Update & Create Actions in Web Services

I’ve recently started to integrate our Dynamics ERP into our new eCommerce website and noticed that when an update request is made that it will update the requested fields, but all other fields in the schema are then erased. Is this normal? I would expect an update operation to only update what I tell it to and to leave the rest in tact. Does it work differently?

And while I’m at it, how do you enable custom fields to be updated via web services if they aren’t in the schema? Can the schema be customized and hosted elsewhere? When I try to access schema’s hosted by MS I am always given an error.