Razib Khan One-stop-shopping for all of my content

October 21, 2012

Dogs, domesticated before agriculture

Filed under: Domestic dogs,Ecology — Razib Khan @ 10:27 pm

The domestication of the dog is a complex and unresolved topic. But at this point I am convinced that this is one domestication event which well predates agriculture. To some extent this is common sense. There are tentative archaeological finds of domestic dogs in the New World almost immediately after widespread human habitation of the Western hemisphere, >10,000 years ago. More concretely domestic dog DNA has been retrieved from ~9,250 year old coprolites in Texas. The distinctiveness of the New World dogs is well attested genetically. Eskimo dogs for example are nested in a well diverged clade with “ancient dogs” (e.g., Basenji), indicating their early separation from the main Eurasian stock. Additionally, from talking to a dog geneticist I am to understand that the Eskimo dogs themselves are likely new arrivals, and superseded older dog lineages in the far north.

These results suggest to me that by the late Pleistocene many of the hunter-gatherer populations of Eurasia existed in a symbiotic relationship with domestic dogs. I suspect that the dog lineages in fact predate the late Pleistocene, and the earliest tendencies toward coexistence with humans may date to before the Last Glacial Maximum. And yet an interesting sidelight ...

July 3, 2012

Evolution, not ecology

Filed under: Ecology,Environment,Environmentalism — Razib Khan @ 9:59 pm

One of my major gripes with my friends in ecology is that there is a tendency to look at every problem through the lens of ecological models. Garrett Hardin, who popularized the term “tragedy of the commons” is an exemplar of this. People in ecology often get irritated by the public confusion between it, a positive scientific discipline, and environmentalism, a normative set of beliefs (it doesn’t help when some environmentalist groups have names like “ecology movement”). But the fact is there are deep commonalities in terms of prior assumptions by both ecologists and environmentalists. Despite evolutionary ecology, the reality is that ecologists seem to be characterized by a mindset which posits limits to growth and a finite set of responses to the challenges of scarce resources. That is, the Malthusian paradigm.

I bring this up because despite the similarities between ecology and economics it strikes me that ecologists often have a difficult time admitting that the parameters of the model which they think they have a good grasp of may not always be fixed. Incentives and innovation can shift the dynamics radically. Consider George Monbiot’s about ...

May 5, 2011

Why the Amazon Rainforest is species rich

Filed under: Amazon Rainforest,Amazonia,Diversity,Ecology,Environment,Speciation — Razib Khan @ 3:08 pm


A monkey frog

The Pith: The Amazon Rainforest has a lot of species because it’s been around for a very long time.

I really don’t know much about ecology, alas. So my understanding of evolution framed in its proper ecological context is a touch on the coarse side. When I say I don’t know much about ecology, I mean that I lack a thick network of descriptive detail. So that means that I have some rather simple models in my head, which upon closer inspection turn out to be false in many specific instances. That’s what you get for relying on theory. Today I ran into a paper which presented me with some mildly surprising results.

The question: why is the Amazon Rainforest characterized by such a diversity of species? If you’d asked me that question 1 hour ago I would have said that it was a matter of physics. That is, the physical parameters of a high but consistent rainfall and temperature regime. This means the basic energetic inputs into the biome is high, and its consistency allows the organisms to plan their life schedule efficiently, maximizing the inputs. All ...

April 20, 2011

Evolutionary pressures, within and without

Filed under: Ecology,Evolution,Extinction,Radiation,Speciation — Razib Khan @ 12:16 am


Foraminifera, Wikimedia Commons

The Pith: The tree if life is nourished by agon, but pruned by the gods. More literally, both interactions between living organisms and the changes in the environment impact the pulsing of speciation and extinction.

No one can be a true “Renaissance Man” today. One has to pick & choose the set of focuses to which one must turn one’s labor to. Life is finite and subject to trade offs. My interest in evolutionary science as a child was triggered by a fascination with paleontology. In particular the megafauna of the Mesozoic and the Cenozoic, dinosaurs and other assorted reptilian lineages as well as the hosts of extinct and exotic mammals which are no more. Obviously I don’t put much time into those older interests at this point, and I’m as much of a civilian when I read Laelaps as you are. More generally when it comes to evolution I focus on the scale of microevolution rather than macroevolution. Evolutionary genetics and the like, rather than paleontology. This is in part because I lean toward a scale independence in evolutionary process, so that the ...

