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Life and Earth’s Life Support Systems

What is Life?
The cell is the basic unit of life.  Each cell is encased in an outer membrane or wall and contains genetic material (DNA) and other parts to perform its life functions.  All forms of life:
Have a highly organized internal structure and organization.
Have a characteristic types of deoxyribonucleic acid, or DNA, molecules in each cell.  DNA is the stuff genes- the basic units of heredity- are made of.
Can capture and transform matter and energy from their environment in order to survive, grow, and reproduce.
Can maintain favorable internal conditions despite changes in their external environment if not overwhelmed.
Arise through reproduction
Can adapt to external changes by mutations- random changes in their DNA molecules- and through combinations of existing genes during reproduction.  Most mutations are harmful, but some give rise to new genetic traits that allow the organisms to adapt to changing environmental conditions and have more offspring than those without such traits- a process called natural selection.  When such genetic changes in existing organisms from distinctly different new organisms, evolution has occurred.  Life forms that cannot adapt become fewer and may become extinct.  These ongoing processes have led to the diversity of life forms found on Earth today.  This biodiversity.

Earth: A Dynamic Planet
	We can think of the earth as being made up of several layer or spheres (figure 4-2):
       
- The atmosphere- a thin, gaseous envelope of air around the planet.  Its inner layer, the troposphere, extends only about 17 kilometers (11 miles) above sea level but containd\s most of the planet’s air- mostly nitrogen (78%) and oxygen (21%).  The next layer, stretching 17-48 kilometers (11-30 miles) above Earths surface, is called the stratosphere.  Its lower portion contains enough ozone (O3) to filter out most of the sun’s harmful ultraviolet radiation, thus allowing life on land to exist.

- The hydrosphere- liquid water (both surface and underground), frozen water (polar ice, icebergs, permafrost in soil), and water vapor in the atmosphere.

- The lithosphere- Earth’s crust and upper mantle.  It contains the fossil fuels and minerals we use and the soil chemicals (nutrients) needed to support plant life.

- The ecosphere or biosphere- the portion of Earth where living (biotic) organisms are found and interact with one another and with their non-living (abiotic) environment.

ENERGY FLOW, MATTER CYCLING,AND GRAVITY
Life on earth depends on three connected factors:
- The one-way flow of high quality (usable) energy from the sun.
- The cycling of matter required.
-Gravity

THE SUN: SOURCE OF ENERGY FOR LIFE
The sun light and warms the planet. It also suplies the energy for photosymthesis prosses used by green plants and some bacteria to synthesize the compound that keeps them alive and feed most other organism. Solar energy also powers the cycling of matter and drives the climate and weather system that distribute heat and fresh water over Earth’s surface. It is expected to provide Earth with energy for atleast another 4 billions years.
Moust of what reaches the troposphere is viseble light and infrared rediation (heat) plus a small amount of ultraviolet radietion not absorved by the ozone layey in the stratasphere.
	About 34% of the solar energy reaches the troposphere is reflected right back to space by clouds, chemicals, and dust and by the Earth’s surface of land and water. The ability of surfaces to reflect radiation, called albedo, depends on their color and their texture. Clouds, ice, and snow have a high albedo, while forest have a low one. The albedo of the ocean, which covers three-quarters of the Earth’s surface, depends on the angle at whic the sun’s rays strike.

Nutrient Cycles 
Any chemical element or compound an organism must take in to live, grow, or reproduce is called a nutrient. Those needed in large amounts are called macronutrients. Carbon, oxygen, nitrogen. Elements required by organisms in smallor trace anount are called micronutrients. Examples are iron, copper, zink, chloride and iodine.
	These nutrient elements and their compounds are continuously cycle from the nonliving enviroment (air, water, soil) to living organisms, and back to the nonliving enviroment in what are called nutrient cycle, or biogeochemical cycles driven directly or inderectly by incoming solar energy and gravity, include the carbon, oxygen, nitrogen, phosphorus, sulfer and hydrologic (water) cycles.

