Preferred nectar sources for the monarch butterfly (Danaus plexippus plexippus) along the Great Plains migration pathway

Abstract

Population declines of the monarch butterfly (Danaus plexippus plexippus) in North America have largely been attributed to the distribution and condition of species‐specific preferred nectar sources. In 2020, the US Fish and Wildlife Service (USFWS) listed the monarch butterfly in the US Federal Register as a candidate species under the Endangered Species Act of 1973. The USFWS ranked the availability, quality, and spatial distribution of nectar plants during autumn migration as the fourth most contributing factor to US monarch population declines. During the autumn migration through the Great Plains, monarchs seek nectar plants to accumulate lipid reserves for further migration to and overwintering in Mexico. We applied vegetation and rangeland health data from the US Department of Agriculture, Natural Resources Conservation Service, National Resources Inventory (NRI) to quantify species density and richness of monarch‐preferred nectar plants, associated rangeland conditions, and diversity of nectar sources along this autumn migration pathway. We focused specifically on longitudinal gradients W‐095‐100 and W‐100‐105 and discrete 5° latitudinal–longitudinal cells within those gradients. The respective NRI dataset spans 8211 rangeland sites sampled between 2009 and 2018. Approximately 84.4% of sites in W‐095‐100 and 72.5% of sites in W‐100‐105 contained monarch‐preferred nectar plants. Preferred nectar plants made up 7.4% of 2438 identified plant species in W‐095‐100 and 6.1% of 2371 identified plant species in W‐100‐105. For W‐095‐100, preferred nectar plant densities were highest for the 5° cell covering portions of US states Oklahoma and Kansas and lowest for the 5° cell at the US–Mexico border. In W‐100‐105, preferred nectar plant densities decreased linearly from north to south. Preferred nectar plant densities were greater for 5° cells in W‐100‐105 (50.5 billion plants) as compared with W‐095‐100 (44.4 billion plants). Consistent with trends in preferred nectar source density, rangeland conditions assessed by similarity indices and rangeland health protocols were generally lowest for 5° cells spanning the US–Mexico border. The results provide the most comprehensive assessment to date for preferred nectar sources of the monarch butterfly along the Great Plains autumn migration to Mexico and document generally decreasing nectar sources and habitat conditions at southern latitudes in this ecologically important pathway.

INTRODUCTION

Over the last two decades, eastern and western United States monarch butterfly (Danaus plexippus plexippus; Figure 1) populations have experienced significant declines, and in response, concern and causal factors have stimulated considerable interest, resulting in various hypotheses and conservation efforts (Pelton et al., 2019; Pocius et al., 2017; Thogmartin et al., 2017; USFWS, 2020a; Voorhies et al., 2019; Zaya et al., 2017). In December 2020, the US Fish and Wildlife Service (USFWS) listed the monarch butterfly in the US Federal Register as a candidate species under the Endangered Species Act of 1973 (USFWS, 2020b). Monarch declines in the United States have been attributed to two main hypotheses: (1) the milkweed (Asclepias spp.) limitation hypothesis and (2) the migration mortality hypothesis (Agrawal & Inamine, 2018).

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Understanding and assessment of both monarch decline hypotheses clearly require knowledge of the distribution and condition of milkweeds and other monarch-preferred nectar sources. The milkweed limitation hypothesis cites declining milkweed populations attributed to multiple factors such as urbanization, agricultural practices, and use of herbicides (Pleasants & Oberhauser, 2013; Taylor et al., 2020; USFWS, 2020a, 2020b). Monarch larvae are obligate specialists subsisting on the leaves of at least 80 plants in the subfamily Asclepiadoideae (Greenstein et al., 2022), including honeyvines (Cynanchum laeve), twinevines (Funastrum spp.), and milkvines (Matelea spp.), but utilizing most frequently dozens of milkweed species (Ackery & Vane-Wright, 1984; Tracy et al., 2022). The second hypothesis states that mortality during autumn migration is higher due to a possible decline in nectar resources over the last two decades (Agrawal & Inamine, 2018; Inamine et al., 2016). Spaeth et al. (2022) assessed current trends in milkweeds on private rangelands in the central and western United States and estimated the total density of milkweeds to be 4.1 billion plants across 13.2 million ha spanning that domain. The study found that seven US states (Kansas, Texas, Nebraska, South Dakota, North Dakota, Oklahoma, and Montana) contained 88.8% of the total rangeland hectares where milkweeds were present, all spanning substantive portions of longitudes W-095 and W-105 along the monarch Great Plains migratory pathway. Similar information is needed for monarch-preferred nectar sources beyond only milkweeds to address these hypotheses for monarch declines in the United States.

In particular, more information is needed surrounding the abundance and availability of monarch-preferred nectar plants and the year-to-year autumn migratory conditions that affect the flowering of those plants that subsequently foster migration survivability (Agrawal & Inamine, 2018; Inamine et al., 2016; Ries, Taron, & Rendhauser, 2015; Ries, Taron, & Rendo'n-Salinas, 2015; Saunders et al., 2019). Nectar is vital as it is the primary nutrition source for adult monarchs (Fallon et al., 2016a, 2016b; USDA-NRCS, 2018a, 2018b). Beyond milkweeds, Asteraceae species are found to be the most common of the various important nectar plant species for monarchs (see Figure 1, monarchs nectaring on three Asteraceae species) (Agrawal, 2017; USDA-NRCS, 2018a, 2018b). The abundance of all nectar plants, especially during autumn migration, is variable from year to year due to seasonal weather conditions. Monarch butterflies accumulate lipids during autumn migration to Mexico and depend on this source of energy during a 5-month overwintering period (Alonso-Mejía et al., 1997) (Appendix S1: Figure S1). Thus, the prairie provinces along the Great Plains migratory pathway in the United States ostensibly provide the important nectar sources for the autumn migration and are host to a diversity of forb and subshrub species important to pollinating insects. With that said, these prairies have experienced substantial decreases in land area since Anglo-European settlement and conversion to cultivated cropland (Samson & Knopf, 1994). The tallgrass prairie once occupied 97 million ha of the North American Great Plains but has declined to about 3% of the original area due to conversion to other land uses, that is, principally cropland (Samson & Knopf, 1994; Wagle & Gowda, 2018). Since 2000, new US federal policies, changes to commodity markets, and biofuel production have resulted in conversion of 2.3 million ha of intact grasslands to cultivated cropland (largely corn production, 2008–2012) (Lark et al., 2015). The impact of these landscape-level changes on preferred nectar sources remains a key concern for monarch habitat in the central United States (Agrawal, 2017).

