john hawks weblog

paleoanthropology, genetics and evolution

plants

  • Tartar control and Neandertal plant use

    Tue, 2011-01-04 23:44 -- John Hawks

    Dental plaque is a biofilm made up of bacteria adhering to the enamel surface of the teeth. Plaque is soft but over many days can gradually calcify. The hardened plaque, called calculus (or dental tartar) can build up in layers. This forms an ideal surface for further plaque formation and can damage the connective tissue between the teeth and gums -- so dentists and dental hygienists work hard to remove tartar.

    Despite these risks to dental health, calculus formation is a natural process in populations of humans and animals. Teeth from archaeological sites often have calculus adhering to them. This calculus contains partially mineralized bacteria, organic material, epithelial cells and fragments of foods ingested by the individual during her life.

    Recently, archaeologists and paleontologists have begun to examine calculus samples microscopically to identify the traces of ancient foods. Phytoliths are microscopic structures made of silica or calcium oxalate, many of which are distinctive to species or genera of plants. These durable inorganic structures can persist for hundreds of thousands of years. Many kinds of plants store their starches in granules that can also persist over long periods of time. These granules differ in form among families of plants. Starch granules also display characteristic changes when they are heated or cooked in liquid. Hence they can provide evidence of cooking practices by ancient people.

    Amanda Henry and colleagues [1] scraped a bit of calculus off the teeth of Shanidar 3, from the north of Iraq sometime around 50,000 years ago. This skeleton is housed at the Smithsonian National Museum of Natural History, and figured in the 2009 story about a projectile wound between its ribs ("Real stories of the Neandertal CSI"). The Shanidar skeletal remains are generally called Neandertals on the basis of their morphology. This case is better for Shanidar than for near-contemporaries in the Levant such as Amud or Kebara, and may reflect connections with populations to the north in the Caucasus. No genetic sampling has been done on the Shanidar sample.

    Neandertals are known for a diet stereotype -- they ate a very high proportion of meat. This stereotype is rooted in fact: the majority of Neandertal sites show a clear reliance on large mammal acquisition. Bison, horse, red deer and other large mammals are represented, often with a statistical preference toward one of these species at a given site. The faunal remains from many Neandertal sites are consistent with an ambush hunting strategy, with a higher proportion of prime age adult animals than found among persistence hunters or scavengers. The stable isotope record in Neandertal teeth so far seems consistent with an estimated 90% or more meat consumption, leaving relatively little dietary intake from plant foods.

    This is an odd picture for a hunter-gatherer: all hunting and little gathering. Most living hunter-gatherers rely on plant foods to buffer the risks of hunting. Many eat far more calories from plants than from meat. Plant processing in the archaeological record is well-known from among the earliest archaeological traces ("Plant processing with early Oldowan tools") and continued throughout the Paleolithic. The question at hand is specific to Neandertals -- how dependent were they on plant foods, and how much would they have been specifically adapted to meat acquisition and consumption? Some high-latitude hunting groups, such as the Inuit, do maintain very high meat consumption, and the Neandertals may have relied on this strategy in Europe.

    In contrast to Europe, a pattern of plant exploitation has long been known in the Middle Paleolithic of the Levant, at sites like Amud and Kebara that many argue were occupied by Neandertals. For example, Marco Madella and colleagues [2] describe phytoliths from the soils of Amud Cave. They demonstrated that these people were probably gathering seeds from grasses, systematically enough to concentrate the phytoliths from mature grass panicles in the sediments. Efraim Lev and colleagues [3] described charred plant remains including legumes and pistachio nuts from the Mousterian levels of Kebara Cave. Middle Paleolithic people in the Levant were using plants in quantities as great as can be shown at any contemporary sites anywhere in the world.

    Henry and colleagues help to put the seeds in the mouths of the Shanidar Neandertals. The Shanidar 3 calculus samples yielded substantial evidence of barley consumption. Many of the starch granules were clearly cooked:

    The overall pattern of damage to the starch grains matches most closely with that caused by heating in the presence of water, such as during baking or boiling, rather than “dryer” forms of cooking like parching or popping (38). The finding of cooked Triticeae starches on the Shanidar teeth reinforces evidence from other studies (13) that suggest that Near Eastern Neanderthals cooked plant foods.

    They report that 42% of the starch granules on these teeth are consistent with damage from cooking, suggesting that cooking was a systematic strategy for plant exploitation in these people. They also find other plants besides grains -- including legume seeds and tubers of some kind, and phytoliths from date palms.

    In addition to the Shanidar 3 skeleton, Henry and colleagues examined calculus samples from the two skeletons from Spy, Belgium. This gave them the opportunity to examine plant consumption in a European context. These teeth included starch molecules in relatively large numbers, mostly derived from some kind of plant underground storage organs (USOs) which the authors tentatively identify as a water lily. At least one starch granule of a sorghum or related grass seed is also present, along with a few other unidentifiable starches and no phytoliths.

