TEN WORST INVASIVE SPECIES

You may be familiar the amusing “highlight” reel on ESPN ranking the ten worst sports plays of the week. In recognition of Invasive Species Week and considering that the Sacramento-San Joaquin Delta is one of the most invaded estuaries in the world, we bring you the “Lowlight Reel” – Invasive Species Week Edition. Following are ten of the worst invaders continuing to plague the California.

While so far only detected in Northern California lakes, this carnivorous predator poses a serious threat to the Delta. Like many other voracious predators further up the list, the increasingly altered conditions of the Delta are perfectly suited for the Northern Pike. Water officials fear the detrimental effect this potential invader could have on struggling native species such as smelt, salmon and steelhead.

The 20-pound rodent has remained in the headlines recently for it’s swift and concerning infiltration of the Delta and surrounding regions. The beaver like invader has caused concern due to its ability to devour almost all vegetation and burrow through already unstable levees. If only we could point them towards some of the invasive plants further up the list.

Introduced to the Delta in the early 19Th century by restaurants in the area to replace the declining red-legged frog population, this non-native quickly took over the food chain. The ferocious frog has been known to wreak havoc on native species including red-legged frogs, yellow-legged frogs, Delta smelt, salmon, and steelhead.

A new entrant to the Delta’s hotbed of more than 250 invasive plants and animals, the Limnobium laevigatum was first detected in the Delta in 2008 quickly putting the Department of Boating and Waterways on high alert. While not yet as prolific as another invasive plant further up the list, the Spongeplant can withstand frost and quickly creates large mats of vegetation from a single floating, inch tall plant.

A veteran to the Delta invasive species scene, Egeria densa is thought to have been introduced to the region after being sold in local aquarium shops. The weed settles just below the surface, restricting movements of native endangered fish such as the smelt as well as boaters. The only method currently used to eradicate the unwelcome traveler is herbicides, since mechanical removal poses serious threats to smelt that may be hiding or tangled below.

While seemingly innocuous, the Corbula Amurensis have an insatiable appetite for plankton, which is a key part of the Delta food chain and diet for delta smelt. The small but mighty invader has altered entire food webs, harming smelt, salmon and other native Delta species.

Originally introduced to the Delta for their sportfishing appeal, this aggressive predator thrives in the highly altered conditions of the Delta that have become more like a lake over the years. Making itself at home in the region, the black bass has been known to feed on endangered smelt and salmon smolt.

Introduced in the 1960s deliberately to control certain insects, the Mississippi Silverside have outstayed their welcome according to many, including the recently published Analysis of Limiting Factors Across the Life Cycle of Delta Smelt which names the invasive fish as a key limiting factor to smelt recovery. The fish feed on larval smelt, as well as zooplankton – a major food source for endangered native fish.

A well known invasive plant in the Delta, this non-native ornamental plant can double in size every ten days in warm weather conditions, quickly becoming a dense floating mat of vegetation up to six feet thick. Water hyacinth can limit biological diversity by restricting other species of plants and blocking access to water for fish and wildlife or eliminating plants these organisms depend on for shelter and nesting. Water managers have spent millions of dollars attempting to eradicate the pesky plant with herbicides and mechanical harvest.

Like the black bass, the striped bass was introduced to the Delta for sport fishing. In the decades since then, the prolific predator has been credited for killing up to nine out of ten juvenile salmon before they even reach the Delta.

There you have it. The Delta is teaming with more than 250 invasive species, ranging from relatively innocuous to lethal. Making up an estimated 95 percent of the aquatic biomass in the region, invasive species continue to be one of the most detrimental stressors plaguing the Delta. Sadly, our scientific understanding of the impacts of these invasive species is limited and we have almost no ability to eradicate or even control the overwhelming number of invaders.

The billions of dollars we have spent, and will continue to spend on habitat restoration, in the Delta watershed may be of marginal value if the government agencies responsible for the health of the ecosystem do not place a higher priority on addressing the scourge of invasives.