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Body size is a defining phenotypic trait of organisms, and evolutionary patterns within and between taxa provide some of the best-known examples of ‘rules’ in evolution: Bergmann's rule (Bergmann 1847), island rule (Foster 1964; Lomolino 1985) and Cope's rule (Stanley 1973). Cope's rule suggests that body size tends to increase over evolutionary time (Stanley 1973), but in the past decade, there has been a resurgence of interest in the evolution of dwarfism (nanism), with studies of a variety of vertebrate taxa (Brown et al. 2004; Kottelat et al. 2006; Kraus 2011; Glaw et al. 2012; Rittmeyer et al. 2012). Studies of fish are conspicuous by their paucity (although see Landry & Bernatchez 2010; Moles et al. 2010; Macqueen et al. 2011), despite the fact that the world's smallest vertebrate is a fish (Kottelat et al. 2006). In addition to changes in body size through time, it is well documented that body size shows particular evolutionary lability on islands (Lomolino 1985; Lomolino et al. 2012; although see Meiri, Raia & Phillimore 2011). A common pattern among vertebrate taxa is that small species get bigger, while big species get smaller (the island rule, Lomolino 1985). This pattern is not seen in all terrestrial vertebrates and has been hardly examined in fish (although see Herczeg, Gonda & Merila 2009). Despite the interest in documenting patterns of body size variation, we still have a poor understanding of what causes dwarfism, beyond the fact that it often occurs on islands (Lister 1989; Brown et al. 2004; Glaw et al. 2012; although see Lomolino et al. 2012).
Previous work suggests that patterns of body size evolution, especially on islands, are driven by the interplay between resources, competitors and predators (Case 1978; Raia, Barbera & Conte 2003; Raia & Meiri 2006; McNab 2010; Lomolino et al. 2012), but the detail remains generally unresolved. Body size therefore provides a good example of how piecemeal is our understanding of the ecological and environmental conditions that drive evolution (MacColl 2011). This is surprising, given that it is a question that vexed Darwin (1859). Darwin clearly thought that the struggle for existence, and hence evolution, was more likely to be driven by biotic factors like predation and competition than by abiotic factors like climate or other aspects of the physical environment (Chapter 3, Darwin 1859). However, in his entangled bank metaphor and elsewhere, Darwin also recognised the ‘infinitely complex relations to other organic beings and to external nature’ that drive the struggle for existence.
A life-history perspective provides a useful theoretical framework for thinking about how different factors may drive evolution (Palkovacs 2003). In general, reductions in predation (which are thought to be usual on islands) increase survival rates, which is expected to lead to longer life and larger body size. At the same time, decreased competition on islands can increase resource availability, which is expected to lead to increased growth rates and larger body size (Stearns & Koella 1986; Palkovacs 2003). Despite Darwin's observations, what has seldom been taken into account in empirical studies of body size evolution is that different environmental factors may interact, both in their effects on each other, and on phenotype (Stearns & Koella 1986; Grether et al. 2001). For example, resource availability may be greater where predators are more common, either because predators reduce competitor density or because productive environments support more predators (Holt 1977). Such relationships between environmental variables, across populations, can be described by a (co)variance matrix that MacColl (2011) has called the ‘O’ matrix. Relationships between environmental variables may conceal or exacerbate the apparent role of different factors in driving evolution and make it very difficult to interpret the real significance of the many correlative studies that have examined only single putatively causative factors. Because of this, definitive tests of the factors that drive evolution should ultimately rely on experimental manipulation of environmental conditions (putative selective agents) and quantification of subsequent changes in traits. However, the factorial experiments that are necessary should ideally be multi-generational and are therefore difficult and time-consuming to carry out for anything other than the shortest-lived species. Observational evidence obtained from thorough ecological studies can provide insights into the relationships between variation in body size across taxonomic units and systematically quantified environmental variation (Michaux et al. 2002; White & Searle 2007; Li et al. 2011) and should help the design of future experiments. Observational studies should measure all of the different factors that are hypothesised to affect body size. They require large sample sizes of independent populations so that they can control for the collinearity of environmental factors across populations (Graham 2003) and their potentially confounded effects on evolution (MacColl 2011).
Standard univariate statistical models are not especially good at disentangling multiple interacting processes (Grace 2006), especially when putative mechanisms operate at different levels, for example when underlying environmental variation determines the occurrence of predators and competitors, which in turn determine phenotypic evolution. As hypotheses about the causes of (body size) evolution are inherently complex and multivariate, here we use structural equation modelling to understand how different environmental factors interact (Grace 2006). Specifically, we use a simple structural equation model (SEM) in which abiotic, environmental variables can contribute to variation in body size either directly or indirectly through their influence on biotic variables (competition and predation), which in turn have a direct effect on body size evolution (Fig. 1, see Methods).
