Ecosystems

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ECOSYSTEMS

 An ecosystem is the unit composed of all the living things in

a single place at a given time, in addition to, the important

non-living components of the system.

 Nonliving components include sunlight, rainfall, silica and

clay particles in the soil, the air, the water in the soil, etc.

 Thus, an ecosystem encompasses all aspects of a

biological community, in addition to factors such as rates

of CO2 uptake, rates of nitrogen fixation from the

atmosphere, precipitation, seasonal flooding and its effects

on nutrients, etc.

 Ecosystems vary in size. Like communities, small

ecosystems are “stacked” within larger ones, and the

boundaries are sometimes diffuse.

 The biosphere the largest and most encompassing

ecosystem we know-it encompasses all the plants and

animals on Earth.

Energy and Biomass

 Much of ecosystems ecology concerns itself with the flow of

energy and biomass.

 Nutrient cycling and energy flow are common to all

biological communities.

 These phenomena are both a consequence, and a function

of biological communities.

 The complex matrix of interactions among members of a

community expends energy, as well as passing it from

one member to the next through trophic interactions.

 Likewise, biomass is constantly recycled through

production, predation, herbivory, and decomposition.

 Energy the ultimate energy source for almost every

The sun is

ecosystem on earth.

 Hydothermal vent communities are a partial exception-

(they rely on geothermal energy, but still depend upon

oxygen fixed by photosynthetic organisms).

 Energy enters ecosystems via photosynthesis (or, in a few

exotic excosystems, chemosynthesis).

 Organisms that bring energy into an ecosystem are called

producers.

 Producers include green plants, algae, cyanobacteria,

etc..anything that can make its own energy from nonliving

components of the environment.

 Organisms continuously use energy.

 All metabolic processes consume energy in

some way, and in each reaction, much of it is

effectively “wasted”…

 ..this is one reason why rapid metabolism makes us

homeothermic-the waste heat from metabolic

processes, mostly as molecular motion, warms our

bodies.

 Ultimately, all biological energy radiates into the

environment as infrared light (a by-product of

respiration).

 Much energy is lost every time it passes from

one trophic level to the next.

 Energy does not recycle.

 it must be continually replenished from the

sun.

 Autotrophs fix their own energy from inorganic

sources.

 Autotrophs are the producers in an ecosystem.

 Heterotrophs depend upon energy and carbon

fixed by some other organism

 they are consumers, detritivores, or decomposers.

 (A mixotroph is gets its energy from inorganic

sources, but relies of organic sources of carbon.)

 A food web is a schematic diagram that

describes the patterns of energy flow in

an ecosystem

 Every instance of predation, herbivory, and

parasitism is a trophic interaction that moves

energy from one organism to another.

 Decomposition is also a trophic interaction that

uses up the energy left over in dead bodies of

organisms.

 A food chain is one path through a food

web, from bottom to top.

 Because energy is lost at each step, food chains have a

limited number of links.

Matter

 Unlike energy, matter recycles through

ecosystems.

 Atoms of every biologically important element constantly

recycle through ecosystems, into the abiotic component of

the biosphere, and back into living systems.

 Elements are passed from one organism to another via trophic

interactions, or are taken directly from the environment.

 Via the process of decomposition, each element ultimately

becomes nonliving, and has the potential to re-enter the

biosphere again.

 Thus, each element has its own biogeochemical cycle-these

can take days, years, or eons, depending upon the element

and the circumstances.

Biomass

 Biomass can be defined as the weight of living

matter (usually measured in dry weight per unit

area).

 A pyramid of biomass is a figure that quantifies the

relative amounts of living biomass found at each

trophic level.

 In most ecosystems, the amount of biomass found in

each trophic level decreases progressively as one

moves from the bottom to the top of the food chain.

Pyramid of biomass for a pond. (Source: Data from Whittaker,

R.H. 1961. Experiments with radiophosphorus tracer in

aquarium microcosms. Ecological Monographs 31:157-188).

 Primary consumers eat producers.

 They generally possess significantly less biomass than

producers.

 Plants have evolved numerous mechanisms to protect their

tissues from consumption by herbivores and pathogens

 In most ecosystems only a small amount of producer biomass is

eaten.

 Significant losses of biomass occur because of digestive

inefficiencies, and return of CO2 to the atmosphere via

respiration.