The purpose of this study was to characterize the density, richness, and diversity of monarch butterfly-preferred nectar plants along the autumn migratory pathway through the Great Plains Region. Our specific objectives were to (1) quantify species density and richness of monarch-preferred nectar plants, (2) assess how nectar plant abundance may be affected by rangeland condition and health, and (3) evaluate alpha and beta diversity of monarch-preferred nectar sources along two latitude/longitude gradients and in 10 5° latitudinal–longitudinal cells spanning this ecologically important monarch migratory pathway. We used the US Department of Agriculture (USDA), Natural Resources Conservation Service (NRCS), and rangeland National Resources Inventory (NRI) dataset to address the above objectives. The respective sampling protocols and measures used are described below.

METHODS

Designating plant species as preferred nectar plants of the monarch butterfly

To assess the density, abundance, and diversity of monarch nectar plants, we first needed to determine which of the thousands of plant species from the Great Plains are preferred nectar plants of the monarch butterfly. We referred to monarch plant lists created for the Northern Plains (USDA-NRCS, 2018a) and Southern Plains (USDA-NRCS, 2018a). The intent was to include all herbaceous plant species that are native to the region and are of high or very high value to monarchs. The lists were determined by identifying “value to monarchs” and preference as exhibited by the frequency of monarch visits to those plants. The primary “value to monarchs” source of information that NRCS used was the Xerces Society's Monarch Nectar Plant Database (Xerces Society for Invertebrate Conservation, 2023). The Xerces Society created this database in 2015 to serve as a repository of monarch nectar plant use observations from throughout the United States, and to our knowledge, it is the most complete database of its kind. The Monarch Nectar Plant Database currently has thousands of records of monarch nectar use that involve observations of tens of thousands of monarch butterflies. After selecting all plant species listed in the NRCS monarch plant lists for the Northern Plains and Southern Plains, we reviewed the Monarch Nectar Plant Database and found additional plant species from these two regions that we consider to be preferred monarch nectar plants but that were not listed in the aforementioned lists.

Study plots and NRI field sampling protocols

Following the selection of the preferred nectar plant species within the Great Plains Region, we employed available ground-based data from the USDA-NRCS NRI dataset. The NRI data collection has been ongoing for over 65 years and has evolved from simpler qualitative assessments in the early 1980s to now robust quantitative field methods applied on 0.164-ha circular georeferenced plots (NRI sample points) since 2003 (Spaeth et al., 2003, 2005; USDA-NRCS, 2023). The NRI is conducted on nonfederal lands in the United States in cooperation with Iowa State University's Center for Survey Statistics and Methodology, Ames, Iowa, USA. The spatial coverage of the NRI dataset includes all 17 US states west of 95° latitude using a stratified two-stage unequal probability area sampling methodology (see Nusser et al., 1998; Nusser & Goebel, 1997 for NRI statistical design). Each NRI sample point has designated permanent latitude and longitude coordinates and has a statistical weighting factor where a single point represents several hectares on the landscape (Breidt & Fuller, 1999; USDA-NRCS, 2023; Yu et al., 2017).

Our study area includes all NRI sample points in the longitude gradient of West −95° to −100° and West −100° to −105° (abbreviated as W-095-100 and W-100-105, respectively). These two gradients include important migratory routes of monarchs to and from Mexico through large areas of rangeland (Figure 2). There was a total of 8211 individual field NRI sample points in our study area, 3291 in W-095-100 and 4920 in W-100-105. Field NRI inventories are conducted during optimum phenological growth periods, when the emergence of the majority of plant species has occurred. Sampling early in the spring and late in the autumn is not optimum as some species have not emerged and/or have senesced in the autumn, risking misidentification. Thus, the authors do not mean to imply that the species recorded in the NRI sample points are actively being visited/used as nectar sources at the time of collection, but the plant species presence does indicate that the species were available to monarchs at the NRI sample point, although a phenological mismatch is possible during the autumn migration. All plants within each plot are identified to species except that genera are identified when plant senescence inhibits identification. The USDA-Plants Database (USDA-NRCS, 2020) is used for taxonomic conventions. Within the USDA-NRCS NRI plot, a full plant census is conducted where species density is estimated using five classes: (1) 1–10 plants, (2) 11–100 plants, (3) 101–500 plants, (4) 501–1000 plants, and (5) >1000 plants (USDA-NRCS, 2023). Because these density classes represent a range, values for low, midpoint, and high categories for each plant species can be compiled for respective plant density assessments (Spaeth et al., 2022). No low, midpoint, or high estimates are possible for class five (>1000 plants), and therefore, a value of 1000 is used for that class. The final plant density counts are determined by multiplying the plant density per hectare by the statistical weighting factor (in hectares) (Nusser et al., 1998; Nusser & Goebel, 1997; Yu et al., 2017).

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Rangeland condition/health at each NRI sample point is evaluated by three protocols: (1) evaluation of apparent rangeland trend (USDA-NRCS, 2022); (2) similarity index; and (3) interpreting indicators of rangeland health (IIRH) (Pellant et al., 2020). Apparent rangeland trend is a subjective determination that defines the direction of change from historic plant community (HPC) reference conditions. Apparent trend classes are categorically coded as follows: the site is trending toward the HPC, 3; no apparent shift from HPC, 2; and the site plant community trending away from the HPC, 1. The USDA-NRCS defines similarity “as the present state of vegetation on an ecological site compared with species composition (current year's growth) based on the reference historic plant community.” Examples for calculating the similarity index as used in the NRI are given in the USDA-NRCS National Range and Pasture Handbook (USDA-NRCS, 2022). Lastly, IIRH is a quantitative plus expert-evaluated qualitative assessment on 17 field-based indicators, which are used to determine attributes of biotic integrity, hydrologic function, and soil and surface stability (Pellant et al., 2020). The status of each of the three attributes is determined from the indicators and is based on the degree of departure from a historical reference condition using the scale: none-to-slight, slight-to-moderate, moderate, moderate-to-extreme, and extreme (Pellant et al., 2020; USDA-NRCS, 2022). The broad concept of IIRH is to provide a needed and time-efficient assessment on how well ecological processes—such as the water cycle, energy flow, and nutrient cycling—are functioning at a site. Lepak et al. (2022) provide an additional overview on IIRH and the value of these types of assessments in land evaluation and management.

Variable derivations and analyses applied in the current study

NRI sample point summaries and statistical analyses were performed for longitude gradients W-95-100 and W-100-105, as well as for 5° latitudinal–longitudinal cells within those longitudes. The two longitudinal gradients were separated into 5° latitudinal–longitudinal grid cells to aid in interpretation along the Great Plains autumn monarch migratory pathway. All analyses in this study were completed using SAS software, version 9.4 (SAS Institute, 2013). Six alpha diversity-related indices were calculated from the NRI plant census density estimates for each NRI sample point. The objective of including these indices is that they all evaluate the data from different mathematical perspectives. The alpha diversity indices (Ludwig & Reynolds, 1988) chosen were as follows:

• Species richness (S): the number of species in the stand or community.
• Simpson's index $$$ \left(\uplambda ={\sum}_{i=1}^s{p}_i^2\ \right) $$$: probability of two individuals selected at random belonging to the same species—index of dominance, insensitive to rare species.
• Shannon–Weaver $$$ \left({H}^{\prime }=-{\sum}_{i=1}^s\left({p}_i\times \ln {p}_i\right)\right) $$$: measure of species diversity within a given community—alpha diversity, increases with more diversity.
• Hills diversity N1, ($$$ {e}^{H^{\prime }} $$$): measures the number of abundant species.
• Hills diversity N2, $$$ \left(1/\uplambda \right) $$$: measures the number of very abundant species.
• Modified Hill's ratio evenness ($$$ \mathrm{E}5=\left(\left(1/\uplambda \right)-1\right)/\left({e}^{H^{\prime }}-1\right) $$$): approaches zero as a single species becomes more dominant.