    The authors cannot conclude much about the importance of these plant foods to the overall diet. The remains of starch grains and phytoliths tell us about diet breadth but not the proportions of different foods. They do note that nitrogen stable isotopes are most informative about protein-rich food sources, so that a substantial consumption of starchy plants such as grains and USOs might be hidden by isotope analysis. Their main conclusion is about dietary flexibility and the sophistication of Neandertal foraging strategies:

    These lines of evidence [cooking and processing of grains] indicate Neanderthals were investing their time and labor in preparing plant foods in ways that increased their edibility and nutritional quality (24, 45). It should also be noted that date palms and possibly other un- identi␣ed plants have different harvest seasons than barley and legumes, a factor that may suggest that the Shanidar Neanderthals practiced seasonal rounds of collecting and scheduled returns to harvest areas. Overall, these data suggest that Nean- derthals were capable of complex food-gathering behaviors that included both hunting of large game animals and the harvesting and processing of plant foods.

    I expect that quite a bit more evidence from dental calculus will be forthcoming. The study of microfossils from a broader range of sites will help to give a picture of the local resource exploitation. It may not be possible to get an estimate of dietary proportions from these kinds of evidence, but I imagine that similar comparisons of calculus samples from other animals will provide some useful context for the human numbers. In addition, calculus has become a promising source of DNA recovery, from the epithelial cells trapped in its calcified matrix. This has a good chance of recovering ancient DNA from specimens that have not previously yielded any successful extraction.

    References

    1. Henry AG, Brooks AS, and Piperno DR. 2011. Microfossils in calculus demonstrate consumption of plants and cooked foods in Neanderthal diets (Shanidar III, Iraq; Spy I and II, Belgium). Proceedings of the National Academy of Sciences [Internet] 108:486–491. Available from: http://dx.doi.org/10.1073/pnas.1016868108
    2. Madella M. 2002. The Exploitation of Plant Resources by Neanderthals in Amud Cave (Israel): The Evidence from Phytolith Studies. Journal of Archaeological Science [Internet] 29:703–719. Available from: http://dx.doi.org/10.1006/jasc.2001.0743
    3. Lev E, Kislev M, and Baryosef O. 2005. Mousterian vegetal food in Kebara Cave, Mt. Carmel. Journal of Archaeological Science [Internet] 32:475–484. Available from: http://dx.doi.org/10.1016/j.jas.2004.11.006
    Tags: 
    plants
    Spy
    Europe
    Neandertals
    Shanidar
    teeth
    diet
    Iraq

  • Mailbag: Grasses and bedding

    Tue, 2009-12-22 09:12 -- John Hawks

    Re: MSA sorghum use and starches adhering to tools non-obvious for seed processing:

    Dear John Hawks: Just saw your item about grains of Sorghum on Ngalue
    cave articfacts, and the puzzle about the abundance of grass in the
    cave. It reminded me of the pleasant grass that covered the hut floors
    I slept on during a field trip to the mountains of Pakistan. Could the
    grass in the cave have had similar uses? (Just a thought, no reply
    needed).

    We have a lot of evidence of grasses being brought into caves because of phytoliths in grass leaves and stems. The Neandertals at Kebara cave, for example, were apparently bringing in grass to use for bedding or floor covering.

    In this case, it is starch granules that are adhering to the tools, which would be coming from the seeds themselves, if I understand correctly. That seems unlikely to be seeds that are just adhering to stems and leaves used for other purposes, it wouldn't be enough.

    Tags: 
    plants
    diet

  • Seeds of MSA diet breadth

    Sat, 2009-12-19 20:56 -- John Hawks

    Julio Mercader reports in a short Science paper that the MSA stone artifacts from Ngalue cave, Mozambique, preserve thousands of grains of sorghum starch, along with a few other grasses and palm pith.

    The role of starchy plants in early hominin diets and when the culinary processing of starches began have been difficult to track archaeologically. Seed collecting is conventionally perceived to have been an irrelevant activity among the Pleistocene foragers of southern Africa, on the grounds of both technological difficulty in the processing of grains and the belief that roots, fruits, and nuts, not cereals, were the basis for subsistence for the past 100,000 years and further back in time. A large assemblage of starch granules has been retrieved from the surfaces of Middle Stone Age stone tools from Mozambique, showing that early Homo sapiens relied on grass seeds starting at least 105,000 years ago, including those of sorghum grasses.

    This is another of those very interesting technical developments in archaeology. The use of grass seeds may not be surprising in itself. Some think that australopithecines were eating grass seeds for a substantial amount of their diet; some (notably Clifford Jolly and Jonathan Kingdon) have suggested that grass seeds were one of the resources that prompted the evolution of bipedality. The dental reduction in early humans doesn't argue strongly against seed consumption; they are an important part of the diet for many recent hunter-gatherers including Australians. But it's nice to see a direct confirmation that humans were gathering seeds relatively intensively.

    How intensive? Well, there is a slight problem:

    It is not clear why the tools should be mostly coated with grass starches and not so much with other types of starch. It is possible that high-starch–bearing grass refuse built up considerably in the cave’s main chamber at times of human occupation, thus coating both tools that were used in the processing of grass seeds and others that were not.

