Photo: Longhorns, Austin, Texas. © 2017 Delena Norris-Tull

Grazing solutions for management of invasive plants

​Summaries of the research and commentary by Dr. Delena Norris-Tull, Professor Emerita of Science Education, University of Montana Western, September 2020, updated November 2024.
 
Livestock as important dispersers of native seed
 
Couvreur, et al., 2004, examined the impact of large herbivores on the spread of native seed in Europe. They point out that, “In postglacial times, wild animals were probably key long-distance dispersal vectors through external…and internal…seed dispersal.” Today, movement of wild animals between natural areas is highly restricted, due to habitat losses, animal extinctions, and greater distances between natural areas. As a result, modern agricultural practices have prohibited many natural native seed dispersal processes. “Seed dispersal has now become a bottleneck for vegetation development and restoration of isolated (semi-)natural relicts, particularly for [native] species with short-lived seed banks.” Couvreur, et al., 2004, examined present-day long-distance seed dispersal processes. They focused on domesticated large herbivores, grazing in nature preserves in Belgium. “Grazing is a major management measure to maintain the semi-natural vegetations types in these reserves.” Domesticated herbivores, including mostly cattle, but also horses, sheep, donkeys, and goats, are introduced into the nature preserves in low densities. The herbivores are moved through the various reserves over the spring and summer. Couvreur, et al., 2004, collected seed from the fur of 82 animals, cattle, horses, and donkeys, that had been moved between numerous nature reserves. They germinated the seeds of 52 native species representing 23 plant families. 2522 seeds germinated successfully. The germination rates ranged from 6.3% to over 50% per species. 67% of all seedlings emerged from only five species. 90% of all seedlings emerged from ten species. Cattle dispersed more seeds than horses. They concluded that livestock can be effective at facilitating long-distance seed dispersal between nature reserves separated by large distances. Thus, when moved between reserves regularly, livestock are effective substitutes for wild mammals that previously had been the major seed dispersers. They concluded that livestock can be effective grazers for use in ecological restoration projects. In this study, they did not look at the viability of seed that had passed through the bodies of the grazing animals.
 
Hayes and Holl, 2003, examined the impact of cattle grazing on California coastal prairies. They surveyed the composition of vegetation communities in 25 paired grazed and ungrazed sites. They concluded that, “Native annual forb species richness and cover were higher in grazed sites… Exotic annual grass and forb cover were [also] higher in grazed sites. Native grass cover and species richness did not differ in grazed and ungrazed sites, but cover and species richness of native perennial forbs were higher in ungrazed sites.” They concluded that “cattle grazing may be a valuable management tool with which to conserve native annual forbs in [this particular] ecosystem but that grazing differentially affects the various life-history guilds…” Their results show that the use of grazing as a management tool is a complex enterprise and “highlight the importance of considering the adaptation of vegetation communities to disturbance in making recommendations for grazing management.”
 
While livestock can be effective in helping to restore native seed to natural habitats, livestock grazing also has been one of the human-induced factors that has led to increases in weeds in many rangeland soils. Just as livestock can move native seeds across the landscape, they also can move the seeds of invasive plants across the landscape.
 
Sheley, et al, 1996, also point out that livestock can increase weeds, if allowed to graze when seeds are present on the weeds. A few studies have been conducted to examine whether seeds remain viable after consumption by livestock.
 
Viability of noxious weed seed in livestock
 
In a study in southwestern Montana, Olson, Wallander, and Kott, 1997, compared leafy spurge seed extracted from sheep feces and sheep wool, with seed collected from the field. They found that seed that had been digested by sheep had a much lower germination rate than seed collected from the field. But sheep also pick up a lot of seed in their wool, and thus can potentially spread viable weed seed. Fortunately, seed embedded in wool tends to cling to the wool until time for shearing. Olson, et al, 1997, also found that when leafy spurge seed capsules are immature, less than 13 days old, they lose their viability after ingestion by sheep.
 
