Andrea Belgrano

Trophic cascades promote threshold-like shifts in pelagic marine ecosystems

Fisheries can have a large impact on marine ecosystems, because the effects of removing large predatory fish may cascade down the food web. The implications of these cascading processes on system functioning and resilience remain a source of intense scientific debate. By using field data covering a 30-year period, we show for the Baltic Sea that the underlying mechanisms of trophic cascades produced a shift in ecosystem functioning after the collapse of the top predator cod. We identified an ecological threshold, corresponding to a planktivore abundance of ≈17 × 1010 individuals, that separates 2 ecosystem configurations in which zooplankton dynamics are driven by either hydroclimatic forces or predation pressure. Abundances of the planktivore sprat above the threshold decouple zooplankton dynamics from hydrological circumstances. The current strong regulation by sprat of the feeding resources for larval cod may hinder cod recovery and the return of the ecosystem to a prior state. This calls for the inclusion of a food web perspective in management decisions.

Body-size distributions and size-spectra: universal indicators of ecological status?

The sizes of individual organisms, rather than their taxonomy, are used to inform management and conservation in some aquatic ecosystems. The European Science Foundation Research Network, SIZEMIC, facilitates integration of such approaches with the more taxonomic approaches used in terrestrial ecology. During its 4-year tenure, the Network is bringing together researchers from disciplines including theorists, empiricists, government employees, and practitioners, via a series of meetings, working groups and research visits. The research conducted suggests that organismal size, with a generous helping of taxonomy, provides the most probable route to universal indicators of ecological status.

The Eco-field Hypothesis: Toward a Cognitive Landscape

Cognition is recognized as an essential component of the living strategies of organisms and the use of cognitive approaches based on an organismic-centered-view is discussed as a strategy to aid the advancement of landscape ecology to a more independent scientific discipline. The incorporation of the theory of information, the theory of meaning and the Umwelt, and the biosemiotic models into the landscape ecology framework is described as the necessary step to create a common paradigmatic background and operational tools to develop basis for a cognitive landscape ecology. Three cognitive landscapes (neutrality-based landscape, individual-based landscape and observer-based landscape) have been described as the result of distinctive mechanisms to extract information from a cognitive matrix based on a growing literature of (bio)semiotic exchange. The eco-field hypothesis is presented as a new possibility to describe landscape processes according to an organismic-centered-view. The eco-field is defined as a spatial configuration carrier of a specific meaning perceived when a specific living function is activated. A species-specific cognitive landscape is composed of all the spatial configurations involved for all the living functions for a particular organism. Eco-field hypothesis offers a detailed vision of (habitat) environmental requirements and creates a novel conceptual bridge between niche, habitat, Umwelt and the methodological approaches of spatial ecology. Finally the eco-field hypothesis promises a new testing ground for experimental investigations in landscape ecology and in related disciplines including environmental psychology, cognitive ethology, cultural ecology, landscape aesthetics, design and planning.

The eco-field: A new paradigm for landscape ecology

In the spirit of the theory of biocomplexity and of the non-linear emergent characters of ecological systems, the eco-field is a new paradigm that integrates the vision of the landscape as a neutral matrix (like a habitat) in which organisms are living, and contemporarily as a product of the human mind. Eco-field is defined a ‘cognitive field’ created by the interference between functional traits and the ‘real world’. Species-specific environmental suitability is the result of the quality of the different eco-fields and the landscape becomes a cognitive entity. The eco-field paradigm can be extended to the emergent properties of the systems. The eco-field of emergences is the geographic space in which the emergent properties appear. The eco-field of organisms and the eco-field of emergences, like results of aggregated entities, have in common the multidimensionality of landscapes, refusing the vision of landscape like a neutral geographic matrix for organisms and processes.

Climate change causing phase transitions of walleye pollock (Theragra chalcogramma) recruitment dynamics

In 1976 the North Pacific climate shifted, resulting in an average increase of the water temperature. In the Gulf of Alaska the climate shift was followed (i.e. early 1980s) by a gradual but dramatic increase in the abundance of groundfish species that typically prey on pre-recruitment stages of walleye pollock. In the present study we used a previously parameterized model to investigate the effect of these climate and biological changes on the recruitment dynamics of walleye pollock in the Gulf of Alaska. Simulations covered the 1970–2000 time frame and emphasized the medium-to-long temporal scale (i.e. about 5–10 years) of environmental variability. Results showed that during periods characterized by high sea surface temperature and high predation on juvenile pollock stages, recruitment variability and magnitude were below average, and recruitment control was delayed to stages older than the 0-group. Opposite dynamics (i.e. high abundance and variability, and early recruitment control) occurred during periods characterized by low temperature and predation. These results are in general agreement with empirical observations, and allowed us to formulate causal explanations for their occurrence. We interpreted the delay of recruitment control and the reduction of variability as an effect of increased constraint on the abundance of post age-0 stages, in turn imposed by high density dependence and predation mortality. On the other hand, low density-dependence and predation favoured post age-0 survival, and allowed for an unconstrained link between larval and recruitment abundance. Our findings demonstrate that the dominant mechanisms of pollock survival change over contrasting climate regimes. Such changes may in turn cause a phase transition of recruitment dynamics with profound implications for the management of the entire stock.

CHIHUAHUAN DESERT KANGAROO RATS: NONLINEAR EFFECTS OF POPULATION DYNAMICS, COMPETITION, AND RAINFALL

Using long-term data on two kangaroo rats in the Chihuahuan Desert of North America, we fitted logistic models including the exogenous effects of seasonal rainfall patterns. Our aim was to test the effects of intraspecific interactions and seasonal rainfall in explaining and predicting the numerical fluctuations of these two kangaroo rats. We found that logistic models fit both data sets quite well; Dipodomys merriami showed lower maximum per capita growth rates than Dipodomys ordii, and in both cases logistic models were nonlinear. Summer rainfall appears to be the most important exogenous effect for both rodent populations; models including this variable were able to predict independent data better than models including winter rainfall. D. merriami was also negatively affected by another kangaroo rat (Dipodomys spectabilis), consistent with previous experimental evidence. We hypothesized that summer rainfall influences the carrying capacity of the environment by affecting seed availability and the intensity of intraspecific competition.

Trophic indicators in fisheries: a call for re-evaluation.

Mean trophic level (MTL) of landings and primary production required (PPR) by fisheries are increasingly used in the assessment of sustainability in fisheries. However, in their present form, MTL and PPR are prone to misinterpretation. We show that it is important to account for actual catch data, define an appropriate historical and spatial domain, and carefully consider the effects of fisheries management, based on results from a case study of Swedish fisheries during the past century.

Ecosystem-based management for marine fisheries : an evolving perspective

Foreword Alec MacCall Introduction Andrea Belgrano and Charles W. Fowler Part I. Current Forms of Management: 1. Food-web and climate-related dynamics in the Baltic Sea: present and potential future applications in fish stock assessment and management Michele Casini, Christian Mollmann and Henrik Osterblom 2. Northwest Atlantic ecosystem based management of fisheries Jason S. Link, Alida Bundy, William J. Overholtz, Nancy Shackell, John Manderson, Daniel Duplisea, Jonathan Hare, Mariano Koen-Alonso and Kevin Friedland 3. Alaska marine fisheries management: advancements and linkages to ecosystem research Patricia A. Livingston, Kerim Aydin, Jennifer L. Boldt, Anne B. Hollowed and Jeffrey M. Napp 4. A pragmatic approach for ecosystem-based fisheries assessment and management: a Korean marine ranch ecosystem Chang Ik Zhang and Suam Kim Part II. Elements of Importance to Management: 5. Unintended consequences sneak in the back door: making wise use of regulations in fisheries management Anne Maria Eikeset, Andries Richter, Florian K. Dickert, Dorothy Dankel and Nils Chr. Stenseth 6. Population dynamic theory as an essential tool for models in fisheries Mauricio Lima 7. Recovery of former fish productivity: philopatric behaviors put depleted stocks in an unforeseen deadlock Henrik Svedang, Massimiliano Cardinale and Carl Andre 8. Boundary shifts: from management to engagement in complexities of ecosystems and social contexts Peter J. Taylor 9. Civil society and ecosystem-based fisheries management: traditional roles and future opportunities Tundi Agardy Part III. Using Patterns: 10. Science and management: matching the questions Charles W. Fowler and Larry Hobbs 11. Sustainability, ecosystems and fishery management Charles W. Fowler and Shannon McCluskey 12. On the path to holistic management: ecosystem-based management in marine systems Andrea Belgrano and Charles W. Fowler Afterword Keith Brander Index.

How Fisheries Affect Evolution

Commercial fishing alters the genetic traits of fish stocks. The extensive exploitation of marine resources by modern fisheries (see the figure) has wide-ranging effects on marine ecosystems. Across the worlds oceans, size-selective harvesting by commercial fisheries has been a key driving force behind changes in phenotypic traits such as body size and age at maturation (1–3). These changes have altered the trophic structure of the affected ecosystems, disturbed predatorprey relationships, and modified trophic cascade dynamics (3, 4). Phenotypic changes can involve both ecological and evolutionary reactions to the effect of fishing, and there has been much debate about the relative roles of these reactions. This is important because genetic changes could result in long-term reductions in catches. Recent work has provided evidence for fisheries-induced evolutionary changes, with important implications for the sustainability of fisheries.

Oceans under the macroscope

Phytoplankton are marine algae that support all ocean life, so it is important to understand the processes that control their distribution, abundance and diversity. Macroecology offers a way to do so.