Hawaii: An Island Paradise Overrun with Cats

Feral Cats on the Island of Hawaii

What images spring to mind when you image vacationing in Hawaii?  Traipsing through the lush rainforest?  Snorkeling in the crystal-clear water?  Reclining in a hammock as the warm breeze gently caresses your sun-kissed skin?  These are images that surge through most people’s minds as they imagine their dream vacation on this secluded Eden.  However, what most people do not imagine is what they are most likely to experience: frequent encounters with many of the island’s 16,700 feral cats.

Wild cats have become a major problem on islands over the past few decades and can annually cause billions of dollars’ worth of damage through predation and disease transmission.  Seeking to mitigate this problem in the Hawaiian Islands, Cheryl Lohr and colleagues from the University of Hawaii at Mānoa explored whether it is more cost-effective to control feral cat populations through trap-neuter-release (TNR) programs, or trap and euthanize (TE) programs.  In their September 2012 (“on-line early”) article in Conservation Biology, Lohr et al. determined that TE programs are almost always more effective than TNR programs.  However, the current high rate of domestic cat abandonment can severely diminish the impact of both programs.

In this study, Lohr et al. used systems-modeling software to approximate changes in the abundance of the feral cat population over the next 30 years in response to each management method, and calculated the costs and benefits associated with each.  The costs of the current TNR program include trapping, sterilization, and micro-chipping (for future identification), and the subsidization of veterinary care and food for the rest of the cats’ lives (during which they are under the care of a registered volunteer).  Conversely, the costs associated with the TE program include trapping, euthanasia, and professional wages (unlike the TNR program, which mainly utilizes volunteers, the TE program is chiefly staffed by professionals).  Due the lack of published data concerning feral cats, the only benefit incorporated into this analysis was reduced predation on Wedge-tailed Shearwaters (Puffinus pacificus).  Abandonment rates were then incorporated into the simulations by adding 0%, 1%, and 10% of the initial feline population back into the model each year.

Using hundreds of simulations, it was demonstrated that under ideal conditions (the abandonment rate was 0% and 30,000 cats were trapped in the first year) the trap and euthanize program had a 75% chance of eliminating the feral cats within two years; under these same conditions, it took the trap-neuter-release program 30 years to accomplish the same results.  When the abandonment rate was increased to 10%, the TE program was still able to eliminate the feral cat population in 2 years, but the populated regenerated within 6 years, and the program had to be repeated.  Conversely, with a 10% abandonment rate the TNR program was never able to eliminate the feral cat population within the considered 30 year time frame.  Overall, even if the TE program has to be repeated every 6 years, the cost of this management program over 3 decades was $4.06 million cheaper than management with the TNR program.  The reasons why the TNR program might not be as effective as the TE program might be due its reliance on volunteers (as opposed to paid professionals) and because the current sterilization rate is so low, that enough cats are not able to be sterilized for mortality to exceed fecundity.  The only simulation in which TNR proved to be more cost-effective was when the number of cats fell below 1670 individuals, a number which will take decades to achieve without the TE program.

One of the major challenges in implementing this research is widespread urbanization in the Hawaiian Islands, a phenomenon which has forced feral cats into frequent contact with humans.  This proximity has allowed most of these felines to become semi-feral, meaning that they depend on the food and water provided by humans (through garbage or purposeful feeding) for survival.  Due to their frequent interaction, much of the Hawaiian population has become loath to support euthanization programs, even though they have shown to be the most effective method for feline control.  Unfortunately, unless Lohr’s proposed TE program, or a more rigorous TNR program, is implemented within the next few years, Hawaii’s feral cats will continue to precipitate the extinction of many of the ecosystem’s unique species.  With this consideration, future researchers should focus on developing a more effective TNR program, methods to promote increased public support of euthanization programs, or methods to decrease feline abandonment.

Despite these complications, this research is significant because it strongly suggests that the environmental and economic benefits of removing feral cats from small, tropical islands are much greater than the fiscal costs of removal programs.  Therefore, the establishment of TE or TNR programs on these islands could likely become a useful method for conserving local biodiversity.  In terms of future research, the authors suggest that the effectiveness of feral cat removal programs be examined on islands of varying size and climate (such as New Zealand and Australia) to see if these results remain consistent.

By: Lauren Lyssy

Journal Reference:

  1. Lohr, C. A., Cox, L. J., and Lepcyzk, C. A.  2012.  Costs and Benefits of Trap-Neuter-Release and Euthanasia for Removal of Urban Cats in Oahu, Hawaii.  Conservation Biology.  doi: 10.1111/j.1523-1739.2012.01935.x.


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This entry was posted in Conservation Biology Posts, Conservation Blogs 2012-2013. Bookmark the permalink.

3 Responses to Hawaii: An Island Paradise Overrun with Cats

  1. Pam Murray says:

    We wouldn’t kill the homeless, would we?


    • Judy says:

      I’m with you Pam. They are living creatures. Also the above does not take into account the vacuum effect. TNR works, it does take longer but there are many instances where its been effective. Humans are irresponsible and create this situation. they should be required to spay neuter their animals and not allow them outside. Those who claim to love cats yet let them roam should be fined.


  2. Eden says:

    Pam, comparing domestic cats to humans is rather pointless.

    Ben, they should be in a home, but for those who are free-roaming, they shouldn’t be outside. They need to be removed. Can’t tell you just how tiresome this. Blah, blah, ‘TNR has been proven over and over as the ONLY way to end feral populations’. Saying so, does not make that true. In FACT, you are highly mistaken. Name ONE peer reviewed paper to substantiate your claims. TNR has taken place for over 20 years and supposedly has so so so many volunteers and so much funding – so WHERE are all these colonies that have ENDED through TNR? I can think of a whole whopping TWO that get cited by TNR folks. One in Newburyport which took well over a decade to expire, and removal for adoption for half of those cats in the beginning (which is not TNR) helped.

    Adopt out what you can, remove the rest and euthanize.

    Not everyone thinks re-abandoning domestic cats is humane or compassionate.

    You did hit one thing on the nose – cat overpopulation will not improve until people become more responsible pet owners. And that AIN’T gonna happen if we condone a method that ships them back to the streets to live and die. TNR encourages abandonment.


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