Saturday, November 3, 2012

Through the belly of the tortoise, passes the wise seed

Hitching a ride with a tortoise is a speedy way to travel- at least for Galapagos plants living rooted to the spot. According to research published in the Journal of Biogeography's special issue on how island plants spread their seeds, forty-five different plant species on Santa Cruz Island may take advantage of Galapagos tortoise mobility and undiscriminating appetite to move their offspring from place to place.

How does one go about discovering such a thing? You've got to be willing to wade through a bunch of crap... literally. In a series of cleverly planned experiments, mostly involving tortoise poo, the researchers were able to piece together which plants were being transported, how far they might be traveling, and how well seeds survived the harrowing journey through a digestive tract without the aid of Ms. Frizzle's magic school bus.

First, the scientists tramped around the island, picking up 120 piles of tortoise feces, which were later carefully dissected to find all of the tiny seeds (sometimes more than 1000!) that might be hiding inside. In order to find out how far these seeds may have traveled, they attached GPS tags to 14 tortoises and recorded how far they wandered over a period of a year. Of course, a seed probably doesn't sit in a tortoises' stomach for a whole year, so the researchers needed a way to calculate how far on average a tortoise moves from the time a seed is eaten until it pops out the other end. By feeding over 15,000 little plastic seed-sized beads to 19 captive tortoises, the researchers were able to monitor, to the hour, how long a seed takes to pass from food to feces (usually about 12 days- quite some time compared to a human's speedy 1-3 days). Combining these digestion times with the GPS records of where the tortoises walked showed that about half of seeds eaten by a tortoise are likely to travel more than 1/2 a kilometer.

Finally, the scientists compared whether seeds that had been eaten by tortoises were more or less likely to sprout than seeds taken directly from their fruits. Some plants with especially hard seeds might actually sprout better after being digested because the acidity and abrasion in the stomach wear away outer protective layers that prevent a seed from starting to grow. Not to mention that seeds ending up in a mound of dung are surrounded by lots of fresh fertilizer. But, for Galapagos plants at least, tortoise digestion doesn't seem to help or hinder a seed's ability to sprout, nor does landing in a warm pile of poo.

Studying Galapagos tortoise feces may seem esoteric, to put it kindly, but life on earth might look quite different if it weren't for animals spreading seeds from the fruits they eat. This is especially true for islands, whose stationary flora required an improbable leap across miles of ocean.

You can find this article at:

ResearchBlogging.orgStephen Blake, Martin Wikelski, Fredy Canbrera, Anne Guezou, Miriam Silva, E. Sadeghayobi, Charles B. Yackulic, & Patricia Jaramillo (2012). Seed dispersal by Galapagos tortoises Journal of Biogeography, 39, 1961-1972 DOI: 10.1111/j.1365-2699.2011.02672.x

Thursday, October 11, 2012

To Lilliput and Brobdingnag, Plantae style

Islands are quirky. They don’t always play by the same rules that continents do. And even though continents are just really big islands, their large size means that evolution managed to fit many forms of life onto them, all eating, parasitizing, and competing with each other. Not so on islands. Only a “lucky” few species cross the waters to reach those isolated bits of land.  And when they get there, there are fewer predators, fewer competitors, fewer parasites, and ironically, more ecological space available.

This extra space and safe-haven from enemies has led biogeographers to hypothesize several patterns for how animals and plants on islands differ from their close relatives on the mainland. In a recent issue of Global Ecology and Biogeography, three scientists in New Zealand test one such pattern, somewhat arrogantly named, ‘The Island Rule’. The island rule posits that when small mammals colonize islands, they evolve to become larger because they don’t have to hide from predators, while large colonists evolve to become smaller because they don’t have as much food available. The generality of this “rule” has been debunked for mammals- mice and rats get larger (of course they do), artiodactyls (such as deer) get smaller, but this has more to do with  what type of animal they are than with how large they began.

