Question 1

Sea otters in marine habitats along the west coast of North America were hunted to near extinction.This caused sea urchins to grow exponentially.They overgrazed and nearly destroyed kelp beds, which provided habitat and food for many fish and other species.After kelp was destroyed, many fish species also declined.This suggests that sea otters act as a _______ species and their loss set off a(n) _______.

Accepted Answer

Sea otters act as a keystone species, meaning their presence has a significant impact on the entire ecosystem. Their loss set off a trophic cascade, causing a chain reaction of ecological effects throughout the food web.

Question 2

Which factor is characteristic of disturbances within a community?

Accepted Answer

Disturbances within a community can kill some species but provide space and resources for others. This is known as an intermediate disturbance hypothesis, which suggests that moderate levels of disturbance can increase community diversity by creating opportunities for colonization and reducing competition among species. Disturbances can also vary in magnitude and be caused by abiotic or biotic factors, and may not always be predictable or small-scale in their geographic impact.

Question 3

Low levels of chemical pesticides occur in water supplies such as ponds and lakes, which provide habitats for many fish species.This results in death of some fish fry (first-year fish), loss of some insects and invertebrates that serve as fish food, and depleted oxygen concentrations due to decomposition of algae killed by herbicides.These effects suggest that the presence of low-level pesticide residues in water supplies acts as a _______ to fish populations.

Accepted Answer

Low-level pesticide residues in water supplies act as a stress to fish populations, causing negative effects such as death of fish fry, loss of food sources, and decreased oxygen levels.

Question 4

The major process that occurred on the Pumice Plain on Mount St.Helens in the years after its eruption was

Accepted Answer

The Pumice Plain on Mount St. Helens experienced primary succession after its eruption. This is because it was a new, barren area with no previous plant or animal life, and over time, pioneer species gradually colonized the area and created a new ecosystem.

Question 5

Two species of gobies (small fish) live on a coral reef.The two species use similar habitats and compete equally for vacant corals.The "winner" is the goby that arrives first.Juveniles and adults of the two species have the same relative abundance.When one species is removed experimentally, either species can colonize the vacant coral.These factors indicate that the presence of the two species on the reef can best be described by a model called the

Accepted Answer

The fact that both species use similar habitats, compete equally for vacant corals, can colonize vacant corals when the other species is removed, and have the same relative abundance indicates that the presence of the two species on the reef can be best described by a lottery, or neutral, model. In this model, the species coexist by random chance rather than through direct competition or niche differentiation.

Question 6

In Yellowstone National Park, elk preferred to browse on specific plants, such as willow and aspen trees.The rapid increase in elk after wolves were removed resulted in what change, if any, to the community structure of the area?

Accepted Answer

The rapid increase in elk population led to excessive browsing on willow and aspen trees, which eventually caused a decline in their populations. With the depletion of these trees, the forest ecosystem transitioned to a meadow habitat. This is an example of how changes in the population of one species can have ripple effects on the entire ecosystem.

Question 7

Refer to the diagram, showing changes occurring during succession as the glacier in Glacier Bay, Alaska, melted over the past 200 years.   The moraines, or gravel deposits exposed by the retreating glacier, increase in age of exposure from left to right.Which statement best describes how the vegetation changes during succession?

Accepted Answer

The diagram shows an increase in vegetation abundance, diversity, and longevity over time as the glacier retreats and exposes new land. This is a typical pattern of succession, where new and more complex ecosystems develop over time.

Question 8

The greatest noticeable change in the community structure of a natural community would most likely occur as a result of

Accepted Answer

A large-scale and rapid disturbance, such as a forest fire, can quickly alter the physical and biotic components of a natural community, leading to significant changes in the community structure. This can include changes in the relative abundance of different species, alterations to the physical environment, and disruptions to the inter-species interactions that make up the community. While other factors may also exert influence on community structure, few are as disruptive or immediate in their effects as large-scale disturbances like forest fires.

Question 9

Tropical rainforests have hundreds of tree species.It has been hypothesized that the model that best describes the causes of species richness in these tree communities is the neutral, or lottery, model.This is because rainforest trees likely

Accepted Answer

The neutral or lottery model suggests that individual trees in a tropical rainforest use available resources equally and at random, which contributes to the high species richness by minimizing competitive exclusion among species.

