Final answer:
Fleas are ectoparasites, living on the surface of their host and dependent on the host for their nutrients and resources like blood.
Step-by-step explanation:
Fleas are ectoparasites, which means they live on the surface of their host. They depend on the host for their nutrients and resources, such as blood. They are specialized in inhabiting the host's surface for at least part of their life cycle, where they consume host-derived resources like blood, secretions, keratin, and skin cells.
Fleas are often considered in the context of transmitting invasive pathogens, while their presence as ectoparasites rarely causes substantial fitness costs or health issues for their hosts. Understanding the natural history and evolutionary ecology of parasites is important for public health planning.