Livestock grazing is the widespread agricultural use of natural and seminatural landscapes throughout the world. Although estimates vary, as much as 50 percent of the world’s land area is grazed by domestic livestock [1
]. People have relied on grazing livestock as a source of high-quality protein for thousands of years, especially in ecosystems not usable for cultivation due to a lack of water, poor soils, harsh climate, rough topography, or high elevation. Throughout their history, grazing domestic livestock have generally shared grasslands with wild grazers and a host of other wild animals. Traditionally taking place on lands that are not arable, livestock producers are adapted to rearing animals on lands in a seminatural condition, sometimes with natural or anthropogenic fire, and other land management practices to keep woody vegetation in check and improve the forage for both wild and domestic grazers.
Despite technological and production shifts initiated in the twentieth century, the life cycle of commercial beef cattle in the United States still most often includes a significant period on grazing lands [2
]. Such lands have been promoted for conservation through land sharing under the rubric of “working landscapes” [4
]. Land sharing, which encompasses wildlife-friendly farming practices, integrates biodiversity conservation with agricultural production on the same land [6
]. To better understand the relationship between livestock grazing and species conservation from land sharing, this study assesses the current impacts of livestock grazing, detrimental and beneficial, on the conservation of federally listed plant and animal species in California as stated in listing documents published in the United States Federal Register.
Listing documents used to implement the US Endangered Species Act of 1973 (ESA) identify plants and animals vulnerable to extinction, designate their critical habitat, and inform their recovery, including recognizing threats to the species and their habitats. The United States Fish and Wildlife Service (USFWS) administers and enforces the ESA for terrestrial species. The USFWS is required to use scientifically valid information to describe reasons for a species’ demise and recommend actions for its recovery. The descriptions as outlined in Section 4
(a) (1) of the ESA consider five factors: (A) habitat loss, (B) overutilization, (C) disease or predation, (D) inadequate regulatory mechanisms, and (E) other natural or human-made factors affecting a species’ survival. The impact of livestock grazing on a listed species and its associated habitat is included in the USFWS’s analysis of the five factors when livestock grazing occurs with the species or within its habitat. The information provided is the current state of knowledge and continually changing; when new information is learned about a species’ needs and survival, the five-factor information is updated through 5-year reviews and recovery plans.
A previous highly cited study, Wilcove et al. [8
], used ESA listing information published in the US Federal Register to quantify threats to listed species. They found 22% of all US-listed endangered, threatened, or candidate species (n = 1207) were impacted by habitat degradation or destruction resulting from livestock grazing. This finding is similar to Czech et al. [9
], which compared Federal Register documents and World Wildlife Fund compendium data for causes of the endangerment of 877 species. They found 16.0% were threatened by livestock grazing and 20.8% of the species by ranching, respective of the data source.
Both Wilcove et al. and Czech et al. provided little explanation of livestock grazing’s role in species decline. Wilcove et al. [8
] categorized grazing as an extractive land use along with logging and mining. Czech et al. [9
] found a strong relationship between grazing threats and non-native species, which they noted was at least, in part, explained by grazing’s modification of plant and animal community composition. However, there is a growing body of research from grazed lands worldwide that recognize the potential of domestic animal grazing to modify or maintain habitat in ways that benefit certain species [10
]. In recent reviews, for example, positive impacts from livestock grazing maintaining open habitats were found for amphibian [10
] and small mammal [11
Livestock grazing affects biota and their ecological systems in varied and complex ways. Through consumption, physical impact, and nutrient redistribution, grazing livestock can change vegetation, soils, and habitats [17
]. Although these effects of grazing are well-known, whether they have negative or positive impacts in a particular ecosystem and on a specific plant or animals species depends on the species, the ecosystem, the current environmental conditions, as well as the management of the livestock and their grazing [18
]. In terms of livestock’s influences on ecosystems and threatened and endangered species, herbivory should not be treated as a “black box”.
Despite the varied and contrasting impacts of grazing, to date, there is no assessment of threats to imperiled species that considers livestock grazing’s varied impacts on specific species and their habitats, positive as well as detrimental. If nothing else, Wilcove’s concerns with the importance of conservation on private lands and the need for active management [21
] calls for such an assessment since so much land in a natural and seminatural state is owned by ranchers, who rely on grazing for income and use it as a tool of active habitat management.
California is well suited for such an assessment. First, with a varied climate and topography and a growing population, the California Floristic Province, which includes most of the state and small parts of adjacent areas in Oregon, Nevada, and Baja California, Mexico, is one of the world’s most biologically rich and endangered ecoregions; it is a globally recognized biodiversity hotspot [22
]. Six thousand one hundred forty-three plant species are native to the province; 42% of these species are endemic [23
]. California has more federally listed species (282 in 2017) than any other state in the continental United States. With so much biodiversity at risk, threats to species and recovery activities have been identified for a large number of plants and animals across many species groups.
