Epiphyte metapopulation persistence after drastic habitat decline and low tree regeneration: time-lags and effects of conservation actions.
Abstract
Old trees have declined in Europe due to agricultural intensification and forestry. For shade-intolerant epiphytic species occurring on old trees in semi-open landscapes, host tree numbers have further decreased because of shading by developing secondary woodland. Moreover, in this habitat, regeneration that could replace the extant old trees is low. This suggests that epiphytic species associated with old trees are declining. However, for species with low extinction rates, the decline may be slow and hard to elucidate. We investigated the persistence of five old-oak-associated epiphytic lichens with different traits by simulating metapopulation dynamics using Bayesian incidence function models for dynamic landscapes. With an oak-rich landscape as a reference, we investigated effects of (i) drastic habitat decline, (ii) conservation actions such as clearing around trees or increased regeneration rate, (iii) low tree regeneration and (iv) clearing and increased regeneration after 100 years of low regeneration. After drastic habitat decline, the number of occupied trees continued to decrease, displaying long time-lags before reaching new metapopulation equilibriums. Lichen extinction risks increased with decreasing habitat and were highest for species that only colonise very old trees or have large dispersal propagules. In landscapes with low tree densities, conservation actions had only minor effects on lichen extinction risks. Low tree regeneration rates increased lichen extinction risks, but species declines were slow. Conservation actions that increased regeneration after 100 years of low regeneration decreased the extinction risks to very low levels. Synthesis and applications. Due to low rates of local extinction, epiphytes display long time-lags to reach new equilibriums after habitat loss. Thus, we should expect ongoing declines in epiphyte metapopulations in landscapes where old trees have recently declined. Slow extinction gives an opportunity to improve persistence by conservation actions, but the success depends on species traits and the current density of old trees. In landscapes with many old but few young trees, epiphytes may persist if conservation actions quickly address the need to increase tree regeneration rates. The best conservation approach for long-term persistence of epiphytic lichens is to ensure regular tree regeneration in landscapes with a current high density of old trees.