sunday times: impact of deer on forest invertebrates

December 4, 2005

in Science

The December 2005 issue of the journal Conservation Biology has a paper on a topic near and dear to my heart (heh-heh):  deer overpopulation.  In case you missed previous installments, there was the overview of the ecological problems of too many deer, their impact on songbird populations (scroll to bottom of post), and how too many introduced deer actually extirpated black bears on an Canadian island.

The current paper also examined islands in Canada, this time the Queen Charlotte Islands, BC.  The authors looked at islands on which Sitka Black-tailed Deer (Odocoileus hemionus sikensis) had been introduced for less than 20 years, more than 50 years, as well as deer-free islands. They sampled invertebrates at the forest edge and in the interior, in the litter and below the browse line. 

Not surprisingly, abundance and diversity decreased with increasing length of browsing history, especially at the forest edge, in the vegetative layer, representing the destruction of the understory plant cover by deer.  Primary consumers which depend on the vegetation depleted by deer were most affected; true bugs (Heteroptera) were absent from islands which had a long browsing history.  Forest litter invertebrates were less impacted, with only gastropods (such as snails) decreasing with increased browse time. 

My immediate thought was how this impacts the animals that feed on these now-missing invertebrates.  The authors echoed my concerns, stating that "Such reductions most likely affects groups such as insectivorous songbirds."  The implications for my own research on migratory songbirds in urban areas leaped out at me:

  • The decreases of invertebrates are more pronounced at the forest edge.
  • Urban habitats are often characterized by more edge habitat, and urban forest remnants are already oversimplified and stressed by invasive species, human use, and other factors.
  • Fewer deer have higher impacts in small urban forest fragments, and their populations increase quickly in the absence of hunting and predators.
  • Other studies, as well as my own research, indicate that migratory birds preferentially use edge habitats during migration.
  • Therefore, the reductions of invertebrates in urban forests due to deer browsing may be even greater than those found in this study, and have magnified impacts on birds that depend on these forest fragments during migratory stopovers.

Forest litter invertebrates are also important food items for migratory birds, but I didn’t take a lot of consolation in the fact that they were less impacted in this study.  The islands examined here were dominated by Sitka spruce.  While I admit I am not very well-versed in coniferous versus deciduous forest litter composition, my hunch is that the litter in an eastern deciduous forest probably has a richer, and certainly different, invertebrate fauna than in a coniferous forest. Deer might have an even more profound impact on leaf litter invertebrates in a deciduous forest setting than they did in the coniferous forest islands.

Allombert, S., S. Stockton, and J.-L. Martin.  2005.  A natural experiment on the impact of overabundant deer on forest invertebrates. Conservation Biology 19: 1917-1929.

Aydin December 5, 2005 at 5:42 pm

More deer means less snails? Interesting. I have to read that paper.

Thanks for posting this.

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