Why are alien species a problem?

Answer 1

invader species can cause problems in many ways. A good example is the snakehead fish, they are so feirce that they kill (not eat) every thing in their path (they heve also killed pets and kids) so they destroy the food web! So yeah they kill off the food webs or a part, wich than kills the rest of the food web.

Answer 2

Alien species can cause loss of habitat or destroy indigenous populations because the alien
species has a higher reproductive rate or can out compete the indigenous species.

Examples: Domestic cats (an alien species) have devastated the song bird population is many regions of the US.
Rabbits have a much higher rate of reproduction than the marsupial mammals that fill the same niches in Australia resulting habitat destruction. Inadequate predation was another contributing factor.
The release of Large constrictors into the Everglades is devastating bird populations in
Florida.
Various species of carp have been introduced into the US and are migrating up the Mississippi. Populations of indigenous fish species (Paddle fish and Gizzard Shad) cannot compete with this introduced fish.
They are also a hazard to humans boating in the waterways because of their habit of jumping when startled.