Ecology:
Grazing

Grazers in Prairies

Coastal prairies may have evolved with intense levels of grazing and browsing by large and small animals. In prehistoric times, coastal prairies were alive with large migratory and resident herbivores including mammoths, horses, camels, llamas, and bison that became extinct in the late Pleistocene. In the last 10,000 years, more familiar herbivores, such as Elk (Cervus canadensis nannodes)elk, Black-tailed deer (Odocoileus hemionus columbianus), and Pronghorn antelope (Antelocapra americana), were abundant. With the arrival of the first Spanish colonists in the 1700s, new grazers entered California grasslands: domestic cattle, horses and sheep, and other grazers, such as elk, declined.

Throughout these periods, small grazers, such as Pocket gophers (Thomomys bottae), Black-tailed rabbit (Lepus californicus), Brush rabbit (Sylvilagus bachmani), California meadow vole (Microtus californicus), and diverse insect fauna, in coastal prairies have continued to persist. The effect of their grazing is often underestimated. For example, Pocket Gophers have high energy requirements and can consume from 18-49% of above ground vegetation and more than 30% of below ground primary production (e.g. roots, bulbs, rhizomes) in some areas (Andersen and MacMahon 1981; Foster and Stubbendieck 1980; Williams and Cameron 1986).

Survival Strategies for Grazing

Grassland plants have evolved with animal disturbances and have developed strategies to evade or persist in spite of tissue loss. Likewise, animals have adapted to counter these challenges from their food plants. Some plants and animals have intimately coevolved together so that one or both may be dependent on the other for its livelihood. In coastal prairies, coping mechanisms that species use to deal with the effects of grazing include:

• Avoidance
• Concealment of Reproductive or Growth Tissues
• Compensation and Resistance
• Chemical or Structural Defenses

Avoidance

• Flowering culms (stems) lean obliquely or horizontally from the plant.

Concealment of Reproductive or Growth Tissues

• Many perennial grassland plants survive grazing because they have perennating structures or storage organs (e.g. rhizomes, tap roots, tubers, bulbs, corms) that lie below the ground. When the top portion of the plant is destroyed by grazing, the plant retains the ability and resources to re-sprout and grow. During the long summer drought, the underground structures store the energy needed to grow when the rains begin. The growing parts of grasses (basal meristems) are at the base of the plant. When the tops are grazed, the plant continues to grow from the bottom.
• The stems conceal hidden seeds that form at the base of the plant.
• California oatgrass (Danthonia californica) produces from one to eight cleistogamous (hidden self-fertilized auxiliary seeds) inside the flowering stem just above the nodes (Prasad, et al. 2005). This means that the plant can still produce seeds even if the terminal seed heads are eaten away.
• Sun cups (Taraxia ovata, formerly Camissonia ovata) also hide their seeds at the base of the plant; the seeds develop in the ovary which is located below the soil level at the bottom of the long stem-like flower tube.

Compensation and Resistance

• Grazing stimulates the formation of new stems (tillers) in bunchgrasses and stimulates the growth of horizontal stems of rhizomatous grasses (rhizomes and stolons).
  • Rangeland specialists refer to the new growth stimulated by grazing or cutting as “stooling”; tufted hairgrass (Deschampsia caespitosa) “reproduces well by stooling and is resistant to heavy grazing”(Sampson, et al. 1951).
  • Some grasses form a mat-like sod when moderately heavily grazed.
• Many grasses, such as California oatgrass (Danthonia californica) have stems that readily disjoint at the nodes, freely breaking away from the plant leaving the other stems and roots intact. This allows seeds attached to the stem either in the flowers or those hidden side of the stem to be transported by grazing and other passing animals where they are dispersed into other areas, if not eaten with the stem.
• Cattle and other large animals assure good germination in large seeded grasses such as California oatgrass (Danthonia californica) by trampling the seed into the ground (Sampson, et al. 1951).

Chemical or Structural Defenses

• Awns—many grasses have sharp needle-like awns on their flower spikelets that deter potential grazers.
• Spines—the flower heads of coyote thistle (Eryngium armatum) have sharp, spiny bracts.
• Hairs reduce palatability of plants—e.g. Trifolium spp., velvet grass (Holcus lanatus).
• Toxins—some plants produce toxic compounds that give them a disagreeable taste, act as a poison or are hard to digest (Lyons and Hanselka n.d.).
• Silica—
  • Grasses have cell walls composed of silica that helps them to tolerate and sometimes avoid grazing by making the leaves coarser and less palatable to some insects and animals. Grasses may have evolved to incorporate silica in their tissues over 65 million years ago as a response to dinosaur herbivores (Sullivan 1996).
  • Grazers, such as horses, cows, and deer have evolved hypsodontic teeth in order to process the abrasive silica rich grasses. Hypsodontic teeth have high crowns that extend into the jaw and emerge to replace wear. Modern horses that do not graze on enough grass must have their teeth filed down to prevent their teeth from growing too long, which causes dental problems.

Grazing as a Management Tool

Grazing can have positive, negative, or neutral effects on grassland plants and animals dependent on the species and how grazing is managed (Hatch, et al. 1999; Hayes and Holl 2003; Lyons and Hanselka n.d.). To complicate matters domestic livestock have different dietary and topographic preferences and behavior patterns that vary by species, breed and even by individual (Bush n.d.; Huntsinger, et al. 2007).

Different species respond in different ways to varying timing, intensity, and duration of livestock grazing. Hayes and Holl (2003) recommend using a mosaic of grazing and other disturbance regimes across the landscape as appropriate to maintain biological diversity in coastal grasslands. They studied the effects of livestock grazing on coastal prairie plants in various coastal prairies from northern Mendocino County to Moro Bay, San Luis Obispo County. They found that livestock grazing has varied effects on plants depending on various factors including the type of species (grass, forb; native, introduced) and the life history strategy (annual, perennial):

• Native annual forbs increased in grazed sites compared to ungrazed sites,
• Exotic annual grasses and forbs also increased in grazed sites,
• Native grasses performed similarly in grazed and ungrazed sites,
• Native perennial forbs were more abundant in ungrazed sites.

Removal of domestic grazing not only can change the plant community but can adversely affect some of the animals that inhabit coastal prairie (Marty 2005). Other species, such as pocket gophers, increase when grazing is removed, presumably due to increased foliage for forage and cover.