As the pace of environmental change increases, there is an urgent need for quantitative data revealing the temporal dynamics of local communities in tropical areas. Here we quantify the stability of avian assemblages in the highly threatened, but poorly studied, Andean biodiversity hot spot. We evaluated the temporal variation in species richness and community composition of local bird assemblages in three habitat types (native forest, introduced forest, native shrub) using a unique, relatively long-term data series from Cajas National Park and Mazán Reserve in the Southern Andes of Ecuador. We sampled birds with mist nets using a standardized protocol over 11 years, from 2006 to 2016. Species richness remained stable over time across habitats, but community composition changed in the native forest. In particular, we observed taxonomic reordering in the native forest, in which the evenness in the distribution of abundances of taxa decreased over time. This finding is consistent with other studies where species richness remained constant over time while community composition changed. Our study highlights the value of long-term studies in the tropical Andes as we show that species composition of birds in a montane forest is changing, consistent with global trends in biodiversity change.

This study was conducted in Cajas National Park and the adjacent Mazán private reserve in the high Andes of Azuay province, Ecuador Mazán reserve has 2700 ha (02.870° S, 79.120° W) of mostly native montane forest, with páramo grasslands occupying the upper ridges of the valley. Selective logging occurred in Mazán >30 years ago, but today it is under strict protection that permits only limited scientific activity. In Cajas National Park, we worked in the U-shaped Llaviuco valley (02.840 S, 79.160° W) at the eastern border of the park. Recreational activities (trekking) are common in this valley, but are limited to selected trails. Most of the forest in this valley was cleared for cattle ranching, but since its incorporation into Cajas National Park in 1996 and the removal of the cattle, vegetation has naturally re-established.

Within Mazán and Llaviuco, we placed three sampling stations in areas with different habitat types: native montane forest (hereafter native forest), mixed stands of mature Eucalyptus sp. and Pinus patula with a native understory (hereafter introduced forest), and early successional vegetation (hereafter native shrub). The native forest was located in Mazán at an elevation of 3200 m. a.s.l.. Common tree species around the sampling station included Hedyosmum cumbalense, Symplocos quitensis, and Myrcianthes sp., with an understory mainly composed of Miconia bracteolata, Viburnum triphyllum, and Oreopanax avicenniifolius. The canopy reached 10-15 m. The introduced forest was also located in Mazán at 3100 m. a.s.l.. The canopy here varied between 15-20 m, and species common in the understory included Rubus sp. and Baccharis sp. The native shrub was located in Llaviuco Valley at an elevation of 3150 m. a.s.l., and is successional stage of native montane forest. Common shrub species were Barnadesia arborea, Brachyotum sp., Rubus floribundus, and Salvia corrugata. The canopy was low (< 2-3 m), with isolated taller trees mainly along small creeks that crossed our sampling area. There has been little change in the structure of the vegetation in all habitat types over the period of this study.

Sampling Protocol

At each sampling station, we captured birds by placing 20 mist nets (12 m x 32 mm mesh) along edges of narrow trails (1 m aprox.) in fixed locations for two consecutive days (Figure 2). Nets were open from dawn to 17:00 of day 1, and dawn to 11:00 of day 2. All birds were uniquely banded with a numbered aluminum metal band to identify recaptured individuals. Birds were sampled three times annually from 2006 to 2016 at each sampling station. To cover the climatic seasonality of the study area we sampled once in the main wet season (within April 1^st - May 15^th), once in the dry season (within July 15^th - August 15^th), and once in the secondary rainy season (within November 9^th - December 18^th). The total number of sampled hours varied somewhat among sampling periods because we did not operate nets during rainy conditions.

The data shown has not been corrected by sampling effort. Data for the Endangered species Metallura baroni has been removed, following guidelines of the National Park. Rare species were removed from the data set