Individual Characters that Affect Resistance

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Various aspects of the life history of individual members of the population will make them more or less resistant to disturbance. And even within a species and therefore life history, there can be individual variation. These factors can be broken down into at least five categories:

1. Behavioral responses (that allow the organism to adjust its position etc. with respect to a disturbance)

2. Resistant stages (which allow an organism a temporal opportunity to escape harsh climate)

3. Physiological traits (biochemical mechanisms which allow an organism to repel harsh conditions and organisms through chemical means)

4. Morphological traits (physical characteristics that enhance resistance to environment and other organisms)

5. Developmental responses (which involve reproductive responses or life-stage adaptations that may allow populations to resist harsh environments).

Some examples of these categories of character are given in Table 1. Behavioral responses are widespread for animals especially, and include fossorial (burrowing) behavior of a range of insects, reptiles, amphibians, and mammals to avoid extremes of weather. Many taxa have resistant stages, such as endoparasites that have a resistant cyst as part of their life cycle, often to resist strong acid conditions in vertebrate guts. Physiological responses include a suite of terrestrial plants and marine algae that produce distasteful or poisonous secondary metabolites to avoid grazing predation. External morphological features to resist abiotic and

Table 1 Examples of individual mechanisms that promote resistance to environmental conditions and population buffering capacity

Trait

Example

Organism

Consequence

Behavioral

Resistant stages

Physiological

Morphological

Developmental plasticity

Burrowing to escape extreme climate Flexible migration patterns Adjusting home range or territory

Cyst formation Dormancy, estivation Seed bank

Urea sac

Secondary metabolites to reduce predation Desiccation resistance -waxy cuticle presence Lipid buildup Pesticide resistance

Hard exoskeleton/shell Helmet development in high-predator lakes Thorns

Embryo suspension/

Anoestrus Tadpole development vs. pond drying

Giant burrowing frog Bluefin tuna, hummingbirds Thrushes, raptorial birds

Malaria protozooans, Artemia

Lungfish, water-holding frog, hummingbirds European perennial grass (Holcus sp.), many plant species Water-holding frog

Marine algae Insects

Hummingbirds Various insects

Intertidal gastropod snails Daphnia spp.

Acacias

Red kangaroo (Megaleia rufa) Helioporous sp. frog

Survive drought and fire

Avoid cooler temperatures of ocean currents or cold fronts, maintain swimming/flying efficiency Defend variable resource levels

Prevent removal from host body, survival when saline ponds dry up Survival in suspended state during stressful season

Allows rapid recolonization after adult dieback

Allows hydration in ground during drought conditions

Prevents grazing by fishes

Water balance

Energy for long flights

Resist crop spraying; population survival

Resist desiccation mortality Reduces fish predation

Prevents grazing by most herbivorous mammals Birth postponed in drought conditions

Able to complete life cycle in sporadic rain areas biotic environments are common and include Daphnia, freshwater crustaceans that may have a pronounced helmet that appears to reduce predation risk, but only in lakes that have a history of predator presence. Finally, there are many instances where organisms are able to switch their life histories to respond to temporally variable and extreme environments. Red kangaroos have the ability to suspend embryo development if extreme arid conditions are encountered, while tadpoles of several frog species can vary the rapidity of their development to metamorphosis depending on rainfall and pond persistence.

All of these dictate important factors like dispersal ability, maturity, longevity, fecundity, and growth rate which will determine how an individual responds to disturbance (lives or dies, or sublethal effects on growth and fecundity). Note that plastic phenotypic variation among individuals within populations (developmental, behavioral, or physiological), which may or may not have a genetic basis, will lead to variation in the buffering capacity of a population to environmental change. In some instances, such as the increased appearance of coral bleaching worldwide, previous buffering capacity of taxa has been lost as water temperatures reach their physiological thermal maxima over a period apparently too short for adaptation to occur. Here, indirect effects on reef fish communities are apparent (see Box 1).

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