At least, that’s according to a survey cited by epiphenom. The least-religious professors at U.S. universities are, in fact, the psychologists — almost 50% are atheists, and about another 10% are agnostic. Biologists are close behind, actually, but are more likely to be agnostic (about 35%) than out-and-out atheist (about 25%).
Category Archives: science
How fast do ecosystems recover from disturbance? It’s complicated.
In the 21st century, human activity promises to impact the natural world on an unprecedented scale. In order to decide where to focus conservation effort, one thing we need to know is how permanent the damage from a forest clear-cut or a collapsed fishery actually is. A paper in this week’s PLoS ONE looks at natural systems’ ability to recover after human and natural disturbances, and the authors say the results are hopeful. I’m not so sure.
The authors, Jones and Schmitz, assemble a meta-dataset of ecological studies published from 1910 to 2008, all examining the recovery of either ecosytem functions (like total nutrient cycling rates) or plant or animal diversity following disturbances as diverse as hurricanes and oil spills. They then calculated the proportion of measured variables that recovered, or failed to, within the period studied by each paper in the dataset, how much the measured variables had been altered by the disturbance, and how long it took before they returned to their pre-disturbance state.
The results are complicated, to say the least. For example, here’s Figure 2, which charts the times to recovery for variables measuring animal community recovery (black bars), ecosystem function (white bars) or plant community (gray bars), broken down by ecosystem type in the top panel, and by disturbance type in the bottom panel:
The authors’ conclusion? There is “no discernable pattern.” Which I can’t really dispute — recovery times look highly idiosyncratic. An ANOVA performed on the data finds significant effects of ecosystem type and disturbance type, but what does that tell us? Different ecosystems recover differently. Forests take the longest to recover, which makes sense given that trees grow slowly, and succession from clearcut to mature forest can take centuries. Similarly, ecosystems experiencing multiple types of disturbance took the longest to recover.
Of the ecosystems that do recover, the authors point out, recovery occurs comparatively rapidly:
Among studies reporting recovery for any variable, the average recovery time was at most 42 years (for forest ecosystems) and typically much less (on the order of 10 years) when recovery was examined by ecosystem. When examined by perturbation type, the average recovery time was no more than 56 years (for systems undergoing multiple interacting perturbations) and typically was 20 years or less …
The authors then perform a regression of the strength of disturbance (i.e., how much the measured variables changed due to disturbance) against the time needed for recovery. The data set is necessarily small, because not many studies follow an ecosystem all the way from disturbance to complete recovery, and they find a significant effect of disturbance strength on recovery time mostly because of a single data point.
Jones and Schmitz conclude from this dataset that ecosystem recovery from human disturbance is frequently possible within human lifetimes, especially if we put in the effort for restoration. I’ll buy that; but I think the more important lesson to draw from this paper is that, after a century of watching the natural world respond to human activity, we still can’t predict what the results of our actions will be. It shouldn’t need saying, but when we fiddle with our life-support systems, we must proceed cautiously.
Reference
Jones, H., & Schmitz, O. (2009). Rapid recovery of damaged ecosystems PLoS ONE, 4 (5) DOI: 10.1371/journal.pone.0005653
Oh, hey
So I just noticed that this was my 50th post through the Research Blogging content aggregator. I joined Research Blogging last July, and it’s been nothing but good for traffic to D&T — and maybe more importantly it’s one of the first places I check when I want to see what other scientists are blogging about.
I’ve now tagged every piece submitted through the system, and will continue to do so for organizational convenience. It’s variable output, quality-wise, but pretty representative of my free-time science reading, which is what I aim for my scientific blogging to be.
In social courtship, it pays to be a good wingman
The search for a mate is traditionally a selfish enterprise. After all, the ultimate goal is reproduction, and — barring any effect of kin selection — natural selection only cares about how many babies you make, not how many you help to make. This is fundamentally a biological question, though, and if there’s a universal rule in biology, it’s that nature is good at making exceptions.
One such exception is the wire-tailed manakin. A study in the latest Proceedings of the Royal Society seems to show that male manakins can boost their own mating success by helping other males attract mates [$-a]. Manakins are a family of brightly-colored neotropical birds, and the males of many manakin species attract females by putting on dancing displays, as seen in this video:
(I seem to recall that there’s also some excellent footage of manakin dancing in David Attenborough’s The Life of Birds.)
