( d a hybrid of the two species that is l...)Invasive species alter habitat to their benefit

In the marshes she studied, Farrer found that hybrids were more common than native cattails. She also noted that the areas of each marsh with lots of hybrid cattails had higher nutrient levels and heavier mats of dead cattail stems than areas with only native wetland plants. The plants growing in these invaded areas also were different, with fewer classic wetland species, such as bulrushes, rushes, and sedges, and more typical land plants like grasses, asters, and goldenrods.

Next, Farrer did transplant experiments to figure out whether the invaders were causing the changes she observed. She set up four study plots in a previously uninvaded section of marsh. In one, she transplanted live hybrid cattails; in the second she added litter---the mats of dead stems that accumulate around hybrids. A third plot received both live hybrids and litter, and the fourth was left alone.

Litter accumulation was the deciding factor, she found. "Plots with the litter treatment had higher levels of nitrogen in the soil and higher turnover rates of nitrogen, along with much lower light levels and lower soil temperatures," Farrer said. "So the litter was creating a pretty different environment."

When she tallied other plants in the experimental plots, she found that both the diversity and the density of native species were lower when litter was present. But while native plants suffered, invaders prospered. "The hybrid plants performed better with litter addition," Farrer said. "They obviously aren't hindered by the litter, and the increase in nutrients may help them grow larger."

The results suggest that invasive cattails set in motion a feedback loop that helps them gain a stronghold. "The invasive cattails change the ecosystem through litter accumulation, producing an environment in which native plants don't perform we
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Source:University of Michigan