A multivariate ANOVA (MANOVA) performed on the six indices listed above provided the ability to detect when groups differ among a number of dependent variables (O'Rourke et al., 2005). The reason to go beyond single variable ANOVA was to avoid Type I errors that can arise from conducting separate ANOVA on each variable (Tabachnick & Fidell, 2007). All variables used in the MANOVA were checked and transformed for normality where necessary. Our null hypothesis is that all treatment groups (latitude/longitude 5° grid cells in W-095-100 and W-100-105) are equal for the six dependent variables.

Additionally, the beta diversity of preferred nectar plant species (Whittaker, 1975), based on presence or absence, was evaluated for each NRI sample point using the Jaccard coefficient (Anderson et al., 2011; Chiclana et al., 2013; Jaccard, 1912; Ludwig & Reynolds, 1988). The Jaccard's coefficient ranges from zero to one and is defined quantitatively as: $$$ J=a/\left(a+b+c\right) $$$ where a and b are the number of observations (species) shared between two groups respectively, and c is the number of observations unique to both groups. Beta diversity (Jaccard's coefficient) was evaluated for key nectar species and is based on the premise that there are a myriad of biotic and abiotic factors influencing the distribution, abundance, and interactions across species (Ludwig & Reynolds, 1988).

RESULTS

We present plant species data for the two longitudinal gradients (W-095-100 and W-100-105) and 10 5° latitude/longitude cells. The Xerces Society's Monarch Nectar Plant Database used in this study included 252 preferred nectar plant species. Of the 252 species on the list, 197 preferred nectar plants were identified on NRI sites throughout the two longitudinal gradients. Of the 197 species, 148 were forbs or herbs, 8 were shrubs, 19 were subshrubs, 1 was tree, and 21 were classified as mixed growth form (due to identification to genus only). The percentage of sites in W-095-100 and in W-100-105 with at least one preferred nectar plant species was 84.4% and 72.5%, respectively. Native plants comprised 87.8% of the identified preferred nectar plants (173 spp.), while the other remaining identified preferred nectar plants were introduced species (1.0%, 2 spp.) and those classified as mixed growth form species (11.2%, 22 spp.). The number of preferred nectar species by plant life duration class was 132 for perennials, 31 for annuals, 12 for biennials, and 22 for mixed annuals/biennials/perennials.

The total numbers of plant families, genera, and species (whether used by monarchs or not) and corresponding plants categorized as monarch-preferred nectar plants in longitude gradients W-095-100 and W-100-105 are provided in Appendix S1: Table S1 and briefly summarized here. Longitude gradients W-095-100 and W-100-105 had similar numbers of total and preferred nectar plant families (~145 and 14, respectively), genera (~731 and 48, respectively), and species (~2405 and 162, respectively) (Appendix S1: Table S1). We further quantified plant families, genera, and species based on 5° latitude/longitude cells within longitudes W-095-100 and W-100-105 (Appendix S1: Table S2). Representative total and preferred nectar plant families, genera, and species were greatest in latitude/longitude N35-40/W-095-100 and were generally greater for mid-latitudes spanning Kansas and Oklahoma, western portions of South Dakota and Nebraska, eastern portions of Colorado, northeastern portions of New Mexico, and northern Texas, USA, with respect to the fully sampled domain (Appendix S1: Table S2).

The family Asteraceae contained the greatest number of preferred nectar plants across all sampled longitude gradients (Table 1). In W-095-100, species with ≥5% frequency (based on midpoint total plant densities) were Symphyotrichum ericoides (14.6%), Vernonia baldwinii (6.3%), Amorpha canescens (5.3%), and Gaillardia pulchella (5.0%) (Appendix S1: Tables S3 and S4). A. canescens is a member of Fabaceae, the other three species are Asteraceae. S. ericoides, V. baldwinii, and G. pulchella are native forbs; A. canescens is a native subshrub. G. pulchella is an annual and the other three species are perennials. The value of these plants for nectar quality for monarch butterflies is as follows: V. baldwinii is “high,” A. canescens is “moderate to high,” and S. ericoides and G. pulchella are “moderate” (Table 2). In longitude gradient W-100-105, many of the preferred nectar species were members of the Asteraceae family (Appendix S1: Table S5) and those with ≥5% frequency (based on mid-value densities) were S. ericoides (12.5%), Helianthus annuus (8.1%), Phlox hoodia (5.8%), Echinacea angustifolia (5.3%), and Liatris punctata (5.1%) (Appendix S1: Table S6). All of these species are native perennials, except for the annual H. annuus. H. annuus is classified as “very high” nectar value, E. angustifolia and L. punctata are classified as “high” nectar value, and P. hoodia is not characterized in terms of nectar quality.

TABLE 1 List and rank of total number of species and frequency of occurrence of preferred nectar plant (PNP) families in longitude gradients W-095-100, W-100-105, and W-095-105.

Latitude gradient	Family	No. species	PNP species frequency (%)
W-095-100	Asteraceae	120	65.93
Asclepiadaceae	24	13.19
Lamiaceae	9	4.95
Polemoniaceae	6	3.30
Verbenaceae	5	2.75
Fabaceae	4	2.20
Boraginaceae	3	1.65
Apiaceae	2	1.10
Apocynaceae	2	1.10
Rubiaceae	2	1.10
Campanulaceae	1	0.55
Capparaceae	1	0.55
Grossulariaceae	1	0.55
Polygonaceae	1	0.55
Scrophulariaceae	1	0.55
W-100-105	Asteraceae	98	66.67
Asclepiadaceae	20	13.61
Lamiaceae	8	5.44
Polemoniaceae	5	3.40
Verbenaceae	4	2.72
Boraginaceae	3	2.04
Fabaceae	3	2.04
Apiaceae	1	0.68
Apocynaceae	1	0.68
Capparaceae	1	0.68
Grossulariaceae	1	0.68
Polygonaceae	1	0.68
Rubiaceae	1	0.68
W-095-105	Asteraceae	136	68.00
Asclepiadaceae	24	12.00
Lamiaceae	9	4.50
Polemoniaceae	8	4.00
Verbenaceae	5	2.50
Fabaceae	4	2.00
Boraginaceae	3	1.50
Apiaceae	2	1.00
Apocynaceae	2	1.00
Rubiaceae	2	1.00
Campanulaceae	1	0.50
Capparaceae	1	0.50
Grossulariaceae	1	0.50
Polygonaceae	1	0.50
Scrophulariaceae	1	0.50

TABLE 2 Frequency of plant species occurrence (≥5%) based on all NRI^a sample sites, percentage of total preferred nectar plant density based on midpoint estimate, nectar value for monarchs, and average bloom period for the southern latitude/longitude cells N25-30/W-095-100 and N30-35/W-095-100.