Second, despite California’s rich biodiversity, non-native annual grasses and forbs or broad-leaved plants are widely established and dominant across most of California’s rangelands. These annuals are part of a novel ecosystem with large numbers of species from Mediterranean environments worldwide, many originating in the “Fertile Crescent”, a domestication hearth [24
]. Biological introductions, both intentional and unintentional, are pervasive, impacting native species and ecosystems in California and throughout the world [27
Third, like much of the western United States, livestock grazing across the state is extensively managed, with about 40% grazed by livestock to some extent [28
]. While California’s Mediterranean climatic zone, which includes grasslands, hardwood woodlands, and chaparral, provides most of the forage consumed by livestock, two other zones also support extensive livestock production. The cold desert steppe, which is characterized by sagebrush grassland and pinyon-juniper woodlands, and the warm desert, which includes barren lands and, at higher elevations, montane meadows and conifer forest, provide seasonal grazing, primarily for cattle [29
Another reason that California is well suited for an assessment of grazing impacts on listed species and associated habitats is that it is a natural laboratory to study land sharing. Although livestock grazing in the western United States has a relatively short history (<300 years) compared to Europe and Asia, native species found on California’s rangelands evolved with herbivory by now-extinct megafauna, including medium to large herbivores, such as ground sloth, bison, camel, horse, mammoth, mastodon, and ox [30
]. Like in other parts of the world, conservation efforts in California increasingly consider livestock grazing a way to maintain extensive natural landscapes and their native diversity [31
], expanding available habitat well beyond the possibility for nature reserves. Land sharing provides an income to landowners through livestock production and reduces the risk of development or land-use conversion [28
]. While land sparing strategies emphasize separating nature conservation and agriculture, relying on intensification of agriculture on smaller areas to “spare” conservation areas, land sharing strategies seek to accomplish both biodiversity conservation and agriculture within the same landscape [33
]. This strategy is often presented as requiring trade-offs, where either agricultural yields or species conserved are reduced [34
]. For instance, Butsic and Kuemmerle [35
] have suggested considering land-sharing and land-sparing across a spectrum where agricultural yields and species conservation are optimized depending on the ecosystems and species. Given the limited feasibility of intensifying agricultural production on many grazing lands [36
], including California’s rangelands, livestock grazing may provide a decisive land-sharing opportunity where conservation is not compromised for agricultural production and species conservation and recovery for some species is enhanced.
A quantitative assessment from USFWS listing documents for multiple species across a wide variety of habitats grazed by domestic livestock allows detection of patterns that may explain the varied and sometimes contradictory responses from livestock grazing observed and often reported in the literature. This assessment of all federally listed species in California addresses the following questions (i) What is the role of grazing in the conservation of federally listed species? (ii) Does grazing’s role in species conservation differ by taxonomic groups and across different ecosystems? Moreover, (iii) What are the specific reasons that plants or animals are threatened by or benefit from grazing? These three questions allow us to understand better land sharing’s potential for conservation on livestock grazing lands in California. It is important to keep in mind that “grazing” occurs on all rangelands, by everything from caterpillars to elephants depending on location. However, in this paper, the term grazing refers to the agricultural pursuit of managed livestock grazing unless otherwise stated.
2. Materials and Methods
Federal Register documents published by the USFWS for all federally listed threatened or endangered species (182 plants and 100 animals) in California as of 1 December 2017, were reviewed. A searchable database of ESA documents developed by the Center for Conservation Innovation of the Defenders of Wildlife [37
] was initially used to identify species associated with grazing. ESA listing documents search through the database included the final rule for listing, 5-year reviews, and recovery plans.
Federally listed species were determined to be associated with grazing if “graz*”, “overgraz*”, or “trampl*” appeared in the text of at least one of the listing documents associated with an imperiled species in California (Table 1
). Mentions of “grazing” and “trampling” were verified as being related to domestic livestock versus wild or feral animals. The term “livestock” was searched, but it did not yield additional species affected by grazing livestock or ranching. Other terms for herbivory, “browsing” and “foraging”, were also searched but were not found to be used in the USFWS documents reviewed to describe interactions with livestock.
