Charophytivory, Playas y Papalotes, a Local Paradigm of Global Relevance

Abstract

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 upon closer inspection this flora is seen to consist of two, strictly non-overlapping segments, one of 12 taxa, the other of nine. The larger group of species is confined to a vast series of shallow, ephemerally inundated depressions known as playas. These normally fill with run-off rainwater in late May or early June, remain flooded through early to mid-September before eventually drying to the curling polygon stage near the end of September. Approximately 8 months later the cycle is repeated. The remaining nine charophytes are confined to permanently inundated sites, the most common of which are livestock-watering windmill complexes, known as papalotes. Rarely, if ever, are members of the playa-12 encountered in papalotes or members of the papalote-9 in playas. The underlying bases for this ‘two flora’ dichotomy stem from the contrasting amphipod populations engendered by the two habitat types. Most freshwater amphipods (scuds), including the common North American herbivore Hyalella azteca (Saussure), cannot withstand complete desiccation which, accordingly, prevents its colonisation of playas. By contrast, scuds often reach exceptionally high densities in the associated concrete or metal tanques of papalotes, since such specialised habitats are usually free of vertebrate predators, e.g. birds, fish, turtles, salamanders. Scud- resistant charophytes are larger and slower to develop gametangia, features inimical to success in short- lived playas. By contrast, precocious charophyte opportunists–typical of playa floras–cannot withstand the grazing pressures of papalote (or papalote-like) environments. While charophytivory is assumed to be of worldwide occurrence and significance, it has been clearly demonstrated only in the unique juxtaposition of ‘playas y papalotes’.