Ecology
| I | INTRODUCTION |
Ecology, the study of the relationship of plants and
animals to their physical and biological environment. The physical environment
includes light and heat or solar radiation, moisture, wind, oxygen, carbon
dioxide, nutrients in soil, water, and atmosphere. The biological environment
includes organisms of the same kind as well as other plants and animals.
Because of the diverse approaches required to
study organisms in their environment, ecology draws upon such fields as
climatology, hydrology, oceanography, physics, chemistry, geology, and soil
analysis. To study the relationships between organisms, ecology also involves
such disparate sciences as animal behavior, taxonomy, physiology, and
mathematics.
An increased public awareness of environmental
problems has made ecology a common but often misused word. It is confused
with environmental programs and environmental science (see Environment).
Although the field is a distinct scientific discipline, ecology does indeed
contribute to the study and understanding of environmental problems.
The term ecology was introduced by the
German biologist Ernst Heinrich Haeckel in 1866; it is derived from the Greek
oikos (“household”), sharing the same root word as economics.
Thus, the term implies the study of the economy of nature. Modern ecology, in
part, began with Charles Darwin. In developing his theory of evolution, Darwin
stressed the adaptation of organisms to their environment through natural
selection. Also making important contributions were plant geographers, such as
Alexander von Humboldt, who were deeply interested in the “how” and “why” of
vegetational distribution around the world.
| II | THE EARTH'S BIOSPHERE |
The thin mantle of life that covers the earth
is called the biosphere. Several approaches are used to classify its
regions.
| A | Biomes |
The broad units of vegetation are called
plant formations by European ecologists and biomes by North American
ecologists. The major difference between the two terms is that biomes include
associated animal life. Major biomes, however, go by the name of the dominant
forms of plant life.
Influenced by latitude, elevation, and
associated moisture and temperature regimes, terrestrial biomes vary
geographically from the tropics through the arctic and include various types of
forest, grassland, shrub land, and desert. These biomes also include their
associated freshwater communities: streams, lakes, ponds, and wetlands. Marine
environments, also considered biomes by some ecologists, comprise the open
ocean, littoral (shallow water) regions, benthic (bottom) regions, rocky shores,
sandy shores, estuaries, and associated tidal marshes.
See also Chaparral; Coral Reef;
Estuary; Marine Life; Marshland; Peatland; Savanna; Shore Life; Tundra.
 | |||
| Parts of Ecosystem | Secondary consumers are a diverse group of animals—some eat primary consumers and some eat other secondary consumers. Those animals that eat smaller primary consumers include frogs, snakes, foxes, and spiders. Animals that eat secondary consumers include hawks, wolves, and lions. | Primary consumers are animals that feed on plants. This group includes some insects, seed- and fruit-eating birds, rodents, and larger animals that graze on vegetation, such as deer. When primary consumers eat primary producers (plants), the energy in plant cells changes into a form that can be stored in animal cells. Primary producers....Plants are primary producers. All life in an ecosystem depends on primary producers to capture energy from the Sun, convert it to food that is stored in plant cells, and pass this energy on to organisms that eat plants. | Decomposers include worms, mushrooms, and microscopic bacteria. These organisms break down dead plants and animals into the nutrients needed by plants to survive. |
B |
Ecosystems |
A more useful way of looking at the
terrestrial and aquatic landscapes is to view them as ecosystems, a word
coined in 1935 by the British plant ecologist Sir Arthur George Tansley to
stress the concept of each locale or habitat as an integrated whole. A
system is a collection of interdependent parts that function as a unit
and involve inputs and outputs. The major parts of an ecosystem are the
producers (green plants), the consumers (herbivores and
carnivores), the decomposers (fungi and bacteria), and the
nonliving, or abiotic, component, consisting of dead organic matter and
nutrients in the soil and water. Inputs into the ecosystem are solar energy,
water, oxygen, carbon dioxide, nitrogen, and other elements and compounds.
Outputs from the ecosystem include water, oxygen, carbon dioxide, nutrient
losses, and the heat released in cellular respiration, or heat of respiration.
