Habitats of North Carolina
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Early Succesional and Semi-Natural Habitats
Open Apocynaceous Forblands
General Description The key environmental factor for this habitat is the presence of Milkweeds, Dogbanes, or other plant species belonging to the family Apocynaceae. This particular habitat includes open to semi-open fields, barrens, and woodland glades. Species more closely associated with bottomlands or closed-canopy hardwoods are treated separately.

The Determining Species of this habitat include the most wide-ranging members of the Apocynaeceae, along with their herbivores, many of which feed on several species or different genera of the Apocynaceae. The co-evolution of these plants and their herbivores has been exceptionally well-studied, involving the evolution of latex and toxic cardenolides on the part of the plants and the ability to handle the latex and sequester or detoxify the cardenolides on the part of the insects. The use made by the insects of the cardenolides for their own protection, both as larvae and adults, has also received a great deal of research, including the evolution of aposematic coloring and associated Mullerian and Bayesian mimicry complexes.

Most famous of all of the adaptations of Milkweed-feeding species are the migrations of the Monarch Butterflies. This phenomenon itself depends on the protection offered by the milkweed-derived toxicity of the Monarchs: their flights over thousands of miles of bird-inhabited country would not be possible without this pre-adaptation. As is true for neo-tropical migrant birds, Monarchs belong to a tropical subfamily of butterflies and their great powers of flight allow them to make use of the rich areas of milkweeds that occur much farther north than the butterflies can survive through the winter. The end result has been the evolution of a vast two-way migration that extends across thousands of miles and involves multiple generations of the Monarchs. Not only are excellent biological compasses required to make this trip, but also the ability locate a very small wintering area in the mountains of Mexico, which none of the individuals who reach that site have ever been to before. This is one of the most spectacular biological events on earth but is now becoming increasingly threatened due to loss and degradation of the Monarch's habitats, including their breeding habitats that are discussed here.

Abiotic Factors Geographic Regions: High Mountains to the Lower Coastal Plain. USDA Hardiness Zones: 6-8. Landform: ridges, open slopes, bottomlands. Soil Moisture: dry to mesic Soil Texture: loamy to sandy. Soil pH: acidic to circumneutral. Soil Nutrients: poor to rich. Microclimate: warm to cool, dry to mesic. Flood Frequency: rare except in bottomlands, where at least occasional Flood Duration: hours to days. Presence of Pools: uncommon. Fire Frequency: frequent to uncommon. Drought Frequency: rare to occasional. Insolation: partial to full sun.

Biotic Structure Key Species members of the Apocynaceae must be present, particularly Asclepias and species. Vegetation Type: herbland. Strata: trees and shrubs are usually absent, although individual trees or small shrubs may be present. Organic Shelter, Foraging, and Nesting Structures: the animal species belonging to this habitat have little need to hide, due to the protection the alkaloids they acquire by feeding on their poisonous host plants. They are usually aposematically colored and stand out against the foliage; they are also typically sedentary or slow-moving making little or no effort to escape when disturbed.

Co-evolved Species Groups The associations between the species associated with this habitat are some of the best-known and well-studied co-evolved complexes of plants and herbivores. Additionally, the similarity in coloration between the various species of herbivores represents a Mullerian Mimicry Complex, itself a high co-evolved complex.

Phagic and Competitory Symbioses:
Apocynaceae species // Cycnia tenera-Euchaetes egle-Danaus plexippus
Apocynum cannabinum // Cycnia collaris-Cycnia inopinatus-Cycnia oregonensis-Marmara cf. apocynella-Spargaloma sexpunctata-Saucrobotys futilalis

Mullerian Mimicry Complexes: Euchaetes egle (larvae)-Labidomera clivicollis-Tetraopes melanurus-Tetraopes tetrophthalmus-Danaus gilippus-Danaus plexippus-Lygaeus kalmii-Oncopeltus fasciatus

