Burrowing Birds, Biocrust, and Biodiversity: A Microclimate Story

Nolana humifusa (Solanaceae) Photo by Michael Wolf licensed by GNU Free Documentation License

Peru’s coastal deserts are some of the driest places on Earth. Most of the water they receive comes not from rain but rather fog rolling in off the ocean. These fog-fed habitats are known as Lomas and they support a surprising diversity of plant species. Still, life in the Lomas is no treat so plants growing there need a bit more than a tough disposition to get by. Many components of the Lomas flora rely on favorable microclimates to survive long enough to reproduce. Recently it has been found that a few species of burrowing birds are responsible for creating some of these favorable microclimates.

The beneficial effects of burrowing or “fossorial” animals on plant diversity has many examples in nature. This is especially true in harsh climates. The act of burrowing disturbs the surrounding soil and can expose nutrient-rich soils as well as increase hydrology. However, more than just mammals burrow. As such, researchers wanted to investigate the role of burrowing birds on Lomas plant diversity.

A pair of burrowing owls (Athene cunicularia) Photo by Ron Knight licensed by CC BY 2.0

The birds in this study consist of one owl - the burrowing owl (Athene cunicularia), and two species of miner birds (Geositta peruviana & G. maritima). Instead of nesting in trees, which are few and far between in such arid habitats, these birds nest in the ground. To do so, they excavate burrows. As they excavate, these birds break up the thin biocrust of cyanobacteria that carpets undisturbed stretches of sand. This biocrust is an immensely important component of the local ecology. It stabilizes sandy soils and increases their fertility. It also has a considerable impact on water infiltration, runoff, albedo, and temperature of the soil.

The greyish miner (Geositta maritima)

The coastal miner (Geositta peruviana) Photo by Berichard licensed by CC BY-SA 2.0

Taken together, it is easy to see how large patches of biocrust can either promote or inhibit plant germination and growth. Some species perform well under such conditions while others do not. This is why researchers were so interested in burrowing birds. By breaking up the biocrust and constructing mounds outside of their burrows, these birds are changing the microclimates of the surrounding area. This creates a heterogeneous patchwork of soil types that in turn influence the plant species that can grow and survive.

It turns out, burrowing birds on the Peruvian coast are having considerable effects on local plant diversity. By studying the soil properties around burrows and comparing it to undisturbed soil patches nearby, researchers were able to show that the plant communities living in these areas are in fact different. For starters, despite undisturbed soils having far more seeds in the soil seed bank than burrow mound soils, far more plants germinated and grew on the mounds than in the biocrusts. Also, though the seed bank of the mounds was largely comprised of similar species to that of the undisturbed soils, the seeds of species that produce bird-dispersed berries such as Solanum montanum were more abundant in the mound soil.

Fuertesimalva peruviana (Malvaceae) Photo by Jose Roque licensed by CC BY-SA 3.0

In terms of seedlings, mound soils not only exhibited higher seedling emergence, they also exhibited a higher species richness than the undisturbed biocrust soils nearby. The benefits of growing in the mound soils were most apparent for three plant species in particular: Cistanthe paniculata (Montiaceae), S. montanum (Solanaceae), and Fuertesimalva peruviana (Malvaceae). It appears that these species are much more likely to germinate and survive in and around the burrows than they are in the surrounding landscape. Such a boost to growth and survival, however marginal, means a lot in such a harsh, uninviting landscape.

Even more incredible is how specific burrow microclimates can be. Plants growing on the mounds didn’t do so in a uniform way. Instead, tiny variations in the soil of the burrow mound appeared to make a huge difference for plants. Soils near the entrance of an active burrow are disturbed far more often than soils on the backside of the mound. As such, more plants were found growing on the backside of the mound, demonstrating yet again how slight improvements in favorable microclimates can have astounding impacts on plant survival and diversity.

A. Soil profiles of the studied treatments. B. Landscape of the study area. The lower site of the hills is covered in biocrust except where it is disturbed by birds' burrows (Bioperturbation labeled in the picture). C. Dark cyanobacterial biological soil crust that covers the study site. D. Burrowing owl Athene cunicularia standing on its bioperturbation. [SOURCE}

The reason some plants do much better in disturbed soils over those covered in cyanobacteria biocrust are still not entirely clear. It is likely that some plants simply can’t break through the biocrust as they germinate. It is also possible that the seeds of some of these species simply can’t break through the biocrust to even make it into the soil seedbank. Not only would this cause them to blow around, it also means that they aren’t contacting the soil enough to imbibe water and germinate. Despite containing fewer seeds, the act of digging a burrow may loosen up the soil enough so that seeds are properly buried and thus can maintain good soil to seed contact for long enough to promote germination and growth.

