Niches and community interactions Chapter 4, section 2 Honors bio

Think about it If you ask someone where an organism lives, that person might give you an answer such as “on a coral reef” or “in the rainforest.” If you ask someone where a person lives, they may say, “in California” or “in Illinois.” They will typically answer with an environment or a location, but ecologists need more information than that to understand why an organism lives where it does. What else do they need to know?

tolerance Organisms occupy different places because each species has a range of conditions under which it can grow and reproduce. This is called tolerance: the ability to survive and reproduce under a range of environmental circumstances. Organisms have an upper and lower limit of tolerance for every environmental factor. If temperature, for example, extends in either direction beyond an organism’s optimum range, the organism will experience stress. Why?

Tolerance The organism will experience stress because it will have to expend more energy to maintain homeostasis, so there isn’t as much energy left for growth and reproduction. The tolerance a species has for environmental conditions will help to determine it’s “address” or habitat—the general place where an organism lives.

Defining the niche Ecologists don’t just look at where a species lives—they also study a species’ ecological “occupation,” where and how it “makes a living.” A niche describes not only what an organism does, but also how it interacts with both biotic and abiotic factors of the environment. The textbook definition of a niche is as follows: The range of physical and biological conditions in which a species lives and the way the species obtains what it needs to survive and reproduce.

Resources and the niche The term resource can be applied to any necessary part of life: water, nutrients, light, food, space, etc. What are some resources plants need? What about animals?

Physical aspects of the niche Part of an organism’s niche involves the abiotic factors it requires for survival. If you look at an amphibian, for example, most of them absorb and lose water through their skin. Due to that, they must live in moist places.

Biological aspects of the niche This is going to include the biotic factors the organism needs to survive. Examples: When/how it reproduces, what it eats, how it obtains food, etc. Birds on Christmas Island (a small island in the Indian Ocean) all live in the same habitat, but they eat fish of different sizes. They also feed in different places. What does this sound familiar to?

competition When organisms attempt to use the same limited ecological resource in the same place and at the same time, competition occurs. Plants can compete for water and nutrients in the soil. Animals compete for things such as food, mates, places to live, etc. Intraspecific competition: occurs among members of the same species Interspecific competition: occurs between members of different species

The competitive exclusion principle If there is direct competition between two species, there will almost always be a winner and a loser—the losing species will die out. The competitive exclusion principle states that no two species can occupy exactly the same niche in exactly the same habitat at exactly the same time. If two species attempt to occupy the same niche, one will eventually out-compete the other. Remember Darwin’s Survival of the Fittest…..?

Dividing resources Instead of competing for similar resources, species usually divide them. For example, the three species of North American warblers live in the same trees and feed on the same insects, but one species feeds on high branches, another feeds on low branches, and the third feeds in the middle. Each of these species has its own niche, which was likely brought about by past competition. By causing species to divide resources, competition helps determine the number and kinds of species in a community and the niche each species occupies.

Predation, herbivory, and keystone species Virtually all animals, since they are not producers, must eat other organisms for energy and nutrients. This is why predator-prey and herbivore-plant interactions are important in shaping communities.

Predator-prey relationships An interaction in which one organism (the predator) captures and feeds on another organism (the prey) is called predation. Predators can affect the size of prey and populations in a community and determine the places prey can live and feed. Birds of prey can play an important role in regulating populations of the organisms they feed off of.

Herbivore-plant relationships These are as important as predator-prey interactions. An interaction in which an organism (the herbivore) feeds on producers (plants) is called herbivory. Herbivores can affect both the size and distribution of plant populations in a community and determine the places that certain plants can survive and grow. Many herbivores are eliminating their favorite food plants from many places across the U.S., so it is important to have a balance between the two types organisms.

Keystone species Sometimes changes in the population of a single species, often called a keystone species, can cause dramatic changes in the structure of a community. In the cold waters off the Pacific coast of North America, for example, sea otters devour large amounts of sea urchins. Urchins, in turn, are herbivores. Their favorite food is kelp. A while ago, sea otter populations neared extinction. What would happen to the rest of their ecosystem? Why?

symbioses Symbiosis: any relationship in which two species live closely together. Symbiosis means “living together.” Biologists recognize three main classes of symbiotic relationships in nature: mutualism, parasitism, and commensalism.

mutualism Mutualism: a relationship in which both organisms benefit. For example: A sea anemone’s sting has two functions: to capture prey and to protect the anemone from predators. The clownfish is immune to those stings, so they seek shelter in the anemone when they feel threatened. On the flip side, clownfish chase away other fish if those fish are trying to snack on the anemone.

parasitism Parasitism: the relationship in which one organism lives inside or on another organism and harms it. The parasite gets all of its nutrients from the host. They generally weaken but do not kill their host, which is usually larger than the parasite. For example: tapeworms live in the intestines of mammals, where they absorb large amounts of the host’s food. Fleas, ticks, lice, and leeches are examples.

commensalism Commensalism: a relationship in which one organism benefits and the other is neither helped nor harmed. For example, barnacles (small marine animals) will attach themselves to the side of a whale. They perform no known service for the whale, nor do they harm it. However, the barnacles benefit from the constant movement of water while the whale is swimming, because their food is in the water.