© Mike Checkett / Ducks-Unlimited

Overview

Biodiversity, or the variety of plants, animals, fungi and microorganisms found in nature, is a critical natural resource for the health of the planet and human society, including agriculture. Supporting diverse organisms requires diverse habitats, many of which can be found in and around farms. Sustainable, productive farming systems ultimately rely on biodiversity. For example, native pollinators provide most of the crop pollination and support resilience where domesticated honeybee populations are facing threats. Integrated pest management is an agricultural management strategy that relies on ecosystems that support sufficient populations of natural pest predators to reduce threats to crops. 

Society at large shares a vested interest in biodiversity. In fact, few sustainability issues are as visible and understandable to people than the preservation of wildlife habitat. Most farms are located in rural landscapes and tend to be near natural forests, prairies, wetlands or deserts that give wildlife a place to forage for food, breed and nest. Farms share these spaces with outdoor enthusiasts who value such areas for hunting, fishing and enjoying nature. Understanding how biodiversity in these regions has changed over time can inform strategies for preserving habitats and managing lands to support species and ecosystems at risk. SECTION 1: Field to Market's Biodiversity Metric One of Field to Market's goals is to support diverse species and ecosystems by conserving and enhancing habitats across U.S. agricultural landscapes. The Fieldprint® Platform assesses biodiversity using the Habitat Potential Index (HPI). HPI scores the potential for a given farm to provide wildlife habitat on land or in the water. HPI scores range from 0 to 100 and measure the level of opportunity to improve or maximize habitat potential. 

The metric provides separate scores for cultivated cropland, pastures and non-cultivated lands such as forests and wetlands, plus an aggregated score for the whole farm. Higher scores are desirable and indicate a greater potential to support wildlife habitat. Scores less than 50% represent significant opportunities for improving habitat potential, whereas values of 50%-80% indicate moderate realized potential and scores greater than 80% demonstrate farms that have maximized opportunities for biodiversity to flourish. Guidance is provided to farmers and advisers on how to maximize the habitat potential of lands that they manage to improve on the score over time. SECTION 2: Biodiversity Indicators Biodiversity and habitat potential are inherently local and challenging to assess at the macro-scale. There is a wide diversity in management practices that influence habitat potential, and we currently do not have nationally available aggregate data that allow trends in such management to be tracked. In the previous edition of the National Indicators Report (Field to Market, 2016) we reviewed change in land cover as an important indirect factor for consideration of habitat and biodiversity potential. We used the USGS Land Cover Trends report to assess changes in the 1980 to 2000 period and additional scientific literature on land cover and land use change. We found increases in overall cropland in the early 1980s, followed by a loss of cropland in the 1990s to urban areas and grasslands, and finally with recent information indicating a new expansion of cropland in the years since 2008 at the expense primarily of grasslands (Field to Market, 2016). 

Since 2016, new resources for assessing biodiversity trends directly have been released. The most authoritative scientific sources are from the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) which has released a regional assessment for the Americas (IPBES, 2018) as well as a report on ecosystem services and food production (IPBES, 2017). IPBES is an independent, intergovernmental body established in 2012 to draw from expertise across scientific disciplines to catalyze the implementation of knowledge-based policies among governments, the private sector and civil society. Other sources of information for this section include scientific research articles and reports from the U.S. Department of Agriculture. SECTION 3: Biodiversity in North America North America supports a wide diversity of species, with approximately 13,000 species of plants, 650 birds, 450 mammals, 300 amphibians and 430 reptile species. However, the overall trends indicate that this diversity is threatened - across the Americas, one quarter of all species are threatened or face the risk of extinction. In North America, more than 75 species of freshwater fishes have become extinct since 1950 (IPBES, 2018).

Trends in Crop Diversity

Agricultural lands can support biodiversity through growing diverse species for food production and taking other measures to support both habitat above ground and habitat for soil micro-organisms that are important for soil health. However, from 1978 to 2012, the diversity of crops grown in the United States dropped significantly (Figure 2.1.1). The lowest crop diversity is found in the American Corn Belt, where high-yielding corn and soybean varieties dominate the agricultural landscape. The trend toward corn and soy is also evident in the Eastern Uplands region of the U.S. which includes portions of Ohio, Pennsylvania, West Virginia, Virginia, Tennessee, Kentucky, Alabama, Mississippi, Oklahoma and Arkansas. Crop diversity remained fairly high and consistent in regions growing most of the fruits and vegetables in the country, as seen in California, Oregon, Washington, New York, Pennsylvania, New Jersey and Massachusetts. The Mississippi Portal region is the only region in the nation to record greater crop diversity over time, attributed, at least in part, to declines in cotton acreage, which may have opened to the door to more diverse crop rotations (Aguilar et al., 2015). Figure 2.1.1. Number of crop species for six ERS Farm Resource Regions for Census of Agriculture years. Numbers represent county averages.

