Invasive Species

Invasive Species:  Our Accidental Attack On Nature

The story of invasive species and their alteration of native habitats is one of the most disappointing tales to be told of human interaction with nature. It is full of surprises, unsolved mysteries, scientific research, and reactive management that too often exemplifies the adage, “too little too late.” This post summarizes the main elements of the plot and its conclusion. It begins about 500 years ago when people started crossing the oceans and taking boatloads of new species to North America and other parts of the world. Some of the new species invaded native habitats, replaced the natives, and became permanent residents. Of all the things people have done to the wildlife and wildlife habitat of the Agua Fria River Basin—road and building construction, ranching, logging, hunting, farming, fertilizing, recreation, burning, and water use—the most destructive has been the accidental introduction of invasive plants and animals.

Horehound (Marrubium vulgare) seeds.  This Asian invader is common in the arid uplands of central Arizona.

Horehound (Marrubium vulgare) seeds. This Asian invader is common in the arid uplands of central Arizona.

The photo shows the dry seeds of Horehound (Marubium vulgare).  The seeds have small hooks that catch on clothing and animal fur. Horehound is a small perennial shrub that forms pure stands when native vegetation is removed by livestock. The seeds often create persistent mats in animal fur, and they are irritating when they get in your socks. Cattle will eat a little Horehound when the plants are young, but they don’t eat enough to prevent the plant’s spread.

Continue:  Invasive Species Dispersal

When people began traveling across the great desert, jungle, and ocean barriers that had segregated the globe into separate biospheres, they took along a plethora of problem plants and animals. Some of the transported species flourished in their new homes. Familiar examples include the European Hare in Australia, Kudzu in the Southeastern U. S., and Tumbleweed in the Western U. S. The Center for Invasive Species and Ecosystem Health (, reports that introduced species in the U. S. that have invaded native habitats include more than 1500 animal species and 1200 plant species. And like the water in a sinking ship, the number of invasive species continues to rise.

Invasive Species Establishment

After introducing invasive species, people unwittingly help them spread and get established. We spread seeds, eggs, and whole organisms on the hulls of boats, in the treads of tires and boots, and in the hair of our herds of cattle and sheep. After helping invasive species spread, we help them get established. Every dam creates a new habitat. Every hiker’s boot, cow’s hoof print, and OHV tire tread breaks the soil surface and provides a site for invasive plants to germinate. Perhaps the greatest offense against nature in the Agua Fria River Basin has been the failure to realize what we are doing and adopt land use policies that would end our contribution to the spread and establishment of invasive species.

Invasive Species Become Permanent Residents

Some of the plants and animals that people introduce, disperse, and help get established are able to replace native species and take over native habitats. The invasive species are more aggressive than the natives. All weeds, whether they are native or introduced, have the ability to spread and grow rapidly after a disturbance. One thing our native weeds didn’t do was provide large amounts of fine fuel that increased the frequency and size of fire. The loss of 100,000,000 acres (yes, that’s one hundred million acres) of native grassland, shrubland, and woodland in the Great Basin Desert of Nevada and surrounding states to the small invader known as Cheatgrass (Bromus tectorum) has occurred because Cheatgrass provides more fine fuel than native plants ever did. This lets fires become so large and frequent that there is no time for native grasses, shrubs, and trees to recover. Cheatgrass and several similar invaders are present in the Agua Fria River Basin. You can see an example of what they do along I-17 around the Sunset Point rest stop where a perennial grass/shrubland has become a weed desert (Location:  34.187329,-112.134597). South from Sunset Point, I-17 enters Sonoran Desert vegetation dominated by Saguaro, the tall cylindrical cactus that bears the creamy white Arizona State Flower, and Palo Verde, the small green tree with no commercial value that was adopted by Arizona as its State Tree. My friend Jeff Steele and I have studied the effects of fire in the Sonoran Desert for almost 40 years. We use repeated observations in permanently staked sample plots to follow post-fire recovery. We have learned that fire kills most of the native plants, and they reseed and grow slowly.  Too slowly.  The very next year after a fire, introduced invasive plants spring up from existing seeds and spread across the site.  Within just a year or two, the site is ready to burn again.  For photographs and discussion of similar fire-weed disasters throughout North American deserts see the article by Turner et al. (2010) in the references.  The two photos below, taken from the same spot in 1901 and 2008, illustrate Cheatgrass’s conquest of the Great Basin Desert. The scene in the second photo gives a forecast of what is in store for most of the world’s arid shrublands.