The major driving force is solar energy.
| C | Energy and Nutrients |
Ecosystems function with energy flowing in
one direction from the sun, and through nutrients, which are continuously
recycled. Light energy is used by plants, which, by the process of
photosynthesis, convert it to chemical energy in the form of carbohydrates and
other carbon compounds. This energy is then transferred through the ecosystem by
a series of steps that involve eating and being eaten, or what is called a food
web. Each step in the transfer of energy involves several trophic, or feeding,
levels: plants, herbivores (plant eaters), two or three levels of carnivores
(meat eaters), and decomposers. Only a fraction of the energy fixed by plants
follows this pathway, known as the grazing food web. Plant and animal
matter not used in the grazing food chain, such as fallen leaves, twigs, roots,
tree trunks, and the dead bodies of animals, support the decomposer food
web. Bacteria, fungi, and animals that feed on dead material become the
energy source for higher trophic levels that tie into the grazing food web. In
this way nature makes maximum use of energy originally fixed by plants.
The number of trophic levels is limited in
both types of food webs, because at each transfer a great deal of energy is lost
(such as heat of respiration) and is no longer usable or transferable to the
next trophic level. Thus, each trophic level contains less energy than the
trophic level supporting it. For this reason, as an example, deer or caribou
(herbivores) are more abundant than wolves (carnivores).
Energy flow fuels the
biogeochemical, or nutrient, cycles. The cycling of nutrients begins with
their release from organic matter by weathering and decomposition in a form that
can be picked up by plants. Plants incorporate nutrients available in soil and
water and store them in their tissues. The nutrients are transferred from one
trophic level to another through the food web. Because most plants and animals
go uneaten, nutrients contained in their tissues, after passing through the
decomposer food web, are ultimately released by bacterial and fungal
decomposition, a process that reduces complex organic compounds into simple
inorganic compounds available for reuse by plants.
| D | Imbalances |
Within an ecosystem nutrients are cycled
internally. But there are leakages or outputs, and these must be balanced by
inputs, or the ecosystem will fail to function. Nutrient inputs to the system
come from weathering of rocks, from windblown dust, and from precipitation,
which can carry material great distances. Varying quantities of nutrients are
carried from terrestrial ecosystems by the movement of water and deposited in
aquatic ecosystems and associated lowlands. Erosion and the harvesting of timber
and crops remove considerable quantities of nutrients that must be replaced. The
failure to do so results in an impoverishment of the ecosystem. This is why
agricultural lands must be fertilized.
If inputs of any nutrient greatly exceed
outputs, the nutrient cycle in the ecosystem becomes stressed or overloaded,
resulting in pollution. Pollution can be considered an input of nutrients
exceeding the capability of the ecosystem to process them. Nutrients eroded and
leached from agricultural lands, along with sewage and industrial wastes
accumulated from urban areas, all drain into streams, rivers, lakes, and
estuaries. These pollutants destroy plants and animals that cannot tolerate
their presence or the changed environmental conditions caused by them; at the
same time they favor a few organisms more tolerant to changed conditions. Thus,
precipitation filled with sulfur dioxide and oxides of nitrogen from industrial
areas converts to weak sulfuric and nitric acids, known as acid rain, and falls
on large areas of terrestrial and aquatic ecosystems. This upsets acid-base
relations in some ecosystems, killing fish and aquatic invertebrates, and
increasing soil acidity, which reduces forest growth in northern and other
ecosystems that lack limestone to neutralize the acid.
See Carbon Cycle; Nitrogen
Cycle.
| III | POPULATIONS AND COMMUNITIES |
The functional units of an ecosystem are the
populations of organisms through which energy and nutrients move. A population
is a group of interbreeding organisms of the same kind living in the same place
at the same time (see Species and Speciation). Groups of populations
within an ecosystem interact in various ways. These interdependent populations
of plants and animals make up the community, which encompasses the biotic
portion of the ecosystem.
| A | Diversity |
The community has certain attributes,
among them dominance and species diversity. Dominance results when one or
several species control the environmental conditions that influence associated
species. In a forest, for example, the dominant species may be one or more
species of trees, such as oak or spruce; in a marine community the dominant
organisms frequently are animals such as mussels or oysters. Dominance can
influence diversity of species in a community because diversity involves not
only the number of species in a community, but also how numbers of individual
species are apportioned.
