Ecoregions: Southern California and the Northern Baja Coast

Joshua trees and yucca moths: coevolved… and codependent

This post is the first installment in a series that will follow the PCT Class of 2026 on their trek north through eight unique ecoregions (as defined by the Environmental Protection Agency). For each region, we’ll highlight one example of mutualism, or a mutually beneficial symbiotic relationship between two or more native species. We hope that this series will remind readers that no single species can exist by itself—humans, least of all.

Don’t be deceived by the disparity between the chalky Southern California orange that greets hikers this time of year, and the dense green waiting for them up north: each landscape along the Pacific Crest Trail teems (and swarms, and crawls) with the kinds of huge and tiny hopes, perils, and innovations found in any ecosystem. Still, hikers in Southern California won’t need to look hard to spot one of the species we’re highlighting today. Without Joshua trees (Yucca brevifolia, also commonly called yucca palms) standing sentinel over their much shorter relatives elsewhere in the yucca genus, the desert would be unrecognizable (and even less shady).

Dr. Jeremy Yoder, an associate professor in the Department of Biology at California State University Northridge, quipped recently that we have a “small, boring-looking moth” to thank for the ubiquitous trees. Two species of yucca moth (Tegeticula synthetica for Western Joshua trees, and Tegeticula antithetica for Eastern Joshua trees) are Joshua trees’ sole pollinators.

That means that, unlike most flowering plants, which rely on a variety of moths and other insects to distribute their pollen, Joshua trees have “little guided missiles for pollinators,” Dr. Yoder said. The entire yucca plant genus and yucca moth family are similarly interdependent: neither group of species could exist for longer than a season without the other. Some yucca moth species are able to pollinate more than one yucca plant species, Dr. Yoder explained, but Joshua trees and their own personal pollinators “have been together for as long as there have been Joshua trees.”

While most pollinator insects distribute pollen inadvertently as they go about collecting nectar from flowers, yucca moths’ pollination seems to be “as deliberate as anything an insect can do,” Dr. Yoder said. The females’ highly specialized mouth parts allow them to gather pollen from yucca flowers’ ovaries and insert it directly into the flowers’ stigmas. They don’t do it without a favor in return: as they gather pollen, the female moths lay eggs in the flowers’ ovaries. The growing caterpillars will feast on the developing ovules, or future seeds, which were made possible by their mother’s not-so-selfless act of pollination. (There are usually not enough caterpillars to eat all of the ovules, but when this happens, the yucca is able to kill off its own overloaded flower.)

“The flipside of being tightly coevolved is that you’re also tightly codependent,” Dr. Yoder said. Any risk posed to one of these species is a risk to the other species. Through community science initiatives, PCT hikers can help Dr. Yoder and his team continue to learn about Joshua trees and yucca moths, and about the risks posed to them by threats such as extreme weather events, human development, and invasive species. Dr. Yoder encouraged hikers to use the online platform iNaturalist to document Joshua trees and yucca moths along the trail. “We can use every bit of data we get,” he said.

The state of California is working with tribal leaders on the Lone Pine Paiute-Shoshone reservation to develop Joshua tree conservation plans, the first of which was adopted last summer.

Dr. Yoder mused that “animal pollination means that some other species is responsible for a big element of what defines a species,” that is, the ability to reproduce on its own. At first, a tightly coevolved relationship like the one between yucca plants and yucca moths may seem to call that definition of a species into question. However, anyone who’s had the privilege of observing an ecosystem like Southern California’s will recognize that these plants and their pollinators are just one ultra-magnified example of the ways in which all species rely on one another. If we didn’t, none of us would be a species at all.

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