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1Elementa: Science of the Anthropocene • 3: 000041 • doi: 10.12952/journal.elementa.000041
elementascience.org
Enhancing agroecosystem performance and
resilience through increased diversication
of landscapes and cropping systems
Matt Liebman 1* • Lisa A. Schulte2
1Department of Agronomy, Iowa State University, Ames, Iowa, United States
2Department of Natural Resources Ecology and Management, Iowa State University, Ames, Iowa, United States
*mliebman@iastate.edu
Over the past two decades, ecologists have gained a considerable amount of insight concerning the eects
of biological diversity on how ecosystems function. Greater productivity, greater carbon sequestration,
greater retention of nutrients, and greater ability to resist and recover from various forms of stress, including
herbivorous pests, diseases, droughts, and oods, are among the eects of increased biological diversity noted
in a recent review by Cardinale et al. (2012). e latter eect, often called resilience, is particularly important
in managed social-ecological systems, including agroecosystems (Walker and Salt, 2006). In addition to
being better able to withstand and recover from disturbances due to pests, weather, and other biophysical
factors, resilient agroecosystems can be less susceptible to uctuations in production costs and market prices
(National Research Council, 2010; Kremen and Miles, 2012).
In general, the relationship between biological diversity and ecosystem function resembles an asymptotic
hyperbola (Cardinale et al., 2012). That is, increases in the number of species present in an ecosystem from
a very low level to some intermediate level engender large changes in ecosystem function, whereas increases
in species richness above some intermediate, and undetermined, value engender smaller effects.
Another way to look at biodiversity-ecosystem function relationships is through the lens of losing species
diversity. Professor Shahid Naeem of Columbia University uses the following analogy to examine how many
species might be lost from an ecosystem before critical functions are no longer available: Imagine you have
a computer on your desk that works well. Now open it, reach in with a needle nose pliers and randomly
remove five of the many parts of the motherboard. Do you expect the computer to continue to function well
after the loss of those parts?
The development of modern, industrial agriculture has been characterized by large reductions in biological
diversity, both across landscapes and within farming systems (DeFries et al., 2004; Vandermeer et al., 2005).
This loss of biodiversity is particularly evident in the U.S. Corn Belt. Where species-rich prairie grasslands,
wetlands, and oak savannas once grew, corn and soybean now dominate (Klopatek et al., 1979). Farming
systems that once contained small grains, hay, and pasture in addition to corn and soybean now contain almost
exclusively the latter two crops (Hatfield et al., 2009; Brown and Schulte, 2011; Johnston, 2013). In Iowa,
which has lost proportionally more area of its native vegetation than any other U.S. state (Klopatek etal.,
1979), corn and soybean now occupy 63% of the state’s total land area and 82% of its cropland (National
Agricultural Statistics Service, 2014).
Simplification of crop and non-crop vegetation in the Corn Belt has been a strategy pursued through
decisions and actions of individual farmers and through federal and state policies, with a goal of producing
huge amounts of corn, soybean, chickens, cattle, hogs, ethanol, and farm revenue (Durrenberger and Thu, 1996;
Secchi et al., 2009; Nassauer, 2010; McGranahan et al., 2013). It has also been concomitant with simplification
of management strategies and increases in scale ( Johnston, 2013; McGranahan, 2014). Nonetheless, despite
impressive gains in farm productivity and revenue, Corn Belt agricultural systems and the region’s residents
are threatened by a number of emerging and continuing challenges, including soil erosion, water quality
degradation by nutrient and pesticide emissions, greater prevalence of herbicide-resistant weeds, volatility
in production costs and crop prices, loss of knowledge and infrastructure to support diverse markets, and
declines in rural community vitality (Tegtmeier and Duffy, 2004; Alexander et al., 2008; Sullivan et al., 2009;
Brown and Schulte, 2011; Sprague et al., 2011; Mortensen et al., 2012; Heathcote et al., 2013). Perhaps most
Domain Editor-in-Chief
Anne R. Kapuscinski, Dartmouth
Associate Editor
Ricardo J. Salvador, Union of
Concerned Scientists
Knowledge Domain
Sustainability Transitions
Article Type
Commentary
Part of an Elementa
Forum
New Pathways to Sustainability
in Agroecological Systems
Received: September 24, 2014
Accepted: January 15, 2015
Published: February 12, 2015
Diversity affects agroecosystem performance and resilience
2Elementa: Science of the Anthropocene • 3: 000041 • doi: 10.12952/journal.elementa.000041
emblematic of the Corn Belt’s environmental effects is the annual discharge of nearly one million metric tons
of nitrogen into the Gulf of Mexico from agricultural lands lying upstream in the Mississippi River Basin,
leading to a large coastal hypoxic zone (Alexander et al., 2008; Broussard and Turner, 2009).