Trends in Bird Abundance and Diversity in North America

The conversion of lands to agriculture in North America has contributed to declines in bird species-between 1966 and 2013, populations of bird species associated with farmland declined by 74%. The most significant decline has been in populations of aerial insectivores, or birds that eat insects on the wing. No single factor has been identified for this decline, but important contributing factors include inadequate insect prey populations on and around farms which is linked to several factors, including agricultural insecticide use, compromised or insufficient surface water, and lack of suitable habitat and forage (Stanton et al., 2018). 

Lands fragmented by agriculture provide less suitable habitat for diverse bird populations. Smaller, discontinuous grassland habitats produce greater edge effects, which leads to greater pressure on bird populations by nest predation compared to larger continuous grassland areas (Stanton et al., 2018). In addition, shrinking wetlands has led to fewer habitats for riparian birds, such as red-wing blackbirds whose young depend on larger habitats to provide more available food. Rangeland or pasture lands, particularly when planted to multi-species grasses, offer better bird habitat than those cultivated in row crops. Farm operations can also cause direct harm to birds through pesticide exposure, soil preparation, planting, tillage, mowing and other harvest practices that can directly kill birds and destroy nests. 

Farmers can reduce these risks and support avian biodiversity in their regions with the right information. For example, in soybean fields bird mortality can be almost eliminated by delaying planting by two weeks, which allows time for the young to fledge before the equipment destroys the nests. However, there are tradeoffs with production as such a delay may lower crop yields. Greater density and diversity of nesting birds can be found in no-till corn and soybeans, compared to those that are conventionally tilled, most likely because the standing vegetation offers greater cover from predators. In forage crop production, planning the timing of mowing to avoid cuttings during the breeding season can reduce the risk to birds. This can be a challenge in some systems, such as for cool season forages because they produce more biomass earlier in the season, sometimes necessitating mowing before young birds have fledged. Mowing exposes young birds to direct destruction from equipment and exposure to predators resulting from less plant dense plant cover. Wet years, though they depress yields, are associated with greater nest success in certain bird species due to fewer and later cuttings of forage crops (Stanton et al., 2018).

Trends in Pollinator Abundance and Diversity in North America

Pollination is a necessary process for crop seed production and may be mediated by non-living factors such as wind or water, or by living pollinators, including bees, butterflies and birds. Due to this, pollination was the subject of a special report by IPBES (IPBES, 2017). Worldwide, approximately 75% of all cultivated crops depend on living pollinators. In 2012, the value of pollination for food production was estimated at $351 billion globally. Although some commodity crops commonly cultivated in North America self-pollinate, like cotton and soybeans, or are wind-pollinated like corn and sorghum, other crops such as alfalfa and many specialty crops depend on insect pollinators for reproduction. 

In the United States, managed pollination by domesticated, non-native honeybees (European or Western honeybees and Asian or Eastern honeybees) is common. Commercial production of cherries, blueberries, almonds, tomatoes and watermelon are dependent on managed Western honeybee hives. These hives have been under threat of colony loss, which has been escalating since 1990. These losses have been largely attributed to infestation by the parasitic varroa mite, which feeds on bee pupae and transmits viruses that compromise the bee's immune system leaving them vulnerable to other transmittable disease (IPBES, 2017). 

Although a few species of domesticated honey and bumble bees are widely deployed to pollinate crops, there are at least 20,000 species of other organisms that provide pollination services, many of which are wild bees. Between 2008 and 2013, wild bee abundance decreased by 23% in the United States, mostly in the agricultural lands of the Midwest, Great Plains and Mississippi River valley. Threats to wild and domesticated bees are tied to land use changes, changing climate and pesticide toxicity, among others. IPBES (2017) also found that using an integrated approach to pest management can decrease the amount of insecticide applied and reduce the risk of exposing pollinators. 

Land use changes lead to habitat fragmentation and loss for wildlife, including pollinators. A USDA report (Hellerstein et al., 2017) reviewed the impacts of these land use patterns on pollinators and found that the ability of a given area to support pollinators is highest in forests, rangeland, rural roadsides and certain farmland that produces sunflowers and berries. This ability is lower in lands producing cotton, soybeans, nuts and grapes, and further reduced in farmland producing corn, wheat, rice, barley and sorghum. While there is not a clear national trend, regional patterns in this ability as measured by a Forage Suitability Index (FSI) have been observed. Nationwide, from 2002 to 2012, FSI remained the same in 85.8% of land, improved in 6.9% and decreased in 7.3%. Decreases were concentrated in areas of Central California, Montana, North and South Dakota, Iowa, Illinois, Indiana, northeast Nebraska, northwest Kansas and along the Mississippi River, with the most significant decreases occurring in the Dakotas, which are primary summering grounds for honeybees. The greatest improvements from 2002 to 2012 were observed in Washington, Nevada, eastern Kansas, Oklahoma and eastern Texas (Hellerstein et al., 2017).

