A Deep Dive (Pun Intended) into Aquatic Milkweed Biogeography and its Implications for Monarchs

Monarch Caterpillar Chomping on the Stems of Roadside Aquatic Milkweed

Aquatic milkweed's evergreen habit and current distribution play a significant role in the lifecycle of monarch butterflies in the southeast, and raises questions about resident monarchs, migrating monarchs, and the great monarch migration itself. The following is (just about) everything one would need to know to understand where and why aquatic milkweed occurs where it does, and how this milkweed's habit AND habitat are important to monarch butterflies.

Regulatory Classification of Aquatic Milkweed

 

Asclepias perennis is classified by state and federal agencies as a wetland obligate (OBL) species.  Regulating agencies utilize this species to aid in wetland delineations and determinations. Despite this regulatory classification, it is important to note that Asclepias perennis is not an obligate to wetland soils because of a biological requirement for hydric soils. Indeed, aquatic milkweed can be cultivated in mesic soils away from wetlands. However, Aquatic milkweed is an obligate to the greatly-reduced competition afforded by their ability to grow in saturated soils under closed canopies, often submerged underwater for portions of the year.

Essentially, if a wild population of aquatic milkweed is encountered, the population is probably within a jurisdictional wetland regulated by state and federal agencies. Aside from its wetland classification, aquatic milkweed does not hold any other sort of listing status; e.g. endangered, threatened, etc.

 

Aquatic Milkweed Habitats

There are primary and secondary habitats for Asclepias perennis. A majority of populations and individuals occur in the species’ primary habitats. It is important to note that the species is versatile in habitat utilization; thus, the species can occur within or at the edges of virtually any wetland community type that occurs within drainage districts or low-lying sheetflow platforms. 

Primary Natural Community Types

-          Rivers and Streams - alluvial stream, blackwater stream, spring run

-          Forested Wetlands - floodplain swamp, hydric hammock, coastal hydric hammock, alluvial forest, basin swamp (in Big Bend and Southwest Florida), sloughs (in North Florida and surrounding coastal plain)

-          Pine Flatwoods - Wet Flatwoods

-          Sinkholes and Outcrops - sinkholes (when connected by karst lens to disappearing stream)

 

Secondary Natural Community Types

-          Forested Wetlands - dome swamp (when adjacent to sheetflow or drainage features), basin swamp (conditions same as dome swamp), freshwater tidal swamps (when associated with freshwater drainage systems), bay swamp (same as dome swamp), prairie hydric hammock, bottomland forest, stringer swamp

-          Non-Forested Wetlands - wet prairie (in Peninsular Florida), basin marsh, depression marsh (chiefly southwest Florida within range), floodplain marsh, freshwater tidal marsh, slough marsh (in Peninsular Florida)

-          Pine Flatwoods - wet flatwoods, cabbage palm flatwoods

-          Ponds and Lakes - river floodplain lake, sinkhole lake

-          Sinkholes and Outcrops - limestone outcrop

Aquatic Milkweed Population Dynamics

Aquatic milkweed’s niche revolves around its ability to recruit and grow in wetlands where competition is excluded by long hydroperiods, minimal sunlight, and flowing water.

Generally, aquatic milkweed can be detected in any wetland community type (within its range in Florida) where conditions exclude aggressive competition by herbaceous and woody species, and water flow velocity is sufficient to transport aquatic milkweed seeds to bare soil in some sort of “seed trap.”

As with many other native plant species, aquatic milkweed’s population dynamics vary between different regions within its range. Geology and hydrology present different niches across aquatic milkweed’s range. Predicting and identifying the regional differences (and similarities) in aquatic milkweed’s spatial distribution plays a significant role into the ecology and migration of monarch butterflies in the southeastern United States.

Drainage District Population Dynamics - Floodplains

Quintessential Drainage District Habitat: Floodplain swamps and bottomlands along riparian corridors of rivers.

