RADIATION ECOLOGY 1043
this problem. Much more is known about the radiosensi-
tivity of organisms exposed to radiation doses which are
much higher than we expect to contend with in the normal
environment. In general, higher animals are far more sensi-
tive to radiation than are lower animals, and the very young
and the aged are more sensitive than mature, healthy ani-
mals. For example Table 2 gives estimated acute doses of
gamma of X-radiation necessary to kill 50% or more of
the adult members of several groups of organisms. These
data should be considered only as an indication of rela-
tive radiosensitivity as they represent generalized ranges.
Half the humans exposed to a single dose of 500 R will
die. For other mammals the lethal dose ranges from less
than 300–1200 R. Frogs and newts can survive higher
radiation levels, depending on their body temperatures at
the time of exposure. Insects can survive doses of up to
100,000 R in a few instances; most have lethal doses in the
10,000–20,000 R range. These kinds of data do not reflect
the more complex responses of organisms subjected to ion-
izing radiation under natural conditions. For example, most
organisms go through several stages of development from
egg to adult. These stages may take place in different parts
of the ecosystem. Likewise, the radiosensitivity of these
organisms may differ in different stages of the life cycle.
In radioresistance groups such as insects, 10% or less of
the lethal dose to adults may be effective at juvenile or egg
stages. For example, in the bagworm a dose of 450 R is
sufficient to kill 50% of 1-day old eggs, whereas a dose of
approximately 10,000 R was required to produce the same
effect in the larvae.
Effects other than lethality also may be produced by radi-
ation, especially in ecosystems where all organisms are linked
through various interactive processes. Aside from genetic or
reproductive effects, changes in number, growth, disease resis-
tance, life span or response to physical environmental factors
are of interest to the ecologist. Radiation-induced changes
can affect the role of organisms or populations within the
ecosystem. Predator–prey relationships, food chain transfers,
and other ecological processes which depend on the continu-
ing interaction between different organisms may be altered
by the impact of ionizing radiation.The effect of ionizing radiation on plants has been
studied both outdoors and in greenhouses. One indoor test
field consists of 10 acres of land with a^60 Co source located
at the center. It is installed in a vertical tube, which can be
raised to different heights for irradiation and then lowered
by remote control into a lead case when not in use. Various
species of plants grow in the soil in concentric circles around
the source. Each species is arranged in a wedge-shaped area
so that the plants are located at various distances from the
source and receive various intensities of radiation. Plants are
exposed to radiation for 20 hr a day.
Radiation effects on plants are complex and depend on a
number of factors, including the plant species, the maturity of
a plant, its physical condition, the parts of the plant exposed
to radiation, the kind and amount of radiation, and the rate
at which the radiation dose is applied. Woody plants gener-
ally are more sensitive to radiation than are herbaceous plants
(Table 2). Gymnosperms are more sensitive to radiation than
angiosperms. A pine tree shows severe growth inhibition at a
level of about 10 R/day, while the same degree of inhibition in
a gladiolus plant requires about 5000 R/day. Some ecologists
have speculated that radiation from a nuclear attack would
destroy all pine trees and other gymnosperms in irradiated
areas, leaving other plants relatively unharmed.
It is possible to predict some radiation effects in plants.
The meristematic or growth regions in plants are the most
radiosensitive tissues. It is the absorption of radiation energy
in these regions that alters plant growth and development.
Ecologists and botanists have shown that the response of
plants to ionizing radiation is directly proportional to the
interphase chromosome volumes in meristematic tissues.
That is, plant species with large chromosomes are more sen-
sitive; those with small chromosomes are more resistant to
radiation. In general, this is an extremely useful concept, and
it has been applied to predict and assess probable radiation
effects on vegetation (natural and agricultural) from military
uses of nuclear devices.
Seeds are far less sensitive to radiation than are growing
plants. A stand of pine trees exposed to a total of 12,000 R of
gamma radiation was 90% destroyed, yet 95% of the seeds
taken from cones on the same trees were viable. The high
resistance of seeds to radiation damage is probably associated
with their low water and oxygen content. The sensitivity of
dry seeds varied widely among species, however. Lily seeds
show practically no ability to sprout after receiving a dose of
2000 R. Yet the seeds of other plants seem to be stimulated to
sprout more vigorously than normal under the same amount
of radiation or more. Such differences favor the growth of
certain species over others in areas where radiation is a factor
in the environment.EFFECTS OF RADIATION ON ECOSYSTEMSThe effects that large scale ionizing radiation such as from
a nuclear attack would have on plants and animals living
together in an ecosystem have concerned radioecologists
ever since the first use of atomic bombs. Several studies haveTABLE 2
Comparative radiosensitivity of groups of organismsGroup Lethal dose rangea (rads)
Bacteria 100,000–1,000,000
Insects 5,000–100,000
Fish 1,000–300
Mammals 300–1,200
Herbaceous plants 5,000–70,000
Coniferous trees 800–3,000
Deciduous trees 4,000–10,000
a Estimated acute whole body gamma radiation doses
required to kill 50% or more of the adult organisms.C018_001_r03.indd 1043C018_001_r03.indd 1043 11/18/2005 11:04:28 AM11/18/2005 11:04:28 AM