Woodpeckers and non-excavator birds in logged sites: same problem... same solution?

Abstract

Piedmont forests, the lower vegetation stratum of Andean forests in NW Argentina, have been severely disturbed with ~90% of its original range transformed into agricultural, live-stock pastures, industrial, and urban areas. One main current human activity in piedmont forest remnants is selective logging, legal and illegal, and always without criteria for biodiversity conservation. As part of a project aiming to preserve the biodiversity of piedmont forests, we show a study in progress on cavity-user birds, emphasizing on woodpecker species. We have conducted 3 field seasons at 3 undisturbed sites (US) and 4 logged sites (LS) with comparable sample effort, performing point counts to detect differences in cavity-user bird abundances, and looking for cavities used by birds (i.e. nests or roosts). We applied network theory to analyze these bird-tree interactions, i.e. nestwebs. We found that 14 of 22 cavity user bird species (e.g. toucans, parrots, flycatchers, woodcreepers, and woodpeckers) were significantly less abundant in LS than in US; particularly, from 4 woodpecker species occurring at our study area, only the White-barred Piculet (Picumnus cirratus) did not show significant differences in abundance between sites, and the Golden-olive Woodpecker (Colaptes rubiginosus) was not detected at LS. We found 143 cavities (i.e. interactions) in 14 tree species used by 14 bird species in US, and 66 cavities in 12 tree species used by 13 bird species in LS. Difference in the amount of interactions between US and LS was mainly due to woodpecker incidence: 108 vs 35 interactions respectively. Several network parameters, such as connectance, dominance, evenness, and robustness against tree species extinction simulations, were similar between US and LS. Woodpecker cavities were only occasionally used by non-excavators, both in US (18.7% of non-excavator interactions) and LS (19.4%). We found differences in the following aspects: 1) key tree species, determined by strength index, were snags (i.e. standing dead trees), Calycophyllum multiflorum and Amburana cearensis (an endangered Fabaceeae) in US, and snags, Anandenanthera colubrina and Astronium urundeuva in LS; 2) the whole nestweb, as well as the 4 woodpecker species, were more generalists in US than in LS, probably caused by a shortage of suitable trees in LS; and 3) US had 2 interaction modules: one formed by excavators (woodpeckers plus a trogon) that excavated their own cavities in snags and in living trees, and the other by non-excavators mainly with decay-formed cavities in living trees, but also in snags; LS had a third module in which a few non-excavator species, absent in US, constituted a separate module with decay-formed cavities in snags. Our results show that selective logging is heavily influencing the woodpecker occurrence in LS, and that actions tending to preserve woodpeckers could have no impact on non-excavator cavity-users in our study site.