The METAL theory is an unifying theory that aims to explain the arrangement of life in the oceans and seas and to understand and anticipate how marine biodiversity may be altered by natural climatic/environmental variabilities and anthropogenic climate change. The theory posits that both biological and ecological systems tend to be relatively quickly in equilibrium with their climatic and environmental regimes of variability; the rapidity depending on the taxonomic groups under study. The theory also enables us to understand the relative success of ecological niche models or species distribution models to estimate the past, present and future species spatial distribution (Araujo and Guisan 2006) and suggests that these works can be extended from the species to the community and ecosystem levels in space and time (Beaugrand 2014; Beaugrand et al. 2014a).
In the past, useful empirical patterns on how species or communities can be affected by environmental changes have been detected but these patterns were not tested against a solid theoretical basis (Beaugrand 2015). The METAL theory allows the establishment of predictions that can then be tested using observed data. By this way, the theory could radically change the way we investigate the effects of both climate and environmental changes on marine biodiversity. The theory may also unify the whole fields of ecology devoted to the understanding of life distribution in both the terrestrial marine ecospheres.
The METAL theory is primarily deterministic, but a stochastic component could be implemented in the different models used so far. Using time series investigated to characterise the 1976/77 Pacific regime shift, Hsieh and colleagues (Hsieh et al. 2005) showed whereas time series of physical variables (e.g. Pacific Decadal Oscillation index, Southern Oscillation Index) were linear stochastic, biological time series (e.g. CALCOFI copepods, sockeye salmon, chum salmon) exhibited a nonlinear signature. These results supported the idea of a nonlinear amplification of stochastic hydro-climatic forcing.
Even though stochastic effects due to complex abiotic and biotic interactions throughout a species’ life cycle, and from demographic effects that control vital processes (e.g. fecundity, survival), make it difficult to forecast the response of species and ecosystems to climate change (Boyce et al. 2006; Keith et al. 2008), results from the METAL theory suggest that a significant proportion of biogeographical, phenological and long-term community shifts are deterministic and predictable at some observed emergent spatio-temporal scales. A fixed (non-linear) ecological niche offers a way to understand how communities and their species may respond to environmental variability and global climate change. Although the ecological niche is already applied to anticipate the response of a species distributional range to environmental changes by means of Ecological Niche Models (Araujo and Guisan 2006), the concept of the ecological niche has never been used to link phenological, biogeographical and community shifts, which are the three main documented responses to climate change so far (Beaugrand et al. 2008; Beaugrand et al. 2002; Edwards and Richardson 2004; Parmesan and Yohe 2003). The METAL theory provides an explanation for why climate-induced long-term environmental changes -especially temperature changes- and changes in species and communities, are so often tightly correlated (Kirby and Beaugrand 2009). We saw in the first seven chapters of the book the often strong relationships between marine biodiversity and both the climate and the environment. Both the arrangement of biodiversity and its spatial and temporal changes are in part the product of the interaction between the species ecological niche and both climatic and environmental changes in time and space. At the community scale, a large part of climate-caused long-term community shifts is the result of climate-modulated environmental changes on individual species niche, which explains why many species remain stable during an abrupt ecosystem shift, and why some may react earlier than others (Beaugrand 2004).
2. Current limitations and assumptions
A complex ecological space
Ecosystems and their biodiversity are influenced by many environmental parameters; we could even say an infinite number of dimensions. It is impossible to use all niche dimensions and so it is important to select a few that can control a large part of the species spatial distribution. The METAL theory outlined here used a reduced number of ecological dimensions. Simplification is needed but on the other hand oversimplification could become hazardous. Climate is among the most important parameters controlling species spatial distribution (Lomolino et al. 2006). Pearson and Dawson wrote ‘It is a central premise of biogeography that climate exerts a dominant control over the natural distribution of species’ (Pearson and Dawson 2003). The climate variability hypothesis states that the latitudinal range of species is primarily determined by their thermal tolerance (Stevens 1989). Temperature is indeed a key variable in the marine realm because it is the result of many hydro-climatic processes (Beaugrand et al. 2008) and because the factor exerts an effect on many fundamental biological and ecological processes (Beaugrand 2015; Sunday et al. 2012). This assumption especially holds true for thermal-range conformers such as marine species (including invertebrates and fish) whose physiological thermal amplitude closely determines their latitudinal range at large spatial and temporal scales (Sunday et al. 2012). Phenological, biogeographical and long-term community shifts have often been correlated to temperature (Beaugrand et al. 2002; Edwards and Richardson 2004; Luczak et al. 2011; Mackas et al. 2012). Other environmental parameters (photosynthetically active radiation, bathymetry, chlorophyll-a) have also been considered in models testing the METAL theory. These environmental parameters were particularly important in reconstructing seasonal patterns of abundance. When the spatial and temporal scales diminish, the number of ecological dimensions should increase and therefore an application of the METAL theory at smaller spatial scales may necessitate the consideration of more ecological dimensions.
The METAL theory does not consider species interaction yet. In our simplest models, we assume that the spatial and temporal abundance of a species is mainly determined by its fundamental niche i.e. the niche without the effect of dispersal or species interactions such as competition, predation and facilitation (Hutchinson 1978). Species interactions are however often involved in the positive feedbacks that shift a system from one state to another (Scheffer 2009). Because of these species interactions, climate-induced species shifts may propagate through the food web (Kirby and Beaugrand 2009; Luczak et al. 2012). Bottom-up and top-down controls, trophic cascades and amplifications have been observed (Beaugrand 2015). Trophic interactions can be implemented in the METAL theory and will probably lead to an increased sensitivity of the system to the environment and climate.