Frogs face fatal fungal foes

Pathogens are organisms that cause disease, and like all organisms, they obey evolutionary principles. Pathogens that survive and reproduce successfully in a particular environment will have more offspring than those that are less successful, thereby passing on those traits that promote successful reproduction to future generations. The problem is that many pathogens change their environment in a way that makes their environment less hospitable for their own survival or reproduction. For example, the fungal pathogen Batrachochytrium dendrobatidis (Bd) causes chytridiomycosis in its amphibian host, which may severely reduce the host population size to the point where few individuals survive. If the host population goes extinct, then there are no hosts for the fungal offspring to infect.

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Scanning electron micrograph of Batrachochytrium denbdrobatidis spore. Credit: Dr. Alex Hyatt, CSIRO Livestock Industries’ Australian Animal Health Laboratory.

Fortunately for Bd, but unfortunately for amphibians, there are several ways out of this conundrum. One approach is a reduction in pathogenicity so that a pathogen’s host species is able to tolerate the infection (and of course, natural selection will at the same time favor an increase in the host species’ tolerance for the pathogen). A second approach is to broadcast a wide net by infecting many different species. That way if one host species goes extinct, there are always many other species to infect. Bd infects over 500 species of amphibians, and has been implicated in the extinction of over 100 amphibian species, and the severe decline of an additional 100 species.

Ben Scheele and his colleagues wanted to know why the endangered northern corroboree frog, Pseudophryne pengilleyi, was declining in southeastern Australia. Several previous studies showed that many corroboree frog populations declined or went extinct in that region over the past 20 years, while the abundant common eastern froglet, Crinia signifera, showed no signs of decline over the same time period. Pilot studies showed that eastern froglets were heavily and commonly infected with Bd. The researchers reasoned that eastern froglets could be acting as a reservoir for Bd, so that corroboree frog populations are being decimated by association with Bd-infected eastern froglets.

Female Ppen copy Hunter

Female Pseudophryne pengilleyi. Credit: David Hunter.

Preliminary surveys indicated that the decline of corroboree frogs was not uniform across the study site; in fact there were some newly discovered populations that were doing very well. The researchers defined three types of sites in their research area. Absent sites (40 in total) had corroboree frogs in 1998, but the population went extinct by 2012. Declined sites (17 in total) had a greater than 80% decrease in abundance since 2000. New sites (25 in total) were newly discovered since 2012, and had much higher population densities than declined sites.

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Study area in southeastern Australia, showing locations of Absent, Declined and New sites.

Unfortunately, it is impossible to visually distinguish an infected frog from an uninfected frog, at least until the few hours before death. But the researchers needed to be able to tell if a frog had chytridiomycosis. So they collected skin swabs from the frogs during the breeding season – only working at night to ensure cool humid conditions which minimized frog stress. They then did real time PCR on these samples to quantify the intensity of Bd infection.

Scheele and his colleagues had three important questions they were now prepared to answer. First, how prevalent is Bd in these two species? They found that infection rate was much higher in eastern froglets (79.4%) than in corroboree frogs (27.3%). The intensity of infection (measured by the number of fungal spores) was also much greater in eastern froglets than in corroboree frogs.

Second, do eastern froglets act as a reservoir for Bd, leading to infection and decline of corroboree frog populations? As we discussed earlier, the two species coexist at some sites, but not at others. If eastern froglets act as a reservoir for Bd, we would expect corroboree frogs to have higher infection rates at sites they share with eastern froglets, than they do at sites without eastern froglets. In support of this prediction, Bd prevalence in corroboree frogs was 41.4% at sites with eastern froglets, but only 2.6% at sites with no eastern froglets.

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C. signifera (left) and P. pengilleyi spending quality time together in a P. pengilleyi nest. Credit: David Hunter.

