Ecosystems Part 1

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Ecosystems

What is an ecosystem?

the combination of all living things and the nonliving surroundings with which they react

not physically isolated, but have boundaries more or less arbitrarily defined by the scope of a given inquiry
linked by a web of interrelationships, it is the links that we investigate when we study an ecosystem

biosphere=the sum of all the ecosystems
community=the living things in an ecosystem, such as "the forest community" or a subset, such as "the rodent community" or "the treetop community"
population=the group of organisms of a single species within a particular ecosystem or locale
a population, community, ecosystem or the biosphere are examples of systems which have properties their components do not

a population has a growth rate, age distribution, etc; an individual does not
a community has a species diversity, a population does not

Energy flows

Photosynthesis

Summary reaction:

6CO₂ + 6H₂O + sunlight(energy) ® C₆H₁₂O₆ + 6O₂

(NOTE: we might want to write it this way:
6CO₂ + 12H₂O^* + sunlight(energy) ®
C₆H₁₂O₆ + 6O^*₂ + 6H₂O)

goes on in the chloroplasts of green plants

facilitated by a substance called chlorophyll

the plants give us animals two gifts by way of this process:

carbohydrates, sometimes called "fixed carbon" or "reduced carbon" and
oxygen

the complementary process is called respiration:

C₆H₁₂O₆ + 6O₂ ® 6CO₂ + 6H₂O + metabolic energy

except in bacteria and such, respiration goes on in the mitochondria of cells (bacteria cells do have respiration, but they don't have mitochondria; in fact, there is evidence that mitochondria were originally symbiotic bacteria inside other cells)

some organisms, primarily bacteria, carry on respirations without O₂, such respiration is called anaerobic respiration

anaerobic organisms use some alternative electron acceptor (oxidizing agent)

for example, sulfate-reducing bacteria use sulfate; nitrate-reducing bacteria use nitrate

guys which can't use oxygen (and may be poisoned by it) are called obligate anaerobes
guy which have an option to use it or not are called facultative anaerobes
anaerobes include some bad guys like the bacteria that cause tetanus (Clostridium tetani) or botulism (Clostridium botulinum) and some good guys like coliform bacteria and some bacteria that aid in biodegradation

a special type of anaerobic respiration is fermentation, which allows the production of bread and beer

Food chains

the simple transfer of energy from across a series of populations (plant eaten by herbivore eaten by predator...) is called a food chain
a complex of interlinked food chains is called a food web
the foundation of either is the autotrophs=self-feeding photosynthesizers
everything else is a heterotroph=consumer of other organisms
energy flows "up" through trophic levels of a food chain (but some is lost at each level)
the grazing food web moves matter and energy from green plants through herbivores to carnivores
the detritus food web cycles dead or excreted organic matter; moves matter back to lower trophic levels and recycles some energy (for example, when we eat mushrooms)

macrodecomposers=earthworms and some beetles
microdecomposers=bacteria, fungi, and mold

the marine food web:

base are the phytoplankton (tiny floating plants)
next up are the zooplankton (tiny floating animals)
zooplankton eaten by small fish (nekton) and filter feeders
big fish eat little fish etc.

Energy conversion

the ability of an ecosystem to utilize sunlight to convert abiotic matter to biomass (living matter) is called its productivity

expressed as mass per area per time

eg: grams/square meter/year

dieters are also familiar with the notion of equating a given biomass with a certain quantity of energy or "Calories"

thus productivity can also be expressed as energy per area per time: kcal/m2/day (the Calorie of nutrition is the kcal of science)
this way of expressing productivity makes since when we realize that a given quantity of sunlight shines on a given area in a given time

gross primary productivity is absolute rate at which plants photosynthesize
they use some of the fixed energy in their own respiration; the balance is called net primary productivity

the productivity of different ecosystems varies (see also Fig 3-14, p 67, in the 7^th edition):

Type of Ecosystem	Productivity (10³ kcal/m²/yr)
desert	<0.5
deep ocean	<1
grasslands	0.5 to 3.0
deep lakes
mountain forests
some agriculture
continental shelf
moist forests	3 to 10
shallow lakes
moist grasslands
most agriculture
some estuaries	10 to 25
springs
coral reefs
bayous

we each consume about 730 * 10³ kcal/yr; so we need at least about 150 m² each

much of the world is covered with low productivity ecosystems

what is the efficiency of green plants?

