May 2019
Cats – The Invasive Species in Your Backyard

Partly based on this article at

Domesticated cats (Felis catus) are now listed as one of the top 100 invasive species worldwide by the International Union for the Conservation of Nature. They are directly responsible for the extinction of a number of animal species around the world. A recent study found that in the U.S., the popular pet is estimated to kill 1.3 - 4.0 billion birds and over 6.3 - 22.3 billion mammals annually, and that doesn't even include reptiles and amphibians. The study concluded that "free-ranging cats cause substantially greater wildlife mortality than previously thought and are likely the single greatest source of anthropogenic mortality for US birds and mammals." While the biggest threat is posed by feral cats — domesticated breeds that don't have an owner and aren't socialized to humans — even common house cats that are well cared for and fed will hunt and kill if let outside. (See this article and this webpage on research using cameras attached to house cats for an interesting view of their world as they roam.) Cats also transmit diseases. In 2014, of the domestic animals that contracted rabies in the U.S., which can then be spread to the local wildlife and humans, roughly 60 percent of them were cats.

And yet, we humans have been happy to make this invasive species feel at home. We let them wander outdoors, and even feed feral cats.

Like nearly all invasive species, cats have rapid rates of reproduction. Females can start breeding at just 6 months old and can breed every 4 months, producing up to 12 kittens every year. In just the last 40 years, the number of domestic cats across America has tripled. While it's difficult to get an accurate count of feral cats, estimates suggest that today there are at least 30 million of them roaming our streets and neighborhoods. An additional 40 million pet cats have regular access to the outdoors. There are many colonies of feral cats that are maintained by people.

Managing cats in the United States is a controversial topic. Given the clear ecological damage they cause, it's not surprising that many managers of parks and other green spaces would like to be rid of them. One way to do so is of course to kill them, but an alternative approach advocated by animal rights groups and others is the trap-neuter-return (TNR) program, in which feral cats are trapped and neutered and then returned to their colonies. They argue that this is humane as it allows the cats to live while reducing the population over time, reducing the impact on wildlife. Critics of these programs point out that the "humane" part of the argument is moot because the neutered feral cats are vulnerable to disease and injury that can't be treated – the cats are being left to "fend for themselves". For natural resource managers, it is this "fending for themselves" that makes the neutered cats problematic – they are still out there killing wildlife. The same study that determined the ecological damage cats cause in the U.S. noted, "Claims that TNR colonies are effective in reducing cat populations, and, therefore, wildlife mortality, are not supported by peer-reviewed scientific studies." And in many cases, even if the feral cats were eliminated, the domesticated cats that people allow to roam would still negatively impact those natural areas.

In contrast to the U.S., Australia has largely accepted that cats are such an extreme threat to its native fauna that it has set a goal of removing two million cats between 2015 and 2020, according to an excellent article in the New York Times. Cats are the likely main cause of the extinction of 22 of the 34 mammal species that have gone extinct there. And you may think much of Australia is desert and inhospitable to cats, but it is estimated that a whopping 99.8% of the country hosts cats!

So, if you have a cat, don't let it be part of the problem. Don't let it roam outside (the best option), or place a bell or other noise-maker on it. Be sure it is neutered so that it can't contribute to the feral cat population. Finally, volunteer to educate your neighbors about the dangers cats pose, and to help manage feral cats in your neighborhood.

Related: New Zealand Research Explores How to Manage Feral Cats.

feral cats
Credit: Denise Gregoire, U.S. Geological Survey

Feral Cats in trash
Credit: Pest Solutions

feral-domestic-cat-catpures-juvenile-Southern-brown bandicoot
Feral cat with a juvenile southern brown bandicoot (Isoodon obesulus). Photo: Sarah MacLagan

cat eating bird
Feral cat eating a bird. Credit: CNN

Zebra Mussels Found in More Texas Lakes

The Texas Parks and Wildlife Department (TPWD) reported this month that four Central Texas lakes received upgraded zebra mussel classifications this month. Three of them have been added to the list of nine "positive" lakes where zebra mussels have been detected on more than one occasion: Lake Dunlap, Lake Granger and Lake Walter E. Long. The fourth is the newest addition to the list of five "suspect" lakes where zebra mussels or their larvae have been found only once in recent history, Lake Placid. Currently, 15 Texas lakes are "infested" with an established, reproducing population of zebra mussels.

