Shaw’s Agave – A Species on the Edge

March 15, 2017 - Shaw’s agave (Agave shawii) is a species of concern that is literally and figuratively on the edge along the Southern California coast. It is in the northernmost part of its range in San Diego, where one of the last native populations occurs on the Point Loma Peninsula. It occupies the rare coastal sage scrub community near the boundaries of Cabrillo National Monument. Plants are sometimes found at the edges of eroding sea cliffs.

A native population of Agave shawii on the eroding sea cliffs of the Point Loma Peninsula. NPS

While Shaw's agave is somewhat abundant in Baja California, its distribution in the United States is very limited. Baja populations may become threatened because their desirable coastal habitat could be subject to development or ranching. Here at Cabrillo National Monument our concern is with reproduction.

Scattered across the Cabrillo landscape, the conspicuous Shaw’s agave highlights the plight of global pollinators and the plants that rely on them. Recruitment of new individuals to the Point Loma agave population has declined to zero. Cabrillo and San Diego Natural History Museum biologists are now trying to find out why.

Many potential pollinators visit the agave flowers but few seem to be doing any actual pollination. NPS

Specifically, they have initiated efforts to:

• search for the missing pollinator, the Mexican long-tongued bat (Choeronycteris mexicana);
• assess the potential for other pollinators;
• test the effects of hand pollination;
• grow new individuals in greenhouse conditions; and
• cage young plants to test for herbivory.

Researchers hand-pollinate agave flowers to test  for fruit production and seed viability. NPS

So far, biologists have documented 11 species of bats, but no Mexican long-tongued bats. Hand-pollination trials have not seemed to increase Shaw's agave fruits or viable seeds. All insect and bird visitors appear to not act as pollinators.

Greenhouse growing of viable seeds has been highly successful, however. The National Park Service has grown over 200 individuals through their first year. There also does appear to be herbivory of young seedlings, but these trials are still underway.

Agave seedlings growing in the greenhouse. Greenhouse growing of viable seeds has been highly successful. NPS

This work supports a federal effort to understand and protect native plant communities and species that depend on them. The National Park Service strives to foster resilient ecosystems, ensuring the well-being of current and future generations.

Prepared by the Mediterranean Coast Inventory and Monitoring Network.

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