Correct answer: (4) equilibrium

Equilibrium is achieved by an ecosystem if the competing factors in the environment are balanced, thus maintaining the number and types of species over the years. Hence, there will be minimal evolution, which usually brings about formation of new organisms.

Incorrect answers:

(1) feedback

Feedback refers to situations that happened due to a similar event that happened in the past.

(2) global instability

When there is global instability, the population of species dramatically change over time.

(3) environmental change

Environmental change, such as climate change, provides a risk to the survival of the population of species, causing their numbers to change over time.

Correct answer: (2) become extinct over time

Extinction refers to the complete loss of a species in the biosphere. Extinction can be caused by predation, habitat destruction, demographic phenomena, disease, coextinction, and drastic environmental changes such as climate change. When species fail to adapt with these factors, their population starts to decrease until such time when the last species dies and, thus, become extinct.

Incorrect answers:

(1) develop many mutated cells

Mutation is the rare change in the DNA of genes that ultimately creates genetic diversity. When there is lack of variation in a species, then mutated cells would most likely not develop.

(3) begin to reproduce sexually

Organisms that reproduce asexually cannot change their mode of reproduction automatically unless they undergo speciation, or the formation of new species as form of adapting to the changes in the environment.

(4) develop resistance to diseases

Correct answer: (4) no competition between species

An ecosystem can be considered stable when the resources in the surroundings are enough to maintain diversity and populations. This is usually achieved before the ecosystem reaches its carrying capacity, or the maximum population size that can be supported by the available resources. There is no competition among species when there is ample amount of resources in the ecosystem to make populations maintain their numbers.

Incorrect answers:

(1) predators that outnumber their prey

When predators outnumber the prey, there would be substantial competition on the prey in order for the predators to survive. This will cause the decrease in the number of predators. Therefore, the ecosystem is not stable or in equilibrium.

(2) a continual input of energy

There is a continual input of energy when autotrophic organisms continuously produce energy through sunlight. However, the continuous input of energy cannot be equated with the continuous flow of energy (Did you mean "the continuous inflow of energy cannot be equated with the continuous outflow of energy"?). A stable ecosystem is characterized by the continuous flow of energy from one trophic level to another.

(3) limited autotrophic nutrition

Correct answer: (1) stable ecosystem that can last for many years

The community illustrated went through an ecological succession, or transition of species compositions over time. The community passed through a sequence of predictable transitional stages until it reached a relatively stable state called a climax community. During this stage, there will be minimal changes in the composition of species in the community because the resources are abundant and the environment is suitable for the survival of most species.

Incorrect answers:

(2) lost heterotrophs that cannot be recovered

Heterotrophs are organisms that obtain organic food molecules by eating other organisms or their by-products. When a climax community is been reached, all the trophic levels are filled up to maintain the stability of the community. If heterotrophic organisms are absent, climax community would not be achieved.

(3) long-term rise in environmental temperatures

Temperature is an abiotic factor that depends on the amount of sunlight that can enter on the earth's atmosphere. A climax community is usually characterized by the predominance of trees that serve as shade for shade-loving plants and other forms of wildlife that cannot tolerate the extreme heat of the sun. Thus, when climax community is reached, the temperature under the canopy of the trees is decreased.

(4) forest consisting of only producers and decomposers

Correct answer: (1) bacteria and fungi

Ecological niche is the sum total of an organism's utilization of the biotic and abiotic resources of its environment. Since both bacteria and fungi belong to the same trophic level (decomposers), they most likely compete with each other in habitat and on resources in order to survive.

Incorrect answers:

(2) deer and wolf

Deer and wolf have different niche basically because they have different modes of nutrition. Deer are herbivorous, meaning they eat plants to get energy; while wolf are omnivorous, eating meat of other organisms to derive energy.

(3) tree and fungi

Tree and fungi also belong to different niches because they have different ways of using up the resources around them. For instance, trees use the sunlight to convert carbon dioxide and water to glucose and oxygen. On the other hand, fungi, which are decomposers, absorb nutrients from  nonliving organic material and convert them into inorganic forms.

(4) deer and bacteria

Correct answer: (1) ecological succession

A previously disturbed area may be colonized by a variety of species, which are gradually replaced by others. Such change in species composition over ecological time represents the process known as ecological succession. In the situation given, the railroad, which was previously a bustling place, became abandoned. This caused the community to change. It transformed from a community of weeds into a community dominated by aspen trees.