To dance for females, male manakins gather at locations called “leks,” where most try to establish a small territory to perform. Among wire-tailed manakins, though, some males will team up to dance — presumably because if one brightly-colored male jumping around on a branch is attention-grabbing, two or three are even more so. But in these “coordinated displays,” one performer, the socially dominant one, is most likely to mate with the females who like the performance. So what’s in it for the other guys?
There seem to be two possible (though not mutually exclusive) explanations [$-a]: (1) that the mate-attracting dancing does double duty to establish social dominance relationships among males, and (2) that, even if it wins fewer mates than the “lead” role, being a supporting player in a successful cooperative display means better mating prospects than trying to go it alone. To try and disentangle these two possibilities, the new study’s authors followed the behavior of wire-tailed manakins at several leks for four years, building a “social network” of male-male cooperation at the leks and counting the offspring each male bird by taking DNA fingerprints of the males and of newly-hatched chicks in the nests of females who attended each lek.
Although the most reproductively successful males at each lek were all territorial, defending their own spot at the lek and dominating other males who joined in the display on that territory, non-territorial “floater” males tended to make more babies if they joined in more displays. In fact, the number of offspring produced was best predicted by the number of cooperative display interactions in which a male joined, whether he had his own territory or not. This complements an earlier study by the same group [$-a], which showed that a male’s “tenure” — how long he had been dominant in a territory within a lek — was the best predictor of mating success, but that a male’s rise through the social hierarchy at a lek was facilitated by cooperative interactions with other males.
In short, male manakins seem to help each other in mating displays for essentially selfish reasons. Being a supporting dancer has a coattail effect, earning more mates than trying to go solo, and it helps young males improve their social status toward the day when they can establish their own display territory.
References
Prum, R.O. (1994). Phylogenetic analysis of the evolution of alternative social behavior in the manakins (Aves: Pipridae). Evolution, 48, 1657-75 DOI: http://www.jstor.org/stable/2410255
Ryder, T., McDonald, D., Blake, J., Parker, P., & Loiselle, B. (2008). Social networks in the lek-mating wire-tailed manakin (Pipra filicauda) Proc.R. Soc. B, 275 (1641), 1367-74 DOI: 10.1098/rspb.2008.0205
Ryder, T., Parker, P., Blake, J., & Loiselle, B. (2009). It takes two to tango: reproductive skew and social correlates of male mating success in a lek-breeding bird Proc. R. Soc. B, 276 (1666), 2377-84 DOI: 10.1098/rspb.2009.0208
When the going gets tough, C. elegans gets sexy
The trouble with sex, from an evolutionary perspective, is that it’s expensive. Not just in terms of the efforts a sexually-reproducing organism has to go through to secure a mate; every offspring produced by sexual reproduction bears half the genome of each of its parents, compared to an asexual offspring, which bears a complete copy of its only parent’s genome. So, in terms of natural selection, an asexual critter gains twice as much reproductive fitness for each offspring it produces — asexual critters should overrun sexual competitors.
And yet they don’t. Sex is widespread in the animal kingdom, and common in the plant kingdom (although many plants can switch between sexual and asexual reproductive strategies). Many explanations have been proposed for this quandary; most of them have to do with the idea that sometimes it’s useful to mix your genome with someone else’s. The current front-runner hypothesis is that sex basically helps to separate useful genes from damaging ones [PDF], making sexual offspring more fit, on average. A different (but not mutually exclusive) possibility is that by mixing up genomes, sex can help generate the genetic variation necessary for a population to evolve in response to environmental stress. This might explain a discovery reported in this month’s issue of Evolution: that stressful conditions trigger the normally hermaphroditic nematode Caenorhabditis elegans to begin reproducing sexually [$-a].
The study’s authors subjected three experimental lineages of C. elegans to stress — starvation — triggering the worms to produce semi-dormant larvae called “dauer.” They then relieved the stress by transferring the population to a new food source. Some experimental treatments were kept well-fed after one period of dauer; others were repeatedly starved. Two of the three experimental lines responded to repeated episodes of dauer by producing male offspring instead of hermaphrodites.
