Many organisms (of which humans are prime examples) eat from multiple levels of the food chain and, thus, make this classification problematic. A carnivore may eat both secondary and tertiary consumers, and its prey may itself be difficult to classify for similar reasons. Organisms showing both carnivory and herbivory are known as omnivores. Even herbivores such as the giant panda may supplement their diet with meat. Scavenging of carrion provides a significant part of the diet of some of the most fearsome predators. Carnivorous plants would be very difficult to fit into this classification, producing their own food but also digesting anything that they may trap. Organisms that eat detritivores or parasites would also be difficult to classify by such a scheme.
Why are species can be difficult to classify because they?
eat from multiple levels of the food chain

Sociologists and criminologists have not reached consensus on the explanation for the dramatic decrease in the city's crime rate. Some attribute the phenomenon to new tactics used by the NYPD, including its use of CompStat and the broken windows theory. Others cite the end of the crack epidemic and demographic changes, including from immigration. Another theory is that widespread exposure to lead pollution from automobile exhaust, which can lower intelligence and increase aggression levels, incited the initial crime wave in the mid-20th century, most acutely affecting heavily trafficked cities like New York. A strong correlation was found demonstrating that violent crime rates in New York and other big cities began to fall after lead was removed from American gasoline in the 1970s. Another theory cited to explain New York City's falling homicide rate is the inverse correlation between the number of murders and the increasingly wetter climate in the city.
Being exposed to what type of pollution has been theorized to increase aggression?
lead

The presence of pain in an animal cannot be known for certain, but it can be inferred through physical and behavioral reactions. Specialists currently believe that all vertebrates can feel pain, and that certain invertebrates, like the octopus, might too. As for other animals, plants, or other entities, their ability to feel physical pain is at present a question beyond scientific reach, since no mechanism is known by which they could have such a feeling. In particular, there are no known nociceptors in groups such as plants, fungi, and most insects, except for instance in fruit flies.
An animals physical and behavioral reactions may be inferred to indicate the presence of what?
pain