Alien invasions are among the five most important direct drivers of biodiversity loss according to the Millenium Ecosystem Assessment, the others being habitat change, climate change, overexploitation, and pollution. Invasive alien species may displace native species, modify habitats, change community structure, affect ecosystem processes or wider ecosystem functioning, impede the provision of ecosystem services, and cause substantial economic losses. There are many publications in the scientific literature on the impacts of alien invasions. See e.g.:

Blackburn TM, Cassey P, Duncan RP, Evans KL, Gaston KJ (2004) Avian extinction and mammalian introductions on oceanic islands. Science 305:1955–1958

Pimentel D, Zuniga R, Morrison D (2005) Update on the environmental and economic costs associated with alien-invasive species in the United States. Ecological Economics 52: 273–288

Vilà M, Basnou C, Pyšek P, Josefsson M, Genovesi P, Gollasch S, Nentwig W, Olenin S, Roques A, Roy D, Hulme PE (2010) How well do we understand the impacts of alien species on ecosystem services? A pan-European, crosstaxa assessment. Frontiers in Ecology and the Environment 8(3):135–144

I´m with Dr Sihag, it´s a complex problem and depends on your situation. Most alien plants reduce biodiversity yet their effects on ecosystem functions such as N cycling or productivity are uncertain (see Vila et al. 2011-ecology letters). For example, in a study with Acacia saligna in Tunisia it has been found positive effects of this introduced alien plant for soil attributes and neighbour vegetation (Cortina et al. J. Arid Env. don´t remember the year). Negative effects, however, are reported in a lot of places too.

Following suggestions of Dr. de Wit, this is not a simple issue and should be treated with an integrated ecological-economic approach. First, you need to know the ecological effects (positive or negative) of Tithonia diversifolia on biodiversity, productivity and other important functions relevant for the stakeholders. And then decide which is the most comprehensive solution that satisfies both the sustainability of the system and the use and benefits stakeholders can extract from it. Although it comes from a different discipline, I would suggest to use a similar approach than Eldridge et al. 2011 (ecology letters). These authors, in the last table of the paper, summaryze positive/negative/neutral effects of shrub encroachment and associate them with the ecosystem functions and land uses that could be maximized or damaged under each scenario.

Actually, I think the reason for the introduction of the exotic Tithonia diversifolia into the ecosystem of the southwestern Nigeria is because of the great potential they have in revitalizing soil conditions especially fertility improvement. They have been reported to increase soil fertility with time when fallow period is long. But the way this invasive species are spreading is enormous and indigenous species that also provide good ecosystem services could be threatened and become extinct.with time.

Invasive species are invasive and should be dealt with special care. they can be more aggressive in introduced habitat and may eliminate the original native species through competition and allelopathic effects. paper mulberry tree (Broussonetia papyrifera) introduced to Islamabad Pakistan not only became the dominant species but also enhanced pollen allergy. Now at the lower strata Parthenium hysterophorus is occupying every nich. in my openion invasive species or aggressive colonizers should not be introduced to any place where vegetation is present. however, in bare land that are degraded to the extent not to support plantation, there these may be tried with caution.

It is true that "Invasive species are invasive and should be dealt with special care. They can be more aggressive in introduced habitat and may eliminate the original native species." Even if they don't eliminate native species, certainly there will be change in native species thorough pollinator network whereby native species will receive in all likelihood as against invasive species genetic information of the later. It will bring change in ecosystem over time. However, change may be positive as well as negative. It is complex process and little researched, while biotic and abiotic stresses on native species or assemblage or community are well documented and investigated.

most of the findings indicate that exotic species always harmful for local populations. In India effect of eucalyptus plantations on water table ,soil fertility is well studied. Eucalyptus an exotic in India is considered to be responsible for declining water table.

similarly invasion of lantan camara, parthenium (congress grass) are also found to be harmful for the local vegetations.

In this topic I have experience in Cocos Island National Park in Costa Rica; the Cocos Island is a oceanic island (500 Km from de Pacific coast of Costa Rica), and this is full of invasive species (mammals as feral pigs, dears, goats, cats and rats; and crop plants like coffe and grasses). For example, in some sectors the natural understory is being replaced by a thick layer coffee shrubs (Coffea arabica L.), this layer prevented seed germination of native plants and it’s a strong competition by water, nutriments and light. Coffea is native of rainforest in the highlands of Ethiopia (and highland areas of Sudan and Kenya), similarity ecological conditions found in Cocos Island. On the side of mammalians, the rats dispersed coffee seed far away of the mother plant, the feral pigs dig the soil and disturb the native forest structure, and the deers prevent the natural regeneration of old cleared sites more than 120 years ago.

This was my experience http://www.youtube.com/watch?v=ve8Y0onrRO4

Here in the Philippines, most invasive exotic species have caused the extinction of some native species in an ecosystem because of human introduction and natural disturbances. It has also affected the daily life of hundreds of fishermen. An example would be the flood caused by Typhoon Ondoy in 2009 that carried away janitor fish, knife fish and Chinese snake turtle from commercial shops and private owners into Laguna de Bay. About 17 native species became extinct because of these non-indigenous species, and some of them have been destroying the nets and gears of the fishermen. It does not just decrease the biodiversity within Laguna de Bay, but it has also caused problem to the fishermen.

We learned in class that unless an invasive species has a predator, it will prey on the native species and will continue to grow in population. Since the invasive species in this case outgrew the native dominant species, it is possible that it has very few or no predators in the habitat. This could cause the elimination of one species it preys upon or could even lower resources for the other native species which would also affect the overall population of each adversely affecting the ecology.