Interspecies relationships

Types of Interspecies Relationships

Interactions between species are usually classified into amensalism, competition, predation, herbivory, parasitism, commensalism, and mutualism. With the last three coming under the term symbiosis.

Symbiosis

Symbiosis refers to species that live together. It encompasses a wide range of interactions. The three main types of symbiosis are:

• Mutualism

• Commensalism

• Parasitism.

Mutualism

Mutualism benefits all parties. Some species can’t live without their partners. Others can still live but not very well. Some common examples include flowers being pollinated by bees or leafcutter ants and fungus. The ants feed on the fungi they sow.

Fig. 1 – Bee pollinating a flower. Source: unsplash.com

Commensalism

Commensalism is a symbiotic interaction in which one party gets a benefit from the interaction and the other species remains unaffected. This is particularly hard for ecologists to identify as indirect effects on other species can occur.

An example is orchids which grow on branches of large trees without affecting them. Humans also facilitate commensalism by leaving food waste around for rodents that feed on it.

Commensalism can be further defined as phoresis and tenancy.

Parasitism

Parasitism involves an individual who benefits from the relationship and another who is negatively impacted - the parasite and the host. An ectoparasite (eg. fleas and lice) is located outside of the host whereas an endoparasite is located inside the host (eg. tapeworm).

Amensalism

Amensalism is a type of biological interaction where one species causes harm to another organism without any cost or benefits to itself. There are two modes of amensalism:

• Competition: One organism deprives a smaller, weaker organism of food or habitat. E.g. Cattle trample on grass and crush it. They do not benefit from this action or are harmed in the process.

• Antibiosis: An organism is damaged or killed by chemicals released by other organisms. E.g. Penicillium secretes penicillin which kills bacteria.

Neutralism

Neutralism is a relationship in which both species remain unaffected. Many ecologists debate that neutralism is unlikely, as the presence of one organism must affect, to some extent, the other.

However, some bacteria such as genera Lactobacillus and Streptococcus coexist without affecting each other.

Competition

Competition exists when organisms compete for the same limited resources. It negatively affects both parties, with the weaker competitor being affected to a greater extent. Examples of resources species compete for include:

• Food

• Light

• Water

• Habitat

To remind you from the previous sections, when the competition is between members of different species it is called interspecific.

The law of competitive exclusion states that 'No two species can occupy the same niche indefinitely in the same habitat.' It was created by a guy called Gauze who set up cultures of two Paramecium caudatum and Paramecium aurelia which are two closely related species from the Protozoa group (since-celled eukaryotic organisms). He found that when grown separately they flourished but when grown together, one was eliminated. He then conducted further experiments and found that by adjusting the conditions, he could change which species were eliminated.

The two main types of competition are:

• Competition for interference (fighting directly for the limited resource).

• Competition for exploitation (when two or more species use a resource in common).

Often, one species will displace another and the competition will end or one party may evolve to allow both species to coexist in the same niche.

Predation

Predation occurs when one organism (the predator) consumes another organism (the prey). The consequences for the predator are positive and for the prey - negative.

In a healthy ecosystem, the numbers of predators and prey remain fairly constant.

A less balanced ecosystem would occur if there is a change in abiotic factors such as water, sunlight and temperature. Or there could be biotic factors such as a pathogen or a new predator.

In a predator-prey cycle:

1. The number of predators increases because there is more prey.

2. The number of prey then reduces because there are more predators.

3. The number of predators then reduces because there is now less prey for predators so more competition for food.

Over time, prey and predators cause the evolution of each other. Predators adapt for more successful hunting and prey adapt to avoid being eaten and survive long enough to breed.

Table 1. Predator vs prey interactions

Fig. 2 – A lion, which is one of the main predators in savannah ecosystems. Source: wikimedia.com

Herbivory

When an animal consumes a plant, it is called herbivory. A herbivore doesn’t always kill its prey, only if it consumes parts of photosynthetic organs that affect the plant.