Finally, the researchers want to identify conditions that will promote corroboree frog recovery. They approached this quantitatively by modeling the probability of a site being classified as Absent, Declined or New, in relation to eastern froglet abundance. Based on their survey data of 81 sites, those sites with the highest eastern froglet abundance are most likely to be classified as Absent (corroboree frog extinction), while sites with very few eastern froglets are most likely to be classified as New (thriving corroboree frog populations).

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Probability of a site being classified as Absent, Declined or New, based on eastern froglet abundance. Data are log transformed. Dashed lines are 95% confidence intervals.

Scheele and his colleagues conclude that eastern froglets are a reservoir host for Bd, and have played a major role in the decline in corroboree frog populations. The researchers point out that, in general, areas lacking reservoir hosts may provide endangered species with refugia from infectious disease. For managing endangered species, conservation biologists should carefully monitor sites for the presence of reservoir hosts so they don’t reintroduce rare and endangered animals into locations where they will be attacked and killed by pathogens.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Scheele, Ben C., David A. Hunter, Laura A. Brannelly, Lee F. Skerratt, and Don A. Driscoll. “Reservoir‐host amplification of disease impact in an endangered amphibian.” Conservation Biology 31, no. 3 (2017): 592-600. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2017 by the Society for Conservation Biology. All rights reserved.

Fires foster biological diversity on the African savanna

As an ecology student back in days of yore, I was introduced to the classic mutualism between ants and swollen-thorn acacia trees. In this mutually beneficial relationship, ants protect acacia trees by biting and projecting very smelly substances at hungry herbivores, and by pruning encroaching branches of plant competitors. In return for these services, acacia trees provide the ants with homes in the form of swollen thorns, and in some cases also provide food for their defenders.

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Swollen thorns of Acacia drepanlobium occupied by C. nigriceps. Credit: Ryan L. Sensenig.

I always assumed there were limits to what these ants could do. I knew that elephants were a constant problem for trees trying to get established on the African savanna. I figured, wrongly, that ants could not do much to counter a determined thick-skinned elephant. But as Ryan Sensenig describes, ants will swarm any intruding elephant, rushing into the elephant’s very sensitive trunk and mouth, biting it and, in some cases, exuding a chemical compound that is very offensive to an elephant’s keen sense of smell. So don’t mess with these ants if you can help it!

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The Laikipia Plateau has one of the few growing elephant populations in East Africa. Credit: Ryan L. Sensenig.

Fires play an important role in savanna ecosystems, killing many trees before they can get established, and creating a mosaic of burned and unburned areas which vary in grass quality and quantity, and in the abundance of acacia trees (and other species as well). Recently burned grasslands tend to be lower in grass abundance and higher in grass nutrient levels. In a previous study of controlled burns, Sensenig and his colleagues showed that larger animals, such as elephants, tended to graze in unburned areas, which had more grass – albeit of lower quality. Returning seven years after the burn, he was surprised to find that elephants, which eat both trees and grass, had shifted to the burned sites in preference to unburned sites. He thus wondered whether fire was having an impact on the ant-acacia mutualisms that defend acacias from elephants and other large herbivores.

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Sunset strikes an Acacia xanthophloea on Mpala Research Centre in Laikipia, Kenya. Credit: Ryan L. Sensenig.

Ants do not share trees. In Mpala Research Centre in the Laikipia Plateau of Kenya, there are four mutually-exclusive species of ants that live in Acacia drepanolobium trees: Crematogaster sjostedti, C. mimosae, C. nigriceps, and Tetraponera penzigi.

Sensenig and his colleagues wanted to know whether the controlled burns had a long-lasting effect on ant species distribution on acacia trees. The researchers surveyed 12 plots that had been burned seven years previously and an equal number of unburned plots to see how burns affected which ant species were present.

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Goshen College research students estimate ant densities on Acacia drepanolobium trees in the Kenya Longterm Exclosure Experiment. Credit: Ryan L. Sensenig.

They found that C. nigriceps was more common in acacias from burned areas while the other three species were more common in trees from unburned areas.

