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

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 press) Contrasting effect of habitat area and connectivity on evenness of flower-visiting insect communities. Ecography

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