If you have ever seen a Torrey pine in San Diego, you might wonder what it’s doing on a list of rare and endangered species. In that county, Torrey pines get their own reserve, containing thousands of individual trees. 

​

However, that doesn’t mean the Torrey pine is safe. The only “wild” population of Torrey pines live on that reserve, and on Santa Rosa Island, one of the Channel Islands off the coast of California. Beyond that, a few Torrey pines live in arboreta and botanical gardens, such as the one at the Fullerton Arboretum. According to the International Union for the Conservation of Nature (Farjon, 2013), there likely aren’t more than 5,000 of these pine trees left in the world; for an entire species, that isn’t many. 

​

While being restricted to 5000 individuals is bad, what makes the problem worse is that most of the Torrey pine trees sit inside the San Diego reserve. This restriction on the population puts them at an even greater risk. The high density of Torrey pine trees, with no other tree species present, makes them vulnerable to what are called density-dependent factors. Density-dependent factors are things that affect survival or reproduction that are dependent on the density of the population. Some density-dependent factors are positive. Reproducing or surviving a windstorm might be easier in a large group. Others are negative. Negative density-dependent factors include being found by predators or spreading disease more quickly; the likelihood of these negative things often increases at higher densities. 

​

In the case of the Torrey pines in San Diego, the close proximity of the trees, and monotypic stands (groups containing only one species), increases the likelihood that dangers like disease and fire can spread easily. Since most of the Torrey pines individuals are in this situation, this makes the problem even worse. If many of those Torrey pines were to die in a disease outbreak or fire, then most of the species would be gone. Because it could only take a single event to kill most members of this species, Torrey pines are considered to be endangered.

​

However, there is a bright side. Population sizes of Torrey pines have increased since scientists began monitoring them. In 1898, a science writer named Constance G. Dubois suggested that there could only be a few hundred trees in the area that would one day become Torrey Pines State Reserve (DuBois, 1898). That’s not nearly as many as the 3,000+ individuals currently estimated, which suggests that the population has been growing since that area was protected. Additionally, scientists have been able to produce stronger, healthier trees by interbreeding the trees on Santa Rosa Island with trees from the mainland (Hamilton et al., 2017). This cross-breeding is called genetic rescue and it improves the chances that the hybrid offspring will survive what the genetically distinct parents could not. They may be in a precarious position right now, but Torrey pines are not beyond saving.

 

 

 

Works Cited

​

DuBois, C. G. (1898). The Torrey Pine. The Asa Gray Bulletin, 84-85.

 

Farjon, A. 2013. Pinus torreyana. The IUCN Red List of Threatened Species, 2013

 

Hamilton, J. A., Royauté, R., Wright, J. W., Hodgskiss, P., & Ledig, F. T. (2017). Genetic conservation and management of the California endemic, Torrey pine (Pinus torreyana Parry): Implications of genetic rescue in a genetically depauperate species. Ecology and evolution, 7(18), 7370-7381.