Question 10

In the traditional definition of succession (which may not always be achieved), a climax community is characterized by

Accepted Answer

A climax community is characterized by stability, meaning that the community has reached a state of dynamic equilibrium where populations are relatively stable and in balance. This stability is often achieved after a series of successional stages.

Answer: B
 A climax community may have low diversity, as the dominant species in the community often become established through competition and tend to exclude other species. However, this is not always the case, and some climax communities can have high diversity.

Answer: C
 A climax community is not necessarily immune to disturbances, but it is typically better able to withstand and recover from disturbances compared to earlier successional stages.

Answer: D
 The presence of mostly annual plants is not a characteristic of a climax community, as these plants typically have a short life span and are not well adapted to long-term survival and stability.

Answer: E
 A climax community may develop after either primary or secondary succession, depending on the specific circumstances and ecological context.

Question 11

Animals were instrumental in helping recolonize during primary succession after the eruption of Mount St.Helens.Based on scientists' observations, those animals most able to help in the early recolonization were those that could

Accepted Answer

The answer of Animals were instrumental in helping recolonize during...

Question 12

Many biotic factors have the potential to affect community structure.Such biotic factors include all of the following except

Accepted Answer

Biotic factors are living organisms or their products that affect another organism or ecosystem. A hurricane, which is an abiotic (non-living) factor, does not qualify as a biotic factor.

Question 13

Which statement best explains the difference between primary and secondary succession?

Accepted Answer

Primary succession occurs when there is no existing soil or living organisms present due to a disturbance, such as a volcanic eruption or a glacier receding, while secondary succession occurs when some organisms are still present after a disturbance, such as a forest fire or a flood.

Question 14

In which case would secondary, rather than primary, succession occur?

Accepted Answer

Secondary succession occurs in areas where a disturbance has happened but soil and some life forms still exist, such as after a storm that blows down trees. Primary succession, on the other hand, occurs in lifeless areas where soil has not yet formed, such as beside a newly erupted volcano, a new sand dune, a receding glacier, or a newly formed island.

Question 15

In which situation would primary succession occur?

Accepted Answer

Primary succession occurs in a previously uninhabited and barren environment, such as a newly exposed rock surface after a glacier retreats.

Question 16

A saguaro cactus in the Sonoran Desert takes 80 years to become mature and begin flowering.During that time, woodpeckers drill cavities into the cactus and provide nesting sites for up to 30 bird species.After it begins to flower, the cactus also provides nectar for bats, bees, and doves, and fruit for various other birds and insects.Based on these interactions, the saguaro cactus could best be described as a

Accepted Answer

The saguaro cactus acts as a foundation species by creating and defining the ecosystem structure, providing essential resources like nesting sites and food (nectar and fruit) for a variety of species. It also acts as an ecosystem engineer because it modifies the environment in a way that creates new habitats for other species, such as the cavities created by woodpeckers that are used by other birds.

Question 17

Following disturbance to a community, succession—characterized by a change in community structure over time—can eventually result in a relatively stable stage called a

Accepted Answer

A climax community is a relatively stable and mature community that undergoes little changes over time. It is the result of succession, whereby the community structure changes over time in response to disturbances. The climax community represents the end point of succession, where the community structure is well-established and relatively stable.

Question 18

A volcanic eruption occurs.Which resulting condition would likely be considered a stress, rather than a disturbance?

Accepted Answer

The answer of A volcanic eruption occurs.Which resulting condition would...

Question 19

A tornado rips through an area of the Midwest.Its winds destroy many trees, fields, and crops.Heavy rains flood lowland areas.This situation would be described ecologically as a

Accepted Answer

This situation would be considered a disturbance ecologically as it is a strong, short-term disruption to the ecosystem caused by an external force. The tornado, winds, and heavy rains are all examples of disturbances.

Question 20

Lottery (neutral) models of species diversity in a community are most likely to operate when

Accepted Answer

Lottery models of species diversity operate when all species have an approximately equal chance of obtaining resources, which results in a "random" selection of species within the community. The other options (A, C, D, and E) do not fit the conditions for lottery models of species diversity.

Quiz 56: Communities