Building Blocks of an Argument

Premise

Part of the core of the argument; present in every argument
Supports the author's conclusion
Can be a fact or an opinion; can be a description, historical information, statistical or numerical data, or a comparison of things

Background

Not part of the core; often present, but not always Provides context to help understand the core
Almost always fact-based; can be in almost any form: historical information, numerical or
other data, descriptions of plans or ideas, definitions of words or concepts, and so on


Back to the question, try to eliminate the first sentence (Considering it as Background information)

The Chinese White Dolphin is a territorial animal ghat rarely strays far from its natural habitat in the Pearl River Delta. In recent years, increasing industrial and agricultural runoff to the Delta's waters has caused many white dolphins to perish before they reach breeding age.Unless legislation is enacted to ensure there is no further decline in the Delta's water quality, the Chinese white dolphin will become extinct.

Now without the first sentence, the arguement becomes shaky as the first question that will come to mind is , Why cant they move away from the Delta waters and continue breeding ?

The first sentence acts as the authors Arguement to this question hence it is a premise not a background information...