Much of the dysfunction of industrial agriculture in the Corn Belt derives from the low levels of biologi-
cal diversity now present across landscapes and within farming systems in the region (Broussard and Turner,
2009; Liebman et al., 2013; Asbjornsen et al., 2014). Of particular importance is the fact that shallow-rooted,
short-season crops like corn and soybean have replaced native, perennial species whose deep roots and long
growth period from early spring to late fall are much more effective in holding soil in place, promoting water
infiltration into soil and transpiration into the atmosphere, fostering carbon sequestration and nutrient retention,
and providing habitat for pollinators, biological control agents, and a host of other organisms (Asbjornsen et
al., 2014). The consequences of this shift in vegetation are illustrated by results from an experiment conducted
in Illinois comparing nitrate-N losses to drainage water from two annual crops—corn and soybean—and a
reconstructed, multispecies prairie community harvested for biomass. After a two-year establishment period
for the perennial prairie species, loss of leached N was 9- to 18-fold greater from the annual crops than from
the prairie community (Smith et al., 2013).
The effects of integrating diverse, deep-rooted communities of perennial plants into landscapes and
watersheds dominated by row crops are being investigated in experimental watersheds in central Iowa in
which strips of reconstructed prairie have been interwoven into corn and soybean fields (Figure 1). As shown
in Figure 2, there was a 95% reduction in sediment export, a 90% reduction in total phosphorus export, and
an 85% reduction in total nitrogen export from watersheds containing 10% prairie when compared to 100%
row-crop watersheds managed without tillage (Helmers et al., 2012; Zhou et al., 2014). Additional benefits
Figure 1
Science-based Trials of Rowcrops
Integrated with Prairie Strips
(STRIPS) experiment at the Neal
Smith National Wildlife Refuge,
Iowa.
In the STRIPS experiment, small
amounts of reconstructed native
prairie vegetation have been integr-
ated into row crop elds to improve
the performance and increase the
resilience of agricul tural water-
sheds. Image credit: Anna
MacDonald.
doi: 10.12952/journal.elementa.000041.f001
Figure 2
Summary of performance results
from the STRIPS experiment.
Ratio of performance indicators
in watersheds with prairie strips
(10% prairie strips and 90% row-
crops) to performance indicators
in watersheds without prairie
strips (100% row-crops). Soil
sediment data are from Helmers
et al. (2012); phosphorus,
nitrogen, and rainfall runo data
are from Zhou et al. (2014); crop
data are unpublished; bird data
are from MacDonald (2012);
and plant data are from Hirsh
etal. (2013). Data were collected
during 2008-2012 from three
replicate watersheds for each
treatment.
doi: 10.12952/journal.elementa.000041.f002
Diversity affects agroecosystem performance and resilience
3Elementa: Science of the Anthropocene • 3: 000041 • doi: 10.12952/journal.elementa.000041
for biodiversity conservation of plants and birds have been documented in these experimental watersheds
(Figure 2; Hirsh et al., 2013; Liebman et al., 2013). The average annual cost of treating a farm field with
prairie conservation strips ranges from $60 to $85 per treated hectare, making it one of the least expensive
conservation practices available to landowners and farmers (Tyndall et al., 2013).
Results from the prairie strips study emphasize that biodiversity-ecosystem function relationships can be
highly non-linear: small changes in the proportion of land area used for prairie rather than row crops gave
disproportionately large conservation benefits. Such benefits may be increasingly important as a shift in the
region’s climate regime toward a greater frequency of high intensity rainfall events threatens agroecosystem
resilience by increasing soil erosion and crop damage, even in zero-tillage systems (Rosenzweig et al., 2002;
Angel et al., 2005; Pryor et al., 2014). On the other hand, though yields of corn and soybean per cropped hectare
were unaffected by the presence of the prairie conservation strips, total production of corn and soybean were
reduced 10% due to the substitution of prairie vegetation for crops. Thus, increases in soil, water, and nature
conservation involved a trade-off with crop production. While the prairie strips study compares row-crop
fields containing diverse, native, perennial vegetation with crop fields without diverse prairie communities,
other studies have shown that compared with annual crops, monocultures of perennial species can also confer
substantial environmental benefits; the type and level of benefit varies with plant species and management
(Asbjornsen et al., 2014).
