Active opportunist species as potential diagnostic markers for comparative tracking of complex marine ecosystem responses to global trends

ICES Journal of Marine Science ICES Journal of Marine Science (2014), 71(8), 2281– 2292. doi:10.1093/icesjms/fst242 Contribution to the Special Issue: ‘Commemorating 100 years since Hjort’s 1914 treatise on fluctuations in the great fisheries of northern Europe’ Food for Thought Active opportunist species as potential diagnostic markers for comparative tracking of complex marine ecosystem responses to global trends Andrew Bakun* Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, USA *Corresponding author: tel: +1 305 967 4711; fax: +1 305 421 4600; e-mail: Bakun, A. Active opportunist species as potential diagnostic markers for comparative tracking of complex marine ecosystem responses to global trends. – ICES Journal of Marine Science, 71: 2281 – 2292. Received 9 December 2013; accepted 26 December 2013; advance access publication 7 February 2014 . As it becomes ever clearer that on longer time scales marine ecosystems function as non-linear “complex adaptive systems”, potential consequences of global change become obscured within a maze of multiple possibilities. This essay attempts to route one pathway to a potentially more robust conceptual synthesis, employing the globally important example of anchovies and sardines as a model. Expressly, the anchovy emerges as an efficient specialist of neritic origin. In contrast, the sardine’s oceanic-based adaptations equip it to deal with intermittent episodes of poorly productive conditions and to take advantage of associated reduction in predation pressure on early life stages of their offspring. Based on the overall synthesis, the nimble, wide-ranging, actively opportunistic sardine appears notably well equipped to deal with climate-related disruptions and dislocations and even to profit from their adverse effects on predators and competitors. Global-scale multispecies population synchronies in the 1970s to the mid-1980s suggest that a variety of different species types might be flagged for investigation as perhaps embodying similar “active opportunist” attributes. If so, events and anecdotes might, as global changes proceed, be viewed within a developing universal framework that could support increasingly effective transfers of experience and predictive foresight across different species groups and regional ecosystems. Keywords: climate change, earth systems models, global climate models, representative concentration pathways. Introduction In the century that has passed since Johan Hjort’s (1914) seminal publication, much has been learned about the intricate linkages of physical and biological mechanisms operating within the structures of marine ecosystems. Even so, there has been precious little success in applying that progress towards improving fisheries management and marine conservation efforts, i.e. towards the “ultimate”-level goals of long-term preservation of the traditional economic and esthetic benefits that have come to be expected to flow forth from the world’s marine ecosystems. Perplexingly, our current understanding of the dynamics of these systems remains rife with puzzles and paradoxes (e.g. Bakun, 2011, 2012), and actionably reliable prediction continues to evade us (Houde, 2008). This is much in contrast to many other fields of modern science in which the spread of successful applications has been explosive. Certainly, part of the problem must be that we humans are not marine organisms, but are terrestrial ones. Our experiences, as well as our intuitions, are overwhelmingly terrestrially based. Not surprisingly, our conventional conceptual frameworks for understanding how marine ecosystem processes may influence population dynamics are, both at Hjort’s time and at present, rather simple reflections of terrestrial analogies. But the “marine world” is something qualitatively different and in many ways “beyond our human experience” (Bakun, 1996). For example, in terms of such references as numbers of trophic levels or varieties of dynamical feedbacks involved, marine systems are in nearly all cases much the more complex. Moreover, as historical data records have lengthened, it has become ever more evident that on longer time scales, marine ecosystems seem to be functioning as classical complex adaptive systems (Levin, 1998, 1999), characterized by dynamical non-linearities # International Council for the Exploration of the Sea 2014. All rights reserved. For Permissions, please email: 2282 (Hsieh et al., 2005) and self-enhancing feedback loops (Bakun and Weeks, 2006). Dynamical systems of this type tend towards chaotic, non-predictable behaviour, calling into question the very efficacy of scientific management efforts. In particular, since non-linear mathematics do not necessarily yield unique solutions, potential consequences of various aspects of global change tend to become diffused in a nebulous array of multiple possibilities. Certainly, once it is realized that conventional assumptions of system stationarity are becoming increasingly untenable, the sorts of simple linear models (e.g. fitting of indices of reproductive success to one or several indices of correlated environmental or ecosystem mechanisms) that have become traditional in fisheries science, lose their credibility as reliable harbingers of the future. Fortunately, such a pessimistic perspective is contradicted by certain striking regularities and temporal-spatial correspondences that seem to evoke a more promising viewpoint. Among the most remarkable of these have involved interactions between the anchovy (genus Engraulis) and sardine (genera Sardinops and Sardina) species groups. Anchovies and sardines coexist at the crucial wasp-waist position (Rice, 1995; Bakun, 1996, 2006a; Cury et al., 2000) in the trophic structures of a surprising variety of important regional marine ecosystems. Their populations tend to be large, often comprising the greater part of the entire animal biomass of the local ecosystem in which they function. Therefore, they tend to support quite massive fisheries, such that variations in production from the anchovy –sardine pair have often been dominant factors in the overall variability in world fish production. In turn, the very efficient and powerful fisheries focused on these stocks exert important controls on their population dynamics. Notably, the two species groups have historically exhibited a tendency to alternate in ascendancy at the wasp waist of a given regional ecosystem on time scales of several years to several decades, generating differing ecosystem effects depending on which may be dominant at a particular time. For example, anchovies are largely zooplanktivores, whereas sardines are more omnivorous in that they have greater capability to effectively consume phytoplankton as well as zooplankton. Thus, anchovy dominance may promote higher ratios of phytoplankton to zooplankton (Cury et al., 2000), perhaps therefore favouring eutrophication and associated hypox (...truncated)