Latitude/longitude cell	Plant species	Frequency (%)	Mid-point density (%)	Monarch nectar valueb	Bloom period
N25-30/W-095-100	Monarda punctata	12.3	15.0	High	Early-Mid
Helianthus spp.	9.7	14.6	High	Early-Mid-Late
Gaillardia pulchella	9.1	10.2	Moderate	Early-Mid
Rudbeckia hirta	6.2	2.8	Moderate	Mid
Asclepias spp.	5.2	3.1	High to very high	Early-Mid
Verbesina virginica	4.9	3.6	Very high	Late
Monarda spp.	4.9	10.5	High	Early-Mid
N30-35/W-095-100	Verbena halei	16.7	6.7	Moderate	Early-Mid
Gaillardia pulchella	15.9	18.0	Moderate	Early-Mid
Liatris punctata	15.4	5.3	High	Mid-Late
Centaurea americana	12.3	4.8	Moderate	Mid
Asclepias spp.	12.2	2.4	High to very high	Early-Mid
Symphyotrichum ericoides	12.0	8.7	Moderate	Mid-Late
Vernonia baldwinii	9.4	2.4	High	Mid
Coreopsis tinctoria	9.2	8.2	Moderate	Mid
Rudbeckia hirta	7.4	4.6	Moderate	Mid
Asclepias asperula	6.3	0.5	Very high	Early-Late
Cirsium spp.	6.3	1.3	High	Early-Mid
Monarda punctata	5.5	1.9	High	Early-Mid
Cirsium undulatum	5.0	1.4	High	Early-Mid
Helenium amarum	5.0	3.9	Moderate	Mid-Late

Overall, preferred nectar plant densities were as follows: W-095-100 contained 20.9 billion (low), 44.5 billion (mid), and 68.0 billion (high) estimated plants; and W-100-105 had 23.0 billion (low), 50.5 billion (mid), and 78.1 billion (high) estimated plants (Appendix S1: Figure S2a). Preferred nectar plant densities spanning both latitude/longitude gradients were 43.9 billion (low), 95.0 billion (mid), and 146.2 billion (high). Although W-095-100 contained a greater number of plant species compared with W-100-105 (2438 vs. 2371), longitude W-100-105 contained higher densities of preferred nectar plants based on the low, midpoint, and high-density estimates (Appendix S1: Figure S2a).

We provide the total density of preferred nectar plants (midpoint) in longitude gradients W-095-100 and W-100-105 in Figure 3 as the magnitudes of preferred species densities for 5° latitude/longitude cells. The highest ranking grid cell of preferred nectar species densities was N45-50/W-100-105, with a respective low estimate of preferred nectar species density of 9.1 billion plants, a midpoint of 20.4 billion plants, and a high density of 31.7 billion plants (Appendix S1: Figure S2b). Preferred nectar species densities for the second highest ranking cell, N35-40/W-095-100, were 9.4 billion plants for the low estimate, 20.0 billion plants for the midpoint estimate, and 30.6 billion plants for the high estimate (Appendix S1: Figure S2b). The third highest ranking cell was N40-45/W-100-105, with 7.2 billion, 16.2 billion, and 25.2 billion plants at the low, midpoint, and high estimates of preferred nectar species density, respectively (Appendix S1: Figure S2b). The lowest ranked cells were at southernmost latitude (N25-30) in both W-95-100 and W-100-105 longitudes; low-, midpoint-, and high-density estimates for preferred nectar species in those cells were 2.2 billion, 3.5 billion, and 5.0 billion, and 0.4 billion, 0.7 billion, and 1.0 billion plants respectively (Appendix S1: Figure S2b).

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Focusing on the two southernmost, central latitude/longitude cells of monarch autumn migration in the United States (e.g., N25-30/W-095-100 and N30-35/W-095-100), we further evaluated the presence of dominant preferred nectar plants (Table 2). In latitude/longitude cell N25-30/W-095-100, 39 preferred nectar plants were identified (second lowest number of plants among the 10 cells), with seven species approaching or >5% frequency. Monarda punctata, which has a high nectar value, was found at a frequency of 12.3% and 15.0% in the total midpoint preferred plant density estimates (Table 2). In latitude/longitude cell N30-35/W-095-100, 98 preferred nectar plants were identified, with 14 at or above 5% frequency. The six most frequently occurring species >10% were Verbena halei, G. pulchella, L. punctata, Centaurea americana, Asclepias spp., and S. ericoides (Table 2).

Rangeland conditions/health

Along the Great Plains autumn migratory pathway, both the similarity index and interpreting indicators of rangeland health (RH) attributes indicate improved rangeland conditions and health in northern and mid-latitudes with respect to the southernmost cells (Figure 4a,b). Apparent rangeland trend values for the 10 latitude/longitude 5° cells were all between 1 (moving away from the HPC) and 2 (not apparent or no change is detectable) (Figure 4a). The mean values of similarity index ranged from 8.8% (N25-30/W-095-100) to 40% (N40-45/W-100-105) (Figure 4a,b). Similarity index calculations were lowest in latitude/longitude cells N25-30/W-095-100 (8.8%), N25-30/W-100-105 (12.6%), and N30-35/W-095-100 (17.3%) (Figure 4a,b). These three cells are associated with the southernmost regions of our analysis area and the flight path of migrating monarchs to Mexico in late autumn (Figures 2 and 3). Investigation of the RH attribute biotic integrity shows that N25-30/W-095-100, N30-35/W-095-100, N25-30/W-100-105, and N30-35/W-100-105 grid cells were in the slight-to-moderate departure from reference category (Figure 4b). Ratings for hydrologic function and soil and surface stability also demonstrated higher departure in these southernmost latitude/longitude cells.

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Alpha diversity

Six indices related to alpha diversity were evaluated for each longitude gradient (W-095-100 and W-100-105) and their respective 5° grid cell within the Great Plains migratory pathway. Each index (i.e., richness [S], Hill's N1, Hill's N2, Shannon–Weaver [H′], and Hill's modified evenness [E5]) exhibited a similar trend in the W-095-100 longitudinal gradient, with lower values in the northernmost and southernmost 5° grid cells, but peaking in the mid-latitudes (Figure 5a,b). The same indices, but in the W-100-105 longitudinal gradient, decreased moving along the north-to-south pathway (Figure 5a,b). The average preferred nectar plant richness including total species richness and the number of samples are provided in Figure 5c, where total species richness was found lowest in the northernmost and southernmost 5° grid cells for both W-095-100 and W-100-105 longitudinal gradients.

