Seasonal stoichiometry of terrestrial consumer-resource interactions

Variation in producer stoichiometry influences animal distribution, abundance, and behavior. While spatial variation in producer stoichiometry is widely acknowledged, seasonal variation in producer nutrient content may also strongly affect primary consumers. For many terrestrial ecosystems, nitrogen concentration in leaves declines over the growing season and is associated with increasing carbon:nitrogen (C:N) ratios. As a result, late summer folivores like lepidoptera encounter a very different stoichiometric landscape than that of early summer. Although arthropods are assumed to regulate their elemental composition (to some degree), it is possible that the stoichiometry of these communities also varies seasonally in concert with their resources. To quantify the effects of seasonal changes in foliar stoichiometry on insects, we measured carbon and nitrogen concentrations and C:N ratios in leaves and adult moths in an alvar ecosystem of Ontario, Canada, nearly every week from early May to early October 2023. We found that leaf N concentrations decreased, C concentrations remained stable, and C:N ratios increased in leaves across the growing season. The result was consistent across different light levels though somewhat variable across sampling locations. On the contrary, moth N and C concentrations and C:N ratios varied minimally across the growing season. Furthermore, we found little evidence for consistent seasonal declines in per-individual moth body mass or total moth abundance, and no strong direct association between these moth characteristics and foliar C:N. As a result, there may be a limited physiological cost to acute N limitation in an N scarce landscape. Just how late summer foliage eating arthropods have adapted to high C:N ratios remains a key question, especially since climate change may cause earlier leaf emergence and lengthen periods of consumer-resource stoichiometric mismatch.