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Difficulty: HardCause-and-Effect Relationships

The passage details the ecological consequences of introducing the cane toad to Australia. Based on the passage, arrange the following events in the correct logical sequence of cause and effect, starting with the initial intervention and ending with the final ecological outcome.

Passage
In 1935, Australian agricultural authorities released approximately 3,000 cane toads (*Rhinella marina*) into the sugar cane fields of Queensland. The primary objective was to biologically control the native Frenchi and greyback beetles, which were decimating sugar cane crops. However, the toads bypassed the beetles, which inhabited the upper stalks of the cane, and instead fed on ground-dwelling insects. Lacking natural predators and highly adaptable, the cane toad population expanded exponentially, migrating westward across northern Australia.

As the toads invaded new habitats, they encountered native predators such as the northern quoll (*Dasyurus hallucatus*) and various monitor lizard species. Unaware of the toad's defense mechanisms, these predators consumed the slow-moving amphibians. The toads, however, possess large parotoid glands that secrete highly toxic bufotoxins when threatened. Ingesting even small quantities of these toxins proved fatal to the native predators, who had evolved no evolutionary resistance. Consequently, the rapid expansion of the cane toad led to a drastic decline in native predator populations, triggering cascading imbalances throughout the Australian riparian ecosystems.

  1. 1Cane toads are released into Queensland fields to control agricultural pests.
  2. 2The toads bypass the target beetles, feeding on ground insects and expanding their population westward.
  3. 3Unfamiliar native predators encounter and consume the spreading cane toads.
  4. 4Threatened cane toads secrete lethal bufotoxins from their parotoid glands.
  5. 5Native predator populations experience severe declines, disrupting the ecosystem.

Answer

The correct sequence of cause and effect begins with the release of the cane toads in Queensland (item_1). Since they fed on ground insects instead of beetles, their population grew and expanded westward (item_2). This range expansion led to encounters with native predators who consumed them (item_3). Upon being attacked, the toads secreted lethal bufotoxins from their parotoid glands (item_4). The ingestion of these toxins resulted in fatal poisoning and a drastic decline in the predator populations, causing widespread ecological imbalance (item_5).
The correct order follows the chronological and logical progression of events described in the passage. First, the cane toads are introduced to control agricultural pests. Instead of feeding on the target beetles in the upper stalks, they feed on ground insects, which enables them to reproduce rapidly and migrate westward. During this range expansion, they encounter native predators who try to consume them. The attacks trigger the toads to secrete lethal bufotoxins, resulting in the poisoning and decline of these predator populations, which ultimately disrupts the local food web.

Step-by-Step Solution

1
Identify the initial event in the causal chain.
The initial event is the release of the cane toads to control beetles (item_1).
This human action initiates all subsequent ecological interactions.
2
Trace the immediate consequence of the toads' release and behavior.
The toads feed on ground insects instead of beetles, allowing their population to grow and migrate westward (item_2).
Their diet choice and lack of predators allow them to expand their territory, setting up encounters with new wildlife.
3
Determine the next link in the chain when the toads enter new habitats.
Native predators encounter and prey on the invading toads (item_3).
The expansion of the toad population leads directly to encounters with predators like quolls and monitor lizards.
4
Identify the defensive response of the toads when attacked.
The toads release toxic bufotoxins from their parotoid glands (item_4).
This is a direct physiological reaction triggered by the predators attempting to consume them.
5
Establish the final ecological effect of the chemical defense.
Native predator populations decline sharply, causing ecosystem imbalances (item_5).
Since the predators lack evolutionary resistance to bufotoxins, ingesting the toxins is fatal, causing population collapse.

Key Concept

Identifying a chronological and logical sequence of cause-and-effect events within a reading passage.
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Topics

Cause-and-Effect Relationships

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