Invasional meltdown: exotic deer nibbling on native plants

Red deer stag (Cervus elaphus). Image credit, Mehmet Karatay.The journal Biological Invasions will be publishing a special issue devoted to the topic of exotic herbivores and their ecological impacts. One title that caught my eye deals with the hypothesis of invasional meltdown - a highly dramatic but useful term.

Under invasional meltdown, the activity of one exotic species facilitates the invasion of another. This concept is particularly relevant to herbivores given their ability to cause widespread disturbance to plants through grazing and trampling that may affect some species much more than others.

Researchers wanted to find out whether invasional meltdown has been occurring with the introduction of both red and fallow deer (Dama dama) to Nahuel Huapi National Park in Patagonia, Argentina. The deer were introduced to Isla Victoria, an island in the park between 1917 -1922.

The study authors suspected that selective browsing by the deer has been contributing to the spread of exotic forest plant species, particularly Douglas fir (Pseudotsuga menziesii), at the expense of native evergreen forest plants dominated by Chilean cedar (Austrocedrus chilensis).

To better understand the impacts of the non-native deer, they fenced off five 20-square meter plots from 2002-2006 and compared vegetation with adjacent, non-fenced control areas where deer graze freely.  Specifically, they looked at 1) seedling density and size, 2) sapling density and size, and 3) understory cover.

They found that the density of the exotic Douglas fir and the native Chilean cedar were similar in the fenced and open plots. However, the cedars in the open plots were 77% shorter than they were in the fenced areas while the Douglas fir were only 3% shorter. The authors write:

"Introduced deer had an important negative effect on height growth of both the dominant native and the main exotic tree species. However, the impact is stronger for native Austrocedrus than for exotic P. menziesii. This observation aligns with our previous study showing that deer preferred native over exotic foliage. Height growth inhibition is critical to trees because vertical growth is the main mechanism for escaping herbivory by ungulates."

The study also found that understory cover was greater in the fenced plots owing largely to more extensive native shrubs. However, there was no statistical difference in understory native and exotic species cover in the enclosed versus open plots.  

The authors conclude that these results provide evidence that invasional meltdown may be occurring with exotic deer on the island. They suspect that this is causing widespread changes in local ecological processes but concede that this is unstudied. Nonetheless they make strong recommendations:

"Deer have been in the area for more than 80 years, and the quick response of the plant community to deer removal is remarkable… The fast positive response of native plant species and the insignificant response of exotic plant species to the lack of deer suggest that eradication of deer from Isla Victoria would aid conservation of native communities."

--Reviewed by Rob Goldstein

Relva, M., Nuñez, M., & Simberloff, D. (2009). Introduced deer reduce native plant cover and facilitate invasion of non-native tree species: evidence for invasional meltdown Biological Invasions DOI: 10.1007/s10530-009-9623-0