Introduction

The Rio Grande, born in the high Rockies in southwestern Colorado, meanders more than 1,800 miles across Colorado, New Mexico and Texas before spreading into a grand delta along the Texas/Mexico border. The Lower Rio Grande Valley (LRGV) is comprised of the four Texas counties situated within this delta: Cameron, Hidalgo, Starr, and Willacy. The biological communities that evolved within this delta (known collectively as Tamaulipan brushland) are among the most diverse in North America north of Mexico, containing a striking number of unique plants and animals. Development during the 1900s has threatened the Tamaulipan brushland (on both sides of the Rio Grande) and its associated flora and fauna.

In its efforts to conserve Tamaulipan brushland along the lower Rio Grande the United States Fish and Wildlife Service has defined eleven distinct biotic communities/habitat types of special concern. These include the following:

• Clay Loma/Wind Tidal Flats
• Coastal Brushland Potholes
• Sabal Palm Forest
• Mid-Valley Riparian Woodland
• Mid-Delta Thorn Forest
• Woodland Potholes and Basins
• Upland Thorn Scrub
• Barretal
• Upper Valley Flood Forest
• Ramaderos
• Chihuahuan Thorn Forest

Most of these biotic communities are not found anywhere else north of the Mexican border, and many are endangered or severely threatened. Each possesses unique species found nowhere else in the United States. These range from the most obvious and recognized (Texas Ebony, Texas Olive, Retama), to the most obscure (Star Cactus, Johnston’s Frankenia, Walker’s Manioc, Runyon’s Huaco, Limoncillo, Barreta.) The strategic location of the LRGV between tropical and temperate latitudes, and its unique physiographic characteristics, hosts plant and animal elements of western desert, northern, coastal, and tropical affinities. Because of the LRGV’s rich mixture of different biological elements, many groups of animals and plants are represented by greater numbers of species than in any similar region in North America. Simply put, the LRGV is the most biologically diverse region in the United States.

The LRGV's original Spanish settlers generally depended upon livestock ranching. However, beginning in the 1920s, farmers began to clear native brush for intensive irrigated agriculture. During the last 80 years, an estimated 95% of the original native brush was lost for agriculture and urban development. Of the original 40,000 acres of native Sabal palms, only 40 acres remain. Approximately 99% of the riparian vegetation on the U.S. side of the Rio Grande has been cleared, and the corresponding habitat on the Mexican side of the river is being cut at an accelerated rate. Because flood control structures on the Rio Grande have eliminated the annual floods that shaped the LRGV’s unique flood plain forests, what little forest remains is gradually evolving into a drier, less diverse woodland. Most ominously, the continuing increase in population in the LRGV will only increase the pressure on what little native habitat remains. The Tamaulipan brush of the LRGV is not only the most biodiverse region in the United States, it is also the most threatened.

To better demonstrate the remarkable richness of this region, the following inventories of native wildlife have been compiled. These lists should be considered works in progress, yet in general they represent the state of knowledge as it exists today. In addition, the authors admit to being selective in choosing which groups to illustrate. This is not to say that moths and beetles are not as effective in demonstrating the biodiversity of the region: note that over 900 species of beetles have been catalogued in the Audubon Sabal Palm Grove Sanctuary alone. The groups that have been chosen, however, are recognized by the public at large, and therefore stand a better chance of garnering the public’s attention.