1901. Cedar Mountains by G. K. Gilbert

1901. Shrubland in the Great Basin Desert.

 A barren Great Basin landscape where native vegetation was removed by fire and replaced by Cheatgrass.

A barren Great Basin landscape where native vegetation was removed by fire and replaced by Cheatgrass.


Invasive Species Story End

By unwitting dispersing invasive plants and animals, people have helped a few species permanently eliminate many others.  Some of the invaders are plants that have increased the size and frequency of both lightning and human-caused fires beyond the recovery ability of native species. In many areas the invading plants and animals are now so widespread that there is no practical means to remove them and restore original habitats; the loss of diversity and productivity is permanent.

Invasive Species Afterword

What can we do?  When it’s difficult to get enough people, corporations, and lawmakers to protect polar bears, it seems silly to suggest that they do something about invasive species. Oh, there will always be a few programs, and even serious efforts to control newsworthy species such as the Burmese Python in the Florida Everglades. But concern for wildlife loss to the other 1499 invasive animals and habitat loss to our 1200 invasive plant species is never going to turn many wheels. Perhaps the checklists below will at least satisfy someone’s urge to build a collection. If anyone wants to go down to Black Mesa this spring and check off a few weed species, call me.

Invasive Species References

AZ-WIPG.  [Arizona Wildlands Invasive Plant Working Group.]  2009.   Invasive non-native plants that threaten wildlands in Arizona:  A categorized list developed by AZ-WIPWG.  Online:

Belsky, A. J., and J. L. Gelbard.  2000.  Livestock grazing and weed invasion in the arid west.  OR Natural Desert Assn, Bend, OR.  31 p.

Billings, W. D.  1990.  Bromus tectorum, a biotic cause of ecosystem impoverishment in the Great Basin.  Pages 301-322 in G. M. Woodell, ed.  The earth in transition:  Patterns and processes of biotic impoverishment.  Cambridge University Press, Cambridge, ENG.

D’Antonio, C. M.  2000.  Fire, plant invasions, and global changes.  Pages 65-93 in H. A. Mooney and R. J. Hobbs, eds.  Invasive species in a changing world.  Island Press, Washington, DC  457 p.

Leopold, A.  1949.  A Sand County almanac and sketches here and there.  Oxford University Press, New York.  226 p.

Mack, R. N.  1981.  Invasion of Bromus tectorum L. into western North America:  An ecological chronicle.  Agro-Ecosystems 7:145-165.

Mortensen, D. A., E. S. J. Rauschert, A. N. Nord, and B. P. Jones.  2009.  Forest roads facilitate the spread of invasive plants.  Invasive Plant Science and Management 2:191-199.

NISIMS.  2010.   National Invasive Species Information Management System.

Parker, K.  1972.  An illustrated guide to Arizona weeds with drawings by Lucretia Breazeale Hamilton.  University of Arizona Press, Tucson.  338 p.  (Also available online: 

Rogers, G. F.  1982.  Then and Now: A Photographic History of Vegetation Change in the Central Great Basin Desert.  Univer­sity of Utah Press, Salt Lake City, UT.  188 p.

Rogers, G. F.  1985.  Mortality of burned Cereus giganteus.  Ecology 66:630-632.

Rogers, G. F. 1986. Comparison of fire occurrence in desert and nondesert vegetation in Tonto National Forest, Arizona. Madroño 33:278-283.

Rogers, G. F. and J. Steele.  1980.  Sonoran desert fire ecology: Adaptive strategies of perennial plant species.  U.S. Forest Service, General Technical Report RM 81:15 19.

Schmid, M., and G. Rogers.  1988.  Trend in fire occurrence in the Arizona Upland subdivision of the Sonoran Desert.  Southwestern Naturalist 33:437-444.

Tuner, R.M., R.H. Webb, T.C. Esque, and G. Rogers. 2010. Repeat photography and low elevation fire responses in the southwestern United States. Pages 223-244 in R. H. Webb, D. E. Boyer, and R. M. Turner, eds. Repeat photography methods and applications in the natural sciences. Island Press, Washington DC. 530 p.

Whitson, T. D., L. C. Burrill, S. A. Dewey, D. W. Cudney, B. E. Nelson, R. D. Lee, and R. Parker.  2006.  Weeds of the West.  Western Society of Weed Science, Las Cruces, NM.  626 p.