Species Highlight: Monarch Butterflies

The plight of the monarch butterfly (Danaus plexippus) is well known outside of the agricultural community. The butterfly is famous for long migrations and tendency to overwinter en masse in trees and structures along the coast in California and in the mountains of Central Mexico. Populations of the monarch have dropped precipitously since 1950, which has been directly linked to commensurate declines in the 10 milkweed species utilized as their larval host plants. Declines in milkweed populations are related to land use changes. For example, one of the monarch's preferred hosts, swamp milkweed (Asclepias incarnata), lives in wetlands that have been in decline in the U.S. A study in Illinois found a nearly 94% decrease in common milkweed populations (A. syriaca) after croplands were treated with glyphosate, but no significant losses in areas treated with non-glyphosate herbicides. In Iowa, that trend is replicated: milkweed populations dropped steeply since 2000, with some plant populations extirpated following glyphosate treatments (Zaya et al., 2017). 

Weeds are loosely defined as a plant growing where it is not wanted by the land manager. Allowing milkweed and other weedy plant populations to become established in cultivated areas is not feasible for several reasons. Weeds can compete with crops for space, light, water and nutrients, may interfere with harvest operations and their seeds may contaminate harvests. Landowners and managers are encouraged to allow milkweeds and other pollinator host plants to grow and complete their lifecycles in non-cultivated areas of the farm and adjacent landscapes. SECTION 4: Effects of the USDA Conservation Reserve Program on Biodiversity in U.S. Croplands

© Mark Vandever - USGS

While there are ways in which agricultural lands can be managed to support biodiversity and reduce their impact on important species, setting aside sensitive lands out of production and providing areas in the agricultural landscape for diverse species to flourish can also be an option where it aligns with the needs and goals of a farm operation. Here we look at the trend over time in lands enrolled in the USDA's Conservation Reserve Program (CRP), which was established in 1985 to "retire highly erodible and other environmentally sensitive cropland and pastureland" from production (USDA, 2020). 

Farmers receive yearly rental payments from the USDA over a period of 10-15 years, during which time valuable land cover, such as prairie grass, is re-established. While the principal motivation is to reduce soil erosion, there are additional conservation benefits, including supporting diverse plants and ecosystems. Land area in CRP rapidly increased in the early years of the program and stayed relatively constant from the early 1990s through the mid-2000s (Figure 2.1.2); however, enrollment began to decline after 2007. This decline is partly attributed to higher crop prices, which serve as an incentive to bring the set aside lands back into production, as well as reductions in the funds available for the CRP program. As of November 2020, there were over 300 thousand farms enrolled in CRP, covering over 20 million acres with a total financial outlay of more than $1.8 billion in combined rental, cost-share and incentive payments. Figure 2.1.2. Cumulative enrollment in Conservation Reserve Program How CRP land is managed is important in determining how well it will be able to support biodiversity. For example, in the Southwest High Plains, lands enrolled in CRP are frequently seeded with non-native grasses, resulting in low plant species diversity and low-quality forage and habitat for pollinators. Patches of land in CRP that are greater in area than 100 acres and planted with diverse plant species have been shown to support pollinators and other wildlife (Begosh et al., 2020). SECTION 5: SUMMARY No singular measure can provide an adequate understanding of the trend in biodiversity in and near agricultural fields. However, studies show that bird and insect populations in the U.S. have been declining for decades while at the same time native tallgrass prairies have been nearly eliminated. Further, the diversity of crops grown has declined overall, with the exception of the Mississippi Portal region. Together, these results signal an overall negative trend in biodiversity in and around farms. 

Habitat loss continues to be a primary threat to biodiversity, not just in the U.S., but worldwide. The declining numbers of acres set aside in the Conservation Reserve Program over the past 10 years contribute to challenges of efforts to restore habitat and protect biodiversity on farms. With increasing reliance on chemical management with non-selective herbicides, populations of certain pollinator larval host plant, like milkweed, have been significantly reduced. 

Agricultural landscapes across the U.S. have opportunities to support diverse and native species and ecosystems that provide important ecosystem services to humanity. Understanding the trends in both the diversity and abundance of species can help to identify what management practices lead to the greatest risk of further biodiversity loss and also what can be done to prevent loss and transform landscapes to support regeneration of biodiversity in agricultural landscapes. This section has highlighted some of the notable trends and what those imply for the risks to biodiversity associated with agriculture in the U.S. and begun to explore what management practices can reduce risk and support greater abundance and higher biodiversity of species. Biodiversity is a natural resource concern of national and global consequence, but it is inherently local. Farmers, with a deep understanding of their lands, are well positioned to identify both the risks and the best mitigation strategies that align with using the land to produce food, fiber, feed and fuel.