Latitudinal patterns of species diversity, body mass and herbivory: a comparison between ectotherms and endotherms
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Summary Earth varies widely in characteristics along her latitude, like temperature, precipitation and day length. This has consequences for life along this latitude. In this research, a comparison is made between ectotherms (here fish) and endotherms (here fresh water birds) and how their distribution and plant consumption is influenced by latitudinal patterns of temperatures. I collected data from the encyclopedia of Kear (2005), an article from Wood et al. (2012) and a selection of research papers to create a database of 84, mostly migrating, fresh water bird species, along with their body mass, percentage of vegetal matter in their diet, average latitude and average temperature of their summer breeding area and winter non-breeding area. I compared these data with published patterns for fresh water fish from Gonzalez-Bergonzoni et al., 2012. I drew four main conclusions: (1) Fish seem to follow the general latitudinal pattern of increasing species richness towards lower latitudes, while the number of species of fresh water bird species increases towards higher latitudes. (2) Fish seem to be smaller at lower latitudes, following the temperature-size rule, while no such pattern was found for water birds. (3) Heavier birds (3 kg or more) are almost exclusively herbivore, while lighter birds have a variety of diets. (4) Fish show a higher degree of herbivory towards lower latitudes. A specific part of my data of birds with a diet that consisted of at least 1%, but not more than 50% of vegetable matter, showed a pattern of a higher degree of herbivory towards higher latitudes and lower temperatures. Correlations of other birds with no or more than 50% of vegetable matter in their diet and latitude or temperature were not significant. The reversed latitudinal gradients found for fish and water birds may be due to differences in migratory behavior and thermoregulation. Most ectotherms do not migrate, do not form a protection to extreme temperatures, and are not able to control their body temperature as well as endotherms. Both fish and birds may eat more vegetal matter when having a higher metabolism: fish at higher temperatures, because of competition and their ability to digest vegetable matter better at higher temperatures and birds at lower temperatures, because of the useful expulsion of extra C. Because of limitation in my dataset, further research is needed to confirm this.