    Hmmm. On the one hand, that means pretty intensive grass collection. On the other, if such a substantial fraction of the actual sedimentary debris in the cave was composed of anthropogenic plant waste, it's probably not possible to get an accurate picture of the importance of the seeds as a fraction of the diet. It's a data point: these people, living around this cave, used a lot of Sorghum grasses and processed seeds in some way with stone tools.

    It makes me wonder about what non-stone implements they may have used. Winnowing baskets?

    References:

    Mercader J. 2009. Mozambican grass seed consumption during the Middle Stone Age. Science 326:1680-1683. doi:10.1126/science.1173966

    Tags: 
    plants
    Africa
    MSA
    Mozambique
    Late Pleistocene
    diet

  • The fishy spaces of the Middle Pleistocene

    Fri, 2009-12-18 10:27 -- John Hawks

    In Science this week, Nira Alperson-Afil and colleagues report on recent excavations at Gesher Benot Ya'aqov, Israel. I saw some of this research presented at a conference, and I thought it was quite amazing to see the preservation of organic materials at this site. The Science paper is a good summary of the high points, presented in a very readable way.

    About the site:

    Gesher Benot Ya’aqov is located on the shores of the paleo–Lake Hula in the northern Jordan Valley in the Dead Sea Rift (7). The Early to Middle Pleistocene sediments document an oscillating freshwater lake and represent some 100,000 years of hominin occupation (Oxygen Isotope Stages 18–20) dating to 790,000 years ago (8, 9). Fourteen archaeological horizons indicate that Acheulian hominins repeatedly occupied the lake margins, where they skillfully produced stone tools, systematically butchered and exploited animals, gathered plant food, and controlled fire (7, 10–15).

    The current paper reports on a single occupation level, characterized by a hearth feature and associated plant, animal, and artifactual remains. Some interesting things:

    1. The plant remains:

    Although most taxa indicate wet habitats (e.g., lakes, lake margins, swamps, and near streams), the abundant fruit remains of woodland species such as olive, oak, and officinal storax (Styrax officinalis) imply human involvement, as their habitat was likely located some distance from the lake shore. Edible plants include oak acorns, prickly water lily (Euryale ferox) seeds, and water chestnut (Trapa natans) fruits; these were probably staple foods because of the nutritive value of their starchy nuts. Through roasting, the inedible shell of the nuts can easily be peeled and the tannin content of the acorns reduced. The fruits of the wild grapevine (Vitis sylvestris) and olive, and the leaves of the white beet (Beta vulgaris) and holy thistle (Silybum marianum), may also have been consumed.

    2. Crabs:

    The 17 crab specimens [minimum number of individuals (MNI) = 4 (22)], identified as the extant Potamon potamios, include pieces of the two asymmetric chelipeds, each with a distinctive form of the movable (upper) and fixed (lower) pincer....Of the seven pincers of the large cheliped present in Level 2, six occur around the hearth. These are the only crab remains in this area (fig. S4) (23).

    What's not to like about people eating crabs?

    3. Spatial patterning. There are two distinct areas of the horizon with anthropogenic activities -- the hearth and a second cluster of tools and stone waste flakes, I'm not very excited about the spatial distribution of activities. The story in the news is about how ancient humans knew how to "keep house" -- they're selling it as a major breakthrough in cognitive evolution.

    But the reason why we rarely have archaeological evidence about spatial patterning is that an archaeological horizon doesn't have very good temporal resolution. Here's an alternative scenario to account for the spatial pattern of remains in this horizon: One day, some people came, made tools and ate some fish. Three weeks later, some other people were in the same area, and they stayed for a few days, made a fire, did a bunch of other stuff.

    That's pretty much the spatial pattern that I would find if I went back home to Kansas and checked out campsites around the shore of the local reservoir. Few campsites are occupied for very long, and different people use them over time, sometimes with a fire, often not. Sure, we're cognitively advanced. I'm just not convinced that the spatial distribution of our campsite trash is very good evidence about it.

    Here's what the paper includes about the spatial patterning:

    The evidence from Gesher Benot Ya’aqov suggests that early Middle Pleistocene hominins carried out different activities at discrete locations. The designation of different areas for different activities indicates a formalized conceptualization of living space, often considered to reflect sophisticated cognition and thought to be unique to Homo sapiens (3). Modern use of space requires social organization and communication between group members, and is thought to involve kinship, gender, age, status, and skill (2).

    I think this is weak on two grounds -- first because the archaeology is poor evidence about the formal conception of living space, and second because it's not obvious that there's anything very unique about it.

    Why not unique? Any animal that can make a structure must have some capacity to pattern spatial activities -- if they don't, there's going to be poop everywhere. Conditioned on the fact that a human social group is sharing a single space, and group members are doing more than one activity, I don't see how you would ever expect to find a uniform scatter of evidence of these activities. There will always be some kind of spatial pattern, from the mere fact that two people can't occupy the same space at the same time.

    Remember that Gesher Benot Ya'aqov provides the earliest good evidence of human-controlled fire. It's no coincidence that "spatial patterning" should be found with a fire -- anything that people did anywhere other than by the fire is automatically evidence of a pattern.