All in all it appears that these three bird species are important ecosystem engineers across the Lomas of the Peruvian coast. By creating a patchwork of different soil properties, these birds are essentially creating a patchwork of different habitats that support different plant species. Take the birds away and it is reasonable to assume that plant diversity would decline. This is yet another important reminder of how interconnected the natural world truly is. It is also an important reminder of why habitat, rather than species-specific conservation efforts should be a much higher priority than it is today. Please, support a land conservation agency today!

Photo Credits: [1] [2] [3] [4] [5] [6]

Further Reading: [1]

An Introduction to Hornworts

Anthoceros sp. Photo by Bramadi Arya licensed under CC BY-SA 4.0

When was the last time you thought about hornworts? Have you ever thought about hornworts? If you answered no, you aren’t alone. Despite their global distribution, these tiny plants receive hardly any attention and that is a shame. Hornworts (Anthocerotophyta) have been around for a very long time. In fact, it is likely that they were some of the first plants to colonize the land roughly 300 - 400 million years ago.

To be fair, hornworts aren’t known for their size. They are generally small plants, though their colonies can form impressive mats. To find them, one must try looking in and among rocks, bare patches of soil, or pretty much anywhere enough moisture builds up to supply their needs. They tend to enjoy nutrient-poor substrates but I would hesitate to say that with any certainty. No matter where you live, from the tundra to the tropics, there is probably a hornwort native to your neck of the woods.

Dendroceros sp. Photo by J.Ziffer licensed under public domain

How many different species of hornwort there are is apparently the subject of some debate. Some authors recognize upwards of 300 species whereas others suggest the real number hangs somewhere around 150. Regardless of the exact numbers, hornworts belong to one of six genera: Anthoceros, Dendroceros, Folioceros, Megaceros, Notothylas and Phaeoceros. Fun fact, the suffix ‘ceros’ at the end of each genus is derived from the Latin word for ‘horn.’

The reason they are called hornworts is because of their reproductive structures or “sporophytes.” Similar to their moss and liverwort cousins, hornworts undergo an alternation of generations in order to reproduce sexually. The green gametophytes house the sexual organs - antheridia if they are male and archegonia if they are female. After fertilization, a sporophyte begins to grow, which will go on to produce and disseminate spores. However, the way in which the hornwort sporophyte forms is a bit different from what we see in mosses and liverworts.

Alternation of generations in hornworts. Photo by Mariana Ruiz (LadyofHats) licensed under public domain

Upon fertilization, the zygote begins to divide into a bulbous mass of cells affectionately referred to as "the foot.” This foot remains within the gametophyte throughout the lifetime of the hornwort, depending on the gametophyte for water and nutrients. Even more peculiar is the the fact that the growing point of the sporophyte is at the base rather than the tip. As such, the horn of each hornwort could continue to grow upwards until it is damaged in some way.

The horn itself is an amazing structure. Whereas the outside layers of tissue are merely structural, the internal tissues differentiate into two different types - spores and pseudo-elaters. Pseudo-elaters expand and contract as humidity fluctuates so as the sporophyte splits to release the spores, the pseudo-elaters dehydrate and snap like tiny spore catapults, thus aiding in their dispersal.

Megaceros flagellaris. Photo by Dr. Scott Zona licensed under CC BY-NC 2.0

Of course, reproduction is the main goal but to get to that point, hornworts must grow and mature. How they manage to survive is incredible because it is a reminder that what are often thought of as “primitive” plants are actually far more advanced than we give them credit for. The main body of the hornwort gametophyte is a thin layer of cells that spread out to form a tiny, green mat. This is the structure you are most likely to encounter.