However, the researchers wondered whether the island rule might occur in plants, since other scientists had previously discovered that some weeds colonizing islands evolved into larger woody plants, whereas other plants get smaller. Using plants growing in a botanical garden at the southern tip of New Zealand, they compared the sizes of leafs, stems, and seeds of plant species that were native to small islands that surround New Zealand with the sizes of leaves, stems, and seeds of the closest relatives of these species that were native to the large main islands of New Zealand. Presumably, plants on the smaller islands evolved from ancestors that had come from the mainland of New Zealand, so by comparing the size of pairs of relatives they could see whether species from the smaller islands had changed in size.

Their findings? The island rule really isn’t much of a rule- it’s more like an option in a Choose Your Own Adventure novel. Most of the plants just got larger, regardless of how big they started out. But, even though both leaves and stems evolved to be larger on the islands, the scientists believe that it is pressures on the leaves that drive these changes- the stems only get bigger because they have to support larger leaves. What might make leaves larger on islands? Fewer animals eating them, for one. Small leaves might be one way that plants avoid rapacious salad-eaters.

Based on this, we might be inclined to propose the “half-island rule” for plants in which plants evolve to become larger on islands. Insert big yellow triangular caution sign here. First- this is just New Zealand, and everyone knows New Zealand is weird and populated by a tribe of small folk with big feet. Second- as the island rule (among many others) proves, “rules” in biogeography are great for generating controversy, but don’t have such a great track record for holding up to careful scrutiny. The patterns from which such rules are proposed are easy to demonstrate or refute. The causes for these patterns are conspicuously more elusive, but ultimately more important. 

You can find this article at:

Kevin C. Burns, Nadine Herold, & Ben Wallace (2012). Evolutionary size changes in plants of the south-west Pacific Global Ecology and Biogeography (21), 819-828 DOI: 10.1111/j.1466-8238.2011.00730.x

Sunday, August 5, 2012

Great tits make warmer nests up north

A well-known and highly-contested “rule” among biogeographers is Bergmann’s rule, which basically says that animals in the north have larger bodies than animals in the south because it allows them to stay warmer. Sometimes, this is true.  Sometimes, in order to stay warm, animals behave differently too. 

UK researchers have discovered that great tits and blue tits living in the northern parts of the Isle build warmer nests than those in the south by adding extra feathers, grass or animal hair to the inner cup that holds the eggs. Why study tits? They seem to actually like nesting in wooden boxes that humans make especially for the purpose of studying bird breeding. This makes it easy to borrow their nests when the birds are done with them and test how well nests would insulate their potential residents. The scientists found that the extra weight that birds add in the north was enough to keep up with the gradual decline in spring temperatures from southern sites to more northerly ones.

The sites where the researchers tested nests spanned only 5 degrees of latitude, which corresponds to a change of just 1.5°C in average spring temperature when the birds are laying eggs. It seems incredible that this slight variation has even elicited any reaction by the tits. Clearly, 1.5°C to a bird is not the same as 1.5°C to a human.  If 1.5°C didn’t affect the survival of eggs and chicks there would be no pressure for mother tits to change their nests.  But, because they do, we know that it matters and we also know that additional increases in temperature that result from global climate change will matter too.

Just as baby birds can get too cold, they can also get too hot. To think of it a different way; perhaps birds in the north are not building warmer nests, but it is the birds in the south that are building cooler ones.  A two-year-old knows that there are only so many clothes you can take off to cool off; nests can only get so light before not much is left to hold the eggs. How hot before nests get naked?  2°? 3°? Over the next hundred years, we may find out. Or, we may not- Great tits living as far south as north Africa seem to have found a way to keep their eggs cool enough. Extinction by overheating seems unlikely, but more changes to how the tits build nests, a certainty.

