Sunday, December 18, 2011

Bird poop solves the paradox of the snail

I used to love a certain board game called “Snail’s Pace Race”.  For some reason, the inch-by-inch progress of the colorful wooden snails along a flat piece of cardboard was exciting to my five-year-old brain.  Perhaps it was my aversion to competitive situations- no one loses in a game in which snails race.

As even a five-year-old knows, snails are not exactly skillful travelers.  Yet paradoxically, they can be found all over the world.  How exactly does a snail, of all creatures, get to remote places like islands in the middle of the Pacific Ocean? Even Darwin was puzzled by the incongruity of a widespread animal with such little talent for dispersion.

In a way, plants are also like snails.  They don’t move very well, but some live across areas separated by thousands of miles (see previous post).  In order to accomplish this, plants produce seeds that are much smaller than their adult form and which can be more easily transported by wind or on the fur of animals.  Snails are larger than the typical seed, but perhaps they could still be accidentally blown to new places by strong winds or hitch a ride on the legs on birds.  These events don’t seem very likely, but all it takes is a few successful travelers for the snails to start living in a new place.

A group of Japanese researchers were interested in how frequently these chance movements of snails across large distances might occur, but since it is impossible to track every snail across even a relatively small area, they used a more indirect approach.  All across the island of Hahajima they collected snails and analyzed their DNA.  When snails mate with each other, their offspring have DNA from both parents.  If snails are able to move across the island easily and do not just mate with the individuals they are closest to, then different DNA sequences will get passed all around the island and there won’t be huge differences between the type of sequences that can be found on the northern end versus the southern end.  However, if groups of snails living far apart had very different DNA sequences then this would imply that snails are only rarely transported across the island.

Unexpectedly, the genetic analysis showed that DNA was getting swapped between snails across the island and that distant places did not have different DNA sequences- something must be moving the snails around!  Even more interesting was that places with lots of Japanese white-eyes (a bird that eats snails) had greater variation in DNA sequences among snails living there, which meant that more sex was happening there between snails from different places.  The evidence pointed to the birds as the main snail-movers, but when the researchers looked at the birds’ legs, they never saw little hitch-hikers.

It wasn’t until they looked more closely at the birds’ excrement that the mystery began to unravel.  The snail they were studying (Tornatellides boenigi) is a very small snail, and when the white-eyes eat them, many of the shells come out in the poop still in-tact.  What if some of the shells could come out with both shell and living snail still in-tact? To test this, the researchers went to the Yokohama Zoo and started feeding snails to white-eyes and brown-eared bulbuls (another common bird on Haha-jima).  Incredibly, 15% of the snails came back crawling out of the poop- one even had babies afterward.  Like the Millenium Falcon in the belly of the giant space worm, the snails somehow survived the corrosive acidity of the stomach, though in this case they came out the other end.

Like bats that eat figs and subsequently rain their seeds all across the forest, or humans that have transported corn from its home in Mexico to fields all over the world, this new study shows that predators don’t always have a purely negative influence on the things they eat.

You can find this article at:

ResearchBlogging.orgWada, S., Kawakami, K., & Chiba, S. (2012). Snails can survive passage through a bird’s digestive system Journal of Biogeography, 39 (1), 69-73 DOI: 10.1111/j.1365-2699.2011.02559.x

Sunday, December 4, 2011

Different worms for different gulls

Old wives may say that the early bird gets the worm, but there are many worms that birds may not want to get.  These are the helminths (a.k.a parasitic worms), and whether they are early to the table or late, birds end up with a lot of them.  Helminths come in many shapes and sizes; nematodes are round and generally look like a worm, digeneans have two suckers and are solid all the way through, and cestodes (tapeworms) are basically a miniature toothy q-tip head trailing a toilet paper tail.  Birds and other vertebrate animals have such a diverse collection of these gut parasites that scientists have started to think about the insides of an animal as a mini-ecosystem and are asking whether ecological theory developed for larger animals also works in the same way for parasites.

A study published online this month in Ecography asked whether ring-billed gulls have more similar helminthes in their guts if they live closer to each other or if they are closer together in age.  The idea that as places become farther apart they have more different plants and animals is called ‘distance decay’ and biogeographers have gotten pretty excited about measuring it in the last ten years, even though for geographers it has been the ‘first law of geography’ since 1970.

