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Climate Change 

October 30, 2009

New paper: Ecological modeling of paleocommunity food webs


Roopnarine, P. D. 2009. Ecological modeling of paleocommunity food webs. in G. Dietl and K. Flessa, eds., Conservation Paleobiology, The Paleontological Society Papers, 15: 195-220.

Just published, this paper discusses some of our network work in detail. You download it here or here.

Filed under: Climate Change — Peter @ 8:15 pm

October 27, 2009

The Six Degrees of Responsible Science IV


You are looking at the food web of a terrestrial community from South Africa, some 250 million years ago. This ecosystem was the culmination of more than 50 million years of coevolution among rapidly diversifying groups of forest plants, insects, reptiles, and our proto-mammalian ancestors. This is a very informative way to depict the community, showing the predator-prey relationships among the organisms within the ecosystem. The world at this point in time is perhaps well on its way to our modern world: globally on the cool side and “mammal”-dominated. But it all comes to a rather abrupt end when Siberia opens up. Sometime between 251 and 250 million years ago, a gigantic plume of magma began to erupt from deep within the Earth, reaching the surface in the region roughly equivalent to Siberia in today’s world. Lava flowed onto the surface for thousands of years, eventually reaching a volume almost equal to the entire continent of Europe! The physical and chemical changes on the planet were extreme and profound, as were the extinctions. More than 90% of all species known in the oceans, and more than 70% of animals known from land as fossils, became extinct. It is the largest extinction recorded in the fossil record.

My co-workers and I have modeled the way in which these geophysical changes would have affected our South African community, by constructing detailed, dynamic food web networks. I should point out here that a lot of this work has been conducted by graduate and undergraduate students in my lab, including two interns funded by NSF’s REU Summer Systematics Institute here at the California Academy of Sciences. Students have been involved in every aspect of the work, formulating theory, collecting basic systematic and ecological data, conducting simulations and analyses, and even writing computer code! When we applied our model to the South African community, we discovered a couple of interesting things:

    Extinctions resulting from an increasing shutdown of photosynthesis
  1. One way to examine cascading changes through a network is to simply perturb the network, and observe the chain reaction in a house-of-cards fashion. This is the manner in which currently popular analyses of network robustness have been conducted, such as robustness of the internet or airline transportation systems, in addition to food webs. When we do this with our South African network, the result is a reasonably linear and predictable response, and the implication is that the extinctions could only be explained by an almost complete loss of primary production (plant food). This does not agree with the geological record, so we don’t trust the result. And there is another very good reason to not trust this result, and it’s why we cannot always use such a simplistic approach when thinking about biological, including human, networks. This model assumes that the interactors in the network are unintelligent, that is, they do not respond or adapt to the changes taking place around them.
  2. Notice the difference when we make the ecology "real"

  3. We addressed this by using very simple ecological interactions among the species in our community. The interactions are based on straightforward ecological common sense. The result is strikingly different when we assume that our species would respond ecologically to the other species around them. The result has a tipping point!

We’ll delve into this very interesting result in the next post.

Filed under: Climate Change — Peter @ 9:31 pm

October 13, 2009

Yes, it’s getting cooler…


Good, did that get your attention? I’m interrupting my “Six Degrees” series of posts to address a topic that has been in the news recently. That topic is the supposedly new observation or conclusion that the planet is cooling, in opposition to the warming trend. For example, the BBC ran this piece entitled “What happened to global warming?”. The answer, in a nutshell, is NOTHING. The article has stirred up quite a bit of argument and discussion between proponents and skeptics of global warming. So what’s all the fuss about? I’ll try to explain. But first, let me be blunt: the natural world does not give a fig about our opinions, whether we support (“believe in”) or deny (see here) human-caused global warming.

Okay, so what is this about? The Earth’s climate is a dynamic system, meaning that it evolves over time, and at any given time may be considered to be in a particular state. One of the major drivers or controllers of climate is the world ocean, primarily because the ocean is the planet’s major heat sink. This is the result of water’s high heat capacity. To understand this, think about a pot of water boiling on your stove. You watch it, and it just sits there. Now suppose one stuck one’s hand into the flame (Readers: Do not try this at home). Gee, your hand heated up pretty quickly! Not so with the water. But eventually, the water absorbs enough heat energy to begin it’s transition from the liquid state to a vapour state; that is, it boils. It takes a lot of energy to change the temperature of water. That’s just one wonderful property due to water’s fantastic molecular structure. The world ocean covers 70% of the planet’s surface, and it’s movement is the global movement of heat.

