Nov 15 2010

Avoiding Unintended Consequences

Published by at 7:45 am under Beyond Bounds,Birds of Prey


Chuck Tague sent me news from Florida Audubon of a well meaning plan to control an exotic plant that would have disastrous consequences for the snail kite.

Snail kites are unusual birds of prey with red eyes and deeply hooked beaks that specialize in just one food:  the apple snail, so-called because its shell resembles an apple.

Apple snails live in clean, warm, freshwater lakes and wetlands, so that’s where the snail kite lives too.  Most of the snail kite’s range is in South America.  In the U.S. they are found only in Florida but are increasingly rare and now considered endangered in this country.  Their population dropped from 3,000 in the mid-1990’s to only 700 birds today due to habitat loss, the degradation of the Everglades, and a huge drop in the population of native apple snails. 

But there is one bright spot.  Snail kites also eat the exotic invasive Island Apple Snail (Pomacea insularum), pictured here, that thrives in the presence of a plant called hydrilla.

And that’s where the trouble begins.  Hydrilla is both exotic and invasive.  It jams boat propellers and clogs lakewater habitat, so to make room for navigation and native species the Florida Fish and Wildlife Conservation Commission planned control measures to get rid of it.  The problem is, if the hydrilla is gone, the population of island apple snails will crash and this will starve off the last remaining snail kites in Florida. 

That’s why Florida Audubon mobilized their members to attend a meeting in Kissimmee last week to urge Florida FWC and the U.S. Fish and Wildlife not to enact an aggressive hydrilla control plan at Lake Toho. 

Citizen comments prior to the meeting already helped the situation.  According to a Florida Fish and Wildlife news release, FWC and USFW modified their plan to clear hydrilla only from the navigation channels, making sure that enough hydrilla and apple snails remain to feed the snail kites. 

And so they’ll avoid the unintended consequence of extirpating snail kites from the United States.  

Read more about the snail kite’s tenuous life in FL here.

(photo of Snail Kite by Steve Gosser, photo of Island Apple Snail by Chuck Tague)

7 responses so far

7 Responses to “Avoiding Unintended Consequences”

  1. Mary Ann Pikeon 15 Nov 2010 at 4:07 pm

    I am going to play the devil’s advocate here. Which is better, losing a bird species at the edge of its range, or leaving both an invasive plant and invasive animal to keep the birds in the area? I am vehemently opposed to invasive species. We are losing so much of our native flora because of invasive plants and animals, some of which have been brought to this country intentionally and some accidentally. Around Pittsburgh, you see almost nothing growing along stream banks except Japanese Knotweed. Any native species that would grow along the streams have been aggressively pushed out by the knotweed. And, of course the devastation from insects and disease on our native tree species is appalling.

    Granted, the snail kite population may be keeping down the snail population by its existence. But if you could eradicate the plant and in doing so get rid of the snail population, it seems like this would be an overall better condition for the environment, since hopefully native plants and animals would return in place of the invasives.

  2. Kate St. Johnon 15 Nov 2010 at 7:41 pm

    Very good questions, Mary Ann. “Which is better…?” makes these situations such hard decisions.

    Your comments raise more questions. Got me thinking. I will ask a few of these new questions here for the sake of debate. I don’t have answers, though.

    * My first question is can hydrilla really be eradicated?
    From its description, hydrilla sounds as impossible to get rid of as Japanese knotweed because it reproduces three ways: “Reproduces mainly by regrowth of stem fragments, also by growth of axillary buds(turions) and by subterranean tubers which can remain viable for more than 4 years and can produce more than 6,000 new tubers per square meter.” Yow! (quote is from plants.ifas.ufl.edu)

    * Will cutting the hydrilla make it grow better? I wonder this because of the statement “reproduces mainly by regrowth of stem fragments.” That would be a nasty unintended consequence!

    * How do they get rid of the tubers (if they do)? Is the control measure toxic to anything besides hydrilla? Will removing the tubers decrease habitat for some other native animal/plant/fish that we want to keep?