For gulls living along 300 km of the St. Lawrence River in Quebec, Canada, the differences between the parasites living in different gulls was more related to their age than how far apart they lived from each other.  Older gulls had a wider variety of parasites than younger gulls because they forage for food in many places and ingest different types of parasites, whereas chicks and juveniles don’t venture as far.  However, when the researchers compared the parasites living inside Quebec gulls to those living 3000 km away in Alberta, their differences in parasites were much more related to how far apart gulls lived- age barely had any effect.

This distance decay in parasite communities has also been looked at in the parasites of fish, mammals, and molluscs.  The gull researchers wanted to know whether different characteristics of the host animal, such as its size and mobility, or whether it is homeothermic (warm-blooded) affects how quickly the difference between the parasites of two hosts increases as the distance between them increases.  One might think that animals that migrate across wide areas would have similar parasites even across large distances because each animal is likely to pick up most of the same parasites, whereas mussels (that are affixed to rocks on the ocean shore) could have very different types of parasites living inside them even across short distances.

Intriguingly, this doesn’t actually seem to be the case.  In a preliminary analysis of 25 studies of different host animals, the researchers found that only two factors affected the rate of distance decay in parasites; latitude and how widely across space animals were sampled in the study- host mobility didn’t matter at all.  Studies that analyzed parasites in animals across a large extent (1,000 -10,000 km) tended to find slower rates of distance decay.  Parasites in animals living farther north at higher latitudes had faster rates of distance decay.  That is, there is greater variation in the parasites that animals have over shorter distances. To me, it’s not exactly clear why this may be, but the researchers suggest that it could be because animals living up north have to deal with a wider range of climates.  As more scientists study the ecology of parasites it will be interesting to see whether host characteristics affect other aspects of parasite communities.

You can find this article at:

ResearchBlogging.orgLocke, S., Levy, M., Marcogliese, D., Ackerman, S., & McLaughlin, J. (2011). The decay of parasite community similarity in ring-billed gulls Larus delawarensis and other hosts Ecography DOI: 10.1111/j.1600-0587.2011.07244.x

Monday, November 28, 2011

Supertramps play checkers on remote tropical islands

Biogeographers have a not-so-secret love affair with archipelagos.  Inarguably, islands are sexy.  But, when a bunch of islands get together they become the pieces in an irresistible puzzle of “who lives where and why?”

Consider the facts: Islands are hard to get to; for land-dwelling plants and animals they are stepping stones of potential habitat amidst an endless sea of death.  Therefore, islands close together (such as in an archipelago) should have the same plants and animals living on them, because the hardest part of setting up shop in a group of islands is traversing the distance from the mainland.  However, contrary to expectation, islands in archipelagos often have different biotas and the two-hundred-year-old question is “why?”

You may have heard of Charles Darwin- his answer to the “why” for finches in the Galapagos was evolution by natural selection after a long trip from mainland South America.  Someone you may not have heard of is Jared Diamond, but he also sparked a ferocious debate when he claimed that different bird species in the Bismarck Archipelago live on different islands because they compete with each other.

In 1975, Diamond noticed that certain pairs of bird species make a checkerboard pattern across the archipelago- the two species never live on the same island but do live on neighboring islands.  He suggested that one explanation is that the species are so similar in the types of food or habitat that they use that they compete and one kicks the other off the island.  Of course, there are lots of other reasons why two species don’t occur together on the same island: maybe the two are different and need different types of habitat, or maybe they live in different parts of the archipelago that are two difficult to travel between.  A new study this month in the Journal of Biogeography revisits Diamond’s original bird data from the Bismarck Archipelago and looks more closely at whether his checkerboard patterns really do result from competition.

It’s time for some math.  There were 154 bird species found across all the islands, which means that there are (154 x 153)/2 = 11, 781 possible pairs of bird species.  The researchers found that 1,516 pairs of species followed a checkerboard pattern, but that only 27 of these were between closely related species that are likely to compete with each other.  This number seems small relative to the total number of possible pairs; how do we know that this isn’t the number of checkerboard pairs that would happen just by chance? By using a null model, of course (see previous post), where a computer puts bird species on islands at random and then calculates the number of pairs of species that don’t occur together.  This showed that for several types of birds, there were actually more checkerboard pairs than expected by chance- which means that competition could be the explanation.  Or, it might not be.

When glaciers expanded and the sea-level dropped back in the Pleistocene, many of the Bismarck islands became connected by land, but there were still groups of islands that remained separate.  In many of the checkerboard pairs of species, the species live in different parts of the archipelago that were separated by water during the Pleistocene- this shows that species may not live together because they have not been able to get to the same islands.