The degree of diversification within cropping systems can have important effects on crop productivity. A
recent review by Bennett et al. (2012) found yield reductions from 3 to 57% for major crops grown in short
rotation sequences and monocultures relative to yields in extended rotation sequences that included multiple
crop species. Lower productivity in less diverse rotations was attributed to numerous interactive factors,
including increased prevalence and greater damage from insect pests and weeds, deleterious interactions with
soil microbes and nematodes, soil compaction, nutrient depletion, self-inhibition due to toxic compounds
from plant exudates, and reduced soil water availability. Alternatively, cropping system diversification through
the use of multispecies crop rotations can maintain soil fertility and productivity, suppress pests, and increase
yields even in situations where substantial amounts of fertilizers and pesticides are applied (Karlen et al., 1994).
Rotation systems foster diversity not only in time, but also in space, since different crops within the rotation
sequence are typically grown in different fields on a farm in the same year. Diversification through crop rotation
can be an especially useful strategy in farming systems that integrate crop and livestock production, through
the production of perennial forage crops and the application of manure on crop fields (Russelle et al., 2007).
Table 1. Inputs, aquatic toxicity, yields, weed biomass, and net returns for the three cropping systems in the Marsden
Farm rotation experiment, Boone Co., IA, 2006–2011a
Cropping system
Metrics: 2-year rotation:
Corn-soybean
3-year rotation:
Corn-soybean-oat
+ red clover
4-year rotation:
Corn-soybean-oat
+ alfalfa-alfalfa
Whole rotation:
Mineral N fertilizer inputs, kg N ha-1 yr-1 80 a 9 b 7 b
Herbicide inputs, kg active ingredients ha-1 yr-1 1.78 a 0.07 b 0.05 b
Herbicide aquatic toxicity, comparative toxic units 21973 a 74 b 56 b
Fossil energy inputs, GJ ha-1 yr-1 8.9 a 4.0 b 4.1 b
Labor requirements, hr ha-1 yr-1 1.7 c 2.8 b 3.6 a
Net returns to land and managementb, $ ha-1 yr-1 954 965 913
Crop yields:
Corn, Mg ha-1 12.3 b 12.6 a 12.9 a
Soybean, Mg ha-1 3.4 b 3.7 a 3.8 a
Oat, Mg ha-1 — 3.5 b 3.6 a
Alfalfa, Mg ha-1 — — 8.9
Weed biomass:
In corn, kg ha-1 2.9 6.2 5.5
In soybean, kg ha-1 0.8 2.4 2.1
aWithin rows, means followed by dierent letters are signicantly dierent (P < 0.05); means not followed by letters are statistically
equivalent. Data are from Davis et al. (2012).
bCrop subsidy payments were not included as sources of revenue.
doi: 10.12952/journal.elementa.000041.t001
Diversity affects agroecosystem performance and resilience
4Elementa: Science of the Anthropocene • 3: 000041 • doi: 10.12952/journal.elementa.000041
Davis et al. (2012) found that diversification of simple corn-soybean cropping systems with small grain
crops and perennial forages can be a viable strategy for reducing reliance on mineral fertilizers, pesticides,
and fossil fuel inputs, while maintaining or improving crop yields, profitability, pest suppression, and envi-
ronmental quality (Table 1). Compared with a conventionally managed corn-soybean system, more diverse
rotation systems (corn-soybean-oat/red clover and corn-soybean-oat/alfalfa-alfalfa) treated periodically with
cattle manure used 90% less mineral nitrogen fertilizer, 97% less herbicide, and 54% less fossil energy, while
producing corn yields that were 4% higher, and soybean yields that were 10% higher. Weed suppression was
effective in all systems, but herbicide-related aquatic toxicity was two orders of magnitude lower in the more
diverse systems. When calculated over all crop phases, net returns to land and management were equivalent
for each system, though labor requirements were greater for the more diverse systems.
