[Paleopsych] SW: On Human Impacts on Ecosystems

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Anthropology: On Human Impacts on Ecosystems

[I left out several addresses in recent e-mails. Let me know what dates you 
missed if you want to get these articles.]

    The following points are made by Christopher N. Johnson (Science 2005
    1) What was the impact of early human populations on pristine
    ecosystems? Studies of this question have focused on the possibility
    that humans caused extinctions of large mammals. For example, the
    arrival of modern humans in the Americas ~11,000 years ago coincided
    with the disappearance of mammoths, ground sloths, and many other
    large mammals [1]. However, the role of humans is difficult to
    determine in this case because the climate was also changing rapidly
    as the last ice age came to an end; climate change, not human impact,
    may have caused the extinctions.
    2) Modern humans reached Australia much earlier. Just when they did is
    still debated, but occupation was widespread by 45,000 years ago and
    may have begun several thousand years earlier [2] -- well before the
    climatic upheavals at the end of the last glacial cycle. Australia
    should therefore provide a clear view of the ecological impacts of
    human arrival. But environmental changes following human arrival in
    Australia have been difficult to resolve, because very few precisely
    dated environmental records extend through the middle of the last
    glacial cycle. Recent work. (3) provides such a record based on diet
    reconstructions of the continent's two largest bird species. The
    results indicate that human arrival resulted in a profound
    environmental shift.
    3) Miller et al [3] studied past diets of the emu (Dromaius
    novaehollandiae) and an even larger flightless herbivorous bird, the
    extinct Genyornis newtoni, in the arid and semi-arid regions of the
    south Australian interior. By analyzing carbon isotopes in
    individually dated eggshells, they were able to compare the
    contributions of plants that use the C4 photosynthetic pathway (mainly
    tropical and arid-adapted grasses) and those that use the C3 pathway
    (most shrubs, trees, and nongrass herbs) to the diet of the birds that
    laid the eggs. Their collection of eggshells covers the past 140,000
    years, encompassing the whole of the last glacial cycle.
    4) Miller et al [3] found a sudden change in emu diet between 50,000
    and 45,000 years ago. Before 50,000 years ago, emus had variable
    diets, with a strong contribution from C4 plants; after 45,000 years
    ago, they ate mostly C3 plants. Genyornis eggshells were common before
    50,000 years ago, but they abruptly disappeared at the same time as
    the diet of the emu changed. Before then, Genyornis also ate a mixture
    of C3 and C4 plants, but its diet was much less variable than that of
    the emu through the same period, which suggests that it was a more
    specialized feeder.
    5) These results point to a major change in vegetation. Perhaps
    woodland mosaics, with plenty of grass, were converted into monotonous
    shrubland, or nutritious grasses were replaced by poor-quality
    species, forcing emus to increase their feeding on nongrass species.
    Miller et al [3] also measured carbon isotopes in wombat teeth,
    showing that they changed in the same way at the same time. Nowadays,
    wombats are mainly grazers; the switch in their diet from C4 grass to
    C3 shrubs in the middle of the last glacial period can only be
    explained by a huge change in vegetation.
    References (abridged):
    1. A. D. Barnosky, P. L. Koch, R. S. Feranec, S. L. Wing, A. B.
    Shabel, Science 306, [70] (2004)
    2. J. F. O'Connell, J. Allen, J. Archaeol. Sci. 31, 835 (2004)
    3. G. H. Miller et al., Science 309, 287 (2005)
    4. R. G. Roberts et al., Science 292, [1888] (2001)
    5. F. D. Pate, M. C. McDowell, R. T. Wells, A. M. Smith, Austral.
    Archaeol. 54, 53 (2002)
    Science http://www.sciencemag.org
    Related Material:
    The following points are made by C.M. O'Reilly et al (Nature 2003
    1) Although the effects of climate warming on the chemical and
    physical properties of lakes have been documented, biotic and
    ecosystem-scale responses to climate change have been only estimated
    or predicted by manipulations and models.
    2) Lake Tanganyika in Africa is a large (mean width, 50 km; mean
    length 650 km), deep (mean depth, 570 m; maximum depth, 1470 m) north
    south trending rift valley lake that is an important source of both
    nutrition and revenue to the bordering countries of Burundi, Tanzania,
    Zambia, and the Democratic Republic of Congo. The lake has
    historically supported one of the world's most productive pelagic
    fisheries, and the annual harvest in recent years has been estimated
    to be between 165,000 and 200,000 metric tons, with an equivalent
    value of tens of millions of US dollars.
    3) The lake is oligotrophic and permanently thermally stratified with
    an anoxic hypolimnion. During the cool windy season (May to
