Physical stressors are actions that directly remove or alter habitat. Examples include tilling soil, filling wetlands, or dredging a channel or harbor. Ecosystems are dynamic and possess a certain degree of resilience to recover from disturbances. In fact, natural disturbance is a normal part of ecosystem functioning. Generally, larger and more frequent physical stressors result in more excessive and longer-lasting effects. However, the addition of excessive physical stressors from human activities can sometimes exceed the ability of the ecosystem to rebound. The time required for an area to recover from a physical stressor once it is discontinued will often depend upon the type of ecosystem involved
Aquatic ecosystems are often affected by physical stressors. Erosion from road construction or agricultural activities can create siltation (i.e., sediment buildup) in streams or lakes receiving runoff. Siltation can cause changes to habitat features such as water depth, rooted emergent and submergent rooted plant distribution in a freshwater or saltwater marsh, and loss of refuge or foraging areas for fish fry. Siltation can also result in filling of the interstitial spaces in gravel or cobble materials on the bottoms of water bodies. This can make these areas unsuitable for egg-laying by fishes, can suffocate eggs already present, and reduce or eliminate benthic invertebrates. Water bodies exposed to runoff from soils that are high in clay content can experience increased turbidity for days or weeks after a major rainfall event that, in turn, changes light intensity through the water column. Reduced light penetration can affect a fish's ability to find food and can lower the rate of photosynthesis by aquatic plants.
In addition to disturbing or destroying the immediate habitat(s), activities such as road construction, logging, dredging wetlands, and agriculture can whittle away piecemeal at larger, relatively intact areas. This results in habitat fragmentation. Some species cannot survive if the patch of habitat available to them falls below a certain size, while other species prefer the edges where habitat types change. Thus, habitat fragmentation can result in dramatic changes in community structure by altering the types of species that use fragmented areas. Habitat patches can become so fragmented that they act as islands that isolate populations of species that are unable to traverse inhospitable areas of habitat to interbreed with individuals in other fragments.
Risk assessors should be aware of physical impacts that have occurred at sites, as this may alter the types of evaluations that are conducted. For example, previous activities that removed topsoil from a site will lead to changes in the plant composition and the associated wildlife. It would be inappropriate to conclude that differences between a reference site and the contaminated area are due to contamination alone, unless the effect of the physical alteration of the environment is taken into account. The ecological effects of physical stressors should also be considered when evaluating the effects of remedial alternatives for a site. Removal of soil or trees, filling or dredging of wetlands, and the erosion or siltation impacts of cleanup operations can all have substantial ecological effects that, in some cases, may pose a greater ecological risk than the continued presence of contaminants.