Invasive species appearing out of nowhere. What should I do?

@odole I understand what you mean, thank you very much! I was planning to make the pond bigger once fall comes around, since its too hot to dig right now. What do you think would be the minimal size for the pond to “fend for itself” so to speak? I have no problem with checking on it periodically but for example right now I don’t have much of an enclosure to put the pond animals (just some buckets and I don’t know if they would survive for a few weeks there until the pond dries fully) so would I need to acquaire some equipment in the future like fish tanks and such? Or would just some water buckets like the one in the video be enough if I provided some food like leafs and fruit while the animals are there?

@thelamb I will try first to remove as many mosquitofish as I can and if it does not work I will do as you guys suggest. I want to make sure I have a proper temporary enclosure first to ensure the tadpoles and other critters do not die of lack of food or oxygen while they’re out of the pond.

1 Like

As to the relation between pond size and time of pond survival, I will leave it to experts and experienced people (I wish I was one of them).

I would suggest other points to think about :

Reverse ecology and mass effects

If you dig a pond and then wait what happens, you will likely get the usual pioneer species, which often do not need your help to thrive, besides sometimes being invasive. Perhaps not for ever, but possibly for a long time before any other species arrive there and becomes able to compete with the pioneers.

Thus, whether you create a larger pond, or you just dry this up, consider making your choice of the species you think deserve to be there ; design a possible ecosystem, based on your local natural ponds.

Once you have a design, you could take some eggs from natural ponds, put them in your pond as soon as they will be able to live there. Give them a mass effect advantage over the usual pioneers. Is this called “reverse ecology” ?

Again, I am not an expert, thus in particular please ensure that you can take those eggs without risks and without infringing the law.

Perhaps @thelamb your kind advice again ?

Another possibility : let the pond dry during the summer, perhaps only ensuring that the duration of drought is kept the same every year, consistent with the survival of species that are adapted to defined drought conditions.

Just as an example Sympetrum and Lestes odonates in Europe have a one year life cycle, emerge in early summer but reproduce much later at the end of summer and in autumn.

Involving neighbours

Discuss with your neigbours about how they see the possibility of extra mosquitoes, and recommend them to join iNaturalist !

Have fun and good luck !

1 Like

Thank you for all this great info! I understand what you mean and will definitely give it a try. Im lucky enough that, for some strange reason, I’ve never had issues with mosquitoes on the pond. Whether it is due to animals like tadpoles eating the larvae or just the conditions of the pond there, they’ve just never been atracted to it, so I doubt I would have to warn the neighbors. Sadly I don’t think they are the iNat type, since most of them don’t like our local wildlife except for the birds and maybe the hares (which alas are invasive here). In fact its not uncommon that animals like tegus or opossums get killed if they ever enter someone’s property, even if they don’t have any desire to interact with people.
But going back to your advise, I’ll try and get a decently sized water tank and set up a temporary ecosystem for the natives to live in when the pond is almost dried up (there were some massive rains last night so it may take longer than expected haha). I’ll also talk to a friend of mine who is a biology prof. to see if he knows some other native freshwater fish I could safely add to the pond, since from what I’ve read pearlfish are quite territorial so I can’t add a big number of them to compete with the mosquitofish if they ever come back after the cleanup. Appart from fish and tadpoles, do you think there’s any other type of permanent or semi permanent water dweller that may eat small fish? I doubt the frogs and toads do.

Again thanks so much for your advice. Truth is, since I started the pond to help the native fauna and flora when I found this is a highly invasive species I felt like quite the failure. Hopefully with all this new information I’ll be able to deal with them and correct this mistake!


I did my master’s thesis on priority effects, which I believe is the term you are using. Priority effects mean that the first species to arrive in a newly formed habitat can adversely affect the establishment of later arriving species.


Thank you (my description was just a very partial analogy with chemistry).

See some suggestions above (e.g., @dianastuder), and there are more, yet again you should see with experts, and you already know the “prof” you mention, plus at least two others for two quite fundamental questions (the efficient removal of Gambusia and the priority effects).

Following the above recommendations, why do you not just dry up your pond right now ? Then you will make another pond with time. If it rains too much now, and the season will no longer be dry, then pump the water out when you expect some days without rain - after having selected the animals you would like to keep as mentioned above, and assuming that the Gambusias will be dead after a few days.

The design of your next ecosystem will be quite complex if you wish it to survive well and thrive, thus needing the help of experts. But this is different from removing Gambusias.

