Friday, December 26, 2014

Benny the bark beetle and the not-so palatable pine

‘Tis the season to cut down conifers, wrestle them into over-heated living rooms, and bedeck their branches with boxfuls of childhood ornaments carefully preserved in bits of paper towel. While trees may be unable to mount a defense against indignities perpetrated by saw-wielding bi-peds, they can sometimes hold their own in a contest with enemies of a smaller variety. Moths, beetles and other herbivores intending to lunch on enticing green needles or tender inner bark may be surprised to get a mouthful of nasty chemical compounds or gummy resin produced by these trees expressly for the purpose of deterring such intrusions.

Some trees produce defensive resin and chemicals all the time and are always prepared to ward off enemies, but others have figured out how to conserve their energy and only mount defenses when under attack. Some don’t bother protecting themselves at all. Why the variation? It seems like the most advantageous strategy is to defend when attacked, so why hasn’t this ability developed in all species?

Researchers at the Doñana Biological Station in Spain conducted an experiment to answer this question by observing how defensive abilities have evolved in pine trees. They grew seventeen pine species from North America and Eurasia together in a greenhouse and then tried to elicit a response by exposing them to a chemical that signals the trees that they are under attack. By measuring the base amount of resin and chemicals produced as well as the amount produced after provocation, the scientists could determine which strategy each species uses to defend itself and whether closely related species share similar strategies.

It turns out that the baseline amount of toxic chemicals produced in needles and resin in the stem is a characteristic that is similar among species that are more closely related. So, once acquired, these defenses are retained by species that evolve subsequently. However, the ability to produce more resin or chemicals when under attack appears to crop up randomly on the pine family tree. Despite the potential advantage of being able to conserve energy until it is needed, these “inducible defenses” are not retained when species split. Perhaps there is a hidden cost to maintaining this defensive flexibility? Maybe it is not always beneficial to wait a threat is present before committing resources to defense? The explanation will have to wait for another clever experiment or perhaps a clever model. In the meantime I will give thanks for the docility of my own holiday evergreen with a prayer for the continued absence of human-induced defenses.

You can find this article at:

Carrillo-Gavil an A., Moreira X., Zas R., Gonzalez-Voyer A., Vila M. & Sampedro L. (2014). Phylogenetic and biogeographical patterns in defensive strategies and quantitative allocation to checmical defenses in Palaearctic and Nearactic pine trees, Journal of Biogeography, DOI: 10.1111/jbi.12444

Tuesday, September 9, 2014

A Tale of Two Islands

Despite its title, this story is not exactly about two islands. It’s more about the eight-legged inhabitants of two groups of islands. But fear not arachnophobes, body size and parasitism do not feature in today’s plot. Instead we travel to the Hawaiian Islands in the middle of the Pacific Ocean and the Mascarene Islands in the Indian Ocean to unravel a mystery.

The Hawaiian and Mascarene Islands are similar in many ways. The islands that comprise both archipelagos are generally of a similar size and elevation. Both were formed recently in the last 10 million years by volcanoes pushing their way to the ocean’s surface. Both suffer from similarly mild oceanic tropical climates that support a lush flora and fauna, including many species found nowhere else in the world. A few of these endemic species belong to a group of spiders called Tetragnatha. Hawaii boasts 38 different species of Tetragnatha spiders, but when CNRS scientists from the Université Paul Sabatier in Toulouse, France spent three years looking for spiders in the Mascarene Islands, they only found 3 Tetragnatha species. And so we confront a mystery: why the ten-fold difference in diversity? Shouldn’t evolution proceed in similar ways on two island chains that appear, at first, so similar?

A quick comparison of the locations of Hawaii and the Mascarenes provides a clue. While the Mascarenes are a quick 725 km jaunt from Madagascar, Hawaii is stranded in the middle of the Pacific, 4000 km away from any source of land-based life. So, how does this help solve the mysterious diversity disparity? Consider why isolation might allow the Hawaiian Islands to evolve more Tetragnatha spider species. First assume that more distance means fewer spiders are able to balloon their way across the intervening ocean.  ↑ distance = ↓immigration is a primary tenant of island biogeography. If Hawaii received very few colonizing spiders, then the original few Tetragnatha colonists probably had many opportunities to use habitats and resources that weren’t currently being used by other spiders and so were able to evolve into many different species. (Hawaiianplants and birds are famous for this.) However, in the Mascarenes, if other types of spiders arrived before the Tetragnatha, these opportunities to evolve may not have existed. Furthermore, a constant influx of spiders from Madagascar and subsequent fraternization between Mascarene spiders and the new colonists would make it difficult for the genes of Mascarene spiders to become different from the genes of Madagascar spiders. Without genetic differences, new species cannot form.