From the search results, an Excel database was developed with the document’s title, type, date, web URL, and the associated listed species, as well as statements from the text regarding grazing (Data S1
). The data were sorted by species and date, and the newest listing document associated with each species with a grazing interaction identified. The complete listing documents were accessed from the Environmental Conservation Online System (ECOS), managed by the USFWS [38
]. The most recent listing document was typically a 5-year review, but a recovery plan or the final rule was the latest in some cases. From listing documents, species type (plant or animal), animal species group, plant guild, and date of the latest listing document were recorded for each species. Information on the species’ terrestrial and aquatic habitat was obtained from NatureServe Explorer Species reports under ecological and life history [39
]. NatureServe Explorer Species reports, a product of NatureServe in collaboration with the Natural Heritage Network, are referenced on the USFWS ECOS website as an authoritative source of additional species information. NatureServe reports terrestrial habitat for species based on US National Vegetation Classifications at the formation level [40
]. Some species are found in multiple terrestrial or aquatic habitats, and all habitats were recorded for each species. From the NatureServe listing, the primary terrestrial and aquatic habitats, if applicable, were recorded for each listed species and included in the Excel database (Data S1
The USFWS uses various statements to describe the effects of livestock grazing on listed species and their habitats (Table 1
). Statements on grazing were coded and categorized. A category for a grazing benefit and a grazing threat was independently assigned for each listed plant and animal species. Stated threats from “grazing”, “overgrazing”, or “trampling” were categorized as grazing threats. The threat of “loss or cessation of grazing” was considered to indicate a benefit from grazing. In USFWS documents, this threat results from lack of grazing leading to habitat degradation because grazing is acting to maintain habitat structure or function. “Inappropriate grazing” was typically categorized as both an indication of potential threat and benefit from grazing. The USFWS generally uses inappropriate grazing to mean that grazing at the wrong time or intensity is detrimental to the species or its habitat, while grazing at the right time and intensity may be beneficial. The benefit of appropriate grazing is indicated by statements like, “too little grazing is detrimental to the species or its habitat”.
For species with mentions of grazing in initial listing documents but no statement on grazing’s threat or benefit in the newest listing document, the categories, “no grazing threat” and “no grazing benefit” were used. The category “no grazing threat” also includes species where the USFWS states that “grazing is not a threat”. The statement “grazing is a not threat” typically reflects a change in the USFWS’s understanding of grazing’s effect on a listed species, where previous listing documents had mentioned grazing as a threat to the imperiled species or its habitat.
“No longer a factor”, “other grazing threat”, “no current grazing threat”, and “island species” are additional categories used in this study that describe the stated relationship between grazing and a listed species in listing documents (Table 1
). The USFWS states that grazing is no longer a threat when grazing no longer occurs within the species’ habitat, often due to protections that restrict grazing, e.g., reserve status. “Other grazing threatens” was used when listed plants or animals are not known to be impacted by domestic livestock, but the stated threat is from wild or feral herbivores, horses, or burros. “No current grazing threat” was assigned to species where the presence of livestock grazing was noted as a threat in early listing documents, e.g., USFWS final listing rule, but in the most recent documents, e.g., five-year reviews, livestock grazing was not mentioned.
Both “no longer a factor” and “other grazing threat” describe the threat of grazing to listed species endemic to the Channel Islands off the coast of California, but because of the unique grazing history of these islands, the 21 plants and three animals found only on the islands are categorized as “island species”. Grazing threats were primarily from uncontrolled grazing by feral sheep and goats, deer, bison and elk. These animals were introduced to the islands and were generally not managed; they have now been nearly entirely removed.
Although initial results identified 209 species or 74 percent of the listed species in California where grazing is mentioned, the review of most recent documents found a current livestock grazing relationship with 143 species or 51 percent of the listed species (Table 2
). Further analysis of grazing threats and benefits for this assessment was only considered for species with a current livestock grazing relationship.
The association between grazing’s role on listed species and species attributes, including type, i.e., animal versus plant, animal species groups, plant guilds, and habitat types within terrestrial and aquatic systems, was determined using Pearson’s chi-squared tests. Calculating the chi-squared statistic and comparing it against the chi-squared distribution indicates whether the observed pattern of responses is significantly different from expected if the variables were truly independent of each other [41
]. In this case, Pearson’s chi-squared allows us to determine if grazing’s role as a threat or benefit is independent of species’ type, animal group, plant guild and habitat type.
To illuminate the specific assertions about the nature of grazing impacts beyond the broad categories of threats and benefit, reasons for grazing’s stated role in the newest listing documents were coded and categorized and included in the database (Data S1
). Multiple reasons for grazing’s benefit or threat are recorded for some species. Direct threats to an individual animal or plant or its natal site were differentiated from indirect impacts to habitat or ecological processes, e.g., plant succession, impacts to soil and water quality. All benefits were identified as indirect impacts.