Transcript ecological system

Chapter 41
Key Concepts
41.1 Ecological Systems Vary over Space and Time
41.2 Solar Energy Input and Topography Shape Earth’s
Physical Environments
41.3 Biogeography Reflects Physical Geography
41.4 Biogeography Also Reflects Geological History
41.5 Human Activities Affect Ecological Systems on a Global
Scale
Can basic ecological principles suggest why removing cattle
has not restored grasses to the Borderlands?
Physical geography—study of the spatial distribution of Earth’s
climates and surface features
Biogeography—study of the spatial distributions of species
The explorer/scientists in the 18th and 19th centuries began to
realize that the distributions of species and environments are
linked.
Abiotic components of the environment are nonliving.
Biotic components—living organisms
An ecological system—one or more organisms plus the external
environment with which they exchange energy and materials.
A system is defined by the interacting parts it contains.
Ecological systems can include any part of the biological
hierarchy from the individual to the biosphere.
Each level brings in new interacting parts at progressively larger
spatial scales.
At the smallest scale is the individual organism and its immediate
environment.
Individuals remove materials and energy from the environment,
convert them into forms that can be used by other organisms,
and, by their presence and activities, modify the environment.
Population—group of individuals of the same species that
live, interact, and reproduce in a particular geographic area
Community—assemblage of interacting populations of
different species in one area
Ecosystem-An ecosystem is the relationships between and
among the populations and the abiotic (environmental)
factors
Biosphere—all the organisms and environments of the
planet
Ecologists replace the term “ecological system” with ecosystem
when they are explicitly including the abiotic components of the
environment;
And in particular when considering communities and their
environmental context.
Generally, large ecological systems tend to be more complex
because they have more interacting parts, and larger spatial
scale.
But small systems can also be complex:
The human gut is densely populated with hundreds of microbial
species. These cells far outnumber the trillion or so human cells in the
body.
The mammalian gut environment provides stable conditions and
ample nutrients.
Gut microbes metabolize foods, including some the host cannot
digest, and excrete waste products that provide nutrition to the
host or to other microbes.
Microbial species interact with one another and with host cells
by forming biofilms that coat the gut lining.
Biotic and abiotic components of ecosystems are distributed
unevenly in space, and ecosystems can change over time.
The human gut illustrates this variation—each person has a
unique gut community.
But patterns do exist: gut communities of genetically related
people are more similar that those of unrelated people.
Gut communities in lean people and obese people vary in the
ratio of two bacterial phyla.
When obese people lose weight, their gut community becomes
more similar to that of a lean person.
Bacteria in the phylum Firmicutes are good at breaking down
indigestible polysaccharides and extracting more energy from
food than Bacteroidetes.
In experiments with mice, it has been shown that the gut
community contributes to obesity, along with diet and genetic
factors.
Earth’s environments vary greatly from place to place and also
through time.
On long time scales, the coming and going of oceans, ice ages,
and other geologic events shape environments.
On short time scales, physical conditions depend largely on solar
energy input, which drives the circulation of the atmosphere and
the oceans.
Weather is the state of atmospheric conditions in a particular
place at a particular time.
Climate is the average conditions and patterns of variation over
longer periods.
Climate is what you expect; weather is what you get.
Adaptations to climate prepare organisms for expected weather
patterns.
Earth receives uneven inputs of solar radiation due to its
spherical shape and the tilt of its axis as it orbits the sun.
• It is colder at the poles because there is less solar input: the sun’s rays
are spread over a larger area and pass through more atmosphere.
• High latitudes experience more seasonality—greater fluctuation over
the course of a year.
Solar energy inputs are always greatest in the tropics and
decrease poleward.
This latitudinal gradient drives global circulation patterns in the
atmosphere.
Hadley cells:
The tropical air is warmed, rises, and then cools adiabatically (an
expanding gas cools).
The rising warm air is replaced by surface air flowing in from the north
and south.
The cooling air sinks at 30°N and 30°S.
Hadley cell circulation produces latitudinal precipitation
patterns:
Rising warm tropical air releases lots of moisture as rainfall. The sinking
air at 30°N and 30°S is dry—most of the great deserts are at these
latitudes.
Some of the descending air in the Hadley cells flows towards the
poles, overriding cold, dense polar air that is flowing
equatorward.
The interaction of these warm and cold air masses generates
winter storms that sweep from west to east through the middle
latitudes.
Earth’s rotation adds an east–west component to the north–
south movement of the air masses—the Coriolis effect.
These atmospheric circulation patterns affect climate patterns by
transferring heat energy from the hot tropics to the cold poles.
