Hi Ginger As Jura says, you need to place this in context to allow an interpretation. It is not a sentence; it is half of a sentence, and here is the text, including the previous sentence which allows us to see what the meaning actually is: 'One can also observe that dogs create zones around them . Depending upon his relationship to the dog and the zone he is in , a trespasser can evoke different behavior when he crosses the invisible lines which are meaningful to the dog .' According to the writer, dogs will repeatedly return to places they know, and around these places they create areas which they consider to be their own territory. These are the zones. To the dog, these areas/zones have different amounts of threat to themselves if invaded and so have to be defended. A stranger, approaching or passing through an area that the dog considers to be one of its own areas, will provoke a response in the dog; the level/amount of response depends on how threatened the dog feels.

This is taken out of context, what was written before & afterwards?