Are Brown Anoles in Florida Really Driving Green Anoles to Extinction?

Tell almost anyone in Florida that you’re doing research on brown anoles (Anolis sagrei), and they’ll express some distaste for your study organism. “I don’t like them,” they’ll say, “they’re invasive. Aren’t they driving the native green anoles extinct?” Everyone—literally everyone who has lived in Florida for a whilewill tell you how their backyards used to be full of green anoles (Anolis carolinensis). Today, they report, these green anoles have disappeared and been replaced by the invading browns.

Green anoles, increasingly elusive in Florida

Green anoles, increasingly elusive in Florida

These backyard tales are supported by some scientific evidence for shrinking populations of green anoles . On spoil islands in Mosquito Lagoon, Dr. Todd Campbell documented precipitous declines in green anole densities following the experimental introduction of brown anoles [1]. In southwest Florida, Cassani et al. repeated surveys of reptile and amphibian abundance fifteen years apart, using identical methods in exactly the same locations [2].  They found a drop in green anole numbers and a sharp rise in brown anole numbers between 1995 and 2011. Based on their results, both Campbell and Cassani et al. suggest that the persistence of green anoles in Florida has been jeopardized by the invasion and spread of brown anoles.

But both Campbell and Cassani et al. acknowledge a second possible explanation for the apparent disappearance of the green anoles: the lizards may simply have shifted upwards, out of sight.* As Cassani et al. put it, “the hope remains that these lizards persist in the face of competition and predation from A. sagrei by shifting habitat use.” We already know that green anoles shift upwards at least a bit in the presence of brown anoles, and have evolved morphological features that likely help them survive at these higher perches [3]. Could green anoles have shifted so high as to be nearly invisible to us, from our vantage points near the ground?

A bead-tagged brown anole

A bead-tagged brown anole

When I started studying brown anoles in Gainesville, FL, in 2014, I was convinced that the green anoles were all gone. But as we spent many hours marking individual brown anoles and repeatedly surveying their habitat to re-spot them, we began to spot a few green anoles too. I guessed that these green anoles were the last few holdouts against the invaders, and that we were seeing the same individuals again and again. To prove this, all we needed to do was catch and individually mark these green anoles using permanent bead-tags, in exactly the same way that we were catching and marking the brown anoles. It didn’t seem like too much extra work, so once I realised that my 2015 fieldsite was also home to quite a few green anoles, we began catching and tagging them as well.

In two months of sampling,  we either caught or re-spotted green anoles a mere 52 times. In the same period and location, we caught or re-spotted brown anoles 4369 times, which certainly seems to suggest more brown anoles than green anoles in this site. But to compare the population sizes of brown and green anoles, you need to compare how often you see new, unmarked individuals relative to how often you see already-marked individuals for each species**. In the graphs below, I’ve plotted the total number of observations against the total number of marked individuals for both A. carolinensis and A. sagrei***, and then zoomed in to just the first 52 observations for both.

Accumulation curves

Zoomed in, you notice that the curves for the brown and green anoles look quite similar. If anything, we see more new individuals per observation for green anoles than for brown anoles. Neither of these curves has begun to plateau (i.e. we’re still seeing lots of new individuals), so we cannot quantify the difference in total population size of these two species. But these limited data suggest that this population of green anoles is not doing that badly.

The taller tree was home to at least three green anoles

The taller tree was home to at least three different green anoles

But if the population is doing okay, then why weren’t we spotting green anoles all that often? The most logical explanation is that the green anoles have shifted up to very high perches, and only rarely descend to heights at which we can observe and catch them easily. Moving a bit beyond the numbers, we find another piece of evidence that supports the idea of a perch height shift—of the 40 green anoles we caught, only eight were males!

We know that male anoles usually perch higher than female anoles [4], that female anoles will often search for and feed on insects on the ground, and that females must descend to the ground to lay their eggs. Males, on the other hand, often move to higher perches to display, seem to feed more opportunistically than females, and are not necessarily compelled to return to the ground after they hatch. Though sex ratios can deviate quite a bit from 1:1 in natural populations of anoles [5], it seems unlikely that a population of green anoles could be comprised of one male for every four females. Taking the sex differences in perch height into account, it makes sense that for every female green anole we spotted, there’s a male green anole perching really high up whom we simply did not see.

lizards

None of this means that green anole densities aren’t declining due to the presence of brown anoles in some habitats. In particular, because brown anoles can perch as high as 4 m off the ground, there may be many places in which green anoles previously thrived but where there is simply no “up” for them to escape to once the brown anoles arrive. I suspect that many backyards are exactly such places, and that some reports of local declines in green anole population sizes may in fact be well-founded.

But it’s also certainly possible that, in habitats with sufficiently tall trees, brown anoles are not driving green anoles to extinction. Instead, brown anoles may simply have precipitated a substantial upward shift in the perch height of green anoles towards their ancestral trunk-crown niche. It’s therefore possible that green anoles are thriving, just out of our sight. If that’s the case, then brown anoles don’t deserve quite so much of our animosity after all!