The physical nature of a community is
evidenced by layering, or stratification. In terrestrial communities,
stratification is influenced by the growth form of the plants. Simple
communities such as grasslands, with little vertical stratification, usually
consist of two layers, the ground layer and the herbaceous layer. A forest has
up to six layers: ground, herbaceous, low shrub, low tree and high shrub, lower
canopy, and upper canopy. These strata influence the physical environment and
diversity of habitats for wildlife. Vertical stratification of life in aquatic
communities, by contrast, is influenced mostly by physical conditions: depth,
light, temperature, pressure, salinity, oxygen, and carbon dioxide.
| B | Habitat and Niche |
The community provides the habitat—the
place where particular plants or animals live. Within the habitat, organisms
occupy different niches. A niche is the functional role of a species in a
community—that is, its occupation, or how it earns its living. For example, the
scarlet tanager lives in a deciduous forest habitat. Its niche, in part, is
gleaning insects from the canopy foliage. The more a community is stratified,
the more finely the habitat is divided into additional niches.
 |
| Pyramid of Numbers and Pyramids of Biomass The pyramid of number is thin with hawk since the number of hawks (Tertiary comsumers) is less than what it eats ( i.e. sparrowhawk.) Then the bar of caterpillar is far wide due to its very large presence of population and the the bar of tree is very thin because its only one, very less in numbers as compared to the caterpillars, big tree can hold thousands of them. The pyramid of biomass bar is thin with hawk since the term ,biomass, refers to the mass of a living thing without its water content. since almost 4 kg of Hawk will eat 100s of sparrows (almost 50 plus kgs total) which will eat thousands of caterpillar (almost 2000kgs of caterpillars yuck!) which will consume a tree weighing 2-3000 kgs, (this all consumption is measured for equal given number of time.) hence the bars of pyramid decreases linearly. |
| C | Population Growth Rates |
Populations have a birth rate (the number
of young produced per unit of population per unit of time), a death rate (the
number of deaths per unit of time), and a growth rate. The major agent of
population growth is births, and the major agent of population loss is deaths.
When births exceed deaths, a population increases; and when deaths exceed
additions to a population, it decreases. When births equal deaths in a given
population, its size remains the same, and it is said to have zero population
growth.
When introduced into a favorable
environment with an abundance of resources, a small population may undergo
geometric, or exponential growth, in the manner of compound interest.
Many populations experience exponential growth in the early stages of colonizing
a habitat because they take over an underexploited niche or drive other
populations out of a profitable one. Those populations that continue to grow
exponentially, however, eventually reach the upper limits of the resources; they
then decline sharply because of some catastrophic event such as starvation,
disease, or competition from other species. In a general way, populations of
plants and animals that characteristically experience cycles of exponential
growth are species that produce numerous young, provide little in the way of
parental care, or produce an abundance of seeds having little food reserves.
These species, usually short-lived, disperse rapidly and are able to colonize
harsh or disturbed environments. Such organisms are often called
opportunistic species.
Other populations tend to grow
exponentially at first, and then logistically—that is, their growth slows as the
population increases, then levels off as the limits of their environment or
carrying capacity are reached. Through various regulatory mechanisms, such
populations maintain something of an equilibrium between their numbers and
available resources. Animals exhibiting such population growth tend to produce
fewer young but do provide them with parental care; the plants produce large
seeds with considerable food reserves. These organisms are long-lived, have low
dispersal rates, and are poor colonizers of disturbed habitats. They tend to
respond to changes in population density (the number of organisms per unit area)
through changes in birth and death rates rather than through dispersal. As the
population approaches the limit of resources, birth rates decline, and mortality
of young and adults increases.
| D | Community Interactions |
Major influences on population growth
involve various population interactions that tie the community together. These
include competition, both within a species and among species;
predation, including parasitism; and coevolution, or
adaptation.
| D1 | Competition |
When a shared resource is in short
supply, organisms compete, and those that are more successful survive. Within
some plant and animal populations, all individuals may share the resources in
such a way that none obtains sufficient quantities to survive as adults or to
reproduce. Among other plant and animal populations, dominant individuals claim
access to the scarce resources and others are excluded. Individual plants tend
to claim and hold onto a site until they lose vigor or die. These prevent other
individuals from surviving by controlling light, moisture, and nutrients in
their immediate areas.
Many animals have a highly developed
social organization through which resources such as space, food, and mates are
apportioned among dominant members of the population. Such competitive
interactions may involve social dominance, in which the dominant
individuals exclude subdominant individuals from the resource; or they may
involve territoriality, in which the dominant individuals divide space
into exclusive areas, which they defend. Subdominant or excluded individuals are
forced to live in poorer habitats, do without the resource, or leave the area.
Many of these animals succumb to starvation, exposure, and predation.
