Tales of the Wild

Fallen Giants and the Trees That Need Them

One of the most fascinating environmental dilemmas in recent history is the defunct relationship between one of the world’s most famous trees and gargantuan beasts that disappeared at the end of the last ice age. The famous “tree” in question is the Joshua tree, an ecological gem of the American Southwest. These plants have inspired awe and wonder for as long as people have encountered them, but in recent times, the range of the Joshua tree has been  gradually shrinking. One of the main reasons for this is due to the absence of sloths… giant sloths. 

Nothrotheriops shastensis, a species of giant ground sloth also called the Shasta ground sloth, roamed the American Southwest from the time they migrated to the region some 8 million years ago until their demise more than 11,000 years ago. They are an ancestor of the tree sloths now found in modern day South America, but they were the size of a black bear, weighed over 500 pounds, and had very long claws. Believe it or not, humans interacted with these prehistoric giants for a few thousand years and were possibly the main driver of their extinction due to excessive hunting. It is thought that their extinction has led to some changes in Southwest ecosystems. One of the most notable species that has been affected is the Joshua tree (Yucca brevifolia). 

The range of Joshua trees has been shrinking since the end of the last ice age, the Pleistocene epoch, over 11,000 years ago. Some had hypothesized that it may have been in relation to the changing climate, but further research found that the Joshua tree has lost its main means of seed dispersal. Many of the species that consume the fruits have stomach acid that kills the seeds, preventing germination from the animal’s excrement. On top of that, the species that are able to consume the seeds of the Joshua tree without damaging them, like the desert woodrat, may live their entire lives without traveling more than a couple hundred feet from their dwelling. 

In a Nevada cave in the 1930’s, scientists discovered Joshua tree fruit and seeds that were preserved in excrement from a Shasta ground sloth. This evidence supports the hypothesis that ground sloths were a vital seed disperser of the Joshua tree. Individual sloths could ingest the fruits and travel dozens of miles before excreting them. Since the discovery in the 1930’s, there have been many other similar findings. Joshua trees and other plants that relied on ground sloths, including avocado trees, are victims of evolutionary anachronism, a process where an existing species loses a partner species with which it coevolved.  

Illustration by Carl Buell

While the Joshua tree has suffered from the absence of ground sloths for thousands of years, it faces many more immediate problems. During the 35 day U.S. government shutdown of 2018-2019, Joshua Tree National Park suffered severe damage. Without normal park supervision, many trees were cut down or damaged, trails were littered with garbage and dog excrement, vandals drove off road in sensitive habitat areas, and more. A park official stated that it would take hundreds of years for the ecosystems in Joshua Tree National Park to fully recover. As with many species, climate change is also a significant factor affecting Joshua tree’s health. We can aid all species affected by climate change by letting our government officials know we care about climate change, spreading the word about its effects, and using more clean energy. To learn more or to support Joshua trees, visit the Joshua Tree National Park Association at Joshuatree.org.