Figure 1. The a priori structural equation model of the direct (solid arrows) and indirect (dashed arrows) relationships between abiotic and biotic variables and stickleback body size. Terms in boxes represent variables, and single-headed arrows imply causal links between them. Circles represent error terms for the endogenous variables. The double-headed arrow indicates correlation between the exogenous variables ‘mean pH’ and ‘alkaline metals’, which are assumed to be measured without error.
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Here, we examine the ecological causes of variation in body size in three-spined sticklebacks Gasterosteus aculeatus by quantifying variation in body size, competitors, predators and the abiotic resource environment within an adaptive radiation in an archipelago of lochs (lakes) on the Scottish island of North Uist. Among these lochs, there is a strong axis of variation in pH, associated with variation in the concentration of alkaline metals: sodium, potassium, magnesium and especially calcium (see Methods, Waterston et al. 1979). This, in turn, is associated with variation in productivity (Waterston et al. 1979) and has previously been linked to the evolution of sticklebacks in these lochs (Giles 1983). Because of this previous work, we concentrate on variation in pH and alkaline metals as key aspects of the abiotic resource environment. Alkaline metals are important in major physiological processes, including osmoregulation and the formation of bone, while pH is widely acknowledged as a defining property of freshwaters.
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Our results provide support for Darwin's hypothesis (1859) that biotic factors, associated with variation in the abiotic environment, are more important in explaining evolution than is abiotic variation per se. This is consistent with other recent studies that have investigated multiple causes of body size evolution (Meiri, Cooper & Purvis 2008; Raia & Meiri 2011). Specifically, dwarfism in three-spined sticklebacks is associated with reduced abundance of a smaller competitor species, the nine-spined stickleback P. pungitius, and with low pH indicative of poor resource conditions. Dwarfism also tends to occur where an important predator, the brown trout Salmo trutta, is also small and may act more as a large competitor. The abundance of P. pungitius and the size of S. trutta are themselves related to underlying abiotic environmental variation.
Sticklebacks in freshwater ‘island’ populations almost always evolve to smaller body size than their ‘continental’ marine relatives (this study, Baker 1994; although see Moodie 1972). This effect is especially pronounced in some of the archipelago of freshwater lochs on North Uist, where sticklebacks mature at smaller size than has been recorded anywhere else in their large range (Baker 1994; Baker et al. 2008). Females in the smallest (freshwater) population matured at only 7% of the mass of females in the largest anadromous population. In the size range of the species, the smallest freshwater populations can therefore legitimately be described as dwarfed. However, among the lochs on North Uist, there was no correlation between loch (‘island’) size and the body size of sticklebacks, despite the fact that variation in loch size spanned more than two orders of magnitude. Obviously, a large part of the reduction in size between ‘continental’ anadromous sticklebacks and freshwater populations is associated with change in salinity. However, it is likely that most of this difference is attributable to some other aspect of the change in environments than the change in salinity per se. ‘Resident’ sticklebacks, which are phenotypically difficult to separate from freshwater sticklebacks but live year-round in coastal saltwater lagoons, are substantially smaller than anadromous ones but are nonetheless generally larger than freshwater sticklebacks.
Among freshwater three-spined sticklebacks dwarf sizes are especially likely to evolve where nine-spined stickleback P. pungitius are uncommon or absent, and where pH and the availability of alkaline metals is low. Small size of brown trout may also contribute to the occurrence of dwarfism. This strongly suggests that the evolution of body size among freshwater populations of three-spined sticklebacks is determined mainly by the resource environment: fish are bigger where productivity (as controlled by water chemistry) is higher and smaller where it is lower, but they only evolve to very small sizes in the absence of a smaller competitor species. The relative abundance of competitors (and the size of predators) is itself determined by underlying abiotic variation. This is consistent with emerging ideas that the evolution of body size on islands is more to do with the exact resource and biotic conditions that prevail (especially the presence of competitors and predators) than with being on an island per se (Case 1978; Raia, Barbera & Conte 2003; Raia & Meiri 2006; McNab 2010). It is also consistent with the notion that the evolution of body size in fish may be especially responsive to competitor and predator regimes (Robinson & Wilson 1994; Herczeg, Gonda & Merila 2009).
The differences in body size we recorded are probably at least partly genetic, given differences in size between populations reared in common garden conditions in the laboratory, but differences in growth rates and longevity in the wild also contributed to overall differences between populations. The great majority of breeding fish from acid lochs are just a year old (0+ age class), with only occasional 1+ fish (ADCM unpublished data). In contrast, populations from alkaline lochs often showed evidence of a 1+ and sometimes a 2+ (3 year-old) age class. Most populations show inter-annual fluctuations in body size, which are presumably the result of variation in growth conditions (resources and temperature), but these are generally small in comparison with the differences between populations.