A brown anole perching pretty high.

A brown anole perching pretty high.

References:

[1] CAMPBELL, T.S. 2000. Analyses of the effects of an exotic lizard (Anolis sagrei) on a native lizard (Anolis carolinensis) in Florida, using islands as experimental units. Unpublished Ph.D. Thesis. Knoxville, USA, University of Tennessee

[2] CASSANI, J.R., D.A. CROSHAW, J. BOZZO, B. BROOKS, E.M. EVERHAM, D.W. CEILLEY, AND D. HANSON. 2015. Herpetofaunal community change in multiple habitats after fifteen years in a southwest Florida preserve, USA. PLoS One 10(5): e0125845.

[3] STUART, Y.E., T.S. CAMPBELL. P.A. HOHENLOHE, R.G. REYNOLDS, L.J. REVELL, AND J.B. LOSOS. 2014. Rapid evolution of a native species following invasion by a congener. Science 346: 463-466.

[4] SCHOENER, T.W. 1968. The Anolis lizards of Bimini: resource partitioning in a complex fauna. Ecology 49: 704-726

[5] SCHOENER, T.W., AND A. SCHOENER. 1980. Densities, sex ratios, and population structure in four species of Bahamian Anolis lizards. Journal of Animal Ecology 49: 19-53.

*Cassani et al., in particular, trapped reptiles and amphibians in ground-level traps, and very likely missed many anoles. Campbell, however, did sample in arboreal habitats, and did not find this explanation compelling in the context of his study. Trees on the islands he sampled were relatively short (~6 m), “allowing the vertical habitat to be searched thoroughly with small binoculars and some healthy tree climbing.”

**The logic is this: once you’ve marked every individual in a population, you will only re-spot marked individuals and not see new individuals, and the size of your population  will be equal to the number of individuals you’ve marked. In reality, you’ll almost never mark every individual, but the rate at which you spot new individuals relative to the total number of individuals you observe (new and marked) can still be revealing. Say you have two populations, A and B. If population A is much smaller than population B, then you will reach the point of mostly re-spotting marked individuals and not seeing new individuals more quickly in population A than in population B.

We obviously could not catch every lizard, and we were better at catching brown anoles than green anoles, so don’t use these data for any serious estimates of population size. But, if anything, our relative inability to catch green anoles means that there are more green anoles in this site than we document.

***Sampling for A. sagrei began about a month before sampling for A. carolinensis, explaining the mismatch in numbers between graph and text.

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Are Brown Anoles in Florida Really Driving Green Anoles to Extinction II: What Biogeography and Evolution Have to Say

32 Comments

  1. Brian Hillen

    While I have always had more brown anoles in my backyard than green anoles, these last few months I have seen a significant uptick in green anole presence. For most of the past few years I needed o lucky to see a green anole, but lately, every time I look out a window or walk in the backyard, I see green anoles. I don’t know if it has something to do with the colder winters (which green anoles should be more well adapted for?) or some other cycle I hadn’t noticed before. I haven’t seen a noticeable change in brown or bark anole populations in my yard though.

  2. I should also mention that the “we” here is not royal–thanks to Jon Suh and Claire Dufour for their help in the field!
    Also, check out this earlier blogpost: https://www.anoleannals.org/2014/06/20/newspaper-article-on-brown-anoles-affecting-green-anoles-gets-it-right/.

  3. Very good! But I don’t see any line for carolinensis in your first graph….

    • It’s really tiny! Which is why I zoomed in on the second graph, but here it is again in different colours which should be easier to see.

  4. Bruce B. Collette

    Great! This is just what I predicted in my 1961 Bull MCZ paper, p. 150:
    “the aggressive terrestrial sagrei should tend to drive the more generalized carolinensis out of the terrestrial part of its habitat.”

  5. Nathan M

    Has anyone studied the impact of A. porcatus on A. carolinensis in South Florida? They may be driving carolinensis out of that part of their range and perhaps hybridizing as well. A. porcatus seems to offer a serious threat as well.

  6. Yes, I see it now: thanks Ambika!

  7. David

    Just wondering as well if the brown are mixing with the green, producing a new hybrid?

    thanks for the study, that’s a lot of work. we have so many brown anole lizards, they scatter everywhere we walk in our central Florida area. I rarely see green ones. maybe one or two a month. maybe im not looking in the right places.

  8. Hi. Great article. I have noticed this, as well, and even saw a green anole chase off a brown anole. I’d never seen that before. I have been adding trees and large shrubs and also more native plants than non-native. These two changes in my city garden may account for the rise in green anoles on my lot. Just as adding nectar and hosting plants for butterflies, it may be that we should be considering the needs of the green anole. Thank you for your work on this. I adore anoles in general.

