Data from: Threshold elemental ratios and the temperature dependence of herbivory in fishes

1. Herbivorous ectothermic vertebrates are more diverse and abundant at
lower latitudes. While thermal constraints may drive this pattern, its
underlying cause remains unclear. We hypothesized that this constraint
stems from an inability to meet the elevated phosphorus demands of bony
vertebrates feeding on P-poor plant material at cooler temperatures
because low gross growth efficiency at warmer temperatures facilitates
higher P ingestion rates. We predicted that dietary carbon:phosphorus
(C:P) should exceed the threshold elemental ratio between carbon and
P-limited growth (TERC:P) for herbivores feeding at cooler temperatures,
thereby limiting the range of herbivorous ectothermic vertebrates facing
P-limited growth. 2. We tested this hypothesis using the Andean
suckermouth catfishes Astroblepus and Chaetostoma. Astroblepus are
invertivores that inhabit relatively cool, high-elevation, streams while
Chaetostoma are grazers that inhabit relatively warm, low-elevation
streams. We calculated TERC:P for each genus across its elevational range
and compared these values to measured values of food quality over an
elevational gradient in the Andes. We also broadly summarized measurements
of TERC:P across diverse groups of fishes. 3. Supporting our hypothesis,
we found that dietary C:P was predicted to exceed the TERC:P for the
grazer Chaetostoma near the highest elevation where this genus has been
recorded. Conversely, the TERC:P for the invertivore Astroblepus was
consistently higher than that of Chaetostoma and thus its dietary C:P
never approached the TERC:P. We found that, among all fishes, omnivores
had higher average TERC:P than invertivores, and TERC:P did not vary with
temperature. 4. Our results suggest that, at least for Andean suckermouth
catfishes, cool temperatures constrain herbivory at higher elevations.
Increased gross growth efficiency at cooler temperatures evidently
restricts the ability of P-limited consumers to meet P demand. However,
our survey of fish TERC:P estimates suggests that some fishes are able to
circumvent this constraint through behavioral and life history adaptations
that reduce P demand or increase P use efficiency. 5. The physiological
tradeoffs underlying these functional shifts reveal that geographic
dietary patterns can be predicted by stoichiometric theory, but variation
in food quality and consumer traits that reduce P demand and/or increase P
efficiency can create exceptions to these patterns.