December 10, 2010

A cloud forest in….

Filed under: Cloud Forest,Ecology — Razib Khan @ 1:22 am

765px-Salalah_Oman

A “cloud forest”


The lush image above is of a cloud forest biome. Can you guess where it is? The Arabian country of Oman! How’s that for a surprise? I had known of the Green Mountain of northeast Oman, which is ~3000 meters above sea level and receives ~15 inches of rain (enough for shrubby woodland), but was totally ignorant of Salalah mountains in western Oman. Apparently the region catches a bit of the monsoon, and so has a rainy season. And yet the cloud forests receive only ~15 inches of rainfall themselves! (300 mm) But the key is that apparently if you include condensation from fog and such the precipitation triples. So the physical nature of the forest produces a feedback loop which allows it to sustain itself.

Here’s a paper on the Omani cloud forest, how it maintains equilibrium, and possible threats.

Image Credit: storymary

May 4, 2010

The dynamism of nature

Filed under: Biodiversity,Ecology,Environment — Razib Khan @ 12:06 am

On this week’s ResearchBlogCast we discussed Adaptation, Plasticity, and Extinction in a Changing Environment: Towards a Predictive Theory (see my post reviewing it). The basic idea was to discuss a simple mathematical model which treated biological populations as something more than simply static constants buffeted by changes in physical parameters. In particular there’s often an implicit model that species exist at a particular and precise equipoise with an environment, and that when those environmental parameters are shifted that the species is in jeopardy unless it can track its optimal environment through migration.

In some ways this would be mighty convenient for us if it were so. If species were static we wouldn’t have to worry about weeds becoming resistant to pesticide, or diseases wrecking havoc to our crops, and so forth. But biology is dynamic, both on the life history and evolutionary scale. I think it would benefit us to take this into account when we humans consider the value we place on conservation, and the decisions we make to maintain biodiversity. Kevin Zelnio pointed out that there have been worries about the disappearance of charismatic fauna for about a generation now, and though species such as the tiger and elephant are still endangered (and because of their relatively long generation times this is problematic), many species which we were told as children would become extinct by the time we were adults remain a presence today in the wild. Some of this is surely due to conservation after the awareness of the threats, but another issue may be that some of these species are more resilient than we think, or give them credit for. Dave Munger reminded us that in 2007 100,000 Lowland Gorillas “discovered”, tripling the numbers of the species immediately. One way of looking at it is that these gorillas were mighty lucky that they’d been unnoticed…but another issue may be that gorillas coevoled to some extent with hominids and may have some sense where to go to avoid human habitation.

This is not to recommend complacency. And I haven’t even broached the serious normative issues as to the value of biodiversity outside of its human utilitarian consequences. These are points over which reasonable people can discuss and differ. Rather, when we speak of the environmental and non-human life we often speak as if humanity and physical nature are the two active forces operative on a passive and static biological nature. This is obviously not true. Our species’ mastery of the physical sciences in the past 200 years has given us a sense of power over the biological world, but we shouldn’t get complacent, and we shouldn’t dismiss the resilience and cleverness of nature, though that resilience and cleverness does not always redound to our benefit.

April 29, 2010

Modeling the probabilities of extinction

Change is quite in the air today, whether it be climate change or human induced habitat shifts. What’s a species in the wild to do? Biologists naturally worry about loss of biodiversity a great deal, and many non-biologist humans rather high up on Maslow’s hierarchy of needs also care. And yet species loss, or the threat of extinction, seems too often to impinge upon public consciousness in a coarse categorical sense. For example the EPA classifications such as “threatened” or “endangered.” There are also vague general warnings or forebodings; warmer temperatures leading to mass extinctions as species can not track their optimal ecology and the like. And these warnings seem to err on the side of caution, as if populations of organisms are incapable of adapting, and all species are as particular as the panda.

