Introduction

No organism lives completely independently. Every living thing interacts with other organisms in its environment, whether by competing for resources, hunting for food, forming partnerships, or providing habitats for other species. These interactions help shape ecosystems and influence which organisms survive and reproduce.

Some interactions benefit both species, while others benefit one species at the expense of another. By studying these relationships, ecologists can better understand how ecosystems remain balanced and how changes to one species can affect many others.

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Types of Species Interactions

Scientists group interactions between organisms into three major categories:

• Competition
• Predation
• Symbiosis

Each interaction affects the organisms involved in different ways.

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Figure 1. Organisms interact in many different ways, including competition, predation, and symbiosis.

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Competition

Competition occurs when two or more organisms need the same limited resource.

Resources that organisms compete for include:

• Food
• Water
• Sunlight
• Shelter
• Territory
• Mates
• Nesting sites

Competition may occur:

Within the Same Species (Intraspecific Competition)

Members of the same species compete with one another.

Examples:

• Two male deer competing for mates.
• Young trees competing for sunlight.
• Birds competing for nesting sites.

Between Different Species (Interspecific Competition)

Different species compete for the same resource.

Examples:

• Lions and hyenas competing for prey.
• Plants competing for water.
• Foxes and owls hunting the same rodents.

Competition often reduces the amount of resources available to each organism.

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Predation

Predation is an interaction in which one organism (the predator) hunts, kills, and eats another organism (the prey).

Examples include:

• Hawk eating a mouse
• Spider catching a fly
• Shark hunting a seal
• Frog eating insects

Predation helps:

• Control prey populations
• Remove weak or sick individuals
• Transfer energy through food chains

Predators have adaptations such as:

• Sharp teeth
• Claws
• Excellent eyesight
• Camouflage
• Speed

Prey also have adaptations, including:

• Camouflage
• Defensive spines
• Warning colours
• Fast running
• Living in groups

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Figure 2. Predators and prey have adaptations that improve their chances of survival.

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Symbiosis

Symbiosis is a close, long-term relationship between two different species.

There are three main types of symbiosis:

• Mutualism
• Commensalism
• Parasitism

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Mutualism (+/+)

In mutualism, both species benefit.

Examples include:

• Bees pollinating flowers while collecting nectar.
• Clownfish living among sea anemones.
• Nitrogen-fixing bacteria living in plant roots.

Benefits may include:

• Food
• Protection
• Reproduction
• Nutrient exchange

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Commensalism (+/0)

In commensalism, one species benefits while the other is neither helped nor harmed.

Examples include:

• Barnacles attached to whales.
• Birds nesting in trees.
• Orchids growing on rainforest trees.

The larger organism is generally unaffected.

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Parasitism (+/−)

In parasitism, one organism benefits while the other is harmed.

The organism that benefits is called the parasite.

The harmed organism is called the host.

Examples include:

• Tick feeding on a dog.
• Flea living on a rabbit.
• Tapeworm living inside a human intestine.
• Mistletoe growing on trees.

Parasites usually do not kill their hosts immediately because they depend on them for survival.

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Figure 3. The three major types of symbiosis differ in how each organism is affected.

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Comparing the Three Types of Symbiosis

Scientists often use symbols to describe these relationships:

• + = Benefits
• − = Harmed
• 0 = No significant effect

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How Interactions Affect Survival and Reproduction

Species interactions directly influence an organism's ability to survive and produce offspring.

Competition

Can reduce:

• Food availability
• Growth
• Reproductive success

Predation

Can:

• Control population sizes
• Drive the evolution of defensive adaptations
• Influence where prey species live

Mutualism

Can increase:

• Food availability
• Protection
• Successful reproduction

Parasitism

Can:

• Reduce health
• Lower reproductive success
• Spread disease

Healthy ecosystems depend on a balance between all of these interactions.

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Ecosystem Example: An African Savannah

Many interactions occur within a single ecosystem.

One ecosystem contains many different interactions occurring at the same time.

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Figure 4. A single ecosystem often contains many different types of species interactions.

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Worked Example

Question

Identify the type of interaction shown in each example.

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Real-World Connection

Understanding species interactions is essential for conservation and wildlife management. For example, wolves help regulate deer populations, preventing overgrazing and allowing forests to regenerate. Similarly, bees pollinate many crops that humans rely on for food. If important interactions are disrupted—through habitat loss, pollution, or the introduction of invasive species—entire ecosystems can become unbalanced.

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Did You Know?

Cleaner wrasse are small reef fish that remove parasites and dead skin from much larger fish. The larger fish receive a health benefit, while the cleaner wrasse gains a meal. This is one of the best-known examples of mutualism in marine ecosystems.

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Key Terms

Competition – The struggle between organisms for limited resources.

Commensalism – A symbiotic relationship in which one species benefits while the other is unaffected.

Host – The organism that a parasite lives on or inside.

Mutualism – A symbiotic relationship in which both species benefit.

Parasite – An organism that lives on or inside another organism and benefits at its expense.

Parasitism – A symbiotic relationship in which one species benefits while the other is harmed.

Predation – An interaction in which one organism hunts, kills, and eats another.

Predator – An organism that hunts and eats other organisms.

Prey – An organism that is hunted and eaten by a predator.

Symbiosis – A close, long-term relationship between two different species.

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Key Takeaways

• Organisms constantly interact with one another in ecosystems.
• The three major types of interactions are competition, predation, and symbiosis.
• Symbiosis includes mutualism, commensalism, and parasitism.
• Species interactions influence survival, reproduction, and population sizes.
• Healthy ecosystems depend on a balance of interactions between organisms.