Without this transfer, the poles would sink toward absolute zero
in winter, and the equator would reach fantastically high
temperatures throughout the year.
Prevailing surface winds drive the major ocean surface currents,
which carry materials, organisms, and heat with them.
Example: In the northern tropics, the trade winds drag water to the
west; when it reaches a continent, it is deflected northward until the
westerlies drive the water back to the east. The result is a clockwise
gyre.
As they move poleward, tropical surface waters transfer heat
from low to high latitudes, adding to the heat transfer by
atmospheric circulation.
The Gulf Stream and North Atlantic Drift bring warm water
towards northern Europe, warming the air there. The same
latitudes in Canada are much colder.
Deep ocean currents are driven by water density differences.
Colder, saltier water is more dense and sinks to form deep
currents.
Deep currents regain the surface in areas of upwelling,
completing a vertical ocean circulation.
Oceans and large lakes moderate terrestrial climates because
water has a high heat capacity:
Temperature of water changes slowly as it exchanges heat with the air.
Water temperatures fluctuate less than land temperatures, and
the air over land close to oceans or lakes also shows less
seasonal and daily temperature fluctuation.
Topography (variation in elevation) also affects the physical
environment.
As you go up a mountain, air temperature drops by about 1°C for
each 220 m of elevation because rising air expands and cools
adiabatically.
When prevailing winds bump into mountain ranges, the air rises,
cools, and releases moisture. The now-dry air descends on the
leeward side, creating a rain shadow.
Topography also influences aquatic environments:
Flow velocity depends on slope.
Water depth determines gradients of many abiotic factors, including
temperature, pressure, light penetration, and water movement.
Climate diagram—superimposed graphs of average monthly
temperature and precipitation over a year.
The axes are scaled so that it is easy to see the growing season:
When temperatures are above freezing and the precipitation line is
above the temperature line
An organism’s physiology, morphology, and behavior affect how
well it can tolerate a particular physical environment.
Thus, the physical environment greatly influences what species
can live there.
We expect species that occur in similar environments to have
evolved similar phenotypic adaptations.
Early scientist–explorers began to understand how the
distribution of Earth’s physical environments shapes the
distribution of organisms.
Their observations revealed a convergence in characteristics of
vegetation found in similar climates around the world.
Biome—a distinct physical environment inhabited by ecologically
similar organisms with similar adaptations.
Species in the same biome in geographically separate regions
display convergent evolution of morphological, physiological, or
behavioral traits.
Terrestrial biomes are distinguished by their characteristic
vegetation.
Distribution of terrestrial biomes is broadly determined by
annual patterns of temperature and precipitation.
These factors vary along both latitudinal and elevational gradients.
Other factors, especially soil characteristics, interact with climate
to influence vegetation.
Example: Southwestern Australia has Mediterranean climate with hot,
dry summers and cool, moist winters. The vegetation is
woodland/shrubland, with no succulent plants.
The soils are nutrient-poor, and there are frequent fires. Succulents are easily
killed by fires.
Grasslands normally occur where there is not enough
precipitation to support forests, but is more plentiful than is
typical of deserts.
But some grasslands occur in unexpected places, demonstrating
that biome boundaries are not perfectly predicted by
temperature and precipitation—other factors also affect the
vegetation.
Fire is a significant factor affecting vegetation.
Fire rarely kills grasses but often kills shrubs and trees; fires help
maintain grasslands.
Humans have probably used fires for millennia to manipulate
their environment and maintain grasslands.
The biome concept is also applied to aquatic environments.
Aquatic biomes are determined by physical factors such as water
depth and current, temperature, pressure, salinity, and substrate
characteristics.
The primary distinction for aquatic biomes is salinity: freshwater,
saltwater, and estuarine biomes.
Salinity determines what species can live in the biome,
depending on their ability to osmoregulate.
In streams, current velocity is important. Organisms must have
adaptations to withstand flow.
Current also impacts the substrate—whether rocky, sandy, silty,
etc. Substrate also determines what species are present.
Lakes and oceans are divided into water-depth zones.
Nearshore regions (littoral or intertidal) are shallow, impacted by
waves and fluctuating water levels. Distinct zonation of species is
common.
Photic zone—depth to which light penetrates; photosynthetic
organisms are restricted to this zone
In the open-water limnetic zone of lakes and the pelagic zone of
oceans beyond the continental shelf, the prominent
photosynthesizers are phytoplankton (free-floating
photosynthetic organisms).
The aphotic zone is too deep for light penetration and so is
sparsely populated.
