Transcript The effect of afforestation on the biodiversity of Irish spiders

Biodiversity across the forest cycle in
ash and Sitka spruce plantations: effects
of Irish forestry on spider communities
Anne Oxbrough, Tom Gittings, Paul Giller, John O’Halloran
Dept Zoology, Ecology and Plant Science, University College Cork
Xysticus cristatus
©Ed. Nieuwenhuys. Copyright 1996-2002
BIOFOREST
• BIOFOREST aims:
• Investigate the effect of afforestation on the biodiversity of
three major taxanomic groups in Ireland
» Plants
» Invertebrates
» Birds
Why use spiders as an indicator group?
Enoplognatha ovata
©Ed Nieuwenhuyys. Copyright 1996 – 2002.
Spiders:
1. Abundant
2. Positively influenced by vegetation structure
1.
2.
3.
4.
5.
Prey
Web attachment
Hiding places for active hunters
Protecton from predators
Suitable microclimates
3. Adopt many ecological strategies
4. Found in all layers of vegetation
5. Occupy a strategic position food webs
6. Taxonomically well known
Aims of study:
• To investigate changes in spider
communities over the forest cycle
• To investigate the difference in spider
communities in Sitka spruce and ash
• To identify indicators of spider biodiversity
Study sites
• Sitka spruce and ash
- Conifer and broadleaf
- Widely planted
• A variety of age classes
Pitfall traps
Pitfalls
• Plastic cups (9cm depth by 7cm diameter)
• Antifreeze and drainage holes
• Five pitfalls per plot
• Five plots per site
• Plots at least 50m apart
• 2-3 weeks in the ground
• Three changes during the season
Habitat measures
• Cover abundance of plant structure
• Cover of deadwood
• Soil samples
– Organic content
– Moisture content
• Litter depth and cover
Results
Linyphia triangularis
©Ed Nieuwenhuyys. Copyright 1996 – 2002
Results:
• 18730 individuals collected in 139 species
• Species classified by habitat preference:
• 15 forest habitat specialists
• 19 open habitat specialists
Mean species richness of spiders across
the forest cycle. Bar indicate SE
25
Species richness
20
15
10
5
0
Mature
Pole
Prethicket Thicket Closed RePrethicket
maturing opening
Sitka spruce
Closed Semi- Mature
maturing Mature
Ash
Species richness of habitat specialists. Bars indicate SE
Open specialists
6
5
4
3
2
Species richness
1
0
Forest specialists
6
5
4
3
2
1
0
Mature Prethicket Pole Closed Semi- Mature
Prethicket Thicket Closed Rematuring opening
maturing mature
Sitka spruce
Ash
Species assemblages
• Cluster analysis indicated 5 groups:
1) Prethicket mix
2) Prethicket ash
3) Mature ash
4) Closed canopy spruce
5) Thicket/Mature open-spruce
• Ordination revealed similar groups as cluster
analysis
Species NMS of sites with environmental variables
NMS of sites using habitat variables
Mean species richness among
cluster groups. Bars indicate SE
25
20
Species
richness
15
10
5
0
Prethicket
mix
Prethicket
ash
Mature
ash
Closed
Canopy
spruce
Thicket/Mature
open-spruce
Mean species richness of habitat specialists among
cluster groups. Bars indicate SE
Open specialists
5
4.5
4
3.5
3
2.5
2
1.5
1
Species
richness
0.5
0
Forest specialists
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
Prethicket Prethicket
mix
ash
Mature
ash
Closed
canopy
spruce
Thicket/Mature
open-spruce
Trends in vegetation structure among cluster
groups. Bars indicate SE
Ground vegetation
Lower field layer
1
1
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0
0
Percentage
cover
Upper field layer
Canopy
1
1
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0
Prethicket
mix
Prethicket
ash
Mature
ash
Closed Thicket/Mature
canopy open-spruce
spruce
0
Prethicket
mix
PreMature
thicket ash
ash
Closed Thicket/Mature
canopy open-spruce
spruce
Correlations of total S and environmental variables
Cluster group
Environmental
variable
Pearson
correlation
p
(2-tailed)
Prethicket mix
(n=20)
Organic content
0.57
0.009
Lower field layer
0.45
0.05
Prethicket ash
(n=34)
Soil cover
-0.33
0.06
Lower field layer
0.40
0.02
Dead wood cover
-0.50
0.005
Closed canopy
Soil cover
spruce (n=29)
Thicket/mature
Lower field layer
open-spruce (n=44)
-0.47
0.06
0.26
0.06
Correlations of open specialist S and
environmental variables
