Urban frogs shed no blood

Life is a series of tradeoffs. As one example, we humans have the opportunity (if we are fortunate enough to be given choices) to opt for an urban or rural existence. The urban life is quicker-paced, offers more cultural opportunities, and can be annoyingly noisy and polluted. The rural existence is more laid-back, has fewer cultural opportunities, and may provide a peaceful and relatively unpolluted environment. These different environments can profoundly affect how we feel, with some people being stressed-out by cities and others by farms. On a personal level, I was born in New York City and now live in a very small town in Virginia – I was one of the fortunate ones who was given a choice.


Panama City skyline. Credit: Mariordo (Mario Roberto Duran Ortiz)

Heat, light and noise pollution are common in and near cities, and can influence the distribution and behavior of individuals of many different species. But these factors don’t only operate individually; they can work interactively. In other words, someone might not be annoyed by flashing light nor by loud noise, but might find the combination of the two very disturbing. In addition, these factors might not only operate on individuals; they can also affect relationships, or interactions. For example, loud noises generated by natural gas wells have been shown to influence the abundance of seed predators and seed dispersers, ultimately reducing the number of newly-established pine trees.

Armed with this understanding of interactions and relationships, Taegan McMahon and her colleagues wondered how the combination of heat, light and noise pollution might affect urban túngara frogs (Engystomops pustulosus) in comparison to their more rural counterparts. McMahon had observed that urban frogs were not being swarmed by small Corethrella midges that bite them and suck their blood in more rural and forested areas. These midges carry parasites, and if her observation was correct, urban frogs might have lower exposure to some diseases than do their rural counterparts.


Corethrella midge biting a túngara frog. Credit: Taegan McMahon.

The researchers surveyed 49 túngara frog calling sites in urban (Panama City) and rural (near the small town of Gamboa) areas. At each site they counted the number of frogs, number of midges on or above the frogs, the number of frog egg masses (in foam nests), and measured the air temperature, and the light and sound intensity. As expected, urban calling sites were lighter, noisier and warmer. There were slightly more (statistically insignificant) frogs at the urban sites and considerably more egg masses at rural sites. But the dramatic finding was that there were no midges to be seen on or near any urban frogs. So it might have been hot, bright and noisy, but at least those urban frogs were unbitten!

Factor Urban Rural
Light intensity 0.16 ± 0.02 lx 0.11 ± 0.02 lx
Noise intensity 69.0 ± 0.80 dB 59.2 ± 1.00 dB
Temperature 27.6° ± 0.09°C 25.9° ± 0.04°C
Túngara frog abundance 6.09 ± 2.63 frogs/site 4.05 ± 1.11 frogs/site
Foam nest abundance 0.24 ± 0.23 nests/site 2.06 ± 0.74 nests/site
Frog-biting midge abundance 0.00 ± 0.00 midges/site 67.75 ± 43.27 midges/site

Values are means ± standard error.

Analysis of the field survey data showed that temperature did not influence midge abundance but that light and noise were both important. Interestingly, light and noise interacted with each other in an interesting way. At low sound levels (below 65 db) light was important, in that midge abundance decreased at higher light intensity (Figure A). But at high sound levels, it could be pitch black and you would still have no midges (Figure B).


Log(number of midges) in relation to light levels, in field surveys in which noise levels were (A) below 65 db or (B) above 65 db.

Did light and noise somehow influence a midge’s ability to locate a frog? The researchers set up an experiment to see whether midges were attracted to frog calls at low, medium and high light intensities, and low, medium and high sound intensities. The sounds were recordings of Panama City traffic noise. At the same time, the researchers also broadcast the mating calls of túngara frogs at their normal calling intensity (which is remarkably loud for a small animal). They then counted the number of midges attracted to these traps, which were positioned in a rural setting.


Two calling túngara frogs competing for a female’s attention. Credit: Taegan McMahon.

At low light intensities, many midges were attracted to the recorded frog calls, but city noise (low or high) greatly reduced this attraction. At medium light intensity, fewer midges were attracted to frog calls, and again city noise reduced this attraction. Finally, at high light, even fewer midges were attracted to frog calls, regardless of noise.


Number of midges attracted to recordings of túngara frog calls in relation to light and sound intensity.

McMahon and her colleagues conclude that city noise and light pollution work together to disrupt the frog-biting midges host-parasite interaction. However, the overall impact of urbanization on túngara frogs is unclear at this point. Frogs can lose up to 10% of their blood volume to midges in a night of active calling. Frog-biting midges can transmit blood parasites such as Trypanosoma tungarae to túngara frogs, so urban frogs may be liberated from this scourge. A midge-free existence may allow urban male túngara frogs to call louder and for longer periods of time, which would make them more attractive to females. However, loud and long calling has also been shown to attract the túngara frogs’s mortal enemy, the voracious frog-eating bat. The researchers call for more research on how urbanization can affect species interactions, and for greater consideration of how different forms of pollution can interact to influence ecosystem dynamics.

note: the paper that describes this research is from the journal Ecology. The reference is McMahon, T. A., Rohr, J. R., & Bernal, X. E. (2017). Light and noise pollution interact to disrupt interspecific interactions. Ecology, 98(5), 1290-1299. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.

