Soaptree and Aplomados

Soaptree yucca and aplomado falcons are closely associated. Falcons depend on tall yucca for nest sites and yucca also provide habitat to a variety of falcon prey. Soaptree yucca (Yucca elata var. elata) is the dominant perennial arborescent monocot in the northern . Its range matches that of the Chihuahuan Desert (Smith and Ludwig 1978b; Sisson 1983). Its present distribution mirrors that of 10,000-40,000 years ago (Smith and Ludwig 1978b). The soaptree yucca is considered “one of the most important species of the northern Chihuahuan desert,” given the habitat it provides for many faunal species, as well as its large biomass (Smith and Ludwig 1978a). Montoya and Zwank (1995) found that aplomados consistently used yucca for perching, feeding, and roosting.

Livestock grazing poses a significant threat to the value of yucca for falcon nesting and for prey habitat. Recent research has found that falcons in the Chihuahuan desert primarily nest in yucca and that may harm yucca caudices via herbivory, rubbing, and trampling effects (BLM 2001; Young et al. 2001). While rodents and lagomorphs may be damaging to seedlings (Smith and Ludwig 1978b), cattle harm larger with the most potential for falcon nests.

First, studies have found that 75-98% of yucca stalks are consumed in areas grazed by cattle, versus 15% grazed by native ungulates such as pronghorn (Kerley et al. 1993; BLM 2001). On one study site in the Jornada del Muerto, a single bull entered the area and consumed 75% of the flower stalks (Smith and Ludwig 1976). The voracity with which cattle consume yucca and was underscored by Herbel and Nelson (1966: 181):

Soaptree yucca was eaten primarily in the winter and spring, when the faces of the cattle that eat the leaves often become green. Increased use of it was made in the dry winter-spring of 1964 when it sometimes made up 70% of the grazed plants. In late spring and summer, if the plants bloom, the cattle particularly relish the flowers and flower stalks. They will fight for them and, if necessary, stand on their hind legs to reach them.

While flowers on taller yucca may escape cattle browsing (Kerley et al. 1993), they are still vulnerable to cattle damage to the trunk, or caudex, via trampling (see below).

Cattle consumption of yucca flowers and stalks may have long-term impacts on yucca and grassland health. A desert survival adaptation of the yucca is to greatly increase flower production after a wet monsoon and to flower, at most, every two years (Smith and Ludwig 1976; Sisson 1983; Gilstrap Laslei and Ludwig 1985). Grazing during periods of active growth and photosynthesis may reduce energy required for flower production (Kerley et al. 1993; BLM 2001). Therefore, grazing prior to, and during flowering, will lead to a reduction in yucca flowers. This may lead to local extirpations of yucca moths, given its status as an obligatory parasite on yucca seeds (Kerley et al. 1993; BLM 2001). The reduction in moths would, in turn, negatively impact , as yucca moths are the exclusive pollinators of soaptree yuccas (James et al. 1993; Kerley et al. 1993; Johnson 2000; BLM 2001). Consequently,

Local yucca populations may therefore be rendered permanently unfertile and incapable of producing recruits by sexual reproduction, with the associated long term disadvantages of genetic inflexibility (Kerley et al. 1993:16).

In short, harms to yucca negatively impact yucca moths, whose reduction, in turn, limits yucca reproduction.

Harms to yucca also translate to negative impacts on other fauna associated with this plant. On the Jornada del Muerto, cactus wrens have demonstrated a preference for areas with large, mature yuccas that provide feeding and nesting places. In addition, there is a positive correlation between the number of yucca inflorescences per clump and densities of Scott’s orioles. This oriole species feeds on soaptree yucca flowers and usually nests in yucca (Naranjo and Raitt 1993). Other fauna associated with the yucca included members of the insect orders Homoptera, Heteroptera, Lepidoptera, Diptera, Coleoptera, and Hymenoptera; multiple reptile species; and the white-throated woodrat, jackrabbit, and desert cottontail (Campbell and Keller 1932). James et al. (1993) recorded an additional three orders of invertebrates using yuccas: Hemiptera, Thysandoptera, Neuroptera, Araneida, and Orthoptera, for a total of 56 species using yucca during the day and 37 species during the night. Kerley et al. (1993) reported a total of 70 arthropod species associated with yucca flowers, along with birds who use yucca as perches: the loggerhead shrike, northern mockingbird, Swainson’s hawk, and western kingbirds. All of these species may provide prey to falcon (Table 1), with the exception of Swainson’s hawks, who provide falcons with nests. The reduction of these yucca-associated fauna is therefore an additional limiting factor on falcon recovery.

Second, cattle cause breakage of, and damage to, yucca branches and caudices by rubbing against this plant. This is important given the slow-growing nature of yucca, and the role of tall yuccas in providing potential aplomado nests. In New , yuccas tend to vary from 1-9m in height, and some individuals as high as 10.9 m have been reported (Campbell and Keller 1932; Smith and Ludwig 1976; 1978b; BLM 2001). Yuccas grow in height from 0.5 to 1.5 inches per year, with an average annual growth rate of 0.96 inches. Growth rates are lower in drier years (Campbell and Keller 1932; Sisson 1983). Given annual growth rates of only one inch per year, individuals reported at 26-feet-high would be nearly 300 years old (Henry 1995).

Physical trampling by cattle causes yucca trunks to trail along the ground (become procumbent) (Kerley et al. 1993; BLM 2001), which hinders their ability to attain heights sufficient to provide nesting substrates for falcons. Further, the BLM notes that,

As rubbed plants succumb to damage and break off, other plants will be rubbed (perhaps with increased frequency), reducing their life span. At a growth rate of 1in per year (Campbell and Keller 1932), replacement of a nestable YUEL [Yucca elata] lost due to cattle rubbing would take at least 100 years (2001: 10, emphasis added).

Third, cattle trample and degrade soils and vegetation at the base of yucca plants. Yucca provides shade for livestock (Campbell and Keller 1932). As a result, cattle congregate around yucca. According to the BLM (2001), cattle around yucca can impact the health and persistence of yucca plants. Effects include loss of surface , altered ability to absorb nutrients and water, and reduction in plant vigor.

Livestock impacts to yucca and consequent harm to aplomado falcons are not an esoteric concern. In 2001, Raymond Meyer, an environmental consultant conducting aplomado surveys, monitored the pair of aplomado falcons who attempted to nest near Deming, . Meyer expressed concern when he witnessed multiple cattle rubbing against the yucca plant where the falcons were attempted to nest, causing the plant to sway significantly (See Exhibit 1: Raymond Meyer Correspondence) and, as discussed below, the BLM consequently required the grazing permittee to remove his cattle from the area around the nest.

In sum, there are a variety of direct and indirect harms to yucca from livestock grazing which, in turn, impact the aplomado falcon who benefits from nesting substrate on taller yucca and from the variety of avian and invertebrate prey associated with yuccas in the Chihuahuan Desert. These harms are not being adequately addressed by the BLM (See Forest Guardians’ Aplomado Falcon Critical Habitat Petition). Given these multiple harms accruing from livestock grazing, it appears that yucca is, at best, enduring the hardship of grazing, rather than benefiting from it.1 The same may be said of the aplomado falcon.

For latin names of all mentioned species and full citations, see Forest Guardians’ Northern Aplomado Falcon Critical Habitat Petition.

1Smith and Ludwig (1978b: 202) describe soaptree yucca as “possibly being a relic of past grassland vegetation” in the Chihuahuan Desert.