Ecological relationships between poisonous plants and rangeland condition: a review.
In the past, excessive numbers of livestock on western U.S. rangelands, reoccurring droughts, and lack of management resulted in retrogression of plant communities. Poisonous plants and other less palatable species increased with declining range condition and livestock were forced to eat these poisonous species because of a shortage of desirable forage, resulting in large, catastrophic losses. The level of management on most western rangelands has improved during the last 60 years, resulting in marked improvement in range condition; yet losses to poisonous plants still occur, though not as large and catastrophic as in the past. Some poisonous species are major components of the pristine, pre-European plant communities (tall larkspur (Delphinium barbeyi), Veratrum californicum, water hemlock (Cicuta douglasii), bracken fern (Pteridium aquilinum), chokecherry (Prunus virginiana), Ponderosa pine (Pinus ponderosa), and various oak species (Quercus spp.)). Although populations of many poisonous seral increaser species have declined with better management, they are still components of plant communities and fluctuate with changing precipitation patterns (locoweed (Astragalus and Oxytropis spp.), lupine (Lupinus spp.), death camas (Zigadenus spp.), snakeweed (Gutierrezia spp.), threadleaf groundsel (Senecio longolobis [Senecio longilobus]), low larkspur (Delphinium nuttallianum), timber milkvetch (Astragalus miser), redstem peavine (A. emoryanus), western bitterweed (Hymenoxys odorata), orange sneezeweed (Helenium hoopesii), twin leaf senna (Cassia roemeriana [Senna roemeriana]), and white snakeroot (Eupatorium rugosum)). Many of the alien invader species are poisonous: (Halogeton glomeratus, St. Johnswort (Hypericum perforatum), poison hemlock (Conium maculatum), tansy ragwort (Senecio jacobaea), hounds tongue (Cynoglossum officinale), leafy spurge (Euphorbia esula), yellow star thistle (Centaurea solstitialis) and other knapweeds (Centaurea spp.)). Poisoning occurs when livestock consume these plants because they are either relatively more palatable than the associated forage, or from management mistakes of running short of desirable forage.