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A MANOVA was conducted to test whether the respective latitude/longitude cells in W-095-100 and W-100-105 were equal with respect to the aforementioned plant diversity/community variables (S, Simpson's Index, H′, N1, N2, and E5). This analysis revealed a significant multivariate effect for alpha diversity indices for each longitude gradient (W-095-100, Wilks' lambda = 0.67, p < 0.0001; W-100-105 Wilks' lambda = 0.61, p < 0.0001).

Beta diversity

Given the trends in alpha diversity varying along longitudinal gradients, we sought to compare beta diversity of preferred nectar plant families, genera, and species among the two gradients. Beta diversity (e.g., Jaccard's coefficient) between the two longitude gradients (W-095-100 and W-100-105) was as follows: plant families were 87% similar (13% turnover), genera 81% (19% turnover), and species 65% (35% turnover). At a finer scale (i.e., 5° increments in longitude W-095-100), beta diversity of preferred nectar plants (south-to-north) was 50% (N25-30 to N30-35), 80% (N30-35 to N35-40), 80% (N35-40 to N40-45), and 75% (N40-45 to N45-50) (Appendix S1: Table S7). Lower beta diversity was observed in the lower latitudes, whereas similarity increased and remained between 75% and 80% in the higher latitudes. Correspondingly, beta diversity between 5° cells for longitude gradient W-100-105 was 25% (N25-30 to N30-35), 64% (N30-35 to N35-40), 75% (N35-40 to N40-45), and 91% (N40-45 to N45-50). Beta diversity among preferred nectar plants increased from the lower latitudes to the higher latitudes, with a higher degree of similarity in the northernmost latitudes (N40-45 to N45-50).

DISCUSSION

The autumn migration mortality hypothesis (e.g., a lack of nectar sources during autumn migration) is highlighted in the literature as a contributing factor of monarch population decline (Agrawal & Inamine, 2018; Inamine et al., 2016; Taylor et al., 2020), and this hypothesis formed the inspiration for this study. Here, we assessed species richness/abundance, existing rangeland condition and health, and diversity of preferred monarch nectar plants along the Great Plains Region autumn monarch migration route in the United States using USDA-NRCS NRI sample points. It should be noted that the data used in this study are from rangelands only, where cropland, pastureland, forestland, roadside areas, and urban areas are not included.

Preferred nectar plant species density and richness

Evaluating the 8211 random NRI sample points in the two longitude gradients, we did not observe the expected pattern as expressed in species richness in the historical biogeographic literature (Figures 3 and 5c). It has been well-documented that for most groups of organisms, the average number of species per area increases from the poles toward the equator and reaches its maximum in tropical latitudes (Gillman et al., 2015; Wiens & Donoghue, 2004; Willig & Presley, 2018), with some exceptions for select birds, trees, and insects (Araya & Chester, 1993; Kouki et al., 1994). The USDA-NRCS NRI sample points thus offered a unique opportunity to study the biogeographic aspect of monarch-preferred nectar sources. Low densities of preferred nectar plants on rangelands in N25-30/W-095-100 and N25-30/W-100-105 (Appendix S1: Table S2) compared with the more northern cells may be a contributing factor in monarch decline associated with availability, quality, and spatial distribution of migration resources, especially because Texas and northern Mexico are the most important areas for increasing lipid reserves (Brower et al., 2006, 2015). Total species richness in W-095-100 was lower in the southernmost and northernmost latitude/longitude cells, with highest richness at the middle latitude (N35-40; Figure 5c). Further research is needed to determine the associated environmental factors associated with these trends, which is beyond our scope. We do posit however that seasonal climatic regimes/patterns (e.g., potential evapotranspiration, number of wet days per year, etc.) and topographical and habitat heterogeneity are most likely causal factors, especially in the tall grass prairie region of the United States (Gillman et al., 2015; Hawkins et al., 2003; Kreft & Jetz, 2007). The highest species richness (Figure 5) and preferred nectar species (Appendix S1: Table S2) occurred in N35-40/W-095-100, northern Oklahoma and Kansas, highlighting the tall grass prairie as one of the more important regions for monarchs migrating through the Great Plains.

While the tall grass prairies of the central Great Plains are rich with preferred nectar species, a large migration distance and trend of diminishing populations toward Texas of preferred nectar plants is evident in this dataset. The paucity of abundant nectar sources near the US–Mexican border may mean lower lipid storage by monarchs during their autumn migration, in a region where lipid storage is particularly important (Alonso-Mejía et al., 1997). We have several concerns that bear future research: (1) the phenological mismatch between available preferred nectar sources that may not be available later into the autumn migration season; (2) early senescence of preferred nectar plants in warmer, drier portions of the Great Plains Region, especially annuals, in the southernmost grid cells may mean a reduction in lipid stores (e.g., lack of nectar sources), in turn leading to the physiological inability of monarch butterflies to successfully reach their overwintering sites in Mexico; and (3) the inaccessibility of preferred nectar sources in autumn is also exacerbated by early-onset, seasonal drought conditions in Texas coupled with warmer temperatures through climatic change. Although several preferred species may be present along the autumn migration route, the condition and health of the habitat supporting the species also require consideration.

Rangeland condition/health and preferred nectar plant abundance

Rangeland condition (apparent trend and similarity index, collectively) criteria demonstrated that sample locations were moving away from HPC composition. Apparent rangeland trend in the two southernmost 5° grid cells was lower in comparison with the other eight grid cells, with the exception of N45-50/W-095-100 (Figure 4a). Similarity index calculations were also lowest in the southernmost grid cells (Figure 4a). Together, this indicates that the present composition of native plants is highly altered compared with composition expected in HPC reference status. The low similarity may be attributed to numerous factors, such as losses of native species entirely, increases in or invasiveness of certain native species over and above reference status (e.g., juniper [Juniperus spp.], mesquite [Prosopis spp.], and creosote bush [Larrea spp.]; Ding & Eldridge, 2023), and/or the introduction and establishment of non-native exotic species. Within the two southernmost grid cells, there is the potential that sites, where preferred nectar plants were found, have crossed ecosystem state thresholds and a return to reference conditions is unlikely (based on ecological site state and transition models; USDA-NRCS, 2022). Often with such low similarity index values and reductions in apparent trend, native plant species loss, major native composition changes, and community health factors are permanent (Weltz & Spaeth, 2012).

RH, represented by the attributes of biotic integrity, hydrologic function, and soil and surface stability, was shown to have greater departures from reference conditions for the southern versus northern latitudes (Figure 4b). These three rangeland condition criteria indicate particularly low ecological conditions (e.g., slight-to-moderate to moderate departure from reference) for the two most southern latitude/longitude grid cells near the US–Mexico border. Improvements in RH/conditions and preferred nectar plant richness are not likely going to change through natural inherent community resilience factors and plant successional drivers, suggesting intervention and restoration are likely necessary.