Young, J. A., R. A. Evans, and J. Major. 1971.  Alien plants in the Great Basin.  Journal of Range Management 25:194-201.

Invasive Animals Checklist

I couldn’t find an invasive animal list for the Agua Fria River Basin. The list below is for the state, but it includes only those species with profiles by the National Invasive Species Information Center of the U. S. Department of Agriculture. I don’t know how many species are not included. Two important species that were not included, Silver Carp and Northern Crayfish, are listed by the Arizona Game and Fish Department as members of the state’s 10 most unwanted species. I added them to the list and marked them and other members of Arizona’s 10 most unwanted with an asterisk (*) in the check box. I couldn’t find a ranking of levels of concern.

Invasive Animals of Arizona

Latin Name Common Name
Aquatic Animals
Var. Asian Carps
* Hypophthalmichthys molltrix Silver Carp
(Alosa pseudoharengus) Alewife
(Bythotrephes longimanus) Spiny Water Flea
(Carcinus maenas) European Green Crab
(Channa argus) Northern Snakehead
(Corbicula fluminea) Asian Clam
(Didemnum vexillum) Sea Squirt
* (Dreissena bugensis) Quagga Mussel
(Dreissena polymorpha) Zebra Mussel
(Eriocheir sinensis) Chinese Mitten Crab
(Gymnocephalus cernuus) Eurasian Ruffe
(Hemigrapsus sanguineus) Asian Shore Crab
(Lithobates catesbeianus) Bullfrog
(Monopterus albus) Asian Swamp Eel
(Myocastor coypus) Nutria
(Neogobius melanostomus) Round Goby
(Orconectes rusticus) Rusty Crayfish
* Orconectes virllis Northern Crayfish
(Petromyzon marinus) Sea Lamprey
(Phyllorhiza punctata) White Spotted Jellyfish
* (Potamopyrgus antipodarum) New Zealand Mud Snail
(Pterois volitans) Lionfish
(Pylodictis olivaris) Flathead Catfish
(Rapana venosa) Veined Rapa Whelk
(Styela clava) Clubbed Tunicate
Invertebrate Species
(Adelges tsugae) Hemlock Woolly Adelgid
* (Aedes albopictus) Asian Tiger Mosquito
(Agrilus planipennis) Emerald Ash Borer
(Anastrepha ludens) Mexican Fruit Fly
(Anoplophora chinensis) Citrus Longhorned Beetle
(Anoplophora glabripennis) Asian Long-Horned Beetle
(Apis mellifera scutellata) Africanized Honeybee
(Bemisia argentifolii) Silverleaf Whitefly
(Cactoblastis cactorum) Cactus Moth
(Ceratitis capitata) Mediterranean Fruit Fly
(Coptotermes formosanus) Formosan Subterranean Termite
(Diaphorina citri) Asian Citrus Psyllid
(Diuraphis noxia) Russian Wheat Aphid
(Epiphyas postvittana) Light Brown Apple Moth
(Halyomorpha halys) Brown Marmorated Stink Bug
(Heterodera glycines) Soybean Cyst Nematode
(Homalodisca vitripennis) Glassy-Winged Sharpshooter
(Ips typographus) European Spruce Bark Beetle
(Lissachatina fulica) Giant African Snail
(Lymantria dispar) European Gypsy Moth
(Maconellicoccus hirsutus) Pink Hibiscus Mealybug
(Pectinophora gossypiella) Pink Bollworm
(Scirtothrips dorsalis) Chilli Thrips
(Sirex noctilio) Sirex Woodwasp
* (Solenopsis invicta) Red Imported Fire Ant
(Tomicus piniperda) Common Pine Shoot Beetle
Vertebrate Species
(Boiga irregularis) Brown Tree Snake
(Rhinella marina) Cane Toad
(Sturnus vulgaris) European Starling
(Sus scrofa) Wild Boar

Invasive Plants of the Agua Fria River Basin Checklist

Information on invasive plants was better. I used a recent botanical survey by staff of the Desert Botanical Garden (DBG), and lists compiled by Tonto National Forest (TNF) and the Arizona Wildlands Invasive Plant Working Group (AZ-WIPWG) to compile a list of invasive plants in the Agua Fria National Monument near the center of the Basin. The Monument is large and covers most of the habitats found in the Basin. The list includes rankings of the threat posed by each species. The species of Highest Concern were designated by Andrew Salywon of DBG, and AZ-WIPWG.

