Higher herbivory rates in lower-latitude populations of pokeweed (Phytolacca americana)
The latitudinal diversity gradient is a striking and consistent biogeographic pattern, but the processes that generate it are not well understood. The “biotic interactions hypothesis” states that biotic interactions are more important drivers of adaptation in the tropics, while abiotic factors are more important in the temperate zone. This pattern could lead to greater tropical diversification rates because biotic interactions can coevolve, hastening adaptive divergence (and therefore speciation) of allopatric populations. Support has been found for greater importance of biotic interactions at lower latitudes, particularly for mutualistic interactions that are more prevalent in the tropics, but there is mixed evidence for some antagonistic interactions such as herbivory.
We tested whether herbivore pressure is greater at lower latitudes along a gradient from 27°N to 42°N in pokeweed (Phytolacca americana, Phytolaccaceae). Pokeweed is a widespread, herbaceous, short-lived perennial pioneer species native to the eastern US. At five sites between Michigan and central Florida, we measured herbivory rates over time on young and mature leaves in late summer 2014. Most studies that estimate herbivory in the temperate zone use standing measurements on mature leaves, an approach that may greatly underestimate herbivory (e.g., it misses leaves that are completely consumed).
For young leaves, mean herbivory rates were higher in the three southern populations (3-5% leaf area consumed per day) compared to the two northern populations (1% per day). Compared to young leaves, mean herbivory rates were much lower for mature leaves at most sites (0-1% per day). The highest mean herbivory rate (5% per day on young leaves) was observed at the second most southern site, so the pattern was not a true gradient. The study will be repeated in 2015 with two populations at each latitude to capture potential site-specific variation in herbivory. If we base our analysis on standing damage, the results varied over time, and were inconsistent with results based on herbivory rates.
Our results support the herbivory pattern predicted by the biotic interactions hypothesis, but the underlying ecological mechanism driving greater herbivore pressure at lower latitudes is not yet clear. Our next step is to compliment estimation of herbivore pressure in the field with palatability trials to measure constitutive plant defense in the greenhouse. In addition, we will compare plant-herbivore interactions in pokeweed to its tropical congener, Phytolacca rivinoides, to expand the geographic scale of our test of the biotic interactions hypothesis.