Another peculiar commonality of the 27 checkerboard pairs of closely related species is that in almost all cases one of the species lives only on small islands where there aren’t many other bird species.  These ‘supertramps’, as Diamond called them, may be weak competitors that can only live in places not already usurped by other species.  The prevalence of supertramps in pairs of species that form checkerboards means that, despite 35 year of controversy, competition may actually be a good explanation, at least in part, for why islands have different bird species.  As for what’s responsible for the other (much larger) part, geography, evolution, and chance appear to be the major culprits.

You can find this article at:

ResearchBlogging.orgCollins, M., Simberloff, D., & Connor, E. (2011). Binary matrices and checkerboard distributions of birds in the Bismarck Archipelago Journal of Biogeography, 38 (12), 2373-2383 DOI: 10.1111/j.1365-2699.2011.02506.x

Sunday, November 20, 2011

Salmon like it cold, catfish like it hot

How many fish can you name?  Five? Fifty? How about all 829 species native to the rivers and lakes of the continental U.S. and Canada?  Naming all those species is impressive, but the fifteen-or-so minutes that it takes to do so would be a bit like watching water boil… only longer.  Names only become interesting if we know something interesting about the species they pertain to.

In the case of North American freshwater fish, one thing we do know is where each of them live (or lived, for 19 species that recently kicked the bucket); Lawrence Page and Brooks Burr recently published a brand-new field guide that has maps of the ranges of all of these fish species.  But still, this isn’t all that earth-shattering to anyone but an ichthyologist or sport fisher with a checklist.  What’s actually interesting is what Page and another biologist (Jason Knouft) did with these range maps.

By laying all of the maps on top of each other (using a computer, of course) they were able to show that native fish follow both a latitudinal and longitudinal diversity gradient; there tend to be more species of freshwater fish in the lakes and rivers in the southern and eastern parts of the continent than in the northern and western parts.  People have been studying latitudinal diversity gradients for a long time and have come to realize that, even though it’s a cool pattern, the fact that there are more species closer to the equator really doesn’t tell us much about why there are more species closer to the equator.

If the fish are split up into the different families to which they belong (e.g. basses, sculpins, catfishes, perch, trout, etc…) we can learn a lot more about why fish diversity varies so greatly between different places.  Different fish prefer different types of environments; salmon like it cold, but catfish like it hot.  When the researchers correlated temperature and other environmental variables with the diversity of each of these groups, it came as no surprise that trout and salmon (Salmonidae family) diversity is higher in the frigid north and that catfish diversity is higher in the balmy south.  In fact, each fish family had different aspects of the environment that they keyed in to.  Not many suckers live in the mountains, but minnows can be found high or low.

In the case of North American fish vs. Latitude, the latitudinal diversity gradient appears to be losing- the pattern appears to be more of an accident of which families of fish are most prevalent and less of a general pattern found across all types of fish.  Most of the differences in diversity between the north and south stem from differences in the environment and the ability of different groups of fish to survive and diversify in these environments.  In the end, latitude itself doesn’t actually have much to do with it.

You can find this article at:

ResearchBlogging.orgKnouft, J., & Page, L. (2011). Assessment of the relationships of geographic variation in species richness to climate and landscape variables within and among lineages of North American freshwater fishes Journal of Biogeography, 38 (12), 2259-2269 DOI: 10.1111/j.1365-2699.2011.02567.x

Sunday, November 13, 2011

Climate, not space, gives trees more room to range

Everything does not live everywhere; there are no baobab trees in Canada, nor caribou in Florida.  This is not a particularly profound statement.  But, for some of my former Malawian high school students, who had never traveled farther than 20km from their home or watched a nature documentary, it came as a revelation.  “But why, madam?” they would ask me.

Simplistically, the answer has two parts.  One, places are far away from each other, and two, places have different climates; therefore, living things have limited areas where they live because they either can’t get to new places or because when they get there they can’t survive.  Taking this one step further, it follows that species that can survive in a wider range of environments should also have larger ranges.  But, how can we actually determine whether this is true?

One pattern that seems consistent with this idea, is the observation that species that live farther north tend to have larger ranges.  This pattern has been called ‘Rapoport’s Rule’, though it is neither a general rule nor does is belong to Rapoport.  One explanation for why it (sometimes) occurs is that species living farther north have to be able to deal with a wider range of temperatures throughout the year.  This broader tolerance would allow them to live across a wider geographic area.  But again, how would we actually test this?

