Browsing history

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

Menu: Home; Haida Gwaii; Introduced Species; RGIS; What did we learn?; Research; Publications; RGIS symposium; School curriculum; Contacts

English
Français

Effect of browsing history on the vegetation - Effect of chemical defences on deer browsing - Effects of reduced plant abundance on pollination - Effect of hunting on tree regeneration - Effect of prolonged deer population reduction on the vegetation

Effects of different browsing histories on the understory

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

Aim and scope

A dendrochronology study (see Estimating time since deer colonization) has shown that Laskeek Bay is a "natural laboratory" that consists of adjacent islands with different browsing histories: no deer, deer present for less than 20 years, deer present for more than 50 years at time of study,

This situation allowed RGIS to study the relationship linking browsing history to vegetation diversity and structure in two types of habitats:

the forest understory (interior)
the shoreline vegetation (forest edge)

return to top of page

Where and when

In 2000, Steve Stockton and co-wokers sampled the vegetation in permanent plots in the forest interior and along the shoreline on 7 islands situated in Laskeek Bay:

3 islands had never been colonised by deer: Low, Lost, South Low
2 had been colonised for less than 20 years: West Skedans, South Skedans
2 had been colonised for more than 50 years: West Limestone, Haswell.

lenght of deer presence on Laskeek Bay islands

return to top of page

How

15 plots were established on each island:

In the forest interior, 5 circular plots, 10 meters in radius, were placed at least at 50 meters from the forest edge and from one another.
On the shorelines, 10 circular plots, 10 meters in radius, were distributed randomly along the shoreline.

In order to describe vegetation diversity and to estimate plant abundance we recorded the percent cover of each species per vegetation layer.

The vegetation layers were: ground: 0-50 cm transition: 50-150 cm shrub: 150-400 cm Deer feed mainly between 0 and 150 cm but are also able to push shrubs over in order to reach the upper branches.

In each strata and for each species, the observer estimated the percentage of plot area that would be covered by the shadow cast on the ground by the foliage present in the strata (any plant species with foliage in different vegetation layers would thus be represented by several percent cover estimates). Percent cover was estimated with the help of a cover chart, like the one shown here.

return to top of page

Main Results

A simple visual comparison of forest interior or shorelines vegetation between islands with and without deer already shows dramatic differences in plant cover.


Sword ferns in forest interior on an island without deer (Lost)	Browsed and dead sword ferns in forest interior with deer (West Skedans)

Grassy shoreline on an island without deer (Lost)	Barren shoreline on an island with deer (West Limestone)

An analysis of the vegetation samples collected showed that changes in vegetation cover, diversity, structure and composition were associated with length of deer presence.

Changes in vegetation abundance associated to deer presence

Overall vegetation cover often exceeded 80% in the lower vegetation layers of forest interior plots on islands without deer and was less than 10% on the islands with deer for more than 50 years.

Changes in vegetation diversity associated to deer presence

At the plot scale, plant species richness was about 20 to 50% lower on islands colonized by deer than on islands without deer.

Changes in vegetation structure and composition associated to deer presence

A Principal Component Analysis is a statistical procedure that allows to represent complex data on community composition in a two dimensional space and relative to habitat variables.
In this analysis, vegetation plots have been positioned on the figure's plane according to plant composition and percent cover. The closer a plot is to another on this plane the more similar their understory vegetation.
We used different symbols (squares and triangles) for plots from different habitats (forest interior = squares, or forest edge = triangles) and different colours for different browsing histories.

Plots with positive scores on the horizontal axis are characterized by high values for vegetation cover and species richness in the understory. Plots with negative scores have low values for vegetation cover and species richness.
Plots with positive scores on the vertical axis are rich in shrubs and ferns, those with negative scores are rich in forbes or grasses.

This analysis suggests that when time since deer colonisation increases:

The abundance of shrubs, ferns, and flowering plants decreases to the benefit of a few herbaceous plants
The contrast in vegetation composition between the two types of habitats also decreases when time since deer colonisation increases (the spread of the plots is dramatically reduced)

return to top of page

Conclusion

The longer deer are present on an island:

The less vegetation there is
The lower the diversity of vegetation
The simpler vegetation composition and structure

The consequences for biodiversity are:

a risk of species loss
altered habitat with possible consequences on the fauna,
altered ecosystem functions such as nutrient and water cycling

return to top of page

Scientific Publications

Stephen A. Stockton, Sylvain Allombert, Anthony J. Gaston, Jean-Louis Martin, 2005, A natural experiment on the effects of high deer densities on the native flora of coastal temperate rain forests, Biological Conservation 126 (2005) 118–128 – PDF
Martin J.L., Stockton S.A., Allombert S. and Gaston A.J. 2010. Top-down and bottom-up consequences of unchecked ungulate browsing on plant and animal diversity in temperate forests: lessons from a deer introduction. Biological Invasions. 12 : 353-371 - PDF

return to top of page