You can find this article at:

ResearchBlogging.orgM. C. Mainwaring, I. R. Hartley, S. Bearhop, K. Brulez, C. R. du Feu, G. Murphy, K. E. Plummer, S. I. Webber, S. J. Reynolds, & D. C. Deeming (2012). Latitudinal variation in blue tit and great tit nest characteristics indicates environmental adjustment Journal of Biogeography DOI: 10.1111/j.1365-2699.2012.02724.x

Tuesday, July 24, 2012

It takes two to pollinate

Networking is not just a thing that computers and college counselors are into; birds, bees and trees do it too.  Pollination networking, that is. The pollination of all the world’s flowers is not the sole job of the European honey bee; many other insects, birds, and even little furry mammals carry pollen from flower to flower. But, not every pollinator can visit every type of flower. A hummingbird’s long bill is just not ideal for sucking nectar out of a pansy and you won’t see a bumble bee fitting his bumble into a heliconia. Some plant pollinator partners get so close that they don’t share the job with anyone else, like the Yucca moth or the fig wasp.

If you go out to a field (or forest, or mountaintop) and sit for many many hours watching which pollinators visit which species of plants then you can draw what is called a pollination network. This is just a set of dots, one for each plant and pollinator species, with lines connecting the plants and pollinators that do the deed.

Dots and lines are pretty simple, but with a little bit of math, can tell a detailed story about how plants and animals interact with each other.  Want to know whether each pollinator tends to specialize on a particular plant? Calculate the average number of pollinators that are linked up with a single plant. If it is low, then pollinators are specialists, and if it is high they are generalists, visiting any kind of plant. Another calculation, “modularity”, tells whether specific groups of pollinators tend to visit specific groups of plants, with little socialization across cliques. This might happen if similar pollinators are better suited to pollinating similar flowers.

These calculations, called ‘network properties’ can be used to compare pollination networks from different places, even if the species that live in these places are completely different. And finally, all this biology culminates in a fascinating geographical question:  Do plants and pollinators interact in similar ways around the globe, or is there a pattern in the way theses interactions vary?

Two researchers at Aarhus University in Denmark compiled a database of more than fifty pollination networks from all around the world to find out how interactions between plants and pollinators change from the tropics to the temperates, from mountain tops to mountain bottoms, on islands versus mainlands, and among hot, cold, wet and dry places.

In the tropics, plants and pollinators tended to specialize more than at temperate latitudes. Circular reasoning would argue that the specialization between pollinators and plants has helped generate the high diversity seen in the tropics and that this high diversity has forced pollinators and plants to become more specialized to avoid competing with their neighbors. We can leave it to the chicken to sort it out.

Another interesting pattern is that, once above 500 meters in altitude as you go up there tend to be fewer pollinators for each plant and pollinators become more generalists, visiting many different plants. This could be because it is colder at higher altitudes and fewer animals (especially cold-blooded insects) are available to act as pollinators. Also, the higher elevations tend to have more variable environments, so that a pollinator that depends on one type of flower for food may find itself out of luck in a year that that plant produces few flowers.

Networks don’t just happen between plants and pollinators- they occur among  all living things that interact, such as predators and prey or diseases and hosts.  Ecologists have used these tools for many years to understand how particular ecosystems work. This research shows how biogeographers can take the reins and learn how ecosystems compare. 

You can find this article at:

ResearchBlogging.orgKristian Trojelsgaard, & Jens M. Olesen (2012). Macroecology of pollination networks Global Ecology and Biogeography : 10.1111/j.1466-8238.2012.00777.x

Monday, April 30, 2012

Essence of a term paper (deconstructed).

In honor of final exams week.  

Disclaimer: Hypotheses in this essay are mostly conjecture and not vetted by the scientific community. Please use the original sources (in parenthetical citations) in your own research and contact the blog author with further question.

On a recent visit to the North Carolina Aquarium’s nature trail at Pine Knoll Shores on the outer banks, I fell under the thrall of an astonishingly vibrant lichen and after snapping an embarrassing number of pictures immediately went in search of the name of this beauty. It did not take much research to learn that this lichen was Pyrenula cruenta, since, as Harris (1987)puts it, “Homo lichenologicus, like much of the animal kingdom, is attracted to bright colors.” However, despite the ease with which I discovered its identity, there was not much more to be found about the habits and relations of this rather conspicuous lichen.

