Fog-mediated energy and nutrient fluxes through food webs in coastal desert ecosystems

Background/Question/Methods

Ecosystem biogeochemistry is often constrained by local energy and nutrient availability and the requirements of the organisms that process, store and transfer these resources. In many coastal and montane ecosystems, fog water accounts for a large percentage of the total input of chemical elements. Fog can therefore affect the structure and functioning of these ecosystems. Seasonal changes in fog nutrient inputs affect plant nutrient content, which in turn impact in herbivore population dynamics. Further, fog-deposited nutrients may affect the magnitude and/or the pathways of resource flows within food webs. Thus, both nutrient supply and trophic relationships control the amount of energy and nutrients that are transferred from plants to the top predators. We used stable isotope analysis to identify and quantify changes in energy and nutrient flows within food webs in response to variation in fog nutrient supply. We quantified the relative importance of CAM (Bromeliad) and C₃ (lichen) derived resources for consumers by measuring the carbon (d¹³C) and nitrogen (d¹⁵N) isotope ratios of primary producers, arthropods, and lizards from fog-dependent ecosystems of the hyper-arid Atacama Desert, Chile.

Results/Conclusions

We found that desert food webs are linked to marine ecosystems through fluxes of water and nutrients from sea to inland areas. During the fog season, higher nutrient inputs increased the N content of primary producers. C₃-lichens showed significantly higher N content and lower C:N ratios than CAM plants during both dry and fog seasons. Stable carbon isotope ratios showed that consumers have mixed sources of CAM and C₃ derived resources with a stronger reliance on CAM-derived carbon and nutrients than on C₃ resources in both seasons. The proportion of C₃ derived resources tended to increase in all food webs during the dry season, when C:N ratios of CAM tissue are higher. Nevertheless, CAM-derived resources largely fuel these food webs. We hypothesize that the lower reliance of the consumers on nutrient-richer lichen biomass is due to their high concentration of secondary metabolites. This could have affected the consumption of lichens by herbivores, and consequently trophic pathways of resources. These findings provide insight into the linkage between adjacent ecosystems and highlight the role of stable isotopes in the understanding of the fluxes of energy and nutrients within food webs.