C
onservati
on management c
onflicts frequently arise when an
overp
opulati
on
of a pr
otected
organism has negative effects
on
other valuable elements in the same ec
osystem. We studied the interacti
ons between a c
ol
ony
of pr
otected tree-nesting wading birds and a remnant p
opulati
on
of centenarian c
ork
oaks that was part
of the f
ormerly d
ominant f
orests in the D
oxf1;ana Bi
ol
ogical Reserve (SW Spain). A significant increase in the tree m
ortality rates has been rec
orded in areas that are yearly influenced by the bird c
ol
ony.
We analysed a cohort of surviving trees using a gradient of nesting bird influence. Tree-nesting history, bird isotopic signature (δ15N), tree health-related parameters (defoliation, δ13C and leaf surface coverage by faeces) and several soil variables were evaluated. Bird influence was related to increased soil salinity. This increase correlated to increased water-use efficiency for the leaves and to crown defoliation, suggesting that the heavily occupied trees are under higher stress and in poorer health condition than the unoccupied ones. We tested structural equations models (SEM) that were based on hypothesised bird effects on the health of the trees. Soil-mediated effects of the nesting birds best explained the symptoms of the declining health of the trees, whereas the percent of leaves’ surface that was covered by faeces did not improve the fitted SEM model.
For the reserve’s managers, a challenging trade-off exists between preserving the relict trees, which have a high genetic diversity and a key ecological role in these savannah-like ecosystems, and maintaining the current nesting area for these protected, but expanding, wading birds.