Monarch and waterfowl habitat loss from recent cropland expansion

Transcription

1 Monarch and waterfowl habitat loss from recent cropland expansion Research Summary by Tyler Lark, University of Wisconsin-Madison Available at 2/28/2018 Milkweeds and Monarchs: Land conversion s contribution to the disappearance of an iconic pollinator s vital habitat An estimated 223 million milkweed stems were lost on converted grasslands, wetlands, and shrublands across the Midwest (Figure). The largest number of stems were lost in South Dakota, North Dakota, Iowa, and Missouri, due to a confluence of high rates of conversion to cropland as well as high proportions of conversion from land enrolled in the Conservation Reserve Program (CRP)--areas which harbor much greater densities of milkweed stems per acre than pasture, hayland, or other non-crop uses. The number of stems lost due to recent cropland expansion represent roughly 17% of the 1.3 billion stems that were estimated to remain in the region as of 2014 (Thogmartin et al., 2017a). Figure: Milkweed stems lost due to conversion of grasslands, shrublands, and wetlands to corn and soy production in the Midwest,

2 The large number of milkweeds lost due to land conversion reflects the significant decline in CRP acreage over the past decade, and the concomitant loss of pollinator habitat. Our estimate of annual loss of milkweeds due to land conversion is nearly 15 times larger than that found by Pleasants (2016). This discrepancy arises primarily from a difference in accounting, in that the previous work included the loss of only non-crp grasslands in their enumeration of land use change impacts (CRP was fully accounted for, but only in their estimate of milkweeds remaining, not milkweeds lost). By extending that analysis to consider the full contribution of both CRP and non-crp land conversion, we show both the catastrophic decline of milkweed since 2008 as well as the full significance of land use change relative to other drivers of milkweed loss. Most notably, the loss of milkweed due to recent land conversion is 26% as large as the mass extirpation of milkweeds due to the adoption of glyphosatetolerant crops and herbicide application that occurred between 1999 and 2014 (Pleasants, 2016). Given that the loss of milkweed due to herbicide-tolerant crop adoption has been the single largest driver of recent Monarch population declines (Stenoien et al., 2018; Thogmartin et al., 2017b), the additional strain posed by grassland habitat conversion to cropland threatens to even further imperil the species population. The estimated loss of milkweed due to cropland expansion is expected to have considerable impacts on Monarch survival and recovery efforts, as milkweeds (Asclepias species) are the sole host plant for monarchs (Thogmartin et al., 2017a). Current Monarch conservation targets call for a doubling of existing milkweed populations, or the addition of another 1.3 billion stems. However, these recovery efforts will first need to overcome losses from ongoing crop expansion. If land conversion were to continue along similar trends and at the recent rates of roughly 1 million acres per year nationwide (Lark et al., 2018), additional restorations would be needed to make up for the approximately 20 million stems lost each year in the Midwest before making progress towards the additional 1.3 billion stem goal. Given that Photo credit: Tyler Lark projected scenarios to achieve the restoration target already call for an all hands on deck approach to increase milkweed populations in every land sector to meet the initial goal (Thogmartin et al., 2017a), there is little room to accommodate further losses from the conversion of land to cropland. Thus, the continued loss of existing habitat may pose a serious threat to monarch population restoration and would likely need to be accounted for in order to fully reach established goals.

