ScienceWeek

SCIENCEWEEK

ScienceWeek
August 15, 2003
Vol. 7 Number 33A

An Online Digest of Research in the Sciences

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I am a passenger on the spaceship, Earth.
Richard Buckminster Fuller (1895-1983)

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Section 1

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Part A - Symposium: Ecosystems

1. Introduction
2. Plant Roots and Ecological Diversity
3. Urbanization Effects on Tree Growth in the Vicinity of New
York City
4. Scaling Metabolism from Organisms To Ecosystems
5. Biodiversity: A Global View Of Forest Canopies
6. On Multidecadal Change in the Pacific Ocean
7. On the Economic Value of Ecological Stability

Notices and Subscription Information

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Section 2

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1. INTRODUCTION

ON THE BIOSPHERE AND ECOSPHERE

Individual organisms interact with their biotic environment
(other individuals) and with their abiotic environment (the non-
living parts of the ecosphere). A group of interacting
individuals belonging to the same species is a "population"; a
group of interacting individuals belonging to different species
is a "community". In both populations and communities, the
"groups" are tightly linked associations of organisms, rather
than loose associations of individuals that happen to live in the
same neighbourhood.

Communities are sometimes called "biocoenoses". Individuals and
communities live in particular physical surroundings known as the
"biotope". "Ecosystems" are individuals or communities
interacting with their physical environment -- biocoenoses plus
biotopes. Biotopes will normally contain several distinct
habitats that occur as patches in the landscape. The patch
distribution was originally determined largely by topography and
soils, but human activities have produced "holes" in biotopes so
that many patches are now isolated. Each species living in a
patch has particular habitat requirements.

Communities and ecosystems range from microscopic to global. The
tiers in the community hierarchy are local communities,
communities, biomes, zonobiomes, and the biosphere. There are no
specific names for equivalent units in the ecosystem hierarchy,
but the terms "local ecosystems" (which occupy landscape
patches), ecosystems, "geozonal ecosystems" (equivalent to
zonobiomes), and "ecosphere" could be used.

The biosphere is made of living things. It includes lower parts
of the air, the oceans, seas, lakes, and rivers, the land
surface, and the soil. Some organisms live deep in the
lithosphere and in sediments beneath the deep ocean floor. All
life interacts with its surroundings. This pervasive interaction
of living and non-living kingdoms creates and conserves an
ecosphere, a zone fit for terrestrial-type life forms.

A "biome" is a regional community of animals and plants. The
humid temperate zone of western Europe is an example. It supports
a deciduous forest biome, with areas of heath and moorland. The
equivalent term for a community of plants is a "plant formation".
A community of animals at the biome scale has no special
designation; it is simply an animal community. Smaller
communities within biomes are normally based on plant
distribution and are called "plant associations". A "zonobiome"
consists of like biomes. Its phytogeographical equivalent is a
formation-type. It has no zoogeographical counterpart. The broad-
leaved temperate forests of western Europe, North America,
eastern Asia, southern Chile, south-east Australia and Tasmania,
and most of New Zealand comprise a humid temperate zonobiome.
Zonobiomes are also called "ecozones" and "ecoregions".

Adapted from: Richard J. Hugget: Environmental Change. Routledge
1997, p.262.
More information at:
http://www.amazon.com/exec/obidos/ASIN/041514521X/scienceweek

ON POPULATION ECOLOGY

The following points are made by O.N. Bjørnstad and B.T. Grenfell
(Science 2001 293:638):

1) The last 100 years of studies in population ecology has been
dominated by a nested set of debates regarding the importance of
various dynamical forces.

2) The first controversy concerned the relative impact of biotic
versus abiotic control of population fluctuations. The key
question was the relative importance of "noise" (small-scale,
high-frequency stochastic influences) versus climatic forcing
(larger-scale, often lower-frequency signals) versus nonlinear
interactions between individuals of the same or different
species.

3) The second question concerned the impact of intrinsic (i.e.,
intraspecific) processes, as opposed to extrinsic or community-
level interactions, an argument that has been particularly heated
with reference to population cycles.

4) A third debate, nested within the latter, concerns the
"dimensionality" of population fluctuations; given that most
populations are embedded in rich communities and affected by
numerous interspecific interactions, can simple (low-dimensional)
models involving one or a few species capture the patterns of
fluctuations?

5) All these questions have been studied through a number of
detailed analyses of specific systems in which theoretical models
are linked with long-term studies (often 10 or more generations)
through time series analysis.

6) There has been much parallel and intertwined development of
these three dynamical themes, and history testifies to a
succession of popularity of the various positions (1). Crudely
summarized, early focus on extrinsic influences was replaced by
the "density-dependent paradigm" (2) in the 1950s and 1960s. This
accelerated in the late 1970s, with May's cri de coeur (3) about
the potential of dynamical complexity even in simple models,
leading to a focus in the 1980s on nonlinearity and the detection
of deterministic chaos (Taken's embedology, Lyapunov exponents,
etc.).