Competition among members of different
species results in the division of resources in a community. Certain plants, for
example, have roots that grow to different depths in the soil. Some have shallow
roots that permit them to use moisture and nutrients near the surface. Others
growing in the same place have deep roots that are able to exploit moisture and
nutrients not available to surface-rooted plants.
| D2 | Predation |
One of the fundamental interactions is
predation, or the consumption of one living organism, plant or animal, by
another. While it serves to move energy and nutrients through the ecosystem,
predation may also regulate population and promote natural selection by weeding
the unfit from a population. Thus, a rabbit is a predator on grass, just as the
fox is a predator on the rabbit. Predation on plants involves defoliation by
grazers and the consumption of seeds and fruits. The abundance of plant
predators, or herbivores, directly influences the growth and survival of the
carnivores. Thus, predator-prey interactions at one feeding level influence the
predator-prey relations at the next feeding level. In some communities,
predators may so reduce populations of prey species that a number of competing
species can coexist in the same area because none is abundant enough to control
the resource. When predators are reduced or removed, however, the dominant
species tend to crowd out other competitors, thereby reducing species
diversity.
| D3 | Parasitism |
 |
Life Cycle of the Malaria Parasite
Malaria is an infectious disease
caused by a one-celled parasite known as Plasmodium. The parasite is
transmitted to humans by the bite of the female Anopheles mosquito. The
Plasmodium parasite spends its life cycle partly in humans and partly in
mosquitoes. (A) Mosquito infected with the malaria parasite bites human, passing
cells called sporozoites into the human’s bloodstream. (B) Sporozoites travel to
the liver. Each sporozoite undergoes asexual reproduction, in which its nucleus
splits to form two new cells, called merozoites. (C) Merozoites enter the
bloodstream and infect red blood cells. (D) In red blood cells, merozoites grow
and divide to produce more merozoites, eventually causing the red blood cells to
rupture. Some of the newly released merozoites go on to infect other red blood
cells. (E) Some merozoites develop into sex cells known as male and female
gametocytes. (F) Another mosquito bites the infected human, ingesting the
gametocytes. (G) In the mosquito’s stomach, the gametocytes mature. Male and
female gametocytes undergo sexual reproduction, uniting to form a zygote. The
zygote multiplies to form sporozoites, which travel to the mosquito’s salivary
glands. (H) If this mosquito bites another human, the cycle begins
again.
|
Parasite
| I | INTRODUCTION |
Parasite, organism that lives in or on a second
organism, called a host, usually causing it some harm. A parasite is generally
smaller than the host and of a different species. Parasites are dependent on the
host for some or all of their nourishment. For example, a tapeworm, a flattened
worm that lives in the gastrointestinal tract of mammals, lacks an intestine of
its own and must absorb predigested food from the intestine of its host. This
food is the tapeworm’s only energy source for growth and reproduction.
Parasitism affects most life forms, from bacteria infected by the viruses known
as bacteriophages, to humans, who are subject to more than 100 parasites known
to cause disease.
| II | TYPES AND FORMS OF PARASITES |
Parasites come in a variety of forms. Many
arthropod parasites, including mites, ticks, and mosquitoes, cause a number of
debilitating animal and human diseases. Certain plants, including mistletoe and
dodder, parasitize other plants to obtain water and nutrients. Microscopic
parasites include single-celled protozoans such as amoebas and sporozoa, fungi,
and bacteria, which can infect animals and plants. Viruses are entirely
parasitic, able to survive and reproduce only within other living organisms.
Parasites that live on the inside of the
host’s body are known as endoparasites, while those that live on the
outer surface of their hosts are known as ectoparasites. This distinction
reflects adaptations made by the parasite to overcome certain barriers to
parasitism. For example, when invaded by a parasite, a host often triggers an
immune response, a cellular reaction that works to destroy the invader.
Parasitic worms, including flatworms (soft-bodied worms, such as tapeworms and
flukes) and roundworms (thin, unsegmented worms, also called nematodes) are
endoparasites, usually living in the intestines, lungs, liver, or other internal
organs of their hosts. These worms have developed adaptations that enable them
to avoid the host’s immune response, such as during a developmental stage when
they are protected by a cyst wall or an outer surface that constantly changes,
thereby making it difficult for the host immune system to target the parasite
for attack.