1 Like

I see what you mean. My professor friend told me that Gambusia are naturalized here by now, and that if it was his pond he would just leave them be. He also said that frogs and great kiskadees would feed on them occasionally, as well as odonate nymph (but again I haven’t seen any inside the pond, tho maybe im not looking well enough) so now im not sure what I should do. Would maybe taking out some of the gambusia periodically and adding pearlfish be a good idea to counter-balance their population?
And in regard to priority effects, what I don’t get is how if the gambusia arrived to the pond months after it was created and filled with different animals how they manage to have such a population boom. Is it maybe because they are in an ecological niche that no other species was in? They seem to be feeding mostly on plant material, although tadpoles feed on it too. Or is it just that there were enough resources and lack of predators that they could just expand without much concern?

Again, thanks so much for your advice. Im sorry if it seems like im wasting your time with all this questions, your answers really help me out tho! Ecosystems and their management are way more complex than I ever expected them to be, so it has proven quite a challenge for me to strike the balance between letting nature do her thing and making sure its all going as planned. I’d love to be able to manage a bigger piece of land in the future and this small pond is giving me a taste of what that would be like, so I hope all that im learning thanks to you guys will be useful then. Again, thanks so much for taking the time to explain all of this.

mosquitofish are native in my area. there’s a local park / nature center that i go to once in a while that has lots of different ponds with different conditions, and all of the ponds have mosquitofish in them. from what i can tell, bigger predator fish – if they exist in your area (and your pond is big enough for them) – are going to be the best biological control, but they still won’t be able to eradicate. birds, frogs, dragonflies, raccoons, etc. might eat an occasional fish, but they barely make a dent in mosquitofish populations. i’ve seen some of those ponds dry up, and then snakes come in and eat all the fish, but then when the ponds fill up again, the fish find a way back. the staff at the park tell me they’re not moving the mosquitofish in. it’s possible it’s random people doing it, but i think birds are more likely vectors, or else the fish are swimming in themselves during floods maybe.

regarding population booming, i know folks who raise mosquitofish, and they can easily fill giant tanks with mosquitofish in a few months, starting with just a few fish, since each female can produce tons of babies over and over. i’ve never raised them myself, but the numbers i see on Wikipedia are roughly 2-4 week pregnancy, with 60 babies per batch, and then 3-4 weeks to maturity, under ideal conditions.

i would guess that if you don’t know how the fish got in your pond, but there are likely sources nearby, then eradicating them from your pond is ultimately going to be wasted effort, if you don’t / can’t address other potential source bodies of water.

that said, if you’re going to remove mosquitofish from your pond, i think it might be interesting to put them in a small white plastic tub nearby to see what might come to eat them. (you can poke some very small holes in the side of the tub to make sure that the tub won’t overflow and let the fish out when it rains, and you can put some support around the tub to make sure an animal won’t accidentally tip it over.)

1 Like

@abrub thank you for your kind words. It was a pleasure to try to help. Just another general comment : you are the only one that will ultimately judge whether any choice is a waste of time or the only available approach to achieve your goals.

Experts are not expected to make any choice for you, they will help you understand mechanisms and predict more likely consequences for different options.

Also I usually consider that if I expect to experience say 100% chances of failure alone, I will still get 50% failure if helped by experts ! Required but not sufficient…

And finally beware of misleading definitions when used loosely. Try to stick to and to clearly qualify definitions that are of practical interest for you.

And again, your project is very interesting and your motivation inspiring. Keep going!


Before you let it go dry (if you do), I wonder if it would be interesting to look at samples of the water and sludge at the bottom under magnification to see what micro-organisms may be living in the pond? Then, when the pond is re-established, take a look at the new samples for comparison?

Do you have a microscope or possibly could use one at a local school?

I used to have a little ornamental pond with plants and goldfish and it was truly amazing to look at the multitude of smaller life forms under magnification (I used a microscope at the local nature center).
There were so many plants and critters in my pond* (I used a biological filter, no chemicals). I did not even need to feed the gold fish - eventually, they preferred the natural diet they found in the pond to commercial pond food.

*by contrast, when I took samples from the nearby reservoir to view under the nature center’s scope, all I usually saw was dead plant matter. I was surprised, as there are lots of birds and fish and turtles that live in the reservoir, but very little too see in the microscope compared to my patio pond.

1 Like

@teellbee I will be sure to check if I can find someone with a microscope that I can borrow. I doubt I will get access to any in a school since they are all closed due to covid. Also, it seems so strange that the water reservoir didn’t have any microscopic organism, aren’t those usually needed in order to have bigger animals? Cause the way I see it, microscopic critters are food for small fish, which are in turn food for bigger fish and so on. Perhaps people have been feeding the fish and turtles in the reservoir instead and that’s why they can thrive regardless?