Is this a general phenomenon? Do the species that colonize isolated islands subsequently evolve into more species than they would on less isolated islands? That’s not something we can conclude from just one example, which is why scientists test general ideas by looking for lots of examples. Ideas about evolution inspired by the natural world can also be tested in more artificial settings- for example, by watching how viruses and bacteria evolve when they’re put into different situations. Real islands, like Hawaii and the Mascarenes, are often referred to as ‘natural experiments’ because their isolation from other land allows us to compare one to another to test how ecology and evolution play out in different situations. This is the core of biogeography- using the natural world to understand how geography shapes life around us. But, it can be tricky to make conclusions when islands differ in more than just the way that we are interested in, or when factor X (e.g. ‘isolation’) may affect factor Y (e.g. ‘evolution of more species’) in several ways (e.g. ‘new ecological opportunities’ and ‘reduced gene flow from colonists’). I like this paper because it shows how observations of nature inspire “rules” of biogeography that we think are generally true. Now, can we figure out how to test the logic of these biogeographic rules with an un-natural experiment? Could we disentangle the effects of new ecological opportunities from reduced gene flow for the process of evolution on isolated islands? Bring on the microbes and computers!

You can find this paper at:

Casquet J., Corinne Cruaud, Frédérick Gavory, Rosemary G. Gillespie & Christophe Thébaud (2014). Community assembly on remote islands: a comparison of Hawaiian and Mascarene spiders, Journal of Biogeography, n/a-n/a. DOI:

Friday, August 8, 2014

For a wider range, pick a perfect partner

Clearing out the poison ivy from my backyard last weekend, I alternated between swatting mosquitos swarming around my temptingly bare calves and worrying that the subsequent transfer of ivy juices would lead to unfortunate consequences in the not-so-distant future. Times like these I wish that mosquitos would just make like a giant panda.

For that matter, why do some species live almost anywhere, while others struggle to hold on in a little piece of habitat? Sometimes a species is distributed far and wide simply because it’s a hardy traveler that can get swept up by winds, water or waterfowl and transported across the landscape. Other times it’s because a species is good at living just about anywhere- maybe it’s a vigorous grower that sucks up resources and just keeps on multiplying, or maybe it’s just plain hardy, able to cope with a wide range of environments.

Lichens aren’t exactly known for their prodigious growth, but they can survive in many environments and pump out tiny spores that float long-distances on air currents. Yet, they’ve also got another advantage that arises from the fact that a ‘lichen’ is not one species, but many. Most of a lichen’s ‘body’ is formed by one fungal species and inside this structure live many kinds of bacteria, algae, and other fungi. The algae photosynthesizes to provide food for the lichen, but scientists aren’t yet sure what exactly the other parts do. Such mutualisms, where two (or more) species have to swap resources or services in order to survive, are often thought of as fragile. Not only do conditions have to be good for one species, they have to work for all the species that participate in the mutualism. New research on lichens by Silke Werth and Victoria Sork, published last month in the American Journal of Botany, suggests that with a little bit of flexibility mutualism could actually be the key to thriving across a wide range of environments.

Lace lichen (Ramalina menziessi) is like a filmy mesh scarf that can be found draped from tree branches up and down the coast of western North America, from the hot dry desert of Baja California to the wet rainforests of Southeast Alaska. How can it cope with such different environments? Werth and Sork uncovered a couple key clues by looking into the genetics of the alga and fungus from lace lichens living throughout this range. It turns out that both the alga and fungus have different genetic signatures depending on the region they come from, which could be because they have adapted to the different environmental conditions found in the regions, or it could just be because the lichens are separated by a wide distance and rarely get to exchange genes. By investigating a bit further, the researchers uncovered a remarkable pattern: differences among individual lichens in the genetic signatures of the algae within the lichens are related to local climate variation and even more so to the tree species that the lichen grows on. That is, the algae are more similar to one another when they come from lichens living on the same tree species in similar environments. But, this wasn’t the case for the fungus, even though the alga and fungus inhabit the same lichen body!

These genetic patterns suggest that different descendants on the alga’s family tree have adapted to different climates and to the environmental conditions found on different tree species. The fungus, on the other hand, is just along for the ride and partners with whichever alga can be found on the tree that the fungal spore lands on. Thus, specialization on the part of the alga, and flexibility on the part of the fungus, has made lace lichen a successful inhabitant of trees up and down the western coast of North America. For lace lichen, a little flexibility makes mutualism an advantage, rather than a liability.

Flexibility may be the key to conquering a broad geographic area for many organisms, not just lichens or other mutualists. For example, plants that can change how they grow based on the environmental conditions they encounter (say, growing more leaves in low light to capture more of said light) can often be found across a broader range of habitats (check out Sonia Sultan’s cool case study with ‘plasticity’ in Polygonum). Perhaps giant panda’s need a lesson in dietary flexibility, but in this case I suspect the root of the panda’s distributional woes lie in our own species’ amazing adaptability.