Nitrogen nurses

Alfred Lord Tennyson puzzled over the conflict between love as a foundation of Christianity, and the apparent violence of the natural world.

Who trusted God was love indeed

And love Creation’s final law

Tho’ nature, red in tooth and claw

With ravine, shriek’d against his creed

The good poet would be relieved to learn that modern ecologists have uncovered a softer, gentler side of the natural world – facilitative interactions, in which one species (the facilitator) helps out a second species. In many, but not all, cases, the second species also helps out the first species. Ecologists describe these mutually-beneficial interactions as mutualisms. As an example, Mimosa luisana is a mutualist with Rhizobium bacteria, providing the bacteria with root nodules to live in and carbohydrates as an energy source, while receiving ammonia (NH3) that the bacteria fix (convert) from atmospheric N2. A second type of mutualism, a mycorrhizal association, is a very common facilitative interaction between plants and fungi, which grow alongside or within the plant roots. In many mycorrhizal associations, the plant provides carbohydrates to the fungi, which import and share nutrients and water.

Mimosa plant

Mimosa luisana. Credit: Leticia Soriano Flores, algunos derechos reservados (CC BY-NC)

Alicia Montesinos-Navarro and her colleagues, and researchers before them, noticed that in arid and semi-arid environments, plant-plant facilitation was most common between two plant species that were structurally and functionally very distinct, and that tended to be very distantly related to each other. In particular, M. luisana tends to associate with many different species of plants, including many cacti that look nothing like it, and are very distantly related. M. luisana is called a nurse plant, because other species tend to grow under its branches, which shade the soil and reduce water loss from evaporation. Recent work by Montesinos-Navarro and her colleagues showed another benefit of nursing – some plants receive nitrogen from these nurse plants via the network of mycorrhizal fungi.

Traditionally, ecologists have argued that associations between distantly-related plants occur because the plants have very different ecological niches, using different resources in different ways, so they are not competing with each other. Montesinos-Navarro and her colleagues argue that a second process might be important in this and other systems. Close relatives of M. luisana might tend to have high nitrogen levels and thus not benefit from nitrogen transfer from the nurse plant, while more distantly-related plants might tend to have lower nitrogen levels and thus benefit from any nitrogen arriving from M. luisana. They explored this hypothesis in the semi-arid Valley of Zapotitlan in the state of Puebla, Mexico.


Study site dominated by the columnar cactus Neobuxbaimia tetezo, Credit: Alicia Montesinos-Navarro.

Measuring nitrogen transfer from the nurse plant to the recipient is not the world’s easiest task. Fortunately there is a rare form or isotope of nitrogen, 15N, which can be distinguished from the more common 14N. The researchers soaked M. luisana leaves in urea that was made up of primarily 15N, and the leaves took up the urea. Consequently, any exported nitrogen would contain a disproportionately high concentration of 15N, resulting in high 15N levels in the recipient plant. They then measured 15N levels in 14 different species of plants that used M. luisana as their nurse. The researchers were able to test two hypotheses. First, they could see whether close relatives to M. luisana tended to have higher N-levels than more distantly related species. Second they could see whether distant relatives tended to receive more nitrogen from nurse plants than did close relatives.


Mimosa luisana branch taking up 15N-labeled urea. Credit: Alicia Montesinos-Navarro.

The graph below summarizes the results. The y-axis measures how much the 15N level in the facilitated species increased by the end of the experiment (15 days). The x-axis measures the evolutionary relationship between M. luisana and the facilitated species – more precisely how long ago the two species shared a common ancestor. Lastly, the size of the dot measures the initial difference in leaf N-levels between M. luisana and the facilitated plant.

Ecology Fig 2

Influence of evolutionary relationship between M. luisana and the facilitated species (x- axis) and nitrogen gradient – the initial difference in nitrogen levels between the two species (size of dots) on the amount of nitrogen imported by the facilitated species.

Several trends are evident. First, close relatives of M. luisana tended to have similar leaf nitrogen values to M. luisana (medium sized dots), while distant relatives tended to have much less nitrogen than M. luisana (largest dots). Second, the most distant relatives tended to have the greatest increase in their 15N levels, which indicates that they received the greatest nitrogen export from their nurses.

One question is how the nitrogen is transported. Montesinos-Navarro and her colleagues describe how they treated soil with a fungicide, presumably killing the mycorrhizae, which resulted in a substantial reduction in nitrogen transport. This suggests that the mycorrhizal network is important for nitrogen transport. But more pressing is what do the nurse plants get out of the relationship. The researchers suggest that the recipient plants may provide M. luisana with either water or phosphorus, both of which may be in short supply in arid environments.

This study indicates that we need to look beyond traditional niche theory, and may need to  dig deeper to understand the structure of plant communities, and how facilitative interactions can explain the coexistence of very distantly related plants.

note: the paper that describes this research is from the journal Ecology. The reference is MontesinosNavarro, A., Verdú, M., Querejeta, J. I., & ValienteBanuet, A. (2017). Nurse plants transfer more nitrogen to distantly related species. Ecology, 98(5), 1300-1310. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.