Fish aren’t herbivores in cold climates because they can’t meet their phosphorus needs

Eric K. Moody, Nathan K. Lujan, Katherine A. Roach and Kirk O. Winemiller

Scientists have long recognized that, unlike (warm blooded) endotherms like mammals, herbivorous (cold blooded) ectotherms like fishes and invertebrates are rare in colder climates. However, the reason for this pattern was not clear. Since the metabolic rates of ectotherms are reduced in cooler climates, we hypothesized that this reduced metabolism may constrain their ability to persist on an herbivorous diet. This is because plant material is high in carbon-rich molecules like glucose but low in other important nutrients such as phosphorus relative to an animal-based diet. In warm climates, fishes may be able to consume food at high rates to meet both their metabolic and phosphorus needs, while in cool climates their reduced metabolism may prevent them from eating enough food to acquire sufficient phosphorus.

We tested this hypothesis by studying suckermouth catfishes in the Andes mountains. There, an herbivorous group of fishes lives in warmer, low elevation streams while an insectivorous group of fishes lives in cooler, high elevation streams. We used a model of the threshold elemental ratio, a measure of food quality where animal growth switches between carbon- and phosphorus-limitation, to predict when herbivory may become inviable. The model predicted that growth of the herbivorous catfishes should become limited by phosphorus near the maximum elevation where they occur, suggesting that an inability to meet phosphorus needs in cooler streams limits the distribution of these fishes. We also examined broader patterns in the threshold elemental ratio among fishes. We found that within species, the risk of phosphorus-limited growth decreases with increasing temperature, but species-specific differences prevented a broader pattern from emerging among all species. Our results indicate that cool temperatures are a constraint on herbivory in ectotherms like fishes, but individual species can adapt in ways that allow them to maintain an herbivorous diet in cooler places.

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