7) Research has focused on two fronts in the past decade: (i) the
impact of large-scale climatic forcing, coinciding with the rise
in popularity of climate change studies through the early 1990s,
and (ii) stochastic nonlinear models that combine the nonlinear
deterministic and (largely) linear stochastic theories. The goal
in synthesizing these approaches in recent years is to understand
how population fluctuations arise from the interplay of noise,
forcing, and nonlinear dynamics. The comparable importance of
deterministic and stochastic forces makes ecological dynamics
unique. In particular, the interaction between noise and
nonlinear determinism in ecological dynamics adds an extra level
of complexity compared with the largely stochastic dynamics of,
say, economic systems or the largely deterministic dynamics of
many physical and chemical processes.(4,5)

References (abridged):

1. S.E. Kingsland, Modeling Nature (Univ. of Chicago Press,
Chicago, 1985)

2. C.J. Krebs, Wildl. Res. 22, 1 (1995)

3. R.M. May, Nature 261, 459 (1976)

4. M.J. Caley et al., Annu. Rev. Ecol. Syst. 27, 477 (1996)

5. O.N. Bjørnstad, J.-M. Fromentin, N. C. Stenseth, J. Gjøsæter,
Proc. Natl. Acad. Sci. U.S.A. 96, 5066 (1999)

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2.. PLANT ROOTS AND ECOLOGICAL DIVERSITY

The following points are made by Peter D. Moore (Nature 2003
424:26):

1) The subterranean life of plants is not easily documented.
Take, for instance, the question of how root activities may
determine the composition and diversity of plant communities.
Above ground there is an observable struggle for light, but what
goes on in the soil may be of greater consequence in determining
diversity. Such is the conclusion reached by from recent
experiments(1).

2) Understanding the factors that control diversity is high on
the agenda for ecologists and conservationists, and different
factors evidently operate at different scales. For example, at a
continental scale the areas that have the greatest diversity of
species are often those with the highest levels of primary
production. The productive forests of the equatorial regions
support more species than do less productive systems in the cool
temperate zone. In North America, the areas that are richest in
tree species correspond to those regions (such as the south-east)
where forest productivity is highest(2). But at the habitat scale
this correlation does not apply. A reed bed may be highly
productive, but it is relatively non-diverse, and productive
grasslands are usually lower in plant diversity than less fertile
ones(3). Competition seems to lie at the heart of this problem,
with robust and productive plants eliminating those that are less
able to acquire the resources that they need. The most obvious of
these resources are sunlight, nutrient elements and water, the
first being tapped by shoots, the latter two by roots.

3) Ecological research tends to concentrate on above-ground
processes, because they are more straightforward to study.
Rajaniemi and colleagues(1), however, have attempted to
manipulate shoot and root competition separately so as to analyze
the influences on ecosystem diversity. The first conclusion from
these intact plot experiments is that fertilization does indeed
reduce plant diversity, as might be predicted for this scale of
study. However, when Rajaniemi et al. examined the relative
impacts of shoot competition and root competition on diversity,
they found that virtually all the depression in diversity is a
consequence of the effects of roots: the long-held assumption
that shoot growth and competition for light lead to exclusion of
species when productivity increases is not supported by this
work.(4,5)

References (abridged):

1. Rajaniemi, T. K., Allison, V. J. & Goldberg, D. E. J. Ecol.
91, 407-416 (2003)

2. Currie, D. J. & Paquin, V. Nature 329, 326-327 (1987)

3. Grime, J. P. Plant Strategies, Vegetation Processes, and
Ecosystem Properties (Wiley, Chichester, 2001)

4. Newman, E. I. Nature 244, 310-311 (1973)

5. Kemp, D. R. & King, W. McG. in Competition and Succession in
Pastures (eds Tow, P. G. & Lazenby, A.) 85-102 (CABI,
Wallingford, 2001)

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ROOT COMPETITION CAN CAUSE A DECLINE IN DIVERSITY WITH INCREASED
PRODUCTIVITY

The following points are made by T.K. Rajaniemi et al (J. Ecology
2003 91:407):

1) Plant community theory often invokes competition to explain
why species diversity declines as productivity increases.
Competition for all resources might become more intense and lead
to greater competitive exclusion or, alternatively, competition
for light only could become more intense and exclude poor light
competitors.

2) To test these hypotheses, the authors constructed communities
of seven old-field species using combined monocultures.
Constructs experienced no interspecific competition, only shoot
competition or only root competition, with and without
fertilizer. Diversity in these limited interaction communities
was compared to diversity in unfertilized and fertilized mixtures
of the seven species.

3) Diversity decreased with fertilization in mixtures and in
communities with only root competition. Shoot competition had
small effects on the community and did not contribute to changes
in diversity with fertilization.

4) The authors conclude: Root competition may strongly impact
plant community structure in unproductive communities where light
never becomes limiting, or under non-equilibrium conditions
following human disturbances.

