Frogs and spiders, oh my! Surprising food web dynamics in Neotropical swamps

 

When Science Met Nature…

Jessica’s Science Blog- 10.01.17

Frogs and spiders, oh my! Surprising food web dynamics in Neotropical swamps

For this month’s extra special guest post, we’ll dive into the wild world of Neotropical swamps. The author and illustrator of my October piece is Dr. Kristiina Hurme, a behavioral ecologist at UC Berkeley (and my dear friend and colleague pictured above) who studied parental care in frogs for her doctoral research. This post was inspired by conversations Dr. Hurme and I had years ago about weird things we witnessed while doing field work. By the end of this post, you may agree that truth is sometimes stranger than fiction when it comes to predator-prey interactions in the swamp. I want to stress that Dr. Hurme’s observations of spiders and frogs are 100% real and unembellished and that her illustrations are based on actual photos of these interactions. Some of the details of these food web dynamics may not have ever been scientifically documented. So, strap on your mental headlamp and let’s fly to Panama!

***************************************************************************************************************************

Are you my mother?

I spent a lot of time in the swamps at night in Panama focusing on parental care in frogs. Along the way, I got to observe the natural history of many other organisms. The most interesting- and sometimes endearing- was that of spiders. So, I spent all my time thinking about what makes a good frog momma, but I also saw spiders being really good mommas and carrying around their egg sacs and often their young. I’m not sure about any parental care after that, but they definitely transported the eggs.

***************************************************************************************************************************

Female L. insularum guarding newly hatched school of tadpoles. Photo by Michael Starkey.

In most species of frogs, the tadpoles are solitary, so they’re very skittish and not very active because they don’t want to draw a lot of attention to themselves. But in my species, Leptodactylus insularum, they make huge gregarious schools and they’re very active. But, they use strength in numbers to hope that they’re not the one caught by a predator. Therefore, there’s actually a high level of mortality by predators that focus on movement, such as spiders, which sit at the surface and sense movement in the water.

Spiders prey on tadpoles and other spiders

Whenever I found a school of tadpoles in the swamp, there would be a high number of spiders present (with tadpoles as prey) right next to the school. A tadpole school is this weird thing where the water is  boiling with tadpoles. So if they pass by where a spider is sitting and waiting, he’s almost guaranteed he’s gonna catch a tadpole in this mess. So, I very rarely found spiders with other species of tadpoles because it’s hard to predict where that one individual tadpole’s going to come up to the surface. It’s only at later stages that they need to even breathe air. Spiders probably have a hard time catching tadpoles generally, but not when they encounter the species I was studying.

Male fishing spider with L. insularum tadpole.

It was neat to see spiders catching tadpoles, because sometimes they’re pulling up a tadpole that’s even bigger than them. You can imagine tadpoles are a pretty good meal. So once a spider catches a tadpole, it looks like it’s set for a while. He’s gonna suck it dry from the inside, so there will be this trail of happy spiders with their tadpoles. You can tell where the school had been wherever there was a high concentration of spiders with juicy tadpoles they were chowing down on. So, I came to notice that there were always spiders killing tadpoles, any time you went out in the swamp. But, of course there was probably a spider like every six inches throughout the whole surface of the swamp. There was a high density of spiders.

 

 

 

The cool part was that I would find spiders eating other spiders….and spiders eating larger frogs….and then really large frogs eating spiders. At first I thought the food web was pretty simple, with spiders eating tadpoles. But then, I got some really cool observations.

Once I found a large wandering spider (family Ctenidae) with a juvenile frog. That made me sad, because it’s so hard for a tadpole to be the one that survives past metamorphosis. He made it to about teenager size (half as large as an adult), and that’s when the spider killed him. So, I was pretty bummed, but I was taking pictures. And then I watched an even larger spider come up and attack the first spider, to the point that then he left the frog behind. So, the original spider got killed by the larger spider and then that poor frog was left in the swamp.

I’m sure it wasn’t wasted, I’m sure someone eventually ate him, but it made me feel bad because my frogs have such a hard time making it to adulthood.

