Nolina interrata

This is a very rare species, restricted to only 10 known, extant subpopulations. Three of these are situated in Mexico, near Ensenada, La Mission and a more recently discovered site close to the USA border. Each of these subpopulations are very small and are estimated to have fewer than 25 plants. The Mexican population is therefore not thought to consist of more than 75 individuals (CITES 2007).

In the USA, this species is known from seven extant subpopulations, all falling within a small area of around 15.6 km² in the foothills of San Diego County. Here, the total population was estimated in 1995 to be around 9,000 individuals (U.S. Fish and Wildlife Service 1995). However, this is a dioecious species whose main reproductive mechanism is asexual and therefore the number of plants which have originated from seed is unknown and likely to be considerably lower (CITES 2007). For example, a population of 2,000 rosettes was observed flowering in 1985 and 1986 following a wildfire in 1984, though no seeds were produced as all flowers were female. Electrophoretic studies indicated that 50+ distinct clusters of rosettes were genetically identical at 15 loci, suggesting that what appeared to be a subpopulation of over 50 individuals may actually have been a large clone of a single female plant (Bauder 1993). However, for the purposes of this assessment in terms of considering the number of mature individuals, it is assumed here that all clones are capable of asexual reproduction and are therefore here considered as mature individuals. 

Three of the USA subpopulations are considered to be large (>1,000 individuals) and these are situated at Sycuan Peak Ecological Preserve (over 1,000 individuals), McGinty Mountain Ecological Reserve (3,900 to 4,570 individuals) and Dehesa Mountain South Crest Preserve (1,680 individuals). The most recent survey of these subpopulations was carried out in 2014. The remainder of the subpopulations are considered to be small (<1,000 individuals) with 52 individuals at one site and over 120 individuals at a second site. This latter subpopulation is located only 0.3 miles away from the larger subpopulation in the reserve and the two may functionally exist as a single subpopulation (Conservation Biology Institute 2015). During the 2014 survey, only five of the seven known extant Californian subpopulations were successfully surveyed as the remaining two subpopulations are situated on private land and were not accessible at the time. However, they appeared to be small and previous surveys in 1980 and 1977 gave estimates of 5 to 10 individuals and 50 individuals for these sites (Conservation Biology Institute 2015). A further two subpopulations were also surveyed based on existing records or information which indicated their presence. Neither subpopulation could be located during the survey, indicating that they have either been extirpated or were mistaken observations of the morphologically similar chaparral yucca (Hesperoyucca whipplei). Development and habitat loss around the site may have been a factor in the possible loss of this species (Conservation Biology Institute 2015).

The current population trend of this species is unknown, but it is likely to be vulnerable to declines due to stochastic events such as wildfires, which are projected to cause a future continuing decline. The likelihood of threats other than fire impacting this species should be partly mitigated by current legislation and conservation actions in place for it. Though two-thirds of the USA population is being conserved on protected land, these subpopulations are still vulnerable to severe fires which are likely to cause declines. For example, the 2003 Cedar Fire resulted in a population reduction from 4,647 individuals in 2002 to 1,680 individuals in 2014, representing a reduction of almost 64% within this subpopulation (Conservation Biology Institute 2015). The subsequent invasion of its habitat by non-native species may prevent the full re-establishment of this species post-fire. However, there is potential for it to potentially re-populate historically occupied areas if a natural fire regime can be restored. Though more precise estimates of population size are needed from the two subpopulations on private lands and also from Sycuan Peak Ecological Reserve, the overall population size of this species is estimated to be between 9,000 and 10,000 mature individuals as a maximum.