if we assume that half of the 2 cal/cm²/min put out by the sun reaches the earth's surface for 12 hours, 300 days per year:

2 cal

60 min

12 hr

300 d

10⁴ cm³

1 kcal

4,320 * 10³ kcal

cm² min

hr

day

yr

m²

10³ cal

m² year

based on the productivity of moist grasslands, this gives us an efficiency of about (10/4320 *100%) = 0.23%

published values range from about 0.13 percent in the ocean to about 0.3 percent on land; thus only a small fraction of sunlight enters the food chain

The LAWS of thermodynamics, especially the second law, explain why the trophic levels form a pyramid

First Law: energy cannot be created or destroyed, only converted from one form to another = "you can't get something for nothing"
Second Law: every energy transformation results in some energy being lost as waste heat = "you can't even break even"

this law is used to calculate the maximum theoretical efficiency of a heat engine (like in a steam train or a car)

the formula E% = 100% * (T_hot-T_cold)/T_hot gives the efficiency if all mechanical losses (eg, friction) were eliminated
unless the cold side (the condenser) were at absolute zero, there would alway be some inefficiency
thus this law guarantees that no heat engine (like a car's internal combustion engine or an electrical plants steam turbine) will ever be 100% efficient

there is an energy price on every action (some energy becomes unusable); unless this price is paid, the action will not occur
hence, second law losses are also evident in the flow of energy through trophic levels in a food web

the transfer of energy from one trophic level to the next is not 100 percent efficient; at each level energy:

can be used for respiration by the organisms on that level

organisms must expend energy just to stay alive!

can enter the detritus web as the organisms fail to assimilate the energy they ingest or die and decay
can be passed on to the next level when organisms are consumed and assimilated (only about 10 to 20 percent)
because of this loss, the amount of biomass that can be produced at higher levels is less (the diet for a small planet phenomenon: you can feed 20 people corn or one person hamburger)
in contrast, though probably not significant, is the fact that smaller organisms need to eat more per unit body mass (hence "eating like a bird" is a misleading expression)

Ecological efficiency

the ratio of energy available divided by energy received
an important notion, especially in a managed ecosystem (ie a farm)

Physical limits

the living things in an ecosystem cause some changes to the environment that change its suitability for the species in question and for others but physical conditions can impose insurmountable constraints

Abiotic elements

physical or environmental factors which influence or limit the living things in an ecosystem
Liebig's Law (the law of the minimum) = when a physical necessity is not present in a locale in sufficient quantity to support a population, that population cannot exist there
Shelford’s Law of Tolerance = any factor outside the optimum range for an organism (at any time of the year) will put stress on that organism and will limit the growth of the individual and the species

critical factor: the factor which most determines the presence/size of a population in a habitat or ecosystem

indicator species can tell us something about critical physical factors

carp can survive in water with essentially no dissolved oxygen
locoweed can survive in soils with high selenium content
the plants take up selenium and use it in place of sulfur in certain amino acids
cows that eat the plant have the nselenium lodge in their brains and go crazy

some related terms:

optimum or optimal range refer to the value or set of values for some parameter which best support the life and growth of a particular species
range or zone of tolerance: the set of values for some parameter which can be withstood by an organism without actually dying; the extreme values are called the limits of tolerance (Shelford’s law)
the zone of stress is those values of a parameter which do not kill an organism outright but which cause reduced growth, limited or no reproductions, etc

The main abiotic factors are:

Water/Rainfall

a biggy, water is the sine qua non of life

terrestrial life became possible when plants and animals learned how to carry water with them

the amount and distribution of rainfall can have a big effect on living things

Fig 2-20 on p 44 shows the interaction of rainfall and temperature

water is sometimes considered to be the principle factor with temperature modifying the effects of rainfall

in case of extreme cold, temperature dominates

desert creatures have a variety of behavioral and physiological adaptations to conditions of low humidity

nocturnalism
tough outer coatings
rain requirements for germination

freshwater creatures subject to salt loss

special kidneys

saltwater creatures subject to water loss

salt glands
tough coatings
chemical adaptations (urea in sharks)

water is not only a mineral nutrient, it can be a physical substrate that determines the form of plants and animals

