The harmful effects of ionizing radiation apply to all living organisms, but the sensitivity to radiation varies from species to species over a wide range. Generally, the higher the species on the evolutionary scale the greater the sensitivity. In plants the sensitivity appears to be related to the volume occupied by the chromosomes in the nucleus of the cell; the larger the chromosome volume, the smaller the dose required to produce a given degree of damage. As a consequence of this, after exposure to a high dose of radiation some animal or plant species will suffer much more than others and this may seriously upset the ecological balance. For example, the killing of birds may result in a large increase in insect populations, which are much less sensitive to radiation, and this in turn would cause enormous damage to plants. A rapid increase in insect population, particularly disease vectors, would also have serious effects on the health of both man and animals.
The main effect of fall-out on animals is to cause external exposure by the gamma rays emitted from radioactive substances, but the consumption of contaminated grass would in addition produce an internal exposure from the beta-rays. Beta-burns may also occur if fall-out particles remain on the skin of the animal. They could cause mucosal burns in the mouths of ruminants, leading to starvation. As in man, whole body exposure of animals to doses in the range of 1-10 Gy may result in death within a few weeks. The LD-50, that is, the dose which will cause 50% mortality in the irradiated animals, varies between the species of domestic animals, being lowest in sheep and highest in poultry.
The main difference between animals and plants is the larger range of sensitivities to radiation observed in plants. Different species may differ in their sensitivities by a factor of 500; if algae are included the variation may be 5,000 fold. Moreover, a given species may itself have a wide range of sensitivities, up to a factor of 50, depending on the different stages of growth. Apart from the dose itself, the effect of radiation on plants may be influenced by many environmental factors. In the case of fall-out, and additional factor is the season of its occurrence; for example, food crops irradiated in the seedling stage will be exposed for a longer time and will therefore receive a larger dose of radiation than if the fall-out occurred near harvesting time. On the other hand it would be impossible to gather in the harvest if the fall-out came down at that time. Exposure to large doses of radiation will kill plants. The lethal doses are much higher than for animals: for food crops, even for the most sensitive plants, the LD-50 is about 10 Gy and it goes up to about 200 Gy. At smaller doses the effects of exposure are reduced yield and height; both are dose-rate dependent, the effect being smaller the lower the rate at which the dose was delivered. Yield is more severely affected in the early reproductive state. Plants with a growing season limited by climatic conditions may produce no yield at all, even if they survive. Flowering and ripening of fruit is delayed by exposure to radiation. Exposure of seeds produces mutations, most of which are deleterious. Among trees, conifers are very sensitive to radiation, whereas deciduous trees are less sensitive. The LD-50 values for exposed trees range from 20 to 100 Gy. Grasses are more radiation-resistant and a dose of 200 Gy is needed to destroy grassland.