Biological diversity contributed in multiple ways to the successful functioning of the more diverse rotation
systems examined in this experiment. For example, though oat added relatively little revenue to the more
diverse systems (Liebman et al., 2008), it served as an effective companion crop for establishing red clover
and alfalfa, thereby minimizing erosion and reducing weed growth in the absence of herbicides. Forage crops
were generally less profitable than corn (Liebman et al., 2008), but their inclusion in the more diverse systems
allowed substantial reductions in the amount of mineral nitrogen fertilizer used for corn production (Fox
and Piekielek, 1988; Morris et al., 1993) and contributed to greater nitrogen retention (Drinkwater etal.,
1998; Tomer and Liebman, 2014). Integration with livestock, through forage harvest and manure return,
fostered nutrient balance and further reduced production costs (Davis et al., 2012). Finally, diversifying
the corn–soybean system with small grain and forage crops increased the diversity of habitats available to
insects and rodents that preyed upon weed seeds, which is likely to have stabilized seed predator popula-
tions and increased their effectiveness in suppressing weed population growth under conditions of reduced
herbicide inputs (Westerman et al., 2005; Heggenstaller et al., 2006; O’Rourke et al., 2006; Williams et al.,
2009). Spreading the burden of weed control over multiple tactics through diverse rotation systems and their
attendant management practices is a key strategy for retarding the evolution of herbicide resistance in weeds,
and is particularly relevant to the management of glyphosate-resistant weed species, which are increasingly
prevalent in the Corn Belt and which present a clear example of a human-induced challenge to agroecosystem
resilience (Mortensen et al., 2012).
Although we have presented just two case studies of how diversification might be used to enhance agro-
ecosystem performance and resilience in the U.S. Corn Belt, studies conducted in other regions also support
diversification as a key principle underpinning the design of multifunctional agroecosystems that provide a
wide range of goods and services while protecting environmental quality (Altieri, 1995; Kremen and Miles,
2012; Asbjornsen et al., 2014). Additional options for diversifying landscapes and cropping systems include the
use of mixed species pastures for dairy and beef production (Sulc and Tracy, 2007); perennial grains for food
and feed production (Cox et al., 2006); cover crops to fill otherwise unoccupied temporal niches (Snapp et
al., 2005); dedicated perennial grasses and native mixed-species communities for biofuel feedstock production
(Heaton et al., 2013); herbaceous and woody species for reconstructing wetlands (Zedler, 2003) and riparian
corridors (Schultz et al., 2004); and trees for agroforestry plantations ( Jose et al., 2012).
Given the broad portfolio of diversification options that are, or soon could be, technically feasible, how
might greater diversification be implemented? Currently, weak markets and a lack of marketing infrastructure
impede the production of ‘alternative’ crops in areas dominated by only one or two commodity crops. Thus,
in addition to the need to supply farmers with necessary technical information and inputs for producing
non-traditional crops, planning for generating a critical mass of producers and the development of expedited
paths to markets are needed. This is particularly true in the case of ‘second-generation’ bioenergy crops, for
which new biomass collection strategies and processing facilities are needed (Heaton et al., 2013).
Failure to recognize and prevent the costs of environmental degradation incurred by current patterns of
agricultural land use penalizes citizens downwind and downstream of regions of intensive commodity pro-
duction, as well as those in future generations dependent upon unpolluted air, clean and abundant freshwater
resources, productive soils, abundant pollinators, and other components of resilient ecosystems. Diversifica-
tion of agricultural landscapes and cropping systems offers one of the best and most accessible strategies for
resolving the seemingly intractable tension between agricultural production and environmental quality (Boody
et al., 2005; Jordan and Warner, 2010; Liebman et al., 2013; Asbjornsen et al., 2014; McGranahan, 2014).
