Another problem caused by urban growth is the replacement of native plant species by "weedy" non-native species. Studies have shown the number of non-native species increases with increasing urbanisation in an area, while the number of native species decreases. In fact, a majority of the natural plants in a development site is often removed and replaced with shrubs and grasses that provide minimal habitat value for native animals. Usually, these grasses are non-native or worse, they are invasive.

Not all non-native species are bad for the environment. Some non-native garden plants, for instance, provide food for native insects. However, some non-native species are "invasive". Invasive species are organisms (plants or animals) that do not naturally occur in a specific area AND whose introduction does or is likely to cause economic or environmental harm or harm to human health. The Quercus Project has a thorough overview of invasive species in Ireland and is a good source for information on invasive species in both Northern Ireland and the Republic.

Invasive species can affect native organisms in a number of ways including the following (adapted from the Quercus report on Invasive Species in Ireland):

• Competition--Competitive impacts arise when non-native invaders and native species compete for resources
• Predation--Non-native species may kill and/or eat native species, or themselves sustain higher populations of native or non-native predators.
• Alteration of habitat form or function--Introduced species alter the water table, fire regime, soil properties or vegetation structure and can make habitats unsuitable for native species

Rhododendrons (Rhododendron ponticum) and Giant Hogweeds (Heracleum mantegazzianum) are examples of non-native plant species introduced to Ireland as ornamental garden plants but have since been classified as invasive. Rhododendrons invade important and fragile Irish ecosystems such as bogs and heath where they out-compete native plants. Rhododendrons are extremely difficult to control. The same can be said about the Giant Hogweeds. Giant Hogweeds pose a health hazard to humans as skin contact with the sap of the plant causes irritation, particularly in direct sunlight, and often requires hospital treatment.

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