Some of this effect was due to males’ better ability to survive dauer state than hermaphrodites. A large portion was because hermaphrodites became more likely to mate with males (with a possibility to produce male offspring) following dauer, though. This kind of facultative sex takes the best of asexual and sexual reproduction — the twofold fitness benefit of asexual reproduction most of the time; and the improved response to natural selection associated with sex in stressful conditions, when it’s needed most.
References
Keightley, P., & Otto, S. (2006). Interference among deleterious mutations favours sex and recombination in finite populations Nature, 443 (7107), 89-92 DOI: 10.1038/nature05049
Morran, L., Cappy, B., Anderson, J., & Phillips, P. (2009). Sexual partners for the stressed: Facultative outcrossing in the self-fertilizing nematode Caenohabditis elegans.
Evolution, 63 (6), 1473-82 DOI: 10.1111/j.1558-5646.2009.00652.x
Getting away from it all: Why are invasive species invasive?
When humans move from place to place, we almost always bring other organisms with us. Sometimes it’s intentional — domestic animals carried along with Polynesian colonists, for instance. Just as often, it’s accidental, as with mice stowing away on Viking longships. A lot of these introduced species have done so well in their new habitats that they become invasive, outcompeting natives and disrupting local ecosystem processes. But the species that go crazy-invasive — the cane toads and the purple loosestrife — are probably only the very successful subset of the species that hitch rides in cargo holds and ballast tanks. What sets the dangerously successful invasive species apart from others?
A new dataset published in last week’s PNAS suggests that it may be an interaction between available resources in a new habitat and a lack of compatible pathogens [$-a]. This is an amalgam of two hypothesized causes for successful invasion: access to new resources, and escape from antagonistic species. Focusing on European plant species that have successfully invaded North America, the authors, Blumenthal et al., assembled records of viral and fungal infections on each plant species in its native range, and in North America. They classified the plant species based on the habitats each occupies — wet vs. dry, nitrogen-rich vs. -poor — and on whether the plants tended to grow slowly or rapidly. This is because plant species adapted to rich, wet environments are generally thought to evolve fewer defenses against infection and herbivores; they can “afford” to grow new tissue instead of fight to keep it.
If resource availability interacts with freedom from infectious agents to spur a successful invasion, then invasive plants adapted to rich conditions should tend to host more pathogens in their home ranges than they do in their introduced range; and this difference should be less pronounced in invasive plants adapted to dry, resource-poor conditions. This is exactly what the analysis found — plants adapted to richer habitats saw a larger reduction in the number of pathogen species attacking them in their new ranges than plants adapted to less-productive conditions.
This is a valuable result for its basic application — helping to predict which introduced species are likely to become invasive, and target them for eradication efforts before they become well-established. But it also provides us with an insight into how evolution works. Many authors, particularly G.G. Simpson and Dolph Schluter, have described ecological conditions that set the stage for adaptive radiation — the rapid diversification of a lineage into many species — which sound a lot like the “ecological release” that invasive species seem to experience.
Rapid evolutionary diversification may be triggered by the evolution of a key innovation; by colonization of a new, empty habitat; or the removal of antagonistic species (usually by their extinction). These three classes of conditions are closely related, and they can be mimicked, or even replicated, when humans move species to new habitats [$-a]. Blumenthal et al. suggest, for instance, that species invasions entail both colonization of a new habitat and escape from pathogens. This is a broad observation; a good next step would be to directly compare natural selection acting on invasive plants in their native and introduced ranges. Through day-to-day processes like this, the specific ecology of a species can ultimately shape its evolutionary fate.
Reference
Blumenthal, D., Mitchell, C., Pysek, P., & Jarosik, V. (2009). Synergy between pathogen release and resource availability in plant invasion. Proc.Nat. Acad. Sci. USA, 106 (19), 7899-904 DOI: 10.1073/pnas.0812607106
Vellend, M., Harmon, L., Lockwood, J., Mayfield, M., Hughes, A., Wares, J., & Sax, D. (2007). Effects of exotic species on evolutionary diversification. Trends Ecol. & Evol., 22 (9), 481-8 DOI: 10.1016/j.tree.2007.02.017
Seed dispersal by ants: A lousy way to travel, a good way to diversify
New in the always open-access PLoS One: turns out that a great way to make new species, if you’re a plant, is to have your seeds dispersed by ants. This is because ants aren’t very good at seed dispersal.