Lacey, et al, 1992, also found greatly reduced leafy spurge seed germinability (percentage of seeds that sprout) and viability (percentage of seeds that are alive) after ingestion by both sheep and goats. Germinability and viability were lower after ingestion by sheep than by goats. A high percentage of the ingested seeds were never excreted, thus leafy spurge populations can be reduced by grazing, as long as livestock are not moved to un-infested pastures too soon. It takes up to 9 days for leafy spurge seed to be excreted from sheep and up to five days from goats. The longer the seed remains in the livestock, the lower is its germinability and viability.

Invasive plants that protect native plants from large herbivores

 Odour, et al. (2018), discovered a case in which an invasive plant provided a benefit for native plant communities. Opuntia ficus-indica, a prickly pear cactus from Central America, has become an invasive species around the world, including in the Mediterranean, and many subtropical and topical climates. Introduced to Kenya in the early 1940s for use mainly as a hedge plant, today, it occurs in thick stands.

Odour, et al. (2018), conducted an extensive native plant survey in Nairobi National Park, in paired invaded and uninvaded plots in a savannah ecosystem in Kenya. While Opuntia species are readily eaten by large herbivores in their native habitats in the Americas, in Kenya, Opuntia spines discourage large herbivores from eating the prickly pear pads. Within this park the most common herbivores are "wildebeest, zebra, eland, Thomson's gazelle, giraffe, Grant's gazelle, Africa buffalo, and black rhinoceros."

"When these invasive plants are relatively unpalatable, they may act as nurse plants by reducing herbivore damage on cooccurring native plants, thereby structuring native plant communities. However, the potential for unpalatable invasive plants to structure native plant communities has been little investigated. Here, we tested whether presence of an unpalatable exotic invader Opuntia ficus-indica was associated with the structure of native plant communities in an ecosystem with a long history of grazing by ungulate herbivores."

Odour, et al. (2018), surveyed 17 transects, 1000m long. They compared plot pairs (one invaded plot and one uninvaded plot, 2-5m apart). The plot pairs were 100m apart along the transect. All plant species were recorded and plant covers estimated. They sampled 27 paired plots. Within the invaded plots, the Opuntia cover ranged from 1% to 65%.

"Native species covers were used to calculate indices of community diversity, namely species richness (S), Shannon–Wiener diversity index (H0) and evenness (J). The species richness index S is a measure of the number of unique species within a community, whereas Shannon–Wiener index H0 represents the diversity within each community. Species evenness J is a measure of the relative abundance of the different species making up the richness of a community."

The results revealed "a total of 219 native plant species (54 grass species and 165 forb species) from 50 families. Plots invaded by O. ficus-indica had, on average, a higher number of native plant species than adjacent uninvaded plots (mean species richness S: 25 in invaded vs. 20 in uninvaded plots)... Similarly, invaded plots had significantly greater Shannon–Wiener diversity H0 than adjacent uninvaded plots... Presence of O. ficus-indica was significantly associated with a compositional change in native community assemblages... However, the relative abundance of native species did not differ between the two plot types (similar mean species evenness J between invaded and uninvaded plots)... There was no significant correlation between native plant diversity and percentage plot cover by O. ficus-indica."

​Odour, et al. (2018), state that the data suggests "that the invader O. ficus-indica may, in fact, play an important role in maintaining species and functional diversity within the ecosystem. As long as grazing continues in this ecosystem, patches invaded by O. ficus-indica may serve as a source of propagules to adjacent uninvaded habitats where grazing-sensitive species are not able by themselves to persist under intense grazing."

Few studies have examined the possible positive benefits of an invasive plant species. I would contend that humans are not smart enough to successfully remove invasive plant species without further damaging the ecosystems in which they have become naturalized.

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Copyright: Dr. Delena Norris-Tull, July 2020. Management of Invasive Plants in the Western USA.

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