THE REALM OF ECOLOGY 
Ecology deals mainly eith interactions among organisms, populations, communities, ecosystem, and the ecosphere. An organismis is any form of life. Estimate range from 5 million to 100 million, most of them insects, microscopic organisms, and tiny sea creatures. So far biologists have identefied and named only about 1.4 million species.
	A population is a group of individuals of the same species occupying a given area at the same time. Examples are all sunfish in a pond, while oak trees in a forest, and people in a country. Individuals vary slighttly in their genetic makeup so that they don’t all look or behave exactly alike-some called genetic diversety. 
	The place where a population (or an individual organism) normally lives is known as its habitat.  Populations of all the  species occupying a particular place make up a community or biological community. 	
	An ecosystem, recall, is a community of different species involved in a dynamic network of biological, chemical, and physical interactions that sustain it.
	Climate - long term weather- is the main factor determining what type of life, especially what plants will thrive in a given land area.  Biologist have divided the terrestrial (land) portion of the ecosphere into biomes, large regions such as forests, deserts.  Each biome consists of many ecosystems.  

BIODIVERSITY 
	Biological diversity, or biodiversity consists of the forms of life that can best survive the variety of conditions currently found on Earth and includes genetic diversity, species diversity, and ecological diversity.
	Species diversity is the variety of species on Earth and in different habitats of the planet.  Ecological diversity is the variety of forests, deserts, grasslands, streams, lakes, oceans, and other biological communities that interact with one another and with their nonliving environment.

TYPES OF ORGANISMS
	All organisms except bacteria are eukaryotic:  Their cells have a nucleus and several other internal parts surrounded by membranes.  Bacterial cells are prokaryotic:  They have no distinct nucleus or other internal parts enclosed by membranes.

Monera (bacteria and cyanobacteria) are single-celled, prokaryotic, microscopic organisms.

Protista (protists) are amoebas and slime molds.

Fungi are mushrooms, molds, and yeasts.

Plantae (plants) 

Animalia (animals) most called invertebrates, have no backbones.  Sponges, jellyfish, worms, anthropods (insects, shrimp, spiders), mollusks (snails, clams, octopuses), and echinoderms (sea stars, sea urchins).  The vertebrates, animals with backbones.

LIVING COMPONENTS OF ECOSYSTEMS
	Living components of ecosystems can be broken down into two parts:  the living or biotic and the nonliving or abiotic water, air, nutrients, and solar energy.  Living organisms in ecosystems are usually classified as either producers or consumers, based on how they get food. 
	Producers--sometimes called autotrophs (self-feeders)--can make the organic nutrients they need from simple inorganic compounds in their environment.  In most terrestrial ecosytems green plants are the producers.  In aquatic ecosystems most of the producers are phytoplankton, floating and drifting bacteria and protists, mostly microscopic.  Only producers make their own food.



HAVE YOU THANKED INSECTS TODAY?
	Insects play important and largely unrecognized roles.  A large fraction of Earth’s plant species depend on insects for pollination and reproduction.  Plants also owe their live sto the insects that help turn the soil around their roots and decompose dead tissue into nutrients the plants need.  In turn, we and other land-dwelling animals depend on plants for food, either by eating them or by consuming plant-eating animals.
	Indeed, if all insects were to disappear, we and most of Earth’s amphibians, reptiles, birds, and other mammals would become extinct within a year because of the disappearance of most plant life.  Earth would be covered with dead and rotting vegetation and animal carcasses being decomposed by unimaginable hordes of bacteria and fungi.  The land would be largely devoid of animal life and covered by mats of wind-pollinated vegetation and clumps of small trees and bushes here and there.
	This, however, is not a realistic scenario.  Insects, which originated on land nearly 400 million years ago, are so diverse, abundant, and adaptable that they are virtually invincible.  Some have asked whether insects will take over if the human race extinguishes itself.  This is the wrong question.  The roughly billion billion insects alive at any given time around the world have been in charge for millions of years.  Insects can thrive without new comers such as us, but we and most other land organisms would quickly perish without them.  


	Most producers use sunlight to make organic nutrients by photosynthesis.  Although hundreds of chemical changes take place in sequence during photosynthesis, the overall net change can be summarized as follows:

	carbon dioxide + water + solar energy		glucose + oxygen

	6 CO2	     + 6 H2O + solar energy		C6H12O6 + 6 O2


	A few producers, mostly specialized bacteria, can convert inorganic compounds from their environment into organic nutrients without sunlight--a process called chemosynthesis.  In this case the source of energy is heat generated by the decay of radioactive elements deep in the earth’s core and released at hot-water vents in the oceans’s depths.  In the pitch-dark around such vents specialized producer bacteria use the heat to convert dissoved hydrogen sulfide (H2S) and carbon dioxide into organic nutrient molecules.  
	All other organisms in ecosystems are consumers, or heterotrophs (other-feeders), which get their organic nutrients by feeding on the tissues of producers or other consumers.  There are several classes of consumers: 

Herbivores (plant-eaters) are called Primary comsumers  because they feed directly on other producers.

