Have you ever wondered why there are livestock on some DFG ecological reserves? We use them as a management tool to control unwanted plants. Many of the grasses and broad-leaved weeds you see in the Central Valley were introduced into California in the nineteenth century. These plants (which may be called “non-native,” “introduced,” “alien,” or “exotic”) came from Europe and Asia. They made their way to California in the hair and guts of livestock or in their feed. Non-native plants found the climate and soils of California very hospitable. Few of their natural enemies found their way to California, so non-native plants became firmly established and now overrun our natural lands. Perhaps you have heard of some non-native grasses such as ripgut brome, Italian ryegrass, or foxtail barley, or their broad-leaved compatriots which include yellow star-thistle and tumbleweed (AKA Russian thistle).
California native species and natural land suffer a variety of problems due to non-native plants. One problem arises because non-native plants grow in dense stands, whereas our native plants are often more widely spaced. Small critters such as lizards and kangaroo rats need bare ground to run away from predators, but the dense grasses slow them down and make them more vulnerable to predation. Another problem occurs because non-native plants tend to start growing earlier in the season than most natives. The non-native plants deplete precious moisture from the soil, causing the native plants to die before they bloom. In addition, non-native grasses also make natural lands more vulnerable to wildfire because they leave behind so much dry fuel after the rainy season ends.
We can’t hope to eliminate most non-native plants because they are so widespread and so firmly established. Instead, we try to minimize their impacts through management. That’s where the livestock come in. Cattle, sheep, and in some cases, goats, are used to graze back the non-native plants beginning early in the growing season. Timing of livestock grazing is critical to ensure that native plants have a chance to complete their life cycles before the growing season ends. We determine the grazing seasons for each area and each year based on annual rainfall as well as the type of non-native plants and native species present. Livestock grazing is not appropriate in all circumstances. For example, livestock may do more harm than good in areas that are moist year-round, such as springs.
How ironic that livestock, which facilitated the invasion of non-native plants into California, are helping us to keep them under control 150 years later!