Although current rangeland similarity to the native plant community may be low as compared with historical/Pre-Anglo-European settlement, other land cover types (e.g., urban open spaces, roadside communities, pasturelands, etc.) may be contributing additional supplementation of nectar availability for the monarchs in the autumn (Agrawal & Inamine, 2018). More directed, ground-based research in these southernmost regions would help to ascertain existing status and density of “most valuable” preferred nectar plants and the realized impacts on monarch migration and habitat suitability. Furthermore, concerned citizen scientists can assist in providing valuable information about preferred nectar status to the Xerces Society's Monarch Nectar Plant Database or to other online, community science and social network programs such as iNaturalist.

Diversity versus richness of preferred nectar sources

The southernmost latitudes within the Great Plains autumn migratory route were found to have the lowest species density and richness of monarch-preferred nectar plants and exhibited rangeland conditions/health that departed from historical references. While the number of ostensibly available nectar sources is important, there also should be a diversity of nectar sources for the resilience of these landscapes to disturbance such as drought and climatic change. Collectively, alpha diversity was lowest in the northernmost and southernmost grid cells for W-095-100, but interestingly, H′, beta diversity, species richness, and the total number of species decreased from north to south in W-100-105 (Figure 5). The southernmost grid cells (e.g., N25-30/W-100-105, N30-35/W-100-105) contained relatively high preferred nectar plant H′ despite low richness, and the lowest total number of species (Figure 5b,c). We highlight this because the relatively few species recorded were preferred monarch nectar species and diverse in number. A similar pattern between preferred nectar plant species richness and total species compared with diversity was evident in the northernmost latitudes (e.g., N40-45 and N45-50) for the longitudinal gradient W-095-100. With continued alterations to historical climatic regimes, regions (e.g., grid cells) with more monarch nectar plant diversity may be poised to remain resilient to disturbance, while providing nectar availability, even if the total number of plant species is low. This aligns with the notion that ecosystem resiliency hinges on increased biodiversity (Oliver et al., 2015) but presumes that natural rangeland plant communities remain intact without further degradation or land cover conversion (e.g., rangeland to monoculture cropland).

Additional analyses beyond this effort are needed to pinpoint the most at-risk habitats of preferred nectar plants for the monarch butterfly along the autumn Great Plains migration route. Our findings here do suggest, however, that special attention should be given to locations within the latitude/longitude 5° grid cells of N25-30/W-095-100, N30-35/W-095-100, N25-30/W-100-105, and N30-35/W-100-105 due to the proximity with the Mexico overwintering site, potential plant phenological mismatch with the lipid requirements needed during autumn, and the declines in rangeland condition/health in the southernmost latitudes. Second, the grid cells of N40-45/W-095-100 and N45-50/W-095-100 need further evaluation on the specific a/biotic drivers of low preferred nectar plant density and total number of species, despite having relatively high species richness.

CONCLUSIONS

We address one of several key components in the decline of monarch populations in the United States: the availability, quality (as it relates to rangelands), and coarse spatial distribution of autumn monarch migration nectar resources. The USFWS (2020a) summarized and percent ranked the 11 factors associated with the decrease of monarch populations in the United States, noting availability, spatial distribution, and quality of milkweeds (most important factor, 25%), and availability, quality, and spatial distribution of migration resources (i.e., nectar resources) (fourth most important, 12%) as key factors. Undoubtedly, all 11 factors listed by the USFWS contribute to the causality of monarch decline, but there are still many unknowns as to their additive and interacting effects. Inter- and intra-annual variability in climate in concert with various natural and anthropogenic perturbations throughout the vast range of migrating monarch populations make predicting the trajectories of plant and invertebrate species composition and suitable habitat on rangelands a difficult task (Deng et al., 2014; Hobbs & Huenneke, 1992; Symstad & Jonas, 2011). Our work here provides a comprehensive quantitative assessment of preferred nectar sources for the monarch butterfly along the autumn migration route in the Great Plains Region and thereby contributes to the broader assessment of monarch habitat in North America.

In all, the abundance of monarch butterfly-preferred nectar plants on private rangelands in the Great Plains Region, USA, was generally greater in northern and mid-latitudes compared with the southernmost latitudes. Rangeland conditions and health declined north to south along both W-095-100 and W-100-105 longitudinal gradients when compared with a historical reference. In the southernmost latitudes, where rangeland conditions/health and abundances were lowest, preferred nectar plant diversity may still be sufficient to maintain resiliency to disturbance. Beyond our study goals and objectives, there remains the need to identify critical habitat areas within the autumn migration route for conservation of monarch butterfly resources. Future efforts may employ the USDA-NRCS NRI sample points with remote sensing and geospatial information related to climate, topographic, and edaphic conditions for developing habitat suitability models of monarch-preferred nectar plants, including those areas at risk of loss to climatic change and land use/cover conversion (e.g., rangeland to cropland).

ACKNOWLEDGMENTS

The authors are grateful foremost to NRCS NRI data collection teams and state and regional range specialists, soil scientists, and biologists. Enlightening and helpful discussions with other federal agency biologists, and university biologists, botanists, and researchers were highly informative and valuable. The USDA is an equal opportunity provider and employer. Mention of a proprietary product does not constitute endorsement by USDA or the US government and does not imply its approval to the exclusion of the other products that may also be suitable. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US government.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflicts of interest.

DATA AVAILABILITY STATEMENT

Data are available from Natural Resources Conservation Service (NRCS), with restrictions, including nondisclosure agreements, licensing, confidentiality, and other agreements as mandated by law. For more details regarding the associated dataset, NRCS National Resource Inventory (NRI), please see the NRCS NRI website and respective contact information at: . Requests and inquiries can be directly communicated to NRCS by email at [email protected].

References

Ackery, P. R., and R. I. Vane‐Wright. 1984. Milkweed Butterflies: Their Cladistics and Biology. Ithaca, NY: Cornell University Press and Comstock Publishing Associates.

Agrawal, A. A. 2017. Monarchs and Milkweed. Oxford: Princeton University Press.

Agrawal, A. A., and H. Inamine. 2018. “Mechanisms behind the Monarch's Decline: Migratory Failure May Contribute to the Dwindling of this Iconic Butterfly's Population.” Science 360: 1294–1296.

Alonso‐Mejía, A., E. Rendon‐Salinas, E. Montesinos‐Patiño, and L. P. Brower. 1997. “Use of Lipid Reserves by Monarch Butterflies Overwintering in Mexico: Implications for Conservation.” Ecological Applications 7: 934–947.

Anderson, M. J., T. O. Crist, J. M. Chase, M. Vellend, B. D. Inouye, A. L. Freestone, N. J. Sanders, et al. 2011. “Navigating the Multiple Meanings of β Diversity: A Roadmap for the Practicing Ecologist.” Ecology Letters 14: 19–28.