In the most recent issue of Ecography, Xavier Morin and Martin Lechowicz describe their clever way of figuring out whether the Rapoport effect they found across all 598 of North America’s native tree species actually resulted from tolerance of annual temperature variability, as opposed to just chance.  One alternative hypothesis they had to rule out was the possibility that the trees’ ranges were larger farther north simply because there is more land area in the northern parts of North America than there are in the southern parts.

On a computer, the scientists made an outline of North America and then randomly dropped the tree species onto the continent and let them spread.  When the trees were allowed to spread anywhere, the range sizes that came out of the simulation were much different than the actual ranges sizes for real North American trees and there was no relationship between the size of the range and how far north it was.  However, when the simulation was constrained so that each fake species was given a temperature tolerance and only allowed to spread to places within that temperature range, then the simulation produced range sizes that were larger further north.

This type of null modeling, where a computer makes a simulation that is used to test whether patterns observed in nature could come about just because of chance, is becoming much more popular with all kinds of scientists now that fast computers are cheaper and easier to use. 

You can find this article at:

ResearchBlogging.orgMorin, X., & Lechowicz, M. (2011). Geographical and ecological patterns of range size in North American trees Ecography, 34 (5), 738-750 DOI: 10.1111/j.1600-0587.2010.06854.x

Sunday, October 30, 2011

In matters of home, lichens follow their algal hearts

Many couples would probably agree that deciding on a place to live together is about as easy as tying your shoes using one hand and one foot- and that’s when the two parties are both human.  Consider what life is like for a lichen, a creature made up of organisms from two different kingdoms of life.  A lichen is a fungus that obtains nutrients from an algae growing inside of it- a symbiotic lifestyle that benefits the fungus, but not necessarily the algae.

It the past, biologists thought that it was the fungal partner who determined where the lichen grew, since it is the fungus that makes up most of the body of the lichen.  However, a new study in Molecular Ecology adds to the growing body of evidence that, it may actually be the algae who hold the reins.

When researchers at Charles University in Prague took samples of the DNA of Asterochloris algae living in Lepraria lichens from central Europe and California, they found that the most closely related algae were not the ones found living in the same species of lichen.  Instead, algae from different lichen species living in similar habitats were more closely related to each other.  For example, algae from lichens living in locations exposed to the sun and rain were more closely related to each other than they were to algae from lichens living in sheltered humid environments.

What this shows is that different algal species have preferences for different environments and if a fungal partner wants to live in a particular environment, it has to ally itself with an alga that also wants to live in that environment.  If the fungus attempts to grow somewhere that the alga doesn’t like, the alga will probably die, taking the fungus along with it.

Why is this biogeography?  Scale up the microclimatic preferences of the algae to the macroscale patterns of climate found across the earth.  Different algae have been found in lichens from the tropics versus the temperate zone versus more polar regions; and on a smaller scale, different algae are found in lichens at the tops of mountains than at their bases.  To figure what causes a lichen to live where it does, the most pertinent question may be, “Who’s your alga?”

You can find this article at:

ResearchBlogging.orgPEKSA, O., & ŠKALOUD, P. (2011). Do photobionts influence the ecology of lichens? A case study of environmental preferences in symbiotic green alga Asterochloris (Trebouxiophyceae) Molecular Ecology, 20 (18), 3936-3948 DOI: 10.1111/j.1365-294X.2011.05168.x

Monday, October 24, 2011

Floral gladiators evolve faster in South Africa than Rome

Gladiolus is latin for ‘little sword’, a fitting name for a plant whose flowers grow along giant spikes.  While these plants are ubiquitous in gardens across the United States, they actually are complete foreigners.  Of the 260 species around the world, almost all are from southern Africa, and none are from North or South America.

Even in the parts of the world where Gladioli are native, the distribution of their diversity varies hugely.  There are 106 species of Gladiolus in the Cape Floristic Region of South Africa and only 7 in the Mediterranean region of Europe, even though the Mediterranean region is 25 times larger than the Cape.  The two areas have similar climates and ecosystems, so what explains the 15-fold difference in diversity for these plants?