Of particular importance is the fact that substantial numbers of agricultural stakeholders are interested in
reconfigurations of landscapes and cropping systems in ways that enhance resource conservation and biodiver-
sity. Nassauer et al. (2011) examined the attitudes of Iowa farmers and farmland investors toward alternative
land management systems ranging from maintenance of status quo patterns of corn and soybean production
to a shift toward greater perennial cover, either as a part of rotational grazing systems or through greater use
of conservation buffer strips. Under the assumption that all scenarios were equally profitable, less than 25%
of the farmers and fewer than 10% of the investors ranked the status quo scenario most preferable. Boody
etal. (2005) conducted a statewide survey in Minnesota to determine how much residents were willing to
pay to reduce environmental impacts of agriculture in a manner consistent with the effects of greater planting
Diversity affects agroecosystem performance and resilience
5Elementa: Science of the Anthropocene • 3: 000041 • doi: 10.12952/journal.elementa.000041
of small grains and forages in corn and soybean-based cropping systems, and including more pastureland
and more perennial conservation buffers in the overall landscape. Respondents indicated that they would be
willing to pay an average of $201 per household annually to achieve reductions in soil erosion, nutrient runoff,
flooding, and greenhouse gas emissions from Minnesota farmland, while gaining increases in wildlife habitat.
Often forgotten in the discourse over alternative paths in U.S. agriculture is the fact that there is already
substantial public investment in maintaining the status quo of land use and commodity crop production.
That investment could be shifted toward the types of diversification practices named here. Between 1995 and
2012, U.S. farmers received $231 billion in federal crop subsidies, supported by tax dollars, for a narrow group
of commodity crops and insurance that promoted production of those crops, compared with $39 billion in
federal conservation payments (Environmental Working Group, 2014). Shifting commodity crop and insurance
subsidies toward conservation and ecosystem service payments could provide strong financial incentives for
farmers to increase crop and non-crop diversity at targeted locations within agricultural landscapes, while
maintaining farm income. Diversification’s documented effects on natural resource conservation and protection
indicate it could benefit both farmers and society at large by enhancing the resilience of cropping systems to
climate change and other large-scale environmental stresses (Kremen and Miles, 2012). Moreover, a shift of
support from commodity and insurance subsidies to payments for agricultural conservation programs, farm-
derived ecosystem services, new and expanded market opportunities, and reoriented research and extension
activities could generate additional benefits, including a more stable and secure food supply, cleaner air and
water, larger wildlife populations, and improved outdoor recreational experiences (Iles and Marsh, 2012).
ough understanding of the eects of diversication in agroecosystems has expanded considerably in
recent years, substantial knowledge gaps remain, especially with regard to questions of scale, appropriate
domains of inference, and the impacts of social and economic factors. Questions we feel especially important
to address include the following:
• Whatfunctionalrolesdoparticularspeciesplayinimprovingagroecosystemperformanceandresilience,
and how can this information be used to design agroecosystems that will promote high yields and
conserve resources over the long term?
• Towhatextentdotheplot-levelstudies(i.e.,<0.5 ha) that have dominated agroecological research
to date reect the eects of biodiversity on ecological function and resilience at farm, landscape, and
regional scales?
• Towhatextentaretheresearchresultsfromoneregiontransferrabletoothers?
• Whatisthevalueofan agroecologicalapproachto agriculture,suchasincreasedcropand non-crop
diversity, in comparison to other approaches?
• Whatkindsofpolicies,informalgovernancestructures,andeducationalactivitiessupporttheadoption
by farmers of low-external-input, diversied agroecological approaches in dierent settings around the
world?
• Whatstrategiescanbeemployedtogarnergreatercooperationbetweenscientists,farmers,andother
stakeholders in answering these questions at farm, landscape, and regional scales?
We welcome submissions to this forum addressing these and other salient questions on how diversication
and other agroecological approaches can enhance farm and landscape performance and resilience.
Returning to Professor Naeem’s analogy, there is widespread recognition that computers play an impor-
tant role in our everyday lives. What we tend to forget is that healthy food, farms, and farm landscapes are
far more important, garnering benets at individual to global scales and for present and future generations.
While the challenge of sustaining highly functional, resilient agroecosystems cannot be reduced to protect-
ing a motherboard from damage, it is not intractable. Indeed, tangible and practical approaches are already
in hand. Concerted eort to build upon current agroecological foundations, especially the importance of
enhancing biological diversity, is likely to yield a highly desirable future for all.
References
Alexander RB, Smith RA, Schwarz GE, Boyer EW, Nolan JV, et al. 2008. Dierences in phosphorus and nitrogen delivery
to the Gulf of Mexico from the Mississippi River Basin. Environ Sci Technol 42: 822–830.
Altieri MA. 1995. Agroecology: e Science of Sustainable Agriculture. 2nd edition. Boulder, Colorado: Westview Press.
Angel JR, Palecki MA, Hollinger SE. 2005. Storm precipitation in the United States. Part II: Soil erosion characteristics.