Typical Floodplain Habitat Meandering Through Uplands - Washington County, Florida

Drainage district populations typically begin within drainage headwaters where measurable flow rates originate and terminate where the respective drainage terminates, which is usually in a floodplain mouth in a coastal district. Populations occur in three types of coastal plain drainage systems – alluvial, blackwater, and spring-run systems. Asclepias perennis is absent from seepage stream systems, such as steepheads springs and acid seepage bogs in West Florida.

In riparian districts and their associated drainage systems, occurrences of Asclepias perennis are confined to relatively narrow, sinuous distributions that meander through upland communities. Due to the species’ reliance on flowing water for seed dispersal and recruitment, population expansion is limited by the seasonal high water (SHW) elevation of the respective drainage basin. Thus, expansion or seed dispersal into uplands occurs infrequently or never. Seedling recruitment is typically proximal or downstream from established populations. Seedling recruitment is virtually never upstream from source populations.  An inference gained from this information is that populations within discrete drainage districts generally do not exchange genetic material (outcross) with populations in adjacent drainage basins; e.g., Wakulla River and St. Marks River populations do not engage in genetic exchange north of their confluence. A secondary inference is that monarch and queen larval host habitats can form distributions that mirror riparian corridors inland through upland habitats. 

Drainage district sloughs, oxbows, and backwaters are ideal habitats for aquatic milkweed, particularly where backflow from the associated river or creek enters the slough. Such sites are dominated by Nyssa, Betula, Taxodium, and Planera. Great examples exist along the Ochlockonee River. South Florida sloughs are typically not associated with primary drainage features with high flow rates and are generally south of the range of Asclepias perennis; thus, sloughs are not habitat for the species in the southern-half of Florida.

 Drainage district populations are associated with “seed traps”, whereby the water-dispersed seeds of aquatic milkweed are captured within depressional features occupied by colonial plant species. Lizard’s tail (Saururus cernuus) serves as a primary seed trap for A. perennis. Colonies of various Carex, Eleocharis, Juncus, and Rhynchospora species serve as secondary seed traps in the upper reaches and backwaters of drainage districts. 

 

Depression Within Floodplain - Lizard's Tail Milkweed Trap

Primary overstory species associated with drainage district populations are Fraxinus spp., Nyssa spp., and Taxodium spp. Stations of canopy species with high basal areas; e.g., high densities of Taxodium spp. with copious root knees present seed trap opportunities.

Aquatic Milkweed Within High Quality Cypress Knee-Lizard's Tail Seed trap

Human-derived structures also serve as seed traps Engineered dynamics can be utilized to explain dispersal and establishment trends of Asclepias perennis populations in engineered ditches and drains with periodic, flowing water; e.g., Asclepias perennis populations within the Morris Bridge Road ditches through the Hillsborough River floodplain.

As a note, culverted or bridged roads through floodplain populations can produce positive or negative impacts on aquatic milkweed populations, largely dependent upon the extent of the road design through wetland habitats. For example, Morris Bridge Road in Tampa, Florida has caused river flow to “backup along the north side of the road, thereby dispersing aquatic milkweed seeds north and south into a ditch before the water bottlenecks into the actual river course under the bridge. The consequence of this is an abnormally high density of aquatic milkweed on one side of the bridge, and a very low density on the other side.

 

Aquatic Milkweed Population Largely Confined to Engineered Seed Traps on East Side of Bridge

Aquatic Milkweed Establishment in Culverted Ditch through the Hillsborough River Floodplain

Drainage District Population Dynamics - Coastal Populations

Near the terminus of a drainage system at the coast, aquatic milkweed populations occur (and end) where floodplain swamp natural communities transition into freshwater tidal swamps and freshwater tidal marshes. In these natural community transitions, Asclepias perennis populations may be infrequently found in atypical, open habitats dominated by Amphicarpum muehlenbergianum, Cladium spp., Ludwigia repens, Sagittaria spp., and Pontederia cordata. In these coastal, non-forested wetlands, Asclepias perennis is infrequently sympatric with Asclepias lanceolata or Asclepias incarnata, particularly where floodplain swamp forest transitions into floodplain marsh and/or hydric hammock. Populations will typically be limited to ecotonal transitions due to aggressive competition from herbaceous wetland species, increasing salinities, and fire suppression.

Coastal Rivermouth Population - Where Floodplain Forests Transition into Non-Forested Wetlands

Basin-Platform Populations

Quintessential Basin-Platform Habitat: Basin swamps, depression swamps and hydric hammocks embedded within a connected, intermittent creek systems, or below the seasonal high water (SHW) elevation in drainage basins or on karst platforms below the SHW.

Typical Basin-Platform Habitat Diffuse Across Landscape - Levy County, Florida

Basin Platform Landscape with Braided, Intermittent Streams Connecting Wetlands through Flatwoods to Primary Rivers - Southwest Florida

Aquatic Milkweed Within Seasonally Connected Slough Through Flatwood-Hydric Hammock Complex - Seasonally Connects to Myakka River

Basin-platform (BP) population dynamics are more complicated than drainage district populations. BP populations are not confined into narrow distributions by surrounding uplands, and populations can be found in a higher diversity of habitats.

Basin-Platform populations of Asclepias perennis occur where landscape-scale surficial sheet flow events occur within low-lying, broad geophysical districts. Such landscape features were formed by ancient shallow bays, seas, barrier island accretion, and carbonate platforms. The natural communities that occur at the surface of these features are variable, and often correlate with the elevation, overburden of sand deposits, and proximity of limestone to the surface. The Big Bend drowned karst district, the Green Swamp Basin, and the Peace River Basin are examples of regions where basin-platform populations occur.

Basin-Platform Populations – Escarpment Populations

BP populations are frequently adjacent to or between ancient sand ridges and escarpments and can also form at fracture points in limestone deeply seated under sand ridges. Basin populations occur on soils where subsurface limestone strata are typically deep enough to not influence surface soil chemistry. Subsurface limestone underlying escarpment soils are typically karst with many voids and input/output hydrological features.

Where surficial karst features are present at the surface, Asclepias perennis populations occur in surficial openings within “disappearing stream” districts (e.g., the Aucilla River), where seeds are dispersed underground via subterranean river-flow and deposited at soil accumulations within other surficial openings downstream).

Aquatic Milkweed Population Within a Karst Window of the Subterranean Aucilla River

Where escarpments do not produce surficial karst, the upland escarpments transition into basin wetlands and wet flatwoods; these communities are in-turn drained by networks of braided and anabranching creeks collecting surface/sheetflow waters. These minor creeks typically collect and drain into regional or localized watersheds. One can expect to find blackwater streams and sporadic spring-runs coalescing with each other here. Aquatic milkweed can be found in virtually any natural community with seasonal sheetflow in such districts.

Examples include the Green Swamp Basin, San Pedro Bay, the Withlacoochee-Lake Panasoffkee Basin, the Wekiva River Basin. Such places are dominated by low-lying wetland natural communities and surface waters.

Basin-Platform Populations – Platform Populations

Platform Populations tend to be coastal and near-sea level.

Platforms usually have limestone near the surface but lack significant karst features such as sinkholes and major spring complexes. Hydric hammocks and wet flatwoods dominate such areas and are connected with each other (and coastal natural communities) via braided or anabranching creek networks. Many of these creek systems are unnamed.

Platform populations of aquatic milkweed - such as populations within the drowned karst region of the Florida Big Bend - are associated with limestone seed traps within coastal hydric hammocks, where water-dispersed seeds accumulate along limestone features (and along wood debris or large trees) during sheet flow events and flood stages. 

Aquatic Milkweed Recruitment Along a Limestone Boulder in a Surficial Limestone Seed trap

Aquatic Milkweed and Isolated Wetlands

The species is not likely to be detected in herbaceous-dominated, fire-dependent wetlands, even if water flow/velocity are sufficient for seed dispersal. At the margins between forested and non-forested wetlands in drainage districts, aquatic milkweed can occasionally recruit into marshes, wet prairies, graminaceous karst plugs, edge marshes and other non-forested wetlands; the species is otherwise absent from non-forested wetlands.

Hydroperiod, low flow rates and fierce herbaceous competition exclude Asclepias perennis from the majority of freshwater marsh community types. However, the species occurs infrequently in depression marshes embedded in floodplains, hydric hammocks and karst drainage districts (such as graminaceous karst plugs or edge marshes). The species can also be found infrequently in non-forested wetlands within river/creek mouths at the coast (where they transition into brackish river marshes.

Aquatic milkweed is not present in genuinely isolated forested wetlands, even if the wetland is forested with associated canopy species. Basin swamps and isolated depression swamps (such as dome swamps) generally lack the input-output flow rates and riparian connectivity to support long-term populations of Asclepias perennis. Basin swamps, depression swamps and hydric hammocks embedded within a connected, intermittent creek system or below the SHW elevation in floodplains (as is prevalent in the Big Bend Region) are exceptions with occasional populations but are still not primary habitat due to low flow rates, isolation, and herbaceous competition.  

Declining Populations in Natural Habitat and Increasing Populations in Engineered (Roadside) Habitat

In a 10-year population survey of aquatic milkweed in Florida, results showed that a majority (57%) of populations were found exclusively in ditches along the side of the road, or (at a minimum) were rare in adjacent habitats.

At survey sites where aquatic milkweed was prevalent throughout intact habitat (at 43% of assessed site), the populations were within habitats with minimal impacts to wetland hydrology. Data tables for this information can be found here.

Results of 10-Year Assessment Show Aquatic Milkweed Leaning Towards Roadsides

It is unclear why the species has become roadside-only in many places, as the species is not dependent upon fire to maintain most of its habitat. Survey data suggests that hydrological modification to associated wetlands is strongly associated with aquatic milkweed becoming roadside only. 3 consequences of roadside ditching or bridge-building have been implicated in producing roadside populations of aquatic milkweed:

1 - Roadside ditching lowers water levels and velocities in the adjacent wetland

2 – Velocities become higher where water is directed into bottlenecks through roadside ditches, culverts, or under bridges, thereby depositing seeds into these sites instead of into downstream floodplain habitats.

3 – Reductions in wetland water levels and soil mounding to support bridges allows for recruitment of plant species into the margins of floodplain (except for the lowermost reaches) that compete and exclude aquatic milkweed.

Aquatic milkweed is found in roadside ditches that do not have suitable (adjacent) habitat. In some surveyed populations, it was evident that suitable natural communities had been removed during the course of private development. In other cases, it appeared that populations were the result of waterflow in the roadside ditches, transporting seeds away from native habitats. In this case, the engineered ditches are acting as analogues of creeks.

Roadside populations of aquatic milkweed in Florida, Louisiana, and Texas provide great examples of the aforementioned dynamics.

Based on survey results of aquatic milkweed populations in healthy natural communities, it is assumed that populations that are not impacted by direct and adjacent hydrological engineering features are secure, so long as landscape scale reductions in water table or SHW levels do not occur.

Conservation of Aquatic Milkweed in Roadside Ditches

 Roadside ditches are arguably the greatest repository of the highest numbers of plants in many parts of their range. Significant hydrological modifications to sheetflow and drainage are ongoing across the region, and many populations are relegated to ditches and swales. Additionally, overwithdraw of the aquifer has created significant reductions in hydrological output, lowering the SHW level to lower elevations, isolating wetlands that previously incurred inputs of water. High-magnitude springs have ceased to flow across the peninsula in the last 50 years, and many populations may have been extirpated or relegated to nearby ditches. Populations in drainage district habitats are generally secure. However, basin-platform populations are declining in the peninsula due to vanishing sheetflow events, ditching, logging, and conversion to pine plantation or development.

Herbicide as an Emerging Threat

In what we refer to as the “herbicide revolution” of the 2010s, The Milkweed Foundation began to document increasing impacts to roadside populations of aquatic milkweed due herbicide applications. State and local agencies, and linear facility companies have shifted into herbicide utilization (and reduced mechanical utilization) for economic reasons, and it has become very common for broadcast applications of herbicide to impact miles of roadside aquatic milkweed habitat. Conferring with agencies, politicians, and power company representatives will be necessary to prevent the extirpation of a majority of roadside aquatic milkweed populations.

7/19/2023 - Largescale Herbicide Impacts to a Roadside Aquatic Milkweed Population

Monarch and Queen Butterfly Habitat

Aquatic milkweed is a preferred larval host by monarch and queen butterflies. Long-term population surveys demonstrate high rates of utilization of the species by these insects.

In addition to being an evergreen milkweed that provides large, year-round biomasses of larval host material, the microclimates and native plant communities of aquatic milkweed habitat provide preferable conditions for monarchs and queens.

Microclimates within Aquatic Milkweed Habitats

The moderating nature of water and the closed canopy of many types of aquatic milkweed habitats provide opportunities for monarchs and queens to escape adverse temperature conditions in adjacent communities and/or uplands. In the dormant season, these habitats are warmer than the surrounding landscape, and cooler in the summer.

Temperature Readings Within Aquatic Milkweed Habitat Showing Moderation from Uplands

Abundance of Nectar Sources for Adult Butterflies

A high diversity of flowering species utilized by adult monarchs and queens is present with aquatic milkweed habitat. Notably, numerous species and genera provide flower nectar resources in the dormant season. The following is a list of wildflower species that monarchs have been observed utilizing in aquatic milkweed habitat:

Binomial	Common Name	Winter Resource	Notes
Acer rubrum	red maple	Yes	Important Resource in Jan/Feb
Baccharis glomeruliflora	silverling	Yes	Important Resource in Jan/Feb
Bidens alba	beggarticks	Yes	Important Resource in Jan/Feb
Bidens mitis	smallfruit beggarticks	Yes	Important Resource in Jan/Feb
Clematis spp.	pine hyacinth	Yes	Important Resource in Jan/Feb; Notably C. baldwinii & C. crispa
Crataegus spp.	all hawthorne species	Yes
Ditrysinia fruticosa	gulf Sebastian-bush	No
Eupatorium perfoliatum	rough boneset	No
Eutrochium fistulosum	Joepyeweed	No
Helianthus agrestis	southeastern sunflower	Yes
Helianthus angustifolius	narrowleaf sunflower	Yes
Iris spp	all iris species	Yes	Notably I. savannarum
Justicia americana	American waterwillow	No
Justicia angusta Justicia ovata	looseflower waterwillow	Yes
Melanthera nivea	snow squarestem	Yes
Lobelia cardinalis	cardinalflower	No
Lonicera sempervirens	coral honeysuckle	No
Packera glabella	butterweed	Yes	Important Resource in Jan/Feb
Physostegia spp.	all dragonhead species	Yes
Rubus spp	all blackberry species	Yes	Important Resource in Jan/Feb
Saururus cernuus	lizard’s tail	Yes
Smallanthus uvedalia	bearsfoot	No
Solidago fistulosa	pinebarren goldenrod	No
Solidago rugosa	wrinkleleaf goldenrod	Yes
Symphiotrichum spp	new world asters	Yes	Important Resource in Jan/Feb in Peninsula
Viburnum obovatum	Walter’s viburnum	Yes	Important Resource in Jan/Feb
Viola spp.	all violets	Yes	Important Resource in Jan/Feb

Generalizations About the Relationship Between Aquatic Milkweed and Monarch Butterflies

 

-         At least in the coastal plain, moderate conditions and evergreen aquatic milkweed allow for potential year-round occupancy of monarch butterlies.

 

-          Aquatic milkweed facilitates year-round occupancy north of temp zone 9 in Florida, with non-migratory reproduction being documented statewide, year-round.

 

-         Coastal hibernaculums for monarchs facilitated by moderated temperatures coincide geographically with abundant aquatic milkweed habitat and large biomasses of native milkweed.

 

-         Monarch preference for aquatic milkweed is demonstrable.

 

-         Cultivation of tropical milkweed should be discouraged, but not on the basis of its evergreen nature.

 

-         Aquatic milkweed populations are large and stable in communities with no hydrological impairments; but are increasingly shifting towards roadside dominance due to widespread water flow manipulation and/or flood control.