One of the old biogeographical rules (the Island-rule) predicts that small creatures becomes giants, while huge beasts dwarfs after colonising small islands. One possible reason suspected behind is the overly simplified biological environment of small islands resulting in relaxed pressure from predators and interspecific competitors, letting animals to evolve towards an “optimal” (probably clade-specific) intermediate size. Such rules are formed originally in an interspecific, within-clade context, but their rationale can be used in fine-scale intraspecific studies as well.
Here, we tested if nine-spined sticklebacks (Pungitius pungitius) evolved larger in isolated small ponds (often representing the only fish species) when compared to the common lake or marine habitats. We measured fish body size in nine isolated ponds (surface area < 5 ha) populations, and compared them to what we found in four marine and five lake populations. Our findings were in line with the expectations: pond fish were considerably larger than lake or marine fish, sometimes reaching giant sizes (we are holding the world record with 115 mm; note that the normal size is ca. 5 cm). Size-at-age analyses in a subset of populations indicated that the larger size in ponds are a common result of higher growth rates and extended longevity in the wild. A common garden study revealed that pond fish grow two to three times heavier bodies during 36 weeks (while reaching adult size) than marine fish, suggesting genetic basis for the pattern found in the wild. Interestingly, in two geographically independent cases (distance > 500 km) when a common stickleback predator (brown trout, Salmo trutta) was introduced to the ponds recently, sticklebacks became smaller than in nearby ponds lacking predators. Further, nine-spined sticklebacks found in sympatry with three-spined sticklebacks (Gasterosteus aculeatus; three-spined sticklebacks were larger-bodied and found in larger numbers than nine-spined sticklebacks) were much smaller than in ponds where it was the only fish species.
We suggest that predation and interspecific competition are important forces shaping insular body size evolution, and that the relaxation of these forces acting against large body size can result in gigantism in small species. Further, even the smallest island can become “mainland-like” when predatory or superior competitor species are present. Our previous studies suggest that besides interspecific interactions, the elevated intraspecific competition in ponds (Herczeg et al. 2009a) that is strong enough to present a serious energetic constraint (for pond populations only!) in high densities (Gonda et al. 2009; Herczeg et al. 2009b) might also select for larger body size.
Herczeg G, Gonda A, Merilä J (2009) Evolution of gigantism in nine-spined sticklebacks. Evolution, in press.
Herczeg G, Gonda A, Merilä J 2009a: Predation mediated population divergence in complex behaviour of nine-spined stickleback (Pungitius pungitius). J. Evol. Biol. 22:544-552.
Herczeg G, Gonda A, Merilä J 2009b: The social cost of shoaling covaries with predation risk in nine-spined stickleback (Pungitius pungitius) populations. Anim. Behav. 77:575-580.
Gonda A, Herczeg G, Merilä J 2009: Habitat-dependent and -independent plastic responses to social environment in the nine-spined stickleback (Pungitius pungitius) brain. Proc. Roy. Soc. B. 276:2085-2092.