 Assimilation efficiencies for most terrestrial herbivores range from 20 to 60

percent. Some invertebrates do better than that..some do not.

 A very large proportion of the assimilated biomass is lost through the

process of respiration, so only a small amount of the biomass is available to

the next level.

 Secondary consumers consume primary consumers.

 Tertiary consumers consume secondary consumers, and so forth.

 Not all organisms at one level are eaten, because of

defensive mechanisms-and predation is only one way to

die.

 Defensive adaptations include the ability to fly and run,

body armor, quills and protective spines, and camouflage.

 In general, carnivores have higher assimilation

efficiencies than herbivores. These range from 50 to 90

percent.

 Only a small fraction of the assimilated energy becomes

carnivore biomass because of the metabolic energy

needs of body maintenance, growth, reproduction, and

locomotion.

 Most food chains have at most

four or five trophic levels.

 The amount of biomass found

at each trophic level is small

relative to amount found at the

next lowest level.

 This is because less energy is

available to successive

consumers.









http://www.bioquip.com

 Decomposers, scavengers, saprophytes, and

detritivores are organisms that eat dead organic matter.

 Detritivores eat the dead bodies of living things, such

as carrion, leaf litter, etc..

 “Scavenger”s are animals that eat dead animals.

 Decomposers are microscopic organisms that break

down organic compounds into nonliving, inorganic

precursors.

 Saprophytes are organisms that feed on dead

organic matter, this term is usually applied to fungi

or bacteria, but there are plant saprophytes as well

Primary Productivity

 Primary productivity is the amount of biomass

produced through photosynthesis per unit area and

time by producers.

 It is usually expressed in units of energy (e.g., joules

/m2 day) or in units of dry organic matter (e.g., kg /m2

year).

 Globally, primary production amounts to 243 billion

metric tons of dry plant biomass per year.

 The total energy fixed by plants in a community through

photosynthesis is referred to as gross primary

productivity (GPP).

Net vs. Gross Primary Productivity

 Most gross primary productivity is used via respiration by

the producers themselves.

 Subtracting respiration from gross primary production gives

net primary productivity (NPP)

 NPP represents the rate of production of biomass that is

available for consumption (herbivory) by heterotrophic

organisms (bacteria, fungi, and animals). It is also easier to

measure, because it tends to accumulate over time.

 Problem:

 A plot of Panicum sp. grass has a gross

primary productivity of 10,700 kcal/m2year.

The grass respire approximately 9,100

kcal/m2year.

 What is the net primary productivity?

 Answer:

 10,700kcal/m2year - 9,100

kcal/m2year=1600kcal/m2year.







 Problem:

 The field is 10m x 10m. Over the course of

one year, what is the total net primary

productivity for the field?

 Answer:

 100m2 x 1600kcal/m2year=1.6x105kcal/year.





 Problem:

 If Panicum grass has an energy value of

6kcal/gram, and all of the primary

productivity were to accumulate as biomass,

how much biomass (expressed as dry weight)

will have accumulated in the field over the

course of 1 year?

 Answer:

 (1.6x105kcal/year x 1 year)/(6kcal/gram)=2.67x104

grams or 267kilograms.



 Problem:

 Suppose herbivores (wild mules) eat ALL this

biomass, and assimilate 10%. The respiration of the

mule is 15kcal/kilogram day.

 Would this field be sufficient to support a 150 kilogram

mule?

 Answer:

 The mule would assimilate (1.6x105kcal/year x

10%)=1.6x104kcal/year.

 Over the course of the year, the mule would

require 15kcal/kilogram day x 365 days x 150

kilograms=8.21x105kcal.

 The field is not nearly enough. This is why large

herbivores move around so much.

Communities Differ in their Productivity



 Globally, patterns of primary productivity vary both spatially

and temporally.

 The least productive ecosystems are limited by heat

energy, nutrients and water like the deserts and the polar

tundra.

 The most productive ecosystems have high temperatures,

plenty of water and lots of available soil nitrogen.

Productivity is high in areas of oceanic upwelling-

oceanic producers, which include diatoms,

dinoflagellates, cryptomonads, and other algae-

require nutrients

Nutrient Cycling

 Each biologically important element has nutrient

cycle.