“Memorial Day weekend is the unofficial kick-off to boating season in Texas, and while we want everyone to have a great time, we also want them to avoid giving free rides to invasive species when they travel to new lakes,” said Brian Van Zee, TPWD Inland Fisheries Regional Director. “The best way to help keep Texas lakes fun for everyone and prevent the spread of destructive invasive species is to clean, drain and dry your boats and equipment – every time.”

For more information, please see TPWD's news release.

Credit: Amy Benson, U.S. Geological Survey

tpwd-logo-large to Move to the Texas Invasive Species Institute at San Houston State University

During the 2019 Fiscal Year, will be moving from the University of Texas at Austin Lady Bird Johnson Wildflower Center to the Texas Invasive Species Institute (TISI) at Sam Houston State University. The website and the TISI website ( will be merged, with the intention of improving the user experience, including on mobile devices. As far as the other programs offered by and TISI, we will uphold the same high standards you've come to expect for our presentations, free workshops, and other outreach. We are excited about these coming changes!

ti logo

tisi logo Receives APHIS-Farm Bill Funding for FY2018!

We are very pleased to announce that United States Department of Agriculture's Animal and Plant Health Inspection Service (APHIS) awarded the Texas Invasive Species Institute funding from the Farm Bill to enhance the's Sentinel Pest Network and Invaders of Texas Citizen Scientist Programs, and TISI's invasive species outreach programs on pest insects. The award supports the combining of the two programs, as described above, as well as the two programs' public education efforts, including free workshops and webinars, and of course the iWire.

The TISI and teams and cooperators are excited to continue this engaging 14-year partnership with USDA-APHIS to prevent the introduction and spread of invasive species.

Follow this link to learn more about the FY19 Farm Bill, and this link for the list of funded projects.

red imported fire ant


New Bill Awaiting Governor's Signature Makes It Easier to Harvest Feral Hogs

According to this article by Texas Sports Nation, "It will be cheaper for some hunters to take feral hogs (Sus scrofa) under terms of SB 317, which the legislature adopted and awaits action by the governor. The legislation exempts persons who hunt feral hogs on private property with the consent of the landowner from having to hold a valid Texas hunting license... SB 317 is set to become effective Sept. 1."

feral hogs and piglets
Credit: TPWD

New Bill Awaiting Governor's Signature Puts Funding for State Parks in the Constitution

The same article by Texas Sport Nation describes new legislation that will put a change in the Texas constitution before the voters in November that will affect funding for Texas parks and thus potentially funding for invasive species management in the parks. "Perhaps the most significant legislation directly affecting outdoor recreation will not be officially decided until November and take effect two years down the road. With passage of Senate Joint Resolution 24 and Senate Bill 26, Texans will have the opportunity to vote on an amendment to the Texas Constitution that would dedicate all money generated through the state sales tax on purchases of sporting goods to fund Texas parks and the state’s historical commission."

Current legislation allows the appropriation of 100% of TPWD's promised portion of the sales tax revenue but does not require it, which has led to erratic, unpredictable and less than the full funding. "Under the proposed amendment, Texas Parks and Wildlife Department would [automatically] receive 93 percent of the $170 million to $180 million annually collected from sales tax paid on sporting goods, with 7 percent going to the Texas Historical Commission."

"Lacking reliable, sufficient funding, Texas’ 95-unit state park system has struggled, saddled with a backlog of scores of millions of dollars in deferred maintenance and damages caused by a series of natural disasters ... over the past decade.


Invasive Spotlight:
(Hydrilla verticillata)

Hydrilla is an aquatic perennial plant that grows underwater (submersed). Considered one of the world's worst invasive plants, it can quickly form dense underwater stands that raise water pH and temperature, and lower dissolved oxygen, affecting populations of fish and other organisms. It promotes mosquito habitat. It can make it difficult to boat, fish, and swim, and can potentially clog dams and water intakes.

Typically rooted, hydrilla has slender ascending heavily branched stems up to 9 m (30 ft) long. Its leaves are whorled, 3-8 per whorl, 2-4 mm (0.1-0.2 in) wide and 6-20 mm (0.2-0.8 in) long, bearing visible teeth along the margins (serrated) and usually 1-4 small conical bumps or teeth along the underside midrib, which is often red. It is easy to confuse hydrilla with native elodea (Elodea canadensis) and non-native Brazilian waterweed (Egeria densa). However, neither has bumps or sharp teeth along the underside leaf midrib, elodea leaf margins are smooth and those of waterweed have minute teeth, and waterweed leaves are longer, 2-3 cm (0.8-1.2 in) long.