SensenigFig2

Why were there more C. nigriceps ants in previously burned areas? One explanation is that perhaps C. nigriceps is better at avoiding getting burned by fire, or else is better at recolonizing after a fire. To look for species difference in response to fire, the researchers simulated fires by burning elephant dung and dried grass in 3-gallon metal buckets, creating a small sustained smoke source. They stationed observers every 50 meters along a 500 meter transect for the first experiment, and a 1.8 km transect for the second experiment. They then measured ant-evacuation rate by counting the number of ants moving down the trunk. There were some very pronounced differences, with C. nigriceps having the highest evacuation rate, C. mimosae also showing a strong smoke response, and the other two species showing little evidence of any response.

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Evacuation rate for each species in response to smoke.

C. mimosae generally prevails when it battles a colony of C. nigriceps. These results indicate that the subordinate C. nigriceps is able to maintain its presence in the community, in part, by taking advantage of its superior performance when it encounters a fire. The researchers also found some evidence that C. nigriceps is better at recolonizing after a fire than is C. mimosae. So despite being the subordinate species, C. nigriceps is abundant in this ecosystem.

Returning to those elephants, the researchers describe one final experiment in which some plots had a series of fences that excluded herbivores, while other plots were open to herbivores, including elephants.

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In this experiment, as well, there were burned and unburned plots. In general, there were more ants present when herbivores were present, as the trees invested more in swollen thorns and in ant food (in the form of nectar) to attract protective ants. In addition, ants were more abundant in unburned plots than in plots that had been previously burned, with the exception of C. nigriceps when herbivores were excluded.

Ecologists have long known that fire maintains savanna ecosystems by preventing the grasslands from being overgrown by trees. This study shows that fires shift ant community structure in favor of the subordinate ant species (C. nigriceps), resulting in a higher diversity of ant species overall. The researchers suggest that if fires become more common in savannas, elephants may become more attracted to acacias that harbor a reduced (or nonexistent) cast of defenders, which could lead to a further reduction in the abundance of acacia trees and their mutualistic ants.

note: the paper that describes this research is from the journal Ecology. The reference is Sensenig, R. L., Kimuyu, D. K., Ruiz Guajardo, J. C., Veblen, K. E., Riginos, C., & Young, T. P. (2017). Fire disturbance disrupts an acacia ant–plant mutualism in favor of a subordinate ant species. Ecology, 98(5), 1455-1464.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.

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.

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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.

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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).

FrogFig1

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.

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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.

FrogFig2

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.

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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.

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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.

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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.

Spotted salamander performance per polymorphism persistence

His first winter at the University of Mississippi Field Station, Matt Pintar was wading through some ponds where he noticed a large number of egg masses. Clear jelly surrounded most of these egg masses, but a whitish jelly encased some of them. These egg masses were produced by the spotted salamander, Abystoma maculatum, which immediately made Pintar wonder why these differences exist within this species. Biologists use the term “polymorphism” to describe a situation like this, in which two or more forms (poly = multiple, morph = form) exist within a population.

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White egg mass (left) and clear egg mass (right). Credit: Matt Pintar

Could it simply be random chance that there were two egg mass morphs? Or was one morph better than the other in getting fertilized by the appropriate sperm, or in keeping the eggs together? Alternatively, perhaps one morph was better at providing nutrients or protecting against predators. The puzzle is that if one morph was superior to the other, then that morph would be favored by natural selection, should outcompete the other, and ultimately cause the second morph to go extinct. So why did both morphs persist in this population of spotted salamanders?

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Adult male spotted salamander. Credit Matt Pintar

Pintar embryos

Recently hatched larvae. Credit Matt Pintar.

Pintar and his colleague Willliam Resetarits Jr. thought it most likely that the polymorphism was a chance event that provided no benefit to the salamanders. But they did consider the alternative that one morph might be better in some conditions, while the other morph was better in other conditions. Surveys done about 25 years ago suggested that the polymorphism might be connected to differences in water chemistry, so Pintar and Resetarits decided to explore this possible link with a combination of observations of natural ponds and field experiments on artificial ponds.