 A nutrient cycle is the path of an element from one

organism to another, and from organisms into the

nonliving part of the biosphere and back.

 Nutrient cycles are sometimes referred to as

biogeochemical cycles, reflecting the fact that chemicals

are cycled between biological organisms, and between

organisms and the geologic (physical) environment.

C, H, O, N

 Carbon, hydrogen, oxygen, and nitrogen make up

most of the biological molecules found in living

organisms. These elements are passed from

organism to organism by chemical conversion

processes, which occur in food webs.

 They are also converted from non-living forms to

living forms by photosynthesis and nitrogen fixation,

and from living forms to non-living forms through

cellular respiration.

Reservoirs

 The non-living forms of carbon, hydrogen, oxygen,

and nitrogen form huge reservoirs in the physical

environment. For instance, nitrogen makes up

78% of the atmosphere as N2, and hydrogen

comes from water.

 In ecosystems ecology, a reservoir is a supply of a

biologically meaningful element that is not easily obtainable

by living organisms.

 Elements can have multiple reservoirs

Carbon

 Most of the material substances that make up living

organisms consist of organic compounds of carbon. In

contrast, carbon is relatively scarce in the nonliving part of

the Earth.

 Carbon exists in the non-living environment as carbon

dioxide in the atmosphere, dissolved carbon dioxide

(HCO3-, etc.) in the ocean, and as carbonates in the Earth’s

crust.

 It is also locked in fossil deposits, and embedded in the

ocean floor as deposits of methane anhydride.

 Carbon cycles between the

living and nonliving

components of the

biosphere.

 The most important reservoir

for carbon is the atmosphere:

 Although CO2 makes up less than

one percent of the atmosphere, it

is very important to the

biosphere.

 Much of the carbon in your

body was part of the

atmosphere, some of it

relatively recently.

 When you decompose, it will

return to the atmosphere.

Fixation, Fixation

Carbon in this sense, means capture and

conversion to a biologically useful form.

 Eg., water does not need to be “fixed”, neither does

sodium, but carbon and nitrogen do.

 CO2 is fixed by plants during photosynthesis.

 Photosynthesis converts atmospheric CO2 into

organic carbohydrates by combining them with

water, also from the nonliving part of the biosphere.

 This process requires the input of specific light photons,

which plants capture with the pigment chlorophyll.

 Once fixed by plants, CO2 is passed up the food

chain by trophic interactions such as herbivory and

predation.

Respiration



 Most organisms, including plants, respire.

 Respiration liberates carbon back into the atmosphere and

provides energy to the organism.

 CO2 enters the atmospheric reservoir.

 If it is not eaten and respired, or decomposed, organic

carbon may become buried and enter a carbon reservoir

in the soil, or ultimately fossilize.

 Carbon that is "fixed" can also return to

the atmosphere if the plant material is

burned, either naturally, or through

human activities.

 Even ancient plant and animal

material that contains carbon that

was fixed millions of years ago can

be returned to the atmosphere by

burning fossil fuels.

 Carbon can also be recycled back into

the atmosphere through volcanic

activity.

 As a tectonic plate goes underneath

a continent, superheated oceanic

material upgasses through

geological vents and reenters the

atmosphere.

Carbon, Global Warming,

Anthropogenic Climate Change

 CO2 has a crucial role in the climate of the Earth because

it is quite transparent to light at the visible wavelengths,

and relatively opaque to infrared light.

 Gasses with this property are called greenhouse gasses,

because they tend to trap heat, forcing a higher

equilibrium temperature.

 Methane, and CFC’s are also greenhouse gasses, but CO2 is the

most important because it occurs at higher concentrations.

 Geological periods of low CO2 concentration (such as the

present) are strongly correlated with low global

temperatures, higher CO2 is strongly correlated with

higher global temperatures.

 Additionally, sudden increases in CO2 can be linked to

a sudden warming of the climate.

 Such an event occurred in the Miocene, 15 million

years ago.

 There is very solid evidence that CO2 concentrations

have increased significantly over the course of the last

150 years.

 This is partially due to the burning of fossil fuels, and

partially due to deforestation.

 By cutting and burning of forests, the carbon that

once was locked in the trees is released into the

atmosphere.