J Appl Meteorol 44: 947–959.
Asbjornsen H, Hernández-Santana V, Liebman M, Bayala J, Chen J, et al. 2014. Targeting perennial vegetation in agricul-
tural landscapes for enhancing ecosystem services. Renewable Agriculture and Food Systems 29: 101–125. doi: 10.1017/
S1742170512000385.
Bennett AJ, Bending GD, Chandler D, Hilton S, Mills P. 2012. Meeting the demand for crop production: the challenge
of yield decline in crops grown in short rotations. Biological Reviews 87: 52–71.
Boody G, Vondracek B, Andow DA, Krinke M, Westra J, et al. 2005. Multifunctional agriculture in the United States.
BioScience 55: 27–38.
Diversity affects agroecosystem performance and resilience
6Elementa: Science of the Anthropocene • 3: 000041 • doi: 10.12952/journal.elementa.000041
Brown PW, Schulte LA. 2011. Agricultural landscape change (1937–2002) in three townships in Iowa, USA. Landscape
Urban Plan 100: 202–212.
Broussard W, Turner RE. 2009. A century of changing land-use and water-quality relationships in the continental US.
Frontiers in Ecology and the Environment 7: 302–307.
Cardinale BJ, Duy JE, Gonzalez A, Hooper DU, Perrings C, et al. 2012. Biodiversity loss and its impact on humanity.
Nature. doi: 10.1038/nature11148.
Cox TS, Glover JD, van Tassel DL, Cox CM, DeHaan LR. 2006. Prospects for developing perennial grain crops. BioSci-
ence 56: 649–659.
Davis AS, Hill JD, Chase CA, Johanns AM, Liebman M. 2012. Increasing cropping system diversity balances productivity,
protability and environmental health. PLoS ONE. doi: 10.1371/journal.pone.0047149.
DeFries RS, Foley JA, Asner GP. 2004. Land-use choices: balancing human needs and ecosystem function. Frontiers in
Ecology and the Environment 2: 249–257.
Drinkwater LE, Wagoner P, Sarrantonio M. 1998. Legume-based cropping systems have reduced carbon and nitrogen
losses. Nature 396: 262–265.
Durrenberger EP, u KM. 1996. e expansion of large scale hog farming in Iowa: e applicability of Goldschmidt’s
ndings fty years later. Hum Organ 55: 409–415.
Environmental Working Group (EWG). 2014. Farm payments, United States summary information. EWG: Washington,
D.C. http://farm.ewg.org/region.php?ps=00000&statename=theUnitedStates.
Fox RH, Piekielek WP. 1988. Fertilizer N equivalence of alfalfa, birdsfoot trefoil, and red clover for succeeding corn crops.
J Prod Agric 1: 313–317.
Hateld JL, McMullen LD, Jones CS. 2009. Nitrate-nitrogen patterns in the Raccoon River Basin related to agricultural
practices. J Soil Water Conserv 64(3): 190–199.
Heathcote AJ, Filstrup CT, Downing JA. 2013. Watershed sediment losses to lakes accelerating despite agricultural soil
conservation eorts. PLoS ONE 8(1): e53554. doi:10.1371/journal.pone.0053554.
Heaton EA, Schulte LA, Berti M, Langeveld H, Zegada-Lizarazu W, et al. 2013. Managing a second-generation crop
portfolio through sustainable intensication: examples from the USA and the EU. Biofuels, Bioproducts, and Bioren-
ing. doi: 10.1002/bbb.1429.
Heggenstaller AH, Menalled FD, Liebman M, Westerman PR. 2006. Seasonal patterns in post-dispersal seed predation
of Abutilon theophrasti and Setaria faberi in three cropping systems. J Appl Ecol 43: 999–1010.
Helmers MJ, Zhou X, Asbjornsen H, Kolka R, Tomer MD, et al. 2012. Sediment removal by prairie lter strips in row-
cropped ephemeral watersheds. J Environ Qual 41: 1531–1539. doi: 10.2134/jeq2011.0473.
Hirsh SM, Mabry CM, Schulte LA, Liebman M. 2013. Diversifying agricultural catchments by incorporating prairie
buer strips. Ecological Restoration 31: 201–211.
Iles A, Marsh R. 2012. Nurturing diversied farming systems in industrialized countries: how public policy can contribute.
