Impacts of habitat fragmentation on plant and insect communities: beyond species richness!

Part 1 Using traits to understand impact of habitat fragmentation on plant communities: local vs. dispersal processes Seminar outline Part 2 Impact of habitat fragmentation on changes in relative abundance of flower-visiting insects

2. Selection of traits linked to clear ecological hypotheses: Using traits 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 Response How does the trait modify the response to the environmental change? “Traditional approach” “Our approach” Test interactions between traits

Impact of habitat fragmentation on 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

Impact of grassland fragmentation on plants Large number of studies testing area and connectivity effect on overall plant species richness AreaConnectivity Plant species richness Metapopulation ecology has mainly considered mobile animals and therefore stressed the importance of dispersal processes However...

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 Source population Occupied patch Unoccupied patch Dispersal processes Local within patch processes

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!

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?

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)

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) Area Species richness Tall Short Connectivity Species richness Annual Clonal... ?

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 bank 5. Seed number Area Connectivity Species richness

Results: trait effect

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)

Plant sensitivity to habitat fragmentation Results match well with other recent studies Lindborg et al. (2012) Ecography

Plant sensitivity to habitat fragmentation Results match well with other recent studies Montoya et al. (2008) Science Negative Ω j implies a negative response to habitat loss

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

What about changes in relative abundance?

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

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

Species evenness has been used more often as a driver of ecosystem functioning rather than as a community response Aims Evenness Which are the effects of habitat fragmentation on abundance patterns of flower-visiting insects? Fragmentation ?

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

Evenness profile 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.

Background: General predictions Evenness Connectivity Local processes promoting evenness: -Larger habitat diversity in large patches -Lower inter-specific competition in large patches Evenness Area Dispersal processes promoting evenness: -Larger exchange of individuals between patches Aim: to test these predictions using a large empirical data set

Data Ten grassland networks (7 for butterflies and 3 for wild bees) Habitat area Habitat connectivity Orthogonal gradients in area and connectivity Transect counts Proportional sampling Patch

Results Increasing importance of changes in dominant species Species evenness Area Increasing area Slope ±CI 95%

Results Increasing importance of changes in dominant species Connectivity Species evenness Slope ±CI 95% Increasing connectivity Weaker effect for bees than for butterflies

Which are the underlying mechanisms?

Fragmentation modifies the specialization distribution Area % Generalist spp. Area % Generalist spp. Area and specialization ButterfliesBees (Central foragers) P<0.01

% Mobile spp. Area % Mobile spp. Area and mobility Same patterns for species mobility (body size) Small patches host less sedentary species than large patches P<0.01 What about connectivity?

% Generalist spp. Connectivity % Mobile spp. Connectivity-evenness relationship No patterns for bees P<0.01 Connectivity % Mobile spp. Connectivity % Generalist spp. Negative relationship for butterflies

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

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

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

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