Environmental interaction
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Environmental Interaction. Examining some of the ways that organisms respond to stimuli in their environment. Stimulus and Response. One criterion for defining life is that living organisms react to stimuli .

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Environmental Interaction

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Environmental interaction

Environmental Interaction

Examining some of the ways that organisms respond to stimuli in their environment.

Stimulus and response

Stimulus and Response

  • One criterion for defining life is that living organisms react to stimuli.

  • Stimulus (stim-yoo-lus) [plural stimuli]: Something that evokes or causes a response in an organism. Ex: a pin poke

Stimulus and response1

Stimulus and Response

A stimulus causes a response in a living organism. Notice the response from the pain of being stuck with a large needle.

Any action or behavior directly connected to a stimulus is called a response

Environmental interaction

Plant Stimulus & Response

Plant stimulus and response

Plant Stimulus and Response

  • For an organism to respond to a stimulus, it first must be able to sense it.

  • Ex: A plant must be able to sense where the most light is coming from.

Plant stimulus and response1

Plant Stimulus and Response

  • When a plant grows toward or away from a stimulus, it is called a tropism.

  • Ex: When plants grow toward a light source, it is known as “phototropism”

  • (photo = light)

These house plants are growing toward their light source (a nearby window), a response known as phototropism.

Plant stimulus and response2

Plant Stimulus and Response

  • In plants, no matter how a seed is placed in the ground, the roots always grow down & the shoots always grow up. How?

Environmental interaction

  • Plants sense the tug of gravity and send their roots toward it and their shoots away from it. Because this growth response is based on sensing gravity, it is known as “gravitropism”.

Plant stimulus and response3

Plant Stimulus and Response

  • In hydrotropism, a plant’s roots grow in response to a water source.(i.e- the roots grow towards the water)

  • Some plants (such as mesquite trees) have roots that can go over 150 ft. in search of water

Plant stimulus and response4

Plant Stimulus and Response

  • Plants sometimes change the position of their leaves so that they can catch more or less of the sun’s rays.

  • Since this growth response is based on the sun, it is called “heliotropism”

  • (Helio = Sun)

In heliotropism, a plant adjusts the tilt of its leaves relative to the sun’s rays.

Plant stimulus and response5

Plant Stimulus and Response

  • During periods of drought, plants often need to conserve water, so they tilt or fold their leaves sideways to avoid direct sunlight.

This plant has folded its leaves up to avoid direct sunlight & conserve its moisture. This is a from of heliotropism

Plant stimulus and response6

Plant Stimulus and Response

  • Tropisms are caused by plant hormones called auxins.

  • Auxins are plant hormones that regulate growth of plant cells. When cells get more auxin, they grow larger (and faster).

These house plants are growing toward their light source (a nearby window), a response known as phototropism.

Plant stimulus and response7

Plant Stimulus and Response

  • In phototropism, auxin hormones concentrate in the side of the stem furthest from the light. This causes those cells to grow longer, and consequently “bend” the stem towards the light.

Auxins are the plant hormones responsible for tropisms such as phototropism; shown here

Environmental interaction

A quick illustration of how auxins affect phototropism. The red lines represent auxinlevels: when more auxin hormones are present, the cells are stimulated to grow longer, pushing the shoot tip toward the light source.

Environmental interaction

Animal Stimulus & Response

Animal stimulus and response

Animal Stimulus and Response

  • A taxis is anytime that an animal moves toward or away from a stimulus. (i.e.- moving in response to a stimulus)

  • Animals have some marvelous sensory apparatus which enable them to sense environmental stimuli.

The leaf shaped antennae on this moth allow it to detect trace amounts of pheromones in the air.

Animal stimulus and response1

Animal Stimulus and Response

  • A taxis differs from a tropism because tropisms involve growth in response to a stimulus, whereas taxis involves movement of the entire organism.

Mosquitoes find a meal by sensing CO2 in the air with their antennae and flying towards it; a good example of chemotaxis.

Animal stimulus and response2

Animal Stimulus and Response

  • In chemotaxis, an animal (or bacteria) moves in response to a chemical that it senses in its environment.

  • “Chemo” = chemical

  • “Taxis” = movement

An E. coli bacteria swims toward a source of sugar, a response known as chemotaxis.

Animal stimulus and response3

Animal Stimulus and Response

  • Phototaxis is when animals move in response to light.

  • Ex: Many bugs are attracted to light sources such as porch lights, etc. Pill bugs will move out of the light & into shady areas.

  • “Photo” = light

Bug zappers take advantage of the fact that many bugs exhibit phototaxis towards light sources.

Animal stimulus and response4

Animal Stimulus and Response

  • Animals exhibit responses FAR more complex than tropisms or taxis. Animals need to be able to be able to find food, water, shelter, sense danger, etc. Thus, they have highly specialized sense organs to help them accomplish this.

Animal stimulus and response5

Animal Stimulus and Response

  • Animals have some marvelous sensory apparatus which enable them to sense environmental stimuli.

  • Name some of the sensory apparatus that animals are equipped with…

An excellent example

An excellent example…

  • Our eyes sense & process visible light.

  • Specialized cells in the eye called “rods” distinguish how bright a light source is, while “cones” tell variations in colors. The optic nerve then transmits this info to the brain.

An excellent example1

An excellent example…

  • Nocturnal animals typically have bigger eyes than their diurnal counterparts, and their eyes generally have more rod photoreceptors. These two specializations allow them to see better in the dim light of night.

The sugar glider, a nocturnal marsupial, has become a popular pet in the U.S. and Australia.



  • Animals don’t just sense & monitor their external environment, but also their internal environment so that they can keep it relatively stable.

