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.

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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

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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.

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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).

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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.

Metallic starlings: a rain of terror

I am a slow learner. Several times in the past few years I have paddled my canoe under a particular sycamore tree in the New River in Radford, Virginia. Each time I do so, I am greeted by large numbers of cormorant poop bombs dropped by the dozens of cormorants that seem to find that particular tree to their liking, and this particular canoeist not to their liking. Fortunately, cormorants have bad aim, but unfortunately it is not that bad.

Daniel Natusch and three other researchers wanted to know how an analogous form of nutrient enrichment from large colonies of nesting Metallic Starlings (Aplonis metallica) affects the nearby ecosystem in a tropical Australian rainforest. They were interested in this question because it was obvious that the ground below the nesting colony trees was basically devoid of vegetation; they describe it as “an open moonscape”, contrasting sharply with the thick rainforest nearby. Other studies have shown that nutrient enrichment from bird guano leads to increased vegetation density – so why is this ecosystem different?

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Lockerbie Scrub rainforest, in Cape York Peninsula, Australia, showing colony tree with dead zone (left) and a continuous rainforest (right)

 

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Dan Natusch conducts herpetological research with his son Huxley. Credit: Jessica Lyons

 

 

 

 

 

 

 

 

 

 

 

 

The researchers compared the biological, chemical and physical environment underneath 27 different colony trees to the environment underneath a randomly chosen tree 100-200 meters from the colony tree. As expected, they found very little vegetation near colony trees, in contrast to relatively dense vegetation near the randomly chosen trees.

 

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Vegetation cover (left) and number of live stems (right) in relation to distance from the colony or randomly chosen tree (Point 0 on X-axis).  Negative numbers are downslope and positive numbers are upslope from the tree.

 

Soil analyses showed that the soils under the colony trees had much higher concentrations of important nutrients. For example, phosphorus levels were more than 30 times greater, and ammonium nitrogen was about four times greater under colony trees than under the randomly chosen trees. The researchers wondered whether these nutrient levels were so high that they were toxic to vegetation. That would account for the dead zone under the colony trees. An alternative hypothesis is that animals (pigs and turkeys in particular) may be attracted to these high nutrient areas under the colonies, and may either kill germinating plants by eating or trampling them.

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Feral pigs (Sus scrofa) rooting and trampling under a colony tree. Credit Daniel Natusch

To test both hypotheses, at the beginning of the breeding season the researchers covered a portion of the colony tree region with metal cages (exclosures) that prevented turkeys and pigs from gaining access. They discovered a much greater number of seedlings under the exclosures in comparison to the areas where turkeys and pigs could access the seedlings.

natuschfig7They concluded that nutrient levels were not toxic to seedlings, but that pigs and turkeys were either eating or trampling the seedlings as they emerge. As you can see, the number of exclosure seedlings dropped sharply in July, in part because rainfall declines sharply in June, which leads to high plant mortality, particularly in the unshaded dead zone. But in addition, feral pigs broke into all of the exclosures that summer to access the seedlings and the nutrient-rich soil.

Do these dead zones actually benefit the starlings in any way? One possible advantage is that dead zones prevent snakes from climbing nearby trees and vines to gain access to the nests that are located high in the canopy of the colony tree. However there is good evidence that colony trees suffer high mortality, as 10 of the 27 colony trees died within three years of the study. Trees that fall during the nesting period could lead to the failure of all of the nests within that colony tree.

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A scrub python (Morelia amethistina) puts the squeeze on a juvenile Metallic Starling. Credit Daniel Natusch.

 

Why do we find dead zones beneath colonies of Metallic Starlings, and increased plant growth rate, larger plant size and greater plant diversity beneath the colonies of several other bird colonies? Most previous studies have looked at sea-bird colonies on small islands that have few terrestrial herbivores, so germinating seedlings are relatively undisturbed. This study occurred in a continuous forest in tropical Australia, which harbored a large population of hungry herbivores. These contrasting findings show the important role of environmental context for understanding how ecological interactions will play out. Given that we humans are continually adding nutrients to our environment (through natural bodily function and when we fertilize our fields), we need to carefully consider the biotic and abiotic players in the ecosystem, so we can predict the effects we are having on the environment.

note: the paper that describes this research is from the journal Ecology. The reference is Natusch, D. J. D., Lyons, J. A., Brown, G. P., & Shine, R. (2017). Biotic interactions mediate the influence of bird colonies on vegetation and soil chemistry at aggregation sites. Ecology 98(2): 382-392. 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.