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3. URBANIZATION EFFECTS ON TREE GROWTH IN THE VICINITY OF NEW
YORK CITY

The following points are made by J.W. Gregg et al (Nature 2003
424:183):

1) Urbanization of the globe is accelerating, with potentially
large impacts on vegetation in cities and surrounding areas.
Urban air contains high concentrations of many gaseous,
particulate and photochemical pollutants (such as NOx, HNO3, SO2,
H2SO4, O3 and volatile organic compounds)(1,5); and urban soils
are high in heavy metals and can be more hydrophobic and acidic
than surrounding rural environments(2). Although many of these
contaminants have detrimental effects on plant growth, urban
environments also have higher rates of nutrient and base-cation
deposition(1,5), warmer temperatures (urban "heat-island"
effect)(3), and increased CO2 concentrations(4) -- factors that
often, but not invariably, enhance plant growth.

2) Given the potential for interactions among all factors and the
relative absence of studies examining more than two or three
factors in combination, understanding the net effect of multiple
anthropogenic environmental changes in an urban environment and
the relative importance of the individual factors remains a major
challenge.

3) In summary: Plants in urban ecosystems are exposed to many
pollutants and higher temperatures, CO2 and nitrogen deposition
than plants in rural areas(1-5). Although each factor has a
detrimental or beneficial influence on plant growth, the net
effect of all factors and the key driving variables are unknown.
The authors grew the same cottonwood clone in urban and rural
sites and found that urban plant biomass was double that of rural
sites. Using soil transplants, nutrient budgets, chamber
experiments and multiple regression analyses, the authors
demonstrate that soils, temperature, CO2, nutrient deposition,
urban air pollutants and microclimatic variables could not
account for increased growth in the city. Rather, higher rural
ozone (O3) exposures reduced growth at rural sites. Urban
precursors fuel the reactions of O3 formation, but NOx scavenging
reactions resulted in lower cumulative urban O3 exposures
compared to agricultural and forested sites throughout the
northeastern USA. The authors suggest their study shows the
overriding effect of O3 despite a diversity of altered
environmental factors, reveals "footprints" of lower cumulative
urban O3 exposures amidst a background of higher regional
exposures, and shows a greater adverse effect of urban pollutant
emissions beyond the urban core.

References (abridged):

1. Lovett, G. M. et al. Atmospheric deposition to oak forests
along an urban-rural gradient. Environ. Sci. Technol. 34, 4294-
4300 (2000)

2. Pouyat, R. V., McDonnell, M. J. & Pickett, S. T. A. Soil
characteristics of oak stands along an urban-rural land-use
gradient. J. Environ. Qual. 24, 516-526 (1995)

3. Peterson, J. T. The Climate Of Cities: A Survey Of The Recent
Literature. NAPCA Pub. No. AP-59 (US Department of Health,
Education and Welfare, 1969)

4. Idso, C. D., Idso, S. B. & Balling, R. C. Jr An intensive two-
week study of an urban CO2 dome in Phoenix, Arizona, USA. Atmos.
Environ. 35, 995-1000 (2001)

5. Gatz, D. F. Urban precipitation chemistry: A review and
synthesis. Atmos. Environ. B Urban Atmos. 25, 1-16 (1991)

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ATMOSPHERIC DEPOSITION TO OAK FORESTS ALONG AN URBAN-RURAL
GRADIENT

Environ. Sci. Technol 2000 34:4294

The following points are made by G.M. Lovett (Environ. Sci.
Technol 2000 34:4294):

1) To determine the patterns of atmospheric deposition and
throughfall in the vicinity of a large city, bulk deposition, oak
forest throughfall, and particulate dust deposition, the authors
measured at sites along a transect within and to the north of New
York City. Concentrations and fluxes of NO3-, NH4+, Ca2+, Mg2+,
SO42-, and Cl- in throughfall all declined significantly with
distance from the city, while hydrogen ion concentration and flux
increased with distance from the city. Most of the change in
concentrations and fluxes occurred within 45 km of the city.

2) Throughfall deposition of inorganic N was twice as high in the
urban sites as compared to the suburban and rural sites. Bulk
deposition patterns were similar to those of throughfall, but
changes along the transect were much less pronounced. The water-
extractable component of dust deposition to Petri plates also was
substantially higher in the urban sites for Ca2+, Mg2+, SO42-,
NO3-, and Cl-. The dust particles had little alkalinity,
suggesting that alkaline aerosols were neutralized by acidic
gases in the atmospheric.

3) The authors propose that dust emissions from New York City act
like an "urban scrubber", removing acidic gases from the
atmospheric and depositing them on the city as coarse particle
dry deposition. Despite the urban scrubber effect, most of the
dry deposition of nitrate was from gaseous nitrogen oxides, which
were in much higher concentration in the city than in rural
sites. The authors suggest that excess deposition of nutrients
and pollutants could be important for the nutrient budgets of
forests in and near urban areas.

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SOIL CHARACTERISTICS OF OAK STANDS ALONG AN URBAN-RURAL LAND-USE
GRADIENT

The following points are made by R.V. Pouyat et al (J. Environ.
Quality 1995 24:516):

1) Urban-rural land-use gradients are environmental gradients
determined by human-built structures and human activity. Although
gradients of land use are readily measurable, little is known
about the effects of urbanization on forest soil properties.