That’s why I pointed to a recent paper in PLoS Biology, Adaptation, Plasticity, and Extinction in a Changing Environment: Towards a Predictive Theory below. I am somewhat familiar with one of the authors, Russell Lande, and his work in quantitative and ecological genetics, as well as population biology. I was also happy to note that the formal model here is rather spare, perhaps a nod to the lack of current abstraction in this particular area. Why start complex when you can start simple? Here’s their abstract:

Many species are experiencing sustained environmental change mainly due to human activities. The unusual rate and extent of anthropogenic alterations of the environment may exceed the capacity of developmental, genetic, and demographic mechanisms that populations have evolved to deal with environmental change. To begin to understand the limits to population persistence, we present a simple evolutionary model for the critical rate of environmental change beyond which a population must decline and go extinct. We use this model to highlight the major determinants of extinction risk in a changing environment, and identify research needs for improved predictions based on projected changes in environmental variables. Two key parameters relating the environment to population biology have not yet received sufficient attention. Phenotypic plasticity, the direct influence of environment on the development of individual phenotypes, is increasingly considered an important component of phenotypic change in the wild and should be incorporated in models of population persistence. Environmental sensitivity of selection, the change in the optimum phenotype with the environment, still crucially needs empirical assessment. We use environmental tolerance curves and other examples of ecological and evolutionary responses to climate change to illustrate how these mechanistic approaches can be developed for predictive purposes.


Their model here seems to be at counterpoint to something called “niche modelling” (yes, I am not on “home territory” here!), which operates under the assumption of species being optimized for a particular set of abiotic parameters, and focusing on the shifts of those parameters over space and time. So extinction risk may be predicted from a shift in climate and decrease or disappearance of potential habitat. The authors of this paper observe naturally that biological organisms are not quite so static, they exhibit both plasticity and adaptiveness within their own particular life history, as well as ability to evolve on a population wide level over time. If genetic evolution is thought of as a hill climbing algorithm I suppose a niche model presumes that the hill moves while the principal sits pat. This static vision of the tree of life seems at odds with development, behavior and evolution. The authors of this paper believe that a different formulation may be fruitful, and I am inclined to agree with them.

journal.pbio.1000357.e001As I observed above the formalism undergirding this paper is exceedingly simple. On the left-hand side you have the variable which determines the risk, or lack of risk, of extinction more or less, because it defines the maximum rate of environmental change where the population can be expected to persist. This makes intuitive sense, as extremely volatile environments would be difficult for species and individual organisms to track.Too much variation over a short period of time, and no species can bend with the winds of change rapidly enough. Here are the list of parameters in the formalism (taken from box 1 of the paper):

ηc – critical rate of environmental change: maximum rate of change which allows persistence of a population

B – environmental sensitivity of selection: change in the optimum phenotype with the environment. It’s a slope, so 0 means that the change in environment doesn’t change optimum phenotype, while a very high slope indicates a rapid shift of optimum. One presumes this is proportional to the power of natural selection

T – generation time: average age of parents of a cohort of newborn individuals. Big T means long generation times, small T means short ones

σ2 – phenotypic variance

h2 – heritability: the proportion of phenotypic variance in a trait due to additive genetic effects

rmax intrinsic rate of increase: population growth rate in the absence of constraints

b – phenotypic plasticity: influence of the environment on individual phenotypes through development. Height is plastic; compare North Koreans vs. South Koreans

γ – stabilizing selection: this is basically selection pushing in from both directions away from the phenotypic optimum. The stronger the selection, the sharper the fitness gradient. Height exhibits some shallow stabilizing dynamics; the very tall and very short seem to be less fit

Examining the equation, and knowing the parameters, some relations which we comprehend intuitively become clear. The larger the denominator, the lower the rate of maximum environmental change which would allow for population persistence, so the higher the probability of extinction. Species with large T, long generation times, are at greater risk. Scenarios where the the environmental sensitivity to selection, B, is much greater than the ability of an organism to track its environment through phenotypic plasticity, b, increase the probability of extinction. Obviously selection takes some time to operate, assuming you have extant genetic variation, so if a sharp shift in environment with radical fitness implications occurs, and the species is unable to track this in any way, population size is going to crash and extinction may become imminent.

