Keywords are what search engines and abstracting services such as Bioabstracts use to alert interested scientists who are conducting literature searches that your paper may be of interest.  Typically,  < 6 words/phrases are permitted.  Choose words/phrases that are succinct and pertinent.  I find it helpful to think about what terms I would type into a search program and expect to come across the paper that I am writing.  Often one of the terms used is the species of interest while others focus on the larger questions under study.