Chapter 57: Species Interactions (Part 2, Week 4) Flashcards
[Start 57.3 Mutalism and Commensalism]
What is mutalism and commensalism?
In mutualism, both species gain from the interaction. For example, in mutualistic pollination systems, the plant benefits by the transfer of pollen, and the pollinator typically gains anectar meal.
In commensalism, one species benefits, and the other remains unaffected. For example, in some forms of seed dispersal, barbed seeds are transported to new germination sites in the fur of mammals. The plant species benefit by dispersing their progeny, but the mammals are generally unaffected.
What are three different types of mutualisms that occur in nature?
Resource-based mutualisms - A mutually beneficial interaction in which both species receive a benefit in the form of resource transfer of energy and nutrients.
Defensive mutalisms - A mutually beneficial interaction often involving an animal defending a plant or herbivore in return for food or shelter.
Dispersive mutalism - A mutually beneficial interaction often involving plants and pollinators that disperse their pollen, and plants and fruit eaters that disperse the plant’s seeds.
What are examples of resource-based and defensive types of mutalisms? Think ants.
Resource based - Ants and fungi - ants dont eat the leaves but chew them up and store them in the den. There, the fungi grows and elminates other fungi and disperses more. The fungi creates structures that the ants can eat.
Defensive - Ants and aphids - aphids feed on plant sap and are defensless. Ants protect them because the aphids excrete a fluid called honeydew that the ants eat.
What are the two types of dispersive mutualism?
Obligatory mutalism - An interaction in which two mutualistic species cannot live without each other.
Facultative mutalism - An interaction between mutualistic species that is beneficial but not essential to the survival and reproduction of either species.
What are the two ways plant species in the area promote the pollinator’s species fidelity (faithfulness to a person, cause, or belief, demonstrated by continuing loyalty and support.)?
Synchronized flowering and sequential flowering
The plant should provide just enough nectar to attract a pollinator’s visit. From the pollinator’s perspective, it would be best to be a generalist and obtain nectar and pollen from as many flowers as possible in a small area, thus minimizing the energy spent on flight between patches.
This suggests that although dispersive mutualisms are beneficial to both species, the species’ optimal needs are quite different.
What is one of the best examples of commenalism?
Phoresy - A form of commensalism in which individuals of one species use individuals of a second species for transportation.
[Start 57.4 Bottom-Up and Top-Down Control]
How can ecologists determine which factors, along with abiotic factors such as temperature and moisture, are the most important in affecting population sizes?
Some ecologists stress the importance of so-called bottom-up factors, such as plant quality and abundance in determining the sizes
of herbivore populations and the populations of predators that feed on them.
Plant quantity or quality limits the density of herbivores, such as grasshoppers, which, inturn, sets limits on the abundance of predators, such as spiders. Other ecologists stress the importance of top-down factors
In this case, the spiders play a major role in determining the size of grasshopper populations, which control the size of the plant population.
You apply fertilizer to a bush, and this increases spider density on the bush. What type of control is occurring?
Fertilizer increases plant quality and hence herbivore density, which, in turn, increases the density of spiders. This is bottom-up control.
Current thinking is that both bottom-up and top-down control are important in affecting population sizes. Their relative importance depends on the environment and the types of species interactions thatare involved. In this section, we will briefly discuss some of the evidence for bottom-up and top-down control.
What are two lines of evidence suggest that bottom-up control is important in limiting population sizes?
Not all of the energy in food resources can be used by the consumer.
For example, when a herbivore eats plant material, some ofthe energy is lost in the form of heat. This phenomenon, based on thethermodynamic properties of energy transfer, suggests that the quantity and quality of plants may be limiting for the population sizes of all other species that rely on them.
Second, much evidence supports the
nitrogen-limitation hypothesis that organisms select food in terms of the nitrogen content of the tissue. Thisis largely due to the different proportions of nitrogen in plants andanimals
Animal tissue generally contains about 10 times as much nitrogen as plant tissue. For this reason, herbivores favor high-nitrogen plants.
What does top-down models suggest?
Top-down models suggest that predators control populations of their prey (ultimately, herbivores) and that these herbivores control plant populations.
The wolves reduced the density of elk, which in turn promoted the growth of two major elk food plants: aspen and willow.
Increased plant availability also increased the abundance of songbirds, especially obligate willow specialists, such as the American redstart. Beaver abundance also increased due to agreater availability of willow.
