003, Christensen et al., 2014a, Christensen et al., 2015), to analyse the impact of management scenarios such as the establishment of marine protected areas, and to assess the correlation of spatial distributions of marine species and fishing effort (Walters, 2000, Martell et al., 2005, Fouzai et al., 2012). Ecospace can also be used to develop spatial optimization routines (Christensen et al., 2009) and to assess the impact of climate change on marine ecosystems by linking Ecospace with low trophic level models (Fulton, 2011) or by driving Ecospace directly with external spatial–temporal data (Steenbeek et al., 2013, Steenbeek et al., 2016).
Despite their capabilities, available tools offered limited capacity to model cumulative impacts of stressors and produce integrated assessments that take both food web dynamics and spatial–temporal environmental and human variability into account (Steenbeek et al., 2013). To overcome some limitations, the habitat foraging capacity model was recently added to Ecospace (Christensen et al., 2014b). This new development offers the ability to spatially drive foraging capacity of species from the cumulative effects of multiple physical, oceanographic, and environmental conditions (such as depth, bottom type, temperature, salinity, oxygen concentrations and primary production). The habitat foraging capacity model runs in Ecospace in conjunction with the food web and fisheries dynamics. This development, in combination with the spatial–temporal framework module (Steenbeek et al., 2013), bridged the gap between envelope environmental models and classic food web models (Christensen et al., 2014b, Christensen et al., 2015).
In this study we applied the new habitat foraging capacity model (Christensen et al., 2014b) to study the distribution of three commercially important fish species (European hake Merluccius merluccius, sardine Sardina pilchardus and anchovy Engraulis encrasicolus) of the Southern Catalan Sea (NW Mediterranean Sea, Fig. 1) and evaluate the combined effects of environmental drivers (primary production, salinity, temperature, depth and substrate), in addition to fishing, and food web structure in their dynamics.
The NW Mediterranean Sea is one of the most important fishing grounds of the Mediterranean Sea, particularly the region surrounding the Ebro Delta area (Coll et al., 2012). Important environmental parameters drive the dynamics of commercial species in the area, such as small pelagic fish and important demersal species (Lloret et al., 2004, Palomera et al., 2005, Palomera et al., 2007, Martín et al., 2008, Ospina-Alvarez et al., 2015). Previous studies looking at the temporal dynamics of marine resources identified that both environmental factors, human activities and the structure of the food web were key elements to predict ecosystem dynamics (Coll et al., 2006, Coll et al., 2008, Mackinson et al., 2009).
Section snippets
Study area
Our study area was located on the Catalan Sea (within the Balearic Sea, NW Mediterranean Sea, Fig. 1). The NW Mediterranean Sea is an area of relatively high productivity due to a combined effect of the Northern current and the runoff of the Ebro and Rhone Rivers (Estrada, 1996, Bosc et al., 2004). In the northern area, the continental shelf is narrower, with the Northern current flowing south-westwards along the continental slope, towards the wider continental shelf surrounding the Ebro Delta
Comparison of Ecospace original with the capacity model
As a general trend, the overall biomasses and catches results from the implementation of the spatial dimension showed relevant decreases compared to the baseline Ecosim. Moreover the biomass and catch results for the Ecospace original model (Spatial_EO) were more optimistic than the ones resulting from the new habitat capacity model (Spatial_CM). Under Spatial_EO, the biomass of adult hake, anchovy and sardine showed a decrease of 77%, 38% and 33%, respectively, from 1978 to 2010 (Fig. 3a and
Comparison of Ecospace original with the capacity model
This study is one of the first applications of the new habitat capacity model in Ecospace (Christensen et al., 2014b) and illustrates its capabilities. It shows that the spatial–temporal modelling at the local level is possible if environmental, ecological and human activities are included with enough spatial detail, a challenge that it is present in most of the modelling initiatives to date (Barange et al., 2010).
Species distributions and abundances resulting from the new habitat capacity
Acknowledgements
MC was partially funded by the European Commission through the Marie Curie Career Integration Grant Fellowships – PCIG10-GA-2011-303534 – to the BIOWEB project and by the Spanish National Program Ramon y Cajal. This study is a contribution to the project ECOTRANS (CTM2011-26333, Ministerio de Economía y Competitividad, Spain).
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