3 Table 1: Number of milkweed stems lost due to conversion of grasslands, shrublands, and wetlands to corn and soybeans, by state. Acres of Average stems Stems lost State conversion lost per acre Illinois 128,099 7,240, Indiana 54,282 1,113, Iowa 502,036 33,342, Kansas 180,348 11,793, Kentucky 358,815 5,485, Michigan 51,305 1,313, Minnesota 219,645 9,906, Missouri 539,924 28,234, Nebraska 366,851 22,012, North Dakota 484,860 41,280, Ohio 90,411 3,945, South Dakota 697,229 52,985, Wisconsin 84,396 4,282, State Region 3,758, ,935, Methods for estimating milkweed loss Milkweeds are commonly found in natural and managed grasslands, wetlands, and certain shrublands, but have been nearly extirpated from crop fields since the implementation of herbicide-tolerant crop varieties and associated pesticide application (Thogmartin et al., 2017a). We estimated the total number of milkweed stems lost due to conversion of grasslands, shrublands, and wetlands to corn and soy production in the Midwest using data from (Lark et al., 2018). For wetlands converted to crop production, we assumed an average initial stem density of stems/acre based on (Pleasants, 2016). For grasslands and shrublands converted to crop production, we first estimated for each county the proportion likely to have been enrolled in the Conservation Reserve Program (CRP) prior to conversion using county-aggregated point data from the National Resources Inventory (NRI). Because county-level NRI data on the conversion of CRP to cropland was only available for at the time of analysis (USDA, 2015), we assume the same proportion of conversion from CRP extended throughout our study period of This assumption is supported by overall trends in CRP enrollment, which show that CRP acreage declined nationwide by 5.09 million acres and by another 5.64 million acres (USDA, 2019). Based on Thogmartin et al. (2017) we assume a value of 3.09 stems/acre for grasslands and shrublands not enrolled in the Conservation Reserve Program and a density of stems/acre for those lands enrolled in CRP. We then used the NRI-derived percent of conversion that came from CRP in each county to estimate average stems lost per acre from all converted grasslands and shrublands based on the following equation: Eq 1: Stem density = 3.09 * (1 CRP%) * (CRP%)

4 Where CRP% is the proportion of new crop production that came from CRP within each county. Thus, in counties with no conversion of CRP to cropland, the stem density value used for all grasslands and shrublands converted to cropland was In counties where nearly all conversion to cropland was from CRP, the value was near For most counties, the stem density was an intermediate value, reflecting the mixed sources (CRP and non-crp) of land converted to crop production. To enable equal comparison of total stems and average densities across states, we performed our analysis over the entire 13 state region encompassing the main summer breeding range for Monarchs in the Midwest (Pleasants, 2016), an area once estimated to support over 85% of the breeding population of monarch butterflies prior to the widespread loss of milkweed (Thogmartin et al., 2017a). Note that while our estimated milkweed loss numbers rely on the available data from the scientific literature, there remains substantial uncertainty in the magnitude of milkweed loss reported here. For example, many of the milkweed stem density values were based on observations in Midwestern states located at the interior of the modeled region, and thus values for milkweed stems and losses may be more uncertain and variable around the periphery of the region (Thogmartin et al., 2017a). In addition, our estimates account for only common milkweed (Asclepias syriaca), and thus total and relative loss of all milkweed species may vary. For example, recent field surveys suggest common milkweeds outnumber the next most prevalent variety by over 10 to 1 in CRP landscapes in Minnesota (K. Oberhauser, personal communication), whereas other areas like Kansas and Missouri have higher occurrences of less common species like Asclepias viridis and thus our estimates will be less representative of the changes occurring there (O. Taylor, personal communication). Lastly, the estimates for milkweed stem densities are expected to vary widely from parcel to parcel and across landscape types, and thus the numbers presented here may represent overestimates in some areas and underestimates in others. Waterfowl and land conversion: Ongoing losses of high-quality habitat in the Prairie Pothole Region Grasslands and wetlands in the prairie pothole region (PPR) are particularly valuable for nesting ducks and other waterfowl (Reynolds et al., 2006; Walker et al., 2013). We assessed recent cropland expansion in relation to duck breeding pair accessibility across the PPR and found that areas estimated to provide 138,000 nesting opportunities for breeding pairs were recently converted to crop production. On average, habitat converted to cropland had a modeled duck accessibility of 42.7 breeding pairs per square mile (SD=30.6), which is nearly twice as high as the average for existing croplands (22.9, SD=24.7). This suggest a sizeable reduction in nesting capacity and utilization could be expected upon conversion to cropland. In addition, areas of habitat that were recently converted to cropland had been accessible to an estimated 37% more duck breeding pairs per acre than other remaining habitat that was not converted (31.2 pairs/sq.mi; SD=30.2), indicating that cropland expansion is disproportionately occurring on lands that were providing higher habitat quality and greater wildlife-supporting benefits.

