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Greatest Invasive Species of the San Joaquin River Delta [PICS]

Joe Bartok/Blogspot

These three invasive species have left a Delta ecosystem unbalanced, tarnished, and forever changed.

The San Joaquin River Delta in Sacramento, California is an expansive area that has long been claimed by settlers for its rich fertile soils. It is called home by numerous species, some of which should have never been introduced to the area.

The American bullfrog, red-eared slider turtle, and Mississippi silverside have caused irreversible damage the ecosystem of the Delta.

American Bullfrog


In the early 19th century the red-legged frog, which is native to the Delta basin, was a very popular food item. When the population dipped lower, restaurants in the area turned to an alternative by importing in the very recognizable American bullfrog.

The American bullfrog was originally only found in the eastern parts of North America. They can reach sizes of over six inches in length and are very aggressive predators. They will eat just about anything they can fit in their mouths including birds, other frogs, fish, lizards, mice, and anything else they see fit.

They are also capable of adapting to live in any environment, both man-made or natural, that has water. They have spread from the Delta itself to surrounding ponds, swamps, irrigation ditches, and smaller streams that feed the Delta.

This caused huge problems as live frogs brought in escaped or were set free into the ecosystem. They quickly climbed to the one of the top predators of the food chain. The native species including red-legged frogs, yellow-legged frogs, Delta smelts, salmon, steelhead, and more didn’t stand a chance as the bullfrog’s populations grew and devoured native species, as well as their young.

Upsetting the food chain isn’t their only issue. Red-legged frogs find the large female bullfrogs extremely attractive. This has led to decreased reproduction rates in the species as they males waste their time courting the female frogs of the wrong species.

The bullfrogs are also known to be carriers, but also immune, to a disease known as chytridiomycosis. This deadly disease is brought on by a type of fungus that begins growing on amphibians. In some species it causes only sporadic deaths, while others it carries a 100 percent mortality rate.

In 2012, Santa Cruz banned the sale and import of bullfrogs in city limits to help stop the population decline of native species. In 2014, the California Fish and Game Commission is looking into greater restrictions in importing the frogs or possibly banning all new imports statewide.

Red-eared Slider Turtle


The red-eared slider turtle is the most popular pet turtle in the world. They can grow up to 12 inches long and live up to 30 years. Although they mainly eat aquatic plants, they have been known to eat small fish and frogs when the opportunity arises.

While they are a popular pet, a lot of people don’t like to keep them once they reach a larger size and look for a way to be rid of them. What better way than to set them free into ponds and rivers? While this doesn’t seem like a big deal at the time, they can quickly take over an area where smaller turtles, like the western pond turtle that is native to the Delta, reside.

Both the red-eared slider and the western pond turtle require very much the same habitat to survive. The larger red-eared slider easily outcompetes their smaller relatives for food, space, and breeding areas. This has led to dramatic population decreases for the western pond turtle in the area.

Although is hasn’t yet been proven, many believe the red-eared slider has also spread new diseases to native turtles in the area, helping to contribute to their decline.

Mississippi Silverside


This small, clear fish with silver sides is another native to eastern North America. They can grow up to six inches in length and feast on anything smaller than itself.

Silversides were introduced to the Delta, various lakes, ponds, and streams in California in the the 1960s. They were introduced to help the decline of bothersome insect populations and as a new food source to attract larger fish to the area. But introducing this non-native species soon backfired.

The silversides not only ate the zooplankton and insects in the water, but soon turned their eyes on the eggs and tiny fry of native fish species. The Delta’s Chinook salmon and Delta smelts were soon in trouble as the fish moved into their breeding grounds and devoured up the easy targets.

They soon became a formidable predator for the Delta smelts especially. In some areas of the Delta, the silverside is the most common fish in the area.

At some point we may realize that adding species to ecosystems that have spent thousands of years working to create ecosystem balance is not a great idea, no matter the benefits at the time. Once a species is added, especially one that flourishes quickly, it is extremely difficult to eliminate once again.

However these species were added, whether on purpose or by accident, it is obvious they are there to stay. Maybe in a few hundred years the ecosystem will balance itself back out, even if that means a few species may be wiped out in the process.


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Greatest Invasive Species of the San Joaquin River Delta [PICS]