Alabama is one of the most biologically diverse states in the United States, and with regard to some plants and animals, in the world. This diversity is a product of Alabama's warm, moist climate, its great geologic diversity, and its rich evolutionary past. With more than 4,533 documented species, Alabama ranks fifth among states in terms of overall species diversity, and is first among states east of the Mississippi River. The large western states of California, Texas, Arizona, and New Mexico lead the nation, and fellow southeastern states Georgia (sixth) and Florida (seventh) trail Alabama. Alabama harbors 64 types of terrestrial ecosystems, including 25 forests and woodlands, 11 wetlands, and 7 glades and prairies. The state also supports 77,000 miles of rivers and streams and several dozen marine ecosystems.
Alabama ranks first among states or is at or near the top for several types of animals and plants. It is number one for diversity of freshwater mussels, freshwater fishes (Tennessee is a close second), freshwater snails, crayfish, and freshwater turtles. It is third for combined diversity of amphibians and reptiles and seventh for number of species endemic to the state, that is, species found only within Alabama. Some of the state's most famous endemic species include the Red Hills salamander, Alabama sturgeon, and the Alabama beach mouse. Alabama is also first in carnivorous plant biodiversity and is one of the world's hotspots for these unusual organisms. The extensive cave system in the northeastern part of the state harbors the third most biodiverse cave fauna in the temperate world. Considering that only 10 percent of these caverns have been explored, Alabama's eventual ranking may be at the very top.
The Influence of Climate
Alabama's location on the planet, between 30 and 35 degrees north of the equator, means that it receives an abundance of sunlight, more so than states farther north. Thus, Alabama's ecosystems have a relatively high rate of biological productivity, which is the sum of all energy captured by organisms in an ecosystem over time, including photosynthesis. In addition, this position allows for a relatively long growing season, which further boosts biological productivity. Indeed, the seven U.S. states with the highest biodiversity rankings are found along the nation's southern border.
Abundant water resources also boost biological productivity, and Alabama is very wet. The Deep South competes with the Pacific Northwest as the rainiest region in the country but is much more biologically diverse because of its higher temperatures. The waters of the Gulf of Mexico are an important factor in Alabama's climate, especially the influence of the Loop Current. Throughout the year, this current enters from the Caribbean Sea between Cuba and the Yucatan Peninsula, heads north toward Alabama, then curves to the southeast and exits the Florida straits. In the process, the current delivers warm tropical waters to the Alabama coast. Evaporation from the northern Gulf provides water vapor that falls on the state as rain throughout the year. Approximately 50 percent of the rainfall in the eastern United States is water recently evaporated from the Gulf. A glance at any world map will reveal that the latitudes in which Alabama resides typically harbor deserts. Instead, and thanks to the Gulf of Mexico, the Southeast is lush and wet.
High levels of heat and moisture can also produce volatile weather patterns, but these, too, support the state's species diversity. In particular, convective storms generate lightning, and the resulting wildfires have kept the Southeast burning for millions of years. The role that fire plays in shaping the biodiversity of the region cannot be overstated. Prior to widespread human settlement, most of the state would burn several times a decade. These fires maintained prairies and open woodlands and prevented fast-growing shrubs and trees from overtaking and monopolizing ecosystems. The plentiful sunlight reaching the ground supported hundreds of less-competitive, sun-loving species, especially wildflowers and grasses. These species evolved strategies to cope with the frequent, low-intensity fires. Within the longleaf pine woodland (Alabama's most widespread native ecosystem), it is possible to find more than 30 fire-adapted species per square meter of soil.
The Influence of Geologic Diversity
Alabama's rich geologic diversity has played an even more important role than climate in shaping the state's biodiversity. Variations in soil types, bedrock exposure, and topography affect biological productivity by moderating the availability of heat, light, water, and nutrients. As a rule, and all else being equal, regions with more geologic variation have more species. Alabama has a high degree of geologic variation relative to most other southeastern states because of the many rock types uplifted by the rise of the Appalachian Mountains during the last 170 million years of the Paleozoic Era (542-251 million years ago).
Alabama has many unique ecosystems directly related to the soil type on which they are found. For example, the quartz sand deposits along the coast support dune communities sparsely populated by grasses and shrubs. Though the dune sands are frequently doused by rain, water seeps through them quickly, and the resulting ecosystem is more akin to desert than any other ecosystem in the state. In contrast, the marine chalks of the Blackland Prairie region supported tallgrass prairies before they were largely converted to cotton plantations. Trees were kept at bay by frequent fire and the dense chalk, which was nearly impenetrable to tree roots and groundwater. Instead, grasses and wildflowers more typical of the tallgrass prairies of the Great Plains presided. Scattered throughout Alabama's coastal plains are seepage bogs, which are wetlands created by groundwater forced to remain on the surface by a thick layer of nonporous clay. These sunny wetlands support wildflowers, grasses, and most of the state's roster of carnivorous plants.