In evolutionary terms, making species is like making pancakes; to get more pancakes you either have to work longer or put more batter on the griddle and work faster.  Similarly, for there to be more Gladiolus species in southern Africa, either they have lived in southern Africa longer and have had more time for evolution to produce new species, or evolution of Gladioli proceeds faster in southern Africa than in Europe, so that new species are produced at a faster rate.

Luis Valente from the Imperial College in London and a group of international colleagues constructed a phylogenetic tree from 150 Gladiolus species from in Europe and across sub-Saharan Africa.  This ‘tree’ is like a family tree, showing how closely related all of the Gladioli are to each other.  By coloring the tips of the branches based on what region each species lives, the researchers were able to infer what region each of the ancestors used to live in, all the way back to the first Gladiolus.  From this information they could then calculate how long Gladioli have lived in different regions of Africa and Europe, as well as how quickly each of the branches of the tree found in these regions have produced new species.

So, which hypothesis explains the high Gladiolus diversity in the Cape- was it because of more time or faster evolution?  The answer, as it turns out for most questions in ecology, is both.  It is likely that the first Gladiolus species evolved in the Cape region and only much later did a Gladiolus species reach the Mediterranean and then evolve into the seven species found there.  Thus, Gladiolus in the Cape have had much more time to evolve than Gladiolus in the Mediterranean.  However, they’ve also done it faster- the rate of diversification has been 3-5 times slower in the Mediterranean than in certain parts of the Cape.

Just because Gladiolus appear to have diversified more rapidly in the Cape doesn’t mean that all plants follow this pattern.  In the group of plants called Dianthus (another flower that frequently shows up in people’s gardens), the pattern is just the opposite; Dianthus has evolved faster in the Mediterranean than in the Cape. 

You can find this article at:

ResearchBlogging.orgValente, L., Savolainen, V., Manning, J., Goldblatt, P., & Vargas, P. (2011). Explaining disparities in species richness between Mediterranean floristic regions: a case study in Gladiolus (Iridaceae) Global Ecology and Biogeography, 20 (6), 881-892 DOI: 10.1111/j.1466-8238.2010.00644.x

Sunday, October 16, 2011

Seeing evolution from space

A long long time ago, 25 million years, to be exact, the Andes Mountains in South America began to rise, and as they rose, all of the water in what is now Peru and Ecuador that used to flow northwest had to change directions and head east. This rainwater carried with it lots of dirt, creating a big mud-pie full of cations (i.e. plant nutrients) under a lake on the edge of the ocean.  But, the Andes kept rising and this mud-pie (called the Pebas Formation) drained of water as it was lifted up out of the ocean.  The Amazon River then started to flow east, carrying with it lots of nutrient-poor sand that covered up the Pebas formation.  But, the Andes kept rising, and now, instead of depositing sandy dirt on top of the ground, in the western Amazon the rivers carry dirt away, exposing the older Pebas dirt.  Today, the ground under the Amazon rainforest is like a half-eaten cracker.  In the west, crumby islands of nutrient-poor soil occur on top of the exposed nutrient-rich Pebas layer, but in the east, where erosion has not yet carried it away, the uneaten nutrient-poor soil reigns unmolested.

A human looking down from space upon the Amazon Basin would see a vast expanse of green and would probably wonder why this post just spent the last 200 words talking about dirt when the dominant feature of the Amazon is clearly trees.  The reason is, a special type of satellite imagery called Landsat, can look down from space upon that same vista and say- yup, those are trees, but here’s what kind they are. 

That’s where the geology becomes interesting to a biologist.  An international group of scientists, headed by Mark Higgins, has discovered that Landsat images of the western Amazon Basin show abrupt boundaries between one group of plants and another, and that these boundaries correspond exactly to the underlying geologic border between where the Pebas formation has been exposed and where it hasn’t.  By collecting soil and plant samples from either side of the satellite-predicted boundary, the researchers were able to show that different types of plants grow on the cation-rich soil of the Pebas than grow on the overlying cation-poor soils.

It may not seem like much of a leap to say that geology influences soils, which influence plants.  But, consider the fact that the geology of the Amazon has not remained constant through time.  Higgins and colleagues suggest that when rivers began to erode away the nutrient-poor soil and expose the nutrient-rich soil 5 million years ago, this created a new type of lifestyle for plants to adapt to and may have spurred the evolution of some of the biodiversity that is the signature of the Amazon.  Evidence for this includes the diversification during this time of a groups of plants called Inga that specialize on rich soil, as well as the fact that closely related bird and mammal species from eastern and western parts of the Amazon have only recently become separate species.