2) The authors report that soil properties were quantified in oak
stands (Quercus sp.) along an urban-rural transect in the New
York City metropolitan area. A suite of 25 soil chemical
properties were subjected to a Principal Component Analysis of
ordinate stands. The first principal component (PCI) accounted
for 42.3% of the variation. Positive loadings of PCI corresponded
to high concentrations of Pb, Cu, Ni, Ca, Mg, and K; high total
soluble salt concentrations; high organic matter; high total N;
and slightly more soil acidity.

3) Stands located closer to the urban core has positive loadings
on PIC; sites located beyond 30 km of the urban core had negative
loadings. The variation accounted for by PCI was significantly
explained (P < 0.005) by measures of urban development quantified
along the transect, including percent urban cover (r2 = 0.735),
population density (r2 = 0.700), traffice volume (r2 = 0.778),
and road density (r3 = 0.700).

4) Of the heavy metals measured, Cu and Pb showed a 2.5- to 4-
fold increase in concentration from the rural to the urban land
use types, with maximum concentrations for Cu reaching 49.3 mg kg
and Pb 181.4 mg kg in the urban sites, respectively.

5) The authors suggest that more transects must be established in
this and other metropolitan areas to build a data base to develop
predictive models of ecosystem change, given the amount and type
of urban development in a landscape.

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AN INTENSIVE TWO-WEEK STUDY OF AN URBAN CO2 DOME IN PHOENIX,
ARIZONA, USA

The following points are made by C.D. Idso et al (Atmospher.
Environ. 2001 35:995):

1) The authors report that atmospheric CO2 concentrations were
measured prior to dawn and in the middle of the afternoon at a
height of 2 m above the ground along four transects through the
metropolitan area of Phoenix, Arizona on 14 consecutive days in
January 2000.

2) The data revealed the existence of a strong but variable urban
CO2 dome, which at one time exhibited a peak CO2 concentration at
the center of the city that was 75% greater than that of the
surrounding rural area. Mean city-center peak enhancements,
however, were considerably lower, averaging 43% on weekdays and
38% on weekends; and averaged over the entire commercial sector
of the city, they were lower still, registering 30% on weekdays
and 23% on weekends. Over the surrounding residential areas, on
the other hand, there are no weekday-weekend differences in
boundary-layer CO2 concentration. Furthermore, because of
enhanced vertical mixing during the day, near-surface CO2 concns.
in the afternoon are typically reduced from what they are prior
to sunrise.

3) This situation is additionally perturbed by the prevailing
southwest-to-northeast flow of air at that time of day, which
lowers afternoon CO2 concentrations on the southern and western
edges of the city still more, as a consequence of the importation
of pristine rural air. The southwest-to-northeast flow of air
also sometimes totally compensates for the afternoon vertical-
mixing-induced loss of CO2 from areas on the northern and eastern
sides of the city, as a consequence of the northeastward
advection of CO2 emanating from the central, southern and western
sectors of the city.

4) The authors conclude: Although complex, the nature of the
urban CO2 dome of Phoenix, Arizona, is readily understandable in
terms of basic meteorological phenomena and their interaction
with human activities occurring at the land/air interface.

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4. SCALING METABOLISM FROM ORGANISMS TO ECOSYSTEMS

The following points are made by B.J. Enquist et al (Nature 2003
423:639):


1) Our ability to predict variation in ecosystem processes is
currently limited by our ability to mechanistically link
biological processes across both spatial and temporal scales(4).
One promising approach is to focus on how biotic and abiotic
factors regulate metabolic rates of individuals, which combine to
determine ecosystem flux rates. Metabolism is the fundamental
process dictating material and energy fluxes through organisms.
Recent work shows that most variation in the metabolic rates of
individuals can be quantified on the basis of the combined
effects of two variables, body size and absolute temperature,
using a general model for metabolic scaling.


2) In summary: Understanding energy and material fluxes through
ecosystems is central to many questions in global change biology
and ecology(1-5). Ecosystem respiration is a critical component
of the carbon cycle(1,5) and might be important in regulating
biosphere response to global climate change(1-3). The authors
derive a general model of ecosystem respiration based on the
kinetics of metabolic reactions and the scaling of resource use
by individual organisms. The model predicts that fluxes of CO2
and energy are invariant of ecosystem biomass, but are strongly
influenced by temperature, variation in cellular metabolism and
rates of supply of limiting resources (water and/or nutrients).
Variation in ecosystem respiration within sites, as calculated
from a network of CO2 flux towers(5), provides robust support for
the model's predictions. However, data indicate that variation in
annual flux between sites is not strongly dependent on average
site temperature or latitude. The authors suggest this presents
an interesting paradox with regard to the expected temperature
dependence; nevertheless, the model provides a basis for
quantitatively understanding energy and material flux between the
atmosphere and biosphere.