    September), strong southerly winds tilt the thermocline, causing
    upwelling of deeper nutrient-rich waters at the south end of the lake
    and initiating seiche activity. Cooling during this season also
    contributes to a weaker thermocline, and entrainment of deep
    nutrient-rich waters from the hypolimnion occurs in this time period.
    Overall, these mixing events provide the dominant source of some
    limiting nutrients (P, Si) to the surface waters and are important in
    maintaining the pelagic food web.
    4) The authors present evidence that climate warming is diminishing
    productivity in Lake Tanganyika. In parallel with regional warming
    patterns since the beginning of the twentieth century, a rise in
    surface-water temperature has increased the stability of the water
    column. A regional decrease in wind velocity has contributed to
    reduced mixing, decreasing deep-water nutrient upwelling and
    entrainment into surface waters. Carbon isotope records in sediment
    cores suggest that primary productivity may have decreased by about
    20%, implying a roughly 30% decrease in fish yields. The authors
    suggest their study provides evidence that the impact of regional
    effects of global climate change on aquatic ecosystem functions and
    services can be larger than that of local anthropogenic activity or
    Nature http://www.nature.com/nature
    Related Material:
    The following points are made by David Raffaelli (Science 2004
    1) The accelerated extinctions of species and changes in biodiversity
    are no longer disputed issues. Much effort has gone into quantifying
    biodiversity loss rates for particular animal and plant groups (1).
    Less clear, however, is the impact of such losses on ecosystems,
    especially when many different kinds of species of plants and animals
    are lost simultaneously (2). Yet policy-makers urgently need guidance
    on the effects of multispecies losses if they are to plan for and
    advise on the societal consequences of biodiversity changes. The
    ecological research community has been highly active in attempting to
    provide such guidance (3,4), but many challenges remain. Foremost
    among these is that most real extinction events are nonrandom with
    respect to species identity -- some species are more likely to go
    extinct than others -- whereas research studies often assume that
    extinctions are random.
    2) Solan et al (5) and Zavaleta and Hulvey (6), reporting on work in
    two very different types of ecosystem, reveal that the impact of
    nonrandom species extinctions on ecosystems is markedly different from
    that predicted by scenarios where extinctions are random. These
    studies bring us a step nearer to understanding the impact of
    nonrandom species losses on ecosystems and should help to provide
    policy-makers with a firmer basis for decision-making.
    3) The two studies examine very different habitats (marine versus
    terrestrial), each with different kinds of organisms (sea-bed
    invertebrates versus grassland plants), different ecosystem processes
    (sediment biogeochemistry versus resistance to invasion by exotic
    species), and different types of experimental approaches (data
    analysis and modeling versus controlled experimentation). So it is all
    the more interesting, for scientists and policy-makers alike, that
    both papers arrive at the same conclusion: Nonrandom extinction events
    have impacts on ecosystems that are quite different from those
    predicted by scenarios that assume species extinctions occur at
    4) Solan et al (5) combine into a model a well-documented data set of
    invertebrate communities in marine sediments off the coast of Galway,
    Ireland. This fusion, facilitated by the BIOMERGE initiative, enables
    the authors to predict what will happen to the cumulative effects of
    the small-scale sediment disturbances (bioturbation) caused by the
    movement, feeding, and respiration activities of all 139 species of
    clams, worms, sea urchins, brittle stars, and shrimps present in this
    system if species are lost through impacts such as overfishing,
    habitat destruction, and pollution. The authors scored each species
    for its body size, mobility, and mode of sediment mixing to calculate
    an index of bioturbation potential for different species combinations
    and for different degrees of species richness. In their model, either
    extinction scenarios could be random or losses could be ordered with
    respect to the sensitivity of species to environmental stress, body
    size, and abundance, traits that in turn reflect different kinds of
    References (abridged):
    1. See www.royalsoc.ac.uk/events
    2. See www.millenniumassessment.org/en/index.aspx
    3. M. Loreau et al., Science 294, 804 (2001)
    4. M. Loreau, S. Naeem, P. Inchausti, Biodiversity and Ecosystem
    Functioning (Oxford Univ. Press, Oxford, 2002)
    5. M. Solan et al., Science 306, 1177 (2004)
    6. E. S. Zavaleta, K. B. Hulvey, Science 306, 1175 (2004)
    Science http://www.sciencemag.org

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