Many ectoparasites have developed structures,
such as suckers, hooks, and teeth, which help penetrate the host’s outer
surface. Primitive fishes, such as hagfish and lampreys, use suctionlike mouths
to attach to the outer surface of other fish and suck out nutrients. Some
annelids (segmented worms), such as leeches, are also ectoparasites, using
sucking disks to feed on the blood and tissues of vertebrate hosts.
| III | PARASITE AND HOST RELATIONSHIPS |
Parasites vary in the ways they use their
hosts. Temporary parasites spend only part of their lives in or on their
hosts. Ticks, fleas, mites, and other arthropods, for example, attach to hosts
and then detach to live as free-living organisms. Ticks normally live in woods
and tall grass. To feed they may climb onto a passing dog, sink their mouthparts
into the flesh, drink a small amount of blood, and then drop off the host. Most
flatworms and roundworms are permanent parasites and live their entire
adult lives in their hosts.
Facultative parasites are not
dependent on their hosts for survival. Many leeches will feed on the blood or
tissues of their hosts, but when released in an aquatic environment survive as
free-living organisms. Obligate parasites are totally dependent upon
their hosts for survival and will die without their host. A bacteriophage, for
instance, would be unable to survive and reproduce if it was removed from its
bacterium host.
| IV | LIFE CYCLE OF PARASITES |
In order to survive from one generation to
the next, parasites have a series of distinct developmental stages and hosts
collectively known as a life cycle. Life cycles range from a simple, single host
that is home to the larval and adult stages of a parasite, to the more complex
life cycles requiring one host for the developmental stage of the parasite and a
second host for the adult stage.
Beef tapeworms have a simple life cycle.
These worms form cysts in the muscles of cows. When a human eats infected beef
that is improperly cooked, the cyst enters the human digestive tract and opens
to release a worm that attaches to the wall of the small intestine. The worm
absorbs large quantities of nutrients from the intestines, sometimes causing
malnutrition in its human host. The adult worm releases eggs that are passed out
in the feces where they can infect other animals.
The eye fluke is a good example of a complex
life cycle, although many variations of complex life cycles exist. Adult eye
flukes live in the eyelids of wading birds and release their eggs into the water
when the birds dip their heads underwater to feed. Each egg hatches and releases
a microscopic free-living larva called a miracidium. The miracidium must
penetrate the skin of a specific species of aquatic snail within a few hours or
it will die. Once inside the snail, the miracidium develops into a 1 to 2 mm
(0.04 to 0.08 in) long, saclike stage called a redia. The redia feeds on
snail tissue and buds off other larval stages through asexual reproduction.
A new larval stage called a cercaria
is produced within the redia. The 0.5 mm (0.02 in) long cercaria is a
free-living, nonfeeding, short-lived stage that resembles a tadpole. It migrates
to the surface of the snail's soft tissue and is shed into the environment.
There, it swims and attaches to the surface of a small invertebrate such as a
snail, clam, or crab, and forms a cyst. Wading birds feed on these invertebrates
and become infected when the cyst wall breaks in the bird’s mouth. The released
larva, called a metacercaria, travels through a slit in the back of the
bird’s throat and migrates to the bird’s eye. In the bird’s eyelid it develops
into a mature adult capable of producing eggs and starting the cycle once again.
Other parasites have life cycles that involve
intermediate organisms, or vectors, which carry disease-causing microorganisms
from one host to another. The protozoan blood parasite that causes sleeping
sickness, or trypanosomiasis, infects humans, cattle, and other animals. It uses
the tsetse fly as a vector to carry it from one host to the next. When a tsetse
fly bites an infected animal, it picks up the parasite when it sucks blood. When
an infected fly bites another animal, the parasite enters the bloodstream and
begins to reproduce in the new host.
| V | PARASITES OF ANIMALS |
Animals are infected by many parasites
including protozoans, worm parasites, and arthropod parasites such as mites,
ticks, and fleas. Veterinarians diagnose these parasites in or on pets by
checking the animal for visible parasites or by examining blood, tissue, or
waste products under a microscope. Common worm parasites of dogs and cats
include hookworms, roundworms, and tapeworms.
Hookworm infection occurs when larvae in the
soil penetrate the pet’s skin, move into the bloodstream, and eventually travel
to the intestine. Adult worms mature in the wall of the intestine and feed on
blood from the intestinal lining, sometimes causing serious anemia. Roundworm
infections of dogs and cats occur when these pets eat microscopic worm eggs
present in the soil. The eggs develop larval stages in the intestine and some of
these larvae penetrate the intestinal wall, move into the lungs, are coughed up
and reswallowed, and once again enter the small intestine where they mature into
10 to 15 cm (4 to 6 in) worms. Roundworms compete with the pet for food and may
cause malnutrition.