Inside each cell is a single chloroplast. In most hornworts, the chloroplast does not exist in isolation. Instead, it is fused with other organelles into a structure called a “pyrenoid.” The pyrenoid functions as both a center for photosynthesis and a food storage organ. This is unique as it relates to terrestrial plants but quite common in algae. Another odd fact about hornwort anatomy are the presence of tiny cavities scattered throughout their tissues. These cavities form as clusters of hornwort cells die. They then fill with a special mucilage that appears to invite colonization by nitrogen-fixing cyanobacteria. The cyanobacteria set up shop within the cavities and provides the hornwort with supplemental nitrogen in return for a place to live.

Anthoceros agrestis photo by BerndH licensed under CC BY-SA 3.0

Cyanobacteria aren’t the only organisms to have partnered with hornworts either. Mycorrhizal fungi also enter into the picture. A study done back in 2013 actually found that a wide variety of fungi will partner with hornworts which suggests that this symbiotic relationship is much more ancient and versatile than we once thought. Fungi cluster around parts of the gametophyte that produce root-like structures called “rhizoids,” offering nutrients in return for carbohydrates.

All in all, I think it is safe to say that hornworts are remarkable little plants. Though they can sometimes be difficult to find and properly identify, they nonetheless offer plenty of inspiration for the botanically inclined mind. We can all do better by tiny plants like the hornworts. They have been on land for an incredible amount of time and they definitely deserve our respect and admiration.

Photo Credits: [1] [2] [3] [4] [5]

Further Reading: [1] [2] [3]

Are Algae Plants?

I was nibbling on some nori the other day when a thought suddenly hit me. I don't know squat about algae. I know it comes in many shapes, sizes, and colors. I know it is that stuff that we used to throw at each other on the beach. I know that it photosynthesizes. That's about it. What are algae? Are they even plants?

The shortest answer I can give you is "it depends." The term algae is a bit nebulous in and of itself. In Latin, the word "alga" simply means "seaweed." Algae are paraphyletic, meaning they do not share a recent common ancestor with one another. In fact, without specification, algae may refer to entirely different kingdoms of life including Plantae (which is often divided in the broad sense, Archaeplastida and the narrow sense, Viridiplantae), Chromista, Protista, or Bacteria.

Caulerpa racemosa, a beautiful green algae. Photo by Nhobgood Nick Hobgood licensed under CC BY-SA 3.0

Taxonomy being what it is, these groupings may differ depending on who you ask. The point I am trying to make here is that algae are quite diverse from an evolutionary standpoint. Even calling them seaweed is a bit misleading as many different species of algae can be found in fresh water as well as growing on land.

Take for instance what is referred to as cyanobacteria. Known commonly as blue-green algae, colonies of these photosynthetic bacteria represent some of the earliest evidence of life in the fossil record. Remains of colonial blue-green algae have been found in rocks dating back more than 4 billion years. As a whole, these types of fossils represent nearly 7/8th of the history of life on this planet! However, they are considered bacteria, not plants.

Diatoms (Chromista) are another enormously important group. The single celled, photosynthetic organisms are encased in beautiful glass shells that make up entire layers of geologic strata. They comprise a majority of the phytoplankton in the world's oceans and are important indicators of climate. However, they belong to their own kingdom of life - Chromista or the brown algae.

To bring it back to what constitutes true plants, there is one group of algae that really started it all. It is widely believed that land plants share a close evolutionary history with a branch of green algae known as the stoneworts (order Charales). These aquatic, multicellular algae superficially resemble plants with their stalked appearance and radial leaflets.

A nice example of a stonewort (Chara braunii). Photo by Show_ryu licensed under CC BY-SA 3.0

It is likely that land plants evolved from a Chara-like ancestor that may have resembling modern day hornworts that lived in shallow freshwater inlets. Estimates of when this happen go back as far as 500 million years before present. Unfortunately, fossil evidence is sparse for this sort of thing and mostly comes in the form of fossilized spores and molecular clock calculations.

Porphyra umbilicalis  - One of the many species of red algae frequently referred to as nori. Photo by Gabriele Kothe-Heinrich licensed under CC BY-SA 3.0

Now, to bring it back to what started me down this road in the first place. Nori is made from algae in the genus Porphyra, which is a type of Rhodophyta or red algae. Together with Chlorophyta (the green algae), they make up some of the most familiar groups of algae. They have also been the source of a lot of taxonomic debate. Recent phylogenetic analyses place the red algae as a sister group to all other plants starting with green algae. However, some authors prefer to take a broader look at the tree and thus lump red algae in a member of the plant kingdom. So, depending on the particular paper I am reading, the nori I am currently digesting may or may not be considered a plant in the strictest sense of the word. That being said, the lines are a bit blurry and frankly I don't really care as long as it tastes good.

