Latihan TOEFL Reading 3

For millions of years, land plants have coexisted with underground fungal networks in a relationship that has shaped terrestrial ecosystems. This symbiosis, known as mycorrhizae (mycorrhizae: symbiotic associations between fungi and plant roots), is foundational to the survival of over ninety percent of all land plant species. Historically, botanists viewed plants as individualistic competitors, striving against one another for sunlight, water, and soil nutrients. However, the discovery of mycorrhizal networks has revolutionized this perspective, revealing that plants are interconnected in a complex, subterranean web. Through these fungal connections, plants can share resources, exchange chemical signals, and even alter their physiological behaviors in response to neighboring flora. This cooperative framework challenges traditional ecological models that focus exclusively on competition. The structural foundation of these networks lies in the hyphae (hyphae: tiny, thread-like structures that make up the body of a fungus) that spread extensively through the soil. These microscopic filaments are far thinner than the finest plant root hairs, allowing them to penetrate incredibly small soil pores that plant roots cannot access. By associating with these fungi, plants effectively expand their root surface area by several orders of magnitude. In exchange for the carbohydrates and sugars produced by plants through photosynthesis, the fungi provide their hosts with vital, otherwise inaccessible water and minerals, particularly phosphorus and nitrogen. This mutualistic exchange is not merely a localized transaction; because a single fungal mycelium can connect to multiple plants of the same or different species, it creates a shared pool of resources. The implications of this shared pool are particularly evident in forest ecosystems, where mature canopy trees are linked to struggling understory seedlings. Research conducted by ecologist Mei-Ling demonstrated that older, well-lit trees transfer substantial amounts of carbon, in the form of sugars, to younger seedlings growing in the deep shade. Using isotopic labeling techniques, Mei-Ling traced the movement of carbon from the leaves of mature trees, down through their roots, across the fungal hyphae, and into the tissues of shaded seedlings. This carbon subsidy significantly increases the survival rate of seedlings that would otherwise perish from a lack of sunlight. This network-mediated support system suggests that forests function more like a single superorganism rather than a collection of independent individuals, with resource distribution regulated to maintain overall forest health. Beyond resource sharing, mycorrhizal networks serve as a rapid transit system for information. When a plant is attacked by herbivores or pathogens, it generates defensive chemical compounds, but it also transmits warning signals through the fungal network to neighboring plants. Upon receiving these signals, the unattacked neighbors immediately begin producing their own defensive toxins, such as volatile organic compounds that repel pests or attract the pests' natural predators. This preemptive defense mechanism dramatically reduces the vulnerability of the entire plant community. Consequently, a localized insect infestation does not remain a localized threat; instead, the network mobilizes a collective, coordinated defense across a wide area, showcasing a sophisticated level of communal adaptation. Despite the clear benefits of mycorrhizal networks, evolutionary biologists caution against viewing them through an overly altruistic lens. [1] They do not function as simple charitable organizations designed to support weaker plants. [2] These underground connections are governed not by conscious generosity, but by complex evolutionary pressures where each participant acts to maximize its own fitness. [3] Fungi, for instance, have been shown to restrict resource allocation to plants that do not provide adequate carbon in return, essentially punishing "cheaters" in the network. [4] Similarly, plants can redirect their carbon flow to more generous fungal partners. Therefore, the seemingly cooperative network is maintained by a delicate balance of reciprocal exploitation, where mutual benefit is an evolutionary byproduct of individual self-interest rather than genuine altruism.

The Mediterranean Sea is almost entirely landlocked, connected to the vast Atlantic Ocean only by the narrow Strait of Gibraltar. This geographical configuration makes the basin highly sensitive to geological and climatic changes. Approximately 5.96 million years ago, a dramatic geological event known as the Messinian Salinity Crisis began, during which the Mediterranean Sea became completely isolated from the global ocean. The primary driver of this isolation was tectonism (tectonism: the deformation of the Earth's crust by internal forces), specifically the northward migration of the African tectonic plate toward the Eurasian plate. This collision gradually uplifted the seabed in the region of the Gibraltar arc, narrowing and eventually closing the marine gateways that allowed Atlantic water to flow into the Mediterranean. Once the Atlantic connection was severed, the Mediterranean Sea was subjected to intense evaporation. Located in a warm, semi-arid latitudinal belt, the basin loses far more water to the atmosphere through evaporation than it receives from precipitation and river runoff. Under normal conditions, this water deficit is balanced by a continuous inflow of surface water from the Atlantic Ocean. Without this vital replenishment, the sea level dropped precipitously, falling by as much as 1,500 to 2,000 meters over the course of just a few thousand years. As the water evaporated, the dissolved minerals in the seawater became increasingly concentrated. This process culminated in the deposition of massive quantities of evaporite (evaporite: a mineral sediment remaining after the evaporation of water), primarily gypsum and halite, which accumulated in layers up to three kilometers thick across the dry seabed. This unprecedented desiccation transformed the geography and ecology of the Mediterranean basin. The dry, salt-crusted plain resembled a deep, hot desert basin, situated far below sea level. Rivers that emptied into the Mediterranean, such as the ancestral Nile and Rhône, had to adjust to this drastic drop in base level. They carved immense, deep canyons into the exposed continental shelves, some of which extended hundreds of kilometers inland and were comparable in depth to the Grand Canyon of North America. Furthermore, the dry seafloor acted as a temporary land bridge. Terrestrial animals, including ancestors of modern elephants, hippopotamuses, and various rodents, were able to migrate freely between Africa, Europe, and the Mediterranean islands, dramatically reshaping the terrestrial ecosystems of the surrounding continents. The crisis ended abruptly around 5.33 million years ago with the Zanclean Flood. Tectonic subsidence—the sinking of the Earth's crust—in the Gibraltar region, combined with a rise in global sea levels, finally breached the land barrier. Atlantic water began to spill over the barrier, initially as a trickle but rapidly escalating into a colossal deluge. Geologists estimate that at the peak of the flood, water rushed through the newly formed Strait of Gibraltar at rates hundreds of times greater than the flow of the Amazon River. This massive influx of water refilled the entire Mediterranean basin in a period ranging from a few months to two years. The sudden return of marine conditions brought an end to the terrestrial migrations and initiated a new epoch of marine sedimentation. Scientists have long debated the exact mechanics of how the Mediterranean basin emptied and refilled. [1] Early models proposed a single, continuous event of extreme evaporation that left the basin completely dry for hundreds of thousands of years. [2] However, modern analysis of deep-sea sediment cores has challenged this simplistic view by revealing a far more dynamic, multi-stage history. [3] This newer perspective suggests that the region experienced multiple episodes of partial drying followed by temporary floods before the final Zanclean deluge. [4] These cyclic fluctuations were likely driven by a combination of regional tectonic movements and global sea-level changes.