Insects

The list of Odonata (dragonflies and damselflies) in the LRGV includes 69 species, plus one species known from north and south of the Lower Valley and presumed to occur (Apppendix 1). This figure, representing only four counties, totals 35% of the Texas odonates, approximately 14% of North America's odonates and the list continues to grow. Many of the LRGV's odonates exhibit tropical affinities and are widespread in Mexico; thus, the Lower LRGV's odonate fauna presents, along with the its plants, butterflies, birds, and other biotic elements, a distinctly tropical aspect. Comparatively few species are outlying representatives of northern groups; indeed, many species widespread in the

nearby Hill Country are absent from the Lower Valley. This diversity is remarkably high, given the amount of aquatic habitat the LRGV has lost during the 1900s. Many of the Rio Grande’s resacas (oxbow lakes) have been filled and various sections of the river itself have been turned into reservoirs whose fluctuating wet and dry regimes often spell disaster to aquatic insects. Insecticides and herbicides also take their toll on aquatic life. Nonetheless, these tenacious insects continue to flourish in the LRGV and some of the State’s rarest insects may be found in resacas bordered by Brownsville’s residential subdivisions.

Most species inhabiting the northern portion of the adjacent Mexican State of Tamaulipas have been recorded in south Texas, the Rio Grande acting more as a dispersal corridor than a zoogeographic barrier for aquatic insects. South Texas representatives of Neotropical genera not widespread in the U.S. include Acanthagrion, Brachymesia, Micrathyria, Neoerythromma, Neoneura, Phyllogomphoides, and Tholymis.

Some of the other as-yet unrecorded Neotropical odonates found in Tamaulipas are montane forms; these are unlikely to occur in the floodplain of the Rio Grande, but should be looked for further upriver. John Abbott's range maps were utilized to eliminate certain species of the Tamaulipan Biotic Province not known to occur in the Lower Valley. If surrounding counties were taken into consideration, a few species would be added to the list.

New species of odonates continue to be discovered in the LRGV. The LRGV has been poorly studied in regard to its odonate fauna (compared to mammals, birds, and butterflies, for example), and observers should expect this initial assessment to need amendment on a frequent basis as more attention is paid to these insects. For example, a dragonfly species new to the U.S., Gynacantha mexicana, was first noted in the LRGV as recently as Fall 1998.

Long popular among naturalists, butterflies are finally beginning to attain the status and popularity associated with high profile animal groups such as birds and mammals. During the last decade, tours, conventions, photographic outings, and workshops focusing on butterfly observation and education have become more common. New field guides, videos, and close-focusing binoculars have all enhanced the public’s appreciation of these enchanting insects. Not surprisingly, the LRGV’s array of unique North American habitats attracts an extroardinary diversity of butterflies, often species typically associated with Mexico. Because of its high diversity of tropical species, the LRGV has become a Mecca for butterfly fanciers and their organized activities. Many LRGV residents now landscape with host plants that nourish caterpillars or nectar plants that feed adult butterflies, thus attracting a variety of species to their yards. From such activities, many rare strays have been detected during the past several years.

Based upon a review of existing data, the LRGV Butterfly list includes 294 species (including historic records) observed or collected in the four counties of the LRGV. An additional 10 species [bracketed] are included as dubious records but not calculated into the total. Previous lists of Texas butterflies have expressed strongly differing taxonomic opinions. In turn, these opinions effect the total species count of each list. For this reason, and because new species continue to be discovered in the state, this total must be considered an approximation. By applying a less conservative taxonomic scheme, the list would consist of more than 300 species.

The 294 species included in the LRGV butterfly list represent approximately 40% of all butterflies recorded in North America north of Mexico. Of this total, 70 species, or nearly 10% of the North American Butterfly fauna, have not been recorded for the United States outside of the 4 counties comprising the Lower Rio Grande Valley.

An additional 48 species are not known in the U.S. from more than 5 counties outside of the Lower Rio Grande Valley. Opler (1992) defines the Lower Rio Grande Valley without Willacy County, stating that "more butterfly species, more than 300, have been found in this small area than in all the rest of eastern North America!" Only Texas (the entire state) with 423 species, Arizona with 326, and New Mexico with 318 break the "300 species barrier" (California has but 250 species of butterflies).

In summary, only three states (Texas, Arizona and New Mexico) have documented more butterflies than the LRGV. Just over two-thirds of the 423 species recorded for all of Texas has been recorded in the LRGV and 70 species are exclusively from the LRGV.