    4. Fish. Now if there is one big reason why the spatial patterning is useful, it's the interpretation of the fish remains. It's not in the least bit surprising that there would be a lot of fish remains on an ancient lakeshore. But the remains are clustered into two distinct parts of the site, which happen to be the very two locations that humans were clearly using.

    In other words, once you accept that the archaeology gives you some evidence of where the people were within the site, you can test for association of the fauna and plant remains with the people. The crabs aren't all around the fire because of a failed attempt to stay warm at night; the people brought them there and ate them. The fish remains are clustered around the fire and flintknapping areas because people were eating them.

    Here's a good moral of the Gesher Benot Ya'aqov story: It's now past time to stop talking about whether "pre-modern" humans used aquatic resources. They did, sometimes intensively. I never understood why this argument about seafood and modern humans ever got any traction. We've known for twenty years that coastal Neandertals ate shellfish. We also have known from the numbers in caves near the coast that people never seem to have transported them very far inland. So there was a good reason why you didn't see more evidence of seafood; there just weren't that many sites very near the coast.

    So why was it news when a bunch of coastal African sites started producing evidence of shellfish consumption? Evidence that we already had for coastal Neandertals? I don't understand. Well, here we have people eating crabs and lots and lots of fish, 800,000 years ago. We can add the paper by Jose Joordens and colleagues earlier this year about Trinil (I reviewed it in "The shells of Trinil"), a million years ago or more.

    Another reason why Gesher Benot Ya'aqov is interesting: outside Africa, Middle Pleistocene sites (and Late Pleistocene sites, for that matter) have a fairly extreme bias toward caves and rock shelters. Caves can preserve evidence of within-site spatial patterns, and certainly offer some exceptional opportunities to track human activity over long periods of time. However, humans aren't very likely to have schlepped hundreds of fish from a lakeshore into some remote cave.

    UPDATE (2009-12-18) Thanks to a reader who pointed out a hanging omission; I corrected the text.

    References:

    Alperson-Afil N, Sharon G, Kislev M, Melamed Y, Zohar I, Ashkenazi S, Rabinovich R, Biton R, Werker E, Hartman G, Feibel C, Goren-Inbar N. 2009. Spatial Organization of Hominin Activities at Gesher Benot Ya’aqov, Israel. Science 326:1677-1680. doi:10.1126/science.1180695

    Tags: 
    diet
    Israel
    Middle Pleistocene
    Gesher Benot Ya'aqov
    Levant
    plants

  • Middle Stone Age bed and breakfast

    Thu, 2009-10-15 00:39 -- John Hawks

    On occasion, I point out interesting findings from archaeological chemistry and microscopic study of site formation processes. Last month, I pointed to the ability to distinguish animal and plant fat residues on ancient artifacts. Before that, there was the discovery of flax fibers from the Upper Paleolithic of Dzudzuana Cave, Georgia.

    In July, a paper by Paul Goldberg and colleagues described the "micromorphology" of the sediments from Middle Stone Age levels of Sibudu Cave, South Africa. The excavations at Sibudu have been able to distinguish many distinct stratigraphic units with distinctive spatial locations and compositions. Micromorphology involves looking at these sediments in microscopic detail, picking out small grains of crushed bone, charcoal, plant fibers, phytoliths, and other materials.

    Goldberg and his colleagues were able to make some very cool observations. For one thing, they have charred drips of broiled grease:

    Two types of amorphous organic combustion remains were identified in samples from Sibudu: a type with a typically vesicular texture and a type with a cracked texture. The first type was found as isolated bodies, subrounded with a diameter of 10 µm to 1 mm, and they exhibited no evidence of cell structure. Bubbles or vesicles give the bodies a highly porous nature, and they are often thin walled. The microstructure of these homogenous or finely heterogeneous isotropic particles and their droplet-shaped occurrence suggest that these bodies were originally fluid and that they underwent a degassing process but have since hardened. These bodies resemble char and are probably derived from the burning of flesh or animal fat (104-105).

    Mmmmm...MSA barbecue. The other type of "amorphous organic combustion remains" are charred resins, from trees or seeds. Mmmm...MSA smokehouse.

    Second, they have beds.

    Because of its long fibrous nature, it seems that this material consists of herbaceous plants, possibly some type of sedge, reed, or grass. There is no evidence to suggest that this plant would have grown naturally in the rock shelter, and the presence of clay aggregates derived from the river valley found in association with the laminated plant fibers implies that the grass or reed was transported to the cave from the nearby Tongati River by the shelter’s inhabitants.

    The compact and laminated structure of the organic fibers in this microfacies also suggests that, once brought to the cave, the grass or reed was subjected to compaction, most likely through trampling. Further evidence supporting the interpretation of trampling is seen in the stringers of charcoal, clay aggregates, and burnt bone that define horizontal and subhorizontal surfaces on top of and within the laminated organic fibrous material. Pieces of éboulis and lithic fragments also define surfaces wi thin the microfacies. The fact that this grass or reed was transported to the cave by humans and that once there it was influenced by human trampling suggests that this microfacies represents a type of constructed bedding. If this is the case, then Sibudu contains the oldest evidence for constructed bedding, significantly older than that reported at the open-air site of Ohalo in Israel (Nadel et al. 2004).