References (abridged):

1. Valentini, R. et al. Respiration as the main determinant of
carbon balance in European forests. Nature 404, 861-865 (2000)

2. Giardina, C. P. & Ryan, M. G. Evidence that decomposition
rates of organic carbon in mineral soil do not vary with
temperature. Nature 404, 858-861 (2000)

3. Huxman, T. E. et al. Temperature as a control over ecosystem
CO2 fluxes in a high elevation subalpine forest. Oecologia 134,
537-546 (2003)

4. Rosenzweig, M. L. Species Diversity in Space and Time
(Cambridge Univ. Press, 1995)

5. Baldocchi, D. et al. FLUXNET: A new tool to study the temporal
and spatial variability of ecosystem-scale carbon dioxide, water
vapor, and energy flux densities. Bull. Am. Meteorol. Soc. 82,
2415-2434 (2001)

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TEMPERATURE AS A CONTROL OVER ECOSYSTEM CO2 FLUXES IN A HIGH-
ELEVATION SUBALPINE FOREST

The following points are made by T.E. Huxman et al (Oecologia
2003 134:537):

1) The authors report they evaluated the hypothesis that CO2
uptake by a subalpine, coniferous forest is limited by cool
temperature during the growing season. Using the eddy covariance
approach, the authors conducted observations of net ecosystem CO2
exchange (NEE) across two growing seasons. When pooled for the
entire growing season during both years, light-saturated net
ecosystem CO2 exchange (NEE(sat)) exhibited a temperature optimum
within the range 7-12 degrees C.

2) Ecosystem respiration rate ( R(e)), calculated as the y-
intercept of the NEE versus photosynthetic photon flux density
(PPFD) relationship, increased with increasing temperature,
causing a 15% reduction in net CO2 uptake capacity for this
ecosystem as temperatures increased from typical early season
temperatures of 7 degrees C to typical mid-season temperatures of
18 degrees C.

3) The ecosystem quantum yield and the ecosystem PPFD
compensation point, which are measures of light-utilization
efficiency, were highest during the cool temperatures of the
early season, and decreased later in the season at higher
temperatures. Branch-level measurements revealed that net
photosynthesis in all three of the dominant conifer tree species
exhibited a temperature optimum near 10 degrees C early in the
season and 15 degrees C later in the season.

4) Using path analysis, the authors statistically isolated
temperature as a seasonal variable, and identified the dynamic
role that temperature exhibits in controlling ecosystem fluxes
early and late in the season. During the spring, an increase in
temperature has a positive effect on NEE, because daytime
temperatures progress from near freezing to near the
photosynthetic temperature optimum. During the middle of the
summer an increase in temperature has a negative effect on NEE.

5) The authors suggest that when taken together, the results
demonstrate that in this high-elevation forest ecosystem CO2
uptake is not limited by cool-temperature constraints on
photosynthetic processes during the growing-season, as suggested
by some previous ecophysiological studies at the branch and
needle levels. Rather, it is warm temperatures in the mid-summer,
and their effect on ecosystem respiration, that cause the
greatest reduction in the potential for forest carbon
sequestration.

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5. BIODIVERSITY: A GLOBAL VIEW OF FOREST CANOPIES

The following points are made by C.M. Ozanne et al (Science 2003
301:183):

1) The forest canopy -- defined as the aggregate of all crowns in
a forest stand -- plays a crucial role in the maintenance of
biodiversity and the provision of local and global ecosystem
services. Forest canopies support approximately 40% of extant
species (1–4), of which 10% are predicted to be canopy
specialists (1). Forest canopies also influence the hydrology of
more than 45 million ha of land by controlling evapotransipration
and intercepting up to 25% of precipitation, and their removal
often decreases local rainfall substantially (5). Work at this
challenging frontier only began in earnest in the early 1980s and
has already changed substantially our understanding of key
ecosystem processes.

2) Forest canopies are among the most species-rich yet most
highly threatened terrestrial habitats. Twenty-two of the 25
global "biodiversity hotspots" embrace forest habitat that
combines high levels of endemism (indigenous species) with the
imminent threat of degradation. Knowing the number of species is
fundamental to formulating questions about ecosystem function and
evolution, as well as informing conservation priorities.

3) Global estimates of 30 million to 100 million species, on the
basis of work in tropical canopies, were a key driver in the
formulation of species coexistence and habitat specialization
models. Detailed studies of herbivorous forest insects that
suggest much lower levels of host specificity have recently
resulted in revised estimates of 2 million to 6 million (4),
resolving previous discrepancies between field data, data from
taxonomic collections, and biogeographic estimates. These
studies, which constantly reveal new species, also challenge
equilibrium models of species coexistence (4).

4) A relatively high proportion of invertebrates, about 20 to
25%, are proposed to be unique to the canopy, although this
proportion varies with forest type, canopy structure, and
microclimate and is probably greater than 25% for herbivorous
invertebrates. Ten percent of all vascular plant species are
epiphytic canopy dwellers. This diversity can be attributed in
part to the complex three-dimensional structure of the canopy,
which affords opportunities for niche diversification and
vertical stratification.

5) The forest canopy is the functional interface between 90% of
Earth's terrestrial biomass and the atmosphere. Multidisciplinary
research in the canopy has expanded concepts of global species
richness, physiological processes, and the provision of ecosystem
services. Trees respond in a species-specific manner to elevated
carbon dioxide levels, while climate change threatens plant-
animal interactions in the canopy and will likely alter the
production of biogenic aerosols that affect cloud formation and
atmospheric chemistry.