On the numerator you see that the more heritable variation you have, the higher ηc. The rate of adaptation is proportional to the amount of heritable phenotypic variation extant within the population, because selection needs variance away from the old optimum toward the new one to shift the population central tendency. In other words if selection doesn’t result in a change in the next generation because the trait isn’t passed on through genes, then that precludes the population being able to shift its median phenotype (though presumably if there is stochastic phenotypic variation from generation to generation it would be able to persist if enough individuals fell within the optimum range). The strength of stabilizing selection and rate of natural increase also weight in favor of population persistence. I presume in the former case it has to do with the efficacy of selection in shifting the phenotypic mean (i.e., it’s like heritability), while in the latter it seems that the ability to bounce back from population crashes would redound to a species’ benefit in scenarios of environmental volatility (selection may cause a great number of deaths per generation until a new equilibrium is attained).

journal.pbio.1000357.e002Of course a model like the one above has many approximations so as to approach a level of analytical tractability. They do address some of the interdependencies of the parameters, in particular the trade-offs of phenotypic plasticity. In this equation 1/ω2b quantifies the cost of plasticity, r0 represents increase without any cost of plasticity. We’re basically talking about the “Jack-of-all-trades is a master of none” issue here. In a way this crops up when we’re talking of clonal vs. sexual lineages on an evolutionary genetic scale. The general line of thinking is that sexual lineages are at a short-term disadvantage because they’re less optimized for the environment, but when there’s a shift in the environment (or pathogen character) the clonal lineages are at much more risk because they don’t have much variation with which natural selection can work. What was once a sharper phenotypic optimum turns into a narrow and unscalable gully.

Figure 2 illustrates some of the implications of particular parameters in relation to trade-offs:

paramslande

There’s a lot of explanatory text, as they cite various literature which may, or may not, support their model. Clearly the presentation here is aimed toward goading people into testing their formalism, and to see if it has any utility. I know that those who cherish biodiversity would prefer that we preserve everything (assuming we can actually record all the species), but reality will likely impose upon us particular constraints, and trade-offs. In a cost vs. benefit calculus this sort model may be useful. Which species are likely to be able to track the environmental changes to some extent? Which species are unlikely to be able to track the changes? What are the probabilities? And so forth.

I’ll let the authors conclude:

Our aim was to describe an approach based on evolutionary and demographic mechanisms that can be used to make predictions on population persistence in a changing environment and to highlight the most important variables to measure. While this approach is obviously more costly and time-consuming than niche modelling, its results are also likely to be more useful for specific purposes because it explicitly incorporates the factors that limit population response to environmental change.

The feasibility of such a mechanistic approach has been demonstrated by a few recent studies. Deutsch et al…used thermal tolerance curves to predict the fitness consequence of climate change for many species of terrestrial insects across latitudes, but without explicitly considering phenotypic plasticity or genetic evolution. Kearney et al…combined biophysical models of energy transfers with measures of heritability of egg desiccation to predict how climate change would affect the distribution of the mosquito Aedes aegiptii in Australia. Egg desiccation was treated as a threshold trait, but the possibility of phenotypic plasticity or evolution of the threshold was not considered. These encouraging efforts call for more empirical studies where genetic evolution and phenotypic plasticity are combined with demography to make predictions about population persistence in a changing environment. The simple approach we have outlined is a necessary step towards a more specific and comprehensive understanding of the influence of environmental change on population extinction.

Citation: Chevin L-M, Lande R, & Mace GM (2010). Adaptation, Plasticity, and Extinction in a Changing Environment: Towards a Predictive Theory PLoS Biol : 10.1371/journal.pbio.1000357

April 14, 2010

Shadows of menageries past

Filed under: Ecology,Rare animals,science,Sibree's Dwarf Lemur,Zoology — Razib Khan @ 10:08 am

100413162914-largeI’m still a sucker for stories like this, Only Known Living Population of Rare Dwarf Lemur Discovered:

Researchers have discovered the world’s only known living population of Sibree’s Dwarf Lemur, a rare lemur known only in eastern Madagascar. The discovery of approximately a thousand of these lemurs was made by Mitchell Irwin, a Research Associate at McGill University, and colleagues from the German Primate Centre in Göttingen Germany; the University of Antananarivo in Madagascar; and the University of Massachusetts.

The species was first discovered in Madagascar in 1896, but this tiny, nocturnal dwarf lemur was never studied throughout the 20th century. Following the destruction of its only known rainforest habitat, scientists had no idea whether the species still existed in the wild — or even whether it was a distinct species….

Living today is much more awesome than the 19th century overall, but, we’ve mapped the whole world, and have a good sense of all the large animals (at least the upper bound, unfortunately the number seems to be dropping). Call me mammal-centric, but I feel that we have tapped out most of the zoological wonder of our planet. Is it too much to say that the terrestrial domain now involves mostly the counting of beetles? (I exaggerate!) But sometimes there’s a lemur in Madagascar or a rare ungulate in Vietnam, and we get a sense of the wonder which once was (along with all the -isms which we now abhor!). Could you imagine the blog posts that Carl Zimmer or Ed Yong could have written about the discovery of the Platypus? Actually, they’d probably end up narrating a special on the National Geographic Channel….