    The bedding material was in some instances burned, in some instances swept or trampled in such a way that the regular alignment of the phytoliths was jumbled and disrupted. They interpret this as efforts to "maintain" the site -- in other words, housekeeping:

    What seems a likely and reasonable scenario is that the original organic matter of this laminated layer of sedges, grass, or reeds was completely combusted, resulting in total ashing of the organic material. The calcitic ash in this microfacies was transformed through phosphatization, as evidenced by the presence of a few remnant pockets of phosphate in this microfacies. The fact that large crystals of gypsum often form directly below these phytolith layers provides suggestive evidence for the downward leaching of CO3- or P-rich solutions.

    Just an aside -- that is such interesting chemistry, like the organic materials and ash are melting down into the underlying deposits.

    The association between microfacies 2 and 4 suggests that the sedges, grass, or reeds that were brought into the cave for bedding were usually burned and probably by humans when they no longer used the bedding. This observation explains the sequence seen in samples SS-6 and SS-5 of laminated nonburnt fibrous organic material grading into laminated burnt fibrous organic material with phytoliths (microfacies laminated type 2B); the sequence is finally capped by a layer of laminated phytoliths.

    Why did they burn the stuff? The authors guess that they were trying to cut down on parasites:

    Together, this evidence shows that not only were the occupants of Sibudu bringing grass or reeds into the cave—likely for the construction of bedding—but they were periodically burning them, possibly as a means to remove pests or insects that had colonized the beds. (Smoldering goat dung and organic matter can be observed in many parts of the Middle East, including Hayonim, where tick removal is one of the important objectives; P. Goldberg 1992, personal observation.)

    The MSA at Sibudu dates to between 45,000 and 65,000 years ago, with the best evidence for bedding in the units that OSL puts around 50,000 years ago. The implications of the "site maintenance" and spatial characteristics of the site are mentioned in the paper's conclusion:

    Organization of living space, and particularly a deliberate use of space, has been suggested by Wadley (2001) and also Binford (1996) as an important trait of culturally modern behavior, reflecting a more complex social organization. While the evidence from the laminated units at Sibudu may reflect such organization, the lack of evidence for such spatial organization, such as is the case for the lower homogeneous layers at Sibudu, should not automatically suggest that occupation in these units was any less complex.

    If spatial organization of living space is a "modern" behavioral feature, it is one shared by Neandertals (I noted that briefly in a 2006 post). But then, it's shared by any number of invertebrates, also. I think the interpretation of this kind of behavior will have to wait until we have more sites investigated with comparable methods. As the introduction to the current paper points out, a lot of spatial information could be brought out of these micro-scale studies, if they were conducted routinely.

    References:

    Goldberg P, Miller CE, Schiegl S, Ligouis B, Berna F, Conard NJ, Wadley L. 2009. Bedding, hearths, and site maintenance in the Middle Stone Age of Sibudu Cave, KwaZulu-Natal, South Africa. Archaeol Anthropol Sci 1:95-122. doi:10.1007/s12520-009-0008-1

    Tags: 
    archaeology
    early modern
    South Africa
    plants
    Africa
    MSA
    Synopsis: 
    Archaeological chemistry at Sibudu turns up evidence of grease drippings and bedding.

  • Neandertals, plants, and fish

    Fri, 2009-09-18 08:30 -- John Hawks

    I don't read Spanish well, but I'm going to go ahead and link a news article in a Spanish journal about Neandertal diet and cooking at the Spanish site of El Salt:

    Uno de los casos es la aplicación de la química orgánica en el estudio de la estructura de combustión, conocida como el lugar en donde los neandertales hacían las hogueras para calentarse o cocinar. Ahora "estamos empezando a saber que asaban animales como el ciervo y la cabra", señala Galván. Han tenido conocimiento de esta información a través "de las grasas contenidas en las piedras quemadas procedentes del asado de estos animales", dijo la doctora. Asimismo, también han encontrado grasas de origen vegetal y restos de "espinas de peces quemadas". Y es que los neardentales sabían utilizar todas las materias primas que tenían a su alcance.

    One example [of a "quantum leap" in excavation techniques] is the application of organic chemistry to the study of hearths, used by the Neandertals for heat or cooking. Now "we are learning that they roasted animals like deer and goats," said [Bertila] Galvan. This information was obtained from "the fat contained in burned rocks from cooking these animals," said the doctor. In the same way, they also found fats of vegetal origin and remains of "burned fish bones." And that shows that the Neandertals knew how to use all the raw materials available to them.

    Not much more than that, but I think it's very interesting in light of last week's story about flax fibers. The point is that these microscopic and chemical excavation techniques are able to find some surprising information -- a process in archaeology that is mirroring the application of similar techniques to dinosaurs. Results like these show the great promise of such analysis, or the reanalysis of existing samples. It seems like a very propitious time to be trained in chemical techniques to apply to archaeological sites.