Sunlight/Day length/Temperature

sunlight varies greatly even within a given area (varies with height, topography)

available sunlight limits photosynthesis

duration of daylight (photoperiod) has regulatory effects on plants and animals as well as limiting the photosynthetic abilities of plants
temperature is partly a function of sunlight and daylength

cold blooded animals require high temperatures
warm blooded animals produce the body heat they need to survive in cold temps, have some adaptations (hibernation) to get them through the winter

Wind/Air pressure

wind distributes seeds, carry birds
wind can move soil and affect moisture
violent winds (hurricanes, tornadoes) can damage the physical substrate etc.
air pressure decreases with altitude

a minimum air pressure is needed for life

Soil nutrients

limit plant growth

Biochemicals

Organic molecules

organic compounds:

contain carbon
are synthesized by living things (or organic chemists)
usually contain stored chemical energy

there are 92 natural elements, most of which are rare (especially those heavier than iron)
plants use 15 or so elements, animals 25 or so, but most of the atoms in living things are carbon, hydrogen, and oxygen

carbohydrates and lipids contain no other elements

lipids, by the way, are not hydrocarbons; they are carboxylic acids which have a hydrocarbon tail hanging off -COOH head

the membrane around each cell is a "lipid bilayer"
brain cells have a large area of cell membrane, so when someone calls me a "fat head" I take it as a compliment

proteins also contain a fair amount of nitrogen (about 1/7 by mass) and a small amount of sulfur

proteins themselves contain no other elements, but they may have "prosthetic groups": other specialized biomolecules bound to the protein to carry out specific tasks

hemoglobin is bound to heme which contains iron
vitamin B12 contains cobalt and is thought to work with certain proteins
chlorophyll contains magnesium

nucleic acids and similar compounds contain phosphorus
a few other elements are bound in biomolecules, sometimes only during their synthesis
sodium, potassium, chlorine, and for the most part calcium are not found within biomolecules, but they exist as ions free in solution in and around animal cells

Mineral cycles-Beneficial elements (biogeochemical cycles)

just as energy moves in chains and webs, so do elements
in a biogeochemical cycle, an element moves from a reservoir stage, where large amounts reside outside the biomass, into nonliving and living things and back

Carbon cycle (p 69)

the basic element of life; chemically speaking, "organic" means "containing carbon"
the fixing of atmospheric carbon dioxide in photosynthesis is the basis of the terrestrial food chain
CO₂ is released to back to the atmosphere by respiration, decay, combustion (atmosphere is about 0.038% CO₂)
CO₂ dissolves in water to form carbonate, used by algae in photosynthesis

some carbonate is used in forming shells and reefs, or otherwise precipitates out; carbonate in this form can become fixed in sedimentary rocks (eg limestone)

Oxygen cycle

released by plants from water (H₂O) during photosynthesis (atmosphere is about 20% O₂)
recombined with hydrogen and carbon to form water and carbon dioxide during cellular respiration and other metabolic processes
involved in inorganic cycles of metals

Nitrogen cycle (p 71)

nitrogen makes up about 78% of the atmosphere but is unusable to most organisms in its atmospheric form (N₂ gas)

some organisms (some bacteria, fungi, and blue-green algae) can form ammonia (NH₃) from N₂
this process of biological nitrogen fixation is very important
legumes have a symbiotic relationship with some nitrogen fixing bacteria that grow in nodules in the roots and produce available nitrogen for the host plant

legumes thus can grow in nitrogen-poor soils and in fact can restore nitrogen to them

other little critters live on their own and fix nitrogen

most plants can't use ammonia but rather need nitrate

nitrate is produced in the soil by nitrifying bacteria and is readily taken up, stored and used by plants

nitrogen is essential for protein and some other biomolecules
ammonia and nitrate present in the soil from nitrogen fixation, excretion or death by organisms, can be released back to the atmosphere by bacteria that carry out denitrification

some of the discarded nitrogen remains in sedimentary form before denitrification

fertilization and sewage disposal can upset the nitrification/denitrification balance and disrupt ecosystems