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Ponds at the University of Mississippi Field Station. Credit Matt Pintar.

Nutrient levels of ponds at the field station are influenced by two major factors: the type of surrounding habitat and the duration of time each pond holds water over the course of the year (the hydroperiod). Ponds surrounded by trees, as opposed to grass, have higher nutrient levels courtesy of tree leaves that fall into the ponds and leach out their nutrients. Many of these ponds dry out in the summer, so ponds with a longer hydroperiod will have more time to receive and leach nutrients from organic matter.

Ecologists often use water conductivity as a general measure of pond nutrient levels. Ponds with high conductivity have higher nutrient levels than ponds with low conductivity. Pintar and Resetarits sampled the water from 55 ponds and counted the number of white and clear egg masses from 40 ponds that had egg masses in 2015 and 2016. They found a striking relationship between conductivity and egg mass morph. White egg masses were much more common in low-nutrient (low conductivity) ponds.

Fig2A

Proportion of white egg masses in relation to water conductivity (a measure of nutrient level)

The researchers further explored this relationship by setting up artificial ponds that contained either low or high nutrient levels (obtained by putting leaf litter into some of the pools), and inoculating each pond with both white and clear egg masses. Pintar and Resetarits made sure that larvae were not mixed up once they hatched. They then measured the effects of both nutrient levels and morph on many variables associated with growth and development in these artificial ponds.

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Some of the artificial ponds used for controlled experiments on the effects of nutrient levels. Credit: Matt Pintar.

In general, eggs from white masses had a significantly higher rate of hatching (about 80%) at both nutrient levels than did eggs from clear masses (about 60%). But eggs from white masses took longer to hatch (Figure (c) below). Importantly, larvae from white masses tended to grow better under low-nutrient conditions than did larvae from clear masses. In contrast, larvae from clear masses grew better under high-nutrient conditions than did larvae from white masses (Figures (d, e, f) below).

Fig1C-f

The relationship between nutrient level and (c) days to hatching, (d) snout-vent length (the distance between the tip of the snout and the cloacal opening), (e) total length and (f) body mass.

These findings indicate that the polymorphism is advantageous in environments with considerable variation in nutrient levels. The white morph tends to do well at low nutrient levels, while the clear morph does better at higher nutrient levels. Pintar and Resetarits suggest that these differences in growth and development are likely to translate to higher adult survival and reproductive rates. The researchers used population modeling to demonstrate that under realistic conditions in which some individuals migrate from one pond to another, both morphs will persist indefinitely in ponds of varying nutrient levels.

We still don’t know why the two morphs perform differently under these different conditions. We do know that the outer jelly layer of white egg masses have white crystals made of proteins that are not water soluble, while the outer jelly layer of clear egg masses have smaller water soluble proteins. Pintar speculates that the firmer consistency of white egg masses could cause them to degrade more slowly and to retain their constituent nutrients more effectively than do the clear morphs.

note: the paper that describes this research is from the journal Ecology. The reference is Pintar, Matthew R., and William J. Resetarits Jr. “Persistence of an egg mass polymorphism in Ambystoma maculatum: differential performance under high and low nutrients.” Ecology (2017). The print version will probably come out in May or June of this year. Meanwhile, you can access it here. 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.

Mustard musters its troops

North American forests are being invaded. The invading forces use chemical warfare to attack the native inhabitants and to repel counterattacks by hostile enemies. As it turns out, the invader is the humble garlic mustard, Alliaria petiolata, which releases toxic chemical compounds into the soil that reduce the growth rate of many native plant species, and has strong chemical defenses that makes it unpalatable to most herbivores.

