Ecological Principles - SAT Subject Test in Biology

Abiotic factors are non-living and molds are living.

Ecosystems are composed of communities and communities are composed of populations. A biome is a larger unit than an ecosystem, as it includes multiple ecosystems.

The characteristics listed describe a savanna. Deserts can also have pronounced periods of rain and dryness but are not dominated by grasses and not populated by large migratory animals. Tropical rainforests and deciduous forests are dominated by trees and tundras tend to be colder and may not receive much rainfall, but often have moist soil due to low evaporation rates.

The taiga biome is the largest biome in the world and is characterized by short, rainy summers coupled with long, freezing winters. Due to its extremely cold temperatures, the taiga's flora or plant life consists of mostly coniferous trees that can survive on the scarce nutrients available in the cold climate of the biome. In contrast, the tundra is frozen year-round and permafrost prevents any trees from growing. Deciduous forests, as the name suggests, sports mainly deciduous trees as its plant life. The alpine biome consists mostly of the areas right below the snow lines of mountains and is generally dry year-round. The chaparral is a relatively small biome that is characterized as being mostly hot and dry year-round, with mild winters.

R-strategists are known for producing large numbers of offspring, though few live to maturity. This is a result of low parental investment, competition, etc. Despite such a large number of offspring, the gestation period for these species is generally shorter than that of K-strategists. For instance, the gestation period of mice is much shorter than the nine months seen in humans.

External process also affect the ecosystem. For example, climate is a very important external factor that will affect temperature and precipitation within the ecosystem.

"k" and "r" refer to two different reproductive strategies. K-strategists tend to occupy a more stable environment and have a larger body size as adults and at birth, fewer offspring, longer life spans, more efficient use of energy, later age of reproduction, slower maturation, more parental care, and longer gestation times. The question asked which characteristic is not generally true of k-strategists, and the correct answer is "shorter life spans," which describes r-strategists instead.

Density-dependent factors are factors that affect population growth only when the population reaches a certain level, and are strongest in a large, dense population and weakest in small, scattered populations. Disease, parasitism, competition, and predation all vary with population density. Natural disasters are the one choice that are density-independent, meaning they affect all populations similarly regardless of size/density.

Abiotic factors are the non-living components of an ecosystem. Biotic factors are living things, such as vegetation (the correct answer because it is the only living thing among the answer choices and therefore NOT abiotic).

This is an example of ecological succession. While climax community may seem like a reasonable answer, the text describes the entire process from barren rock onwards and does not specifically state that new species are no longer appearing on the island and that the community is stable. Evolution occurs within a species, and while the species on the island may change, the species themselves are not described as changing. This question mentions nothing about genetics, thus we cannot determine whether or not a population bottleneck (a reduction in population that is sudden and reduces genetic diversity) is occurring and we have no evidence that one is.