Another time, I saw a male fishing spider (family Pisauridae), the smaller one that’s more yellow with a white stripe, holding a tadpole but he wasn’t eating it. He was drumming on the surface of the water with his legs while holding the tadpole. I kept watching him and watching him. I realized that a female spider was approaching. So, he started to approach her very cautiously (because spiders kill each other). He held the tadpole out in front of him with his other legs put up in the air, as if he wanted to keep as much of his body away from her. He slowly inched forward, with the tadpole close. Luckily for him, she grabbed the tadpole and stared eating it. At which point, he flipped her over and used his little forelimbs, the pedipalps, to inseminate her on both sides. So, he got his copulation with his nuptial gift. And then, risky as he was, he stole the tadpole from her and ran away. So, I don’t know if he was gonna use the tadpole on another female, but he definitely made out with his life and his meal.

Male L. insularum eating Ctenid spider.

Like I said, I saw almost always spiders eating tadpoles, but one time I did find an adult male L. insularum frog with a really big spider (family Ctenidae) in his mouth. So, the food web is interconnected in many ways. Also, I once saw an adult female L. insularum frog eating a small tree frog. Swamps are dangerous places. Anyone who’s bigger eats anyone who’s smaller across many different taxa.

************************************************************************************************************************************************************************************************

 

Maternal care and offspring development

In terms of cool facts about the female frog and her parental care, the females will guard the schools, including the foam nests. So, when the pair makes the foam nest, it’s only around for about two days, which is very fast in terms of frog development. The female will sit inside, and if you come close she’ll jump out and hiss and splash and bite to try to protect her eggs. This seems to be aimed only at vertebrate predators. I never saw her react to any sort of spider or invertebrate predator….that’s only thinking about one tadpole…she does not react at all. If my body was hidden and she just saw my hand- as if I were a snake or a turtle head- then she would aggressively attack my hand to keep me away. I think she’s waiting for a vertebrate predator that could pick off her entire school, which could be up to 10,000 tadpoles.

From day two to day 17, the tadpoles are constantly active, constantly feeding. By 17 days, they’re metamorphosing into tiny frogs. This is really fast. For context, the fastest development of the desert frogs is about 11 days, so this is really really fast.

Other frogs in Panama might take two to three months to develop, but of course you’re risking that the water is going to dry up. In these flooded grasslands, where my frogs are breeding, water drying up killed at least half of the schools. They never even made it to day 17 because the swamp dried up. But interestingly, only about half of the schools ever had a female stay behind to protect them. I don’t know what a female needs in terms of the energy resources to stay and defend her school. But, I found that none of the schools that didn’t have a mother guarding them survived to metamorphosis. So, if your mother didn’t stay around to protect you, there was zero survival.

Close up of school of L. insularum tadpoles.

Females might also be important in leading schools around to different spots within the swamp. So, females can dig little tunnels to connect drying puddles. Maybe they know something about where there’s better nutrients or better places to avoid predators, either vegetation to hide from vertebrates or less invertebrates. For invertebrates, it seems like being the first frog to colonize after the rains come and the water fills up, you have an advantage because there are fewer insects living in the swamp. Later in the season, the insects are in later stages or adults that can chow down on the tadpoles much more readily. In the beginning, it’s like a novel environment every time it rains. If you can time it with breeding, right when the pond fills up, there might be less predation from invertebrates. At least those are my feelings from being out there.

 

 

There’s tons of natural history. I felt like I could do a whole other Ph.D. or at least tons of art and photography projects just on documenting spiders. I have a great appreciation for spiders after being with them in the swamp for about a year.

Notes on natural history

Spiders are carnivorous throughout their entire lives but almost all tadpoles are herbivorous and almost all adult frogs are carnivorous. We couldn’t see tadpoles preying on anything, but there are tadpoles (especially fast-developing ones) that will scavenge if there’s a dead animal in the pond. But, from what I could tell my species does not do that.

Also, parental care is really rare in frogs and in the few that do provide parental care, it’s split between whether it’s the mother or father that provides the care. In my species, it happens to be the mother. The males are really muscular and have thumb spines, probably for fighting and wrestling. Someone has seen them wrestling. The males are covered in scars, which I assume is from fighting for good spots within the swamp where they call. In three years, I only saw one pair in amplexus and it was a really big, muscular, macho male.

 

A pair in amplexus.

Did you enjoy this story? Be sure to check out my previous posts:

SEP: The Long Trail

JUN, JUL, AUG: Tales from a far-flung forest: parts 1, 2, and 3,

MAY: We can be heroes! Finding your community on iNaturalist

FROM 2016: The health benefits of yoga: A scientific perspective.

 

 

 

 

Comments are Disabled