Parasitism and Mutualism: Host Relationships, Types and Ecological Effects

Item 16: Parasites

Parasitism is an interaction in which the consumer-type organism, the parasite, feeds on a resource of another species, the host, which is harmed while the parasite benefits. Parasite-host associations are intimate, and the host serves as habitat and food for the parasite, which is usually the larger partner.

Parasitism differs from predation: parasites usually do not kill their host immediately, and they are typically smaller than their host. A predator, by contrast, causes immediate death of its prey.

Types of Parasites

By size:

• Microparasites: micro-sized, very numerous within the host, intracellular, and generally without specialized transmission mechanisms from one host to another.
• Macroparasites: larger, less numerous, and not intracellular (they live in cavities). They frequently have specialized transmission mechanisms.

By location:

• Endoparasites: live inside the cavities and body tissues of the host.
• Ectoparasites: live on the external parts of the host's body.

By host status:

• Necrotrophic: the first stage of the relationship consists of feeding on detritus, with the host already dead.
• Biotrophic: the host is alive.

Types of Hosts

• Intermediate hosts: hosts in which parasites pass part of their life cycle. They do not undergo several phases of asexual reproduction in these hosts, though they may undergo metamorphosis.
• Definitive hosts: hosts in which the parasite reaches sexual maturity and carries out sexual reproduction.

Effects of Parasites on Hosts

• Infectious process: the parasite infects and may provoke the death of the host.
• Loss of condition: although it does not cause death, it may cause malnutrition and reduce the growth rate.
• Morphogenesis: changes in body form, and in some cases the host reaches larger sizes.
• Loss of reproductive capacity: castration, which may be total in some crustaceans.
• Metabolic changes: loss of photosynthetic capacity.
• Behavioral modifications: for example, in snails, under normal conditions they tend toward dark, humid areas; if parasitized, they move toward light and into zones that increase the degree of parasitization.

Parasitism Measures

• Prevalence: the percentage of individuals of a host population that harbour parasites.
• Intensity: the number of parasites per host.

Factors Affecting Parasitization

• Host density: higher density leads to higher degrees of parasitism, since parasites transmit more easily.
• Behavior and abundance of vectors: for example, in malaria, the mosquito vector transmits the parasite between individuals.
• Rank and distribution of hosts, age and size of hosts: older hosts and larger hosts tend to carry more parasites.
• Environmental factors, intra- and interspecific competition among parasites, and evolutionary factors.

Mathematical Models of Parasitism

• Ecological models: analyze the dynamics of the interaction between the two populations, parasite and host.
• Infection models: study the infection process and parasitation. There is no single model, but hosts can be grouped into pre-parasitized, parasitized, and post-parasitized stages.

Evidence of Host-Parasite Interaction and Parasite Importance

Evidence includes the abundance of parasite species and the specific adaptations of parasites:

• Morphological: flattened bodies, structures for attachment.
• Functional: coupling and interference cycles with the host's life, often increased anaerobic metabolism, and mechanisms to evade host defenses.

Host-specific adaptations include:

• Functional: immune system responses and behaviors, such as association with cleaner organisms.

Evidence from experiments:

• Natural: parasitization dynamics influence host population dynamics; at various population levels, parasites may or may not influence the host.
• Manipulation of hosts: deep parasitization occurs in some cases, though not in superficial ones.

Topic 17: Mutualism

Symbiosis is an intimate association between two species. Within it are included parasitism and mutualism, the type of symbiosis in which both species involved benefit.

Types of Mutualism

By obligatoriness:

• Obligate mutualism: in some communities, one can find organisms that are modified corporally in a profound way; the partners cannot live without the mutualistic relationship.
• Facultative mutualism: the symbiotic relationship may not exist; the species involved can live without each other. For example, cleaner fish can survive without obtaining food from cleaning other fish, so both are facultative.

Mandatory or optional: each can function without the other, or can live alone or live on the mutualism.

By specificity:

• Selective: the organisms live in a habitat determined by one species; none of them will live with another organism.
• Diffuse: several different species serve as mutualists.

By benefit:

• Protection-food: one organism defends another, and the other reciprocally provides food.
• Protection-habitat: for example, clownfish are protected by sea anemones, which they in turn protect from predators.
• Signal-food-habitat: for example, bioluminescent bacteria living in certain organisms.
• Food-cleaning: one organism obtains food by cleaning parasites from the other, which is cleaned.

Mathematical Models of Mutualism

There are models based on Lotka-Volterra equations:

dN₁/dt = r₁ N₁ [1 - (N₁ - β N₂) / K₁]

dN₂/dt = r₂ N₂ [1 - (N₂ - β N₁) / K₂]

The isoclines usually give models of limited realistic importance.

Evidence of Mutualistic Interactions

• Diversity and ubiquity of mutualism.
• Experiments (natural or manipulative): species decline when the diversity of cleaner organisms disappears.
• Natural example: zooxanthellae and corals.