lawn? vernal pool

are the fill-ins done by nature? or by humans? i notice bare patches in my lawn's at home in the late winter in Northern California's Sonoma County.... but my lawn is not really a lawn... it is native vernal vegetation that i mow and call lawn...My Property is part of the Laguna de Santa Rosa (for more info ck out the web site www.lagunadesantarosa.org) and now, after my 2 year working retreat experience on my late father's New Stone Age Farm in Southern California's San Bernardino County, where the soil, weather and vegetation is quite different than in Sonoma County....i will continue to leave the 'lawn' natural and note what species are growing ....