Mutalism/Competition

Macroparasites vs. Microparasites (both are small relative to host size)

Macroparasites: Large species such as arthropods and tape worms.

The whale tapeworm Tetragonoporus calyptocephalus is a giant parasite that lives in the intestines of whales and can reach 130 feet long.

Microparasites: Microscopic, such as bacteria and viruses.

A virus is an obligate intracellular parasite because it cannot replicate or reproduce outside of a host cell, relying instead on the host's cellular machinery.

Many species are host to more than one specialized parasite species.

• It’sunusualforanysingle organism to be host to all potential parasite species.

• The Unusual Number Of parasites for trees includes many species of insects that feed on sap of trees (e.g., aphids and cicadas). Some consider such insects to be parasites.

Parasites specialize on many different parts of the body, internal and external.

Ectoparasites vs. Endoparasites

Ectoparasites live on the outer body surface of the host.

Endoparasites live inside their hosts, within cells or tissues, or in the alimentary canal.

Endoparasite dispersal presents significant challenges.

• Endoparasites often disperse through hostile environments between multiple hosts specific to certain life stages.
  • Primary host (or definitive host) is where the parasite reaches its adult stage and reproduces sexually.
  • Secondary host (or intermediate host) is where the parasite develops or undergoes a life cycle phase but does not reach sexual maturity.

• Endoparasites produce enormous numbers of offspring to compensate for the very low probability that young will successfully disperse to new host.

• Asexual reproduction is common due to the low probability of finding a mate.

Some parasites have evolved to alter the behavior of their hosts in ways that promote the completion of their life cycle.

Many parasites manipulate primary hosts to attract secondary hosts.

Parasite-host systems: a balancing actThe host-parasite interaction represents a battle between parasite

virulence and host defenses.

Host defenses:

 Physical defenses – e.g. tough skins/shells

 Immune system responses – Specialized immune cells identify, engulf and destroy microparasites.

 Encapsulation – Blood forms capsules around smaller parasites, a defense more common in insects.

 Biochemical defenses – secondary compounds common in plants Parasites:

• Try to multiply rapidly before an immune response can be deployed.

• Under selective pressure not to be so virulent as to kill their host, as the host must stay alive at least until transmitting offspring to new host.

Interspecific competition is – / – interaction between two species.

Competition among individuals may be:

• Intraspecific (same species)

• Interspecific (different species)

Schoener’s (1983) 5 types of competition:

Consumptive - One organism inhibits another by consuming a shared nutrient resource (e.g., two mice species competing for food)

Preemptive - One organism occupies space, preventing another from having it (e.g., two species of barnacle competing for space on rock).

Overgrowth - One organism grows directly over another. (e.g., one tree species blocking another from access to light).

Chemical (allelopathy) - Chemical warfare between competitors (e.g., many Mediterranean desert plants).

Territorial - Aggressive behavioral exclusion of an organism from defended territory (e.g., wolves defending territories from predatory cats).

Encounter - Non-territorial encounters reducing growth.

Consumers compete for resources

Resources are substances or factors that are consumed by an organism to survive and which support population growth with increasing availability. When reduced, the result is reduced population growth.

Examples: food, light, water, nutrients, space

Conditions may affect growth and reproduction, but they are not consumed by organisms, and so are not resources for which species compete.

Examples: temperature, pH and salinity

The outcome of competition is often determined by competition for the limiting resource

• Liebig’s law of the minimum: limitations on growth tend not to be determined by total resources available, but rather by the availability of scarcest resource (i.e., resource that is the limiting factor).

• Each population increases until the supply of the limiting resource becomes depleted.
  
  

Intensity of competition increases with degree of niche overlap and scarcity of associated resources.

Niche overlap:

The greater the niche overlap between two competitor species, the greater the intensity of competition.

Resource abundance:

The lower the abundance of the resources for which the species compete, the greater the intensity of competition.

Rarely does competition have the same effect on competitor species.

Asymmetric competition occurs when the effects of competing species on each other are not equal (this is almost always the case in nature).

Amensalism is the term used to describe the most extreme form of asymmetric competition (i.e., where one species is affected negatively, and the other species is not affected at all).

The ‘superior’ competitor has the largest effect on abundance and distribution of its competitor.

• Fundamental niche - Niche in the absence of competition

• Realized niche - niche with competition.

Mutualists perform complementary functions for each other.

Interactions between species that benefit both participants can lead to coevolution

There are several types of mutualisms.

Trophic mutualism
• help in obtaining energy and nutrients

Defensive mutualism
• help in shelter and defense against enemies

Dispersive mutualism
• help in dispersing individuals, seeds or pollen