The freshwater fish fauna of the LRGV reflects its combined northern, southern, and marine heritage. Tropical representatives including Mexican Tetra and Rio Grande Cichlid swim alongside northern fishes such as Buffalofish and Freshwater Drum. Sharing the waters with them are estuarine species that venture inland such as Striped Mullet, American Eel, and Gizzard Shad. Furthermore, introduced Old World species such as Common Carp and Blue Tilapia contribute to the LRGV’s ichthyological diversity.

Threats to the LRGV’s fishes include overfishing, damage or death from contact with power turbines and poisoning from industrial pollution and pesticide or herbicide runoff. Elsewhere, tropical predators such as Oscar (Astronotus ocellatus) and Peacock Bass (Cichla ocellaris) have escaped the pet trade and become threats to native fishes. Wherever such species are raised outdoors in shallow pools, there is always a risk of accidental dissemination during high water events. Of considerable importance to many species are the agricultural demands on the Rio Grande’s waters. Irrigation results in reduced river flows, which in turn raise water temperatures, lower oxygen concentrations, and strand fishes in isolated pools or channels where they are subject to increased avian predation as well as death from desiccation.

With the worldwide decline in amphibians, the 21 species that breed in the LRGV are of particular importance. Although depauperate in salamanders (their Texas population center lies in the Hill Country), the LRGV supports more than its share of frogs and toads. These 18 frogs and toads account for 25% of all the species of these animals known in North America. Additionally, one peripheral species is likely to be found on private lands in western Starr County. One species, the White-Lipped Frog is endangered in Texas, and three additional species are threatened: Mexican Treefrog, Sheep Frog, and Mexican Burrowing Toad. Two LRGV salamanders are endangered in Texas: The Black-spotted Newt, and the Rio Grande Lesser Siren. Unusual LRGV amphibians include:

• Giant Toad
• Mexican Treefrog
• White-lipped Frog
• Rio Grande Chirping Frog
• Sheep Frog
• Mexican Burrowing Frog
• Black-spotted Newt
• Rio Grande Lesser Siren.

The continued existence of these water dependent species is inextricably linked to wetland habitats (such as resacas) and availability of wet areas.

The 11 species presented in represent approximately 30% of Texas’ 35 turtle species and forms. Half of them are recognized by federal and/or state wildlife agencies as being either threatened or endangered. Turtles of concern in the LRGV include five marine species (Loggerhead, Common Green, Hawksbill, Kemp’s Ridley, and Leatherback), all persecuted for their meat and shells, and the Texas Tortoise, long a prize of the (now illegal) pet trade. Although sea turtles are protected in the U.S. and a few other countries, numbers are still taken for food or drown in shrimp nets. Additionally, their nesting beaches are subject to disturbance by natives and tourists, as well as development as resort communities.

The only crocodilian occurring naturally in the LRGV is the American Alligator. The Spectacled Caiman (Caiman crocodilus) is occasionally seen in Texas, the result of introduction through the pet trade.

Of the 50 species (64 forms) of lizards that occur in Texas, 13 occur in the Rio Grande Delta, with an additional 8 species occurring peripherally in Starr County. Two additional species have been introduced through the pet trade to the LRGV. The lizard population includes interesting species such as the Keeled Earless Lizard, Mesquite Lizard, Rosebelly Lizard, Four-lined Skink, and Laredo Striped Whiptail.

Thirty-three species of snakes occur in the LRGV, including several that are limited in the United States to these four counties. This represents approximately 40% of the Texas snake fauna. Two species are peripheral (and most likely would be found on private lands in western Starr County), and one has been introduced from Florida. Species of concern include Ruthven’s Whipsnake, Mexican Racer, Northern Speckled Racer, Texas Indigo Snake, Black-striped Snake, Northern Cat-eyed Snake, and Texas Scarlet Snake.