  9. I always thought they could change colors. I have seen green ones and identical looking brown ones in the past.
    I still see only green ones at my house in the deep woods.
    There is a new variety that I see every where else but my house. They have a distinct pattern on their backs and have a mean aggressive look.
    What kind are these? I figured they bred with the greens that’s why I don’t see them anymore but at my house.

    • Jonathan Losos

      The ones around your house are the green anoles, which can turn brown. The new ones are the brown anoles.

  10. This Helped Me Alot With My Research. Thanks Alot!

  11. Rebecca

    It is now Feb. 17th in Gainesville FL and the temp is back to 80 degrees. There is not a single brown anole in sight. Did they all freeze this year and will they be returning? My yard was literally crawling with them.

    • I’m seeing a decent number of sagrei here in Volusia county on the east coast, though admittedly not as many as usual. I’ve also seen a few carolinensis out and about these past few days. This was a harsh winter on many fronts.

    • Zach

      We have tons still over near Wards.

    • Wow, that’s certainly likely that the freeze hit their populations hard. Are you seeing lots of greens?

    • Kathryn Madsen Anderson

      I am in Manatee county and had tons of brown anoles until this year. We never had a real freeze here this winter, but I have no explanation why they are no longer here. We used to have tons on the lanai all around the pool and everywhere else. They laid eggs that hatched in the plants and garden etc. but no signs of them now. I know our neighbor has her yard sprayed. I hope they haven’t died off because they naturally kill so many insects. Thoughts? Data?

      • It certainly seems possible that the pesticide could have adversely affected the brown anoles, either directly or indirectly by limiting the food available to them, but I’m not aware of any data that’s been collected that would be relevant. I’ll keep an eye out and get back to you if I find anything, thanks so much for sharing your observation!

  12. Robert Langford

    Great article. It was a cold winter but this guy has been hanging around here for more than a year. So happy he made it through the cold. We enjoy seeing him daily.

  13. Mike

    I would like to know if anyone has tried– and had any success with– buying Green Anoles from online sources and even pet shops, and releasing them in their yards to try and increase their numbers. I still have beloved GA’s about my home of 14 years, and they seem to be holding their own, but only just. I am willing to spend the money to buy some and set them free to bolster the micro-local population, but can anyone please tell me if this might succeed?

    • Ambika Kamath

      My first reaction is that this would not be a good idea, for several reasons. (1) Local adaptation and gene flow–we don’t know where these anoles from breeders originally came from, and what the introduction of their genes into the local population would do. (2) Disease–captive bred anoles may carry diseases that could be introduced to wild populations (3) Whatever force is keeping green anole populations low is probably still operating, and I’m skeptical that an introduced population will thrive. My best suggestion would be to plant/support native vegetation that could grow tall quickly, giving native wild green anoles more habitat where they can avoid interacting with brown anoles.

  14. EC

    Hi. I live in Wilmington, NC and I have been wondering why I have seen only one or two green anoles in my yard this year. Nothing has changed in my landscape; there are plenty of tall trees and other vegetation. I have read the info on this website but I would like to know if there could possibly be others reasons for the scarcity of green anoles.

    • Disease is always a possibility, as well as stochastic (random) variation in population dynamics. Keep an eye on them, maybe you’ll notice other changes with time that might explain this!

  15. Steve

    Hi, I live in Bradenton, Fl. and have a backyard pool and cage. We have many browns all over the pool cage but no green sightings in our yard for the last 10 years. About a week ago I went out the front door and there was a green sitting on my driveway looking at me. Hope he stays for awhile!

  16. buckdupyun

    this is the better of all of these sites i have ever been on for brown/green anoles!!!

  17. Vito Coppola

    Hello my name is Vito and I recently went to visit my parents who moved to Naples in south west Florida. I didn’t see a single green anole but saw plenty of brown all over. This I expected but to my surprise all around her lanai where we spent our morning and nights were clearly crested anoles, and their offspring. I collected some babies being a reptile enthusiast myself, knowing they are invasive and also difficult to find in the pet trade. I know brown anoles are the big bad wolf of the anole world but I was wondering if A. Cristatellus was ever researched or had their invasive range looked into? Ive argues this same theory to a few people and enjoyed reading this well written article on it that I’ll be sending their way lol

    • Vito Coppola

      I’m looking for how to post the photos of the crested anoles I saw there last week but can find the correct button etc. if anyone has any advice feel free to contact me so i can truly share and keep everyone in the know without having To take a strangers word for it.

  18. Gregg L. Friedman MD

    I was born and raised in Miami and I remember in the 1960’s all we had were green anoles in our yard. Around 1970 the brown anoles took over to the point that I thought that the green anoles were gone forever. I am happy to report that since the arrival of the curly tail lizards I have noticed far fewer brown anoles on the ground and far more green anoles in the trees. Is in possible that the curly tail lizards are preying on the brown anoles which is allowing the green anoles to recover? I am curious if anyone else has noticed this trend? Gregg L. Friedman MD

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