While the roundworm enters its host by
ingestion and the hookworm enters by active penetration of the skin, the
heartworm enters its dog host with the help of a mosquito vector. Microscopic
larvae known as microfilariae enter the blood along with mosquito saliva
when an infected mosquito bites a dog. The larvae travel via the blood stream to
the heart and develop into sexually mature male and female heartworms. They grow
5 to 10 cm (2 to 4 in) in length infesting the heart’s chambers and lodging in
the veins that enter the heart.
| VI | PARASITES OF HUMANS |
Humans are subjected to numerous protozoan,
worm, and insect-related parasites. Two of the most damaging human parasites are
the protozoan Plasmodium that causes malaria and the flatworm
Schistosoma that causes schistosomiasis. There are an estimated 400
million to 600 million cases of malaria each year and 200 million cases of
schistosomiasis worldwide.
In malaria, the infective larval stage of the
Plasmodium protozoan is transmitted to humans by the bite of a female
Anopheles mosquito. The larvae undergo asexual reproduction in the liver
producing a cyst that releases new larval stages into the blood stream. Larvae
invade red blood cells and reproduce, eventually rupturing the blood cells. Upon
rupturing, a toxin is released that causes the chills and fever that are the
characteristic symptoms of malaria patients. Drugs such as chloroquine can be
used to prevent infection in the blood. Mosquito control by use of repellents
and pesticides is also helpful in preventing spread of the parasite.
Humans are infected with Schistosoma
when they enter water containing infected snails. The larval stages of this
flatworm develop in the tissues of infected snails and eventually release
fork-tailed cercariae into the water. The cercariae penetrate human skin, lose
their forked tails, enter the blood, and migrate to major veins in the liver,
intestine, or urinary bladder. Within about six weeks of infection, the juvenile
worms develop into sexually mature adults measuring 1 to 2 cm (0.4 to 0.8 in) in
length. The males and females mate and produce microscopic eggs, some of which
migrate to the liver and cause a condition known as cirrhosis. Other eggs move
into the intestine and are passed out in the feces. When untreated human sewage
enters waters containing the snail hosts, the eggs hatch and start a new cycle.
Preventive measures include the use of boots
and gloves or special ointments to block penetration of the larvae into the
skin. Molluscicides (drugs that kill snails) are used to kill infected snails
but they often kill other important fish and invertebrate species. The drug
praziquantel has proved effective in killing Schistosoma in humans,
although some people experience adverse side effects.
| VII | PARASITES OF PLANTS |
Similar organisms that parasitize animals
also infect plants (see Diseases of Plants). Fungi cause the majority of
plant diseases. Although they typically feed on dead organic matter, fungi can
also feed directly on living tissues. The fungus that causes Dutch elm disease
is often carried from tree to tree by beetles. It attacks and eventually kills
by blocking water flow through the plant. Protozoans such as phytoflagellates
can parasitize milkweed plants. Bacterial plant diseases include fire blights,
certain soft rots, and citrus canker. The bacteria that cause these diseases
destroy tissue or block the passage of water through the plant. Numerous
viruses, such as the tobacco mosaic virus, also attack plants. Certain insects
and worms, particularly nematodes, parasitize the roots, stems, and leaves of
plants. They secrete chemicals that induce plant cells around the parasite to
rapidly divide and produce large growths known as galls. Galls formed by the
root knot nematode can cause serious physical damage to the roots of important
crops including tomatoes and potatoes.
Some higher plants feed on other plants and
cause them harm. One group known as hemiparasites, or water parasites, absorbs
water and nutrients from their plant hosts. Witchweed is a hemiparasitic seed
plant that damages sugarcane, corn, and other grasslike crops by attaching
itself to the host’s roots and absorbing minerals and water, eventually killing
the host. Mistletoe, another hemiparasite, parasitizes broadleaf trees including
ash, maple, walnut, birch, and some conifers. Mistletoe roots bore into the
host’s branches in order to draw out water and nutrients. Birds eat mistletoe
berries, which pass through their digestive tracks, are excreted, and sometimes
stick to a tree branch where they produce a new mistletoe bush.
