Impacts of habitat fragmentation on plant and insect communities: beyond species richness!
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Transcript of Impacts of habitat fragmentation on plant and insect communities: beyond species richness!
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Impacts of habitat fragmentationon plant and insect communities:
beyond species richness!
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• Part 1Using traits to understand impact of habitat fragmentation on plant communities: local vs. dispersal processes
Seminar outline
• Part 2Impact of habitat fragmentation on changes in relative abundance of flower-visiting insects
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2. Selection of traits linked to clear ecological hypotheses:
Using traits...
1. Theoretical predictions
3. Test using large scale datasets
Environmental change
Change in trait Composition (e.g. weighted mean)
Environmental change
Trait group A(e.g. Mobile species)
Trait group B(e.g. Sedentary species)
Response
Change in species diversity
Change in species diversity
Response
How does the trait modify the response to the environmental change?
“Traditional approach”
“Our approach”
Test interactions between traits
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Impact of habitat fragmentationon plant communities:
local vs. dispersal processes
Marini L., Bruun H.H., Heikkinen R.K., Helm A., Honnay O., Krauss J., Kühn I., Lindborg R., Pärtel M., Bommarco R. (in press) Traits related to species persistence and dispersal explain changes in plant communities subjected to habitat loss. Diversity and Distributions
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Impact of grassland fragmentation on plants
Large number of studies testing area and connectivity effect onoverall plant species richness
Area ConnectivityPl
ant s
peci
es
richn
ess
Metapopulation ecology has mainly considered mobile animals and therefore stressed the importance of dispersal processes
However...
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Local vs. dispersal processes
For plants, it is expected that species’ ability to both persist locally and disperse are critical in shaping communities
One approach to clarify this is to explore species richness responses to fragmentation for groups of species with shared life-history traits
Sourcepopulation
Occupied patch
Unoccupied patch
Dispersal processesLocal within patch processes
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Local processes
Asymmetric competition for light Plant height (short vs. tall)
Increase dispersal success
Starting hypotheses
Asexual reproduction
Dispersal processes
Persistence in the seed bank
Traits
Clonal vs. annual
Persistent vs. transient
Animal (directional) vs. abiotic agent (random)
Seed number(low vs. high)
Processes favouring species robustness to habitat fragmentation
Careful to avoid collinearity between traits!
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2. To use traits to understand the relative importance of local vs. dispersal processes
AIMS
1. To test for interactions between traits: do any combination of traits provide higher robustness to habitat fragmentation?
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Data
Extinction debt mostly paid in all regions [Krauss et al. (2010) Ecol. Lett.]
Homogenization of taxonomy and plant life-history traits across regions
Orthogonal gradients in area and connectivity (Hanski connectivity index in all regions)
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Methods: Mixed model approach in two steps
Species richness~ Trait*Area, random=~1|country/site
Species richness~ Trait A*Trait B*Area, random=~1|country/site
I. Testing ecologically meaningful interactions between traits
II. Testing interactions between single traits and area (or connectivity)
AreaSpec
ies r
ichn
ess
Tall
Short
ConnectivitySpec
ies r
ichn
ess
Annual
Clonal
...
?
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Results
No interactions between traits
Negative effect of habitat loss but no effect of connectivity
The effect of area was modified by three traits:1. Plant height (short vs. tall species)2. Clonality (annual vs. clonal)3. Dispersal agent (abiotically- vs. animal-dispersed species)4. Seed bank5. Seed number
Area Connectivity
Spec
ies r
ichn
ess
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Results: trait effect
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Plant sensitivity to habitat fragmentation
Higher sensitivity to habitat loss for:
1. Small species (low competitive ability for light)
2. Perennial clonal (trade-off between clonality and dispersal?)
3. Abiotically-dispersed species (random vs. animal directional dispersal)
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Plant sensitivity to habitat fragmentation
Results match well with other recent studies
Lindborg et al. (2012) Ecography
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Plant sensitivity to habitat fragmentation
Results match well with other recent studies
Montoya et al. (2008) Science
Negative Ωj implies a negative response to habitat loss
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Conclusions
Our trait-based analyses gain insights into the potential mechanisms leading to plant extinction due to habitat fragmentation
The importance of within-patch local processes have been probably underestimated in fragmentation research so far
The interaction between local persistence and dispersal shaped plant communities
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What about changes in relative abundance?
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Evenness refers to the relative contribution of each species to the total biomass or number of individuals
Background
Abundance-based measures:-Evenness-Dominance-Species composition-Functional diversity...
Species diversity
Species richness
Species evenness
Evenness
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Impact of fragmentation on evenness of flower-visiting insect communities
Marini L. , Öckinger E., Bergman K.-O. , Krauss J., Kuussaari M., Jauker B., Pöyry J., Smith H.G, Steffan-Dewenter I., Bommarco R. (in prep.) Contrasting effect of habitat area and connectivity on evenness of flower-visiting insect communities
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Species evenness has been used more often as a driver of ecosystem functioning rather than as a community response
AimsEv
enne
ss
Which are the effects of habitat fragmentation on abundance patterns of flower-visiting insects?
Fragmentation
?
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Problems with evenness definition
Looseness of the mathematical definition of evenness: several indexes with different sensitivity to changes in rare or dominant species
The choice of the metric is central in the interpretation of the ecological relationships between environmental drivers and evenness
The most important property is the independence from species richness
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Evenness profile
0 0.25 0.5 1 2 4 8 Inf
-2.0
-1.5
-1.0
-0.5
0.0
alpha
E-alpha
Baz1
Baz1
Baz2
Baz2
Baz3
Baz3
Baz4
Baz4
Baz5
Baz5
Baz1Baz2Baz3Baz4Baz5
Increasing importance of changes in dominant species
From the diversity Rényi profile we derived an evenness profile
Diversity profile: Community A is more diverse than a community B if the diversity profile for community A is everywhere above the diversity profile for community B.
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Background: General predictionsEv
enne
ss
Connectivity
Local processes promoting evenness:-Larger habitat diversity in large patches-Lower inter-specific competition in large patchesEv
enne
ss
Area
Dispersal processes promoting evenness:-Larger exchange of individuals between patches
Aim: to test these predictions using a large empirical data set
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Data
Ten grassland networks(7 for butterflies and 3 for wild bees)
Habitat area
Habi
tat c
onne
ctivi
ty
Orthogonal gradients in area and connectivity
Transect countsProportional sampling
Patch
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Results
Increasing importance of changes in dominant species
Spec
ies e
venn
ess
Area
Increasing area
Slope ±CI 95%
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Results
Increasing importance of changes in dominant species
Connectivity
Spec
ies
even
ness
Slope ±CI 95%
Increasing connectivity
Weaker effect for bees than for butterflies
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Which are the underlying mechanisms?
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Fragmentation modifies the specialization distribution
Area
% G
ener
alist
spp.
Area
% G
ener
alist
spp.
Area and specialization
Butterflies Bees (Central foragers)
P<0.01 P<0.01
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% M
obile
spp.
Area Area%
Mob
ile sp
p.
Area and mobility
Same patterns for species mobility (body size)
Small patches host less sedentary species than large patches
P<0.01 P<0.01
What about connectivity?
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% G
ener
alist
spp.
Connectivity
% M
obile
spp.
Connectivity-evenness relationship
No patterns for bees
P<0.01 P<0.01
Connectivity
% M
obile
spp.
Connectivity
% G
ener
alist
spp.
Negative relationship for butterflies
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Interpaly of local and dispersal processes
Local processes:Inter-specific competition (nesting sites, plant resources etc.)
Different local population growth
Dispersal processes:Inter-patch movements
Sedentary and specialists
Mobile and generalists
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Increasing importance of dispersal processes
Small patches are dominated by generalist immigrants, no viable local populations: minimum area threshold?
Increasing importance of local processes
Increasing connectivity may reduce species dominance by favoring inter-patch dispersal of sedentary and specialist species
Interpaly of local and dispersal processes
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Combinations of species exhibiting true metapopulation dynamics with species with frequent inter-patch movements
Only large patches sustain populations that can be locally dominant
Highly complex processes underpinning abundance patterns
Interpaly of local and dispersal processes
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Conclusions
Pollinators are expected to show drastic changes in evenness (dominance) due to several environmental pressures other than fragmentation
We need to evaluate multiple drivers and their interactions on pollinator evenness!
Pollinator evenness is expected to be strongly related to pollination service