Ecology and Society 17(4): 42. doi: 10.5751/ES-05041-170442.
Johnston CA. 2013. Agricultural expansion: land use shell game in the U.S. Northern Plains. Landscape Ecol 29: 81–95.
Jordan N, Warner KD. 2010. Enhancing the multifunctionality of US agriculture. BioScience 60: 60–66.
Jose S, Gold MA, Garrett HE. 2012. e future of temperate agroforestry in the United States, in Garrity D, Nair PKR,
eds., Agroforestry: e Future of Global Land Use, Advances in Agroforestry 9. New York, NY: Springer: pp. 217–245.
Karlen DL, Var vel GE, Bullock DG, Cruse RM. 1994. Crop rotations for the 21st century. Adv Agron 53: 1–45.
Klopatek JM, Olson RJ, Emerson CJ, Jones JL. 1979. Land-use conicts with natural vegetation in the United States.
Environ Conserv 6: 191–199.
Kremen C, Miles A. 2012. Ecosystem services in biologically diversied versus conventional farming systems: benets,
externalities, and trade-os. Ecology and Society 17(4): 40. doi: 10.5751/ES-05035-170440.
Liebman M, Gibson LR, Sundberg DN, Heggenstaller AH, Westerman PR, et al. 2008. Agronomic and economic per-
formance characteristics of conventional and low-external-input cropping systems in the central Corn Belt. Agron
J 100: 600–610.
Liebman MZ, Helmers MJ, Schulte LA, Chase CA. 2013. Using biodiversity to link agricultural productivity with envi-
ronmental quality: results from three eld experiments in Iowa. Renewable Agriculture and Food Systems 28: 115–128.
MacDonald AL. 2012. Blurring the lines between production and conservation lands: Bird use of prairie strips in row-
cropped landscapes. M.S. esis. Iowa State University, Ames, IA.
McGranahan DA. 2014. Ecologies of scale: multifunctionality connects conservation and agriculture across elds, farms,
and landscapes. Land 3: 739–769. doi:10.3390/land3030739.
McGranahan DA, Brown PW, Schulte LA, Tyndall J. 2013. A historical primer on the U.S. farm bill: Supply management
and conservation policy. J Soil Water Conserv 68: 67A–73A. doi: 10.2489/jswc.68.3.67A.
Morris TF, Blackmer AM, El-Hout NM. 1993. Optimal rate of nitrogen fertilization for rst-year corn after alfalfa.
JProd Agric 6: 344–350.
Mortensen DA, Egan JF, Maxwell BD, Ryan MR, Smith RG. 2012. Navigating a critical juncture for sustainable weed
management. BioScience 62: 75–84.
Nassauer JI. 2010. Rural landscape change as a product of U.S. federal policy, in Primdahl J, Swaeld S, eds., Globalisation
and Agricultural Landscapes: Change Patterns and Policy Trends in Developed Countries. Cambridge, UK: Cambridge
University Press: pps. 185–200.
Nassauer JI, Dowdell JA, Wang Z, McKahn D, Chilcott B, et al. 2011. Iowa farmers’ responses to transformative scenarios
for Corn Belt agriculture. J Soil Water Conserv 66: 18A–24A.
National Agricultural Statistics Service (NASS). 2014. Iowa 2013 Annual Statistical Bulletin. NASS, U.S. Department
of Agriculture, Washington, DC. http://www.nass.usda.gov/Statistics_by_State/Iowa/Publications/Annual_Statisti-
cal_Bulletin/2013/index.asp.
National Research Council. 2010. Toward Sustainable Agricultural Systems in the 21st Century. National Academies
Press, Washington, DC.
Diversity affects agroecosystem performance and resilience
7Elementa: Science of the Anthropocene • 3: 000041 • doi: 10.12952/journal.elementa.000041
O’Rourke ME, Heggenstaller A, Liebman M, Rice ME. 2006. Post-dispersal weed seed predation by invertebrates in
conventional and low-external-input crop rotation systems. Agr Ecosyst Environ 116: 280–288.
Pryor SC, Scavia D, Downer C, Gaden M, Iverson L, et al. 2014. Chapter 18: Midwest, in Melillo JM, Richmond TC,
Yohe GW, eds., Climate Change Impacts in the United States: e ird National Climate Assessment. U.S. Global Change
Research Program, Washington, D.C. http://nca2014.globalchange.gov/report/regions/midwest.