    * Does the presence of the snail kite increase the quality of life and/or tourism for that part of Florida? About tourism: I made a special trip to the lake district of Florida to see the snail kite. Quality of life: I don’t know about everyone else’s quality of life but seeing a snail kite increased mine. ;)

    * Would the snail kite’s absence have a negative economic impact? Does the hydrilla’s presence have a negative economic impact? I certainly don’t know these answers!

    * If we let the hydrilla go will the island apple snails eventually eat enough of the hydrilla to balance it?

    * And finally, for the sake of debate, if hydrilla cannot be eradicated then why not leave some so the snail kite can live there too?

    **p.s. The island apple snails came from pet aquarium “dumps” into the waterways. Amazing to me how many people get rid of pets they don’t want by dumping them outdoors. Did you see those videos of the Burmese python in the Everglades? A very dangerous “dump.” Eeeek!

  3. TJon 17 Nov 2010 at 2:09 pm

    I wonder if the native apple snail was outcompeted by the invasive island apple snail? If so, then reducing the invader population by removing the hydrilla might increase the native snail enough to sustain the birds…

    And I wonder if the hydrilla was also dumped by aquarium owners?

    I have to say, on first glance I think the FWC and USFW made the wrong decision. Hydrilla is probably contributing more to the degradation of the Everglades (listed as one factor in the kite’s decline) than it is to the kite’s tenuous existence.

  4. Hydrillaon 24 Jan 2011 at 10:59 am

    It is said to be that hydrilla have it’s advantages and could be use as a food source. That is a good source of vitamins and minerals.

  5. Alyssaon 09 Mar 2011 at 10:35 am

    I’m very interested in learning about invasives. I’d be interested in knowing where you (Kate St. John) found this information about Snail Kites eating Island Apple Snails and their connection wtih Hydrilla. As far as I know, the Snail Kite has a very specialized beak that will allow them to base their diet on the native Apple Snail, but not a snail as large as the Island Apple Snail (maybe I’m wrong, but I think Scientists are still out to lunch abou the answer). What happens when the Island Apple Snail grows to full size? Will the Snail Kite still eat it? If not, then shouldn’t we kill the Island Apple Snails and Hydrilla, thus allowing a better chance for the native Apple Snails to hold their own (and then Snail Kites would still survive)?

  6. Kate St. Johnon 09 Mar 2011 at 11:12 am

    Snail kites do indeed eat Island Apple Snails. This study from Univ of Florida(http://etd.fcla.edu/UF/UFE0021750/cattau_c.pdf) describes the interaction (I have removed their parenthetical statements to shorten the quote):
    “Relative to native snails, we found that exotic snails require longer handling times, lead to increased drop rates, and result in depressed capture rates; however, we also found that exotic snails provide more energy than natives. Consequently, the effects of the exotic snail on foraging behavior do not have negative energetic repercussions for 15 adult kites. In fact, we found that adult kites are attracted to Lake Tohopekaliga and that the relative contribution of the lake to the range-wide nesting effort increased from 6% to 33% after the invasion of the exotic snail. Conversely, the effects of the exotic snail on juvenile foraging behavior can lead to insufficient daily energy balances and may suppress juvenile survival.”

    So these exotic snails *might* be difficult for juveniles but it sounds like the jury is out.

    Keep in mind that the snail kite’s range is mostly in South America (http://en.wikipedia.org/wiki/Snail_Kite) and includes the native range of the island apple snail. Florida is the northern end of the snail kites’ range, a place where the kites breed and wander. Elsewhere in their range snail kites are not considered endangered.

  7. Kate St. Johnon 09 Mar 2011 at 11:32 am

    p.s. I fixed the broken link to the FWC and USFW news release. It explains their treatment plan more fully.
    And you remarked…
    >shouldn’t we kill the Island Apple Snails and Hydrilla, thus allowing a better chance for the native Apple Snails to hold their own (and then Snail Kites would still survive)?

    I don’t know the answer but I’ll bet there would be an unintended consequence for that plan too.

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