Chronic anthropogenic disturbance drives the biological impoverishment of the Brazilian Caatinga vegetation

1. In addition to acute transformations of ecosystems caused by deforestation, old-growth forests world-wide are being increasingly altered by low-intensity but chronic human disturbance. Overgrazing and the continuous extraction of forest products are important drivers of chronic disturbance, which can lead to the gradual local extinction of species and the alteration of vegetation structure. 2. We tested this hypothesis in the Brazilian Caatinga vegetation, one of the most speciesrich and populated semi-arid regions of the world. Using a multimodel averaging approach, we examined the impact of five recognized indicators of chronic disturbance (i.e. proximity to urban centre, houses, roads, density of people and livestock) on the diversity, abundance and evenness of 30 woody plant communities. We separately tested the response of seedlings, saplings and adults to identify the ontogenetic stages that are most susceptible to chronic disturbance. 3. We recorded over 11 000 individuals belonging to 51 plant species. As expected, most indicators of chronic disturbance were negatively related to species diversity and stem abundance, with a variable effect on community evenness. The density of people and density of livestock were the main factors driving changes in plant communities, with a stronger negative impact on seedling and sapling diversities. Species composition also varied significantly with disturbance indicators, irrespective of ontogeny. 4. Our results show the potential negative impact that chronic disturbance can have on Caatinga plant assemblages and highlight the fact that disturbance resulting from an extractivism-based and subsistence economy are probably driving old-growth forest stands towards shrub-dominated secondary stands. 5. Synthesis and applications. These findings indicate that chronic disturbance should not continue to be neglected and we argue for: (i) research and rural programmes able to support better practices in terms of land use and sustainable exploitation of forest resources, (ii) improved governance and law enforcement to shift extractivism towards sustainable standards, and (iii) expanding the coverage and effective implementation of strictly protected areas.