References (abridged):

1. P. M. Hammond, N. E. Stork, M. J. D. Brendell, in Canopy
Arthropods, N. E. Stork, J. Adis, R. Didham, Eds. (Chapman &
Hall, London, 1997), pp. 184–223

2. D. J. Rodgers, R. L. Kitching, Ecography 21, 392 (1998)

3. D. E. Walter, O. Seeman, D. Rodgers, R. L. Kitching, Aust. J.
Ecol. 23, 501 (1998)

4. V. Novotny et al., Nature 416, 841 (2002)

5. I. R. Calder, Ecology 153, 203 (2001)

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VERTICAL STRATIFICATION OF RAINFOREST COLLEMBOLAN (COLLEMBOLA :
INSECTA) ASSEMBLAGES: DESCRIPTION OF ECOLOGICAL PATTERNS AND
HYPOTHESES CONCERNING THEIR GENERATION

The following points are made by D.J. Rodgers and R.L. Kitching
(Ecography 1998 21:392):

1) The authors describe a complex vertical stratification of
collembolan (springtail insect) assemblages from rainforest leaf
litter samples and identify distinct assemblages associated with
forest floor, lower canopy and upper canopy samples. Leaf litter
samples were collected from the forest floor and deposits of leaf
litter suspended in epiphytes in the canopy of a subtropical
rainforest site at Lamington National Park in southeast
Queensland. The patterns of relationship among assemblages of
Collembola extracted from these samples were examined using a
variety of analyses of a matrix of similarities between samples.

2) The results of analyses showed that forest floor, lower canopy
and upper canopy samples formed discrete groups. These results
permit a discussion of these groups as three distinct collembolan
assemblages. Analysis of the dissimilarities between these
assemblages revealed a gradient of similarity from the forest
floor through the lower to the upper canopy. This gradient
represents a more complex vertical stratification than has
previously been identified in rainforest canopy arthropods.

3) The authors suggest that limitations on the dispersal of some
forest floor species into the canopy may be responsible for this
pattern. The authors also identify a second gradient of
similarities among these assemblages. The authors demonstrate
that dissimilarity among samples from forest floor is
significantly lower than dissimilarity among samples from within
the lower canopy, and that the level of dissimilarity between
samples from within the upper canopy is significantly higher
again.

4) The authors suggest that dispersal barriers and higher
probabilities of extinction in upper canopy collembolan colonies
may be responsible for higher heterogeneity of species
composition and abundance among samples from the upper canopy.
The authors outline a number of testable hypotheses aimed at
determining the importance of these processes in producing the
patterns observed.

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LOW HOST SPECIFICITY OF HERBIVOROUS INSECTS IN A TROPICAL FOREST

The following points are made by V. Novotny et al (Nature 2002
416:841):

1) Two decades of research have not established whether tropical
insect herbivores are dominated by specialists or generalists.
This impedes our understanding of species coexistence in diverse
rainforest communities. Host specificity and species richness of
tropical insects are also key parameters in mapping global
patterns of biodiversity.

2) The authors report an analysis of data for over 900
herbivorous species feeding on 51 plant species in New Guinea and
demonstrate that most herbivorous species feed on several closely
related plant species. Because species-rich genera are dominant
in tropical floras, monophagous herbivores are probably rare in
tropical forests. Furthermore, even between phylogenetically
distant hosts, herbivore communities typically shared a third of
their species.

3) The authors suggest these results do not support the classical
view that the coexistence of herbivorous species in the tropics
is a consequence of finely divided plant resources; non-
equilibrium models of tropical diversity should instead be
considered. Low host specificity of tropical herbivores reduces
global estimates of arthropod diversity from 31 million to 4-6
million species. This finding agrees with estimates based on
taxonomic collections, reconciling an order of magnitude
discrepancy between extrapolations of global diversity based on
ecological samples of tropical communities with those based on
sampling regional faunas.

--------------------------------

THE RELATIVE IMPORTANCE OF TREES VERSUS LIANAS AS HOSTS FOR
PHYTOPHAGOUS BEETLES (COLEOPTERA) IN TROPICAL FORESTS

The following points are made by F. Odegaard (J. Biogeography
2000 27:283):

1) Insect assemblages associated with lianas in tropical forests
are poorly studied compared with those associated with trees. The
importance of lianas for the maintenance of local species
richness of insect herbivores in tropical forests is therefore
poorly understood. With this in mind, a comparative study of the
relative importance of trees and lianas as hosts for phytophagous
beetles was carried out by the author. The study area was located
in the canopy of a dry tropical forest in Parque Natural
Metropolitano, Panama province, Republic of Panama.

2) A crane system was utilized to access the canopy. The number
of species and host specialization of adult phytophagous beetles
associated with twenty-six liana species of ten different
families, and twenty-four tree species of twelve different
families were compared.

3) A total of 2561 host associations of 697 species of beetles
were determined (1339 for trees and 1222 for lianas). On average
55.8 +/- 6.8 beetle species were found to be associated with each
tree species while the comparable number for lianas was 47.0 +/-
6.1.

