Charophytivory, playas y papalotes, a local paradigm of global relevance

The Llano Estacado region of western Texas and adjacent New Mexico has one of the most species-rich charophyte floras known for North America, but upo n closer inspection this flora is seen to consist of two, strictly non-over lapping segments, one of 12 taxa, the other of nine. The larger group of sp ecies is confined to a vast series of shallow, ephemerally inundated depres sions known as playas. These normally fill with run-off rainwater in late M ay or early June, remain flooded through early to mid-September before even tually drying to the curling polygon stage near the end of September. Appro ximately 8 months later the cycle is repeated. The remaining nine charophyt es are confined to permanently inundated sites, the most common of which ar e livestock-watering windmill complexes, known as papalotes. Rarely, if eve r, are members of the playa-12 encountered in papalotes or members of the p apalote-9 in playas. The underlying bases for this 'two flora' dichotomy st em from the contrasting amphipod populations engendered by the two habitat types. Most freshwater amphipods (scuds), including the common North Americ an herbivore Hyalella azteca (Saussure), cannot withstand complete desiccat ion which, accordingly, prevents its colonisation of playas. By contrast, s cuds often reach exceptionally high densities in the associated concrete or metal tanques of papalotes, since such specialised habitats are usually fr ee of vertebrate predators, e.g. birds, fish, turtles, salamanders. Scud-re sistant charophytes are larger and slower to develop gametangia, features i nimical to success in short-lived playas. By contrast, precocious charophyt e opportunists-typical of playa floras-cannot withstand the grazing pressur es of papalote (or papalote-like) environments. While charophytivory is ass umed to be of worldwide occurrence and significance, it has been clearly de monstrated only in the unique juxtaposition of 'playas y papalotes'.