Rosenzweig C, Tubiello FN, Goldberg R, Mills E, Bloomeld J. 2002. Increased crop damage in the U.S. from excess
precipitation under climate change. Global Environ Chang 12: 197–202.
Russelle MP, Entz MH, Franzluebbers AJ. 2007. Reconsidering integrated crop–livestock systems in North America.
Agron J 99: 325–334.
Schultz RC, Isenhart TM, Simpkins WW, Colletti JP. 2004. Riparian forest buers in agroecosystems–lessons learned
from the Bear Creek Watershed, central Iowa, USA. Agroforest Syst 61: 35–50.
Secchi S, Gassman PW, Williams JR, Babcock BA. 2009. Corn-based ethanol production and environmental quality: a
case of Iowa and the Conservation Reserve Program. Environ Manage 44: 732–744.
Smith CM, David MB, Mitchell CA, Masters MD, Anderson-Teixeira KJ, et al. 2013. Reduced nitrogen losses after
conversion of row crop agriculture to perennial biofuel crops. J Environ Qual 42: 219–228.
Snapp SS, Swinton SM, Labarta R, Mutch D, Black JR, et al. 2005. Evaluating benets and costs of cover crops for crop-
ping system niches. Agron J 97: 322–332.
Sprague LA, Hirsh RM, Aulenbach BT. 2011. Nitrate in the Mississippi River and its tributaries, 1980 to 2008: Are we
making progress? Environ Sci Technol 45: 7209–7216.
Sulc RM, Tracy BF. 2007. Integrated crop–livestock systems in the U.S. Corn Belt. Agron J 99: 335–345.
Sullivan DJ, Vecchia AV, Lorenz DL, Gilliom RJ, Martin JD. 2009. Trends in pesticide concentrations in Corn Belt
streams, 1996–2006. Scientic Investigations Report 2009-5132, National Water Quality Assessment Program, U.S.
Department of Interior–U.S. Geological Survey, Reston, VA.
Tegtmeier EM, Duy MD. 2004. External costs of agricultural production in the United States. International Journal of
Agricultural Sustainability 2: 1–20.
Tomer MD, Liebman M. 2014. Nutrients in soil water under three rotational cropping systems, Iowa, USA. Agr Ecosyst
Environ 186: 105–114.
Tyndall J, Schulte LA, Liebman M, Helmers MJ. 2013. Field-level nancial assessment of contour prairie strips for en-
hancement of environmental quality. Environ Manage 52: 736–747. doi: 10.1007/s00267-013-0106-9.
Vandermeer J, Lawrence D, Symstad A, Hobbie S. 2002. Eect of biodiversity on ecosystem functioning in managed
systems, in Loreau M, Naeem S, Inchausti P, eds., Biodiversity and ecosystem functioning: synthesis and perspectives.
Oxford, UK: Oxford University Press: pps. 209–220.
Walker B, Salt D. 2006. Resilience inking: Sustaining Ecosystems and People in a Changing World. Washington, DC: Island Press.
Westerman PR, Liebman M, Menalled FD, Heggenstaller AH, Hartzler RG, et al. 2005. Are many little hammers eec-
tive? Velvetleaf population dynamics in two- and four-year crop rotation systems. Weed Sci 53: 382–392.
Williams CL, Liebman M, Westerman PR, Borza J, Sundberg D, et al. 2009. Over-winter predation of Abutilon theophrasti
and Setaria faberi seeds in arable land. Weed Res 49: 439–447.
Zedler JB. 2003. Wetlands at your service: reducing impacts of agriculture at the watershed scale. Frontiers in Ecology and
Environment 1: 65–72.
Zhou X, Helmers MJ, Asbjornsen H, Kolka R, Tomer MD, et al. 2014. Nutrient removal by prairie lter strips in agricul-
tural landscapes. J Soil Water Conserv 69: 54–64.
Contributions
• Bothco-authorscontributedtothewritingandrevisionofthisarticle,andbothapprovethesubmittedversionfor
publication.
Acknowledgments
We thank Anna MacDonald for use of the Figure 1 photo.
Competing interests
e authors declare no competing interests.
Copyright
© 2015 Liebman and Schulte. is is an open-access article distributed under the terms of the Creative Commons
Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original
author and source are credited.