Stark illustrations of the connection between geology and biological diversity are Alabama's glades. These treeless ecosystems on exposures of bedrock support hundreds of rare and unusual plant species. The glades are only found within the mountainous physiographic regions in the northern half of the state. Much of the ground is exposed rock, but where thin soils develop, an assembly of hardy wildflowers and grasses grow. Trees are unable to gain a foothold, however. Glade types in Alabama, named for the type of rock upon which they are found, include limestone, dolostone, sandstone, and granite because each rock type weathers into a unique soil and supports its own collection of plant species. Most glade wildflowers complete the year's growth and reproduction in a few weeks in late spring, before the glades become unbearably hot. Because many glade wildflower species are not currently found in other ecosystems of the state, the glades contribute significantly to the state's list of endemic plant species.
Geologic processes also created and maintain the state's diverse topography, which is the arrangement of surface features such as mountains, plains, and rivers. Alabama's terrain ranges from the flat Coastal Plain province to the rugged peaks of the Valley and Ridge province in the state's northeast. Each of Alabama's five physiographic provinces has a distinctive topography, and this variation promotes ecological and species diversity. Within these landscapes nest an array of diverse ecosystems, each having a unique combination of soils and topography, and each subsequently supporting a distinctive combination of plants and animals.
In the southern Cumberland Plateau province, for example, different rates of erosion have produced a template upon which many varied ecosystems survive side by side. The region is dominated by broad mountains, such as Sand Mountain and Lookout Mountain, with relatively level upper surfaces. These plateaus persist because a cap of sandstone (a very durable rock), persists near the surface and resists erosion. Lowlands adjacent to the plateaus exist where the sandstone cap was fractured as the Southern Appalachian Mountains grew. Water eroded the fragments away and then cut downward through strata of softer rock, such as underlying shale and limestone.
Up on the plateau surface, weathering of the sandstone produces well-drained sandy soils. Before they were transformed via logging and agriculture, these plateau surfaces supported dry, open woodlands that burned once or twice a decade. Shortleaf pine and a handful of fire-tolerant oak species dominated. Interspersed within these woodlands were dry glades on flat exposures of particularly durable sandstone. These glades support rare plants such as Little River Canyon onion (Allium speculae) and the elf orpine (Diamorpha smallii). Lower areas of the plateau, where erosion has been more persistent, collect groundwater and support bogs with green pitcher plants. At the margin of the plateau, where the sandstone caprock ends, a steep escarpment stretches down to the valley below. The escarpment supports lush broadleaf forests and a handful of other moist ecosystems, such as acidic cliffs and heath bluff forests where fire is absent. The plants and animals living here are different species than those living on the plateau above.
Alabama's varied topography and soils are the product of hundreds of millions of years of geologic history in which an ever-shifting climate, the rise and erosion of mountains, stream and river formation, and changes in sea level have continuously altered habitats and ecosystems. These environmental changes triggered the evolution of many new species unique to the state or the region through a process known as speciation. Speciation can occur via several evolutionary pathways, but the most common method begins when a unified population becomes split into two or more isolated populations because a barrier separating the populations prevents the movement of organisms between the populations. Over millions of years, the isolated populations diverge genetically as they adapt to changes in their unique environments. With enough genetic divergence, these populations become new species. These isolating barriers, such as the expansion of rivers and the rise of mountain ranges, are usually produced through significant geologic change, which is why changes in a region's geology are often linked to the emergence of new species.
Salamanders are one of several groups of southeastern organisms that are famous among biologists for their degree of speciation. The United States has more species of salamander than any other country in the world, and that diversity is centered in the southern Appalachian Mountains, which begin near Birmingham and extend northeastward in the state. These mountains harbor more than 40 species and subspecies of salamander. The salamander species of the family known as Plethodontidae have undergone multiple speciation events and comprise more than half the salamander species in Alabama. Scientists believe the origins of the Plethodontidae date to the Jurassic period (200-146 million years ago), when an ancestral lineage of salamanders became adapted to slightly drier conditions and then spread across the northern hemisphere, including the Appalachians. During warm, wet periods their ranges expanded, but during lengthy cool, dry periods, various populations retreated to and became isolated in mountain valleys. Some of these isolated populations evolved into new species found only in the southern Appalachians.