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Garlic mustard invasion. Credit Pam Henderson

Lauren Smith-Ramesh wondered why garlic mustard was not even more successful as an invader. Its chemical arsenal should allow it to overrun an area, but she (and many other researchers before her) observed that garlic mustard invasions often decline after a while. As part of her investigations into garlic mustard’s use of chemicals to inhibit native plants, Smith-Ramesh collected seeds from plants from different populations. While shaking these seeds into bags, she noticed that web-building spiders often colonized the garlic mustard’s seed-bearing structure (silique). Were these spiders somehow behind the garlic mustard’s surprising lack-of-success?

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Garlic musard silique with web. Credit Lauren Smith-Ramesh

Spiders can benefit plants in several ways. As important predators in food webs, spiders can kill large numbers of herbivorous insects that might otherwise attack a plant. In addition the decaying corpses of their insect prey can add vital nutrients to soils. Garlic mustard does not enjoy these potential spider-associated benefits, because spiders colonize the garlic mustard after it has already gone into decline, and also because garlic mustard is already well-protected (chemically) against herbivorous insects.

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About 60% of the spiders were this species – Theriodiosoma gemmosum. Credit Tom Murray.

Smith-Ramesh first wanted to understand the relationship between seed structures (siliques) and spider abundance. She established three different types of plots that measured 2 X 2 meters: (1) S+, which had mustards with intact siliques, (2) S-, which had mustards with siliques removed, and (3) N, which had no garlic mustard plants at all in 2015. After several months, she collected all spiders from the middle square meter of each plot. Plots with garlic mustard with intact siliques (S+) had, by far, the highest spider density. S- plots had a somewhat higher spider density than N plots, which Smith-Ramesh attributes to spiders wandering in from just outside the S- plots (which tended to have more silique-bearing garlic mustard plants nearby than did the N plots). Based on this experiment Smith-Ramesh concluded that garlic mustard siliques were dramatically increasing spider density.

SR2b use

But did increased spider density in S+ plots reduce the number of herbivorous insects, thereby benefiting nearby native plants? Smith-Ramesh set up insect traps that collected insects over two 48 hour time periods – once in August and again in September – in each of the S+, S- and N plots. Both surveys showed fewest herbivorous insects in the S+ plots. This supports Smith-Ramesh’s hypothesis that native plants are benefitting from higher spider density associated with garlic mustard siliques.

SR 2c use

Next, Smith-Ramesh wanted to know whether the decrease in herbivorous insects benefitted native plant growth. To test this directly, she transplanted three types of native plants into her S+, S- and N plots. One of the species, the Hairy Wood Mint Blephilia hirsuta, enjoyed a 50% biomass boost in S+ plots compared to S- plots. The other two native plants species showed very little effect.

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Smith-Ramesh collecting data with three siliques in the foreground. Credit: Lauren Smith-Ramesh.

Garlic mustard plants with intact siliques also benefitted the soils by increasing the amount of available phosphorus by approximately 60%. This phosphorus may have originated with insect carcasses that made their way into the soil and released their nutrients. In theory, soils with higher phosphorus availability could help support the growth of native plants. Smith-Ramesh plans to explore other plant communities that are suffering from different invasive plants, to see whether these invaders are also inadvertently providing resources or conditions that may undermine the success of their invasion.

note: the paper that describes this research is from the journal Ecology. The reference is Smith‐Ramesh, L. M. (2017). Invasive plant alters community and ecosystem dynamics by promoting native predators. Ecology98(3), 751-761. 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.

Golden Eagles: no tilting at windmills

Todd Katzner and several other scientists were puzzled by a vexing problem. They knew that the wind turbines at Altamont Pass Wind Resource Area (APWRA) were killing large numbers of Golden Eagles that flew into their spinning blades. Yet the population of Golden Eagles in the area had stayed relatively stable over the years despite this unnatural source of mortality. The researchers considered two possibilities. First, this population of eagles may have had unusually high birth rates or unusually low death rates from other sources to compensate for the high windmill-induced mortality. Alternatively, immigrant Golden Eagles might be replacing those killed by turbines.

