ACT Science Practice Test 18

Shebay Park has been the site of ongoing population dynamics studies since the 1960s. Consisting of a group of isolated islands, the park provides ecologists with a unique, closed ecosystem in which to analyze the relationship between predator and prey populations. Figure 4.5 illustrates the food web for the Shebay Park ecosystem.

Figure 4.5

Ecological research in the park has focused mainly on the predator-prey relationship between the jaguar and peccary (a type of pig) populations. In addition to the typical selective pressures each species exerts on the other, scientists have observed specific events over the years that have affected population sizes. The inadvertent introduction of feline leukemia by humans in the late 1980s severely reduced the jaguar population. In 2004, the severest winter on record and an outbreak of ticks did the same to the peccary population. Figure 4.6 compares the annual population sizes for both species observed between 1968 and 2012.

Figure 4.6

An invasive species is a species that is not native to an ecosystem and whose introduction has harmful environmental, economic, and/or human health effects.

Eichhornia crassipes (water hyacinth) is an invasive species of floating aquatic weed found in freshwater waterways in tropical and temperate regions worldwide. It is highly tolerant of fluctuations in water level, nutrient availability, pH, and temperature. This allows it to grow rapidly and outcompete native aquatic plant species for resources. Dense floating mats of E. crassipes further alter aquatic communities by reducing dissolved oxygen levels and access to light. Decomposing matter from E. crassipes mats increases sediment deposition in waterways.

Ecological studies have shown that the growth of a plant can be influenced by competition with different species of neighboring plants. A group of scientists carried out the following studies to determine the effects on the growth of E. crassipes when paired with three other, more benign, aquatic weed species.

Study 1

Scientists collected growth data on E. crassipes mats in the Kagera River in Tanzania. Scientists marked off 1 square meter (m²) sample areas containing E. crassipes alone and in combination with three other aquatic weeds common to the Kagera River.

To determine the effects of the other three weed species on E. crassipes growth, scientists analyzed five growth parameters. Fresh weight was determined by removing and immediately weighing 10 E. crassipes plants from each area. Plant height was measured from the base of the plant to the tip of the tallest leaf. The total number of E. crassipes plants within a sample area was recorded as plant density, which was then multiplied by fresh weight to determine total biomass. The number of leaves per plant was also recorded. Table 5.1 lists the averages for each growth parameter for E. crassipes growing alone and in combination with the three other aquatic weed species.

TABLE 5.1 Kagera River Data

Source: http://www.academicjournals.org/ijbc/fulltext/2011/August/Katagira%20et%20al.htm.

Study 2

Scientists transplanted young E. crassipes, Commelina sp., Justicia sp., and V. cupsidata plants from the Kagera River to a greenhouse. In the greenhouse, E. crassipes potted alone and in combination with the other three weed species were allowed to grow in water from the Kagera River for four months. At the end of the four-month growth period, the parameters of fresh weight, plant height, and leaves per plant were all determined by the same methods used in Study 1.

Table 5.2 lists the averages for each growth parameter for E. crassipes growing in the greenhouse alone and in combination with the other aquatic weed species.

TABLE 5.2 Greenhouse Experiment Data

Source: http://www.academicjournals.org/ijbc/fulltext/2011/August/Katagira%20et%20al.htm.