 Huge stores of fossilized carbon are present within the

Earth’s crust, much of it buried and fossilized during the

Carboniferous period, 200million years ago.

 Liberation of these stores into the atmosphere has the

potential to dramatically change the climate of the

Earth.

 Evidence is mounting that these higher CO2 levels have

already affected the climate of the Earth.

 Some possible effects:

 Higher temps, especially in the high latitudes

 Drier continental interiors

 More unpredictable weather patterns, with more

extreme storms, and extreme heat events

 The potential for tropical diseases to enter

higher latitudes and higher elevations

 The potential for currently farmable areas to

become too dry to farm

 The potential to interfere with oceanic

thermohaline circulation, and cause conditions

in Europe and Eastern North America to

become very cold.

 The potential to interfere with oceanic

productivity through changes in Ph

 The potential for increases in sea level.

N

 N is one of the most common elements that

form biological molecules.

 It is a major component of amino acids, also

a primary constituent of nucleic acids.

The major reservoir for nitrogen is the atmosphere



 N2 makes up 78% of the Earth's atmosphere.

 The majority of living organisms are not able to use it in

that form.

 N2 contains a triple bond between the atoms, it is a very

stable molecule and therefore, biologically inert.

 A large amount of energy is required to break the triple

bond.

 lightning is responsible for converting some of the

atmospheric nitrogen into forms that organisms can

use.

 The process of converting atmospheric nitrogen into

forms that organisms can use is called nitrogen

fixation.

 Although most organisms are not able to convert

nitrogen, there are a few that are able to "fix"

atmospheric nitrogen.

 Some free-living soil bacteria as well as some blue-

green bacteria have the ability to convert nitrogen

into ammonia.

 Nitrogen is also fixed by symbiotic bacteria

that live in and among the root cells of several

types of plants, most notably, the legume

plants such as beans, peanuts, and peas.

Other plants such as alfalfa, locust, and alders

also have root nodules.

 There are a few that are able to "fix"

atmospheric nitrogen.

 These include bacteria in the genus

Rhizobium and Bradyrhyzobium, and also

some cyanobacteria, such as Anabaena and

Nostoc,

 This process, which is energetically

expensive, converts nitrogen into ammonia.

 Other bacteria convert ammonia to nitrates

through nitrification.

 Most plants use nitrogen in the form of

nitrates, though ammonia is also useful.

 Nitrogen fixing bacteria frequently live in mutualistic symbiosis with plants,

notably legumes.

 Thus, legumes can be disproportionately important to

the ecology of a plant community.

 Once nitrogen is absorbed by plants and built into

the plant molecules, the nitrogen can be passed to

consumers and to decomposer organisms through

the food chain.

 Nitrogen can be mineralized and converted to

organic compounds that enter the soil or water

upon their death, or enter as waste through

their digestive tracts.

 These decomposed nitrogen compounds -

ammonia, nitrite, and nitrates, then become

available for other plants to absorb and recycle.

This process is called ammonification.

 Alternatively, other bacteria, known as

"denitrifiers," convert nitrites and nitrates in the

soil to N2O and N2, which returns to the

reservoir in the atmosphere. This process, which

completes the nitrogen cycle, is called

denitrification.

 Certain bacteria convert ammonia to nitrates

through nitrification. Most plants use

nitrogen in the form of nitrates.

 Once nitrogen is absorbed by plants and built

into the plant molecules, the nitrogen can be

passed to consumers and to decomposer

organisms through the food chain.

Water

 The water cycle is one of the most important processes to

living organisms on Earth.

 Water that has evaporated into the atmosphere condenses

and falls as precipitation.

 This precipitation will either run off as surface water and collect as

streams or rivers, or it can seep into the ground and collect in huge

underground rock formations called aquifers, that act much like

sponges.

 The water eventually flows from lakes or streams down into the oceans,

where it can reside for long periods of time, or get evaporated back

up into the atmosphere as water vapor, which collects as clouds.

 A portion of the water absorbed into the ground

is taken up by plants, which use the water to

transport minerals internally as well as to take

part in the photosynthetic process.

 Some of this water is transferred to animals that

feed on plants; from there, water can cycle within

the food web of an ecosystem.

 Water can be given off to the atmosphere by

plant leaves through transpiration, or by

animals through respiration, perspiration, or

excretion.