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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.
1. Abundant
2. Positively influenced by vegetation structure1. Prey2. Web attachment3. Hiding places for active hunters4. Protecton from predators5. 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
Spiders:
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
0
5
10
15
20
25
Prethicket Thicket Closed maturing
Re-opening
MaturePrethicket
Pole Closedmaturing
Semi-Mature
Mature
Sitka spruce Ash
Species
richness
Species richness
Species richness of habitat specialists. Bars indicate SEOpen specialists
Forest specialists0
1
2
3
4
5
6
0
1
2
3
4
5
6
Prethicket Thicket Closed maturing
Re-opening
Mature
Sitka spruce Ash
Prethicket Pole Closed maturing
Semi-mature
Mature
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
0
5
10
15
20
25
Prethicket mix
Prethicket ash
Mature ash
Closed Canopy spruce
Thicket/Mature open-spruce
Species richness
Mean species richness of habitat specialists among cluster groups. Bars indicate SE
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Open specialists
Forest specialists
Speciesrichness
Prethicket mix
Prethicket ash
Mature ash
Closedcanopy spruce
Thicket/Mature open-spruce
Trends in vegetation structure among cluster groups. Bars indicate SE
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.8
1
Percentagecover
Ground vegetation Lower field layer
Upper field layer Canopy
Pre-thicketmix
Pre-thicket ash
Mature ash
Closedcanopyspruce
Thicket/Matureopen-spruce
Pre-thicketmix
Pre-thicketash
Mature ash
Closed canopy spruce
Thicket/Matureopen-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 spruce (n=29)
Soil cover -0.47 0.06
Thicket/mature open-spruce (n=44)
Lower field layer 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 Environmental variable
Pearson correlation
p (2-tailed)
Pure prethicket ash (n= 34)
Twig cover 0.34 0.05
Ground vegetation 0.36 0.04
Leaf litter cover 0.54 0.001
Soil cover 0.45 0.007
Mature ash (n=16) Lower field layer -0.58 0.02
Closed canopy spruce (n= 59)
Upper field layer -0.27 0.03
Thicket/mature open-spruce (n=44)
Twig cover 0.46 0.002
Ground vegetation 0.45 0.002
Upper field layer -0.48 <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 sitesMature 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