Araya, B., and S. R. Chester. 1993. The Birds of Chile: A Field Guide. Santigo: Latour.

Breidt, F. J., and W. A. Fuller. 1999. “Design of Supplemented Panel Surveys with Application to the National Resources Inventory.” Journal of Agricultural, Biological, and Environmental Statistics 1: 391–403.

Brower, L. P., L. S. Fink, R. J. Kiphart, V. Pocius, R. R. Zubieta, and M. I. Ramírez. 2015. “Effect of the 2010‐2011 Drought on the Lipid Content of Monarchs Migrating through Texas to Overwintering Sites in Mexico.” In Monarchs in a Changing World: Biology and Conservation of an Iconic Butterfly, edited by K. S. Oberhauser, K. R. Nail, and S. Altizer, 117–129. Ithaca, NY: Cornell University Press.

Brower, L. P., L. S. Fink, and P. Walford. 2006. “Fueling the Fall Migration of the Monarch Butterfly.” Integrative and Comparative Biology 46: 1123–1142.

Chiclana, F., J. M. Tapia García, M. L. Moral, and E. A. Herrera‐Viedma. 2013. “A Statistical Comparative Study of Different Similarity Measures of Consensus in Group Decision Making.” Information Sciences 221: 110–123.

Deng, L., S. Sweeney, and Z. P. Shangguan. 2014. “Grassland Responses to Grazing Disturbance: Plant Diversity Changes with Grazing Intensity in a Desert Steppe.” Grass and Forage Science 69: 524–533.

Ding, J., and D. Eldridge. 2023. “The Success of Woody Plant Removal Depends on Encroachment Stage and Plant Traits.” Nature Plants 9: 58–67.

Fallon, C., N. L. Adamson, A. Stine, S. Jepsen, and M. Vaughan. 2016a. Monarch Nectar Plants: Northern Plains. Portland: Monarch Joint Venture. USDA‐NRCS, Xerces Society.

Fallon, C., N. L. Adamson, A. Stine, S. Jepsen, and M. Vaughan. 2016b. Monarch Nectar Plants: Southern Plains. Portland: Monarch Joint Venture. USDA‐NRCS, Xerces Society.

Gillman, L. N., S. D. Wright, J. Cusens, P. D. McBride, Y. Malhi, and R. J. Whittaker. 2015. “Latitude, Productivity and Species Richness.” Global Ecology and Biogeography 24: 107–117.

Greenstein, L., C. Steele, and C. M. Taylor. 2022. “Host Plant Specificity of the Monarch ButterflyDanaus plexippus: A Systematic Review and Metaanalysis.” PLoS One 17: [eLocator: e0269701].

Hawkins, B. A., R. Field, H. V. Cornell, D. J. Currie, J. F. Guégan, D. M. Kaufman, J. T. Kerr, et al. 2003. “Energy, Water, and Broad‐Scale Geographic Patterns of Species Richness.” Ecology 84: 3105–3117.

Hobbs, R. J., and L. F. Huenneke. 1992. “Disturbance, Diversity, and Invasion: Implications for Conservation.” Conservation Biology 6: 324–337.

Inamine, H., S. P. Ellner, J. P. Springer, and A. A. Agrawal. 2016. “Linking the Continental Migratory Cycle of the Monarch Butterfly to Understand its Population Decline.” Oikos 125: 1081–1091.

Jaccard, P. 1912. “The Distribution of the Flora in the Alpine Zone.” New Phytologist 11: 37–50.

Kouki, J., P. Pemelä, and M. Viitasaari. 1994. “Reversed Latitudinal Gradient in Species Richness of Sawflies (Hymenoptera, Symphyta).” Annales Zoologici Fennici 31: 83–88.

Kreft, H., and W. Jetz. 2007. “Global Patterns and Determinants of Vascular Plant Diversity.” Proceedings of the National Academy of Sciences of the United States of America 104: 5925–5930.

Lark, T. J., J. M. Salmon, and H. K. Gibbs. 2015. “Cropland Expansion Outpaces Agricultural and Biofuel Policies in the United States.” Environmental Research Letters 10: [eLocator: 044003].

Lepak, N., B. A. Newingham, E. Kachergis, D. Toledo, and J. Moffitt. 2022. “Where Do Qualitative Assessments Fit in an Era of Increasingly Quantitative Monitoring? Perspectives from Interpreting Indicators of Rangeland Health.” Rangelands 44: 39–49.

Ludwig, J. A., and J. F. Reynolds. 1988. Statistical Ecology: A Primer in Methods and Computing, Vol. 1. New York: John Wiley and Sons.

Nusser, S. M., F. J. Breidt, and W. A. Fuller. 1998. “Design and Estimation for Investigating the Dynamics of Natural Resources.” Ecological Applications 8: 234–245.

Nusser, S. M., and J. J. Goebel. 1997. “The National Resources Inventory: A Long‐Term Multi‐Resource Monitoring Programme.” Environmental and Ecological Statistics 4: 181–204.

Oliver, T. H., M. S. Heard, N. J. B. Isaac, D. B. Roy, D. Procter, F. Eigenbrod, R. Freckleton, et al. 2015. “Biodiversity and Resilience of Ecosystem Functions.” Trends in Ecology & Evolution 30: 673–684.

O'Rourke, N., L. Hatcher, and E. J. Stepanski. 2005. A Step‐by‐Step Approach to Using SAS for Univariate and Multivariate Statistics, 2nd ed. Cary, NC: SAS Institute Inc.

Pellant, M., P. L. Shaver, D. A. Pyke, J. E. Herrick, N. Lepak, G. Riegel, E. Kachergis, B. A. Newingham, D. Toledo, and F. E. Busby. 2020. Interpreting Indicators of Rangeland Health, Version 5. Tech Ref 1734‐6. Denver, CO: U.S. Department of the Interior, Bureau of Land Management, National Operations Center.

Pelton, E. M., C. B. Schultz, S. J. Jepsen, S. H. Black, and E. E. Crone. 2019. “Western Monarch Population Plummets: Status, Probable Causes, and Recommended Conservation Actions.” Frontiers in Ecology and Evolution 7: 1–7.

Pleasants, J. M., and K. S. Oberhauser. 2013. “Milkweed Loss in Agricultural Fields because of Herbicide Use: Effect on the Monarch Butterfly Population.” Insect Conservation and Diversity 6: 135–144.

Pocius, V. M., D. M. Debinski, J. M. Pleasants, K. G. Bidne, R. L. Hellmich, and L. P. Brower. 2017. “Milkweed Matters: Monarch Butterfly (Lepidoptera: Nymphalidae) Survival and Development on Nine Midwestern Milkweed Species.” Environmental Entomology 46: 1098–1105.

Ries, L., D. J. Taron, and E. Rendhauser. 2015. “Connecting Eastern Monarch Population Dynamics across their Migratory Cycle.” In Monarchs in a Changing World: Biology and Conservation of an Iconic Butterfly, edited by K. S. Oberhauser, K. R. Nail, and S. Altizer, 268–281. Ithaca, NY: Cornell University Press.

Ries, L., D. J. Taron, and E. Rendo'n‐Salinas. 2015. “The Disconnect between Summer and Winter Monarch Trends for the Eastern Migratory Population: Possible Links to Differing Drivers.” Annals of the Entomological Society of America 108: 691–699.

Samson, F., and F. Knopf. 1994. “Prairie Conservation in North America.” BioScience 44: 418–421.

SAS Institute Inc. 2013. Software Version 9.4. Cary, NC: SAS Institute, Inc.

Saunders, S. P., L. Ries, N. Neupane, M. I. Ramírez, E. García‐Serrano, E. Rendón‐Salinas, and E. F. Zipkin. 2019. “Multiscale Seasonal Factors Drive the Size of Winter Monarch Colonies.” Proceedings of the National Academy of Sciences of the United States of America 116: 8609–8614.

Spaeth, K. E., P. J. Barbour, R. Moranz, S. J. Dinsmore, and C. J. Williams. 2022. “Asclepias Dynamics on US Rangelands: Implications for Conservation of Monarch Butterflies and Other Insects.” Ecosphere 13: [eLocator: e03816].

Spaeth, K. E., G. L. Peacock, J. E. Herrick, P. Shaver, and R. Dayton. 2005. “Rangeland Field Data Techniques and Data Applications.” Journal of Soil and Water Conservation 60: 114A–119A.

Spaeth, K. E., F. B. Pierson, J. E. Herrick, P. L. Shaver, D. Pyke, M. Pellant, and R. Dayton. 2003. “New Proposed National Resources Inventory Protocols on Nonfederal Rangelands.” Journal of Soil and Water Conservation 58: 18A–21A.

Symstad, A. J., and J. L. Jonas. 2011. “Incorporating Biodiversity into Rangeland Health: Plant Species Richness and Diversity in Great Plains Grasslands.” Rangeland Ecology & Management 64: 555–572.

Tabachnick, B. G., and L. S. Fidell. 2007. Using Multivariate Statistics, 5th ed. Boston, MA: Allyn and Bacon.

Taylor, O. R., Jr., J. M. Pleasants, R. Grundel, S. D. Pecoraro, J. P. Lovett, and A. Ryan. 2020. “Evaluating the Migration Mortality Hypothesis Using Monarch Tagging Data.” Frontiers in Ecology and Evolution 8: 264.

Thogmartin, W. E., L. López‐Hoffman, J. Rohweder, J. Diffendorfer, R. Drum, D. Semmens, S. Black, I. Caldwell, D. Cotter, and P. Drobney. 2017. “Restoring Monarch Butterfly Habitat in the Midwestern US: ‘All Hands on Deck’.” Environmental Research Letters 12: 1–10.

Tracy, J. L., T. Kantola, K. A. Baum, and R. N. Coulson. 2022. “Distribution and Phenology of Monarch Butterfly Larvae and their Milkweed Hosts in the South Central US.” Biodiversity and Conservation 31: 1797–1827.

USDA‐NRCS (U.S. Department of Agriculture, Natural Resources Conservation Service). 2018a. Important Plants of the Monarch Butterfly, Version 2, Midwest Region. Washington, DC: US Department of Agriculture, Natural Resources Conservation Service. https://www.nrcs.usda.gov/sites/default/files/2022‐10/_FINAL_MidwestPlantGuide2‐13‐2018_web.pdf.

USDA‐NRCS (U.S. Department of Agriculture, Natural Resources Conservation Service). 2018b. Important Plants of the Monarch Butterfly, Version 2, Southern Great Plains. Washington, DC: US Department of Agriculture, Natural Resources Conservation Service. https://www.nrcs.usda.gov/sites/default/files/2022-10/FINAL_Southern_Great_Plains_ver2_web.pdf.

USDA‐NRCS (U.S. Department of Agriculture, Natural Resources Conservation Service). 2020. The PLANTS Database. Greensboro, NC: US Department of Agriculture, Natural Resources Conservation Service, National Plant Data Team.

USDA‐NRCS (U.S. Department of Agriculture, Natural Resources Conservation Service). 2022. National Range and Pasture Handbook. Washington, DC: US Department of Agriculture, Natural Resources Conservation Service.

USDA‐NRCS (U.S. Department of Agriculture, Natural Resources Conservation Service). 2023. Rangeland National Resource Inventory Field Handbook. Washington, DC: US Department of Agriculture, Natural Resources Conservation Service.

USFWS (U.S. Fish and Wildlife Service). 2020a. Monarch (Danaus plexippus) Species Status Assessment Report, V 2.1. Washington, D.C: US Department of Interior, Fish and Wildlife Service.

USFWS (U.S. Fish and Wildlife Service). 2020b. “Endangered and Threatened Wildlife and Plants; 12‐Month Finding for the Monarch Butterfly.” Federal Register, 50 CFR Part 17 (Docket No. FWS–R3–ES–2020–0103; FF09E21000 FXES11110900000 212).

Voorhies, K. J., J. Szymanski, K. R. Nail, and M. Fidino. 2019. “A Method to Project Future Impacts from Threats and Conservation on the Probability of Extinction for North American Migratory Monarch (Danaus plexippus) Populations.” Frontiers in Ecology and Evolution 7: 384.

Wagle, P., and P. H. Gowda. 2018. “Tallgrass Prairie Responses to Management Practices and Disturbances: A Review.” Agronomy 8: 300.

Weltz, M., and K. Spaeth. 2012. “Estimating Effects of Targeted Conservation on Nonfederal Rangelands.” Rangelands 34: 35–40.

Whittaker, R. H. 1975. Communities and Ecosystems, 2nd ed. New York: MacMillan Publishing Company.

Wiens, J. J., and M. J. Donoghue. 2004. “Historical Biogeography, Ecology, and Species Richness.” Trends in Ecology & Evolution 19: 639–644.

Willig, M. R., and S. J. Presley. 2018. “Latitudinal Gradients of Biodiversity: Theory and Empirical Patterns.” In Encyclopedia of the Anthropocene, Vol. 3, 13–19. Amsterdam: Elsevier.

Xerces Society for Invertebrate Conservation. 2023. Xerces Society Monarch Nectar Plant Database. Portland, OR: Xerces Society for Invertebrate Conservation.

Yu, C., J. Li, and T. Krueger. 2017. Weighting Procedures for the 2016 NRI Rangeland and Pastureland Studies. Ames, IA: Iowa State University.

Zaya, D. N., I. S. Pearse, and G. Spyreas. 2017. “Long‐Term Trends in Midwestern Milkweed Abundances and the Relevance to Monarch Butterfly Declines.” BioScience 67: 343–356.