Nirav Patel
3/15/11
Biogeography
1) Romanuk, Tamara N., Hayward, April., Hutchings, Jeffrey A. 2011. Trophic level scales positively with body size in fishes. Global Ecology and Biogeography (Global Ecol. Biogeogr.) (2011) 20, 231-240.
Tamara N. Romanuk is from the Department of Biology at Dalhousie University, 1355, Oxford Street, Halifax, Nova Scotia, Canada.
April Hayward is a Florida GATOR! Department of Biology, University of Florida, PO Box 118525, Gainesville, FL 32611, USA.
2) The notion of the body size hierarchy correlating with trophic levels within food-web models is often agreed to be a strong theory, but evidence has been ambiguous and the relationship is often considered as weak. Thus, given a set of empirical evidence regarding body sizes in fishes and trophic level scales, is there a positive correlation?
3) Body size often conditions the demand that organisms place on their environment in terms of the use of energy and materials. Interactions between those organisms and the environment are conditioned by body size, for example, body size often constraints the range of prey a predator can consume. Cohen et al (1993), cited the first page of the article, mentions the central assumption that when species are ranked by body size, food webs adhere more closely with a trophic hierarchy in comparison to if the ranks were random. Although seemingly sound, there are many idiosyncrasies in the empirical evidence.
This study prides itself on being unique in providing insight into this particular hypothesis, in that the relationship between trophic level and body size within the most species-rich group of vertebrates, are ideal subjects for broad-scale analysis. Over 28,000 species that occur globally, are distributed over a wide variety of habitats, have a diverse set of morphologies, behaviors, life-history strategies with highly variable diets. Also, adult fishes vary in size by over four orders of magnitude, and a single individual of a species can experience that difference in size over a lifetime. Thus the authors decided to use the existing data they have on fish to examine the soundness of this argument.
4) Utilizing 8361 species of fish and obtaining data on body size and trophic position from FishBase (http://www.fishbase.org). Maximum length is a parameter in fishes and it is highly correlated with metabolic processes and other life-history traits. The metric for body size was maximum total length, which was defined as the longest individual recorded for a given species.
Trophic position was calculated by adding 1 to the mean trophic position, which is weighted by relative abundance, calculated by all the food items consumed by a given species. Thus with a simple analysis, to examine the degree to which phylogeny could influence the relationships between trophic positions and body size, they assigned ranks to the orders of fish.
5) There was a positive correlation between the fish length and trophic position, the relationship being .065 as the slope. There were no significant negative correlations between the two variables were observed for any particular order. Fishes in orders that showed significantly positive trophic level-body size relationships were 86cm smaller than fishes in orders that showed no relation!
6) Given the analysis of a global dataset of 8361 species in 57 orders, there were an identified number of patterns in regards to relations between trophic positions and body size. Body size explained 20% of the variability within trophic position. The scatter was the result of the diverse and sheer number of phylogeny, the authors explain. Variance increased to 37% when considering evolutionary history. Smaller body sizes vary with trophic level in comparison to fishes with larger body sizes.
The analysis, the authors argue, cannot be used to explicitly test the community based hypothesis that other authors have put forth (Arim et al.) which argues that a hump-shaped pattern and trophic level increases with body size for smaller organisms and decreases for larger organisms. The authors contend, however, that the relation between body size and trophic levels was more significant for orders that contained fishes with smaller body sizes. Where this is clear is that the mean body size of 27 orders of fishes that showed a significant positive relation in comparison to trophic level and body size was 86cm smaller in comparison to the mean body size of the 20 orders of fishes that showed no relation between trophic levels and size. The macroecological and evolutionary patters are self-evident within the data.
7.) The study’s findings support recent models that support positive correlation with body size and trophic levels and that differences among orders suggest that this relationship are contingent on evolutionary history. And other physiological/environmental factors might explain the remaining variation within trophic levels.