Open access publications in the SLU publication database

Abstract

Habitat loss is a major cause of species extinction. Old trees have declined, and this continues because of poor management. Examples are development of secondary woodland in semi-open grasslands, and lack of natural tree regeneration. We therefore can expect that species associated with old tree are declining. The aim of this thesis is to increase the understanding about the habitat requirements, colonization-extinction dynamics and persistence of oak-associated epiphytic lichens with different traits. For species with slow dynamics, data on colonization and extinction events are time consuming to collect. We instead fitted Bayesian incidence function models extended to dynamic landscapes to snapshot data of our study species. The results show that all species were restricted to old trees, but the lower age limit when oaks become suitable differed among species. Colonization rates were generally low and explained by connectivity to surrounding occupied trees, in accordance with metapopulation theory. The colonization rates were higher for species with wide niches (colonizing relatively young trees) and small dispersal propagules than for species with narrow niches (only colonizing old trees) or large dispersal propagules. Stochastic extinctions from intact trees were negligible in all study species, and thus, the extinction rate is set by the tree fall rate, which is low, as oaks become old. By using oak surveys from 1830 and 2009 from a landscape where oaks have declined drastically short after 1830, we show that epiphyte occurrence patterns are better explained by the historical configuration of oaks compared to the present configuration. This indicate non-equilibrium occurrence patterns 180 years after habitat decline, and simulations using the fitted metapopulation models suggest even longer time-lags before new equilibriums are reached. The metapopulation extinction risk increased with decreasing habitat amount, and was highest for species with narrow niches or large dispersal propagules (i.e. low colonization rates). Conservation actions in landscapes with a low density of old oaks had only minor effects on epiphyte persistence, while actions in landscapes with many old but few young trees may increase species persistence. The best conservation approach for the long-term persistence of all species is to assure a constant tree regeneration in landscapes with currently high densities of old oaks.