Benthic zone—lake or ocean bottom
Organisms in the deepest oceans (abyssal zone) must have
adaptations to deal with high pressure, low oxygen, and cold
temperatures.
Alfred Russel Wallace advanced the idea of natural selection
along with Darwin.
Wallace studied species distributions in the Malay Archipelago
and observed dramatically different bird faunas on two
neighboring islands, Bali and Lombok.
The differences could not be explained by soil or climate.
He suggested that the deep channel between the islands would
have remained full of water (and a barrier to movement of
terrestrial animals) during the Pleistocene glaciations when sea
level dropped.
Thus, the faunas on either side of the channel evolved mostly in
isolation over a long period of time.
Wallace’s observations led him to divide the world into six
biogeographic regions.
Each region encompasses multiple biomes and contains a distinct
assemblage of species, many of which are phylogenetically
related.
Many of the boundaries correspond to geographic barriers to
movement: bodies of water, deserts, mountain ranges.
Boundaries of some biogeographic regions are related to
continental drift.
Example: Southern beeches (Nothofagus) are found in South America,
New Zealand, Australia, and some south Pacific islands.
The genus originated on the supercontinent Gondwana during the Cretaceous
and was carried along when Gondwana broke apart.
The biogeographic regions occupy land masses that have been
isolated from one another long enough to allow the organisms to
undergo independent evolutionary radiations.
The biotas developed in isolation throughout the Tertiary (65 to
2.6 mya), when extensive radiations of flowering plants and
vertebrates took place.
Continental movement can also eliminate barriers, allowing
biotic interchange.
Examples: when India collided with Asia about 45 mya; when a land
bridge formed between North and South America about 6 mya.
Biogeographers use phylogenetic information, the fossil record,
and geological history to study modern distributions of species.
Geographic areas are superimposed on phylogenetic trees. The
sequence and timing of splits in the phylogenetic tree are
compared with sequence and timing of geographic separations
or connections.
Human activities are altering ecological systems on a global
scale.
Some have suggested we are entering a new geological period,
the “Anthropocene” or Age of Humans.
We are changing the distributions of organisms, vegetation, and
topography, as well as Earth’s climate.
Others suggest the new age should be called the
“Homogenocene,” or Homogeneous Age, because the net effect
of our activities is to make ecological systems less complex and
more uniform.
When we use natural ecosystems for hunting, fishing, grazing, or
logging, we remove particular species and change their
abundances.
If we remove too many, we can even cause some species to go
extinct.
This can change the patterns of interaction among species, and
thereby change how entire ecosystems function.
Human-dominated ecosystems, such as croplands, pasturelands,
and urban settlements now cover about half of Earth’s land area.
These ecosystems have fewer interacting species and are less
complex.
In agricultural lands, monocultures replace species-rich natural
communities.
Diversity of crops planted is also very low: 19 crops comprise
95% of total global food production.
Agricultural systems are more spatially and physically uniform
than natural ecological systems.
Human activities also reduce complexity in natural ecosystems:
• Damming and channelization of rivers
• Pollution and habitat fragmentation
• Overexploitation of wild species
• Introductions of new species
Humans move species throughout the globe, sometimes
deliberately, sometimes inadvertently.
Human-assisted biotic interchange is homogenizing the biota of
the planet, blurring the spatial heterogeneity in species
composition that evolved during long periods of continental
isolation.
New subdisciplines of ecology address how we can preserve
ecological systems and their ability to sustain life on our planet.
Conservation ecology seeks to understand the process of
extinction and ways to prevent extinction of vulnerable species.
Restoration ecology seeks to restore the health of damaged
ecosystems.
Natural systems are sometimes altered so strongly that
extinction will occur unless the systems are restored.
Natural history—observation of nature outside of a formal,
hypothesis-testing investigation—provides important knowledge
about ecosystems
These observations are often the source of new questions and
hypotheses and aid in design of ecological experiments.
Mathematical models and computer simulations are often
needed to study the complexities of ecological systems.
They must be based on natural history knowledge of the system.
The Borderlands are arid—they are at 32° N, where the warm,
dry air of the Hadley cells descends.
Other grasslands around the world occupy different physical
environments, and have different histories.
Why grasslands of the Borderlands have not recovered from
overgrazing may be due to many factors:
• Periodic burning may be needed.
• It may be necessary to restore the original landscape, to return water
and nutrients to the soil that grasses require.
• Interaction between fire and nutrients may influence which vegetation
type will be favored.
• It may be necessary to remove the existing shrubs before other
measures will have a chance of success.