Both Gale Brewer and Jessica Walter have a shared interest in keeping businesses in Manhattan going strong. But Brewer has seen a disturbing trend, particularly on the Upper West Side. Empty storefronts with giant “for rent” signs are popping up with greater frequency, especially on Broadway.  Though Columbus Avenue and Columbus/Amsterdam Avenue BIDs say their vacancy rate is somewhere between five and ten-percent, there are studies that show the rate is more like 20%. In fact, empty storefronts are an equal opportunity blight, affecting shops in poor and wealthy neighborhoods (have a look at Madison Avenue’s empty spaces). So, what is happening? Is the problem greedy landlords? Online shopping? Unprepared business owners? All of the above? We’ll get to the root causes of ground level commercial vacancies, some fixable, some not.