Cluster group
Environmental
variable
Pearson
correlation
p
(2-tailed)
Prethicket ash
(n=34)
Deadwood cover -0.47
0.004
Soil cover
-0.47
0.004
Closed canopy
spruce (n=29)
Canopy cover
-0.31
0.04
Thicket /mature
Open-spruce (n=44)
Ground
vegetation
-0.32
0.04
Correlations of forest specialist S and
environmental variables
Cluster group
Pure prethicket
ash (n= 34)
Environmental
variable
Twig cover
Ground vegetation
Leaf litter cover
Soil cover
Mature ash (n=16) Lower field layer
Closed canopy
Upper field layer
spruce (n= 59)
Thicket/mature
Twig cover
open-spruce
Ground vegetation
(n=44)
Upper field layer
Pearson
p (2correlation tailed)
0.34
0.36
0.54
0.45
-0.58
-0.27
0.05
0.04
0.001
0.007
0.02
0.03
0.46
0.45
-0.48
0.002
0.002
<0.001
Discussion
Meta mengei
©Ed Nieuwenhuyys. Copyright 1996 – 2002
Changes over the forest cycle
• Decrease in overall S in both ash and spruce:
– Decrease in open species
– Increase in forest species
Early stages
• The pre-thicket sites have highest S and the
highest S of open associated species
• Greatest cover of lower field layer vegetation
–
–
–
–
Prey availability
More web attachment points
Hiding places for active predators
Microclimate – stable moisture levels
Effects of canopy closure
• Decrease in lower field layer vegetation
- Reduced light
• Increase in ground vegetation typical of forests
habitats
• Effects on spiders:
- Overall S and open species S is reduced
- Forest species benefit from ground vegetation
Reopening of the canopy
• Mechanisms of reopening
– Thinning
– Wind throw event
– Disease
• Outcomes of reopening
– Increase in lower field layer
– Open species recolonise
– Thinning allows coexistence of both forest and open specialists
– Thicket/mature open-spruce
• Highest total S of all the cluster groups
Differences between ash and spruce
• Prethicket ash and spruce do not form such a
distinct group from each other as mature sites
•
Minimal effect of trees
•
Preplanting habitat type
•
Soil differences
Mature sites
Mature ash distinct in assemblage structure and low S:
• Leaf litter
– Ash and spruce - equally high litter cover
– Ash - low litter depth
– Spruce - high litter depth
– Litter dwelling forest species
• Field layer cover:
– Both spruce and ash have high field layer cover
– Spruce: grass, ferns, brambles
– Ash: Ivy dominated
– Structure of ivy
– Finely tuned vegetation measurements required
Structural indicators of species richness
• Lower field layer cover:
• Important determinant of total spider species
richness
• Diversification of habitat structure
• Evident in more open sites with high species
richness
• Effect of Canopy cover and upper field layer:
• Negative effect on lower field layer vegetation
• Allows colonisation of forest ground vegetation
• Increases S of forest spiders which are positively
correlated with ground vegetation
• Forest associated variables
• Such as:
–
–
–
–
Needle litter
Leaf litter
Deadwood
Litter depth
• Overall negative effect on total and open species
richness
• However it must be noted that forest specialists are
positively affected by these variables
Conclusions
• Sitka spruce and ash have different spider
assemblages which change over the forest cycle:
• Canopy closure has profound effects on spider
communities
• Species richness in spiders is strongly influenced by
vegetation structure
• Sites with a more open canopy contain a more
complex vegetation structure
• Forest species must not be overlooked:
• Open and forest species show opposite trends
over the forest cycle
• Paucity of natural woodlands in Ireland
• Plantations could be a potentially very
important habitat for these species
• Balance between factors affecting open and forest species in
management
• Only ground dwelling spiders studied
• Real data and structural indicators
Acknowledgements
• Many thanks to Myles Nolan, Bob Johnston and
Peter Merrett for help with identification and
verification of difficult specimens
• Thanks to Maire Buckley and Noreen Burke for
help with sample sorting and field work
• Many thanks to Coillte for the use of their forests
during this survey