    Julien Riel-Salvatore has a little bit of context, Anthropology.net has more, and Martín Cagliani has the most direct discussion, although that does raise the Spanish language problem again!

    I'll be waiting for confirmation from other reports from this site, and hope that we can see some replication.

    Tags: 
    Europe
    Mousterian
    plants
    diet
    Middle Paleolithic
    Neandertals
    Spain

  • Weeds and climate change

    Tue, 2008-07-08 23:28 -- John Hawks

    On the subject of weeds and biological invasions, touched on in this week's "Practical Evolution" essay, the NY Times ran a long piece last week about the relationship of weeds and climate change. The story, by Tom Christopher, covers the work of Lewis Ziska, who grows weed plants in different carbon dioxide concentrations. One of the messages is that weeds are poised to take advantage of changes in the environment because of their genetic diversity, while we have bred diversity out of crop plants.

    I enjoyed the discussion of cheatgrass (common in Western Kansas) and this passage about the work of Andrew MacDougall, of the University of Guelph, on an oak savanna in British Columbia:

    MacDougall concluded that rather than serving as drivers of change, the foreign grasses were functioning more in the role of passengers, merely filling in as the natives disappeared. In fact, the foreigners seemed to be serving a stabilizing role. By blocking light from reaching the soil, they inhibited the germination of tree and shrub seeds. Keeping the brush at bay in this fashion preserved the open character of the savanna habitat so that the remnants of the original savanna wildflowers, grasses and wildlife could at least survive. In light of these findings, MacDougall says, he came to believe that the primary cause of the native flora’s decline was human intervention. Before European settlement, fire periodically cleansed the soil surface of dead plant material. Suppression of fire since settlement had allowed a thick layer of litter to accumulate, and the foreign grasses cope better with this than do the natives.

    The end of the story mentions the prospect of kudzu-derived ethanol, as well as introgression of wild plant genes into crops. What more could you ask for?

    Tags: 
    climate
    plants

  • Weed species (part 1)

    Mon, 2008-07-07 20:26 -- John Hawks

    This is the first in a series of essays titled, "Practical Evolution." Here are links to the whole series and the series introduction. I've decided to break the articles up into two parts, so that a full essay will appear in two successive weeks. So if you enjoy the current installment, by all means come back on Friday, when I will follow the threads of dispersal by way of an obnoxious animal pest right back to hominids.

    Dandelion seeds

    Probably the most well-known weed species in America is the one growing so prolifically in my yard: the dandelion. Any kid can tell you why dandelions spread so quickly: Their seeds are carried by the wind.

    But for all their blowing of the puff-heads, as Goodwin calls them, my kids haven't noticed what seems so obvious to me. Dandelions are almost indestructible. Pull them up and their leaves will dry, but the flowers on the dead-looking plants continue to develop seeds and puff out. Leave just a little of the taproot in the ground, and new leaves will sprout hydra-like from all directions. Gretchen drenched a goodly portion of our sidewalk dandelions last year with boiling water, wilting the leaves like spinach. Some of them really died. The rest sprouted right back.

    Even when we're talking about dainty little parasol-like seeds, much depends on the size of the whole package. The part that looks like a "seed," a botanist calls an achene, which is technically a kind of fruit. The sunflower seeds that teenagers spit at softball games are fruits, too, but don't try telling that to their mothers. The dandelion's parachute, or pappus, is connected to the achene by one long shaft. A large, massive achene will tend to fall faster and won't blow very far except in a very strong wind. Sunflowers and coneflowers go for the big achene strategy, which the winter-resident birds really appreciate come January. Dandelions go the opposite way: small achene, big pappus. That's why weedy coneflowers are only a problem within five feet of my perennial beds, while weedy dandelions are in every disturbed field in the country.

    Barkley Sound, Broken Island group

    Barkley Sound, Broken Islands group

    A remarkable botanical field study has been carried out over the last twenty-seven years by Martin Cody, in Barkley Sound, British Columbia. In 1981, Cody began keeping track of the plants on a few of the small islands that dot the Sound. After a few years, the study had changed into something grander than a simple plant census: Cody's 2006 book on the subject begins, "This is a book about a field study that just grew and grew." For more than twenty years, Cody and his students have counted plants on hundreds of tiny islands, most under a few hundred square meters in size. Many of these islands are so small that a species may disappear from them entirely, only to be re-established later by new colonists. With his careful censuses, Cody could determine when populations were newly founded, and when they became locally extinct.

    By 1996, Cody had amassed information about weedy plants that had colonized dozens of the tiny islands of Barkley Sound. These plants all originally had come from Europe, including dandelions, their close relative cat's-ear (Hypochaeris radicata), wall lettuce (Mycelis (Lactuca) muralis), and woodland groundsel (Senecio sylvaticus). That means that all the plants are recent colonists, and they have all been carried to the islands by the wind, each using similar seed packages that include an achene and pappus.