A lichen is a fungus that grows symbiotically with an algae or cyanobacteria (see previous post), but most people know of them as “that grey thing growing on that tree over there… wait, maybe that’s moss”. Those who paid attention in high school biology might remember the crucial role lichens played in the evolution of dark-colored moths in Europe during the industrial revolution (link to Heidi’s Blog). But, for the most part, lichens are not exactly well known- even the red ones.

Pyrenula cruenta and Pyrenula cruentata are bright red because they produce anthraquinone pigments (Brodo et al. 2001). Although these are the only two red Pyrenula lichens, lichens in this group often go by such appealing names as ‘rash lichens’ or ‘pox lichens’ because they grow inside the top layer of a tree’s bark, causing it to be slightly or vividly discolored with brown or black pimples.

Aptroot (2012) has recently published a worldwide key that can be used to identify Pyrenula lichens- all you need is a microscope, a few chemicals, and years of training as a lichenologist. After sorting through 745 potential names, he pared the group down to 169 species, most of which are found in the tropics. There are however, a handful of Pyrenula lichens that seem to avoid tropical areas; for example, Pyrenula pseudobufonia lives in eastern North America and East Asia and Pyrenula nitida lives in Europe. My question was, why are most Pyrenula lichens spread across the tropics, while some are only found in temperate areas? I didn’t find an answer, but I did find a hypothesis.

The clues to a hypothesis come from who’s related to whom and where these relatives live. Unfortunately, no one has worked out the Pyrenula family tree using genetic data, but those scientists who have studied lichens intensively are able to propose which species may be closely related based on physical characteristics. For example, Harris (1989) believes that Pyrenula cruenta and Pyrenula cruetata are each other’s closest relative. Not only are they both red, but the only other character that differentiates them is how the cells in their spores are arranged (it is tempting to conjecture that this trait is under simple genetic control, i.e. easily mutated). Furthermore, Pyrenula cruentata only lives on the southern tip of Florida and in the northwest Caribbean, a small subset of Pyrenula cruenta’s range, meaning that it may have recently evolved from a Pyrenula cruenta living in this area.

But, back to the main question- where did the temperate Pyrenula lichens come from? Both common and rare species of Pyrenula tend to be widely distributed across the tropics (Aptroot 2012) and in North America, the number of Pyrenula species is highest in south Florida and declines northward (Harris 1989). This suggests that Pyrenula originated in the tropics, perhaps around the Tethys Ocean, which wrapped around the equator during the Cretaceous, as has been suggested for other lichens distributed across the tropics (Galloway 1991). However, Harris (1989) has hypothesized that the Pyrenulaceae family of lichens, of which the Pyrenula are members, first evolved in the northern hemisphere because some of the oldest members of this family occur in the north, rather than the tropics.   

So, here’s the hypothesis: Pyrenula lichens first evolved and diversified during the Cretaceous when the world was tropical and the extensive coastlines around the Tethys Ocean created widely dispersed maritime environments that these lichens seem to prefer. When the climate later cooled, only a few of these lichens were able to adapt and remain in the northern temperate regions.

One way to test this hypothesis would be to use genetic data to construct a phylogenetic tree of who’s more closely related to whom. A tropical origin for Pyrenula would be supported if the Pyrenula lichens that live in temperate areas are younger species that have recently shared a common ancestor with a tropical species.

The relatives of Pyrenula psuedobufonia provide some evidence that this hypothesis might be true. Harris (1987) believes that Pyrneula pseudobufonia is closely related to Pyrenula fetivica, Pyrenula nitidula[1], and Pyrenula cocoes, all of which have pantropical distributions (i.e. live all across the tropics). The existence of many other tropical Pyrenula lichens that are more distantly related means that this group probably has a tropical origin and that Pyrenula pseudobufonia has recently evolved to be able to live in the temperate zone. Pyrenula pseudobufonia’s closest relative is probably Pyrenula occidentalis which is found in the Pacific Northwest of the United States. But, these two species may have diverged from each other fairly recently since the only difference between them (other than where they live) is that Pyrenula occidentalis no longer produces lichexanthone, a chemical that fluoresces yellow when UV light is shined on it (in fact, this is how lichenologists tell the two species apart).