Incorrect answers:

(2) biological evolution

Biological evolution refers to the natural genetic change in a species over time. The weed did not genetically change to become an aspen tree, thus the situation given is not an example of evolution.

(3) genetic variation

Genetic variation pertains to the difference in genetic makeup of a certain group of species.

(4) heterotrophic nutrition

Correct answer: (1) A stable ecosystem can be altered, and then it can recover to a point of stability.

The graph illustrates an ecosystem that has been disturbed. This has caused a decline in the population of species. After some time, another community starts to inhabit the ecosystem and starts to grow in numbers, until it reaches the climax community, which is the most stable stage of the ecosystem.

Incorrect answers:

(2) An ecosystem remains unchanged.

If the ecosystem remains unchanged, then the stability curve must be drawn as a strait horizontal line.

(3) The stability of an ecosystem remains unchanged but its biodiversity decreases.

Ecosystem stability depends on the biodiversity or number of species found in the ecosystem. Thus, once biodiversity decreases, ecosystem stability is also changed.

(4) A stable ecosystem cannot recover after it is altered.

An ecosystem has a natural way of recovering from any disturbance to continue life. This phenomenon is called ecological succession. Natural changes and species replacements occur in an ecosystem over time.

Correct answer: (1) Organism X is positively affected by the relationship and organism Y is negatively affected.

At interaction 1, the predator (organism X) benefits by killing the prey (organism Y) for food. At interaction 2, the parasite (organism X) benefits from the host (organism Y) by living on it or by getting nutrition from it. In both interactions, the predator and the parasite (both represented as organism X) benefit in their relationship with prey and host (organism Y), respectively.

Incorrect answers:

(2) Organism X is negatively affected by the relationship and organism Y is positively affected.

Prey and host (organism Y) both do not benefit in their relationship with predator and parasite (organism X), respectively. Moreover, the interactions could be detrimental to organism Y.

(3) Both organisms are positively affected by the relationship.

Mutualism is the type of interaction where both species benefit from the relationship. In, predation and parasitism, the relationship is beneficial to one and detrimental to the other.

(4) Both organisms are negatively affected by the relationship.

Correct answer: (1) Algae > Protozoa > Bacteria

An energy pyramid represents the flow of energy in an ecosystem from one trophic level to another.  In this pond food web, the algae serve as producers, thus they are at the bottom of the pyramid. The chemical energy they have produced is transferred to the protozoa upon consumption of the algae. The protozoa now serve as the consumers, together with all the other organisms that feed on other living organisms for survival. Bacteria, which is at the topmost level of the energy pyramid, feed on the dead organism, thus serving as decomposers.

Incorrect answers:

(2) Algae > Snails > Diving beetles

Snails and diving beetles both serve as consumers in this ecosystem because they both feed on living organisms. The topmost level of the pyramid is reserved for decomposers, which consume dead organisms.

(3) Bacteria > Dead organisms > Crayfish

The first level of the energy pyramid is for producers or organisms that can produce their own food through photosynthesis. Bacteria are not a producer because it does not perform photosynthesis. Dead organisms are not part of the energy pyramid because they are not capable of energy transfer. Crayfish must be at the second level of the energy pyramid because they are considered as consumers.

(4) Crayfish > Catfish > Amphipods

Correct answer: (4) Population sizes of species at feeding levels after amphipods will decrease and before amphipods will increase.

When amphipods are removed in this food web, copepods will increase in number because their predators are already gone. On the other hand, the catfish and their succeeding consumers will decrease in number because they have no source of energy.

Incorrect answers:

(1) Population sizes of species at feeding levels both before and after amphipods will decrease.

Copepods will increase in number because there would be no organisms to feed on them. Only the number of catfish and their consumers will decrease because the catfish have nothing to eat for survival.

(2) Population sizes of species at feeding levels both before and after amphipods will increase.

Only the copepods will increase in number because of the elimination of their predator. Catfish and the other consumers will reduce in number because the amphipods, which is their energy source, is gone.

(3) Population sizes of species at feeding levels after amphipods will increase and before amphipods will decrease.