Background Knowledge
For Predators, Parasites and Other Relationships in Book IV
1. Ecosystem: organisms living in a particular environment, such as a forest or a coral
reef, and the physical parts of the environment that affect them. The term ecosystem
was coined in 1935 by the British ecologist Sir Arthur George Tansley, who described
natural systems in “constant interchange” among their living and nonliving parts.
Coral Reef in the Red Sea Coral reefs represents the most complex aquatic
ecosystem found on Earth. Although coral reefs can be found between 30 degrees
north and south latitude, the greatest concentration is found between 4 degrees north
and south latitude in the western portions of all major oceans. There are two broad
categories of coral reefs: shelf reefs and oceanic reefs. Shelf reefs include fringing
reefs, platform reefs, bank reefs, and barrier reefs and are located on the continental
shelf, while oceanic reefs are found off the continental shelf growing around the
margins of volcanic islands. Coral reefs support greater numbers of fish and
invertebrate species than any other ecosystem in the ocean.
Wetlands are complex ecosystems that provide spawning and nursery grounds
for saltwater and freshwater fish, habitat for more than half of the migratory birds in
the United States, and plants both exotic and commonplace. In addition to providing
plant and animal habitat, wetlands play a crucial role in flood control and water
filtration.
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The ecosystem concept fits into an ordered view of nature that was developed by
scientists to simplify the study of the relationships between organisms and their
physical environment, a field known as ecology. At the top of the hierarchy is the
planet’s entire living environment, known as the biosphere. Within this biosphere are
several large categories of living communities known as biomes that are usually
characterized by their dominant vegetation, such as grasslands, tropical forests, or
deserts. The biomes are in turn made up of ecosystems. The living, or biotic, parts of
an ecosystem, such as the plants, animals, and bacteria found in soil, are known as a
community. The physical surroundings, or abiotic components, such as the minerals
found in the soil, are known as the environment or habitat.
Any given place may have several different ecosystems that vary in size and
complexity. A tropical island, for example, may have a rain forest ecosystem that
covers hundreds of square miles, a mangrove swamp ecosystem along the coast, and
an underwater coral reef ecosystem. No matter how the size or complexity of an
ecosystem is characterized, all ecosystems exhibit a constant exchange of matter and
energy between the biotic and abiotic community. Ecosystem components are so
interconnected that a change in any one component of an ecosystem will cause
subsequent changes throughout the system.
2. Predators: 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.
3. Carnivore: the general term for any animal that subsists mainly on the flesh of
other animals. More specifically, it refers to any member of the mammal order
Carnivora. The carnivores are at the top of the food chains that make up the food web
of the earth’s life forms. They feed on herbivores, or plant-eaters, which in turn feed
on the plants or dinoflagellates, at the bottom of the food chains, that absorb and store
energy directly from the sun. Carnivores live mainly alone or in small groups and are
not preyed upon except by other carnivores.
This general meaning of the term carnivore also applies more specifically to the
Carnivora, except that some members of the order—many bears, for example—are
omnivorous (eat both animals and plants) and may sometimes even have a largely
vegetable diet. A widespread group, the Carnivora have teeth adapted for grasping and
tearing, highly developed physical coordination and flexibility of behavior, and
extended parent-offspring bonds. The order has been divided into two suborders: the
Pinnipedia, with finlike feet, and the Fissipedia, with pawlike feet. The Pinnipedia are
the sea lions and fur seals (family Otaridae); the true seals (Phocidae); and the
walruses (Odobenidae). The suborder Fissipedia has two superfamilies: Canoidea
(dog, bear, raccoon, and weasel families); and Feloidea (cat, civet, and hyena
families).
The jaws of the Carnivores are powerful and move only up and down on a
transverse hinge with none of the rotary motion found in other animals. The teeth
have been adapted for the feeding habits, so that in most carnivores each jaw contains
six pointed cutting teeth; two strong, sharp, recurved canines; and molar teeth that
have been developed into cutting blades. The number of molars varies among the
different families. The cats have only a single vestige of molar in each jaw, whereas
dogs retain more molars, which they use for bone crushing. The bears (Ursidae),
except for the polar bear, are omnivorous, and their back molars show tubercles on the
crowns, which improve their ability to grind vegetable matter. The digestive system of
carnivores is much less complicated than that of herbivores because it does not have
to break down the cellulose in plant matter.
4. 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.
5. Symbiosis: (Greek symbioun, “to live together”), in biology, term for the
interdependence of different species, which are sometimes called symbionts. There
are three main types of symbiosis, based upon the specific relationship between the
species involved: mutualism, parasitism, and commensalism.
Symbiosis that results in mutual benefit to the interdependent organisms is
commonly known as mutualism. An example of mutualism is the coexistence of
certain species of algae and fungi that together compose lichens. Their close
association enables them to live in extreme environments, nourished only by light, air,
and minerals. Living separately, the alga and fungus would not survive in such
conditions. Another example is the relationship between most mycorrhizae and certain
plants. Mycorrhizae are fungal growths on the roots of such plants as heaths, orchids,
and many conifers. The fungi penetrate the roots of the plants and make soil
nutriments such as nitrogen available to the plants, receiving carbohydrates in return.
