Invertebrates

History of deer colonization - Response of vegetation to deer - Response of animals to deer and squirrel - Deer biology

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Invertebrates - Songbirds - Nest predation and reproductive success

Effects of browsing history on understory invertebrates

Aim & scope - Where & when - How - Main results - Conclusion - Scientific Publications

Aim and scope

Our objective was to analyse the effects of a long browsing history on the abundance (number of individuals) and diversity (number of species) of invertebrates.

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Where and when

In the springs of 2000 and 2001, Sylvain Allombert and co-workers collected insects on 6 islands in Laskeek Bay.

2 (Low and Lost) had no deer
2 (South Skedans and West Skedans) have had deer for less than 20 years
2 (Haswell and West Limestone) have had deer for over 50 years

On each island, insects were collected at 10 sampling stations, 5 in the forest interior and 5 at the forest edge (shoreline vegetation).

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How

2 sampling methods were used according to the type of invertebrates:

Understory invertebrates	Litter invertebrates

Understory invertebrates were collected in 5 forest interior and 5 forest edge plots.	Litter invertebrates were collected in 5 forest interior plots.

Understory strata	Litter

Sweeping net method	Pitfall traps method

In the sweeping net method the observer sweeps the understory vegetation within a 3,6m-radius circular station. A sweeping session lasts 2 minutes in forest edge plots and 3 minutes in forest interior plots (which have a denser understory).	Six pitfall traps were set in each forest interior plot. They were left for 20 days. A pitfall trap consists in two plastic cups fitted together and covered with a light grey ceramic tile supported by small sticks. We filled traps with an equal amount of water and ethylene glycol as a preservative. A few drops of detergent reduced surface tension, facilitating drowning.

Once collected, specimens had to be sorted, counted (to assess invertebrate abundance) and identified (to assess invertebrate diversity).


Emptying a pitfall trap	Sorting insects

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Main Results

Understory invertebrates

Boxes with the samples collected in one sweeping session on the 6 islands studied (without deer, with deer for less than 20 years and with deer for more than 50 years) dramatically differ in the abundance (number of individuals) and diversity (number of species) of invertebrates they contain.

The abundance and diversity of understory invertebrates in forest edge and forest interior plots decreases with increasing length of browsing history.

The diversity of understory invertebrates decreases with increasing length of browsing history in forest edge and forest interior plots.

The abundance of understory invertebrates decreases with increasing length of browsing history in forest edge and forest interior plots.

All the main trophic groups are affected.

Abundance and species richness of herbivorous insects decreases with increasing length of browsing history.
Abundance of the dominant pollinator species (bumblebee) decreases with increasing length of browsing history.

Predatory and parasitic insects, that depend on primary consumers, also decrease in abundance and diversity.


A herbivor : leafhopper	A pollinator : bumblebee	A parasitic wasp : ichneumonid

Litter invertebrates

Results from pitfall traps show a modification in community composition, but without any uniform pattern.

However, it seems that the decrease in the abundance and diversity of terrestrial gastropods is associated to browsing history and cascades into a decrease in the abundance of snail eating insects.

number of terrestrial gastropods species

The number of terrestrial gastropod species decreases with the level of deer impact.

The abundance of snail-eating carabid Cychrus tuberculatus decreases with increasing length of browsing history.


2 common terrestrial gastropods: Ariolimax columbianus (banana slug) and Vespericola columbianus		A snail-eating carabid: Cychurs tuberculatus

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Conclusion

Deer browsing history is associated to changes in understory vegetation abundance and diversity and results that these changes explain the observed changes in the invertebrate community.

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Scientific Publications

Sylvain Allombert, Steve Stockton, Jean-Louis Martin, 2005, A Natural Experiment on the Impact of Overabundant Deer on Forest Invertebrates, Conservation Biology 1917–1929 – PDF

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