G8 GLOSSARY
mature community Fairly stable, self-
sustaining community in an advanced stage of
ecological succession; usually has a diverse array
of species and ecological niches; captures and
uses energy and cycles critical chemicals more
effi ciently than simpler, immature communities.
Compareimmature community.
maximum sustainable yield Seesustainable
yield.
MDC Seedeveloped country.
mesotrophic lake Lake with a moderate
supply of plant nutrients. Compare eutrophic lake,
oligotrophic lake.
metabolism Ability of a living cell or organ-
ism to capture and transform matter and energy
from its environment to supply its needs for
survival, growth, and reproduction.
microorganisms Organisms such as bacteria
that are so small that it takes a microscope to see
them.
migration Movement of people into and out
of specifi c geographic areas. Compare emigration
andimmigration.
mineral resource Concentration of naturally
occurring solid, liquid, or gaseous material in or
on the earth’s crust in a form and amount such
that extracting and converting it into useful ma-
terials or items is currently or potentially profi t-
able. Mineral resources are classifi ed as metallic
(such as iron and tin ores) or nonmetallic (such as
fossil fuels, sand, and salt).
model Approximate representation or simula-
tion of a system being studied.
moleculeCombination of two or more atoms
of the same chemical element (such as O 2 ) or dif-
ferent chemical elements (such as H 2 O) held
together by chemical bonds. Compare atom, ion.
more developed country (MDC) Seedevel-
oped country.
mutation Random change in DNA mol-
ecules making up genes that can alter anatomy,
physiology, or behavior in offspring. See mutagen.
mutualism Type of species interaction in
which both participating species generally ben-
efi t. Compare commensalism.
native species Species that normally live
and thrive in a particular ecosystem. Compare
foundation species, indicator species, keystone species,
nonnative species.
natural capital Natural resources and natural
services that keep us and other species alive and
support our economies. See natural resources,
natural services.
natural greenhouse effect Heat buildup
in the troposphere caused by the presence of
certain gases, called greenhouse gases. Without
this effect, the earth would be nearly as cold as
Mars, and life as we know it could not exist. See
global warming.
natural income Renewable resources such
as plants, animals, and soil provided by natural
capital.
natural law Seescientifi c law.
natural radioactive decay Nuclear change in
which unstable nuclei of atoms spontaneously
shoot out particles (usually alpha or beta par-
ticles) or energy (gamma rays) at a fi xed rate.
natural rate of extinction Seebackground
extinction.
natural resources Materials such as air,
water, and soil and energy in nature that are es-
sential or useful to humans. See natural capital.
natural selection Process by which a
particular benefi cial gene (or set of genes) is
reproduced in succeeding generations more than
other genes. The result of natural selection is a
population that contains a greater proportion
of organisms better adapted to certain environ-
mental conditions. See adaptation, biological evolu-
tion, differential reproduction, mutation.
natural services Processes of nature, such as
purifi cation of air and water and pest control,
which support life and human economies. See
natural capital.
negative feedback loop Feedback loop that
causes a system to change in the opposite direc-
tion from which is it moving. Compare positive
feedback loop.nekton Strongly swimming organisms found
in aquatic systems. Compare benthos, plankton.
net energy Total amount of useful energy
available from an energy resource or energy
system over its lifetime, minus the amount of
energyused (the fi rst energy law), automatically
wasted (the second energy law), and unnecessarily
wasted in fi nding, processing, concentrating, and
transporting it to users.
net primary productivity (NPP) Rate at
which all the plants in an ecosystem produce net
useful chemical energy; equal to the difference
between the rate at which the plants in an eco-
system produce useful chemical energy (gross
primary productivity) and the rate at which
they use some of that energy through cellu-
lar respiration. Compare gross primary productivity.
neutron (n) Elementary particle in the nuclei
of all atoms (except hydrogen-1). It has a rela-
tive mass of 1 and no electric charge. Compare
electron, proton.
niche Seeecological niche.
nitrogen cycle Cyclic movement of nitrogen in
different chemical forms from the environment
to organisms and then back to the environment.
nitrogen fi xation Conversion of atmospheric
nitrogen gas into forms useful to plants by light-
ning, bacteria, and cyanobacteria; it is part of the
nitrogen cycle.
nondegradable pollutant Material that
is not broken down by natural processes.
Examples include the toxic elements lead and
mercury. Compare biodegradable pollutant.
nonnative species Species that migrate into
an ecosystem or are deliberately or accidentally
introduced into an ecosystem by humans. Com-
parenative species.nonpoint sources Broad and diffuse areas,
rather than points, from which pollutants enter
bodies of surface water or air. Examples include
runoff of chemicals and sediments from crop-
land, livestock feedlots, logged forests, urban
streets, parking lots, lawns, and golf courses.
Comparepoint source.nonrenewable resource Resource that exists
in a fi xed amount (stock) in the earth’s crust
and has the potential for renewal by geological,
physical, and chemical processes taking place
over hundreds of millions to billions of years.
Examples include copper, aluminum, coal, and
oil. We classify these resources as exhaustible
because we are extracting and using them at a
much faster rate than they are formed. Compare
renewable resource.
NPP Seenet primary productivity.nuclear change Process in which nuclei of cer-
tain isotopes spontaneously change, or are forced
to change, into one or more different isotopes.
The three principal types of nuclear change are
natural radioactivity, nuclear fi ssion, and nuclear
fusion. Compare chemical change, physical change.
nuclear energy Energy released when atomic
nuclei undergo a nuclear reaction such as the
spontaneous emission of radioactivity, nuclear
fi ssion, or nuclear fusion.
nuclear fi ssion Nuclear change in which the
nuclei of certain isotopes with large mass num-
bers (such as uranium-235 and plutonium-239)
are split apart into lighter nuclei when struck by
a neutron. This process releases more neutrons
and a large amount of energy. Compare nuclear
fusion.
nuclear fusion Nuclear change in which two
nuclei of isotopes of elements with a low mass
number (such as hydrogen-2 and hydrogen-3)
are forced together at extremely high tempera-
tures until they fuse to form a heavier nucleus
(such as helium-4). This process releases a large
amount of energy. Compare nuclear fi ssion.
nucleusExtremely tiny center of an atom,
making up most of the atom’s mass. It contains
one or more positively charged protons and
one or more neutrons with no electrical charge
(except for a hydrogen-1 atom, which has one
proton and no neutrons in its nucleus).
nutrient Any chemical element or compound
an organism must take in to live, grow, or
reproduce.
nutrient cycle Seebiogeochemical cycle.nutrient cycling The circulation of chemicals
necessary for life, from the environment (mostly
from soil and water) through organisms and
back to the environment
oil Seecrude oil.
old-growth forest Virgin and old, second-
growth forests containing trees that are often
hundreds—sometimes thousands—of years old.
Examples include forests of Douglas fi r, western
hemlock, giant sequoia, and coastal redwoods
in the western United States. Compare second-
growth forest, tree plantation.