Re-tell Lecture — Ecology

Read the lecture ONCE, then re-tell it in your own words in 40 seconds without looking back. Keystone species and ecosystem stability A keystone species is one whose impact on its ecosystem is disproportionately large relative to its population size. Remove it, and the entire community structure can change dramatically. The classic example is the grey wolf in Yellowstone National Park in the United States. After wolves were hunted to local extinction in the nineteen twenties, elk populations exploded because their main predator had been eliminated. Without predation pressure, elk overgrazed willows and aspens along riverbanks throughout the park. This in turn destroyed critical habitat for beavers, songbirds, and several fish species. Riverbanks began to erode without root systems to hold the soil. When wolves were reintroduced in nineteen ninety-five, elk herds moved more frequently to avoid predation, vegetation slowly recovered, beavers returned to build dams, and rivers gradually stabilised. Ecologists call this cascading effect a trophic cascade: a change at the top of the food chain ripples down through every level of the ecosystem below it.

Read the lecture ONCE, then re-tell it in your own words in 40 seconds without looking back. How invasive species disrupt ecosystems An invasive species is a non-native organism that spreads aggressively in a new environment, often because it faces no natural predators or diseases. The cane toad, introduced to Australia in nineteen thirty-five to control sugar cane beetles, is a well-known example. The toads failed to eat the target beetles but thrived on other prey, and their toxic skin poisoned native predators that tried to eat them. Populations exploded across northern Australia. Invasive species cause damage in several ways: they compete with native species for food and habitat, they introduce new diseases, and they alter physical environments. Islands are particularly vulnerable because their species evolved in isolation and have few defences against newcomers. Controlling invasive species is expensive and rarely fully successful once they are established.

Read the lecture ONCE, then re-tell it in your own words in 40 seconds without looking back. Coral bleaching and reef recovery Coral bleaching occurs when water temperatures rise above the range corals can tolerate. Under stress, corals expel the symbiotic algae that live inside their tissues and provide them with nutrients and colour. Without these algae, the coral turns white and begins to starve. If temperatures return to normal within a few weeks, the algae can recolonise and the coral recovers. Prolonged heat, however, kills the coral outright. Major bleaching events have become more frequent: the Great Barrier Reef experienced mass bleaching in twenty sixteen, twenty seventeen, and twenty twenty. Recovery between events is critical. Healthy reefs can regenerate in ten to fifteen years, but if bleaching occurs every few years, there is not enough time, and the reef degrades permanently into an algae-dominated system with far less biodiversity.