Endemic Castilleja grisea support diverse floral-visitor assemblages with implications for promoting local plant communities

Agapostemon subtilior bee visiting another native bloom, Malacothamnus clementinus (San Clemente island bushmallow). Photo by D.T. Rankin CC-BY

 

by Saldivar et al.

The Channel Islands of California are a biodiversity hotspot and are home to a wide array of unique species found nowhere else. San Clemente Island, one of the southernmost islands in the archipelago, supports a particularly rich native flora. Here, we explore the floral-visitor interactions involving Castilleja grisea (San Clemente Island paintbrush), an island-endemic hemiparasitic plant that has increased in abundance following removal of historic livestock and herbivores. Although this recovery from grazing pressure led to its recent delisting as a federally endangered species, C. grisea remains state listed as rare and is the focus of ongoing post-delisting monitoring.

Using field surveys conducted across the island, we examined how C. grisea contributes to floral visitor diversity and plant-floral visitor network structure within its native habitat on San Clemente Island, California. Despite growing interest in pollinators and conservation of endemic plants, baseline information on visitation patterns and how community composition influences stability of ecosystem services remains limited. Our study addresses this gap by documenting novel plant-floral visitor associations and discussing the potential for C.grisea as an indicator of community health.

We recorded 23 floral visitor taxa visiting 12 plant species, with nearly three-quarters of these visitors interacting with C. grisea. Plots where C. grisea was in bloom supported nearly 50% higher floral visitor species diversity compared to sites without flowering individuals, underscoring its ecological importance. Collectively, our findings suggest that San Clemente Island paintbrush may function as a key species, and that conservation and management strategies focused on its protection may provide broad ecological benefits for island-wide plant and floral visitor communities.

 Read the scientific publication in JPE.

Influence of group size and floral display on pollinator behaviour in Moricandia arvensis

A male of Eucera nigrilabris is visiting a Moricandia arvensis flower 
(from Gomez et al. 2025; https://doi.org/10.1093/aobpla/plae065).

by Torices et al.

Plants often grow in clusters rather than as isolated individuals. We asked whether pollinators respond mainly to how many plants are grouped together or to how many flowers the group displays. To answer this, we created experimental patches of the Mediterranean plant Moricandia arvensis, varying independently the number of plants and the total number of open flowers. We recorded how often insect pollinators visited these patches and how they moved among plants and flowers. Most visits were made by long-tongued solitary bees, especially Anthophora and Eucera, which are the main spring pollinators of this species. The number of open flowers in a patch was the strongest factor attracting pollinators: patches with more flowers received more visits, particularly when they contained many plants. However, individual flowers in large displays received fewer visits on average. The number of plants alone had weaker effects, but it influenced how strongly flower number increased pollinator attraction. In fact, adding flowers made patches much more attractive when those flowers were spread across many plants than when the flowers were arranged in a few plants. Overall, our results show that the way plants are grouped affects pollinator behaviour in ways that cannot be explained simply by the number of flowers within a patch.

Read the scientific article in JPE HERE