The oceans are dynamic; they flow, they mix, they churn. One could say that our understanding of long-term ocean dynamics is still very incomplete, but we do know that there is long-term variability in the states of the world ocean, and hence in climate. Perhaps the most familiar manifestation of this is the El Niño-La Niña quasi-cycle. Another is the less familiar Pacific Decadal Oscillation, or PDO. These phenomena represent natural variability in the ocean’s circulation. The variation translates into climate variability. We’ve seen these cycles operate through the 20th century. The century began a bit on the warm side, but cooled globally, on average, from the 1940′s through 1970′s. The variation is caused by the ocean sloshing back and forth. Now the sloshing is causing cooling once more.

THIS IS UNRELATED TO THE TREND OF HUMAN-DRIVEN GLOBAL WARMING. Let me say this again: This is not a reversal, nor is it refutation of anthropogenic global warming. How do I know this? Science folks. The media really should speak with the scientists doing the research, rather than sensationalizing and immediately seeking the nonsensical sound bytes of dwellers on the fringe. The cooling has been noted for some time (I blogged about it almost a year ago!), and we know that it is being driven by the PDO. Now here’s the cool part. We can tease apart this natural variability and other influences, so-called external forcings. This means that we can look at global temperature, and ask how much of it is due to the ocean driver, and how much to external factors. Should I tell you what the answer is? Can you guess? IT’S STILL WARMING. Overlain on the natural variability of our climate is a clear upward trend through the 20th century.

Don’t believe me? Show your friendly neighbourhood scientists a little love and read one of the original studies here. ‘Nuf said.

Filed under: Climate Change — Peter @ 6:28 pm

October 6, 2009

The Six Degrees of Responsible Science: III

In the previous post I promised to explain the lengthy answer to the Kevin Bacon riddle posed in the first post. Here’s a start…

Let’s take a short trip through food webs, ecosystem collapses, and the bottom-up approach to making new scientists. We’ll begin by looking at tropical coral reefs, the most species-rich ecosystems in modern oceans. Coral reef communities are important reservoirs of biodiversity, and they are of extreme economic importance to many human communities because of tourism and fisheries. They are also one of the most critically endangered ocean systems, and in the last couple of decades we have witnessed the rather sudden transition of many of these systems from diverse, coral-dominated systems, to species-poor, algae-dominated systems. These transitions have been attributed to a number of factors, most notably overfishing of high trophic level species, pollution and pollution-driven diseases, and most recently, ocean warming and acidification because of global warming. But as many scientists have documented, reefs as we know them today are mere shadows of what they were hundreds, and perhaps even thousands of years ago. The footprint of human impact is really a long highway. There is no doubt that transitions are occurring today though, because we are witnessing them. Why now?


Take a look at the heuristic image. This is known mathematically as a catastrophe manifold, and it’s a very useful way to visualize what’s happened recently to some Caribbean reefs. Those reefs, which historically have withstood years of battering hurricanes, are now vulnerable to severe storm damage. The major reason is that the removal of large herbivorous fish, grazers of algae, allows large algae to take over large areas of the reef after a storm has passed through. The algae reproduce and spread much more rapidly than do the coral. Under normal circumstances, suppression of the algae by the fish allow the coral to thrive and construct the important reef habitat. The important take home messages for us are:

  1. 1. The transitions from coral to algae dominance are the result of cumulative impacts, and though the state of the system appears to be stable, an erosion of its internal stability is taking place.
  2. 2. When a transition occurs, it can be quite sudden; it’s a surprise! And indeed, these transitions tend to occur within a few years.

Now the precise manner in which this all happens is of great interest to answering the riddle of Mr. Bacon and science education. It all has to do with networks of interaction and relationships, and the ways in which you can spread, or cascade, through those networks. I’ll illustrate this in the next post by yanking you back into the past, way back (250+ million years), to one of Earth’s most interesting times, the Late Permian.

In the meantime, take a look at some of my recent work on coral reefs. You can also read a rant about networks here.

Filed under: Climate Change — Peter @ 6:53 am

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