If you see a beetle you have never seen in the area before while taking care of your plants in the garden, you might not consider it a big deal. Indeed, too many people, they might just think ‘it’s just another of those annoying pests that harm the wellbeing of my flowers’, but if the person had been an ecologist, he or she might start to panic. For if the small beetle had been invasive, it would be way more than an ordinary pest---it could be unimaginably destructive to the local area. To help you better understand the fear of the ecologist, let me introduce to you the effect of invasive species on the local environment: they(1) crowd out or replace native species (2) change habitats and alter ecosystem function and ecosystem services, and (3) damage human activities.

Before we discuss the effects, a definition is necessary in order to assist you in better understanding what invasive species are. An invasive species is ‘An alien species, any species that is not native to that ecosystem, whose introduction does or is likely to cause economic or environmental harm or harm to human health,’ (OTA.) Non-native species can be added to a community either by natural range extensions or human activity.
Initially, invasive species are a big threat to the local species. There are mainly two ways they harm the native species: invasive species cause disease and act as predators or competitors and outcompete local species. Since residential species have never encountered the new species introduced, they are usually vulnerable to the diseases brought by the incursive species. What’s more, the native species generally have no effective strategy protecting themselves against the invasive species that are predators; at the same time, the local predators have little chance competing against the invasive predators that are equipped with completely unacquainted predatory skills. For example, feral pigs, an invasive species brought to Hawaii by human, will eat almost anything, including native birds, which lead to a sharp decrease in the population of many local bird species. They also compete with native wildlife for food sources such as acorns.  What’s more, Feral pigs spread diseases, such as brucellosis that kills lots of native animals that have no immunization against this alien diseases (Department of Botany.). In fact, the feral pig is only one among many. Worldwide, ‘approximately 42% of threatened or endangered species are at risk primarily due to invasive species’ (OTA.), which bring invasive species the second most threatening factor of species’ extinction.

Being more macroscopic, invasive species can have catastrophic effects on an eco-system as a whole by changing the food web and even alter the condition of the local ecosystem. Due to the effects on local residents mentioned above, the food web will be changed since members of the complex interrelated food chains have been taken out. At the same time, there may not be any restricting for the invaders causing them to reproduce wildly. What’s more, there are some species that have the ability to alter the environmental condition such as the PH value of the soil, which may make it inhospitable for some local species. As an example, according to Wilcove in the book ‘Declining Biodiversity in America's Lakes and Rivers’, water hyacinth is a beautiful aquatic plant, introduced to the U.S. from South America as an ornamental. However, after it was accidently released into the wild, with no predators to control its population and suitable environment, water hyacinths started reproducing wildly, forming dense mats on lake and river surfaces that block out sunlight for submerged plants and aquatic organisms and eventually killing them. As a result, the whole river and lake eco-system is destroyed because of the introduction of one species. In short, only one invasive species has the potential of altering even completely damaging the local ecosystem.

Finally, invasive species damage human activities due to the heavy damages it does on the local environment. They lead to profound costs in lost economic productivity in all areas that depend on nature like agriculture, forestry, fishery and so on. In Brazil, for instance, South African lovegrass has destroyed the pasture value of 10 percent of southern grazing lands and severely damaged the area’s cattle industry. Annual losses are estimated at $30 million a year. The U.S. Department of Agriculture estimates the leafy spurge plague costs ranchers in North Dakota, South Dakota, Montana, and Wyoming more than $144 million a year in losses. The arrival of the cactus moth in Mexico could threaten an $80 million annual industry based on cactus, which is a major source of food for humans and livestock. In fact, ‘invading non-indigenous species in the United States cause major environmental damages and losses adding up to more than $138 billion per year’ (Pimentel et al. 273) the number as high in China. So as you can see, the economic cost of invasive species cannot be overlooked.
All in all, invasive species are extremely dangerous; they wipe out other native species?destroy the native eco-system and have a negative effect on the local people. We should all be fully aware of the severity of the issue and help prevent causing species invasion in everyday lives. For instance, you plant native plants instead of introduced plants and make sure your allopatric pets don’t escape into the wilderness. If we all work together, we will be able to alleviate the damages invasive species.

Works Cited:
OTA. Harmful Non-Indigenous Species in the United States. Washington, DC: Office of Technology Assessment, United States Congress. 1993. Web. 07 Dec. 2015
Wilcove, DS, Bean MJ. The Big Kill: Declining Biodiversity in America's Lakes and Rivers. Washington, DC: Environmental Defense Fund. 1994. Web. 02 Dec. 2015
D, Pimentel et al. “Update on the environmental and economic costs associated alien-invasive species in the United States.” Ecological Economics 5.2 (2005): 273. Web. 27 Nov. 2015
Department of Botany. The effects of feral pigs on a montane rain forest in Hawaii  Volcanoes National Park. Manoa: University of Hawaii 198