Villy Christensen

ECOPATH II — a software for balancing steady-state ecosystem models and calculating network characteristics☆

Abstract The ECOPATH II microcomputer software is presented as an approach for balancing ecosystem models. It includes (i) routines for balancing the flow in a steady-state ecosystem from estimation of a missing parameter for all groups in the system, (ii) routines for estimating network flow indices, and (iii) miscellaneous routines for deriving additional indices such as food selection indices and omnivory indices. The use of ECOPATH II is exemplified through presentation of a model of the Schlei Fjord ecosystem (Western Baltic).

Structuring dynamic models of exploited ecosystems from trophic mass-balance assessments

The linear equations that describe trophic fluxes in mass-balance, equilibrium assessments of ecosystems (such as in the ECOPATH approach) can be re-expressed as differential equations defining trophic interactions as dynamic relationships varying with biomasses and harvest regimes. Time patterns of biomass predicted by these differential equations, and equilibrium system responses under different exploitation regimes, are found by setting the differential equations equal to zero and solving for biomasses at different levels of fishing mortality. Incorporation of our approach as the ECOSIM routine into the well-documented ECOPATH software will enable a wide range of potential users to conduct fisheries policy analyses that explicitly account for ecosystem trophic interactions, without requiring the users to engage in complex modelling or information gathering much beyond that required for ECOPATH. While the ECOSIM predictions can be expected to fail under fishing regimes very different from those leading to the ECOPATH input data, ECOSIM will at least indicate likely directions of biomass change in various trophic groups under incremental experimental policies aimed at improving overall ecosystem management. That is, ECOSIM can be a valuable tool for design of ecosystem-scale adaptive management experiments

A mid-term analysis of progress toward international biodiversity targets

In 2010, the international community, under the auspices of the Convention on Biological Diversity, agreed on 20 biodiversity-related “Aichi Targets” to be achieved within a decade. We provide a comprehensive mid-term assessment of progress toward these global targets using 55 indicator data sets. We projected indicator trends to 2020 using an adaptive statistical framework that incorporated the specific properties of individual time series. On current trajectories, results suggest that despite accelerating policy and management responses to the biodiversity crisis, the impacts of these efforts are unlikely to be reflected in improved trends in the state of biodiversity by 2020. We highlight areas of societal endeavor requiring additional efforts to achieve the Aichi Targets, and provide a baseline against which to assess future progress. Although conservation efforts are accelerating, their impact is unlikely to improve the global state of biodiversity by 2020. Indicators of progress and decline The targets set by the Convention on Biological Diversity in 2010 focused international efforts to alleviate global biodiversity decline. However, many of the consequences of these efforts will not be evident by the 2020 deadline agreed to by governments of 150 countries. Tittensor et al. analyzed data on 55 different biodiversity indicators to predict progress toward the 2020 targets—indicators such as protected area coverage, land-use trends, and endangered species status. The analysis pinpoints the problems and areas that will need the most attention in the next few years. Science, this issue p. 241

Ecosystem maturity - towards quantification

Abstract An attempt is made to rank 41 steady-state models of aquatic ecosystems on the basis of their maturity. Maturity is quantified using several of Odums attributes of ecosystem maturity. It is shown that it is possible to make such a ranking, and that the ranking seems to confirm our intuitive perception of ecosystem maturity. The ranking is compared to rankings based on various ecosystem goal functions. The maturity ranking shows a strong negative correlation with relative ascendency, and thus a strong positive correlation with system overhead, a possible measure of ecosystem stability. The analyses suggest that another goal function, exergy, as calculated here is mainly a function of system biomass, and that it might be appropriate to reconsider the computational aspect of exergy estimation. Most importantly the analyses point to the feasibility of using comparisons of ecosystem models as a tool for enhancing our understanding of ecosystem characteristics, notably sustainability.

Ecospace: Prediction of Mesoscale Spatial Patterns in Trophic Relationships of Exploited Ecosystems, with Emphasis on the Impacts of Marine Protected Areas

ABSTRACT Growing disillusion with the predictive capability of single species fisheries assessment methods and the realization that the management approaches they imply will always fail to protect bycatch species has led to growing interest in the potential of marine protected areas (MPAs) as a tool for protecting such species and allowing for rebuilding populations of target species and damaged habitat. Ecospace is a spatially explicit model for policy evaluation that allows for considering the impact of MPAs in an ecosystem (that is, trophic) context, and that relies on the Ecopath mass-balance approach for most of its parameterization. Additional inputs are movement rates used to compute exchanges between grid cells, estimates of the importance of trophic interactions (top-down vs bottom up control), and habitat preferences for each of the functional groups included in the model. An application example, including the effect of an MPA, and validation against trawl survey data is presented in the form of a color map illustrating Ecospace predictions of biomass patterns on the shelf of Brunei Darussalam, Southeast Asia. A key general prediction of Ecospace is spatial “cascade” effects, wherein prey densities are low where predators are abundant, for example, in protected areas or areas where fishing costs are high. Ecospace also shows that the potential benefits of local protection can be easily negated by high movement rates, and especially by concentration of fishing effort at the edge of the MPAs, where cascade effects generate prey gradients that attract predators out of the protected areas. Despite various limitations (for example, no explicit consideration of seasonal changes or directed migration), the outward simplicity of Ecospace and the information-rich graphs it generates, coupled with the increasingly global availability of the required Ecopath files, will likely ensure a wide use for this approach, both for generating hypotheses about ecosystem function and evaluating policy choices.