    What makes a successful colonist? The first seeds to reach one of these tiny islands have done an exceptional thing: Wind has carried them across a large body of water to reach a tiny speck of virgin soil. Only the tiniest achenes tend to make this unlikely journey, and only if their parachutes are big enough to carry them.

    Cody and his student, Jacob Overton, took some of these seeds from both island and mainland populations and embarked on a series of experiments. First, they set a stopwatch and dropped the seeds to see how fast they fell. The results of this seed drop were pretty much the same as when kids tie little parachutes onto green army men. Generally, the bigger the parachute, the slower the descent. But if the achene is small enough, even a very small pappus may be enough to keep it aloft. The key is the ratio: the volume of the pappus compared to that of the achene.

    Wall lettuce seedhead

    Seed head of wall lettuce (Mycelis muralis) with five achenes remaining

    There's nothing very surprising about this: The wind carries small seeds farther, particularly if they have big parachutes. Plants that bear this kind of seed have only a tiny chance of colonizing an island. But plants with big achenes and little parachutes may have no chance of colonizing new islands at all. So by sorting seeds, the wind and sea also sort the genes of the colonizing plants.

    Indeed, Cody and Overton found that new populations of these plants on islands produce the kind of seeds that occasionally skip across to new islands. For example, the youngest island populations of wall lettuce — less than four years old — have the tiniest achenes. Their parachutes are small, too, but they generate a lot of loft. If the seeds of mainland wall lettuce are like a green army man tied to a kleenex, the first island colonists are like a BB stuck to a postage stamp.

    Wall lettuce populations that manage to stick around on an island make a change. After ten years, they are producing achenes just as big as the mainland populations. But instead of big, fluffy parachutes, they grow smaller, stunted ones. Like green army men tied to smaller and smaller postage stamps, these seeds aren't made for dispersal. They drop fast, right next door to their parents.

    More dispersal is not always better, even if you're a weed. A windy day might carry your seeds far away, but what if you live on a tiny island? Your seeds will be carried right off into the ocean. Worse, even if you live on a large island in Barkley Sound, there may be only a small strip of good habitat. Too far toward the water and you're drowned by storm surges; too far inland and you have to compete with larger, more established plants. For these island weeds, the plants that disperse the least have more new seedlings.

    Cat's-ear

    Cat's-ear

    Adaptable as they are, dandelions have done a poor job colonizing these tiny islands. The best of these weed species have managed to invade one island in five. Dandelions are found on less than a fourth that many. Despite their fame for being windblown, dandelion seeds actually drop faster than the wispy seeds of the other species on these islands. The rare colonists that do reach an island tend to last less than two years.

    Their relative, cat's-ear, has done better. Since it carries its flowers higher, on branched stems, cat's-ear makes more headway into grassy areas away from the beach, giving it a bit more area to work with. As a result, its populations become extinct only a third as much turnover as dandelions. LIke the wall lettuce, the island cat's-ear populations are different from the mainland, sporting big achenes and small parachutes. Dropping their seeds nearby seems to make a big difference as to whether their populations will persist.

    Natural selection seems to have changed the island populations of both these weeds, favoring lower dispersal. But how do we know it is really selection on dispersal, and not just something about the islands affecting seed size?

    Woodland groundsel, another of the weedy asters on the islands, provides one natural experiment. Cody and Overton dropped dozens of groundsel seeds from twenty populations, and found that the sizes of achenes and pappus made no difference at all to the speed that the seeds drop to the ground. Small groundsel achenes drop just as fast as large ones, no less than if Galileo had dropped them from the Leaning Tower. Even if selection had favored faster drop times, woodland groundsel achenes and pappus volumes should not have changed. And in fact, Cody and Overton found that they didn't change: island populations of woodland groundsel have proportions almost exactly the same as the mainland.

    Another experiment has been carried out in the most unlikely location. Hawksbeards (genus Crepis), also relatives of the dandelion, are common weeds spread across the Old World and into North America. Like dandelions, the hawksbeard Crepis sancta can colonize urban habitat, including the tiny patches of bare ground that cities leave in sidewalks for trees to grow. These are the urban equivalent of the tiny islands in Barkley Sound: sometimes far apart, they are separated by stretches of inhospitable concrete. In Montpelier, France, during the last few years Pierre-Olivier Cheptou and his colleagues have performed the urban version of Martin Cody's work: counting plants and collecting seeds from the small hawksbeard colonies around the city. What it lacks in rugged charm, Montpelier makes up in record-keeping — knowing when the sidewalks were built, the researchers could work out the ages of different hawksbeard populations.

    Unlike dandelions, C. sancta produces two different kinds of seeds: one with the achene and pappus carried on the wind, and another with no pappus at all. These seeds without parachutes do not go far; they scatter near the mother plant. That makes them bad for colonizing new patches of ground, but really good for establishing a sustained presence in one patch.

    Compared to the rural C. sancta population, urban plants have more of the no-parachute seeds. These plants cast fewer seeds to the wind, and drop more on the ground to sprout nearby. Like the islands of Barkley Sound, these urban islands select for low dispersal.