  • The process of maintaining a steady internal state despite changing external conditions is known as homeostasis.



  • In humans, homeostasis maintains proper blood sugar levels, body temperature, etc.

  • Ex: When you get too hot, you sweat to cool off. When you get too cold, you shiver to warm up. These actions keep your body temp within an acceptable range.



  • Homeostasis depends upon negative feedback loops.

  • Negative feedback is information that is used to stop the change in a system.

Negative feedback example

Negative feedback example…

  • Take your home as an example: If your thermostat is set for 73, and the temperature in your house drops below 73, the heater kicks in to get the temperature back up over 73. If the temp gets too warm, the thermostat kicks off. Thus, your house stays at an acceptable temp range.

Negative feedback example 2

Negative feedback example 2

  • Running increases the cell’s need for O2. This lowers the O2 levels in the blood, so the brain tells the body to start breathing faster & deeper to bring O2 levels back up to an acceptable range.

Innate responses in animals

Innate Responses in Animals

  • Innate behaviors are those that don’t have to be learned or taught. Instead they are “pre-programmed” in the organism’s central nervous system.

  • Taxis is a type of innate behavior. (Ex: bugs flying towards a light source)

Innate responses in animals1

Innate Responses in Animals

  • A reflex is a type of innate response.

  • A reflex is an automatic response by the nervous system to a certain stimulus. (Ex: hand jerking away from a hot object)

Innate responses in animals2

Innate Responses in Animals

  • Another type of innate behavior is an instinct.

  • An instinct is a complex animal behavior that is triggered by an external stimuli.

A dog that is scared will tuck their tail under instinctually. The tail-tucking is not taught or learned.

Innate responses in animals3

Innate Responses in Animals

  • Ex: Body posturing among dogs is an instinctual response.

  • Dogs use body postures to show other dogs their intentions. These body postures are not learned, but rather are instinctual.

A dog posturing for play (top) and for fighting (bottom)

Innate responses in animals4

Innate Responses in Animals

  • Some organisms releases chemical messengers called “pheromones.”

  • Pheromone messages are instinctual (meaning that they never have to be learned).

When an ant finds food, he leaves a pheromone trail behind him.

Innate responses in animals5

Innate Responses in Animals

  • Bees also use a multitude of pheromones to send messages to each other. (Such as the message to swarm something that is seen as a threat to the hive)

Bees swarming a loud engine. One of the bees was startled by the noise and sent out a “swarming” pheromone.

Environmental interaction

This insect trap uses pheromones to attract insects.

Conditioned responses in animals

Conditioned Responses in Animals

  • Not all automatic responses to stimuli are innate.

  • Automatic responses to stimuli that have been learned are called “conditioned responses”.

A dog that perks up at the sound of the doorbell is exhibiting a conditioned response.

Complex behaviors in animals

Complex Behaviors in Animals

  • Many animals show complex responses to seasonal changes in their environment.

  • One such behavior is migration, when a group of animals moves from one location to another.

Examples of migration

Examples of Migration

Complex behaviors in animals1

Complex Behaviors in Animals

  • Not all migrations cover vast distances:

  • Ex: Robin’s will nest in low trees during the summer & then migrate to a deep forest only a few miles away for winter.

Complex behaviors in animals2

Complex Behaviors in Animals

  • Dormancy is another behavior that is triggered by environmental changes.

  • When an animal goes dormant, it enters a deep sleep-like state.

  • In places where the winter brings snow, some animals hibernate.

Complex behaviors in animals3

Complex Behaviors in Animals

  • Before animals hibernate, they eat large amounts of food to store up enough energy to last through the winter.

  • Ex: Ground squirrels will more than double their weight in the month before they begin hibernation.

Complex behaviors in animals4

Complex Behaviors in Animals

  • Some regions have intense dry seasons, where food and water become very scarce. Animals in these regions will go dormant until water returns; a behavior known as estivation.

When the small lake he is now living in dries up, the lungfish estivatesuntil the rains return.

Complex behaviors in animals5

Complex Behaviors in Animals

  • During the dry season, the lungfish is able to survive by curling itself into a tight ball with its tail covering the eyes. Mud adheres to the body mucus, forming an impervious casing. The lungfish then becomes dormant, or estivates, until the rainy season again fills the pool, softens the mud casing, and releases the fish.

Complex behaviors in animals6

Complex Behaviors in Animals

  • Another complex behavior that is carried out by some animals is the mating ritual.

  • A mating ritual is a set of actions designed to attract a member of the opposite sex.

A tom (male turkey) has bright coloration and struts pontifically in an attempt to impress a mate.

Complex behaviors in animals7

Complex Behaviors in Animals

  • Typically, the male of the species tries to impress the female & prove that he is a suitable mate. If the female is impressed, she may allow the male to mate with her.

A male lobster attempts to impress a mate with his large claws and mating dance.

Examples of mating rituals

Examples of mating rituals:

Complex behaviors in animals8

Complex Behaviors in Animals

  • Territoriality is one of the behaviors that seem to be exacerbated by mating season.

  • Territoriality is the urge to hold and defend an area from other individuals (usually of the same species).

Male frogs will defend a territory and then sing to attract females into the territory for mating.

Example of territoriality sheep

Example of territoriality: Sheep

  • Sheep are very social animals for most of the year, rarely fighting with each other. However, during mating season the male sheep stake out a territory and defend it from all other males that enter it.

Dall sheep rams fighting over territory during mating season.

Example of territoriality sheep1

Example of territoriality; Sheep

  • Schooling is a complex protective behavior exhibited by many different types of fish.

  • Small fish find “safety in numbers” by performing this behavior.

Silver fish schooling off the coast of Ecuador.

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