Seed dispersal by ants, or myrmecochory, works very much like dispersal by fruit-eating birds and mammals: ant-dispersed seeds typically have a fatty attachment, called an elaiosome, that looks tasty to ants. Ants collect elaiosome-bearing seeds, bring them back to their nest, pry off the tasty bit, and then discard the rest of the seed. This leaves the seed safely underground in an ant-midden, ready to germinate — a great way to dodge seed-eating critters and avoid competition from its parent plant and siblings [$-a].
I didn’t learn about myrmecochory until after I’d finished undergrad — which is surprising, because it was going on under right my nose every time I went out into the Appalachian woods near campus. Lots of wildflowers [$-a] have ant-dispersed seeds, including bloodroot, touch-me-not, and good old trillium. It’s an extremely popular dispersal mechanism, having evolved independently multiple times on every continent except Antarctica. Really, me not knowing about myrmecochory is kind of like not knowing about fruit!
Ant dispersal is also associated with increased species diversity. In the new article, Lengyel et al. use a classic analysis method called sister group comparison to test the hypothesis that ant-dispersed plant groups contain more species than the most closely-related plant group. And they do, by a long way: on average, myrmecochorous groups contain twice as many species as their non-myrmecochorous sister groups. Why is this? As the authors conclude, it’s probably a side consequence of ant dispersal — ants don’t move seeds very far from where they collect them.
Recent evidence from genetic studies shows that limited seed dispersal in myrmecochory can lead to strong genetic structure within populations even at spatial scales as small as a few meters. The failure of myrmecochores to maintain gene flow across barriers may lead to reproductive isolation of sub-populations, which may facilitate speciation. [In-text references omitted.]
So myrmecochorous plants, like Appalachian salamanders [$-a] and tropical white-eyes [$-a], make lots of new species not because their unique characteristics give them some adaptive advantage (although, to be sure, there are advantages to ant dispersal), but because ants do a lousy job moving seeds between populations, leaving them free to follow their own evolutionary trajectories.
Lengyel et al. argue that myrmecochory is a key innovation, a trait that helps a group of organisms spread and diversify in the process evolutionary biologists call adaptive radiation. Based on their results, I have to agree — ant dispersal is strongly associated with evolutionary diversification. But the speciation that myrmecochory promotes is an accident, a side effect. We often think of key innovations promoting speciation by adaptive means, by allowing one group of species to outcompete others. Clearly, however, a key innovation can also be a trait that makes the accident of speciation a little more likely.
References
Beattie, A.J., & Culver, D.C. (1981). The guild of myrmecochores in the herbaceous flora of West Virginia forests. Ecology, 62, 107-15 DOI: http://www.jstor.org/pss/1936674
Giladi, I. (2006). Choosing benefits or partners: a review of the evidence for the evolution of myrmecochory. Oikos, 112 (3), 481-92 DOI: 10.1111/j.0030-1299.2006.14258.x
Kozak, K., Weisrock, D., & Larson, A. (2006). Rapid lineage accumulation in a non-adaptive radiation: phylogenetic analysis of diversification rates in eastern North American woodland salamanders (Plethodontidae: Plethodon). Proc. R. Soc. B, 273 (1586), 539-46 DOI: 10.1098/rspb.2005.3326
Lengyel, S., Gove, A., Latimer, A., Majer, J., & Dunn, R. (2009). Ants sow the seeds of global diversification in flowering plants. PLoS ONE, 4 (5) DOI: 10.1371/journal.pone.0005480
Moyle, R., Filardi, C., Smith, C., & Diamond, J. (2009). Explosive Pleistocene diversification and hemispheric expansion of a “great speciator.” Proc. Nat. Acad. Sci. USA, 106 (6), 1863-8 DOI: 10.1073/pnas.0809861105
Carnival of Evolution #11 at Oh, For the Love of Science
The 11th monthly Carnival of Evolution is up at Oh, For the Love of Science. Following Miriam Goldstein’s lead from last month, there’s a cute narrative framing for a long list of interesting posts, ranging from natural history to science education strategies. Lots to read in between undergrad research papers this weekend!
Berry Go Round #16 at Quiche Morraine
The 16th edition botanical blog carnival Berry Go Round is now online at Quiche Morraine, with posts on yuccas and ocotillo, the evolutionary origins of land plants, truffle hunting – and my own recent piece on the possible breakdown of an ant-plant mutualism. Check it out!