Phosphorus cycle (p 69)

reservoir stage is phosphate present in rocks or sediment
phosphate leached from rock is taken up by plants which are eaten by animals
phosphate can be returned to sedimentary form by decay, combustion, or excretion by bats and birds living on high phosphate food

guano (fossilized bat droppings) is a valuable commodity
phosphate released by decaying organisms can be readily retaken-up by plants
phosphate not taken up remains in sedimentary form

phosphate can form water insoluble compounds, such as calcium phosphate, which is essentially unavailable to plants; and thus remains inorganic for long periods of time
phosphorus pollution of fresh water can allow excess algae growth
the dead of the ocean sink to the bottom, where the phosphorus remains until upwelling along the coasts to promote growth

Sulfur cycle

sulfur present as sulfate (SO₄^-2) is available to plants; used in proteins
in the absence of oxygen, bacteria consuming decaying matter release the sulfur as hydrogen sulfide (H₂S) famous for its low-tide or rotten egg smell

some bacteria can use the reduced-sulfur in sulfide as an energy source

sulfur present in fossil fuels becomes sulfur dioxide when burned, which then combines with oxygen and water to form sulfuric acid, causing acid rain

Biomes

large recognizable communities found in different parts of the world

result from the consistent interaction of living things with bands of climate etc

human beings can survive in most conditions, usu. by modifying or creating their own environment; the natural biome can be distorted out of existence

Tundra

found in the northern parts of North America, Europe, and Asia
short summer and growing season, long cold winter
permafrost
plants and animals must grow quickly, complete their business and hunker down

grasses, lichens, sedges, dwarf willow; few woody plants
insects can winter over as eggs or larvae
migratory birds can pass through in the warm weather

very fragile ecosystems, slow to heal

Boreal (northern) forest

south of the tundra: much of Canada, Scandinavia, Russia (where its called "taiga")
evergreen trees

spruce
balsam fir
tamarack

slightly warmer, but still short growing season, much more precipitation (15 to 40 inches per year)
no permafrost, so tree roots can dig in in the summer
the leaf litter decays slowly and renders the soil acidic inhibiting most crops
migratory birds, moose, wolverine, snowshoe hare

Deciduous forest

south of the taiga, extends throughout the northeast US
dominant trees are oak, maple, hickory, beech and other hardwoods

shed their leaves in the fall (=deciduous) and overwinter in a dormant state

high level of precipitation (30 to 60 inches per year), distributed evenly
warm humid summers, cool winters

very dense plant growth

white-tailed deer, ruffed grouse, cottontail rabbit, red fox, raccoon, fox squirrel, wild turkey

Grassland or Prairie

found between the eastern deciduous forest and the western desert of the US (similarly found in the middle of other continents)

in between in climate as well

low rainfall (<30 inches per year)

wet and dry cycles over periods of years

warm summers and cool winters
prairie community found adjacent to forests, maintained by periodic fires; less dense grass found in drier western regions
ideal for crop production

rich soil from decaying grass in the upper soil
light rainfall leaves minerals close to the surface

grazing animals such as the bison and pronghorn antelope

Desert

formed when mountains or other conditions block the flow of water into an area so that potential evapotranspiration exceeds rainfall
very low rainfall (in southwest, <10 inches per year)
two great deserts in the US:

the Great Basin (between the Sierra Nevada and the Rockies)

sagebrush, low shrubs, small conifers and junipers

the low desert (including the Mojave and the Sonoran)

desert shrubs, creosote bush, cacti

desert plant reproduction is triggered by rainfall

Chaparral

Mediterranean climate of coastal California
climate deemed ideal by many people; heavily populated
dry summers, most rain falls during the short winter
fire is common

some seeds germinate only after being burned

huge fires spread quickly up canyons driven by the strong, dry Santa Ana winds

the Oakland firestorm of 1991 was so hot that the roads caught fire

Tropics

found in southern Mexico, Central America, and northern South America, as well as in parts of Africa and Asia
high rainfall (90 to 300 or more inches per year)
two seasons

called "wet" and "dry" though it rains all year

poor soil because the mineral nutrients are almost all contained in the vegetation
very susceptible to human impact

preservation efforts are hampered by development pressures in the relatively undeveloped tropical countries

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Environmental Studies

Environmental Engineering Technology

ANTHONY G BENOIT
ROOM 201B
(860) 885-2386

abenoit@trcc.commnet.edu

2 cal	60 min	12 hr	300 d	10⁴ cm³	1 kcal	4,320 * 10³ kcal
cm² min	hr	day	yr	m²	10³ cal	m² year