    Cheptou and his coworkers were able to go farther than Cody and Overton: They planted the seeds from urban patches in a greenhouse alongside seeds from the rural areas outside of town. The urban/rural differences were still present in the greenhouse plants, showing that growing in a small patch is not enough to change the seeds. Instead, the differences are caused by differences in genes, the result of selection on the urban plants.

    What is more, they grew their own small patches of C. sancta, replicating the conditions of the urban plants. The seed heads of the real urban populations were the equal of those after 12 to 13 generations of selection on their fake urban patches. This rapid pace of selection on the urban plants matches that on the islands of Barkley Sound, where wall lettuce and cat's-ear repeatedly showed strongest changes after only a few generations.

    Cody and Overton 1996, Figure 1

    The cycle of founding new island populations and their subsequent extinction.

    Colonization is a filter: Only certain individuals may have the right combination of traits to cross a significant barrier and inhabit a new place. But the traits that enable individuals to make such crossings are not always very good for staying in the new place.

    For certain species, selection yields a complicated pattern. A plant that is a local failure may be a global success, as it casts its seeds to far-off places, a few of which may grow into significant new populations. Such high-dispersing plants may do very poorly as a new population grows, but may be the only ones with a chance to escape when local extinction looms. High-dispersing individuals are also the ones most likely to carry their genes from one population to another, providing the potential for adaptations across the species as a whole.

    Without dispersal, a species may become moribund, unable to change in the face of new challenges. At the extreme, this has been called "evolutionary suicide." Traits that are selected locally, because they enhance the reproduction of individuals in the species, may nevertheless doom the species as a whole by reducing adaptability to changing conditions. The cost of maintaining the ability to change is a constant unstable balance between dispersal and staying power.

    For the weeds of Barkley Sound, this unstable balance ultimately favors the colonizers. New island populations do not last long. The local selection in favor of lower dispersal helps them to persist for a time, but on the scale of centuries all of them will fail, only to be replaced by new colonists from elsewhere.

    Friday: We turn from plant parachutes to animal dispersal mechanisms. And by "mechanisms" I mean, "legs."

    Notes:

    I am indebted to George Cox, whose excellent Alien Species and Evolution: The Evolutionary Ecology of Exotic Plants, Animals, Microbes, and Interacting Native Species (2004) gives a good account of Cody and Overton's study of island weeds. I can't say enough good things about this book.

    Martin Cody's book, Plants on Islands: Diversity And Dynamics on a Continental Archipelago, gives an account of the history of his field project, as well as reviews of the many areas of ecology that it has helped to illuminate.

    Many readers may notice that there is another factor besides low dispersal potential by which selection might favor large achenes in these plant species. Larger seeds may be advantageous because, with more stored energy, they may tend to germinate more readily or grow more quickly, thus making them more competitive. In this circumstance, selection would pertain to energy allocation or reproductive effort, rather than dispersal. I'll be discussing that form of selection in a later essay. In the present cases, dispersal is indicated as the target of selection, but seed size and energy allocation may also be involved.

    If seed morphology makes so little difference to drop time in Senecio sylvaticus, you may wonder what maintains achene size and pappus volume in this species. At least, that's what I wondered. Without more information it is hard to say, but I would hypothesize that achene and pappus volume may be genetically correlated in a way that is hard to select for one and not the other. In the other species, the selection on achene size and pappus volume is apparently antagonistic -- larger achenes are favored at the same time as smaller pappus. If these two traits are strongly genetically correlated, it becomes harder to select for lower dispersal. It would also be worth investigating whether other traits besides these volumes may have evolved in the island populations.

    The rapid evolution of urban hawksbeard (Crepis sancta) was examined by Cheptou et al. (2008). Pierre-Olivier Cheptou has a webpage at CEFE, including a description of Crepis sancta as a model system.

    Photo credits:

    Dandelion seeds: Photo by Piccolo Namek, on Wikimedia. GPL license.

    Barkley Sound: Photo by Sam Wilson (BaylorBear78) on Flickr. Creative commons noncommercial share-alike license.

    Cat's-ear: Photo by Ian Boyd, on Flickr. Creative Commons noncommercial attribution license.

    Wall lettuce: Photo by Mollivan John, on Flickr. Creative Commons noncommercial attribution license.

    Colonization figure: Figure 1 from Cody and Overton, 1996.

    References:

    Cheptou, P-O, Carrue O, Souifed S, Cantarel A. 2008. Rapid evolution of seed dispersal in an urban environment in the weed Crepis sancta. Proc Nat Acad Sci USA 105:3796-3799. doi:10.1073/pnas.0708446105

    Cody ML. 2006. Plants on Islands: Diversity and Dynamics on a Continental Archipelago. University of California Press, Berkeley, CA.

    Cody ML, Overton JM, 1996. Short-term evolution of reduced dispersal in island plant populations. J Ecol 84:53-61.

    Cox G. 2004. Alien Species and Evolution. Island Press, Washington.

    Tags: 
    island biogeography
    natural selection
    dispersal
    migration
    plants
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