Your can find this article at:

Werth S. (2014). Ecological specialization in Trebouxia (Trebouxiophyceae) photobionts of Ramalina menziesii (Ramalinaceae) across six range-covering ecoregions of western North America, American Journal of Botany, 101 (7) 1127-1140. DOI:

And for a previous post on one of the first papers to show environmental preference in lichen-forming algae, go here.

Sunday, February 9, 2014

Mystery of the Malaysian Mangroves

As an Alaskan child, visiting Florida for the first time when I was 17, mangroves were a fuzzy concept, loosely associated with swamps, alligators, and primordial ooze, and definitely NOT somewhere that I was looking forward to snorkeling. But, once I plunged off the side of the boat and under the two-foot thick opaque layer of vivid green algae, I was entranced by the fairy-tale scene of colorful sponges, tunicates, bryozoans and fishes. The trip was quite educational- I learned that mangroves do not , in fact, produce mangos, and informed that these plants are not one thing, but actually many different (and sometimes unrelated) species. A ‘mangrove’ is like a ‘tree’; it is plant with a special lifestyle, a lifestyle involving lots of salty water and a nice hot climate.

Fast forward ten years, and I am now learning that mangroves are a favored friend of biogeographers. For one, miniature mangrove islands played host to one of the founding experiments in the field of island biogeography. But, more directly, the bull’s-eye of mangrove diversity hovering over southeast Asia was a long-time poster child for the notion that the location where a group of species originates is the location where they eventually reach their highest diversity. How scientists eventually pieced together the evidence to show that the global distribution of mangrove diversity did not result from dispersion out of an east-Asian center of origin, is a classic saga of vicariance versus dispersal (read about it here).

Which finally brings me to today’s mangrove mystery- a story of vicariance and dispersal played out on a smaller stage, around the peninsula formed by Myanmar, Thailand, and Malaysia. During the most recent glaciation, when lots of water was locked up in polar icecaps, the Malay Peninsula was much larger and divided many mangrove species into two separate populations on its eastern and western sides. Individuals descended from each of these population centers now have different sets of genes (this is vicariance). Since we don’t have a historical record from 18,000 years ago, scientists have to think backwards- from the observation that mangroves growing on the east and west sides of the Malay Peninsula have two different sets of genes- to the inference that these two groups were once separated.

A group of scientists who study mangrove genetics set out to document whether a particular species of mangrove (Rhizophora mucronata) also showed evidence of having been historically divided. After sampling trees from all over the peninsula and nearby Sumatra, they did find two distinct groups, but these groups weren’t located where the scientists expected them to be. Instead of eastern and western populations being different, mangroves living on both the eastern and western sides of the peninsula were similar and only a few populations in the west around the Andaman Sea comprised a second group. Even more oddly, this distribution did not adhere to another proposed hypothesis predicting that individuals living closer together should have more similar genes.

What could be going on? To find the answer, the researchers had to look at a map of how water flowed around the peninsula. They discovered that ocean currents prevent water from the Andaman Sea from mixing with strait and South China Sea. Mangroves live on land, but their biology and dispersal are influenced by the ocean, especially for this particular species whose large seeds can probably float much farther than other species. While the currents could transport seeds from the eastern group all around the South China Sea and into the Malacca Strait, the movement of the water also prevented these seeds from making it into the Andaman Sea. So, the working hypothesis is that at some point in the past (maybe during the last glaciation) two groups of this mangrove species were separated and the genetic differences they developed are still maintained because ocean currents keep the two groups from mixing. Yet, the mystery of the Malaysian mangroves isn’t completely solved. The scientist still can’t explain exactly how the two groups arose in the first place.

What we learn from this research is that geography affects species’ genetics. Given enough time and the right circumstances these differences can result in the evolution of new species. Biogeographers typically think that distance and barriers are the two main features of geography that affect evolution and tend to view geological features as their chief determinants. Ocean currents add another layer of complexity, and beg the question ‘what other processes that operate over a shorter time-scale can influence species genetics?’ Where in the terrestrial realm are the analog of ocean currents?

This will be the first in a series of posts exploring how non-traditional factors constrain the geography of genetics. Stay tuned for more on human culture, parasites, and the increasingly fragmented natural world.

You can find this article at:

Wee A.K.S., Takayama K., Asakawa T., Thompson B., Onrizal , Sungkaew S., Tung N.X., Nazre M., Soe K.K. & Tan H.T.W. & (2014). Oceanic currents, not land masses, maintain the genetic structure of the mangrove Rhizophora mucronata Lam (Rhizophoraceae) in Southeast Asia , Journal of Biogeography, DOI: