The N2-fixation trait confers an advantage to not only individual N2-fixing plants, but also to non-fixing plants and herbivores that require high quantities of nitrogen, by increasing the macronutrient availability for the whole system. N2-fixers have been present in all types of ecosystems for millions of years and nitrogen limitation is pervasive across these ecosystems, yet fixers do not consistently outcompete non-fixers. High herbivore pressure may constrain fixers, as herbivores seek nitrogen-rich food. Although many studies have looked at herbivory, the dynamics of herbivory on N2-fixers remain poorly understood, especially in relation to their non-fixing counterparts and nitrogen-rich lifestyles.
To address this knowledge gap, I quantified leaf herbivory and corresponding leaf nitrogen content across 45 families, 82 genera, and 92 species of plants over two dry seasons in California’s Mediterranean climate. Plants were sourced from local organic farms, experimental plots, urban areas, and biological preserves. They included a variety of native, invasive, and crop species. To quantify herbivory rate as percent of leaf area missing per day, I processed all leaf photographs through a customized image processing program. I quantified the corresponding leaf nitrogen content as the percent nitrogen (N) of dry weight.
Results/Conclusions
N2-fixers had a lower mean herbivory rate (0.20% per day) compared to non-fixers (0.24% per day), despite having higher mean leaf nitrogen (2.89% N) than the non-fixers (2.34% N). Leaf herbivory rate was not significantly correlated with leaf nitrogen content. These results indicate that leaf nitrogen content is not a strong predictor of leaf herbivory in agricultural and natural settings. One possibility is that N2-fixers have strong herbivore defenses.
Of the N2-fixer leaves sampled, those that had a leaf nitrogen concentration lower than 2% did not have any herbivory. Similarly, of the non-fixer leaves sampled, only a few of those that had a leaf nitrogen concentration lower than 2% experienced herbivory. These trends may indicate that herbivores avoid plants of lower nutritional quality and there may be a minimum threshold of leaf nitrogen content for herbivore selection.
The results of this study highlight the complex relationship between plant nutritional quality, herbivore selection, and N2-fixation. These findings can be used in modeling trophic interactions in natural systems. These finding also have broad implications for optimal incorporation of N2-fixers in agricultural systems to promote natural pest management.