While it can be complex to measure the effects of nuclear weapons production, use and testing on the environment and individual species and ecosystems, lethal mutations, immunological changes, physical body changes, and increased tumour rates have been observed in a variety of plant and animal species following exposure to nuclear fallout from nuclear testing and nuclear accidents. 

More research is needed, because there are several complicating factors: fallout patterns are complex, and different radioactive particles don’t settle in the consistent ways, making it hard to trace or predict what zones are affected; there can also be lack of support (or in the cases of many nuclear tests, even active blocking of access) for thorough research of the environmental impact on affected sites and beyond; and there are massive differences between species’ ability to sustain toxic and genetic effects caused by radiation, and differences in bioaccumulation rates.  

Bioaccumulation - the gradual accumulation of substances like radioactive elements in an organism, when it absorbs them faster than it can be lost or eliminated by catabolism and excretion - poses a problem at both the species and ecosystem levels. Radioactivity accumulates up the food web and can negatively affect some species—particularly those at the top of the food chain, or those that rely heavily on food items that were strongly affected by the original exposure. 

Studies from the aftermaths of Chernobyl and Fukushima show how bioaccumulation works and varies across species:  

• Mushrooms - including edible species - continued to absorb radioactive material from the soil until 2007. 
• The needles, leaves and twigs from Scots pine trees that have grown in some areas of the exclusion area after the  Chernoby accident show high radiation levels, absorved from the soil. However, through their local deposition, they are believed to restrict the contaminated material from spreading further, which also raises concerns over the potential spreading effects of large scale wildfires.  
• Predatory fish species - like the Trench  (Tinca tinca) -  retained higher radiation levels for a longer time even when those at the bottom of the food chain no longer show effects of radiation exposure (Koulikov 1996). 
• Looking at birds in particular, the different species were not all equally affected by radiation exposure, but on average bird populations in both Chernobyl and Fukushima decreased with increasing levels of background radiation.   
• 10 years after the Fukushima disaster, researchers found that wild Japanese monkeys living in Fukushima City showed high concentrations of 137Cs in their hind limbs, which was likely absorbed by eating buds and bark on local trees, as well as other foods like mushrooms and bamboo shoots, all of which take in radioactive cesium from the ground. 

While the effect of “exclusion zones” - a visible spike in some species’ population sizes following the departure of humans in the area, as seen after Fukushima and Chernobyl -is sometimes cited to minimise concerns over radioactive contamination, such increases - also seen near cities during COVID lockdowns -  is not an immediate indicator that the ecosystem as a whole is healthy, or that these individual animals and populations themselves will not affected by radiation in the long-term. Elevated radiation has been found to negatively affect the abundance of entire species groups, such as insects and spiders, raptors and small mammals.