Can anyone write effectively on any topic or does one need to be an expert in that field to write effectively?

I guess a good writer can always write effectively, but when it comes to some topics which are out of his expertise, he would need to do some research before starting to write.

With regards to writing effectively on a given topic, the writer must be familiar at least, if not experienced to write 'effectively'.  Knowledge of the subject area is important if the writer is to make an unbiased and informative collation of evidences, or opinions.  Writing outside of a subject area can have risks attached where the literature accessed may not not have the appropriate impact the writer wants.  

Any person can write effectively on certain matters provided that the writer has conducted enough research or done extensive study and exposure on the topic.  Otherwise, he/she runs the risk of writing nonsense.

From my perspective, I think that anyone who has acquired the basic skills in a language could be able to convey his/her message to the reader, but writing effectively is a total different subject. In order to write effectively and efficiently, one has to be passionate about writing; he/she must be willing to reveal his/her true thoughts and feelings to the readers. Nevertheless, in order for someone to write effectively in a specific field, such as academic and business writing, one must be equipped with the grammatical and structural knowledge of this particular field.

Communication is the most important thing here. An effective writer is one who communicates well. A good writer maybe able to tackle many topics that may lie outside of their range of expertise but being effective is another ball game. I believe in specialization although dabbling in new waters is good but a jack of all trades will become a master of none.