In parasitism, also known as antagonistic symbiosis, one organism receives no
benefits and is often injured while supplying nutrients or shelter for the other
organism (see Parasite). Parasites include viruses and bacteria that cause many
diseases; certain protozoans that can infect plants and animals; tapeworms and flukes
that infest the intestinal tracks and internal organs of animals; and external parasites
such as lice and ticks. There are also parasitic plants like mistletoe that draw their
nourishment from the branches of other plants.
The type of symbiosis known as commensalism is an association between two
different kinds of nonparasitic animals, called commensals, that is harmless to both
and in which one of the organism benefits. Many commensals are free to separate.
Other commensals function together so completely that they cannot separate. They are
not considered parasitic, however, because they do not harm each other. An example
is a polyp found in deep water off the coast of Newfoundland. It attaches itself to the
shell of a certain species of hermit crab and, by budding, covers the entire shell with a
colony that dissolves the original shell. Because the colony grows at the same rate as
the crab, it furnishes continuous protection, and the crab does not shed its shell at
periodic intervals as it normally would. The polyp, in turn, benefits by moving about
with the crab, thereby obtaining a greater food supply than it would if attached to a
stationary object. Commensalism is most common among marine invertebrates, but it
often occurs among land animals—for example, in the association of ants with other
insects such as aphids and beetles. The association of colon bacteria with humans and
other animals, especially plant-eating animals, is also a type of commensalism.
6. Mammal: animal that raises its young on milk. Most mammals are covered with
hair or fur, and most have specialized teeth that help them to cut or chew their food.
Compared to other vertebrates (animals with backbones), mammals have highly
developed nervous systems, and they show an intelligence and resourcefulness that
few other animals can match. Mammals include some of the most familiar members
of the animal kingdom, such as cats, dogs, elephants, and whales, and also human
beings—a species that now dominates life on earth.
Mammalian Characteristics The class Mammalia has several distinguishing
traits, among them a well-developed brain and high degree of parental care. Its
members, warm-blooded vertebrates, have successfully expanded into habitats all
over the world.
Nursing Young Unique to mammalian females are the milk-secreting, or
mammary, glands that give the class its name. Milk provides complete nourishment,
including water, to the newborn. A mother giraffe usually nurses her calf for 9 to 10
months, until its body can metabolize the leafy diet of its parents. Mammal babies
lacking the sucking reflex at birth generally do not survive.
Four Limbs Most mammals have four appendages, usually legs. These may be
modified for walking upright, as in humans, whose upper legs and knees articulate at
an angle that supports their weight with most stability; swimming, as in whales,
whose vestigial hind legs are still present in the skeleton; or flying, as in bats, whose
metacarpal bones are webbed with skin.
Hair All Over. The bodies of mammals are covered with hairs, filaments of dead
cells strengthened with keratin, that emerge from the skin. Closely spaced, they
constitute a fur coat. Kinky and matted, they form wool. Hair serves as insulation,
protection, water repellent, and, in the case of the porcupine, whose hairs are coarse,
stiff, and pointed, as defense. Some mammals have lost their fur through evolution. A
layer of insulating blubber, for example, takes the protective place of hair in whales.
With the exception of three highly unusual mammals called monotremes, all
mammals give birth to live young. Some young mammals are completely helpless
when they are born, while others are relatively well developed. Despite these
differences, all young mammals initially rely on their mothers for food, and stay with
them until they are ready to fend for themselves. This close link between mother and
offspring produces strong family ties, and allows young mammals to learn by copying
their parents' behavior.
Mammals evolved from a group of reptiles called therapsids. The earliest true
mammals, appearing over 200 million years ago, were only 5 cm (2 in) long and
probably spent at least some of their lives in trees. These tiny mammals resembled
shrews with four roughly equal short legs and sharp claws. Since that time mammals
have evolved an extraordinary variety of body shapes and sizes. Of the approximately
4600 species of mammals alive today, most live on land, and most still move about on
all four legs. But modern mammals also include animals that hop on two legs, ones
that live permanently in water, and ones that can fly. These swimming and flying
species include the world's largest mammal—the blue whale, which can grow over 30
m (100 ft) long—and also the smallest, the Kitti's hog-nosed bat. Discovered in 1973
in the forests of Thailand, this tiny bat is about the size of a bumblebee, and weighs
just 2 g (0.07 oz).
The life spans of mammals vary as enormously as their sizes. Most shrews
survive for less than a year, exhausting themselves in a life of almost ceaseless
activity. By contrast, horses can live about 20 years, chimpanzees can live to be over
50, and elephants can survive into their 60s. Humans have the longest life span of any
mammal, with a few individuals living over 110 years.
Mammals have adapted to some of the most extreme habitats on earth. They are
warm-blooded, or endothermic, meaning that they maintain their body temperature
within a narrow range despite changes in the environment. Polar bears survive on
Arctic ice, while Arctic foxes can sleep on open snow in temperatures as low as –68°
C (-90° F). Camels and kangaroo rats live in deserts, and can tolerate blazing
temperatures that would kill many animals from cooler habitats.
Mammals can also tolerate the thin air of the highest mountains as well as the
crushing pressures of the ocean depths. Yaks, for example, forage for food on
mountain slopes at altitudes of up to 6100 m (20,000 ft), while sperm whales can dive
to depths of at least 2100 m (7000 ft), holding their breath for over an hour. Mammals
also include some of the world's greatest animal migrants. During its yearly migration
from Arctic waters to the coast of Mexico and back, a gray whale may travel 20,000
km (12,500 mi).