Glossary - Agroecology: The Ecology of Sustainable Food Systems - page 373

by a particular species' niche. The niche amplitude
of a generalist species is larger than that of a specialist
species.
niche breadth Essentially a synonym for niche
amplitude .
niche diversity Differences in the resource-use patterns
of similar species that allow them to coexist successfully
in the same environment.
niche See ecological niche.
open pollination The natural dispersal of pollen among
all the members of a cross-pollinating crop population,
resulting in the maximum degree of genetic mixing and
diversity.
organism An individual of a species.
overyielding The production of a yield by an intercrop
that is larger than the yield produced by planting the
component crops in monoculture on an equivalent area
of land.
oxidation The loss of electrons from an atom that
accompanies the change from a reduced to an oxidized
state.
parasite An organism that uses another organism for
food and thus harms the other organism but usually does
not kill the host.
parasitism An interaction in which one organism feeds on
another organism, harming (but generally not killing) it.
parasitoid Insect parasites whose larvae live within and
consume their host, usually another insect.
patchiness A measurement of the diversity of succes-
sional stages present in a specific area.
patchy landscape A landscape with a diversity of
successional stages or habitat types.
percolation Water movement through the soil due to the
pull of gravity.
permanent wilting point The level of soil moisture
below which a plant wilts and is unable to recover.
phenotype The physical expression of the genotype; an
organism's physical characteristics.
photoperiod The total number of hours of daylight.
photorespiration The energetically wasteful substitu-
tion of oxygen for carbon dioxide in the dark reactions
of photosynthesis, which occurs when plant stomata
close and carbon dioxide concentration declines.
photosynthate The simple-sugar end products of photo-
synthesis.
polyploid Having three or more times the haploid
number of chromosomes.
population A group of individuals of the same species
that live in the same geographic region.
potential niche The maximum possible distribution of
a species in the environment.
predation An interaction in which one organism kills
and consumes another.
predator An animal that consumes other animals to
satisfy its nutritive requirements.

prescribed burn A fire set and controlled by humans to
achieve some management objective, such as improving
pasture in grazing systems.
prevailing winds The general wind patterns character-
istic of broad latitudinal belts on the earth's surface.
primary production The amount of light energy
converted into plant biomass in a system.
primary succession Ecological succession on a site that
was not previously occupied by living organisms.
producer An organism that converts solar energy into
biomass.
production Harvest output or yield.
productivity index A measure of the amount of biomass
invested in the harvested product in relation to the total
amount of standing biomass present in the rest of the
system.
productivity The ecological processes and structures in
an agroecosystem that enable production.
protocooperation An interaction in which both organ-
isms are benefited if the interaction occurs, but neither
are harmed if it does not occur.
r -strategist A species that exists in relatively harsh envi-
ronmental conditions and whose mortality is generally
determined by density-independent factors; an r -strategist
allocates more energy to reproduction than to growth.
rainfed agroecosystem A farming system in which crop
water needs are met by natural precipitation.
realized niche The actual distribution of an organism in
the environment (compare with potential niche ).
regolith The layer or mantle of unconsolidated material
(soil and mineral subsoil) between the soil surface and
the solid bedrock of the earth below.
relative humidity The ratio of the actual water content
of the air to the amount of water the air is capable of
holding at a particular temperature.
relative rate of light transmission The percentage of
the total incident light at the canopy of a system that
reaches the ground.
residual soil Soil formed at its current location.
response A physiological change in a plant that is
induced by an outside, usually environmental, condition.
rhizobia Nitrogen-fixing soil microorganisms that form
mutualistic root interactions with plants (primarily
legumes).
safe site A specific location that provides the environ-
mental conditions necessary for seed germination and
initial growth of the seedling.
salinization The process of salt build-up in soils, asso-
ciated with high evaporation following irrigation and salt
deposition at the soil surface.
saltation The transport of small soil particles just above
the soil surface by wind.
saturation point The level of light intensity at which
photosynthetic pigments are completely stimulated and
unable to make use of additional light.