Carnivores (meat-eaters) feed on other consumers.  Those called secondary consumers feed only on primary consumers.  Most secondary consumers are animals, but few such as the Venus flytrap trap and digest insects.  Tertiary (higher level) consumers feed only on other conivores.

Omnivores eat both plants and animals.  Examples are pigs, rats, foxes, cockroaches,and humans.

Detritivores (decomposers and detritus feeders) live off detritus, part of dead organisms and cast-of fragments and waste of living organisms (Figure 4-16).

Decomposers digest the complex organic molecules in detritus into simpler inorganic compounds and obsorb the solulbe nutrients.  These decomposers- mostly bacteria and fungi-(Figure 4-17)are an important sorce of food for worms and insects in the soil  and water.  Detritus feeders,  such as crabs, carpenter ants, termites, and earthworms, extract nutrients from parrtly decomposed organic matter.

Producers and consumers use the chemical enery stored in glucose and other nutrients to drive their life processes.  This enrgy is released by the product of aerobic respiration(not the same as the breathingcalled resiration),

glcose  + oxygen       carbon doixide + water + energy

C6H12O6 + 6 O2       6CO2               +6H2O + energy

NONLIVING PARTS OF AN ECOSYSTEM  
The nonliving things, or abiotic, components of an ecosystem are physical and chemical factors that influence living organism. Important physical factors affecting ecosytems are: 
Sunlight 
Average temperature
Average precipitation
Wind
Latitude
Altitude
Soil
Fire
Water current (aquatic ecosystem)
	The following are important chemical factors affecting ecosystems:
Supply of water and air in the soil (land ecosystems).
Plant nutrients desolve in soil and in water. 
Level of toxic substances.
Salinity of water (aquatic ecosystems)
Level of dissolved oxygem (aquatic ecosystems)


TOLERANCE RANGE OF SPECIES
Each population in an ecosystem has a range of tolerance to variation in its physical and chemical enviroment. 
These observations are summarized in the law of tolerance: The existence, abundance, and distribution of a species in an ecosystem are determined by wether the levels of one or more physical or chemical factors fall within the range tolerated by the species.
Thid agjustment to slowly changing this conditions, or acclimation, is a useful protective device. The next small change triggers a threshold effect, a harmful or even fatal reaction as the tolerance limit is exceeded-like adding that proverbial single straw that breaks the already heavily loaded camel’s back.
	The  threshold explains why many enviromantal problems seem to arise suddently. For example spruce trees suddenly begin dying in droves, but the cause may be exposur to numerous air pollutants for decades. When whole forests die, as is happening in parts of Europe and North America, we’re 10 or 20 years too late to do enything about it.

LIMITING FACTORS IN ECOSYSTEMS
An ecological principle related to the law of tolerance is the limiting factor  principle: Too much or too litte of any abiotic factor can limit or prevent growth of a population even if all other factros. Such a factor is called a limiting factor.
	In aquatic ecosystems salinity is a limiting factor. Important limiting factors for these layers are temperature, sunligth, dissolved oxygen content (the amount of oxygen gas dissolved in a given volume of water at a particular temperature and presure), and availability of nutrients.


FOOD CHAINS AND FOOD WEBS
All organisms, dead or alive, are potential sources of food for other organisms. The sequence of who eats or decomposes whom in an ecosustem is called food chain. Ecologists assign every organism in an ecosystem to a feeding level, or trophic level (from the Greek trophos, “nourishment”), depending on whether it is a producer or a consumer and on what it eats or decomposes.  Producers belong to the first trophic level, secondary consumers to the third trophic level, and so on.  Detritivores process detritus from all trophic levels.  Food chains rarely have more than four trophic levels or energy transfers.  Organisms in most ecosystems form a complex network of feeding relationships called a food web.     

