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.

Scheele fungal spore

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.


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.

crinia and pengilleyi 3

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).


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.

Light levels limit lake phytoplankton response to fertilization

One might naively think that because we humans are land-dwelling creatures, our impact on aquatic ecosystems might be relatively minor. Unfortunately, this assumption is incorrect, as human activities are changing aquatic environments in profound ways that influence how aquatic species survive and interact. Global warming is increasing lake and river temperatures, uncontrolled development is causing some streams to run dry and others to flood, and agricultural practices are adding nutrients to many lakes and streams. Because these human impacts occur simultaneously, it is difficult to evaluate how each factor contributes to the observed changes in species relations.

In northern Sweden, lakes vary naturally in the amount of dissolved organic carbon (DOC) they contain. DOC comes from runoff of decaying plant matter, so lakes surrounded by substantial vegetation, or that experience a great deal of water input (runoff) from the surrounding area, would have higher DOC than other lakes. DOC is potentially very important to lakes, because DOC tends to discolor a lake, which reduces light penetration and slows down photosynthesis. On the positive side, carbon may bond to other molecules such as phosphorus and nitrogen, which are important nutrients that may be in short supply in these relatively infertile lakes.   Anne Deininger and her colleagues focused their studies on two factors: DOC and nitrogen. Most lakes have too much nitrogen, a result of excessive use of nitrogen fertilizers that run off into lakes, so these relatively low-nitrogen lakes provided the researchers with a unique opportunity to see how these two factors, DOC and nitrogen, interacted in a natural ecosystem.

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Low DOC control lake. Credit: M. Klaus

The researchers selected six lakes that varied naturally in DOC levels: two low (~7 mg DOC/liter), two medium (~11 mg DOC/liter), and two high (~20 mg DOC/liter). In 2011 they measured everything possible about each lake: abundance of all of the life forms, DOC, temperature, light levels, nutrients and photosynthetic rates. In 2012 and 2013, they supplemented one of each pair of lakes with nitrogen compounds every one to two weeks. The added nitrogen was equivalent to the higher nitrogen inputs that are experienced by lakes in southern Sweden. And, as you might expect, the researchers continued measuring all factors of interest in both the experimental (fertilized) and control (unfertilized) lakes throughout the year – at least until the lakes froze over.


Anne Deininger (in orange) and Sonja Prideaux collect samples from a lake. Credit: M. Deininger.

Deininger and her colleagues were most interested in differences in the abundance of phytoplankton – small free-floating photosynthetic organisms, because these are the primary producers – the organisms that produce the chemical energy (via photosynthesis) that enters food webs. There are many different types or groups of these phytoplankton; some are flagellated, with hair-like processes that allow them to navigate in the water column. Some are exclusively autotrophs, producing their own energy from photosynthesis, some are primarily hetrotrophic, eating other organisms or the remains of dead organisms, while others are mixotrophs, using both strategies to produce energy. Cyanobacteria are photosynthetic bacteria, while picophytoplankton are phytoplankton of unusually small size.

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Flagellated phytoplankton (Cryptomonas). Illustration by Anne Deininger.

Many important findings are summarized in the graph below. “B” represents the year before fertilization (2011), while “A1” is 2012 (after fertilization – 1st year) and “A2” is 2013 (after fertilization – 2nd year). Remember only the N-lakes were fertilized; the control lakes were simply monitored all three years. One finding is that in 2011, the high DOC lakes had the lowest phytoplankton abundance.  A second is that the low and medium DOC lakes had both flagellated and non-flagellated phytoplankton, while the high DOC lakes were dominated by flagellated phytoplankton.

Moving to the years after fertilization (A1 and A2), you can see that nitrogen fertilization increased phytoplankton abundance, but more so for the low-DOC lake. However, fertilization had little impact on the types of phytoplankton found in each lake; rather it simply increased the abundance of already existing groups.


Mean biomass of major phytoplankton groups in relation to DOC.  Recall that B refers to 2011 (the year before fertilization), while A1 and A2 refer to the two years after fertilization (2012, 2013).

The data can be organized so we can get a better view of what is happening quantitatively. Fertilization increases phytoplankton biomass, but much more for lakes with low DOC levels. In addition DOC appears to decrease phytoplankton abundance.


Deininger and her colleagues conclude that in these northern lakes, phytoplankton production is nutrient-limited at low DOC levels, but becomes limited by light availability in more murky waters. So adding nitrogen increases phytoplankton abundance to a greater extent in low DOC lakes. High DOC lakes have more flagellated autotrophs, as these species can swim to the top of the water column where there is more light for photosynthesis. As needed, flagellated phytoplankton can move lower in the water column where nutrients are more abundant.

The researchers emphasize that the nitrogen experiments were only conducted for two years. They don’t know if, for example, the types of species would change if fertilization continued for more than two years. They also don’t know if after 2013, the communities reverted to their pre-fertilization state, or if biomasses remained higher when nitrogen fertilization stopped. These types of questions are important to pursue because we humans are making drastic changes to most of our aquatic systems in a very uncontrolled manner. We need to understand the effects of these changes to the aquatic environment, and also how we can reverse the effects should they prove to be highly detrimental.

note: the paper that describes this research is from the journal Ecology. The reference is Deininger, A., Faithfull, C. L., & Bergström, A. K. (2017). Phytoplankton response to whole lake inorganic N fertilization along a gradient in dissolved organic carbon. Ecology98(4), 982-994. 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.

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.


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!


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.


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.


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.


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.


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.


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.