Humans have introduced non-native animal and plant species into aquatic and terrestrial habitats around the world at an unprecedented rate. Fish have been accidentally or intentionally introduced into river systems for over 150 years. Introduced fish are known to modify average body sizes in habitats around the world. A new study in the journal Ecology Letters reports that non-native fish are, on average, 12 cm larger than native species. This change in body size can, in many situations, greatly modify aquatic ecosystems.
After cross-referencing freshwater fish data from 1,058 river basins worldwide, the authors found that introduced fish are, on average 12 cm larger than native species. Overall, this increases the average body size of fish in a given assemblage by 2 cm. Remember back in the story about shrinking songbirds when I defined Bergmann's Rule? Its the warmer climates have larger critters rule. The authors here are suggesting that the change in fish body size affects Bergmann's Rule. Additionally, the non-natives alter the functioning of the ecosystem, whether they be predators, herbivores, or detritovores. This alteration of the food chain affects other species within the change as well as the cycling of organic matter within the system. The changes in body size worldwide are likely part of this change.
As you can probably tell there is more to the story. You can find more details in the article itself here: http://www3.interscience.wiley.com/journal/123248911/abstract (DOI: 10.1111/j.1461-0248.2009.01432.x)
(image from animals.nationalgeographic.com)