Bio 100 Ch 31.1 & 31.2 notes

Match the type of interaction with the correct definition

parasitism

mutualism

commensalism

one species benefits, other is not harmed

one organism obtains nutrients from another, harming the other

both species benefit

Parasitism – One organism obtains nutrients from another, harming the other.

Mutualism – Both species benefit.

Commensalism – One species benefits, the other is not harmed.

This is the species upon which most of the community depends.

keystone

native

exotic

The correct answer is: keystone.

A keystone species is one upon which most of the community depends. If it's removed, the entire ecosystem can change dramatically.

• Native species are those that naturally occur in a region.

• Exotic (or invasive) species are those introduced from other regions and can disrupt ecosystems.

This is the level of nourishment within a food web

pyramid level

trophic level

biomass level

The correct answer is: trophic level.

A trophic level refers to a step in a food web or food chain, representing the level of nourishment or energy transfer—like producers, primary consumers, secondary consumers, etc.

• Pyramid level isn't a standard scientific term (though food pyramids illustrate trophic levels).

• Biomass level refers to the total mass of organisms at a certain trophic level, not the level of nourishment itself.

Nitrogen gas is made available to plants through the actions of

humans

animals in the forest

bacteria

The correct answer is: bacteria.

Bacteria, especially nitrogen-fixing bacteria, convert nitrogen gas (N₂) from the atmosphere into forms like ammonia or nitrates that plants can absorb and use.

• Humans can affect the nitrogen cycle (e.g., through fertilizers), but they don’t make nitrogen directly available to plants in natural ecosystems.

• Animals contribute by returning nitrogen to the soil through waste or decomposition, but they don't convert atmospheric nitrogen.

Carbon dioxide gas is made available to plants through the process of

photosynthesis

combustion

diffusion

• Carbon dioxide (CO₂) is in the air.

• Plants take in CO₂ through tiny pores on their leaves called stomata.

• Once inside the plant, the CO₂ is used during photosynthesis — the process where the plant makes its own food (glucose) using:

  • Sunlight (energy),

  • Water (from the roots),

  • and Carbon dioxide (from the air).

So even though diffusion is how the CO₂ gets into the leaf, it’s photosynthesis that actually makes it available to the plant as part of its energy-making system.

Commensalism differs from mutualism in that*

both benefit in commensalism

one benefits in commensalism and other is unharmed

one benefits in commensalism and other is harmed

both benefit in commensalism

Producers form the __________________ level of an ecological pyramid and have the _____________ biomass.*

top, smallest

bottom, largest

top, largest

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Clear form

The correct answer is: bottom, largest

Producers (like plants and algae) form the bottom level of an ecological pyramid and have the largest biomass, because they support all the other levels above them by producing energy through photosynthesis.

Another name for an autotroph would be *

consumer

detrivore

producer

The correct answer is: producer ✅

An autotroph is an organism that makes its own food, usually through photosynthesis (like plants), so it's also called a producer.

Nitrogen fixation is the*

is the conversion of nitrogen in the air to ammonium

is the conversion of ammonium to nitrates

is the conversion of nitrates to ammonium

The correct answer is: the conversion of nitrogen in the air to ammonium ✅

Nitrogen fixation is the process where certain bacteria convert nitrogen gas (N₂) from the atmosphere into ammonium (NH₄⁺), which plants can then use.

This is the limiting inorganic nutrient in ecosystems is*

nitrogen

carbon

phosphorous

The correct answer is: phosphorus ✅

Phosphorus is often the limiting inorganic nutrient in many ecosystems, especially in freshwater environments. Unlike nitrogen and carbon, phosphorus does not have a gas phase and cycles very slowly, making it harder for ecosystems to access.