primary-producer biomass consumed is converted into
new consumer biomass.
Ecological efficiency depends on assimilation effi-
ciency (that portion of the consumed energy assimilat-
ed) and on net production efficacy (that portion of the
consumed energy converted into biomass). Assimilation
efficiencies are greater for carnivores (50–90 percent)
than for herbivores (20–60 percent).
ecological niche The totality of biotic and abiotic
resources an organism interacts with while living in its
environment.
ecological succession A transitional change in the
biological community, where a group of plant and/or
animal species gives way to another set of species over
time, in response to a sequence of events such as fire,
storms, human activities, or other natural or human-
made occurrences. The term also refers to the normal
evolution of a community from pioneer stage to climax
community when equilibrium between species and its
environment occurs.
ecology The study of all life forms and their interac-
tions with their environment.
ecosystem Any natural system—including biotic and
abiotic parts—that interacts as a unit to produce a sta-
ble functioning system through cyclical exchange of
materials.
ectoderm The outer layer of an embryo’s three pri-
mary germ layers (endoderm, mesoderm, ectoderm)
that gives rise to the nervous system and epidermis in
vertebrates.
ectoparasite Aparasite that feeds from the exterior
of its host.
ectotherm Acold-blooded organism that relies on
obtaining its heat from certain behavior techniques and
from the external environment (sun); e.g., snakes, alli-
gators, lizards, fish, or amphibians.EDRF SeeENDOTHELIUM-DERIVED RELAXING FACTOR.effector cell A cell that performs a specific function
in response to a stimulus. A gland or muscle cell that
responds to stimuli from the body. Cells with full
immune functions capable of participating in the
immune response by destroying foreign cells or tissues;
effector lymphocytes can mediate the removal of
pathogens from the body.efficacy Describes the relative intensity with which
AGONISTs vary in the response they produce, even when
they occupy the same number of RECEPTORs and with
the same AFFINITY. Efficacy is not synonymous with
INTRINSIC ACTIVITY.
Efficacy is the property that enables DRUGs to pro-
duce responses. It is convenient to differentiate the
properties of drugs into two groups: those that cause
them to associate with the receptors (affinity) and those
that produce stimulus (efficacy). This termis often used
to characterize the level of maximal responses induced
by agonists. In fact, not all agonists of a receptor are
capable of inducing identical levels of maximal
response. Maximal response depends on the efficiency
of receptor coupling, i.e., from the cascade of events
that, from the binding of the drug to the receptor, leads
to the observed biological effect.EF-hand A common structure to bind Ca2+ in
CALMODULINand other Ca2+-binding proteins consist-
ing of a HELIX(E), a loop, and another helix (F).egg The mature female reproductive cell.Ehrlich, Paul(1854–1915) German/PolishImmunol-
ogist Paul Ehrlich was born on March 14, 1854, near
Breslau, Germany (now Wroclaw, Poland), to Ismar
Ehrlich and his wife Rosa Weigert, whose nephew was
the great bacteriologist Karl Weigert.106 ecological niche