ECOLOGICAL PYRAMIDS
By counting the organisms at each trophic level, ecologists can graph this information to yield a pyramid of numbers for an ecosystem.  For example, a million phytoplankton in a samll pond may support 100 perch, which might feed 1 person for a month or so.  Each trophic level in a food chain or web contains a certain amount of biomass, the weight of all organic matter contained in its organisms.  These data are used to plot a pyramid of biomass.  In a food chain or web, biomass (and thus chemical energy) is transferred from one trophic level to another, with some usable energy lost in each transfer.  Pyramid of energy flow shows that the more trophic levels or steps in a food chain or web, the greater the cumulative loss of usable energy.  The energy flow pyramid illustrates why Earth can support more people if they eat at lower trophic levels by consuming grains directly (for example, rice--human) rather than consuming grain--eaters (grain--steer--human).  This is also why top carnivores like eagles, tigers, and white sharks are sparse in numbers and usually die off in large numbers when the ecosystems that support them are disrupted.

PRODUCTIVITY OF PRODUCERS
The rate at which an ecosystem’s producers capture and store chemical energy as biomass is the ecosystem’s gross primary productivity.  However, since the producers must use some of this biomass to stay alive, what we need to know is an ecosystem’s net primary productivity.  

				rate at which		rate at which
				producers pro-		producers use
				duce chemical	    	chemical energy
net primary productivity = 	energy stored 	    -      	stored in biomass
				in biomass		through aerobic
				through photo-	respiration
				synthesis

	An estimated 59% of Earth’s annual net primary productivity takes place on land, and the remaining 41% in oceans and other aquatic systems.  Estuaries, swamps and marshes, and tropical rain forests are highly productive; open ocean, tundra (arctic grasslands), and desert are the least productive.
	In tropical forests most nutrients are stored in vegetation rather than in the soil.  When the trees are cleared, the nutrient-poor soils are rapidly depleted of their nutrients by frequent rains and by growing crops.  Thus food crops can be grown only for a short time without massive applications of commercial fertilizers.  Humans now use or waste about 27% of the world’s potential net primary productivity (40% for land systems).  		


MATTER CYCLING IN ECOSYSTEMS

CARBON CYCLE 
	Carbon is the basic building of the carbohydrates, fats, proteins, nucleic acids such as DNA and RNA, and other organic compounds necessary for life. The carbon cycle is based on carbon dioxide gas, wich make up almost 0.036% by volume of the troposphere and is also dissolved in water. 
	Producers absorb CO2 from  the atmosphere or water and use photosymthesis to convert it into complex carbohydrates. Then the cells in oxygen-consuming producers and consumers carry out aerobic respiration, which breaks down glucose and athers complex organic compounds and convert the carbon back to CO2 in the atmosphere or in water for reuse by prodecers. This linkage between photosynthesis in producers and aerobic respiration in producers and consumers circulates carbon in the ecosphere and is amajor part of the gloval carbon cycle.
	Carbon dioxide is the key component of nature’s thermostat. If the carbon cycle removes too much CO2 from the atmosphere, Earth will cool; if the cycle generates too much, Earth will get hotter. Some carbon lies deep in the earth In fossil fuels-caol, pretoleum, and natural gas-and  is released to the atmosphere as carbon dioxide only when these fuel extracted and burmed. CO2 also enters the atmsphere from aerobic respiration and from volcanic eruptions, which free carbon from rocks deep in the earth’s crust.
	The oceans also play a major role in regulating the level of carbon dioxide in the atmosphere. Some CO2 stay in the sea, some is removeed by photosynthesizing producers. As water warms, more dissolved CO2 returns to the atmosphere. In marine ecosystems some organisms take up dissolved CO2 molecules from ocean water and form slightly soluble carbonate compounds such as calcium carbonate (CaCO3 ) to build shells and the skeletons of marine organisms.  In fact, most of the earth’s carbon--10,000 times that in the total mass of all life on Earth--is stored in the ocean floor sediments and on the continents.  This carbon reenters the cycle very slowly when some of the sediments dissolve and form dissolved CO2  gas that can enter the atmosphere.  Geologic processes can also bring bottom sediments to the surface, exposing the carbonate rock to chemical attack by oxygen and conversion to CO2 gas.  
	We have disturbed the carbon cycle in two ways that add more carbon dioxide to the atmosphere than oceans and plants can remove:
	
	--  Forest and brush clearing, leaving less vegetation to absorb CO2.

