Immediate Causes of Death

Mortality (from Latin mors, death) is one of the major ecological processes that affect the population dynamics of living organisms and is an important component of population systems. Although most multicellular organisms are genetically programmed to die after the end of reproduction period, death often occurs earlier which may negatively affect population numbers in future generations. Major immediate causes of death include inimical agents, competition, shortage of energy or other resources, and unfavorable environment (Table 1). Immediate cause of death should not be confused with indirect effects of environment. For example, if the humidity accelerates the spread of fungal infection, the immediate cause of death is the fungus rather than humidity. Inimical agents are classified into pathogens, which include viruses, bacteria, or unicellular eukaryotes; parasites, which are multicellular organisms that use their hosts both as habitat and source of food; and predators, which are multicellular organisms that use their prey as

Table 1 Major immediate causes of death in living organisms

Immediate cause of death

Class of causes

Pathogen

Inimical agent

Parasite

Inimical agent

Predator

Inimical agent

Competition, cannibalism

Competition

Food/nutrients

Energy resource

Light

Energy resource

Oxygen

Other resource

Water

Other resource

Shelter/refuge

Other resource

Space/soil

Other resource

Microelements

Other resource

Temperature

Environment

Humidity

Environment

Salinity

Environment

Wind/water current

Environment

Toxic chemicals

Environment

Radiation

Environment

Barriers

Environment

food only. Death due to competition does not include indirect effects of competition (e.g., resource shortage). Mortality from competition or cannibalism is always caused by organisms of the same species, because otherwise it is considered predation.

Many immediate causes of death can be associated positively or negatively with anthropogenic effects. For example, humans reduce the number of wolves and foxes in urbanized areas which reduces the predation rate in deer and rabbit populations. Humans release pathogens, parasites, and predators for biological control of agricultural and urban pests. Agricultural ecosystems provide excessive food supply for many herbivorous insects and reduce their mortality due to food shortage. Forest clear-cuts increase the amount of light that reaches the ground, which causes mortality in plants that cannot tolerate direct sun light, and reduces mortality in other plant species which survive better in open conditions. Also forest cuts remove the food source for herbivores that feed on trees and shelters for many forest animals. Irrigation affects humidity and water supply for many animals and plants which changes their mortality patterns. River dams create barriers for fish movement and causes their mortality. Finally, humans often introduce toxic chemicals into environment (intentionally, e.g., pesticides, or inadvertently, e.g., chemical spills). Radioactivity associated with accidents in nuclear plants may also affect the mortality in animals.

Because of multiple immediate causes of death, mortality is a family of ecological processes rather than a single process. For example, death due to predation and infection by pathogens are two different ecological processes which are affected by different factors. Mortality caused by different species of predators, parasites, or pathogens is also considered as separate processes. Organisms change their susceptibility to inimical agents and harsh environment during their life cycle. Thus, deaths at different stages of development are also considered as different processes. The most spectacular changes in the life cycle occur in insects, which have distinct stages that live in different habitats and consume different resources. As a result, eggs, larvae, pupae, and adults have very different sets of natural enemies, and different requirements to environmental factors. Many animals have specialized stages (e.g., diapause and hibernation) which are not sensitive to resource limitations and to harsh environment. In some species there are differences in mortality rate between males and females, which can be related to reproduction, difference in body size or behavior. In this case, mortality of males and females should be considered as different processes. However, we may decide to combine mortality processes caused by similar natural enemies to simplify the description of population dynamics.

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