You can find this article at:

ResearchBlogging.orgM.A. Higgins, K. Ruokolainen, H. Tuomisto, N. Llerena, G. Cardenas, O. L. Phillips, R. Vasquez, & M. Rasanen (2011). Geological control of floristic composition in Amazonian forests Journal of Biogeography, 38, 2136-2149 DOI: 10.1111/j.1365-2699.2011.02585.x

Sunday, October 9, 2011

Bacteria... in every breath you take

In case you needed a more compelling reason to scoop your dog’s poop than the general good deed of not leaving behind a present for someone’s shoe to discover, consider the fact that what you leave behind on the ground doesn’t always remain on the ground.

One person is likely to breathe in 860,000 bacteria every day1.  Where do these bacteria come from?  What kinds of bacteria are they?  Does where you live affect the kinds of bacteria that are likely to be floating around in the air- and in your lungs?

Scientists from Colorado strained the air for bacteria in Chicago, Cleveland, Detroit, and Mayville, WI over the course of six weeks in the summer and six weeks in the winter in order to answer exactly these kinds of questions.  In 5,000 cubic meters of air (that’s how much a person breathes in about a year and a half) there were 200-300 different kinds of bacteria from at least seven different bacterial phyla.  Side note: Living things that are in different phyla are generally distantly related to each other: for example seas urchins belong to the animal phylum that is most closely related to the animal phylum containing humans- we last shared a common ancestor 600 million years ago.

Even though bacteria in the air were very diverse, the kinds of bacteria found in different cities didn’t vary much.  In fact, there was a much greater change within one city from summer to winter than between two different cities.  In the summer there were generally more bacteria in the air than in the winter, especially in Detroit and Cleveland.  The researchers found that the main sources of airborne bacteria during the summer are likely to be the soil and the surfaces of leaves.  Since leaves aren’t out in the winter and the ground is frozen, this probably decreases the input of bacteria into the air.

So, where do the bacteria found in winter air come from? Dog feces, most likely.  In the winter, but not in the summer, a kind of bacteria called Fusobacteria are very abundant in the air.  These bacteria are commonly found in canine guts, but not in humans or livestock.  Want to avoid the bacteria of fecal origin? Plan your trips to Cleveland or Detroit for the summer.  But to be fair, the origins of aerial bacteria for other cities have yet to be discovered.

1Assuming a constant breathing rate of 12 times per minute, inspiring 0.5 liters each time, with a concentration of 100,000 bacteria per cubic meter of air.

Your can find this article at:

ResearchBlogging.orgBowers, R., Sullivan, A., Costello, E., Collett, J., Knight, R., & Fierer, N. (2011). Sources of Bacteria in Outdoor Air across Cities in the Midwestern United States Applied and Environmental Microbiology, 77 (18), 6350-6356 DOI: 10.1128/AEM.05498-11

Saturday, October 1, 2011

Parasitic worms have center-leaning latitudes

Perhaps the most infamous puzzle in biogeography is the fact that places closer to the equator tend to have more species than places farther from the equator. Why is this true? 212 years have passed since Alexander von Humboldt first noticed this pattern, called the latitudinal diversity gradient, on his voyage to South America and scientists are still not in complete agreement on what causes it.

Not all living thing follow the latitudinal diversity gradient, either. In a paper published in Global Ecology and Biogeography in September, David Thieltges and colleagues discovered that, across Europe, a particular group of parasitic worms called trematodes actually has more species at moderate latitudes than in the south. Of course, since no part of Europe occurs in the tropics, the fact that the scientists didn’t find the traditional latitudinal gradient for trematode diversity in Europe doesn’t preclude its existence for the entire globe.

Trematodes might be bailing off of the latitudinal bandwagon because they aren’t free-living. These parasites must sequentially infiltrate at least two and often three different animals, (such as from a snail, to a fish which is then eaten by a bird) before they can lay thousands of eggs that will start their life-cycle over again. Because they are so dependent on finding hosts, the number of trematode species is more likely to be affected by how many different animal species are available to be parasitized, than aspects of the external environment that vary with latitude.

This is just what Thieltges and colleagues found when they looked at the diversity of animals that trematodes use as a final host (i.e. where they have sex and make eggs). Regions with higher final host diversity had greater trematode diversity. Surprisingly, it is only the variety in final host that matters; snail diversity does not affect trematode diversity, even though trematodes rely on snails to complete the first part of their life cycle.