The state's diverse array of plant species derives largely from the Pleistocene (2.6 million years ago to 11,700 years ago). Many plant species in the Appalachian forests share a peculiar biogeographic pattern in that they are very closely related to species in East Asia. The two regions share more than 100 plant genera, including wildflowers such as mayapple, ginseng, and jack-in-the-pulpit, and trees such as beech, catalpa, chestnut, oak, and tulip poplar. In fact, the two floras are more alike than either is to the neighboring floras of western North America and Europe. The beginnings of this pattern emerged in the Early Tertiary period (65-50 million years ago), when a vast deciduous forest encircled the Northern Hemisphere. Species migrated from continent to continent across temporary land bridges. During the Pleistocene, temperate forests were forced south by the advancing glaciers. In Europe and central Asia, mountains trending east to west blocked the migration of plant species to warmer latitudes, and this type of forest disappeared. In the eastern United States and East Asia, however, north-south trending mountains enabled species to migrate south. For many species, this resulted in their population being split in two, with the frigid arctic temperatures and the oceans keeping these populations isolated from one another. Sufficient time has elapsed for these isolated populations to diverge genetically and evolve into new species.
The most dramatic influence of the southern Appalachians on Alabama's species diversity can be found in its freshwater fauna. When the Appalachians fractured the southeastern landscape into multiple watersheds, this fragmentation created many opportunities for the evolution of aquatic species. Populations of fish, snails, mussels, and crayfish are easily isolated in the shallow streams in the headwaters of major rivers. Owing to that isolation and subsequent genetic divergence, more freshwater species have arisen in the Southeast than in any other place in the temperate world. Alabama is at the center of this aquatic biodiversity hotspot. Speciation was especially rampant in the Tennessee River and the Mobile River Basin. An extreme case is the vermilion darter, a species restricted to one small watershed, Turkey Creek, in Jefferson County.
Biodiversity and the Twenty-First Century
Alabama's ecosystems and species have not been as well-studied as most states. With it now enjoying more scientific attention, there will likely be hundreds of new species discovered in Alabama this century. Just in the first decade, at least 76 species new to science were discovered in Alabama. This list includes the Red Hills azalea; two pancake batfishes; an 11-inch-long Tennessee bottlebrush crayfish; several cave species, including two crayfish; three flesh flies that breed in pitcher plants; several fishes including the Tallapoosa sculpin and the Alabama bass; the cypress floater (a mussel); 32 plants, most being wildflowers; and two trapdoor spiders, one named after musician Neil Young (Myrmekiaphila neilyoungi) and the other named after the Auburn University mascot (Myrmekiaphila tigris).
While there is much to celebrate about Alabama's biodiversity, there are also many disturbing trends. With 90 extinct species, Alabama ranks number two in the United States for total extinctions (species that have disappeared forever). What's more, the state ranks fourth for species in danger of extinction: approximately 1 out of every 7 species in the state is at risk. Many of these species have populations that are declining. Habitat loss and degradation have caused the great majority of these extinctions and endangerments, especially water pollution and the damming of rivers. Non-native invasive species and climate change are novel threats that forcing many species towards endangerment and extinction.
As Alabama's ecosystems lose their species, the quality of ecological "services" provided declines. Ecological services are
beneficial processes that ecosystems provide to for free. The services include supplying freshwater for drinking and irrigation,
providing places for outdoor recreation, and absorbing floodwaters. Ecosystems provide better quality of services when their
flora and fauna are intact. Thus, declines in biodiversity in ecosystems threaten human culture, especially the economy and
quality of life. Conversely, when Alabamians protect the state's biodiversity, they are investing in its future.
Duncan, Scot. Southern Wonder: Alabama's Surprising Biodiversity. Tuscaloosa, Ala.: University of Alabama Press, 2013. (Forthcoming)
Boschung, Herbert T., and Richard L. Mayden. Fishes of Alabama. Washington, D.C.: Smithsonian Books, 2004.
Lacefield, Jim. Lost Worlds in Alabama Rocks: A Guide to the State's Ancient Life and Landscapes. Tuscaloosa: Alabama Geological Society, 2000
Stein, Bruce. A. States of the Union: Ranking America's Biodiversity. Arlington, Va.: NatureServe, 2002.
Stein, Bruce A., Lynn S. Kutner, and Jonathon S. Adams eds. Precious Heritage: The Status of Biodiversity in the United States. New York: Oxford University Press, 2000.
James D. Williams, Arthur E. Bogan, and Jeffrey T. Garner. Freshwater Mussels of Alabama and the Mobile Basin in Georgia, Mississippi, and Tennessee. Tuscaloosa: University of Alabama Press, 2008.
R. Scot Duncan
Published June 12, 2013
Last updated June 13, 2013