For more than 100 years, invasive species have made the Sacramento-San Joaquin Delta their home, disrupting the ecosystem and costing millions of dollars annually in remediation. The latest invader is the nutria, a large rodent native to South America that causes concern because of its propensity to devour every bit of vegetation in sight and destabilize levees by burrowing into them. Wildlife officials are trapping the animal and trying to learn the extent of its infestation.