Parasitism is a type of symbiotic relationship in which one organism benefits at the expense of another. Which of the following examples illustrates parasitism?

Introduction / Context:
In ecology, species interactions are classified into types such as mutualism, commensalism, and parasitism. Parasitism is a relationship in which one organism, the parasite, lives on or in another organism, the host, and obtains nutrients at the host's expense, often causing harm. Many exam questions present specific interactions and ask you to identify which ones are parasitic. This question lists several common host parasite relationships and asks which are examples of parasitism.

Given Data / Assumptions:

• The interactions described involve leeches feeding on blood, ticks on dogs, and aphids on plants.
• In each case, one organism feeds on another over time rather than killing it immediately.
• We assume standard ecological definitions of parasitism.
• The question allows the possibility that all examples may be parasitic.

Concept / Approach:
Parasitism is defined as a relationship where the parasite benefits in terms of nutrition or shelter, while the host is harmed. The host is usually not killed outright, although its fitness can be reduced. Leeches attach externally to a host and feed on its blood, ticks live on the skin of animals like dogs and feed on blood, and aphids feed on plant sap while remaining attached to the plant. All three are classic textbook examples of parasites that obtain nourishment from a host and may weaken it, so each interaction fits the definition of parasitism.

Step-by-Step Solution:
Step 1: Consider leeches. Many leech species attach to vertebrate hosts and suck blood over a period of time. The leech gains nutrition, while the host loses blood and may be exposed to infection; this is parasitism. Step 2: Consider ticks on dogs. Ticks are external parasites that attach to the skin, feed on blood, and may transmit diseases such as Lyme disease. The tick benefits, and the dog can suffer from blood loss and illness. Step 3: Consider aphids on plants. Aphids pierce plant tissues and suck phloem sap, depriving the plant of nutrients and potentially transmitting plant viruses. The aphid benefits, while the plant is harmed. Step 4: Observe that in all three interactions, the host is harmed but not immediately killed, and the parasite gains nutrition in a sustained relationship. Step 5: Therefore, each individual example illustrates parasitism, and the best answer is that all of the above are examples of parasitism.

Verification / Alternative check:
Ecology textbooks often list leeches, ticks, lice, fleas, and aphids as standard examples of parasites in both animal and plant systems. They contrast these with predators, which kill and eat their prey, and with mutualists, where both partners benefit. Diagrams and figures may show ticks attached to dogs or aphids clustered on plant stems under headings describing parasitism. This consistent use of these organisms as teaching examples confirms that all three cases in the options are parasitic relationships.

Why Other Options Are Wrong:
Leeches attach to a host and consume its blood for nutrition is a correct example of parasitism but does not include the other examples, so choosing only this would ignore the other valid parasitic interactions. Ticks living on dogs and feeding on the dogs blood is also a correct example but, again, not the only one. Aphid insects sucking the sap from the stems and leaves of a host plant is a valid parasitic interaction in plant ecology. Since each individual example is parasitic, the most accurate choice is the all of the above option, and the single example options are incomplete.

Common Pitfalls:
Students sometimes confuse parasitism with predation and think that because parasites often do not kill their hosts quickly, they might be commensal or harmless. Others may think only internal parasites like tapeworms count, forgetting that many parasites are external. To avoid these errors, focus on the key idea that the parasite benefits while the host is harmed, regardless of whether the parasite is internal or external. When you see leeches, ticks, and aphids feeding on hosts over time, you can confidently classify these as parasitic relationships.

Final Answer:
The correct choice is All of the above are examples of parasitism, because leeches on hosts, ticks on dogs, and aphids on plants all involve a parasite benefiting at the host's expense.