4) The pooled numbers of phytophagous beetle species associated
with trees and lianas, respectively, were not significantly
different. However, there were significantly more species feeding
on green plant parts on lianas than on trees, and there were
significantly more wood eaters on trees than on lianas.

5) Phytophagous beetles associated with lianas were significantly
more specialized than the tree associates due to a higher degree
of specialization among the species feeding on green plant parts
of lianas. Wood eaters and flower visitors showed no differences
in host specialization on different growth forms.

6) The author concludes: The present study shows that lianas are
at least as important as trees for the maintenance of local
species diversity of phytophagous beetles at this site. The
mechanisms that drive the patterns can only be hypothesized.
Plant architecture, size, and length of growing season are
probably involved. Further studies, should include measurements
of plant traits to elucidate experimentally which mechanisms
drive the patterns. Additional insight would come from similar
studies in other forest types, and also studies of other major
taxonomic groups of arthropod herbivores.

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6. ON MULTIDECADAL CHANGE IN THE PACIFIC OCEAN

The following points are made by F.P. Chavez et al (Science 2003
299:217):

1) Landings of sardines show synchronous variations off Japan,
California, Peru, and Chile (1). Populations flourished for 20 to
30 years and then practically disappeared for similar periods.
Periods of low sardine abundance have been marked by dramatic
increases in anchovy populations (2-5). Several important
conclusions can be drawn from this. First, the mechanism
responsible for the variability must have been similar in all
cases and, some argue, relatively simple and direct. Second, the
variability is difficult to explain on the basis of fishing
pressure. Third, the variability must be linked to large-scale
atmospheric or oceanic forcing.

2) The discovery of these so-called biological regime shifts
preceded the description of the underlying physical variability.
A decade or more after the observations of sardine variations
(1), scientists discovered fluctuations in air temperatures,
atmospheric circulation and carbon dioxide, and ocean
temperatures that were remarkably similar in phase and duration
to the biological records. As a result, it has been suggested
that a regime or climate shift may even be best determined by
monitoring marine organisms rather than climate. Recent
theoretical work supports the idea that complex food webs can
undergo substantial changes in response to subtle physical
forcing.

3) The sardine and anchovy fluctuations are associated with
large-scale changes in ocean temperatures; for 25 years, the
Pacific is warmer than average (the warm, sardine regime) and
then switches to cooler than average for the next 25 years (the
cool, anchovy regime). Instrumental data provide evidence for two
full cycles: cool phases from about 1900 to 1925 and 1950 to 1975
and warm phases from about 1925 to 1950 and 1975 to the mid-
1990s. A wide range of physical and biological time series in the
Pacific Ocean basin show systematic variations on this same time
scale. Anomalies, representing deviations from the mean value,
were negative from about 1950 to 1975 and positive from about
1975 to the middle to late 1990s.

4) In summary: In the Pacific Ocean, air and ocean temperatures,
atmospheric carbon dioxide, landings of anchovies and sardines,
and the productivity of coastal and open ocean ecosystems have
varied over periods of about 50 years. In the mid-1970s, the
Pacific changed from a cool "anchovy regime" to a warm "sardine
regime". A shift back to an anchovy regime occurred in the middle
to late 1990s. These large-scale, naturally occurring variations
must be taken into account when considering human-induced climate
change and the management of ocean living resources.

References (abridged):

1. T. Kawasaki, FAO Fish. Rep. 291, 1065 (1983)

2. D. Lluch-Belda, et al., S. Afr. J. Mar. Sci. 8, 195 (1989)

3. D. Lluch-Belda, et al., Fish. Oceanogr. 1, 339 (1992)

4. R. A. Schwartzlose, et al., S. Afr. J. Mar. Sci. 21, 289
(1999)

5. D. B. Lluch-Cota, S. Hernández-Vázquez, S. E. Lluch-Cota, FAO
Fish. Circ. 934 (1997)

--------------------------------

ON FREQUENCY VARIABILITY IN LARGE MOBILE FISH POPULATIONS

The following points are made by A. Bakun (Progr Oceanography
2001 49:485):

1) The author introduces a conceptual perspective which appears
to convey substantial explanatory power with respect to some
prominent current issues in fisheries ecology, including evident
regime shifts in resource productivity and/or in species
dominance. Underpinning the proposed perspective are two key
ideas. These are the "school trap" concept and the notion of
"affinities" to specific ocean features or locations that may
characterize individual fish. These two ideas lead to a
mechanism, here termed "school-mix feedback", by which mobile
fish populations may automatically track low frequency
environmental and ecosystem variability and make particularly
rapid adaptive adjustments of behaviors and migratory tendencies
to the associated changes in conditions.

2) The mechanism also appears to involve the possibility that a
fish population could thereby fall into a short-period analog to
an evolutionary feedback trap, from which it may not easily
extricate itself without undergoing population collapse.
Analogous adaptive responses to geographically-biased fishery
exploitation may upset the integrity of naturally-evolved systems
and potentially lead to chronic suppression of resource
productivity. Possibilities for innovative adaptive management
actions are suggested by the author. Both heuristic and real
explanatory examples are cited, in most cases dealing with
pelagic fish stocks and upwelling ecosystems.