	--  Burning fossil fuels and wood, which produces CO2 that flows into the
	     atmosphere.     

NITROGEN CYCLE: BACTERIA IN ACTION
	Organisms need nitrogen to make proteins, DNA, RNA, and other nitrogen-containing orgain compounds.  The nitrogen gas (N2) that makes up 78% of the volume of the troposphere cannot be used directly as a nutrient by multicellular plants or animals.  Bacteria conver nitrogen gas into water-soluble compounds, which are taken up by plant roots as part of the nitrogen cycle.
	The conversion of atmospheric nitrogen gas into other chemical forms useful to plants is called nitrogen fixation.  Animals get their nitrogen by eating plants or plant-eating animals.  
	We intervene in the nitrogen cycle in the following ways:
	
	--  Emitting nitric oxide (NO) into the atmosphere when any fuel is burned.  This 
	     nitric oxide combines with oxygen to from nitrogen dioxide(NO2) gas , which can react with vapor to form nitric acid (HNO3). This acid is a component of acid deposition (commonly called acid rain)

-Emitting heat-trapping nitrous oxide (N2O) gas into the atmosphere by the action of bacteria on livestock waste and izers applied to the soil.

-Mining minerals deposits containing nitrate and ammonium ions for fertilizers.

-Depleting nitrate and ammonium ions from soil by harvesting nitrogen-rich crops.

-Adding excess nitrate and ammonium ions to aquatic ecosystems in agricultural runoff and discharge of municipal sewage.  This excess of plant nutrients stimulates rapid growth of algae and other aquatic plants.  The subsequent breakdown of dead algae be aerobic decomposers depletes the water of dissolved oxygen gas, killing great numbers of fish.

PHOSPHORUS CYCLE
	Phosphorus, is an essential nutrient of both plants and animals.  It is a part of DNA molecules, which carry genetic information; other molecules, which store chemical energy for use by organisms in cellular respiration; certain fats in the membranes that encase plant and animal cells; and animal bones and teeth.
  	Phosphorus moves through water, Earth’s crust, and living organisms in the phosphorus cycle.
	Phosphorus released by the slow breakdown, or weathering, of phosphate rock deposits is dissolved in soil water and taken up by plant roots.  Most soils contain little phosphorus.  Thus phosphorus is the limiting factor for plant growth in many soils and aquatic ecosystems.
	Animals get their phosphorus by eating producers or animals that have eaten producers.  Animal wastes and the decay products of dead animals and producers return much of this phosphorus to the soil, to streams, and eventually to the ocean bottom as deposits of phosphate rock.
	Some phosphate returns to the land as guano--the phosphate-rich manure of fish-eating birds such as pelicans and cormorants.  We intervene in the phosphorus cycle chiefly in two ways.
- Mining large quantities of phosphate rock to produce commercial inorganic fertilizers and detergents.
-Adding excess phosphate ions to aquatic ecosystems in runoff of animal wastes.  Much of this nutrient causes explosive growth of cyanobacteria, algae, and aquatic plants, disrupting life in aquatic ecosystems.

SULFUR CYCLE
	Sulfur circulates through the ecosphere in the sulfur cycle.  Much of the earth’s sulfur is tied up underground.
	However, sulfur also enters the atmosphere from several natural sources.  Hydrogen sulfide (H2S, a colorless, highly poisonous gas with a “rotten-egg” smell, comes from active volcanoes and from the breakdown of organic matter in swamps, bogs, and tidal flats caused by decomposers that don’t use oxygen (anaerobic decomposers).  
	In the atmosphere sulfur dioxide reacts with oxygen to produce sulfur trioxide gas (S03), which in reacts with water vapor to sulfuric acid (H2SO4).  These droplets of sulfuric acid and paricles of sulfate salts fall to Earth as components of acid deposition, which can harm trees and aquatic life.
	About a third of all sulfur, including 99% of the sulfur dioxide, that reaches the atmosphere comes from human activities.  We intervene in the atmospheric phase of the sulfur cycle in two ways:
-Burning sulfur-containing coal and oil to produce electric power, which produces about two-thirds of the human inputs of sulfur dioxide
-Refining petroleum, smelling sulfur compounds of metallic minerals into free metals such as copper, lead, and zinc, and performing other industrial processes