Resource Competition Guilds: Several genera of milkweed specialists contain more than one species. These are likely to be close competitors with one another, although in this case they may also mutually benefit by belonging to a Mullerian Mimicry Complex. These include:
Tetraopes melanurus-Tetraopes tetropthalmus
Cycnia collaris-Cycnia inopinatus-Cycnia tenera-Cycnia oregonensis

Determining Species
Tetraopes melanurusBlackened Milkweed BeetleS2S40.00407
Tetraopes tetrophthalmusRed Milkweed BeetleS4S50.00010
Chrysochus auratusDogbane Leaf Beetle SNR (S4S5)
Labidomera clivicollisSwamp Milkweed Leaf BeetleSNR (S4S5)
Danaus plexippusMonarchFSR - G4S4S2S30.01230
Apocynum cannabinum Clasping-leaf DogbaneG5S5S50.00000
Asclepias amplexicaulisClasping MilkweedG5S5S50.00000
Asclepias incarnataSwamp MilkweedG5S5S50.00000
Asclepias syriacaCommon MilkweedG5S5S50.00000
Asclepias tuberosaButterfly MilkweedG5S5S50.00000
Asclepias variegata White MilkweedG5S5S50.00000
Marmara cf. apocynellaGNRSUS1S30.03699
Cycnia collarisG4S2S3S2S30.01230
Cycnia inopinatusUnexpected Cycnia MothG4S2S3S2S30.01230
Cycnia oregonensisOregon CycniaG5S2S3S2S30.01230
Saucrobotys futilalisDogbane Saucrobotys MothGNRS3S5S3S50.00041
Euchaetes egleMilkweed Tussock MothG5S4S40.00041
Spargaloma sexpunctataSix-spotted Gray MothG5S4S5S4S50.00010
Cycnia teneraDelicate Cycnia, Dogbane Tiger MothG5S5S50.00000
Lygaeus kalmiiSmall Milkweed BugSNR (S3S4)
Oncopeltus fasciatusLarge Milkweed BugSNR (S4S5)
Nr = Number of Ranked Species = 21
Ner = Number of Extant, Ranked Species = 17
Nv = Number of Historic and Extirpated Species = 0
Nar = Number of Species at Risk of Extirpation (State rank > S5) = 10
Nss = Number of Secure Species (State Rank = S5) = 7
Pss = Proportion of Secure Species (Nss/Ner) = 0.41176
ENE = Expected Number of Extirpations (Sum of PE) = 0.09128
Average PE (ENE/Ner) = 0.00537
Habitat Risk Index = (Nar+Nv) x Average PE = 10 x 0.00537 = 0.0537

Estimated Risk to the Determining Species For this analysis, we have moved Monarchs from its current state rank of S4 to S2S3. We do not have a separate resident population in the state. Instead, this species is only a transient resident in North Carolina as part of the once vast migratory population found over eastern North America. Within that region, the Monarch has undergone a severe reduction in numbers over the past several decades, which has made it definitely of high conservation concern. That status is better reflected by a rank of S2S3 than S4, which indicates no conservation concern whatsoever.

Most of the other Determining Species are ranked as relatively secure, although a couple of the Cycnia species are rare in the state despite their association with common host plants. The Average Probability of Extirpation for this group is moderately low, the equivalent to a State Rank of S3.
Estimated Risk to the Co-evolved Species Groups All of the Determining Species in this habitat are involved in co-evolved complexes. The ENE and Average PE for the complexes is consequently the same as for the habitat as a whole and the same general comments apply.

Estimated Security of the Habitat With seven species considered to be Secure within the state, the Proportion of Secure Species is relatively high value, 33% . This reflects that this habitat is widespread and consists of either large blocks or clusters of well-connected smaller blocks, which is consistent with its connection to old fields. All of the animal members of this habitat are capable of flight -- exceptionally well-developed in Monarch Butterflies -- and the Milkweeds themselves have highly dispersive seeds. Under these conditions, any extirpation at the local level is likely to be reversed by immigration from nearby sites that escaped a given extirpation event, e.g., a wildfire.