Why are there so many weevils? Coevolution, maybe.
Asked what attributes of the Creator were manifest in the natural world, the 20th-century biologist J.B.S. Haldane is said to have replied, “an inordinate fondness for beetles.” Beetles are, indeed, the most diverse group of animals on earth, accounting for something less than 40 percent out of five to ten million arthropod species, according to one estimate [PDF]. Naturally, evolutionary biologists would like very much to know how there came to be so many beetles* — and a new paper in this week’s PNAS proposes to answer this question for the largest beetle groups, the weevils.
It seems unlikely to be a coincidence that beetles are widely involved in interactions with the most diverse group of land plants, the angiosperms. In a now-classic 1998 paper, which took Haldane’s apocryphal quip as its title, Brian Farrell presented good circumstantial evidence that living and feeding on flowering plants is associated with beetle diversity. Farrell compared the number of species in groups of angiosperm-feeding beetles with the number of species in closely-related groups of non-angiosperm-feeders, and found that angiosperm-feeding groups were more diverse by orders of magnitude [$-a].
A sample of weevil diversity
Photos by Charles Haynes,
janerc, nutmeg66, and
rizalis Malaysian Macro Team.
Interactions between beetles and their host plants could lead to hyper-diversity in two ways. The evolution of new plant defenses and herbivore counter-defenses could generate alternating cycles of diversification in each interacting group [PDF]. Under this process, diversification doesn’t really happen because of reciprocal natural selection between plant and herbivore — it occurs when plants “escape” their herbivores by virtue of a new defense mechanism, and when herbivores exploit a new food resource made available by innovative counter-defenses. Alternatively, plants and beetles might diversify more simultaneously, with natural selection from plants’ defenses actually driving the speciation of the insect populations that eat them, and vice-versa.
The new paper, on which Farrell is senior author, attempts to distinguish between these two possible scenarios [$-a] using a new phylogeny of the Curculionoidea, the superfamily of beetles more commonly known as weevils. Weevils are distinguished by the rostrum, a noselike appendage they use in feeding — and the estimated 220,000 weevil species feed on an enormous array of plant species. Using DNA sequence data, the paper’s authors reconstructed the evolutionary relationships between 135 weevil genera. They then calibrated the resulting evolutionary tree using the known dates of fossil weevils, so that they could compare the dates of origin of major weevil groups to the history of angiosperm diversification.
Based on this analysis, the oldest weevil groups had their origin millions of years before the first flowering plants. Many of the extant species in these groups still feed on gymnosperms, which predate flowering plants. The most diverse weevil families, which feed on angiosperms, did not emerge until well after the first flowering plants appear in the fossil record, and may not have diversified until angiosperms became the dominant land plants. This lag suggests that, at least on a very broad time scale, weevils diversified because of angiosperm diversity, but probably did not contribute much to creating that diversity:
Thus, the extraordinary taxonomic diversity of weevils appears to have been mediated predominantly by the presence of susceptible, abundant, and diverse host resources, and the ability of weevils to use those resources, rather than by the evolution of host taxa themselves.
In the strictest sense, then, it seems that coevolution isn’t responsible for weevil diversity — yet it is hard to conclude much from results at this broad scale. As weevils took advantage of the “ecological opportunity” created by angiosperm diversity, they would have created myriad opportunities for reciprocal natural selection. Patterns of strict-sense coevolution following the initial colonization of angiosperms may only be apparent over shorter time spans.
References
Ehrlich, P.R., & Raven, P.H. (1964). Butterflies and plants: A study in coevolution Evolution, 18, 586-608 DOI: http://www.jstor.org/stable/2406212
Farrell, B. (1998). “Inordinate Fondness” explained: Why are there so many beetles? Science, 281 (5376), 555-9 DOI: 10.1126/science.281.5376.555
McKenna, D., Sequeira, A., Marvaldi, A., & Farrell, B. (2009). Temporal lags and overlap in the diversification of weevils and flowering plants PNAS, 106 (17), 7083-8 DOI: 10.1073/pnas.0810618106
Ødegaard, F. (2000). How many species of arthropods? Erwin’s estimate revised Biol. J. of the Linn. Soc., 71 (4), 583-97 DOI: 10.1111/j.1095-8312.2000.tb01279.x
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* Apart, that is, from the untestable and ultimately unknowable preferences of any putative Creator.