Here’s the original paper: MtDNA and nDNA corroborate existence of sympatric dwarf lemur species at Tsinjoarivo, eastern Madagascar.

Credit: Image courtesy of McGill University

March 29, 2010

Thomas Malthus was right. Mostly

pleistocene_brain_sizeJohn Hawks has an excellent post rebutting some misinformation and confusion on the part of Colin Blakemore, an Oxford neurobiologist. Blakemore asserts that:

* There was a sharp spike in cranial capacity ~200,000 years ago, on the order of 30%

* And, that the large brain was not deleterious despite its large caloric footprint (25% of our calories service the brain) because the “environment of early humans was so clement and rich in resources”

Hawks refutes the first by simply reposting the chart the above (x axis = years before present, y axis = cranial capacity). It’s rather straightforward, I don’t know the paleoanthropology with any great depth, but the gradual rise in hominin cranial capacity has always been a “mystery” waiting to be solved (see Grooming, Gossip, and the Evolution of Language and The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature). Blakemore may have new data, but as they say, “bring it.” Until then the consensus is what it is (the hominins with the greatest cranial capacities for what it’s worth were Neandertals, and even anatomically modern humans have tended toward smaller cranial capacities since the end of the last Ice Age along with a general trend toward smaller size).


But the second issue is particularly confusing, as Blakemore should have taken an ecology course at some point in his eduction if he’s a biologist (though perhaps not). One of the problems that I often have with biologists is that they are exceedingly Malthusian in their thinking, and so have a difficult time internalizing  the contemporary realities of post-Malthusian economics (see Knowledge and the Wealth of Nations: A Story of Economic Discovery).Innovation and economic growth combined with declining population growth have changed the game in fundamental ways. And yet still the biological predisposition to think in Malthusian terms is correct for our species for almost its whole history.*

A “tropical paradise” is only a tropical paradise if you have a modicum of affluence, leisure, and, modern medicine. Easter Island is to a great extent a reductio ad absurdum of pre-modern man and gifted with a clement regime. Easter Island’s weather is mild, the monthly low is 18/65 °C/°F and the monthly high is 28/82 °C/°F. The rainfall is 1,118/44 mm/in. But constrained on an island the original Polynesians famously transformed it into a Malthusian case-study. We literally breed up to the limits of growth, squeezing ourselves against the margins of subsistence.

I can think of only one way in which Blakemore’s thesis that the environment of early humans was rich in resources might hold, at least on a per capita basis: the anatomically modern humans of Africa exhibited bourgeois values and had low time preference. In other words, their population was always kept below ecological carrying capacity through forethought and social planning, since there is no evidence for much technological innovation which would have resulted in economic growth to generate surplus. My main qualm with this thesis is that it seems to put the cart before the horse, since one presupposes that a robust modern cognitive capacity is usually necessary for this sort of behavior.

* Malthus’ biggest mistake was probably that he did not anticipate the demographic transition, whereby gains in economic growth were not absorbed by gains in population.

March 15, 2010

Apartheid of Iberian Neandertals & modern humans

Filed under: Ecology — Gene Expression @ 9:44 pm

The Middle-to-Upper Palaeolithic transition in Cova Gran (Catalunya, Spain) and the extinction of Neanderthals in the Iberian Peninsula:

The excavations carried out in Cova Gran de Santa Linya (Southeastern PrePyrenees, Catalunya, Spain) have unearthed a new archaeological sequence attributable to the Middle Palaeoloithic/Upper Palaeolithic (MP/UP) transition. This article presents data on the stratigraphy, archaeology, and 14C AMS dates of three Early Upper Palaeolithic and four Late Middle Palaeolithic levels excavated in Cova Gran. All these archaeological levels fall within the 34-32 ka time span, the temporal frame in which major events of Neanderthal extinction took place. The earliest Early Upper Palaeolithic (497D) and the latest Middle Palaeolithic (S1B) levels in Cova Gran are separated by a sterile gap and permit pinpointing the time period in which the Mousterian disappeared from Northeastern Spain. Technological differences between the Early Upper Palaeolithic and Late Middle Palaeolithic industries in Cova Gran support a cultural rupture between the two periods. A series of 12 14C AMS dates prompts reflections on the validity of reconstructions based on radiocarbon data. Thus, results from excavations in Cova Gran lead us to discuss the scenarios relating the MP/UP transition in the Iberian Peninsula, a region considered a refuge of late Neanderthal populations.

