Module 8 Lesson 7 - 6
Lesson 7 — Species Interactions and Symbiotic Relationships
Lesson Summary
You should now have come to appreciate the many types of living relationships that can help, harm, or have little effect on interacting species.
Relationships among organisms occur when an organism’s struggle for survival is affected by another organism. In some cases, alliances form that benefit both species; in some, the success of one species is, by definition, the death of the other. The relationships of interest in this lesson are the three kinds of symbiosis: parasitism, commensalism, and mutualism.
A symbiotic relationship ties two specific species together for life, usually in a feeding relationship. In mutualism, the relationship is positive for both species. In commensalism, one species benefits and the other species is unaffected. In parasitism, one member of the pair benefits while the other is harmed.
Another important relationship is that of predator and prey. Predator and prey population cycles keep each other at the carrying capacity of the environment: high prey numbers produce more predators that eat more prey, reducing prey densities back to
normal. Predators improve the prey gene pool by hunting the sick, the weak, and the poorly adapted, sieving out the poor alleles and leaving a stronger, more genetically fit prey population. Most human hunters weaken prey gene pools by killing the
superior members of the population, eliminating the best alleles from the population. Predator-prey population cycles always have more prey than predators, and the numbers of predators follows the trend of the prey species.
All species rely on defenses of some kind to reduce their chances of becoming meals for another species. These can take the form of structural characteristics (thorns, claws), chemical characteristics (plant toxins, venoms) behaviours (territoriality, barking), or colouration (protective colouration, mimicry, and cryptic colouration). These defenses allow organisms from one species to protect themselves and to compete interspecifically with species that share the same trophic level. Slight variations in defenses between individuals of the same species (intraspecific competition) allow some individuals to compete better than their neighbours can, changing allele frequencies in their favour. The greater the scarcity in a habitat, the more competition there is; thus, the faster evolution occurs.