Temporal variation in tree diversity effects on birds and its implications for top-down control of insect herbivores in a tropical system.

Tree diversity promotes predator abundance and diversity, but evidence linking these effects to increased predation pressure on herbivores remains limited. In addition, tree diversity effects on predators can vary temporally as a function of environmental variation, or due to contrasting responses by different predator types. In a multi-year study, we assessed temporal variation in tree diversity effects on bird community abundance, diversity, and predation rates as a whole and by functional group based on feeding guild (omnivores vs. insectivores) and migratory status (migrant vs. resident). To this end, we conducted bird point counts in tree monocultures and polycultures and assessed attacks on clay caterpillars four times over a 2-year period in a tree diversity experiment in Yucatan, Mexico. Tree diversity effects on the bird community varied across surveys, with positive effects on bird abundance and diversity in most but not all surveys. Tree diversity had stronger and more consistent effects on omnivorous and resident birds than on insectivorous and migratory species. Tree diversity effects on attack rates also varied temporally but patterns did not align with variation in bird abundance or diversity. Thus, while we found support for predicted increases in bird abundance, diversity, and predation pressure with tree diversity, these responses exhibited substantial variation over time and the former two were uncoupled from patterns of predation pressure, as well as contingent on bird functional traits. These results underscore the need for long-term studies measuring responses by different predator functional groups to better understand tree diversity effects on top-down control.

Soil salinization disrupts plant-plant signaling effects on extra-floral nectar induction in wild cotton.

Plant-plant interactions via volatile organic compounds (VOCs) have received much attention, but how abiotic stresses affect these interactions is poorly understood. We tested the effect of VOCs exposure from damaged conspecifics on the production of extra-floral nectar (EFN) in wild cotton plants (Gossypium hirsutum), a coastal species in northern Yucatan (Mexico), and whether soil salinization affected these responses. We placed plants in mesh cages, and within each cage assigned plants as emitters or receivers. We exposed emitters to either ambient or augmented soil salinity to simulate a salinity shock, and within each group subjected half of the emitters to no damage or artificial leaf damage with caterpillar regurgitant. Damage increased the emission of sesquiterpenes and aromatic compounds under ambient but not under augmented salinity. Correspondingly, exposure to VOCs from damaged emitters had effect on receiver EFN induction, but this effect was contingent on salinization. Receivers produced more EFN in response to damage after being exposed to VOCs from damaged emitters when the latter were grown under ambient salinity, but not when they were subjected to salinization. These results suggest complex effects of abiotic factors on VOC-mediated plant interactions.

Terpene chemotypes in Gossypium hirsutum (wild cotton) from the Yucatan Peninsula, Mexico.

Cultivated plants of Gossypium hirsutum Cav. (cotton) consistently emit low levels of volatile organic compounds, primarily mono- and sesquiterpenoids, which are produced and stored in pigment glands. In this study, we provide a comprehensive evaluation of the terpene profiles of wild G. hirsutum plants sourced from sites located throughout natural distribution of this species, thus providing the first in-depth assessment of the scope of its intraspecific chemotypic diversity. Chemotypic variation can potentially influence resistance to herbivory and diseases, or interact with abiotic stress such as extreme temperatures. Under controlled environmental conditions, plants were grown from seeds of sixteen G. hirsutum populations collected along the coastline of the Yucatan Peninsula, which is its likely centre of origin. We found high levels of intraspecific diversity in the terpene profiles of the plants. Two distinct chemotypes were identified: one chemotype contained higher levels of the monoterpenes γ-terpinene, limonene, α-thujene, α-terpinene, terpinolene, and p-cymene, while the other chemotype was distinguished by higher levels of α- and ß-pinene. The distribution of chemotypes followed a geographic gradient from west to east, with an increasing frequency of the former chemotype. Concurrent analysis of maternal plants revealed that chemotypes in wild G. hirsutum are highly heritable.