The movement of ancestral species from the tropics to the temperate zone and vice versa is a topic that biogeographers and evolutionary biologists have been getting pretty excited about in recent years. Many scientists believe that the explanation for why some parts of the world have many species whereas others have few is substantially dependent on where lineages of species originated and how they subsequently dispersed. A large tropical group like Pyrenula that includes several temperate species can be an intriguing system in which to study such patterns and how they contribute to the geography of diversity.

Works Cited

Aptroot, A. 2012. A world key to the species of Anthracothecium and Pyrenula. The Lichenologist 44:5–53. doi: 10.1017/S0024282911000624.

Brodo, M. I. M., M. S. D. Sharnoff, and S. Sharnoff. 2001. Lichens of North America1st Printing. Yale University Press.

Galloway, D. J. 1991. Biogeographical relationships of Pacific tropical lichen floras. in D. J. Galloway, editor. Tropical Lichens: their Systematics, Conservation, and Ecology. Clarendon Press, Oxford.

Harris, R. C. 1987. Some distinctive tropical pyrenolichens in the southeastern United States. Evansia 4:28–30.

Harris, R. C. 1989. A sketch of the family Pyrenulaceae (Melanommatales) in eastern North America. Memoirs of the New York Botanical Garden 49:74–107.

[1] Harris (1987) used the names Pyrenula citriformis and Pyrenula plittii, which Aptroot (2012) synonymized with Pyrenula fetivica and Pyrenula nitidula.

Friday, February 24, 2012

Invasion of the Gondwanan Galaxiids

Despite indications to the contrary, this post is not about googly-eyed spaghetti people from beyond our galaxy.  It’s actually about fish, eventually.

Many species live on continents separated by thousands of miles of ocean.  Since one species cannot evolve into being in two separate places at the same time, it falls to biogeographers (not that they mind) to explain where the species originally came from and how it got to be where it lives today.  For example, humans live on practically every parcel of land on Earth and it is now well-accepted that we originated in Africa 200,000 years ago and have since spread across land and sea to pretty much everywhere.

But, what about species that are not so ubiquitous and lack our ingenuity to build boats, coats, and airplanes? If a group of animals is old enough, there may have been no need to find transport across an ocean to explain a modern disjoint distribution because 200 million years ago the continents were all connected! The ancestors of today’s species could have ridden the continents as they moved apart from one another- gradually separately evolving new groups of species.

This is what biogeographers had long thought happened with the Galaxiid fish- a group of unremarkable-looking fish that live in the cold lakes and streams of southern Australia, South America, South Africa, and New Zealand.  As freshwater fish, most species of Galaxiids cannot survive in the ocean because of its high salt content, so it was a bit of a mystery how these closely related fish came to live on such widely separated continents. However, about 180 million years ago, Australia, New Zealand, South America, Africa and Antarctica used to be united in a big southern continent called ‘Gondwana’.  If the ancestor of the Galaxiids lived on Gondwana before it broke apart, this could explain their wide distribution.

In the most recent issue of the Journal of Biogeography, a group of researchers from Australia and New Zealand investigated whether the dates when the continents broke apart matches up with the dates when Galaxiid species lichen on separate continents last shared an ancestor.  Surprisingly they found several cases where species living on Australia and New Zealand had last shared an ancestor as recently as 5-24 million years ago.  Australia and New Zealand were last connected about 80 million years ago, so this means that the species must have crossed the ocean. Even more amazing is Galaxia maculatus, a single species that live in New Zealand, Tasmania, and South America.

