INVASIVE SPECIES AND DROUGHT

Kathryn Thomas, USGS

An invasive species is an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health

How does drought change plant physiology?

Changes

• Slower growth

• Thickened cuticle

• Accumulation of nitrates, oxalates and other chemicals

Outcomes

• Foliar herbicides less effective

• Potentially increased toxicity to grazing and browsing animals

Which invasive non-native plants are drought hardy?

Decreasers

•Annual Grasses (Note: Cheat grass [Bromus tectorum] exploded throughout most parts of Arizona during the 2002-3 winter. It may be that late-winter-early Spring species such as cheat grass do not experience drought stress and they may increase during drought)

•

Increasers

•Leafy spurge

•Squarrose knapweed

•Diffuse knapweed

•Scotch thistle

•Hoary cress

•Jointed goatgrass

•Yellow starthistle

•Kochia

•Russian thistle

•Tamarisk

Changes in the landscape associated with invasive plants

Plant characteristics

•High drought tolerance

•Contributors to fine fuel and flammable biomass

•Rapid colonizers

•Long deep roots particularly for phreatophytes

Possible outcomes: Competitive replacement of native plants

•Plant cover may decrease

•Increased landscape fire vulnerability

•Early succession may be dominated by invasives

•Floodplains/riparian areas are highly impacted

Other environmental influences and invasive plants

•Increased temperature (warmer winters may allow for new invasives to colonize; hotter summers may lead to increased mortality of native plants, opening up the habitat).

•Increased CO2 and NO2

•Grazing/browsing by wildlife and domestic livestock

Possible outcomes

•Elevation expansion of species, including frost sensitive species

•May enhance performance of some invasive plants

•Increased stress on native species, enhanced dispersal from roadsides & water sources

Conclusions

•Some invasive non-native plants, particularly perennials, will be very competitive and successful during drought; annual grasses will be less so

•New landscapes patches will open up for colonization

•New landscape habitats will open up for colonization

•Animal movements may accelerate some dispersal

DISEASES AND DROUGHT

Charles vanRiper USGS

TRENDS

lDiseases and vectors will move up in elevations as temperatures increase

lDisease Vectors will generally become more concentrated around existing water sources

lWildlife, humans and vectors will come into increasing overall contact because of concentrated water sources

PREDICTIONS

Diseases that will Decrease

lPlague: Most likely, drought would decrease plague risk as drought may have a negative impact on reservoir rodent populations and consequently, on flea vectors.  Higher temperatures seem to have a definite negative effect on plague activity as it adversely affects a fleas ability to form Y. pestis blocks. Wetter years can cause an increase in plague activity, but it is usually a delayed effect (1-2 years).

lRocky Mountain Spotted Fever

lHanta Virus: Drought can have a negative effect on hantavirus in wild areas because Peromyscus mouse populations may plummet. However, "articificial" water sources created by human habitation in rural areas can sustain ample mouse populations, especially rural homes, ranches, and farms. So this  reduction in hanta  risk may not be across the board, and we may still see some human cases of HPS in drier years.

Diseases that will Increase

lRabies: Drought can help trigger rabies epizootics in enzootic areas w/ terrestrial animal reservoirs such as foxes and skunks as animals become more concentrated around limited water sources.

lWest Nile Virus: The 1999 WNV outbreak in NYC occurred during drought conditions - perhaps creating highly concentration of organic water habitats which favor Culex pipiens breeding, and other key factors that concentrat birds and vectors in common areas.

New Disease that Might Occur

lMalaria

lChagas Disease

lDengue Fever

lYellow Fever

Unpredictable Diseases

lAborovirus (Mosquito-borne): There are too many variables (preponderance of vector vs. nuisance sp., location, habitat, reservoirs, surveillance bias, etc.) to predict whether arboviruses will be affected by drought

lTularemia: Predictions concerning Tularemia are very difficult. Drought can concentrate rabbit populations somewhat,  and/or reduce populations  (ie, pos. and  negative impacts).  Drought may also reduce breeding habitats  for deerfly vectors.

DISEASE SUMMARY
Consequences of Drought

lSome diseases will remain the same

lSome diseases will increase

lSome diseases will decrease

lNew diseases will enter the system