Photo Credits: [1] [2] [3] [4] [5]

Further Reading: [1] [2] [3] [4]

The Nitrogen-Fixing Abilities of Cycads

Photo by Daderot Public Domain

Long before the first legumes came onto the scene, the early ancestors of Cycads were hard at work fixing atmospheric nitrogen. However, they don't do this on their own. Despite being plentiful in Earth's atmosphere, gaseous nitrogen is not readily available to most forms of life. Only a special subset of organisms are capable of turning gaseous nitrogen into forms usable for life. Some of the first organisms to do this were the cyanobacteria, which has led them down the path towards symbioses with various plants on many occasions. 

Cycads are but one branch of the gymnosperm tree. Their lineage arose at some point between the Carboniferous and Permian eras. Throughout their history it would seem that Cycads have done quite well in poor soils. They owe this success to a partnership they struck up with cyanobacteria. Although it is impossible to say when exactly this happened, all extant cycads we know of today maintain this symbiotic relationship with these tiny prokaryotic organisms. 

Cross section of a coralloid cycad root showing the green cyanobacteria inside. Photo by George Shepherd licensed under CC BY-NC-SA 2.0

The relationship takes place in Cycad roots. Cycads don't germinate with cyanobacteria in tow. They must acquire them from their immediate environment. To do so, they begin forming specialized structures called precoralloid roots. Unlike other roots that generally grow downwards, these roots grow upwards. They must situate themselves in the upper layer of soil where enough light penetrates for cyanobacteria to photosynthesize.

The cyanobacteria enter into the precoralloid roots through tiny cracks and take up residence. This causes a change in root development. The Cycad then initiates their development into true coralloid roots, which will house the cyanobacteria from that point on. Cycads appear to be in full control of the relationship, dolling out carbohydrates in return for nitrogen depending on the demands of their environment. Coralloid roots can shed and reform throughout the lifetime of the plant. It is quite remarkable to think about how nitrogen-fixing symbiotic relationships between plants and microbes have evolved independently throughout the history of life on this planet.

Photo Credits: [1] [2]

Further Reading: [1] [2]

Of Gunnera and Cyanobacteria

Photo by UnconventionalEmma licensed under CC BY-NC 2.0

Nitrogen is a limiting resource for plants. It is essential for life functions and yet they do not produce it on their own. Instead, plants need to get it from their environment. They cannot uptake gaseous nitrogen, which is a shame because it makes up 78.09% of our atmosphere. As such, some plants have developed very interesting ways of obtaining nitrogen from their environment. Some, like the legumes, produce special nodules on their roots, which house bacteria that fix atmospheric nitrogen. Other plants utilize certain species of mycorrhizal fungi. One family of plants, however, has evolved a symbiotic relationship that is unlike any other in the angiosperm world.

A Gunnera inflorescence. Photo by Lotus Johnson licensed under CC BY-NC 2.0

Meet the Gunneras. This genus has a family all to itself - Gunneraceae. They can be found in many tropical regions from South America to Africa and New Zealand. Some species of Gunnera are small while others, like Gunnera manicata, have leaves that can be upwards of 6 feet in diameter. Their leaves are well armed with spikes and spines. All in all they are rather prehistoric looking. The real interesting thing about the Gunneras though, is in the symbiotic relationship they have formed with cyanobacteria in the genus Nostoc.

Traverse section of a Gunnera stem showing cyanobacteria colonies (C) and the cup-like structures (S) where they enter the stem. [SOURCE]

Gunnera produce cuo-like glands that house these cyanobacteria. The glands are filled with a special mucilage that not only attracts the cyanobacteria, but also stimulates it to grow. Once inside the glands, the cyanobacteria begins to grow into the plant, eventually fusing with the Gunnera cells. From there the cyanobacteria earn their keep by producing copious amounts of usable nitrogen and in return, the Gunnera supplies carbohydrates. This relationship is amazing and quite complex. It also offers researchers an insight into how such symbiotic relationships evolve.

Photo Credit: [1] [2] [3]

Further Reading: [1] [2] [3] [4] [5]