5 Figure: Estimated duck breeding pair accessibility of land converted to crop production in the Prairie Pothole Region (PPR) of the northern US, Darker grey background is the PPR. Colored pixels are areas converted to crop production, , categorized by the predicted density of duck breeding pairs available to access the habitat in that location. Methods for estimating waterfowl habitat quality of converted land We used modeled data from Loesh et al. (2018) to approximate habitat accessibility by breeding duck pairs across the PPR (Cowardin et al., 1995; Loesch et al., 2018; Reynolds et al., 2006). Updated land conversion data for was based on approaches of (Lark et al., 2015, 2017) and used to categorize whether land was cropland, non-cropland, or recently converted to cropland (Lark et al., 2018). To calculate the estimated breeding pair accessibility for each land use type, we used the midpoint value for each category range. For example, we use a value of 70 pairs/sq. mi. for the range of For the highest category of >100 pairs/sq. mi. we assumed a value of 110. Total accessibility or nesting opportunities were then averaged or summed for each land use category across the full PPR.

6 Table: Distribution of land in the PPR by modeled duck pair density on each land use type. Distribution of Land in the PPR (acres) Duck Pair Density Recently converted Non-cropland Cropland (pairs/sq.mi.) to cropland ,632,927 24,488, , ,134,631 14,397, , ,407,969 7,389, , ,576,624 5,703, , ,656,079 3,124, , ,621,563 1,737, , ,697, , ,951 Mean (pairs/sq.mi.) Standard Dev References Cowardin L. M., Shaffer T. L. & Arnold P. M. (1995) Evaluations of Duck Habitat and Estimation of Duck Population Sizes with a Remote-Sensing-Based System. [WWW document]. NATIONAL BIOLOGICAL SERVICE FORT COLLINS CO MIDCONTINENT ECOLOGICAL SCIENCE C ENTER URL Lark T., Bougie M., Spawn S. & Gibbs H. (2018) Cropland Expansion in the United States, [WWW document]. University of Wisconsin-Madison URL Lark T. J., Mueller R. M., Johnson D. M. & Gibbs H. K. (2017) Measuring land-use and land-cover change using the U.S. department of agriculture s cropland data layer: Cautions and recommendations. International Journal of Applied Earth Observation and Geoinformation 62: Lark T. J., Salmon J. M. & Gibbs H. K. (2015) Cropland expansion outpaces agricultural and biofuel policies in the United States. Environmental Research Letters 10: Loesch C., Wangler B., Wright N., Pritchert R., Estey M., Niemuth N., Hertel D., Fields S. & Shaffer T. (2018) The Four Square-Mile Breeding Duck and Habitat Survey in the U.S. Prairie Pothole Region: Its History, Use, and Future. Pleasants J. (2016) Milkweed restoration in the Midwest for monarch butterfly recovery: estimates of milkweeds lost, milkweeds remaining and milkweeds that must be added to increase the monarch population. Insect Conservation and Diversity: n/a-n/a Reynolds R. E., Shaffer T. L., Loesch C. R. & Cox R. R. (2006) The Farm Bill and Duck Production in the Prairie Pothole Region: Increasing the Benefits. Wildlife Society Bulletin 34:

7 Stenoien C., Nail K. R., Zalucki J. M., Parry H., Oberhauser K. S. & Zalucki M. P. (2018) Monarchs in decline: a collateral landscape-level effect of modern agriculture. Insect Science 25: Thogmartin W. E., López-Hoffman L., Rohweder J., Diffendorfer J., Drum R., Darius Semmens, Black S., Caldwell I., Cotter D., Drobney P., Jackson L. L., Gale M., Doug Helmers, Hilburger S., Howard E., Oberhauser K., Pleasants J., Semmens B., Orley Taylor, Ward P., et al. (2017a) Restoring monarch butterfly habitat in the Midwestern US: all hands on deck. Environmental Research Letters 12: Thogmartin W. E., Wiederholt Ruscena, Oberhauser Karen, Drum Ryan G., Diffendorfer Jay E., Altizer Sonia, Taylor Orley R., Pleasants John, Semmens Darius, Semmens Brice, Erickson Richard, Libby Kaitlin & Lopez-Hoffman Laura (2017b) Monarch butterfly population decline in North America: identifying the threatening processes. Royal Society Open Science 4: USDA (2015) 2012 National Resources Inventory: Summary Report [WWW document]. Washington, D.C.: Natural Resources Conservation Service, URL USDA F. (2019) Conservation Reserve Program Statistics [WWW document]. Conservation Reserve Program Statistics URL Walker J., Rotella J. J., Loesch C. R., Renner R. W., Ringelman J. K., Lindberg M. S., Dell R. & Doherty K. E. (2013) An Integrated Strategy for Grassland Easement Acquisition in the Prairie Pothole Region, USA. Journal of Fish and Wildlife Management 4: