A sand prairie autumn

Splitbeard bluestem seed headsAsk any astronomer when autumn begins, and they will likely tell you it begins at the autumnal equinox – when shortening days and lengthening nights become equal as the sun crosses over the celestial equator. According to them, fall began this year on September 22 – at 11:44:18 A.M. EDT, to be precise. I agree that autumn begins at a precise moment, but it is not at the equinox. Rather, it is that unpredictable moment when a sudden crispness in the air is felt, when the sky somehow seems bluer and shadows seem sharper, and hints of yellow – ever so subtle – start to appear in the landscape. Butterfly pea blossomIn Missouri, with its middle latitudes, this usually happens a few weeks before the equinox, as August is waning into September. It is a moment that goes unnoticed by many, especially those whose lives and livelihoods have lost all connection with the natural world. To plants and animals, however, it is a clear signal – a signal to begin making preparations for the long cold months of winter that lie ahead. Plants that have not yet flowered begin to do so in earnest, while those that have shift energy reserves into developing seeds. Animals take advantage of their final opportunities to feed before enduring the scarcities of winter, digging in to sleep through them, or abandoning altogether and migrating to warmer climes. Insects begin hastily provisioning nests for their broods or laying eggs – tiny capsules of life that survive the harsh winter before hatching in spring and beginning the cycle anew.

Sand prairie in early September.Sand prairie in early October.  Note abundance of splitbeard bluestem seed heads.Across much of Missouri, in the Ozark Highlands and in riparian ribbons dissecting the northern Plains, autumn brings an increasingly intense display of reds, purples, oranges, and yellows, as the leaves of deciduous hardwoods begin breaking down their chlorophyll to unmask underlying anthocyanins and other pigments. Small southern jointweedIn Missouri’s remnant prairies, seas of verdant green morph to muted shades of amber, tawny, and beige. This subtle transformation is even more spectacular in the critically imperiled sand prairies of the Southeast Lowlands, where stands of splitbeard bluestem (Andropogon ternaries – above) turn a rich russet color while fluffy, white seed heads (1st paragraph, 1st photo) appear along the length of each stem, evoking images of shooting fireworks. Small southern jointweed (Polygonella americana – right) finds a home at the northern extent of its distribution in these prairie remnants and in similar habitats in nearby Crowley’s Ridge, blooming in profusion once the cooler nights arrive. Butterfly pea (Clitoria mariana – 1st paragraph, 2nd photo) blooms add a gorgeous splash of soft purple in contrast to the muted colors of the plants around them.

Kent Fothergill, Ted MacRae, and Rich ThomaAfter first becoming acquainted with Missouri’s sand prairies this past summer, I knew a fall trip (or two) would be in order. The extensive deep, dry sand barrens were ideal habitat for sand-loving insects, including certain spring/fall species of tiger beetles that would not be active during the summer months. The cooler nights and crisp air of early fall make insect collecting extraordinarily pleasurable, so it took little effort to convince friends and colleagues Kent and Rich to join me on another excursion to these extraordinary remnant habitats, along with my (then 8 yr-old) daughter Madison (who would likely characterize this as “tallgrass” prairie). Madison MacRae, age 9 (almost)I was, as ever, on the lookout for tiger beetles; however, temperatures were cool, skies were overcast, and the fall season was just beginning, greatly limiting tiger beetle activity during this first fall visit. We did see one Cicindela formosa (big sand tiger beetle), which cooperated fully for a nice series of photographs. We also found single specimens of the annoyingly ubiquitous C. punctulata (punctured tiger beetle) and a curiously out-of-place C. duodecimguttata (12-spotted tiger beetle), which must have flown some distance from the nearest dark, muddy streambank that it surely prefers. Of greatest interest, we found two specimens of C. scutellaris (festive tiger beetle), which in this part of Missouri is represented by a population presenting a curious mix of influences from two different subspecies (more on this in a later post…). Despite the scarcity of tiger beetles, other insects were present in great diversity, some of which I share with you here.

Ululodes macleayanusThis bizarre creature, sitting on the stem of plains snakecotton (Froelichia floridana), is actually a neuropteran insect called an owlfly (family Ascalaphidae). Looking like a cross between a dragonfly and a butterfly due to its overly large eyes and many-veined wings but with long, clubbed antennae, this individual is demonstrating the cryptic resting posture they often assume with the abdomen projecting from the perch and resembling a twig. The divided eyes identify this individual as belonging to the genus Ululodes, and Dr. John D. Oswald (Texas A&M University) has kindly identified the species as U. macleayanus. As is true of many groups of insects, their taxonomy is far from completely understood. Larvae of these basal holometabolans are predaceous, lying on the ground with their large trap-jaws held wide open and often camouflaging themselves with sand and debris while waiting for prey. The slightest contact with the jaws springs them shut, and within a few minutes the prey is paralyzed and can be sucked dry at the larva’s leisure.

Ant lion, possibly in the genus Myrmeleon.Another family of neuropteran insects closely related to owlflies are antlions (family Myrmeleontidae, sometimes misspelled “Myrmeleonidae”). This individual (resting lower down on the very same F. floridana stem) may be in the genus Myrmeleon, but my wanting expertise doesn’t allow a more conclusive identification [edit 4/12/09 – John D. Oswald has identified the species as Myrmeleon immaculatus]. Strictly speaking, the term “antlion” applies to the larval form of the members of this family, all of whom create pits in sandy soils to trap ants and other small insects, thus, it’s occurrence in the sand prairie is not surprising. Larvae lie in wait beneath the sand at the bottom of the pit, flipping sand on the hapless prey to prevent it from escaping until they can impale it with their large, sickle-shaped jaws, inject digestive enzymes that ‘pre-digest’ the prey’s tissues, and suck out the liquifying contents. Finding larvae is not easy – even when pits are located and dug up, the larvae lie motionless and are often covered with a layer of sand that makes them almost impossible to detect. I’ve tried digging up pits several times and have failed as yet to find one. Larvae are also sometimes referred to as “doodlebugs” in reference to the winding, spiralling trails that the larvae leave in the sand while searching for a good trap location – these trails look like someone has doodled in the sand.

Bembix americanaThis digger wasp, Bembix americana (ID confirmed by Matthias Buck), was common on the barren sand exposures, where they dig burrows into the loose sand. Formerly included in the family Sphecidae (containing the better-known “cicada killer”), members of this group are now placed in their own family (Crabronidae). Adult females provision their nest with flies, which they catch and sting to paralyze before dragging it down into the burrow. As is common with the social hymenoptera such as bees and paper wasps, these solitary wasps engage in active parental care by providing greater number of prey as the larva grows. As many as twenty flies might be needed for a single larva. I found the burrows of these wasps at first difficult to distinguish from those created by adults of the tiger beetles I so desired, but eventually learned to distinguish them by their rounder shape and coarser, “pile” rather than “fanned” diggings (see this post for more on this subject).

Stichopogon trifasciatusRobber flies (family Asilidae) are a favorite group of mine (or, at least, as favorite as a non-coleopteran group can be). This small species, Stichopogon trifasciatus (ID confirmed by Herschel Raney), was also common on the barren sandy surface. The specific epithet refers to the three bands of alternating light and dark bands on the abdomen. Many species in this family are broadly distributed but have fairly restrictive ecological requirements, resulting in rather localized occurrences within their distribution. Stichopogon trifasciatus occurs throughout North America and south into the Neotropics wherever barren, sandy or gravely areas near water can be found. Adults are deadly predators, swooping down on spiders, flies and other small insects and “stabbing” them with their stout beak.

Chelinidea vittigerPrickly pear cactus (Opuntia humifusa) grows abundantly in the sandy soil amongst the clumps of bluestem, and on the pads were these nymphs of Chelinidea vittiger (cactus bug, family Coreidae). This wide-ranging species occurs across the U.S. and southward to northern Mexico wherever prickly pear hosts can be found. This species can either be considered a beneficial or a pest, depending upon perspective. On the one hand, it serves as a minor component in a pest complex that prevents prickly pear from aggressively overtaking rangelands in North America; however, prickly pear is used by ranchers as emergency forage, and fruits and spineless pads are also sometimes harvested for produce. In Missouri, O. humifusa is a non-aggressive component of glades, prairies, and sand and gravel washes, making C. vittiger an interesting member of the states natural diversity.

Ammophila sp., possibly A. proceraThis wasp in the genus Ammophila (perhaps A. procera as suggested by Herschel Raney) was found clinging by its jaws to a bluestem stem in the cool morning, where it presumably spent the night. One of the true sphecid (or “thread-waist”) wasps, A. procera is a widespread and common species in eastern North America. One of the largest members of the genus, its distinctive, bold silver dashes on the thorax distinguish it from most other sympatric congeners. Similar to the habits of most other aculeate wasp groups, this species captures and paralyzes sawfly or lepidopteran caterpillars to serve as food for its developing brood. Females dig burrows and lay eggs on the paralyzed hosts with which the nests have been provisioned. Adults are also found commonly on flowers, presumably to feed on nectar and/or pollen.

Dusty hog-nosed snakeRich is a bit of herpatologist, so when he brought this hog-nosed snake to our attention we all had a good time pestering it to try to get it to turn upside down and play dead. I had never seen a hog-nosed snake before but knew of its habit of rolling over and opening its mouth with its tongue hanging out when disturbed, even flopping right back over when turned rightside up or staying limp when picked up. We succeeded in getting it to emit its foul musky smell, but much to our disappointment it never did play dead, instead using its shovel-shaped snout to dig into the sand. Dusty hog-nosed snake - head closeupWe had assumed this was the common and widespread eastern hog-nosed snake (Heterodon platirhinos); however, in our attempts to turn it over I noticed its black and orange checker patterned belly. I later learned this to be characteristic of the dusky hog-nosed snake (H. nasicus gloydi), only recently discovered in the sand prairies of southeast Missouri and regarded as critically imperiled in the state due to the near complete destruction of such habitats. Disjunct from the main population further west, its continued survival in Missouri depends upon the survival of these small sand prairie remnants in the Southeast Lowlands.

The Loess Hills in Missouri

The term Mountains in Miniature is the most expressive one to describe these bluffs. They have all the irregularity in shape, and in valleys that mountains have, they have no rocks and rarely timber. – Thaddeus Culbertson, missionary, 1852

One of the things I enjoy most about the natural history of Missouri is its diversity. Lying in the middle of the North American continent, it is here where the eastern deciduous forest yields to the western grasslands. Coinciding with this transition between two great biomes is a complex intersection of landforms – the northern plains, recently scoured by glaciers; the southeastern lowlands, where the great Mississippi River embayment reaches its northern extent; the Ozark Highlands, whose craggy old rocks comprise the only major landform elevation between the Appalachian and Rocky Mountains; and the eastern realm of the vast Great Plains. This nexus of east and west, of north and south, of lowlands and highlands, has given rise to a rich diversity of natural communities – 85 in all according to Paul Nelson (2005, Terrestrial Natural Communities of Missouri). Despite the overwhelming changes wrought upon Missouri’s landscape during the past 200 years, passable examples of most of these communities still exist in many parts of the state and provide a glimpse of Missouri’s rich natural heritage.

Last month I talked about the critically imperiled sand prairie community in extreme southeast Missouri. This month, we travel 500 miles to the distant northwestern corner of the state to visit another critically imperiled community – the dry loess prairie. These communities are confined to thin slivers of bluff top along the Missouri River in Atchison and Holt Counties. The bluffs on which they lie are themselves part of a unique landform called the Loess Hills. Like the sand prairies of the southeastern lowlands, this angular landscape owes its birth to the glacial advances of the Pleistocene epoch (2.5 million to 10,000 years ago), when streams of meltwater – swollen and heavily laden with finely ground sediments (i.e., glacial “flour”) – filled river valleys throughout the Midwest during Pleistocene summers. Brutal cold during winter reduced these flows to a trickle, allowing the prevailing westerly winds to pick up the sediments, left high and dry, and drop them on leeward upland surfaces across Iowa and northern Missouri. The thickest deposits occurred along the abrupt eastern border of the Missouri River valley – at least 60 feet deep, and in places up to 200 feet. Loess (pronounced “luss”) is a homogeneous, fine-grained, quartz silt – undisturbed it is highly cohesive and able to stand in near vertical bluffs. It is also extremely prone to erosion, and as a result for 10,000 years now the forces of water have reshaped the Loess Hills into the landform we see today. Loess itself is not rare – thick deposits can be found in many parts of the world and over thousands of square miles across the Midwest. It is here, however, along the western edge of Iowa and northern Missouri – and nowhere else in North America – where loess deposits are deep enough and extensive enough to obliterate any influence by the underlying bedrock and dictate the form of the landscape.

It is this form that makes the Loess Hills so unique. The depth of the soil, its cohesiveness, its natural tendency to slump on steep slopes and sheer in vertical planes, and the action of water over the past several millenia have created a landscape of narrow undulating ridges flanked by steep slopes and numerous side spurs, intricate drainages with sharply cut gullies, and long, narrow terraces called “catsteps” cutting across the steep upper hillsides. It’s a sharp, angular, corrugated landscape, stretching 200 miles north and south in a narrow band of varying width from north of Souix City, Iowa, to its southern terminus in northwestern Missouri. Its western boundary is sharply delimited by the Missouri River valley, where lateral erosion (now halted by channelization of the river) and vertical sheering have created precipitous bluff faces. The eastern boundary is harder to delimit and is dependent upon the thickness of the loess. Deposits that fall below 60 feet in depth are unable to mask and reshape the rolling terrain of the eroded glacial till lying beneath. In general, this happens at distances of only 3 to 10 miles from the western edge of the landform.

Its southern terminus in Missouri, however, is the most arbitrary boundary. Discontinuous patches of deep loess terrain do occur as far south as Kansas City, but the dry hilltop prairies, common in the north, are gradually replaced by woodland in the south and disappear completely just north of St. Joseph. It is this interdigitation of two great biomes – the great deciduous forest to the east, and the expansive grasslands stretching far to the west – that give the Loess Hills such a fascinating natural history. This is due as much to the physical character of the Loess Hills themselves as to their ecotonal position at the center of the continent. Rapid drainage of rainwater off the steep slopes combines with direct sun and prevailing southwesterly summer winds to create very dry conditions on hilltops and south and west facing slopes, especially on the steeper slopes along the landform’s western edge. Such xeric conditions favor the growth of more drought-tolerant species derived from the western grasslands. North and east facing slopes and valley floors, protected from direct sun and drying winds, are able to retain more moisture, favoring the growth of woody plant species more common in the eastern forests. Seasonal moisture also shows a north-south gradient, with southern latitudes receiving higher annual rainfall totals that also favors the growth of woody plants, while the lower rainfall totals further north result in larger, more expansive grassland habitats. The steep slopes and rapid drainage create much more xeric conditions than those found further south in the flat to rolling terrain of the unglaciated Osage Plain, resulting in a more drought-tolerant mixed-grass prairie rather than the tallgrass prairie of western and southwestern Missouri. The distribution patterns of prairie versus woodland are dynamic and ever-changing, influenced by both natural and anthropogenic processes. Climatic conditions over much of the Loess Hills are capable of supporting either community type, both of which repeatedly expand and shrink as the balance tips in favor of one versus the other. In the past, the major influence was shifting periods of greater or lesser rainfall. During drier periods, grasslands expanded and woodlands shrank, finding refuge in only the moistest streamside habitats. Wetter periods allowed woody plants to migrate out of the valleys and up the slopes, especially those facing north and east. One particular very dry “hypsithermal” began about 9,000 years ago and lasted for several thousand years. Tallgrass prairies expanded as far east as present day Ohio, and todays tallgrass praires in the eastern Great Plains were invaded by even more drought-tolerant species from the shortgrass prairies further west. Eventually the hypsithermal abated, moisture levels increased, and the grasslands retreated in the face of the advanding forest. Not all of the drought-tolerant species were driven back, however, and scattered populations of these “hypsithermal relicts” still remain on locally dry sites far to the east of their normal range of distribution. Conspicuous examples of such in Missouri’s Loess Hills are soapweed yucca (Yucca glauca var. glauca) and the leafless-appearing skeletonweed (Lygodesmia juncea) (plant above, flower right). Both of these plants are normally found further west in the mixed grass prairies of the western Great Plains but are considered endangered in Missouri due to the great rarity of the dry loess prairies on which their survival depends. (Incidentally, note the crab spider legs extending from behind the petals of the skeletonweed flower). In total, more than a dozen plant species occurring in Missouri’s dry loess prairies are listed as species of conservation concern, along with one reptile (Great Plains skink) and one mammal (Plains pocket mouse).

As is typical, the insect fauna of the Loess Hills has been far less studied than its plants, but many of the species that have been documented in its prairies also show affinity to the Great Plains fauna. Both soapweed and skeletonweed have insect associates that rely exclusively on these hosts for reproduction, and as a result they are also highly restricted in Missouri. Evidence of one of these – a tiny cynipid wasp (Anistrophus pisum) that forms small spherical galls on the stems of skeletonweed – can be seen in the photo above. However, my purpose for visiting the Loess Hills this summer was to look for the rare and possibly endangered tiger beetle, Cicindela celeripes (see this post). Cicindela celeripes has not yet been recorded from Missouri but is known to occur in the Loess Hills of southwestern Iowa, and while I have not succeeded in finding it (yet!) I did observe several adults of this unusual May beetle species, Phyllophaga lanceolata. This May beetle occurs throughout the Great Plains in shortgrass prairie communities. Larvae feed in the soil on roots of grasses and other plants, while adults feed above ground on flowers and foliage. The heavy-bodied adults are unusual in the genus due to their conspicuous covering of scales (most species of Phyllophaga are glabrous or with sparsely scattered and indistinct setae) and by being active during the day. They are also relatively poorer fliers and are thus usually observed moving about on foot – as seen with this individual who was found on bare soil below a vertical cut. This snakeweed grasshopper (Hesperotettix viridis, ID by Eric R. Eaton) is another species more typically seen in the western United States, although populations have been found from across the continent. Preferred host plants include a variety of asteraceous shrubs, but as suggested by the common name snakeweeds (Xanthocephalum spp.) are highly preferred and account for its greater abundance in the west. Populations in northern and eastern portions of its range, which would include northern Missouri, are considered subspecies pratensis, while the more southern and western populations are considered the nominotypical subspecies. Interestingly (and unlike many grasshoppers), this species is considered beneficial by ranchers, since the plants on which it prefers to feed are either poisonous to livestock or offer little nutritional value while competing with more desirable forage plants for soil moisture. While exploring the upper slopes, I encountered sporadic plants of two of Missouri’s more interesting species of milkweed – whorled milkweed (Asclepias verticillata) and green milkweed (Asclepias viridiflora), raising my hopes that I might encounter one of the many Great Plains species of milkweed beetles (genus Tetraopes). However, the only species I observed was the common milkweed beetle, Tetraopes tetrophthalmus, which occurs broadly across eastern North America on the equally broadly distributed common milkweed (Asclepias syriaca).

It is a familiar refrain, but Missouri’s dry loess hill prairie communities are critically endangered. Historically, these communities were probably never as well developed as those further north, and only a few small remnants remain today due to significant woody encroachment following decades of fire suppression. Much of this encroachment has occurred in the past 50 years – Heinman (Woody Plant Invasion of the Loess Hill Bluff Prairies. M. A. Thesis, University of Nebraska at Omaha, 1982) used aerial photographs to show a 66 percent encroachment of shrubs and trees into the loess hill mixed-grass prairies between 1940 and 1981. Additional threats include overgrazing, erosion, invasion by exotic plant species and homesite development. Fewer than 50 acres of native dry loess hill prairie remain in Missouri – only half of which are now in conservation ownership. The majority of these can be found at Star School Hill Praire and Brickyard Hill Conservation Areas in Atchison County and at McCormack Conservation Area just to the south in Holt County. Controlled burning and selective cutting are being used at these sites to control woody plant invasions, but even these management techniques present challenges. Spring burns have been shown to promote the growth of big bluestem (Andropogon gerardii), which could allow it to encroach drier areas where mid-grasses such as little bluestem (Schizachyrium scoparium) and sideoats grama (Bouteloua curtipendula) typically dominate (Rushin 2005). Increases in tall grasses could shade out and eliminate some of the rarer low-growing forbs such as downy painted cup (Castilleja sessiliflora), locoweed (Oxytropis lambertii) and low milkvetch (Astragalus lotiflorus). Fall or winter burns may be more beneficial to forbs because the plants are allowed to complete flowering and seed set, but the steep slopes on which these communities occur make erosion a potential concern. Clearly, all factors must be considered when designing management plans for this rare and significant slice of Missouri’s natural heritage.

In addition to the links and references provided above, I highly recommend Fragile Giants: A Natural History of the Loess Hills, by Cornelia F. Mutel (1989). All of the above photographs were taken at Star School Hill Prairie Conservation Area on July 12, 2008. Additional photographs of Loess Hill habitats in extreme southwestern Iowa appeared in my earlier post, The hunt for Cicindela celeripes. The plants shown in photographs 5-7 are purple praire clover (Dalea purpurea), white prairie clover (D. candida), and lead plant (Amorpha canescens), respectively. Lastly, I would like to apologize for the length of this post – a consequence of my inability to temper my utter fascination with the natural world and desire to understand the depths its connectedness.