Our data are not obviously consistent with a life-history explanation of body size variation (Palkovacs 2003). On North Uist, adult mortality is high in the acid lochs (the fish are annual) where growth rate is low. The most appropriate theory (Stearns & Koella 1986; Stearns 1992) predicts that where adult mortality is inversely related to growth rate, we should expect organisms to mature later, at smaller size. Instead, on North Uist, they mature earlier at (much) smaller size. It is possible that the shape of the size–age reaction norm (L-shaped) that leads to the Stearns and Koella prediction is different in the lochs on North Uist. For example, if juvenile mortality was also inversely related to growth rate, the expected shape of the reaction norm would be ‘keel’ shaped (Stearns 1992) which would lead to different predictions. However, we consider it to be unlikely that juvenile mortality is higher in the acid lochs, because in these lochs, the sticklebacks generally lay fewer, larger eggs (ADCM, unpublished data), which suggests that juvenile mortality is likely to be lower. Alternatively, size and age at maturity in endothermic organisms can be the result of differences in temperature (Zuo et al. 2012). This seems an unlikely explanation for sticklebacks on North Uist where size variation does not correlate with spot readings of temperature in loch shallows during the breeding season or with loch depth which, in well-mixed lakes like those on North Uist, should be a proxy for annual average water temperature (ADCM, unpublished data).
Our data are also not consistent with a simple interpretation of the effect of competition or predation. Release from competition is normally expected to result in body size increasing, to ‘take advantage’ of an expanded niche. In addition, theory suggests that small predators should favour the evolution of rapid growth and large body size, to allow evolutionary ‘escape’ from predation, whereas large predators should favour rapid maturity at small size (Reznick & Endler 1982). In contrast, on North Uist, release from competition by Pungitius facilitates the evolution of smaller body size. This may be explained if trout are more important as competitors than as predators. Larger trout in the (alkaline) lochs on North Uist certainly eat sticklebacks, but the fact that adult survival of sticklebacks is higher in these lochs than in the acid ones suggests that the effect of trout predation on adult survival is not a primary factor driving body size evolution and that its effect, if anything, may select for larger size (Moodie 1972). We do not yet know whether the small trout in the acid lochs eat sticklebacks, but in other oligotrophic northern temperate and boreal lochs piscivory among (small) trout is rare: they are more likely to be planktivorous or insectivorous (Kahilainen & Lehtonen 2002; Museth et al. 2003). This suggests that small trout also compete with sticklebacks and that this competition may limit the upper size of sticklebacks in resource-poor, acid lochs where trout are small. The absence from acid lochs of Pungitius, which apparently cannot tolerate the environmental or resource conditions, frees up the small body size niche, which is then occupied by three-spined sticklebacks.
Dwarfism may be common among freshwater fish (Riget et al. 2000; Landry & Bernatchez 2010; Moles et al. 2010; Macqueen et al. 2011), at least in North temperate lakes, but there has been little previous study of its causes (although see Riget et al. 2000; Landry & Bernatchez 2010). One previous study of the evolution of gigantism in (nine-spined) sticklebacks (Herczeg, Gonda & Merila 2009) is consistent with ours in that body size variation was principally associated with biotic variation between populations. However, they found that reduced competition and predation led to increased body size. Although this appears to contradict the patterns we observed, in fact it is very consistent given that Herczeg, Gonda & Merila (2009) studied body size evolution in the smaller P. pungitius. Thus, it appears that three-spined and nine-spined sticklebacks have reciprocal effects on each other: absence of the larger three-spined allows nine-spined sticklebacks to become ‘giants’, while absence of the smaller nine-spined allows three-spined to become ‘dwarfs’. This is consistent with the idea that competition is an important agent of character displacement in freshwater fishes (Robinson & Wilson 1994).
The relationships between different abiotic and biotic factors in our study highlight the shortcomings of trying to understand the causes of evolution by examination of single factors and emphasise the importance of quantifying the O matrix of relationships between environmental variables as completely as possible (Graham 2003; MacColl 2011). In our study, decreased competition from one species may go hand in hand with increased competition from another species and reduced resource availability, while improved environmental circumstances are associated with larger predators. Previous studies that have linked predation with body size evolution have usually only characterised the presence or absence of predators or the number of predator species (Michaux et al. 2002; Herczeg, Gonda & Merila 2009; Li et al. 2011). In our study, the large variation in trout size between populations shows that this may be inadequate to fully understand the contributions of variation in predation to evolution of ‘prey’. Indeed, it is quite likely that any indirect measure of predation will be confounded with other environmental factors. Even measures of predator size are unlikely to be a reliable index of the extent of predation: predator diets can vary substantially between populations, depending on what prey is locally available (Kahilainen & Lehtonen 2002; Museth et al. 2003). In future work, we intend to investigate variation in piscivory among North Uist brown trout populations using stomach sampling.
Overall in our study, body size variation within freshwater three-spined sticklebacks was more strongly affected by biotic (competition) than abiotic (pH) variation, but both clearly play a role. Darwin's hypothesis (1859) that biotic interactions have a more important influence on evolution than do abiotic ones may therefore be simplistic. It seems more likely that an ‘entangled bank’ model, in which evolution is driven by complex interactions between ecology and the physical environment, is better able to explain variation in body size in three-spined sticklebacks. This study demonstrates the importance of considering the whole O matrix before drawing conclusions about the causes of body size evolution on islands.