How do ecological backlashes bring imbalance to the ecosystem?

An ecological backlash is "an unexpected and detrimental consequence of an environmental modification which may outweigh the gains anticipated from the modification scheme." (encyclopedia.com)

To illustrate this let's take the Love Canal housing development in Niagara Falls, New York as an example.

• William T. Love

In 1892 landowner and entrepreneur William T. Love, began work on a new city that would be the envy of the world. To power this city he planned to construct hydroelectric dams along a soon-to-be-built canal. However, due to a number of reasons, including stock market crashes and the discovery of AC current, the plan failed. However, a kilometer-long stretch of the canal had been finished.

• First Sale

In 1920 the 'canal' was purchased by the City of Niagara Falls who deemed the pit perfect for the dumping of their toxic chemical waste. Although the city sold the land 20 years or so later, to the Hooker Chemical and Plastics Corporation, it still continued to be filled with toxic waste, so much so that by 1953 almost 22,000 tons of waste had been dumped rendering it full. The company decided to line it with clay and cover it with dirt as a supposed solution to sealing it.

• Unknown consequences

Two years later, the Niagara Falls Education Board bought the strip for one dollar. Within months, 400 children were attending the newly-built school, most of whom lived in the 100 brand new homes surrounding it. Over the next 20 years there were dozens of incidents reporting strange smells and the appearance of a "blue ooze", yet nothing was done.

In 1977, a year after massive floods and a blizzard, record numbers of still births and defects were reported, prompting a national inquiry. A year later the area had been sealed off and been officially declared hazardous.