Alternative attractors in marine ecosystems: A comparative analysis of fishing effects
Introduction
The idea of alternative attractors existing in ecological systems was first proposed in the 1960s (Lewontin, 1969) with a simple question: “Can there be more than one stable community in a given habitat?” Simple abstract theoretical models (Holling, 1973, Noy-Meir, 1975; Glipin and Case, 1976; May, 1977; Case and Casten, 1979; Law and Morton, 1993) have shown that alternative attractors are plausible and indeed might be the common features in ecosystems (Knowlton, 2004), but they are rarely existed in more complex ecological models (Janse, 1997, van Nes et al., 2002, van Nes et al., 2003). van Nes and Scheffer (2004) showed that the alternative attractors might appear robust from interactions between large number of species in response to gradual environmental change or evolution. The existence of alternative attractors in ecosystems has some important influence on restoration ecology (Scheffer et al., 2001, Suding et al., 2004) and conservation of marine ecosystems (Knowlton, 2004). The ecosystems with alternative attractors often show catastrophic shifts and may cause large losses of ecological and economic resources (Scheffer et al., 2001). Furthermore, it requires drastic and expensive intervention to restore the system to a desired state (Maler, 2000).
In oceanography (Steel, 1998, Steel, 2004, Rothschild and Shannon, 2004), ‘regime shift’ is often used in describing abrupt shifts in marine ecosystems. Generally speaking, there are three different types of regime shifts (Collie et al., 2004). The first is a smooth regime shift, which is characterized by a quasi-linear function between the forcing and responsible variables (Fig. 1a). The second is an abrupt shift, which is presented by a nonlinear relationship between the forcing and the response variables (Fig. 1b). The last is a discontinuous regime shift, which involves an abrupt response between alternative attractors (Fig. 1c). In a nutshell, alternative attractor is a discontinuous regime shift which means that ecosystem has different basins of attraction under the same external environmental conditions (Schroder et al., 2005). When the external force perturbs the ecosystem, the system state will transit to the other different stable state.
Although the concept of the alternative attractors is straightforward, understanding and identifying it has proven to be difficult (deYoung et al., 2004). Scheffer and Carpenter (2003) presented that alternative attractors were not easy to be found empirically. However, the recent field studies have provided strong proofs of the existence of alternative attractors in ecosystem. Examples include shallow lakes (Scheffer et al., 1993, Morris et al., 2003), benthic pond food webs (Chase, 2003) and marine ecosystems. Marine ecosystems were among the first to provide potential examples of alternative attractors (Knowlton, 2004). For example, coral reef communities exhibit two alternative states. One is dominated by corals and the other is dominated by seaweeds (Done, 1991, Knowlton, 1992, Hughes, 1994); rocky initertidal and subtidal habitats have also been described as examples of alternative attractors (Johnson and Mann, 1988, Dudgeon and Petraitis, 2001). Soft sediment communities may probably cause alternative attractors, in which one state is dominated by ghost shrimp and the other by bivalves or phoroids (Petersen, 1984).
Until now, most of the oceanographers have taken a mainly empirical approach to identify the alternative attractors by testing the time series of biotic and abiotic variables (Collie et al., 2004). Scheffer and Carpenter (2003) presented a set of criteria for detecting alternative attractors, which can be expressed as three questions (Collie et al., 2004). Here we use one of the three criteria for diagnosing the existence of alternative attractors. The question is listed as follows:
Does the system switch to an alternative state when perturbed? A positive answer to this question indicates the existence of alternative attractors. With the help of the Ecopath with Ecosim (EwE) model, we can use this criterion to find whether there are different stable states in marine ecosystems or not.
It is clearer that the interactions (e.g. predation and competition) in marine ecosystem have much more important influence on driving oceanic community dynamics than what has been previously thought (Verity and Smetacek, 1996). Predation and competition between interspecific and intraspecific play an important role in keeping the structure and function of the ecosystem. Furthermore, these interactions will significantly affect the occurrence of the alternative attractors (van Nes and Scheffer, 2004). Cury and Shannon (2004) showed that different control mechanisms could trigger the regime shifts in marine ecosystems. Gragnani et al. (1999) presented that changing the fishing predation on the phytoplankton would cause the ecosystem to different stable state. One key feature of Ecosim is its ability to allow exploring the implications on system dynamics with different views on how the biomass of different groups in ecosystem is controlled. Therefore, we will test the effect of the control mechanism on the existence of alternative attractors in ecosystem with the EwE model from a different view.
Fisheries can affect the entire food chain, causing profound variation in species abundance on various tropic levels (Cury, 2000, Reid et al., 2000). Furthermore, fishing was used as an indication of perturbation to investigate the alternative attractors in Georges Bank haddock, Melanogrammus aeglefinus (Collie et al., 2004). Fisheries, exploiting the wasp-waist species and occurring at intermediate trophic levels, have a potential disrupting effect on the stability of marine ecosystems (Vasconcellos et al., 1997). Therefore, we change the fishery species from wasp-waist to top predators with fishing mortality as a perturbation to marine ecosystems to test whether there are alternative attractors or not with the diagnosing criterion.
So, the first purpose of this paper is to test the existence of alternative attractors in marine ecosystems, and then we will survey the effect of different flow in control mechanism and the trophic level on the existence of alternative attractors with the aid of the EwE mass-balanced models.
Section snippets
The Ecopath with Ecosim approach
Ecopath with Ecosim is a dynamic simulation tool developed for straightforward construction, parameterization and for the analysis of mass-balance trophic models of aquatic and terrestrial ecosystems (Christensen and Walters, 2004). EwE is a free ecological/ecosystem modeling software suite. It can be used to address ecological questions and to evaluate ecosystem effect of fishing and model effect of environment changes. Many application examples of EwE can be found at http://www.ecopath.org.
Results
When the control mechanism is bottom-up, there are no alternative attractors in the studied ecosystems no matter which group (top trophic or wasp-waist tropic level) is fished. That is to say that the quantity of all the species in the studied models will return to the same stable state after the perturbation (Fig. 3a and b). There is only one same point attractor (1P) in the studied mass-balanced model (Table 2).
For the condition that the control mechanism is mixed, there are alternative
Existence of alternative attractors in marine ecosystems
The presence of alternative attractors has profound implications for ecosystem management as it may imply catastrophic collapse of the system and large restoration efforts (Scheffer et al., 2001). However, manipulating marine ecosystems to demonstrate the existence of alternative attractors is very difficult (Collie et al., 2004). This paper presents a new approach to detect the different stable states in system with the software EwE. The results (see Fig. 3 and Table 2) show that there are
Acknowledgements
This research was supported with funds provided by the National Natural Science Foundation of China (No. 10472077). We thank two anonymous reviews for constructive criticism of the manuscript.
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