Economic Impact

The economic impacts of invasive plants are a results of their effects on agricultural producation as well as activities to control them, eradicate them, and remediate their damages. An impossible loss to monetarily quantify is biodiversity loss, ecosystem degradation, and aesthetic changes, but those are important effects of invasive species. Two studies have attempted to quantify the costs of invasive species in general.

The first was done in 1993 by the Office of Technology Assessment and was done as a part of their report entitled Harmful non-indigenous species in the United States. They estimate that invasive species have cost $97 billion dollars in the United States from 1906 to 1991 (see Table 1). Furthermore, in a worst-case scenario, high-impact species could cost the country an additional $134 billion in the near future. Of the total estimated cost of all nonindigenous species, the OTA attributes $603 million of loss nonindigenous plants and approximately another $3.5-5.3 billion due to agricultural weeds. However, the total costs calculated by the OTA (1993) report is a “minimum benchmark” cost for the 85 years it covers; only 14% of all known harmful nonindigenous species were included in the calculation because estimates for the other species were unavailable. The total estimate also does not include non-market impacts, environmental and health impacts.  The second study was done by Pimentel et al (1999) and estimated that nonindigenous species cost the country more than $138 billion annually, with plants accounting for $34.7 billion of that (See Table 2). Like the OTA (1993) estimated cost, Pimentel et al's figure only includes economic damages and control costs, not environmental and health costs. The difference in the two costs can be explained by the fact that Pimentel et al used ten times the number of species that the OTA report used.

Table 1. Estimated Cumulative Losses to the United States from Selected Harmful Non-Indigenous Species 1906-1991 (Table 2-2 in OTA 1993)

Category	Species analyzed (number)	Cumulative loss estimates (millions of dollars, 1991)	Species not analyzed (number)
Plants (not including agricultural weeds)	15	603	-
Terrestrial vertebrates	6	225	>39
Inserts	43	92,658	>330
Fish	3	467	>30
Aquatic invertebrates	3	1207	>35
Plant pathogens	5	867	>44
Other	4	917	-
Total	79	96,944	>478

 

The OTA report (1993) does discuss the costs of nonindigenous weeds on croplands, but it does not include those costs in its total. Using cost estimates for all weeds (including indigenous weeds) on croplands made by the Weed Science Society of America, the OTA estimate that nonindigenous cropland weeds cause $2-3 billion in crop losses annually. In addition, roughly $1.5-2.3 billion is used on herbicides to kill nonindigenous crop weeds. For instance, Sorghum halepense (Johnson grass) hybridizes with Sorghum bicolor (sorghum, a crop) to produce a useless plant called “shattercane” which outcompetes the desired crop (OTA 1993). Pimentel et al (1999) estimate that weeds cause an overall 12% reduction in crop yields or $33 billion in lost crop annually; 73% weed species are nonindigenous, so nonindigenous weeds cause $24 billion per year in crop losses. They also estimate that nonindigenous weeds account for $3 billion/yr use of herbicides, making the total cost of nonindigenous weeds to United States agriculture $27 billion (Pimentel et al 1999).

Some weeds invade rangelands and outcompete palatable forage for livestock, such as toxic Euphorbia esula (leafy spurge) (OTA 1993). Some are even poisonous. In pastures and rangelands, Pimentel et al (1999) estimate that forage losses are approximately $1 billion per year, while ranchers spend about $5 billion annually controlling nonindigenous weeds on rangelands, where they are often poisonous.

Control of nonindigenous weeds in gardens, lawns, and golf courses costs $1.5 billion annually. Approximately $36 billion annually is spent on the management of these areas annually, and approximately $1.3 billion of that is spent on residential weed, insect, and disease pest control. A large portion of these weeds are exotic (as in dandelion, Taraxacum officinale), about $500 mill is spent on residential exotic weed control and $1 bill on nonindigenous weeds in golf courses (Pimentel 1999).

Nonindigenous plants are often also hosts for pathogens or pests that damage desirable plant species. Berberis vulgaris (barberry) hosts the wheat rust fungus which reduces wheat production. Agropyron dertorum (crested wheatgrass) carries the Russian wheat aphid which is an insect wheat pest (OTA 1993).

 

OTA (1993) also estimates that $100 million is spent annually to control aquatic weeds. Hydrilla verticillata (hydrilla) is an aquatic weed that has spread throughout the country’s waterways, clogging irrigation and drainage canals, degrading water quality, reducing productivity of recreational fisheries, and impeding navigation – particular in Florida (OTA 1993, Clark et al 1998). Controlling Hydrilla verticillata  costs Florida $14.5 million annual, while the recreational losses on just two lakes costs $10 million annually (Pimentel et al).

The OTA (1993) and Pimentel et al (1999) estimates do not include health impacts of invasive species, though they do have potential market impacts. Schinus terebinthifolius (Brazilian pepper tree), which has significantly invaded Florida, causes allergic reaction in many people, including repiratory difficulties and contact dermatitis. Hydrilla verticillata may provide habitat for disease-carrying mosquitoes (OTA 1993).

Lythrum salicaria (purple loosestrife) costs $45 million annually in forage losses and control costs. Lythrum salicaria is spreading at a rate of 115,000 ha/yr and has changed the basic structure of most of the wetlands it has invaded, resulting in biomass reduction of 44 native plants and the reduction of native wildlife that depend on the native plants (Pimentel et al 1999).

Table 2. Estimated annual costs associated with some non-indigenous species introduction in the United States (x millions of dollars) (Table 1 in Pimentel et al 1999)

Category	Non-indigenous Species	Losses and Damages	Control Costs	Total
Plants	25000		subtotal	34,658
Purple Loosestrife		-	-	$45
Aquatic Weeds		$10	$100	110
Melaleuca Tree		NA	3-6	3
Crop Weeds		24,000	3,000	27,000
Weeds in Pastures		1,000	5,000	6,000
Weeds in lawns, gardens, golf courses		NA	1,500	1,500
Mammals	20		subtotal	34,105.5
Wild horses and burrows		5	NA	5
Feral Pigs		800	.5	800.5
Mongooses		50	NA	50
Rats		19,000	NA	19,000
Cats		14,000	NA	14,000
Dogs		250	NA	250
Birds	97		subtotal	2,905.6
Pigeons		1,100	NA	1,100
Starlings		800	NA	800
Reptiles and Amphibians	53
Brown tree snake		1	4.6	5.6
Fish	138	1,000	NA	1,000
Arthropods	4,500		subtotal	19,155
Imported fire ant		600	400	1,000
Formosan termite		1,000	NA	1,000
Green crab		44	NA	44
Gypsy moth		NA	11	11
Crop pests		13,000	500	13,500
Pests in lawns, gardens, golf course		NA	1,500	1,500
Forest pests		2,100	NA	2,100
Mollusks	88		subtotal	6,205
Zebra mussel		-	-	5,000
Asian clam		1,000	NA	1,000
Shipworm		205	NA	205
Microbes	20,000		subtotal	41,200
Crop plant pathogens		21,000	500	21,500
Plant pathogens in lawns, gardens, golf courses		NA	2,000	2,000
Forest plant pathogens		2,100	NA	2,100
Dutch elm disease		NA	100	100
Livestock diseases		9,000	NA	9,000
Human diseases		NA	6500	6,500
Total				$138,299.1