Invasion biologists use the term “naturalisation” to describe when introduced species form self-sustaining wild populations. A subset of naturalised species become so widespread and have such large impacts that they are regarded as pests and weeds. An important step in predicting new pests and weeds is predicting which introduced species will naturalise.
The success of naturalisation of the introduced species depends on factors such as the geological, climatic, and biological conditions of the habitats it is introduced into. The native species present in the habitat also play role in this success (competition for resources).
Australia and New Zealand are closely neighbouring countries separated by 1600 km of sea. The significant differences between two countries include Australia having 29 times more land area, 5 times greater population and very different types of wildlife than New Zealand. Also, the indigenous people of Australia are Melanesian in orgin while New Zealand Maori are Polynesian (see Yahoo! Answers for a wide ranging discussion about the many differences between New Zealand and Australia).
Along with these differences there are similarities which will aid the understanding of this article. Similarities, include the recent European colonisation history and the presence of temperate climate zones (although much of Australia is warmer and drier than any of New Zealand). Surprisingly, naturalisation of introduced plant species turns out to be another similarity between these two countries, despite their many environmental differences.
Jeff Diez, Phil Hulme, Richard Duncan and Jon Sullivan from Lincoln University, and colleagues, have analysed the naturalisation patterns of introduced plants in New Zealand and Australia (Diez et al (2009)).
The results of the study showed that out of the 12927 species that were introduced in both countries the number of them naturalised in both countries are surprisingly similar (Australia 1713 species (13%) and New Zealand 1617 species (13%)). Similar patterns are seen in genera naturalised; out of 2663 genera introduced to both countries, 807 (30%) naturalised in Australia compared 746 (28%) in New Zealand. Of the 155 families introduced to both countries, 152 (98%) naturalised in Australia compared to 155 (100%) in New Zealand.
The analysis of the plant families successfully naturalised in both countries showed that the top four families are the same in both countries: Juncaceae (76% of introduced species in this family naturalised in NZ and 61% in Australia), Poaceae (46%-NZ and 30% Australia), Cyperaceae (42% NZ and 39% Australia), and Amaranthaceae (36% NZ and 43% Australia).
Note that ranking of these families within the top four differs between the countries. Juncaceae and Cyperaceae are the two families which hold the same positions (first and third respectively) in both countries. The other two families Poaceae (second in NZ, fourth in Australia) and Amaranthaceae (fourth in NZ, second in Australia) switch between second and fourth positions depending on the country they are in. A similar pattern is shown with the other families in two countries with some exceptions.
We can conclude that naturalisation patterns of introduced species in both the countries are not so different, most likely due to similarities in colonization history and overlapping temperate climate zones. The study by Diez and colleagues can be used as a guideline for future studies with regards to naturalisation patterns of introduced plant species in these two countries. It also suggests that environmental differences between invaded countries are less important for predicting new naturalisations than differences in the country’s cultures of plant introduction and cultivation.
Diez, J. M., Williams, P. A., Randall, R. P., Sullivan, J. J., Hulme, P. E., and Duncan, R. P. 2009. Learning from failures: testing broad taxonomic hypotheses about plant naturalization. Ecology Letters, 12:1174–1183.