Invasive species: how much is too much?

Lantana camara with crab spider. Image credit, Joaquim Alves Gaspar.When it comes to invasive plant species, how much is too much? Ecologists have suspected that for some exotic species, invasion leads to a non-linear decline in native vegetation.

In other words, at low abundance, an invasive plant may have little impact on species diversity, but after passing some threshold of cover, it may cause native communities to rapidly decline.

Now a study in the journal Biological Conservation finds that this may be the case with lantana (Lantana camara L.), a woody thicket-forming shrub native to South
America and considered one of the world’s worst alien species. Lantana invasion of natural ecosystems has led to declines in native species diversity by limiting recruitment and altering ecosystem structure and function.

Researchers looked at plots with different levels of lantana coverage along the southeast coast ranges of New South Wales, Australia. They found that native species richness remained stable below 75% lantana cover, but declined rapidly above this threshold level, leading to compositional change.

A shortcoming of the study is that it demonstrated correlation between lantana and native species richness but that does not necessarily mean causation. The study authors concede that other factors might be affecting both native species richness and the lantana invasion, though they tried to reduce the influence of potentially confounding variables such as prior forest disturbance.

If a non-linear relationship does exist between some exotic species and native plant diversity, it has important implications for how we manage noxious weeds. Eradication of many invasive species is extremely difficult and costly if not impossible. Controlling weeds below their respective threshold levels may be a more cost effective approach in certain areas.

The researchers warn that since their analysis was at the vegetation community level, it may have overlooked impacts to individual rare and threatened species, which could be negatively impacted by lantana below threshold levels. Therefore, they caution that using impact thresholds as guidance for weed control efforts may not be appropriate in areas with rare species and endangered ecological communities.

--Reviewed by Rob Goldstein

Gooden, B., French, K., Turner, P., & Downey, P. (2009). Impact threshold for an alien plant invader, Lantana camara L., on native plant communities Biological Conservation, 142 (11), 2631-2641 DOI: 10.1016/j.biocon.2009.06.012