HYDROLOGIC CYCLE
	The main processes in this water recycling and purifying cycle are evaporation, transpiration, condensation, cipitation, infiltration, percolation, and runoff downslope back to the sea to begin the cycle again.
	The water cycle is powered by energy from the sun and by gravity.  About 84% of the moisture comes from the oceans, and the rest from land.  The amount of moisture in the atmosphere at any one time is the equivalent of only about 25 millimeters (1 inch) of rainfall spread over Earth’s entire surface.  However, because this moisture is renewed, the average annual rainfall over Earth as a whole is 1 meter (40inches), although actual rainfall varies widely in different places.
	The amount of water vapor air can hold depends on its temperature, with warm air capable of holding more water vapor than cold air.  Absolute humidity is air (usually expressed as grams of water per kilogram of air).  Relative humidity is a measure (expressed in percentages) of the amount of water vapor in a certain mass of air compared with the maximum amount it could hold at that temperature.  For example, a relative humidity of 60% at 27 degrees C (80 degrees F) means that each kilogram (or other mass unit) of air has 60% of the water vapor it could hold at that temperature.
	Winds and air masses transport this water vapor over various parts of the earth’s surface, often over long distances.  Falling temperatures cause the water vapor to condense into tiny droplets taht form clouds or fog.  For precipitation to occur, air must contain condensation nuclei--tiny particles on which droplets of water vapor can collect.  Volcanic ash, soil dust, smoke, sea salts, and particulate matter emitted by factories coal-burning power plants, and vechicles are sources of such particles.  The temperature at which condensation occurs for a given amount of water vapor is called the dew point.
	It takes millions of tiny water droplets adhering to condensation nucleus to produce a drop or rain or a snowflake that will fall to the earth’s surface.  About 77% of this precipitation falls back into the sea, and the rest over land.  
	Some of the fresh water returning to the earth’s surface as precipitation becomes locked in glaciers.  Much of it collects in puddles and ditches, and runs off into nearby lakes and into streams, which carry water back to the ocearns, completing the cycle.  
	Besides replenishing streams and lakes, surface water running off the land also causes soil erosion, which moves various nutrients through portions of other biogeochemical cycles.  Water is the medium that carries rock debris from the mountains into valleys and eventually down to the ocean floor.  It is the primary sculptor of Earth’s  landscape.  Also, acidic rainwater reacts chemically with metallic minerals on Earth’s surface, forming water-soluble metallic salts that are carried by rivers to the sea.  This is one of the reasons the ocean is so salty.  
	Much of the water returning to the land seeps into or infiltrates surface soil layers.  Some percolates downward into the ground, dissolving minerals from porous rocks on the way.  There this mineral-laden water is stored as groundwater in the pores and cracks of rocks.  This underground water, like surface water, flows downhill and seeps out into streams and lakes or comes out in springs.  Eventually this water evaporates or reaches the sea to begin the cycle again.  The average circulation rate of underground water in the hydrologic cycle is extremely slow (300-4,600 years) compared with that of water in lakes (13 years), in streams (13 days), and in the atmosphere (9 days).  The turnover time for water in the ocean is about 37,000 years.
	Withdrawing large quantities of fresh water from streams, lakes, and underground sources.  In heavily populated or heavily irrigated areas withdrawals have led to groundwater depletion or intrusion of ocean salt water into underground water supplies.
	Clearing vegetation from land for agriculture, mining, roads, construction, and other activities.  This reduces seepage that recharges groundwater supplies, increases the risk of flooding, and speeds surface runoff, producing more soil erosion and landslides.

Roles of Species in Ecosystems

TYPES OF SPECIES IN ECOSYSTEMS
One way to look at an ecosystem’s species is to divide them into four types:

-  Native species, which normally live and thrive in a particular ecosystem.
-  Immigrant, or alien, species, which migrate into an ecosytem or which are deliberately or accidentally introduced into an ecosystem by humans.  Some of these species are beneficial, while others can take over and eliminate native species.
-  Indicator species, which serve as early warnings that a community or an ecosystem is being damaged.
-  Keystone species affect many other organisms in an ecosystem.*  For example, in tropical forests various species of bees, bats, ants, and hummingbirds play keystone roles in pollinating flowering plants, dispersing seed, or both.

* Some scientists consider all species equally important, but others consider certain species to be keystone or more important than others.

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