The key to the mystery is the fact that a few of the modern Galaxiids, including Galaxia maculatus, have a special ability called ‘diadromy’, where they are able to live in both freshwater and saltwater (like salmon).  The researchers proposed that the ancestors of some of these fish that now live on separate continents were diadromous allowing them to actually swim between the continents.  Once they got there, most eventually lost the ability to live in the sea.  As for Galaxias maculatus, it probably got swirled around Antarctica from Australia to South America by the WestWind Drift.  Known less romantically as the Antarctic Circumpolar Current, this westward flow of water circles Antarctica and is implicated in the dispersal routes of several other plants and animals.

Not all ancestral Galaxids had to swim to get to their current homes. The research revealed that some of the ancestors of the modern Galaxiids may have ridden the continents as Gondwana was breaking up.  But, contrary to previous belief, these were probably diadromous species that had been able to reach wide distributions across the continent due to their ability to swim across shallow seas.  Biogeographers had thought that species restricted to freshwater (i.e. not diadromous) would be the ones more likely to have arisen from the Gondwanan break-up, since they are less able to disperse.

Although the title of this post suggests otherwise, Galaxids are not, in fact ‘invaders’-  their dispersal to their current homes millions of years ago makes them integral parts of native ecosystems in the southern hemisphere.  The actual “invaders” are a northern group- the salmonids (including the diadromous salmon), which have been introduced by people into many southern hemisphere waterways because they are tasty and fun to catch. Land-locked life no longer need rely on the long-term movement of continents to get around; humans are a new and important route by which animals and plants can travel the world. 

You can find this paper at:

ResearchBlogging.orgBurridge, C., McDowall, R., Craw, D., Wilson, M., & Waters, J. (2012). Marine dispersal as a pre-requisite for Gondwanan vicariance among elements of the galaxiid fish fauna Journal of Biogeography, 39 (2), 306-321 DOI: 10.1111/j.1365-2699.2011.02600.x

Monday, February 6, 2012

Leaf World Tour

Sometimes species are boring- how much does Picea mariana really say about a scraggly looking tree with sharp needles?  Sometimes it can be more interesting to describe plants by their characteristics.  If you are an ecologist- this approach is called ‘functional ecology’, and if you are a biogeographer, well... there isn't a name for it yet, but that hasn't stopped researchers from asking whether there are systematic ways that the traits of plants vary across the globe.
A lot of research has focused on two plant traits called leaf nitrogen content and specific leaf area (SLA).  The first is pretty much what it sounds like, the percentage of a leaf’s dry weight that is from nitrogen.  In leaves, nitrogen is mainly used for proteins involved in photosynthesis, so leaves with higher nitrogen content generally can make sugars more quickly.  Within a single plant, the leaves that get the most light usually have more nitrogen because that’s where chloroplasts are most useful. 

Specific leaf area is the ratio of a leaf’s area to how much it weighs when dried.  Basically, it gives an estimate of how much effort a plant puts into making leaves- plants with thick hard leaves (like a rhododendron) have high SLA and consequently hold onto them for longer since they have invested so much raw material in them.  Alternatively, birch trees have light thin leaves with low SLA and shed them each winter. 

These two traits vary between different species, but they also vary between plants growing in different environments because the strategy that a plant uses to divvy up its limited carbon and nitrogen will depend on how much water it has access to, how long temperatures are warm enough for it to grow, and whether it will be better off growing quickly or growing more slowly. The question is, is there more variation in SLA and leaf nitrogen amongst the plant species that live together in an alpine meadow or between plants living in the meadow and those living in a temperate forest?

Because these traits are easy to measure and lots of plants ecologists have done so, this question is actually not that hard to answer- provided scientists are willing to share their data.  This was the goal of Gregoire Freschet and seventeen other scientists when they pooled their data from 58 sites distributed among nine different biomes around the world.  What they found surprised me- about half of the variation in SLA and leaf nitrogen found among all the species from around the world could be found within a single site.  But, the average levels of these traits also differed between the different biomes that they examined; wet temperate forests in New Zealand had the lowest average leaf nitrogen and SLA (those trees just don’t put much into their leaves!) while the alpine temperate ecosystems in the Caucasas had the highest. 