Index of Habitat Imperilment Due to the low Average PE and relatively small number of Determining Species, the Expected Number of Extirpations is fairly low. Combined with the moderate value of PSS and without any Vanished Species having been identified, the resulting value for HRI falls within our Tier 3 of Conservation Concern (0.5 ≥ HRI > 0.05), indicating a moderate priority for conservation efforts.

Identified Risks This habitat is associated with disturbance-maintained conditions, including both frequent fire and shifting patterns of cultivation, timber harvest, and pasturage. While those conditions were widespread in the past, the suppression of wildfires and intensification of "clean farming" and "improved pastures" has led to significant fragmentation of this habitat, with movements between units becoming increasingly difficult even for its highly dispersive species.

In addition to the general reduction and fragmentation of herb-dominated habitats, the increasing use of herbicides in agriculture -- particularly the glyphosates that are now being heavily applied to field planted with corn, soybean, and other crops that have been genetically modified to be immune to this weed-killer -- has been implicated in the severe decline of both Milkweeds and Monarch butterflies (see Borders and Lee-Mäder, 2014; Brower et al., 2012; Taylor et al., 2020).

Observed Trends The decline of Monarchs was noted as early as the 1990s and has been closely monitored for the past several decades (Brower et al., 2012). Initially, loss of the high elevation forests where they winter in Mexico was believed to be the main factor but now attention has shifted to areas where they spend the winter and/or migrate through on their way to their wintering grounds. Impacts to those areas -- primarily the widespread loss of Milkweed populations from both farmlands and ruderal areas (ditches, roadsides, and powerline rights-of-way) -- are the ones that have direct bearing on the security of this habitat. Although the population trends of Monarchs and Milkweeds have not received as much scrutiny here in North Carolina, the same crops are present and the same herbicides and pesticides used as in the Midwest and it follows that similar impacts should be expected.

Distribution Map
Distribution This habitat occurs across the state. The low level of counties showing a high level of representation is probably due mainly to incomplete survey coverage, particularly for the insect members of this habitat.

Major Conservation Reserves
Priority Areas for Surveys and Conservation Protection
Stewardship and Management Recommendations
References Agrawal, A.A., 2005. Natural selection on common milkweed (Asclepias syriaca) by a community of specialized insect herbivores. Evolutionary Ecology Research, 2005, 7: 651–667. Available online at: https://ecommons.cornell.edu/bitstream/handle/1813/66763/Natural_selection_on_common_milkweed_by_a_community_of_specialized_insect_herbivores.pdf?sequence=1

Betz, R.F., Rommel, W.R. and Dichtl, J.J., 1997. Insect herbivores of 12 milkweed (Asclepias) species. In Proceedings of the Fifteenth North American Prairie Conference (pp. 7-19). Bend, OR, USA: The Natural Areas Association Bend. Available online at: http://www.plantconservation.us/BetzRommel.pdf

Borders, B. and Lee-Mäder, E., 2014. Milkweeds: a conservation practitioner’s guide. Xerces Society for Invertebrate Conservation, 282(9). Available online at: https://www.fws.gov/southwest/es/Documents/R2ES/Pollinators/8-Milkweeds_Handbook_XerSoc_June2014.pdf

Brower, L.P., Taylor, O.R., Williams, E.H., Slayback, D.A., Zubieta, R.R. and Ramirez, M.I., 2012. Decline of monarch butterflies overwintering in Mexico: is the migratory phenomenon at risk?. Insect Conservation and Diversity, 5(2), pp.95-100.

Taylor Jr, O.R., Pleasants, J.M., Grundel, R., Pecoraro, S.D., Lovett, J.P. and Ryan, A., 2020. Evaluating the migration mortality hypothesis using monarch tagging data. Frontiers in Ecology and Evolution, p.264. Available online at: https://www.frontiersin.org/articles/10.3389/fevo.2020.00264/full

Updated on 2023-01-24 21:38:14