Invader versus natives: effects of hydroperiod on competition between hydrophytes in a tropical freshwater marsh.
The La Mancha freshwater marsh (Veracruz, Mexico) has heterogeneous water and ground levels; spatial variation that is reflected in its vegetation's distribution. The invasive grass Echinochloa pyramidalis grows in relatively dry areas, whereas native species, including Sagittaria lancifolia and Typha domingensis, dominate the wettest parts of the marsh. To determine the effects of hydroperiod on competition between the invasive and native hydrophytes, we designed a field experiment based on replacement series of pairs of species under different hydroperiod treatments. The results demonstrate that E. pyramidalis performs best under permanently dry conditions. Under almost permanently flooded conditions, S. lancifolia produced more cover than the invasive grass did, while T. domingensis produced cover and biomass similar to those of the invasive species. Besides the effects of the dominance of E. pyramidalis over the native species, this dominance produced changes in the soil level on a microtopographic scale. Plots under permanently dry conditions and with high E. pyramidalis cover exhibited a slight increase in surface height. We conclude that La Mancha's weed invasion is a consequence of the deliberate introduction of E. pyramidalis into the relatively dry areas of the wetland. This weed, a non-indigenous grass, has gradually advanced towards the wetter areas, changing the marsh's topography and hydroperiod in the process, and inducing changes in species composition.