Altamont Pass Wind Farm

Altamont Pass Wind Farm, California. Credit: Todd Katzner.

This question has important implications for conservation biologists. If immigrant Golden Eagles are replacing those killed by windmills at APWRA, then the apparent stability of the local Golden Eagle population may be at the expense of other populations that are providing APWRA with these immigrants. So, even though APWRA’s windmills are not directly causing local eagle populations to decline, windmills at APWRA (and other windmill sites) may be indirectly leading to a decline in other populations. So Katzner and his colleagues did genetic and molecular analyses of tissues remaining from these killed eagles to learn as much as they could about these eagles and where they came from.

Golden Eagle in flight

Golden Eagle in flight. Credit: Michael J. Lanzone.

The researchers used tissue samples from 67 eagles that were killed at APWRA between 2012-2014. They subjected these tissues to a variety of genetic tests to determine the sex and age of each individual, and to evaluate the genetic differences between individuals killed by the windmills.

In addition, Katzner and his colleagues used stable isotope analysis to evaluate whether the killed individuals were local birds, or immigrants from afar. For this analysis the stable isotope ratio is the ratio of a rare and nonradioactive isotope of hydrogen (2H) found in the sample (feathers of killed birds) in relation to the common isotope (1H). A feather’s stable isotope ratio is very tightly correlated to the stable isotope ratio of the water the bird drinks. The last important point is that different regions of the world have different characteristic stable isotope ratios in rainwater. So if you can determine the stable isotope ratio of a bird’s feather, you can compare it to the world stable isotope ratio map, and determine where the bird most likely spent the previous year (once birds molt, their new feathers assume the stable isotope ratio of their new location). This approach will underestimate the number of immigrants, because some distant locales have a similar stable isotope ratio as APWRA, and birds from those regions will be incorrectly scored as being local.

Stable isotope map

Map of May-August stable isotope ratios (of 2H in rainwater).  Same colors represent similar stable isotope ratios, ranging from relatively high ratios (deep orange), to relatively low ratios (dark blue). I don’t discuss the meaning of the circles and triangles in this blog post.

Based on this analysis, more than 25% of the dead eagles were immigrants to the area, with some birds originating from more than 800 km away. The researchers point out that APWRA might be particularly attractive to eagles looking for a home because it provides two types of resources that are important to these birds – visually open feeding grounds with easily-located prey, and a consistent updraft to facilitate relatively effortless flight.

Fig 3a

Probability that an eagle killed at APWRA was local.  If the probability was less than 0.5 the researchers scored it as immigrant; if greater than 0.5 the researchers scored it as local.

About half of the immigrants that could be sexed were juveniles or subadults. The researchers argue that the apparent stability of the population in the APWRA region is achieved by young immigrants replacing those birds that are killed by windmills.

Fig3b

Percentage of local vs. immigrant (nonlocal) Golden Eagles by age.

Katzner and his colleagues are concerned that APWRA functions as an ecological sink that attracts eagles, primarily from nearby western states, to replace those killed by windmills. High death rates are particularly problematic to slow-growing populations, such as Golden Eagles, which usually lay only two eggs, with generally only one surviving chick per breeding season (the larger chick often kills its sibling). The researchers also point out that windmills also kill many other animals, including numerous bat species, which also have slow-growing populations. They encourage the renewable-energy industry to develop technology that will reduce windmill-induced death. Such efforts are already underway, and there is preliminary evidence that newer generation turbines are reducing Golden Eagle mortality rates.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Katzner, T.E., Nelson, D.M., Braham, M.A., Doyle, J.M., Fernandez, N.B., Duerr, A.E., Bloom, P.H., Fitzpatrick, M.C., Miller, T.A., Culver, R.C. and Braswell, L., 2017. Golden Eagle fatalities and the continental‐scale consequences of local wind‐energy generation. Conservation Biology31(2): 406-415.Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2017 by the Society for Conservation Biology. All rights reserved.