Interestingly, hydrilla exists as monoecious (producing separate “male” and “female” flowers on the same plant) and dioecious (each plant producing only either “male” or “female” flowers) plants. Only the dioecious type grows in Texas and because it produces only “female” flowers, it cannot reproduce by seed. The whitish or translucent 3-petaled-&-3-sepaled flowers are tiny, only 15-30 mm (0.0625-0.125 in) in diameter and float on threadlike stalks that are up to 10 cm (4 in) long.

Fleshy buds up to 5 cm (2 in) long that look like pine cones, called turions, often form at leaf axils, and the plant forms tubers among its roots. No other submersed species produces these structures. Both turions and tubers, along with stem fragments, act as dispersal units that can grow into new individuals.

Hydrilla is on the federal and TDA noxious weed lists and TPWD regulated invasive plant list, yet continues to be sold through aquarium supply dealers and over the Internet.

Learn more about hydrilla at
Credit: Chris Evans, River to River CWMA,

hydrilla on Lake austin

hydrilla comparison to other spp

More News

New Study Explains Why Parrots Ended Up Flocking to Texas
The study compiled sightings of 56 species of parrots in the wild in 43 states. There are now 25 species of “naturalized parrots” established and breeding in 23 states. Texas is No. 3 among the states with the most sightings in the wild of these one-time exotic pets. Learn more at CultureMap-Dallas and

Army Corps Approves $778M Plan to Block Asian Carp Advance with Air Bubbles, Electric Shocks, Noise
The Army Corps of Engineers has sent Congress a $778 million plan to fortify an Illinois waterway with noisemakers, electric cables and other devices in the hope that they will prevent Asian carp (Cyprinus carpio) from reaching the Great Lakes. Learn more at and

For Certain Invasive Species, Catching Infestation Early Pays Off
Researchers have found that invasive species that occupy higher trophic levels than native species are more likely to cause drastically faster, larger negative impacts on the native species than those on the same trophic level. Learn more at

Native Plants Regenerate on Their Own After Invasive Shrubs Are Removed
Results of a new study in a mature, deciduous forest in the Eastern U.S. suggest that invasive shrub removal can make sense, even when active steps to restore the native plant community aren't possible. Learn more at

Ecological Factors Influence the Distribution of Lionfish on Deep Reefs
Diver-led visual surveys at 11 mesophotic reef sites around Bermuda found that high densities of lionfish (Pterois volitans and P. miles) were associated with both higher abundances of prey fish and higher prey fish biomass, and that higher lionfish densities were recorded at sites with lower bottom temperatures. These results suggest that cold-water upwelling may result in higher abundances of prey fish and lionfish. Learn more at

Two North American Species Cause Problems in Other Parts of the World
The red swamp crayfish, Procambarus clarkia, is the most widely spread freshwater crayfish worldwide and is one of the worst invasive species due to its impact on the structure and functioning of freshwater ecosystems. The mosquitofish (Gambusia affinis) is causing problems in Spain. Learn more about the crayfish at and about the mosquitofish at


If you would like to highlight a successful invasive species project or nominate a special person to be highlighted in an upcoming iWire, please send the details to

Sentinel Pest Network and Invaders of Texas Species Workshops

Invaders of Texas workshops train volunteers to become citizen scientists to detect and report invasive species. Workshops, which are free, include information on the Sentinel Pest Network, which serves to increase the awareness and early detection of the Emerald Ash Borer, Cactus Moth, Asian Longhorned Beetle, and other pests of regulatory significance.

Workshops are tailored to meet the interests of your volunteer group, and supplementary session examples include an introduction to the TX Invaders mobile application and the Eradicator Calculator, a feature on designed to help organize and track volunteer-based eradication efforts.

Upcoming Workshops:

Saturday, August 10, 2019
Invaders of Texas Workshop
Location: Houston Advanced Research Center (The Woodlands, TX)
Contact: Teri MacArthur

For more information or to register to attend a free workshop, please visit the Workshop Page.