TOEFL リーディング 模擬試験 5

In the arid and semi-arid savannas of East Africa, survival requires plants to develop sophisticated mechanisms to withstand both climatic extremes and intense herbivore pressure. Among the most remarkable of these adaptations is the mutualistic relationship between certain plant species and ants. These plants, known as myrmecophytes (myrmecophytes: plants that live in a mutualistic association with ants), have evolved specialized structures to host ant colonies. The whistling thorn acacia (*Acacia drepanolobium*) is a premier example of this phenomenon. Dominated by dry, nutrient-poor soils and subjected to grazing by some of the world's largest mammals, this tree relies on a small but formidable ally to ensure its survival: the acacia ant. The mutualism begins with the physical architecture of the whistling thorn acacia. The tree produces large, hollowed-out structures called domatia (domatia: specialized chambers produced by plants to house insects), which are actually modified thorns with swollen bases. These bulbous structures serve as secure nesting sites for the ants, shielding them from the intense tropical heat and potential predators. A single tree can host tens of thousands of ants, distributed across hundreds of these protective chambers. Research led by the biologist Dr. Amara has demonstrated that the development of these swollen thorns is not merely a passive reaction to ant nesting activity. Instead, it is a genetically preprogrammed trait. The acacia allocates substantial metabolic energy to grow these structures even in the absence of ants, indicating a deep-seated evolutionary commitment to the partnership. In addition to shelter, the whistling thorn acacia provides its resident ants with sustained nourishment. The tree features extrafloral nectaries (extrafloral nectaries: nectar-producing glands physically located outside of flowers), which secrete a sugar-rich fluid directly from the stems and leaf bases. Furthermore, the tips of the acacia's leaves produce specialized, nutrient-dense structures called Beltian bodies, which are rich in lipids and proteins. By offering these high-energy food sources, the tree ensures that the ants do not need to leave the branches to forage on the forest floor, where they would be exposed to competition and predation. This continuous supply of food and shelter represents a significant energetic investment for the tree, which must divert resources away from its own growth and reproduction to sustain its tiny defenders. The return on this heavy investment is a highly effective, active defense system. When an herbivore attempts to feed on the acacia's leaves, the disturbance alerts the ants, which emerge from their domatia in massive numbers. They swarm the intruder, biting and spraying formic acid onto sensitive areas such as the eyes, nostrils, and mouths of large herbivores. Even massive animals like giraffes and elephants, whose thick skin is impervious to standard thorns, are deterred by the painful onslaught of thousands of biting ants and will quickly abandon the tree. Beyond deterring herbivores, the ants also act as gardeners. They actively prune encroaching vines and competing vegetation that grow too close to their host tree, preventing these competitors from shading the acacia and robbing it of vital sunlight. [1] The dynamics of this relationship, however, are not static and can shift rapidly depending on environmental pressures. [2] For instance, when large herbivores are excluded from savanna test plots using protective fencing, the whistling thorn acacia reduces its production of swollen thorns and nectar. [3] This reduction in rewards prompts the ants to seek alternative food sources, sometimes leading them to tend to sap-sucking insects that harm the tree. [4] Thus, the stability of this mutualism relies heavily on the ongoing threat of herbivory to keep both partners cooperative.

Karst topography refers to a distinctive landscape shaped by the gradual dissolution of soluble bedrock, most notably limestone, dolomite, and gypsum. This geological process begins when falling rain absorbs carbon dioxide from both the atmosphere and decaying organic matter in the soil, forming a weak solution of carbonic acid. As this acidic water percolates downward through the soil, it encounters subterranean limestone layers. The acid slowly reacts with calcite (calcite: a crystalline form of calcium carbonate that is the chief constituent of limestone), dissolving the mineral and carrying it away in solution. Over thousands of years, this persistent chemical weathering widens tiny fractures and bedding planes within the bedrock, creating an intricate network of underground pathways and fundamentally altering the surface geography. One of the most characteristic surface features of karst terrain is the sinkhole, also known as a doline. Sinkholes typically form in one of two ways: either through the gradual downward dissolution of bedrock from the surface, creating a shallow bowl-shaped depression, or through the sudden collapse of an underground cavern roof that has been hollowed out from below. As these depressions enlarge, they capture surface runoff and direct it into the subterranean network. Consequently, karst regions are frequently characterized by an absence of surface rivers and streams. Instead, surface water quickly channels into disappearing streams—waterways that flow normally across non-soluble rocks but abruptly vanish underground when they encounter highly fractured limestone, continuing their journey through hidden conduits. As acidic water continues its downward journey, it eventually reaches the water table, below which the rock is completely saturated. Within this saturated zone, horizontal water movement dissolves large volumes of rock, forming expansive cave systems. When the water table later drops—due to regional uplift or changing climatic conditions—these caves drain, leaving air-filled chambers. It is within these dry chambers that speleothems (speleothems: mineral deposits formed in caves by the accumulation of dripping water) begin to develop. As groundwater rich in dissolved calcium carbonate drips from a cave ceiling, it encounters an environment with lower carbon dioxide pressure than the soil above. This pressure difference causes carbon dioxide gas to escape from the water droplet, which reduces the water's acidity and forces calcium carbonate to precipitate. Over centuries, these microscopic mineral deposits accumulate to form icicle-like stalactites hanging from the ceiling and mound-like stalagmites rising from the floor. The evolution of a karst landscape proceeds through predictable stages of geological development. In the youthful stage, karst landscapes are dominated by surface sinkholes, with only a few minor underground cavities. As the landscape matures, these sinkholes merge and the underground drainage network becomes fully integrated, leading to the complete disappearance of surface streams and the creation of vast cavern networks. In the final, late-stage phase of karst evolution, almost all of the soluble limestone has been dissolved and carried away. The landscape is transformed into what geologists call tower karst, characterized by steep-sided, isolated limestone hills that rise dramatically from flat alluvial (alluvial: clay, silt, sand, or gravel deposited by flowing water) plains. These residual hills represent the last remnants of a once-continuous limestone plateau, demonstrating the ultimate destructive power of chemical weathering. Because karst aquifers consist of highly open, interconnected conduit networks, they are exceptionally vulnerable to environmental pollution. [1] Unlike sand or clay aquifers, which naturally filter out impurities as water percolates through tiny pore spaces, limestone conduits act like underground pipes that allow water to flow rapidly without filtration. [2] Consequently, a contaminant introduced at the surface can travel miles in a matter of hours, threatening remote drinking water sources. [3] For this reason, tracing the pathways of underground water using non-toxic dyes has become an essential tool in karst hydrology. [4] These dynamic tracing studies allow scientists to monitor flow rates and establish protection zones around vital water supplies.