Representing Density Dependent Consequences of Life History Strategies in Aquatic Ecosystems: EcoSim II

ABSTRACT EcoSim II uses results from the Ecopath procedure for trophic mass-balance analysis to define biomass dynamics models for predicting temporal change in exploited ecosystems. Key populations can be represented in further detail by using delay-difference models to account for both biomass and numbers dynamics. A major problem revealed by linking the population and biomass dynamics models is in representation of population responses to changes in food supply; simple proportional growth and reproductive responses lead to unrealistic predictions of changes in mean body size with changes in fishing mortality. EcoSim II allows users to specify life history mechanisms to avoid such unrealistic predictions: animals may translate changes in feeding rate into changes in reproductive rather than growth rates, or they may translate changes in food availability into changes in foraging time that in turn affects predation risk. These options, along with model relationships for limits on prey availability caused by predation avoidance tactics, tend to cause strong compensatory responses in modeled populations. It is likely that such compensatory responses are responsible for our inability to find obvious correlations between interacting trophic components in fisheries time-series data. But Ecosim II does not just predict strong compensatory responses: it also suggests that large piscivores may be vulnerable to delayed recruitment collapses caused by increases in prey species that are in turn competitors/predators of juvenile piscivores.

Contribution of fish to the marine inorganic carbon cycle.

Oceanic production of calcium carbonate is conventionally attributed to marine plankton (coccolithophores and foraminifera). Here we report that marine fish produce precipitated carbonates within their intestines and excrete these at high rates. When combined with estimates of global fish biomass, this suggests that marine fish contribute 3 to 15% of total oceanic carbonate production. Fish carbonates have a higher magnesium content and solubility than traditional sources, yielding faster dissolution with depth. This may explain up to a quarter of the increase in titratable alkalinity within 1000 meters of the ocean surface, a controversial phenomenon that has puzzled oceanographers for decades. We also predict that fish carbonate production may rise in response to future environmental changes in carbon dioxide, and thus become an increasingly important component of the inorganic carbon cycle.

CHANGES IN MODELS OF AQUATIC ECOSYSTEMS APPROACHING CARRYING CAPACITY

Using a top-down modeling approach, published mass-balance models of trophic interactions and state variables in the western central Pacific Ocean and the northern Gulf of Mexico shelf were used to explore how large increases in top predator biomasses can be accommodated with given primary productions. It appears that the biomasses of top predators in these models can be increased an order of magnitude, which leads to a six- to sevenfold increase in overall consumer biomasses. This results in changes in food web structures that are in agreement with major aspects of E. P. Odums theory of ecosystem development, particularly with regard to features associated with the retention and recycling of detritus. Based on the simulations and Odums theory, we propose a functional definition of carrying capacity: the upper limit of biomass that can be supported by a set primary production and within a variable food web structure is reached when total system respiration equals the sum of primary production and detrit...

Managing fisheries involving predator and prey species

Several management strategies for ecosystems with biological interaction are discussed, including predator removal, predator-prey coexistence, prey exploitation, overexploitation, and introduction of sanctuaries. Some case studies related to ecosystem management are briefly presented; these describe Lakes Victoria and Tanganyika, discarding from shrimp trawl fisheries and the development in the North Sea that led to introduction of multispecies analysis. The concept of ‘fishing down the food web’ is discussed and the average trophic levels at which the fisheries operate in different ecosystem types are estimated based on quantified trophic flow models. On a global level, while on average fisheries operate around two trophic levels above the primary producers, still one third of the catch of the 70 major fish species caught in the world is of piscivorous fish. Using exploitation-predation rate indices for different ecosystem types, the amount of finfish consumed globally by finfish is roughly estimated to be three times the catches of finfish. Finally some implications for the management of ecosystems are drawn up. It makes little difference if short-term prognoses are based on single-species or multispecies considerations. Multispecies models may, however, give the better long-term advice, and adaptive management may facilitate the move towards such long-term goals.

Vertical migrations of herring, Clupea harengus, larvae in relation to light and prey distribution

SynopsisThe influence of light and prey abundance on the vertical distribution of herring larvae was evaluated by three investigations made under calm weather conditions in the North Sea off the Scottish coast. The investigations took place at different time after hatching and the vertical distributions of three size groups of larvae (mean sizes 8,15 and 19 mm) were related to time of day and the vertical distribution of copepods. No migratory behaviour of copepods was observed but their vertical distribution differed between investigations. In the investigation on intermediate sized larvae, copepod density peaked at the pycnocline (40 m). Larvae concentrated at this depth at noon. At dawn and dusk larvae migrated towards the surface and the vertical distributions fluctuated semidielly. In the two other investigations, copepods were homogeneously distributed in the water column and after migration towards the surface at dawn larvae stayed in the upper water column during the day. The observations suggest that the daytime vertical distribution of larvae in calm weather is mainly determined by feeding conditions: the larvae move to depths were light is sufficient for feeding, and refinement within that zone is made according to a compromise between optimal light conditions for feeding and optimal prey densities.