So, just what is diverse, for trematodes? How many different kinds of worms are we talking, here? Thieltges’ source of information for his research was the book Limnofauna Europeae, compiled in 1978 by Joachim Illes, which catalogs pretty much all of the known animals that live in freshwater lakes, streams, and ponds in Europe. According to this book, the fewest number of trematode species occur in Iceland, at a whopping 9 species. And as for the most? You can find 322 trematodes out on the East European Plains. If this sounds like a lot, just consider the fact that at least 18,000 species of trematodes make the world their home.

You can find this article at:

Thieltges D.W., Hof C., Dehling D.M., Brändle M., Brandl R. & Poulin R. (2011). Host diversity and latitude drive trematode diversity patterns in the European freshwater fauna, Global Ecology and Biogeography, 20 (5) 675-682. DOI:

Saturday, September 24, 2011

Warblers fly an unconventional route into the Pacific

Over twenty-thousand islands dot the 63.8 million square miles of the Pacific Ocean, their land surface area covering only half a percent of this massive expanse of water. Yet, land plants and animals have managed to cross thousands of miles of ocean to reach even some of the most remote of these islands.  How did they manage this?  The simplest explanation is that plants and animals started on the larger continents of Asia and Australia, and gradually worked their way east, hopping sequentially from island to island until they eventually reached the far-flung loners in the eastern Pacific.

A paper in this month’s Journal of Biogeography refutes this “stepping-stone” hypothesis for species of reed-warblers living on islands across the Pacific. Cibois and colleagues analyzed mitochondrial DNA from birds in the reed-warbler genus ranging from Australia to Hawaii and Guam to the Pitcairn Islands. Many of the DNA samples came from the toe pads of dead birds in museums that had been collected up to a century earlier, which allowed the researched to use eight warbler species that had already gone extinct. The scientists compared the DNA sequences of birds on different islands to see how closely related they were; the more similar two sequences of DNA, the more recently that two birds shared a common ancestor and therefore came from the same place. If reed-warblers followed the ‘stepping –stone hypothesis’ and colonized the Pacific Islands by gradually hopping eastward from island to the next closest island, then birds living on Australia and other islands close to the Asian continent should be least closely related to birds living on the islands farthest east.

What emerged from the phylogenetic tree of relatedness that Cibois and colleagues constructed was nothing quite so simple. Among the islands of Micronesia, the amount that birds from two islands were related was not correlated with the distance between the islands. And, instead of a gradual progression of relatedness stretching eastward into the Pacific, they found two waves of colonization departing from the island of Guam. One wave spread far out toHawaiiand from there south to the Eastern Polynesian islands, the other traveled southeast into Micronesia and at some point birds from these islands recolonized the continent of Australia! The two waves met up far out east in the Marquesas islands, where today birds that came originally from Polynesia live on the northern islands of this archipelago, while birds from Micronesia lives on southern islands; these 2.2 million-year-distant cousins are separated by just 41 kilometers of water.

All this flying around probably took place over the course of the last couple million years when ice was expanding and contracting across the face of the globe stirring up ocean currents and changing the direction of prevailing winds, which may have facilitated the transportation of birds to remote islands. Although no fossils of reed-warblers exist on Pacific islands, the researchers were able to construct rough estimates of the times when populations on different islands evolved away from each other by counting the number of changes to the DNA and assuming that 2% of the DNA sequence would change every million years. Contrast this to the colonization of the world by modern humans who evolved in Africa 250,000 years ago, reached Australia ~60,000 years ago, and had spread out into the farthest Pacific islands by 1,700 years ago.

So why didn’t the reed-warblers follow the seemingly easier route of jumping from islands to next closest island? No one knows, but it certainly wasn’t intentional on the birds’ part. The colonization of the Pacific islands by various groups of plants and animals is a sizzling topic in biogeography. As scientists study the colonization patterns of a wider variety of organisms, it will be interesting to see whether animals like the reed-warbler that don’t follow the more conventional stepping-stone hypothesis share similar life-styles or other aspects of their biology.

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Cibois A., Beadell J.S., Graves G.R., Pasquet E., Slikas B., Sonsthagen S.A., Thibault J.C. & Fleischer R.C. (2011). Charting the course of reed-warblers across the Pacific islands, Journal of Biogeography, 38 (10) 1963-1975. DOI: