Bee community and trait-based responses to fire in a Mediterranean landscape

Fire is a major global disturbance affecting the evolution of organisms
and shaping plant and animal diversity, especially in fire-prone regions
like the Mediterranean. Yet its impacts on insect-pollinator communities
remain poorly understood. We conducted a 3-year study on a Greek island,
examining spatial and temporal impacts of fire on bee communities and
species’ functional traits. We compared bee diversity and population sizes
in burnt and unburnt sites, by including in our analysis fire severity
metrics using dNBR (differenced Normalized Burn Ratio) derived from
satellite imaging. We show that fire initially led to increased bee
abundance despite having a strong negative impact on floral resources.
Burnt and unburnt communities differed significantly in species
composition revealing that specific bee taxa drove post-fire recovery. In
post-fire year 1, the population spike in burnt sites was driven by an
increased abundance of below-ground nesters, excavators, and trophic
generalists (polylectic); in contrast, the populations of trophic
specialists (oligolectic) were negatively affected. By year 3, most
differences between burnt and unburnt sites had been alleviated.
Multivariate models incorporating fire severity, plant diversity, and
floral resources revealed that the heterogeneity of fire severity, even
within small spatial scales, drove most of the variation in bee
populations, followed by flower numbers. Our findings highlight the role
of fire as an environmental filter, selecting species with specific traits
and shaping distinct post-fire communities. Notably, within three years,
burnt bee communities started converging with the unburnt sites,
suggesting a relatively rapid recovery of functional composition.