ScienceDaily has a lot more. Here's the important point I think:

The samples obtained at Cova Gran using Carbon 14 dating refer to a period of between 34,000 and 32,000 years in which this biological replacement in the Western Mediterranean can be located in time, although the study regards as relative the use of Carbon 14 for dating materials from the period of transition of the Middle to Upper Palaeolithic period( 40,000 and 30,000).

The results also support the hypothesis that there was neither interaction nor coexistence between the two species.

There's long been a model that modern humans replaced Neandertals without coming into direct conflict with them. The model would be that modern humans simply disrupted the ecology which the Neandertals depended upon. It seems a bit too pat for me, but considering the very low population densities of hunter-gatherers, and in particular Neandertals, perhaps it is possible.

Citation: The Middle-to-Upper Palaeolithic transition in Cova Gran (Catalunya, Spain) and the extinction of Neanderthals in the Iberian Peninsula, doi:10.1016/j.jhevol.2009.09.002

Read the comments on this post...

January 26, 2010

Hobbits small brains not so anomolous

Filed under: Ecology — Gene Expression @ 11:35 pm

Is the Hobbit's Brain Unfeasibly Small?:

Brain expansion began early in primate evolution and has occurred in all major groups, suggesting a strong selective advantage to increased brainpower in most primate lineages. Despite this overall trend, however, Mundy and his colleagues have identified several branches/lineages within each major group that have shown decreasing brain and body mass as they evolve, for example in marmosets and mouse lemurs.

According to Mundy, "We find that, under reasonable assumptions, the reduction in brain size during the evolution of Homo floresiensis is not unusual in comparison to these other primates. Along with other recent studies on the effects of 'island dwarfism' in other mammals, these results support the hypothesis that the small brain of Homo floresiensis was adapted to local ecological conditions on Flores."

The paper will show up in BMC Biology at some point. The main question I have is in regards to the purported tool use of the Hobbits. I can believe that a local adaptation toward small brains, Idiocracy-writ large, occurred. Brains are metabolically expensive, and it isn't as if the history of life on earth has shown the massive long-term benefits of being highly encephalized (though I think one can make a case that there has been a modest trend, with primates, and especially H. sapiens as extreme outliers above the trend). But could small brained creatures maintain the relatively advanced toolkit which the Hobbit finds have been associated with? Seems to me that there's a high probability here of some sort of contamination, but I'll be happy to be put in my place by anthropologists in-the-know....

Read the comments on this post...

December 22, 2009

When mammoths roamed (rarely)

Filed under: Ecology — Gene Expression @ 3:02 pm

Brian Switek, The extended twilight of the mammoths:

So, if the team's analysis is correct, both mammoths and horses lived in the interior of Alaska between about 11,000 and 7,000 years ago. This is significantly more recent than the youngest fossil remains of horses and mammoths, dated between 15,000 and 13,000 years ago. There are at least two factors that might contribute to this disparity. The first is that fossils from this more recent time were preserved but have not yet been found. More likely, though, is that the populations of both mammoths and horses had dwindled to the point where fossil preservation was becoming increasingly unlikely. There were so few of them that the death of an individual in circumstances amenable to preservation was becoming rarer and rarer.

Either way, this discovery has important implications for the extinction of horses and mammoths in North America. Based upon the fossil data alone it had been hypothesized that both disappeared around the time that humans became established in North America.* Some have taken this association to suggest that humans engaged in a blitzkrieg in which naive New World megamammals were quickly dispatched by the human hunters. If the new evidence is correct, though, humans did not wipe out horses and mammoths overnight. Instead humans lived alongside dwindling populations in Alaska for thousands of years. Likewise, these new findings also contradict the favored hypothesis of one of the study's authors, Ross MacPhee, who previously proposed that some kind of "hyperdisease" carried by humans (or animals that traveled with humans) quickly wiped out these animals. The pattern of extinction was obviously more protracted.