ScienceWeek http://www.scienceweek.com

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7. ON THE ECONOMIC VALUE OF ECOLOGICAL STABILITY

The following points are made by P.R. Armsworth and J.E.
Roughgarden (Proc. Nat. Acad. Sci. 2003 100:7147):

1) The dynamical stability of populations and ecosystems governs
their responsiveness to fluctuating environmental conditions and
determines with what reliability these natural resources provide
life-sustaining services (1) to society. Population and ecosystem
stability is thus a major structuring theme in ecology (2–5). An
ecosystem that is only weakly stable will vary widely in response
to a changing environment, but one that is more stable can be
relied on to provide ecosystem services consistently. Weakly
stable ecosystems will be prone to species extinctions and thus
could sustain less diverse biotas. Therefore, it is important
that any impacts to the stability of ecosystems be considered
when designing environmental management plans. However, we lack
accounting frameworks that can evaluate ecosystem stability and
factor it into environmental cost–benefit analyses.

2) The authors develop such a framework and use it to illustrate
when ecosystem stability has quantifiable economic value, and how
consideration of that value should change environmental planning.
To illustrate the approach, the authors use a minimal model so
that the ideas remain as transparent as possible. The model
examines an idealized bird species that occupies a terrestrial
reserve. The principles illustrated by this model are
sufficiently general that they will underlie diverse management
decisions ranging from marine reserve design to the management of
pollination services.

3) In summary: Seemingly intangible ecosystem characteristics
that preoccupy ecologists, like ecosystem stability and the
responsiveness of populations to environmental variation, have
quantifiable economic values. The authors demonstrate how to
derive these values, and how their consideration should change
environmental decision making. To illustrate these concepts, the
authors use a simple reserve design model. When resource managers
choose a particular landscape configuration, their decision
affects both the mean abundance of species and the temporal
variation in abundances. Population stability and related
phenomena have economic value, because management actions affect
the variance of ecosystem components. In the example of the
authors, a larger reserve size is recommended when accounting for
the stability of the managed ecosystem.

References (abridged):

1. Daily, G. C. (1997) Nature's Services: Societal Dependence on
Natural Ecosystems (Island Press, Washington, DC)

2. May, R. M. (1974) Stability and Complexity in Model Ecosystems
(Princeton Univ. Press, Princeton), 2nd Ed.

3. Pimm, S. L. (1991) The Balance of Nature? Ecological Issues in
the Conservation of Species and Communities (Univ. Chicago Press,
Chicago)

4. Tilman, D. (1996) Ecology 77, 350-363

5. Ives, A. R. & Hughes, J. B. (2002) Am. Nat. 159, 388-395.[ISI]

--------------------------------

BIODIVERSITY: POPULATION VERSUS ECOSYSTEM STABILITY

The following points are made by D. Tilman (Ecology 1996 77:350):

1) The relationships between biodiversity and stability were
determined by the author for both population and ecosystem traits
in a long-term study of 207 grassland plots. Results demonstrate
that biodiversity stabilizes community and ecosystem processes,
but not population processes. Specifically, year-to-year
variability in total aboveground plant community biomass was
significantly lower in plots with greater plant species richness
both for the entire 11-yr period and for the nine non-drought
years. The change in total plant community biomass from before
the drought to the peak of the drought was also highly dependent
on species richness. For all three measures of total community
biomass stability, multiple regressions that controlled for
covariates showed similar significant relationships between plant
diversity and stability.

2) In contrast, year-to-year variability in species abundances
was not stabilized by plant species richness for either all years
or non-drought years. This difference between species vs.
community biomass likely results from interspecific competition.
When climatic variations harm some species, unharmed competitors
increase. Such compensatory increases stabilize total community
biomass, but cause species abundances to be more variable.

3) The author suggests these results support both the predictions
of Robert May concerning the effects of diversity on population
stability and the diversity-stability hypothesis as applied to
community and ecosystem processes, thus helping to reconcile a
long-standing dispute.

--------------------------------

GENERAL RELATIONSHIPS BETWEEN SPECIES DIVERSITY AND STABILITY IN
COMPETITIVE SYSTEMS

The following points are made by A.R. Ives and J.B. Hughes (Amer.
Naturalist 2002 159:388):

1) Investigating the effect of biodiversity on the stability of
ecological communities is complicated by the numerous ways in
which models of community interactions can be formulated. This
has led to differences in conclusions and interpretations of how
the number of species in a community affects its stability.

2) The authors derive a simple, general relationship between the
coefficient of variation (CV) of combined species densities and
the environmentally driven variability in species' per capita
population growth rates. For a given level of environmentally
driven variability in per capita population growth rates,
increasing the number of species in a community decreases the CV
of combined species densities, provided that species do not
respond to environmental fluctuations in a perfectly correlated
way.

3) Thus, a community with more species of competitors will be
more stable (have lower CV in combined species densities for a
given level of environmental variability) than a species-poor
community, provided that the species in both communities show
equal variability in per capita population growth rates and
provided that species within each community do not show strongly
correlated responses to environmental fluctuations. This
conclusion also applies to "non-interactive" models in which
there is no competition between species.

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