Parsing the Relative Effects of Habitat Amount, Fragmentation, and Habitat Edge on Parasitism Rates in an Experimental Landscape System Kimberly A. With Laboratory for Landscape and Conservation Ecology Division of Biology, Kansas State University Manhattan, KS 66506
[email protected]
Daniel M. Pavuk Department of Biological Sciences Bowling Green State University Bowling Green, OH 43403
102nd Annual Meeting of the Ecological Society of America, Oregon Convention Center, Portland, OR (8 August 2017)
Host-Parasitoid Interactions Question: How does the amount vs. the fragmentation of habitat affect parasitism rates? Habitat fragmentation is expected to negatively affect parasitism rates: • Parasitoids tend to be more negatively affected by habitat loss and fragmentation than their hosts, owing to lower densities • Habitat loss and fragmentation disrupt the movement and search behavior of parasitoids, interfering with their ability to locate and aggregate in response to hosts • Are parasitism rates primarily a response to habitat loss, or to habitat fragmentation, or both?
Habitat Loss vs. Fragmentation • Effects of habitat loss and fragmentation are usually confounded • Many of the supposed measures of fragmentation (decreased patch area and increased patch isolation) could be due to habitat loss alone • Habitat loss and fragmentation are landscape-wide processes and should be studied at the ‘landscape scale’ • Most fragmentation studies are conducted at the patch scale, however (e.g., patch area and isolation effects)
More patches More edge
Fahrig (1997.—J. Wildl. Manage. 61: 603-610)
Fragmentation Experiments – Traditional patch-based approach Adjust patch properties
Patch isolation
to create fragmented landscape patterns (“bottom-up” approach)
Can only explore a small
number of patch configurations in the field owing to space limitations, however
Patch size and number
Smaller patches More patches More total edge
More fragmented
Larger patches Fewer patches Less total edge
Less fragmented
Fragmentation Experiments – Landscape approach
Adjust landscape properties (habitat amount and spatial contagion, H) from the “top down” using fractal neutral landscape models to create patch structure Parse out the independent effects of habitat amount and fragmentation
Smaller patches More patches More edge habitat More fragmented
Larger patches Fewer patches Less edge habitat Less fragmented
Create these fractal landscape patterns in the field With 1997.—Cons. Biol. 11: 1069 – 1080; With & King 1997.—Oikos 79: 219-229.
Experimental Model Landscape System 6 levels habitat amount (10 – 80% red clover)
20% Fragmented
Clumped vs. Fragmented 3 landscape replicates 36 plots total
20% Clumped
16 m Aerial view of 4-ha site near Bowling Green State University in Northwest Ohio With et al. 2002.—Ecological Applications 12: 52-65; With & Pavuk 2011.—Landscape Ecology 26: 1035-1048; With & Pavuk 2012.—Oecologia 170: 517-528; With 2016.—Landscape Ecology 31: 969-980.
Patch properties of fractal landscapes 16
Number of patches
14
(1 - 5 patches/landscape) Clumped Fragmented (1 - 15 patches/landscape)
12 10 8 6
--Fragmented landscapes have significantly more patches, especially 1 border w/matrix (bare ground) — 68% of all edge cells surveyed were from fragmented landscapes — 75% of all interior cells surveyed were from clumped landscapes — No interior cells from fragmented landscapes with 10-20% habitat Interior cell
Edge cell
— Unbalanced design (GLM; Type III SS)—test main effects and two-way interactions only
Significant Effects on Parasitism Rates — Overall parasitism rate = ~0.40 (0.39 + 0.087, N = 36) Source of variation
DF
MS
F
P
Habitat Amount
5
0.103
2.91
0.014
Habitat Fragmentation
1
0.012
0.33
0.567
Edge
1
0.129
3.64
0.057
Amount x Frag
5
0.110
3.11
0.009
Amount x Edge
5
0.094
2.65
0.023
Frag x Edge
1
0.051
1.43
0.232
Model R2 = 0.13; F18,365 = 3.00, P < 0.0001 (GLM; Type III SS)
Parasitism rate
(prop. aphid mummies/10 stems/10% clover cells)
Habitat Amount Effect Greatest difference in parasitism rates occurs between 20% and 50% habitat (0.35 vs. 0.42) 0.50
21.4% increase
AB
0.45 0.40
B AB
A
0.35
12
0.30
72
AB
AB
90
120
60 30
0.25
P < 0.05 LS Means: Tukey-Kramer
0.20 0
10
20
30
40
50
60
Habitat amount (%)
70
80
90
100
Significant Effects on Parasitism Rates — Overall parasitism rate = ~0.40 (0.39 + 0.087, N = 36) Source of variation
DF
MS
F
P
Habitat Amount
5
0.103
2.91
0.014
Habitat Fragmentation
1
0.012
0.33
0.567
Edge
1
0.129
3.64
0.057
Amount x Frag
5
0.110
3.11
0.009
Amount x Edge
5
0.094
2.65
0.023
Frag x Edge
1
0.051
1.43
0.232
Model R2 = 0.13; F18,365 = 3.00, P < 0.0001 (GLM; Type III SS)
Parasitism rate
(prop. aphid mummies/10 stems/10% clover cells)
Habitat Amount x Fragmentation Effect Parasitism rates are generally higher in fragmented landscapes 60% habitat
0.60
6
0.50
30 15
0.40
36
36
45
60
45 60
0.30 6
0.20
30
15
Clumped
0.10
Fragmented
0.00 0
10
20
30
40
50
60
Habitat amount (%)
70
80
90
100
Significant Effects on Parasitism Rates — Overall parasitism rate = ~0.40 (0.39 + 0.087, N = 36) Source of variation
DF
MS
F
P
Habitat Amount
5
0.103
2.91
0.014
Habitat Fragmentation
1
0.012
0.33
0.567
Edge
1
0.129
3.64
0.057
Amount x Frag
5
0.110
3.11
0.009
Amount x Edge
5
0.094
2.65
0.023
Frag x Edge
1
0.051
1.43
0.232
Model R2 = 0.13; F18,365 = 3.00, P < 0.0001 (GLM; Type III SS)
Parasitism rate
(prop. aphid mummies/10 stems/10% clover cells)
Habitat Amount x Edge Effect Parasitism rates are generally higher in edge cells clumped) and 80% habitat (clumped > fragmented) • Greatest effect of edge observed at 40% habitat (edge > interior) and 60% habitat (interior > edge)
Parsing the effects of habitat amount, fragmentation and habitat edge on parasitism rates
• Landscapes transition from edgedominated to interior-dominated cells between 50% and 60% habitat • No effect of fragmentation (P = 0.567) or edge type (P = 0.057) per se on parasitism rates
Prop. edge or interior cells
Crossover in parasitism rates occur around 50% habitat, shifting from higher in fragmented landscapes and within edges to higher in clumped landscapes and within interior cells. Why? 1.0 Edge Interior
0.8 0.6 0.4 0.2 0.0 10
20
40
50
60
Habitat amount (%)
80
• Parasitism rates thus reflect the change in landscape structure that occurs at intermediate habitat levels (40-50-60%): — a shift from mostly edge to mostly interior cells — most edge cells come from fragmented landscapes, which are most numerous