This seems about right. Excuse the analogy, but it sometimes seems that models of human-caused extinction of mega-fauna portray ancient hunter-gatherers as Einsatzgruppen, and the mega-fauna as Jews and Communists. Though genocides of human populations in the concerted manner of the Germans against the Jews, Gypsies and other groups during World War II have occurred periodically, more often what we see is a slow wearing down and attrition of marginal groups at the expense of dominant ones.

It seems a plausible model that when mega-fauna were plentiful hunters would focus on them, but once the mega-fauna became rare naturally the return on investment would decrease and it would become rational to shift to other prey organisms. This implies that many mega-fauna likely persisted in isolated pockets as relict populations, and may have been killed off only far later, or perhaps even succumbed to a natural environmental calamity. In another era the last herds of wild horses would probably have gone extinct due to drought, or perhaps been hunted down by a random group of humans who had no idea that they were decreasing the biological diversity of the planet.

Read the comments on this post...

December 15, 2009

I for one welcome our future cephalopod overlords!

Filed under: Ecology — Gene Expression @ 2:13 am

octpus.pngCool new report in Current Biology, Defensive tool use in a coconut-carrying octopus:

The use of tools has become a benchmark for cognitive sophistication. Originally regarded as a defining feature of our species, tool-use behaviours have subsequently been revealed in other primates and a growing spectrum of mammals and birds...Among invertebrates, however, the acquisition of items that are deployed later has not previously been reported. We repeatedly observed soft-sediment dwelling octopuses carrying around coconut shell halves, assembling them as a shelter only when needed. Whilst being carried, the shells offer no protection and place a requirement on the carrier to use a novel and cumbersome form of locomotion -- 'stilt-walking'.

No surprise that when we are looking to a violation of an old "human exceptional" character (though tool-use seems to have been violated a fair amount now by any interpretation) that the cephalopod would step up to the plate. I've heard of weird behavior by octopuses in laboratories which begs to be anthropomorphized, but no one denies that this is one taxa which has some brains. Who says you need a notochord to be a "higher animal"? Anyone who's read a fair amount of science fiction also is aware that cephalopods are one of the more exotic, but still frequent, candidate earth lineages which might potentially rise to sapience. Fore all the aquatic species who have the glimmer of intelligence cybernetics might offer up some potential avenues of freedom and leveling the playing field with the terrestrials.

Citation: Defensive tool use in a coconut-carrying octopus, Finn, Julian K.; Tregenza, Tom; Norman, Mark D. doi:10.1016/j.cub.2009.10.052 (volume 19 issue 23 pp.R1069 - R1070)

Read the comments on this post...

November 23, 2009

Apocalypse 73,000 B.C.

Filed under: Ecology — Gene Expression @ 10:07 pm

FuturePundit points me to a new paper on the Toba explosion, Environmental impact of the 73 ka Toba super-eruption in South Asia:

The cooling effects of historic volcanic eruptions on world climate are well known but the impacts of even bigger prehistoric eruptions are still shrouded in mystery. The eruption of Toba volcano in northern Sumatra some 73,000 years ago was the largest explosive eruption of the past two million years, with a Volcanic Explosivity Index of magnitude 8, but its impact on climate has been controversial. In order to resolve this issue, we have analysed pollen from a marine core in the Bay of Bengal with stratified Toba ash, and the carbon isotopic composition of soil carbonates directly above and below the ash in three sites on a 400 km transect across central India. Pollen evidence shows that the eruption was followed by initial cooling and prolonged desiccation, reflected in a decline in tree cover in India and the adjacent region. Carbon isotopes show that C3 forest was replaced by wooded to open C4 grassland in central India. Our results demonstrate that the Toba eruption caused climatic cooling and prolonged deforestation in South Asia, and challenge claims of minimal impact on tropical ecosystems and human populations.

The Toba caldera is in Sumatra, but the ashfall in India was on the order of 15 centimeters to 6 feet. This might also be relevant to human evolution, The super-eruption of Toba, did it cause a human bottleneck?.

Citation Martin A.J. Williamsa, Stanley H. Ambroseb, Sander van der Kaarsc, Carsten Ruehlemannd, Umesh Chattopadhyayae, Jagannath Pale and Parth R. Chauhanf, Environmental impact of the 73 ka Toba super-eruption in South Asia, doi:10.1016/j.palaeo.2009.10.009

Read the comments on this post...