There is an important difference between the TNF and AZ-WIPWG ranking systems. Red Brome, for instance, is ranked high (H) by AZ-WIPWG and low (C) by TNF. The high ranking is due to the plant’s heavy production of fine-fuel that facilitates the spread of large fires. The low ranking by TNF is due to the impossibility of eradicating or even controlling the plant because it has so completely saturated the region. The reasons for the ranking of Buffelgrass and Fountain Grass are similar to Red Brome, but neither species is as widespread as Red Brome, and eradication might still be possible.

Plant names are linked to information on the websites of the Southwest Environmental Information Network ( ) and TNF. AZ-WIPWG is abbreviated AZ in the table.

Invasive Plants of the Agua Fria National Monument

Latin name

Common name



Highest Concern

Centaurea melitensis* Malta starthistle A
Oncosiphon piluliferum* Globe chamomile B
Pennisetum ciliare* Buffelgrass C H
Pennisetum setaceum* Fountain grass C H
 Other Weeds of Agua Fria National Monument
Acroptilon repens Russian knapweed A H
  Alhagi maurorum Camelthorn A M
Amaranthus albus* Tumble pigweed
Amaranthus blitoides* Prostrate pigweed
  Arundo donax Giant reed B H
  Asphodelus fistulosus Onionweed A L
Avena fatua* Wild oats C M
Brassica tournefortii* Asian mustard C M
Bromus diandrus ssp. rigidus* Ripgut brome C M
Bromus rubens* Red brome C H
Bromus tectorum* Cheatgrass C H
  Cardaria draba Globe-podded hoary cress A M
  Cardaria pubescens Hairy white-top A M
  Carduus acanthoides Plumeless thistle A
  Carduus nutans Musk thistle A M
  Cenchrus echinatus Southern sandbur A
  Cenchrus spinifex Field sandbur A
  Centaurea biebersteinii Spotted knapweed A M
  Centaurea solstitialis Yellow starthistle C H
  Chondrilla juncea Rush skeletonweed A M
  Chorispora tenella Blue mustard A
  Cirsium arvense Canada thistle A M
Convolvulus arvensis* Field bindweed C M
Cynodon dactylon* Bermudagrass
  Dimorphotheca cuneata White bietou A
Echinochloa crus-galli* Barnyard grass L
  Eichhornia crassipes Water hyacinth H
  Eleagnus angustifolia Russian olive A H
Eragrostis curvula var. conferta* Weeping lovegrass C L
Eragrostis curvula var. curvula Weeping lovegrass C L
Eragrostis lehmanniana* Lehmann’s lovegrass C H
Erodium cicutarium* Stork’s bill
  Euphorbia esula Leafy spurge A H
  Euryops subcarnosus Sweet resinbush A H, ­
  Isatis tinctoria Dyer’s woad A
Kochia scoparia* Kochia A
  Lepidum latifolium Perennial pepperweed H
  Leucanthemum vulgare Oxeye daisy A L
  Linaria dalmatica Dalmatian toadflax A M, ­
  Linaria vulgaris Yellow toadflax A M
  Lythrum salicaria Purple loosestrife A
Marrubium vulgare* Horehound
Melilotus albus* White sweetclover
Melilotus indicus* Indian sweetclover
Melilotus officinalis* Yellow sweetclover C M
  Myriophyllum aquaticum Parrot’s feather H
  Myriophyllum spicatum Eurasian watermilfoil H
Parkinsonia aculeata* Mexican palo verde A
  Peganum harmala African rue A
  Pentzia incana Karoo bush A
  Polygonum cuspidatum Japanese knotweed A
Polypogon monspeliensis* Rabbitsfoot grass
  Potentilla recta Sulfur cinquefoil A
Salsola tragus*and S. Kali Russian thistle C
  Salvia aethiopis Mediterranean sage A
  Salvina molesta Giant salvinia H
Schismus arabicus* Arabian schismus C M
Schismus barbatus* Mediterranean grass C M
Sinapis arvensis* Wild mustard B
  Tamarix chinensis Five-stamen tamarisk C H
  Tamarix parviflora Smallflower tamarisk C H
Tamarix ramosissima* Salt cedar C H
  Ulmus pumila Siberian elm A M

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