The Lower Rio Grande Valley has long been known as one of, if not the, best places in the United States to watch birds. The reason for this interest is the incredible diversity and uniqueness of the birds that can be seen in such a relatively small area. A total of 485 species of birds have been recorded from the LRGV, a number that exceeds any other location of equal size north of the Mexican border.

This total represents 53% of all bird species recorded in North America. In comparison, the total number of species (as of 1998) recorded in Florida is 477, and the total for the bordering state of Louisiana is 450. Only Texas (the state as a whole), California, Arizona, and New Mexico equal or exceed this total.

The Tamaulipan Biotic Province and its special habitats support small populations of several "endemic" species or subspecies. These species are consistently found nowhere else in the United States. Many are restricted to a small geographic area that is related to the availability of a declining habitat such as Sabal Palm forest or riparian woodlands. Endemic subspecies within this province include:

• Texas Red-shouldered Hawk
• Zapata Carolina Wren
• Brownsville Common Yellowthroat.

Of greater interest to birders, however, are the tropical birds that reach their northernmost limit in or just north of the LRGV. Birders travel from all over the world to see these "Valley Specialties." Included in this group of special LRGV birds are:

• Least Grebe
• Muscovy Duck
• Hook-billed Kite
• Gray Hawk
• White-tailed Hawk
• Aplomado Falcon
• Plain Chachalaca
• Red-billed Pigeon
• White-tipped Dove
• Green Parakeet
• Red-crowned Parrot
• Groove-billed Ani
• Ferruginous Pygmy-Owl
• Pauraque
• Buff-bellied Hummingbird
• Ringed Kingfisher
• Green Kingfisher
• Northern Beardless-Tyrannulet
• Brown-crested Flycatcher
• Great Kiskadee
• Tropical Kingbird
• Couch’s Kingbird
• Green Jay
• Brown Jay
• Tamaulipas Crow
• Chihuahuan Raven
• Cave Swallow
• Clay-colored Robin
• Long-billed Thrasher
• Tropical Parula
• White-collared Seedeater
• Olive Sparrow
• Botteri’s Sparrow
• Altamira Oriole
• Audubon’s Oriole

In addition, many species from the tropics have wandered to the LRGV. Although not expected annually, these birds give credence to the notion that the LRGV functions as a major conjunction of tropical bird habitats and ranges. This list of rarities, many of which have only been recorded in the U.S. in the LRGV, includes:

• Jabiru
• White-cheeked Pintail
• Masked Duck
• Snail Kite
• Crane Hawk
• Roadside Hawk
• Short-tailed Hawk
• Collared Forest-Falcon
• Northern Jacana
• White-crowned Pigeon
• Ruddy Ground-Dove
• Ruddy Quail-Dove
• Dark-billed Cuckoo
• Mangrove Cuckoo
• Mottled Owl
• Stygian Owl
• White-collared Swift
• Green Violet-ear
• Green-breasted Mango
• White-eared Hummingbird
• Elegant Trogon
• Social Flycatcher
• Sulphur-bellied Flycatcher
• Fork-tailed Flycatcher
• Rose-throated Becard
• Masked Tityra
• Yellow-green Vireo
• Black-whiskered Vireo
• Gray-breasted Martin
• Orange-billed Nightingale-Thrush
• White-throated Robin
• Rufous-backed Robin
• Black Catbird
• Gray Silky-Flycatcher
• "Mangrove" Yellow Warbler
• Gray-crowned Yellowthroat
• Golden-crowned Warbler
• Rufous-capped Warbler
• Yellow-faced Grassquit
• Crimson-collared Grosbeak
• Blue Bunting
• "Fuertes" Orchard Oriole

Because of its strategic location between tropic and temperate, the LRGV can truly be called the Crossroads of Migration for the Americas. The processions of migrant hawks, shorebirds, waterfowl and other waterbirds, hummingbirds, and songbirds, constitute one of the New World’s avian spectacles. On a single day, a birder can witness migrations involving tens of thousands of hawks, pelicans, and other birds.