Apocalypse 73,000 B.C.

Filed under: Ecology — Gene Expression @ 10:07 pm

FuturePundit points me to a new paper on the Toba explosion, Environmental impact of the 73 ka Toba super-eruption in South Asia:

The cooling effects of historic volcanic eruptions on world climate are well known but the impacts of even bigger prehistoric eruptions are still shrouded in mystery. The eruption of Toba volcano in northern Sumatra some 73,000 years ago was the largest explosive eruption of the past two million years, with a Volcanic Explosivity Index of magnitude 8, but its impact on climate has been controversial. In order to resolve this issue, we have analysed pollen from a marine core in the Bay of Bengal with stratified Toba ash, and the carbon isotopic composition of soil carbonates directly above and below the ash in three sites on a 400 km transect across central India. Pollen evidence shows that the eruption was followed by initial cooling and prolonged desiccation, reflected in a decline in tree cover in India and the adjacent region. Carbon isotopes show that C3 forest was replaced by wooded to open C4 grassland in central India. Our results demonstrate that the Toba eruption caused climatic cooling and prolonged deforestation in South Asia, and challenge claims of minimal impact on tropical ecosystems and human populations.

The Toba caldera is in Sumatra, but the ashfall in India was on the order of 15 centimeters to 6 feet. This might also be relevant to human evolution, The super-eruption of Toba, did it cause a human bottleneck?.

Citation Martin A.J. Williamsa, Stanley H. Ambroseb, Sander van der Kaarsc, Carsten Ruehlemannd, Umesh Chattopadhyayae, Jagannath Pale and Parth R. Chauhanf, Environmental impact of the 73 ka Toba super-eruption in South Asia, doi:10.1016/j.palaeo.2009.10.009

Read the comments on this post...

November 20, 2009

When mammoths roamed

Filed under: Ecology — Gene Expression @ 10:19 pm

Pleistocene Megafaunal Collapse, Novel Plant Communities, and Enhanced Fire Regimes in North America:

Although the North American megafaunal extinctions and the formation of novel plant communities are well-known features of the last deglaciation, the causal relationships between these phenomena are unclear. Using the dung fungus Sporormiella and other paleoecological proxies from Appleman Lake, Indiana, and several New York sites, we established that the megafaunal decline closely preceded enhanced fire regimes and the development of plant communities that have no modern analogs. The loss of keystone megaherbivores may thus have altered ecosystem structure and function by the release of palatable hardwoods from herbivory pressure and by fuel accumulation. Megafaunal populations collapsed from 14,800 to 13,700 years ago, well before the final extinctions and during the Bølling-Allerød warm period. Human impacts remain plausible, but the decline predates Younger Dryas cooling and the extraterrestrial impact event proposed to have occurred 12,900 years ago.

Ed Yong has the bases covered:

What about humans, those pesky slayers of animals? Some scientists believed that North America's Clovis people specialised in hunting big mammals, causing a "blitzkrieg" of spear-throwing that drove many species to extinction. But these hunters only arrive in North America between 13,300 and 12,900 years ago, around a thousand years after the population crashes had begun.

If people were responsible, they must have been pre-Clovis settlers. There's growing evidence that such humans were around, but they weren't common or specialised. They may have contributed to the beasts' downfall, while Clovis hunting technology delivered a coup de grace to already faltering populati0ons.

By analysing the sediment at Appleman lake - spores, pollen, charcoal and all - Gill has replayed the history of the site, spanning the last 17,000 years. Her data rule out a few theories, but as she says, they "[do] not conclusively resolve the debate" about climate causes versus human ones. It's possible that similar studies at different sites and other continents will help to provide more clues.

A complex story like this is perhaps more common than an event such as the extinction of the passenger pigeons. Populations of organisms often go through cycles in census size, whether due to environmental variation or coevolutionary dynamics with parasites. Consider the example of the Tasmania devil, the disease which it is susceptible to is not the doing of humans, but the introduction of dingos (probably by humans) mean that the species is restricted to Tasmania. Additionally, humans have laid claim to much of the habitat of the devil (or what was the habitat of the devil). When a virulent disease hits, the devil has a much smaller margin of error than it had before. It could be that recent megafaunal extinctions are ultimately due to humans, even if they are not always proximately due to humans.

Read the comments on this post...

Powered by WordPress