Ecology - GRE

The over-browsing by the invasive jackrabbit population has resulted in harm being done to the desert plant communities with no negative impact on the rabbits, resulting in amensalism.

Coevolution is a process by which two or more species evolve in response to one another, and generally describes competing species. While the hummingbird and the flower may evolve due to this relationship, coevolution more freqeucntly describes the evolution of defense mechanisms in response to a predator-prey relationship. Parasitism is the relationship in which one species benefits at the expense of the host. This can sometimes lead to coevolution, depending on how the host reacts. Neither species is being harmed in the given scenario, so this is incorrect. Commensalism is when one species benefits, but the other species is not impacted. In this case, both the hummingbird and the flower are benefiting, so this is not our answer. Resource partitioning is when species evolve together to share resources in a sustainable way. The hummingbird is using a resource that the flower provides, but they are not "sharing" in the sense of the definition of resource partitioning. Competitive exclusion describes the phenomenon when two species share the same niche, and the stronger one forces out the weaker competitor.

Mutualism, the correct answer, describes a relationship wherein species exploit each other mutually. In this case, the hummingbird gains nutrition from the flower, and the flower gains a reproductive advantage from the hummingbird.

Beta diversity measures the rate of change in the species composition of one habitat as compared to others. This is called the species evenness of an area. For instance, if one habitat has radically more species than a neighboring habitat, this would result in a high beta diversity. Alpha diversity is measured as the number of species in a uniform area, and gamma diversity is the species richness in a range of habitats (a mix of alpha and beta).

A secondary consumer is a step above the primary consumer (herbivore) on the food chain, consisting of omnivores and carnivores. A mouse that lives off plant matter and is thus a primary consumer. When a snake eats the mouse, it is the secondary consumer in the food web.

With every advancement in the trophic level, energy converts on a ten-to-one scale. For example, ten kilograms of grain fed to a steer produces roughly one kilogram of beef. This is true for every step up the tropic food pyramid.

Less than half of total solar energy is within the photosynthetically active wavelength range, and plants to not absorb all of this energy due to reflection and refraction. Thus, only about 1-5% of incoming solar radiation is actually converted to plant biomass, which serves as the base for all food chains. This is why herbivorous animals generally have to eat extremely high quantities of plants to achieve adequate nutrition.

These are all sources of lost energy in between levels of the trophic pyramid. In the context of herbivores and carnivores: Not all food at each level can be eaten, because some prey escape their predators or they can't be found. When a predator eats its prey, not all of that tissue is digestible, such as cellulose and lignins. Lastly, everything that the predator digests is not used for new growth, and some is lost through excretion and respiration (heat).

Herbivores are very likely limited by the predators in their own food webs, preventing them from completely overtaking the plants that they feed on. This is called the Earth is Green hypothesis, originally proposed by Hairston, Smith, and Slodobkin.

Detritivores are successful because it is much more efficient to digest dung because it doesn't require much extra digesting, as another animal has already done it. Detritivores can generally have a much less complicated digestive system and save themselves the energetically expensive process of digesting new plants or animals. Detritivores are also important to the ecosystem because they cycle the nutrients in dung back into the food chain. There is no evidence that detritivores experience less competition, nor that detritus is in excess to live organisms.

Eusociality is defined by options I, II, and III. A species must cooperate on brood care (think ants bringing food back to the colony), labor is divided by reproductive ability (think sterile worker bees and queens) and the generations must overlap so that the colony is maintained properly.

K-strategist populations are more commonly regulated by density-dependent limiting factors. Their population sizes hover around a carrying capacity that is dependent on factors that increase in severity with the density of the population. On the other hand, r-strategist populations are regulated by density-independent limiting factors. They reproduce rapidly until a density-independent factor causes many of them to die.

Species described as "r-strategist" have a survival strategy of producing large numbers of offspring, a short life expectancy, and typically smaller body sizes. Examples include mice, locusts, and frogs. These species survive by producing lots of offspring, since many individuals don't survive to adulthood.

Species designated as K-strategists thrive through longevity and have a higher survival rate at birth, but produce far fewer offspring. Examples include elephants and whales. Consider an elephant; it may produce relatively few baby elephants, but these baby elephants each have the potential to live relatively long lives. On the other hand, contrast K-strategists like elephants with r-strategists like rats, locusts, and flies. These species are designated as r-strategists because in contrast, they produce numerous offspring, few of which may survive to adulthood, and each one of which reaches maturity quickly and lives a relatively short life. K-strategists in general spend more time than r-strategists caring for and raising their offspring; they have fewer offspring, so they put more care into ensuring the survival of each one. Contrast elephants with frogs to see this difference; elephants care for their young, while frogs lay their eggs and care for their offspring very little, if at all.

Population growth can follow either a J curve trend or an S curve trend. A J curve represents exponential, or unrestricted growth. An S curve represents logistic, or restricted, growth and accounts for the carrying capacity of an environment.

In a J curve model, the population will continue to grow even when it has reached capacity. In an S curve model, the population will stagnate and stabilize at a maximum value when capacity is reached.

A deme is defined as a group of individuals more genetically similar to each other than to other individuals, even if spatially isolated. A group of conspecific individuals separated demographically, genetically, or spatially from other groups is a population**.** A set of spatially disjunct populations in which some immigration still occurs is a metapopulation**.** The other options are not well-defined in population ecology.

Organisms with little to no parental care typically have large amounts of offspring to compensate for high mortality rates of the young, as shown in a type III survivorship curve. These include organisms like trees, which spread large numbers of seeds because the seeds rarely find a good growing spot and mature to adulthood.

Autotrophs use for their carbon source, while heterotrophs use organic compounds from other organisms. Chemotrophs use chemical bonds as their energy source, while phototrophs use light. Combining these ideas we can come up with various terms to describe an organism's nutrition. The bacterium in this question stated to rely on organic chemicals from other organisms as a carbon and energy source. This makes it a chemoheterotroph.

The phosphorus cycle does not contain an atmospheric phase, while both the sulfur and nitrogen cycles do.

Primary succession is characterized by plants establishing themselves in areas that were initially devoid of soil (usually bedrock). Grasses establishing themselves on a newly-formed island that is mostly bedrock is the best fit for this definition.

Primary succession is characterized by pioneer species establishing themselves in areas that are predominantly bedrock and devoid of organic material. The formation of organic material in these areas typically occurs with the accumulation of wind-carried soil erosion, nutrients deposited by flowing bodies of water, and erosion of bedrock. Wildlife populations are also a significant contributor of soil nutrients, with feces being rich in both nitrogen and phosphorus. In contrast, pre-existing nitrogen-fixing plants (or any pre-existing plant communities) are uncommon to nonexistent in an area with no established soil system.