Biology: 17.1 Biological Communities

HowOrganisms Interact

in Communities

Evolution in Communities

When we look at an ecosystem can we say that any one organism is more important than another.

All inhabitants in an ecosystem are bound together in a web of interactions.

To look at an ecosystem we must look at it’s community; all the living organisms that interact with each other in the system.

Interactions Among Species

Interactions among species are a result of a long period of evolutionary adjustment to other species in their community. Both adapt over time to each other.

Interactions Among Species

For example, favorable adaptations appear in flowering plants that promote dispersal of it’s pollen and seeds by other members of it’s community; birds and insects. The bright colors or scents of flowers make them easier to find for birds and bees.

In return, favorable adaptations also appear in the birds and insects that enable them to obtain food from the flowers that they pollinate. A strong sense of smell enables hummingbirds to locate flowering plants.

Interactions Among Species

Because of these evolutionary adjustments through natural selection, close relationships of mutual support develop over time between plant species and the birds and bees that pollinate them.

Both adjust to each other to make their relationship of mutual support tighter as each benefits from each other.

These back-and-forth evolutionary adjustments between interacting members of a community are called coevolution.

Predator and Prey Coevolution

Predation is the act of one organism killing another for food.

Examples include snakes eating frogs, coyote killing deer, or a spider catching a fly.

The predators are the consumers, the hunters, while the prey are the ones consumed, the food source.

predation

parasitism

Predator and Prey Coevolution

Predators and prey both coevolve together.

Predators develop evolutionary traits that make them better hunters; speed, stealth, camouflage, strong scent, claws and sharp teeth among them. This promotes their own survival.

Prey develop keen hearing, strong sense of smell, quick reflexes, and camouflage as well to escape from the hunters. This promotes their own survival.

predation

parasitism

Parasitism

In parasitism, one organism feeds on and usually lives on or in another larger organism. Fleas are an example of a parasite that lives off the blood of it’s host.

Parasites do not kill their hosts but depend on them for their food. Examples of external parasites (parasites that live off the outside of the host) are fleas, lice, mosquitoes and ticks.

Some parasites, such as hookworms, actually live inside the host feeding from the inside.

predation

parasitism

Plant Defenses Against Herbivores

In these relationships of predator and prey, animals develop favorable defense mechanisms to help them escape predators to survive better.

Both predator and prey continually evolve traits that aid their individual survival.

This is true of plants as well as of animals.

The most obvious plant defense mechanisms are the spikes, thorns, and prickles that plants such as cactus develop.

Plant Defenses Against Herbivores

Plants also develop chemical compounds that discourage consumption by herbivores.

These defensive chemicals are called secondary compounds.

Plants such as poison ivy or poison oak are examples of plants with strong secondary compounds.

Mustard plants

Poison ivy

Plant Defenses Against Herbivores

Each plant group produces it’s own kind of defensive chemical.

For example, the mustard plant family produces a group of oils known as mustard oils. These oils are toxic to many varieties of insects.

Some insects however, butterfly larva included, are able to consume mustard plants with no ill affects.

They have developed the evolutionary adaptation to break down mustard oils thus allowing them to use these plants as a food source.

Mustard plants

Poison ivy

Symbiosis

In symbiosis, two or more species live together in a close long-term relationship.

Symbiotic relationships can benefit both organisms or benefit just one of the two, leaving the other unharmed or unaffected.

These small birds live off picking insects from the rhino’s back.

Clown fish live among tendrilsthat would sting other fish. They are immune to the sting.

Parasitism

Parasitism is a form of symbiosis which is detrimental to one of the forms in the relationship.

A parasite draws it’s energy from another host body harming or killing the host in the process.

These small birds live off picking insects from the rhino’s back.

Clown fish live among tendrilsthat would sting other fish. They are immune to the sting.

Symbiosis

It is often easy to determine if an organism is being helped by another in a relationship but it can be difficult to determine if one organism is being harmed or not affected at all by the other in a relationship.

How is this rhino helped by these birds that feed off the insect life that also lives on the rhino?

Does this clown fish help the anemone it lives among or is this a one way relationship?

These small birds live off picking insects from the rhino’s back.

Clown fish live among tendrilsthat would sting other fish. They are immune to the sting.

Mutualism

Mutualism is a symbiotic relationship in which both species benefit from each other.

A well known example of mutualism is the relationship between aphids and ants.

Mutualism

Aphids suck fluids from the sugar-conducting vessels of plants. They extract sucrose and other fluids from this mixture.

In turn, the aphids extrude a substance called honeydew that the ants use for food.

The ants can be found around the aphids, protecting them from other predators, while using their excrement as a food source. Both species benefit from this mutual beneficial relationship.

Commensalism

Commensalism is a third form of symbiosis in which one species benefits while the other is neither harmed nor helped.

Among the best known examples of commensalism is the relationship of the tropical clown fish and sea anemone.

Commensalism

The tentacles of the sea anemone have a venom that will paralyze larger fish that prey upon the clown fish. The clown fish is immune to this venom.

Thus the clown fish spends it’s life in the protection of the coral reef’s sea anemone, feeding where larger fish can not go. Their relationship is an example of commensalism.