In addition to being one of the busiest migration pathways in the New World, the LRGV provides crucial stopover habitat for more than two hundred species of birds from Eastern and Western North America. These birds breed in tundra, northern forest, grasslands, subtropical scrub forest, and virtually all habitats north of the Mexican border. Without such stopover habitat, migration would be difficult, if not impossible for many species that need places to rest, feed, and avoid predators.

Finally, the LRGV is the final migratory destination and wintering area for dozens of species of birds including world class aggregations of some waterfowl and other species.

The combination of high species diversity, a large number of rare, threatened, and endangered species, large concentrations of wintering birds, several "endemic" subspecies, and a migratory pathway of hemispheric importance, makes the LRGV the most important bird area in North America.

Eighty-three species of mammals are known from (or in the case of some marine mammals, expected to occur adjacent to) the LRGV's Willacy, Cameron, Hidalgo, and Starr counties. Another half dozen or so marine mammals including several large whales may occur well offshore in pelagic waters. If the Bottle-nosed Dolphin (frequently seen from land) is retained, and 13 other marine mammals that are rare or usually seen only well away from shore are ignored, the mammal list is left with 70 species. Jaguar and Pronghorn have been extirpated from the LRGV and six mammals were introduced, leaving 62 native species one might have a chance of encountering. These 62 species represent 34% of the 181 mammals known from Texas. Among the state’s mammals, ten are unique to the LRGV and another only occurs as well in the Big Bend vicinity. The unique mammals are Mexican Long-tongued Bat, Southern Yellow Bat, Texas Pocket Gopher, Gulf Coast Kangaroo Rat, Mexican Spiny Pocket Mouse, Coues’ Rice Rat, White-nosed Coati (also in Big Bend region), Eastern Hog-nosed Skunk, Ocelot, and Jaguarundi.

Norway and Roof rats and the House Mouse were accidental Texas introductions that have caused significant damage to structures and foodstuffs, as well as spreading diseases. The South American Nutria was introduced as a furbearer, but has become a pest that ruins earthen dikes and competes elsewhere with the more valuable Muskrat. Feral Pig and Nilgai, both native to the Old World, were introduced as game animals. The former has become a damaging pest in some areas where it has decimated native flora and fauna, and has been the subject of expensive extermination campaigns. Nilgai is restricted to a number of ranches including properties in Willacy County where it is a curious and seemingly benign addition to the fauna. White-tailed Deer and Pronghorn are important game species; the latter has long been extirpated from the LRGV. Others such as the Black-tailed Jackrabbit and the two cottontails are of minor importance as game species; bobcats, coyotes, and birds of prey certainly take many more rabbits than do hunters.

Sightings of native cats are rare and often rank among a naturalist’s lifetime highlights. Unlike East Africa, where a variety of cats may be observed during a short trip, the LRGV’s cats are rare and secretive. All of the LRGV’s cats have been persecuted as the subject of hunting pressure for sport, the commercial (and often illegal) hide market, and, in the case of Jaguar and Cougar, for being suspected predators of livestock. Jaguar (Federally Endangered) has been eliminated from South Texas; Cougar remains a rare resident, and the smaller Ocelot and Jaguarundi (both Federally Endangered) are restricted to dense brush lands along the coast and the relict woodlands bordering the Rio Grande. Only the Bobcat may be considered a locally common and widespread resident.

Many of the remaining species are small, innocuous, and often nocturnal creatures that are rarely observed by humans. Of the ten species listed as unique to the LRGV, Southern Yellow Bat and Coues’ Rice Rat are listed as Threatened in Texas. In reality, any mammal species that depend on dwindling forest habitats along the Rio Grande is seriously threatened by habitat loss due to to farming and commercial development.

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