What common characteristic do parasites exhibit?

Parasites are organisms that cannot live independently and must rely on a host to survive and reproduce. This characteristic is fundamental to their biological nature; they obtain nutrients and shelter from their host, which can be another organism, ranging from humans to animals. The relationship between a parasite and its host can vary, with some causing harm to the host while others may coexist with minimal impact.

The other options do not align with the defining traits of parasites. For instance, the requirement for heat to reproduce is not universally applicable to all parasites, as they can reproduce in various environments depending on the species. Additionally, the assertion that parasites are found only in plant materials is inaccurate; many parasites infect animals and humans, making their presence diverse across different life forms. Furthermore, the notion that they can exist without food sources contradicts their dependence on hosts, as they specifically thrive by deriving nutrition from their hosts. Thus, the requirement of a host is a critical aspect of understanding parasites and their life cycles.