What I found most interesting about this paper was how the authors explained their results.  Rather than try to squish all of their observations into one cohesive theory, they acknowledged that the reasons for particular levels of SLA and leaf nitrogen and the variability of these traits within communities likely differed between biomes and then proceeded to talk about each of them in turn.  In the last decade, numerous groups of researchers have taken the “let’s all share and discover something global” approach.  But in the excitement of forging unified global patterns, it is easy to gloss over what can be learned from how places differ.  These authors do an admirable job of balancing commonalities with idiosyncrasy, showing that collaborative science truly has the ability to turn one person’s ecology into global biogeography.

You can find this paper at:

ResearchBlogging.orgFreschet, G., Dias, A., Ackerly, D., Aerts, R., van Bodegom, P., Cornwell, W., Dong, M., Kurokawa, H., Liu, G., Onipchenko, V., OrdoƱez, J., Peltzer, D., Richardson, S., Shidakov, I., Soudzilovskaia, N., Tao, J., & Cornelissen, J. (2011). Global to community scale differences in the prevalence of convergent over divergent leaf trait distributions in plant assemblages Global Ecology and Biogeography, 20 (5), 755-765 DOI: 10.1111/j.1466-8238.2011.00651.x

Monday, January 16, 2012

How to count ants with plants

For many of us, an ant colony is something that grows between two glass plates when you “just add ants” to a little bit of dirt.  Glass and little red plastic frames being in short supply out in nature, wild ants have to make do with other materials to build their homes. Some ants do house their colonies in dirt, but many others can make a whole colony in a rolled-up leaf or inside an acorn.  These items being abundant in nature, the question arises, “what determines how many ant colonies are out there?”  Certainly not every acorn or leaf could have an ants cozied up inside.

This is the old ‘abundance’ question in ecology, which is usually answered in the following way: the number of animals living in an area is a balance between how many are born and how many die.  But this isn’t very satisfactory- we want to know the ultimate cause, not the proximate mechanism.  A more satisfactory answer may come from energy theory.

One of the first things we learn in our elementary science classes (other than that ants live between glass plates) is that all animals need food to survive.  Although animals each eat different things, the light energy captured by plants was the original source of the energy in their food (except for some crazy creatures that live around hydrothermal vents deep under the ocean).  A simplistic answer to the abundance question is that the number of animals living in an area is determined by how much energy plants make available to them.  Simple answers rarely capture the all the details that make ecosystems interesting to ecologists, but sometimes they are insightfully accurate for biogeographers who study the whole planet.

Michael Kaspari and Michael Weiser recently published research showing that, from Colorado’s alpine tundra to the Peruvian tropical forest, the overall number of ant colonies in a particular place is strongly related to the amount that plants grow (NPP, net primary productivity) relative to the average energy needs of a single colony.  Although the ants were studied 15 years ago, they combined this older data with recent experiments on ant metabolism (how they burn energy) to estimate how much energy a colony needs based on how large the worker ants are.  These energetic requirements were then used to figure out how many colonies the total energy available from plants could support (total plant energy / energy needed for one colony).

What’s particularly cool is that on a species-by-species basis, plant production didn’t do a great job of predicting how many colonies would be found for a particular species; some species had very few colonies even in place with lots of plant production, like tropical forests.  Even the most abundant ant species at each location sometimes had fewer colonies than amount of energy available to them would predict.  It was only at the level of the whole community, when colonies from all ant species were counted together, that the energetic calculations did a good job of predicting the number of colonies. 

The authors think that this could be because each ant species has a specific diet that is different from most other ant species and is only consuming a small fraction of all the energy available out there in the ecosystem.  But, collectively ants eat a wide variety of things (spider eggs, farmed fungus, etc…) and together consume a larger fraction of what’s out there.  This means that the whole community is more limited by the energy available than any one species.