For now I’ve decided to make the pond a bit bigger, since we’re getting a bit of less hot weather so I can dig without risk of overheating, lol. Once the extension is complete I will connect it to the main pond and not fill it with anymore water. That way it will dry faster, I think. If the rains continue filling the pond however I may just stick to manually removing gambusia and try to add pearlfish so they have some competition.

And @odole: thanks again for all the kind advice. I see you seem to like odonates a lot, would you be okay with me occasionally tagging you in any odonate observation I make in the pond? There’s quite the variety! There’s this specially big red dragonfly that visits it all the time, but I haven’t managed to take her picture so far.

1 Like

FWIW, the county rules out feeding wildlife in this park and it is very rarely done. And, you always see the geese, ducks, coots, and piebald grebes diving for food. The reservoir had fish, abundant water birds, turtles, and plants. It is also used by humans for summertime kayaking, paddle boarding, and small boat sailing. The water looked like murky tea, so it seemed promising. I sampled it’s water several different times in different places, including climbing down the bank into the tule and cattails to get a sample around their roots. Nada :roll_eyes:

The lack of microorganisms in that little reservoir’s water remains quite mysterious to me. I did not scoop up sediment from the bottom, and perhaps I should have done that. I wondered if the water had been treated as it is nominally part of the district water supply. I can’t say I think it is likely, given that the water feeds percolation ponds further downstream.

By contrast, my tiny backyard pond water was satisfyingly full of micro-organisms. I used the same microscope to look at all the samples.

Just as an aside, patio pond had a biological filter that required cleaning every few months. I would pull out the filter media and put in a large pail and hose the sludge off. The stuff I hosed off was sprinkled on plants around the patio… it was the very ~best~ fertilizer I have ever used and all organic, too.

I miss having that awesome fertilizer producing itself in my own patio.

If you are going to drain the water anyway, I wonder if you could try a bucket or two of its bottom water on your plants?


Have you considered creating an ecosphere with just some reservoir water and sediment? You could put it inside a sealed jar and see what happens! I’ve never made one, but Life in Jars has a youtube channel dedicated exclusively to that kind of stuff, so that would be a good place to start.
Would the bottom water be any different from the surface water nutrient-wise? I usually splash a bit of pond water on any surrounding plants that seem to be drying up, but it doesn’t help much I think.


Life in Jars has a youtube channeL

Whoa! That video is pretty cool.

1 Like

Did you consider contacting @roget ? He runs ArgentiNat and as far as dragonflies are concerned, he has posted observations for both the only two species found near Buenos Aires that belong to a species-rich South America-endemic genus (Oligoclada), with one such species that even seems endemic to Argentina (Oligoclada rubribasalis) and lives in “shallow marshes”.

These dragonflies might have something to say about Megatherium, Macrauchenia, Toxodon, marsupial saber tooth tigers etc - perhaps with @roget ‘s advice you could go collect their eggs and make them priority species for your pond …

Odonates from 11 other genera are believed to be endemic to Argentina, but apparently either they are very little known, or they live in streams and not ponds, or at most at “lentic environments at stream edges” (Lozano et al. 2020 Int Journal Odonatol, 23 (2): 113–153). Thus Oligoclada could be an ideal start…


I think I phrased my question wrong haha, I meant to say it as in if you would like to see the dragonflies and be tagged whenever I find a new one. As in if you would be interested in seeing the odonates that go to the pond since you said the project was interesting, so I thought you might like to be tagged in future observations. It’s perfectly fine if you prefer not to, tho!
But your idea seems pretty cool. I’ll send roget a message and let him know about the pond and what he thinks I could do to encourage odonates and their survival there. Thanks!

1 Like

Yes do please tag me and keep me informed !

… in addition to endemic species, you could consider other possibilities, for instance looking into a recent study on various wetland habitats in nearby Paraguay. Perhaps you could survey a range of studies of this kind across a range of taxa.

But am I pushing you toward an increasingly promethean endeavour ? I see an increasing need of experts…


Haha it does indeed seem like a lot of information for me to take on. I think for now I’ll work to preserve the pond and maybe expand the range of different “micro-habitats” that form around it. Sadly a few days ago the neighbors “cleaned up” a property that was adjacent to ours in which there was one of the last remaining shrublands around, only leaving a small triangle in the corner where the tractor could not get to. Im not sure if that will affect the odonates, but it has certainly affected the cavy and likely any other animal that lived there.
I swear the more I think about it the more I want to purchase a large piece of land in the Patagonia and make an ecological reserve there, lol. But that would likely requiere tons of experts’ advice as well, wouldn’t it?

For now tho, this are the only two observation of odonates I have in the area so far. Here’s hoping I get to document some more soon!