True plant parasites lack chlorophyll and
cannot photosynthesize. These plants must obtain carbohydrates as well as
minerals and water from their plant hosts. True plant parasites include dwarf
mistletoe, which primarily parasitizes conifers; dodder, which parasitizes
important agricultural crops such as alfalfa, clover, sugar beets, and woody
perennials such as olive trees; and broomrape, which causes extensive damage to
tomato crops.
| VIII | PARASITOLOGY |
There are many research areas in
parasitology practiced by different types of specialists. Microbiologists and
virologists primarily work with parasitic bacteria, rickettsiae, and
viruses. Plant pathologists work with fungi, nematode parasites of plants, and
other plant parasites. Animal parasitologists work with parasitic protozoans,
worm groups, and arthropod parasites. Those who specialize in parasitic
protozoans are called protozoologists whereas those who study parasitic worms
are called helminthologists. Others who examine parasitic insects of humans are
called medical entomologists.
Parasitologists who describe new species of
parasites are known as systematic parasitologists. Parasite immunologists study
ways in which hosts can reject parasites and they also attempt to develop
vaccines against parasites. A growing area of parasitology is ecological
parasitology including mathematical and computer modeling that predicts how
parasites behave in wild populations.
Parasitologists in pharmaceutical
industries develop drugs to prevent, control, and eradicate plant and animal
parasites. Many parasitologists work to discover the complex life histories of
animal and plant parasites whose life cycles remain partially or completely
unknown.
| D4 | Coevolution |
Coevolution is the joint evolution of
two unrelated species that have a close ecological relationship—that is, the
evolution of one species depends in part on the evolution of the other.
Coevolution is also involved in predator-prey relations. Over time, as predators
evolve more efficient ways of capturing or consuming prey, the prey evolves ways
to escape predation. Plants have acquired such defensive mechanisms as thorns,
spines, hard seed-coats, and poisonous or ill-tasting sap that deter would-be
consumers. Some herbivores are able to breach these defenses and attack the
plant. Certain insects, such as the monarch butterfly, can incorporate poisonous
substances found in food plants into their own tissues and use them as a defense
against predators. Other animals avoid predators by assuming an appearance that
blends them into the background or makes them appear part of the surroundings.
The chameleon is a well-known example of this interaction. Some animals
possessing obnoxious odors or poisons as a defense also have warning
colorations, usually bright colors or patterns, that act as further warning
signals to potential predators. See Adaptation; Mimicry.
Another coevolutionary relationship is
mutualism, in which two or more species depend on one another and cannot live
outside such an association. An example of mutualism is mycorrhizae, an
obligatory relationship between fungi and certain plant roots. In one group,
called ectomycorrhizae, the fungi form a cap or mantle about the rootlets. The
fungal hyphae (threads) invade the rootlet and grow between the cell walls as
well as extending outward into the soil from the rootlet. The fungi, which
include several common woodland mushrooms, depend on the tree for their energy
source. In return the fungi aid the tree in obtaining nutrients from the soil
and protect the rootlets of the tree from certain diseases. Without the
mycorrhizae some groups of trees, such as conifers and oaks, cannot survive and
grow. Conversely, the fungi cannot exist without the trees. See
Symbiosis.
| E | Succession and Climax Communities |
Ecosystems are dynamic, in that the
populations constituting them do not remain the same. This is reflected in the
gradual changes of the vegetational community over time, known as succession. It
begins with the colonization of a disturbed area, such as an abandoned crop
field or a newly exposed lava flow, by species able to reach and to tolerate the
environmental conditions present. Mostly these are opportunistic species that
hold on to the site for a variable length of time. Being short-lived and poor
competitors, they are eventually replaced by more competitive, longer-lived
species such as shrubs, and then trees. In aquatic habitats, successional
changes of this kind result largely from changes in the physical environment,
such as the buildup of silt at the bottom of a pond. As the pond becomes more
shallow, it encourages the invasion of floating plants such as pond lilies and
emergent plants such as cattails. The pace at which succession proceeds depends
on the competitive abilities of the species involved; tolerance to the
environmental conditions brought about by changes in vegetation; the interaction
with animals, particularly the grazing herbivores; and fire. Eventually the
ecosystem arrives at a point called the climax, where further changes
take place very slowly, and the site is dominated by long-lived, highly
competitive species. As succession proceeds, however, the community becomes more
stratified, enabling more species of animals to occupy the area. In time,
animals characteristic of later stages of succession replace those found in
earlier stages.