Another fascinating pattern also emerged from this research- even though the number of ant colonies increased in places with more plant production, the total biomass (how much all the ants weigh) did not change.  This seems paradoxical at first since more ants should equal more weight; ten people usually weigh more than one person. But, ants vary in size to a much greater extent than humans do.  The world smallest ant could set up a colony inside the head of the world’s largest ant (antArk). What’s more, ants are generally smaller in tropics where there is greater plant production.  So, colonies become more abundant, but ant’s get smaller, leading to overall ant biomass staying about the same from desert shrublands to highly productive tropical forests.  Sometimes, in biogeography, why things don’t change from place to place is far more interesting than why they do.

You can find this article at:

ResearchBlogging.orgKaspari, M., & Weiser, M. (2012). Energy, taxonomic aggregation, and the geography of ant abundance Ecography, 35 (1), 65-72 DOI: 10.1111/j.1600-0587.2011.06971.x

Sunday, January 8, 2012

Dinos were diverse, too

God creates dinosaurs.
God destroys dinosaurs. 
God creates man. 
Man destroys God. 
Man creates dinosaurs. 
Dinosaurs eat man … 
...woman inherits the earth.*

I never really did think of dinosaurs as actual living creatures.  Maybe it was their starring role in works of obvious fiction*, or perhaps an intervening 65 million years of dino-free history that just did not lend them the same reality as, say, an elephant.  But then I read a new research paper that looked at some of the same patterns for dinosaurs that I, as an ecologist, study in modern-day plants and animals.  And it clicked; dinosaurs are animals too, with their own ecology and hotspots of diversity.

The enlightening paper was by Phillip Mannion and group of European scientists who wanted to know whether dinosaurs had similar patterns of diversity to present-day animals.  Many groups of plants and animals, including dinosaurs’ contemporary descendents, the birds, reach their greatest diversity in the tropics and have fewer species farther away from the equator.  This pattern, called the latitudinal diversity gradient, is so pervasive that it is almost considered a ‘rule’ in biogeography- groups that don’t follow the pattern (see previous post) are regarded as interesting, but anomalous.  The question is, did the dinosaurs that lived a hundred million years ago follow the modern latitudinal diversity rule?

The short answer, that Mannion and colleagues discovered after analyzing a massive database of locations of fossilized dino remains, is ‘no’, they didn’t-  dinosaur diversity peaked between 30-60° latitude in both the northern and southern hemisphere, but was generally lower in the tropics.  This actually isn’t that surprising, given that the earth the dinosaurs inhabited looked nothing like it does today. 

The modern latitudinal peak in diversity around the equator is thought to be caused by a mixture of climate and evolutionary history.  Groups of organisms appear to diversify more rapidly in the tropics, and because of the less stressful climate and lack of glaciers repeatedly plowing across the land (as has happened in the temperate zone for the past 2.5 million years), species are also less likely to go extinct.

The earth the dinosaurs inhabited for 160 million years probably did not have as strong of a change in temperature from the equator to the poles, nor did it have glaciers, or even polar ice caps.  For this reason, the latitudinal climate gradient probably did not influence dinosaur evolution to the same extent that it affected organisms living in the more recent past.  What may have played a stronger role, the authors of this paper hypothesized, was the distribution of land on the Earth’s surface.  The land that the dinosaurs lived on was divided into two large landmasses on either side of the equator- Laurasia and Gondwana.  The reason that there was lower diversity in the tropics may have been because there wasn’t much land there; it is a well-established ecological rule that larger areas have more species.

Regardless of the true reason for higher dinosaur diversity at higher latitudes, its deviation from the modern pattern is a reminder of something invisibly obvious.  The earth now is not how it used to be, nor how it will exist in the distant future.  Which ‘rules’ are based on assumptions of Earth’s current geography and which will remain true throughout time?

*Jurrasic Park. dir. Steven Spielberg. 1993.

You can find this paper at:

ResearchBlogging.orgMannion, P., Benson, R., Upchurch, P., Butler, R., Carrano, M., & Barrett, P. (2012). A temperate palaeodiversity peak in Mesozoic dinosaurs and evidence for Late Cretaceous geographical partitioning Global Ecology and Biogeography DOI: 10.1111/j.1466-8238.2011.00735.x