A trap is a mechanical device that, once set, will close only on objects heavy enough to disengage the trigger. Observing this, those unfamiliar with trapping may assume that traps are not selective; that they will catch anything. This is not a correct assumption unless the trapper - the person required to set the inanimate device in the first place - is removed from consideration. Trap and trapper are part of the same equation; once cannot function without the other. Once this relationship is acknowledged, it is recognized that the trap-trapper unit is actually very selective in terms of what it will catch. Regulated trappers and wildlife researchers invariably set their traps in such a way that only the species (or sometimes even only the individual animal) they are targeting is likely to be captured. The numerous techniques trappers use to ensure their trap sets are selective include the following:

• Location: Where a trap is located determines to a great extent what animals are likely to enter it. Traps may be located underwater, in trees, near den sites, travel routes and loafing areas, or within other specific habitat types where nontarget species are never found or are likely to be found.
  
  
• Type of Trap: The use of certain types of traps virtually eliminates the chance that certain species will be captured. Foxes and coyotes, for instance, will avoid entering box or kill-type traps.
  
  
• Size of Trap: The size of the trap determines to some extent what size animals it will capture.
  
  
• Pan Tension: Pan or trigger tension is adjustable on many traps. As a result, traps are often set so that only relatively heavy animals (such as beavers or coyotes) can spring them.
  
  
• Lure or Bait: Specific baits and lures, often used in conjunction with trap sets, are attractive to specific species of animals. Sweet corn, for instance, is attractive to raccoons, but not to bobcats. Lures in the form of urine or scent gland extracts are particularly attractive to the species from which the scent is derived; may even repel other species.
  
  
• Position of Trigger: Trigger configuration on kill-type traps can be set to allow nontarget species to pass through without setting off the trap.
  
  
• Trap Set: How a trap is handled or placed influences what animals can be captured. Wary species will avoid any trace of human scent, while others such as raccoons and skunks may be attracted to it. Fencing or other obstructions placed around a trap can prevent some species from approaching the trap.
  
  
• Timing: The timing of when traps are set during the trapping season can influence which gender and what age class of animals will be captured.

These same elements, all of which make traps highly selective in terms of what animals they will capture, are used not only in fur harvest trapping, but also in the live capture of animals for research and conservation programs, and for problem animal control and property damage situations.