From seedling to adult assemblages: chronic disturbance drives the biological impoverishment of Brazilian Caatinga flora

In this post, Elâine Ribeiro writes about her recent paper with Víctor Arroyo-Rodríguez, Bráulio Santos, Marcelo Tabarelli and Inara Leal “Chronic anthropogenic disturbance drives the biological impoverishment of the Brazilian Caatinga vegetation”.

Many studies worldwide are devoted to understand the reorganization of biological communities after human disturbances. However, a large proportion of these studies deal with acute (as opposed to chronic) forms of disturbance (Martorell & Peters 2005). Chronic anthropogenic disturbances (CAD) are a kind of disturbance less noticeable, but characterized by frequent transformations that involve lower biomass losses generally caused by overgrazing by livestock, firewood collection, or exploitation of fodder, leaf litter and other non-timber forest products (Singh 1998). Because of their low intensity, CAD do not suddenly change the forest cover and geographic boundaries of the site, leaving the false notion that the ecosystem is unaffected. However, CAD may impose a myriad of negative impacts ranging from population collapse and species extinction at multiple spatial scales to forest replacement by shrub vegetation. The Brazilian Caatinga is an unique vegetation composed by broad mosaics of seasonally dry tropical forest and scrub vegetation (Veloso et al. 2002) located in north-eastern Brazil and continually faces these CAD (Fig 1 & 2).

Figure 1
Fig 1. (a) Study area in north-eastern Brazil, (b) the Parnamirim municipality (in black) within the Pernambuco state (in grey), (c) the study landscape in which we located 30 50×20 m plots and (d) the urban centre (black triangle) (the white areas in (d) represent deforested areas). In (e) a view of the studied site with the Caatinga vegetation is shown (Picture courtesy of Santos, JC).

In this work, Ribeiro and colleagues assessed how important predictors of CAD (e.g. proximity to city, road and house, and density of livestock and people) can affect the wood flora diversity considering seedling, sapling and adult assemblages in order to draw future trajectories of Caatinga vegetation. To record these CAD effects they sampled in 30 plots which showed distinct signs of chronic disturbance predictors in a landscape of approximately 220 km² dominated by old-growth vegetation.

Figure 2
Fig 2. Examples of common anthropogenic chronic disturbances at the Brazilian Caatinga: (a) wood collection from a columnar cacti Pilosocereus pachycladus and (b) extensive goat livestock near to Caatinga vegetation (Picture courtesy of Ribeiro-Neto, JD).

They verified that 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 livestock were the main factors driving changes in plant communities, with a stronger negative impact on the diversity of seedlings and saplings. Species composition also varied significantly with disturbance indicators, irrespective of ontogeny. These changes in species composition suggests that CAD change old-growth Caatinga forests to stands composed by a subset of disturbance tolerant species and these changes occur from seedlings to adults plant communities.

These results show the potential negative impact that chronic disturbance can have on semi-arid ecosystems, such as Caatinga, 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. This scenario suggests that chronic disturbance should not continue to be neglected when assessing the impact of human activities on biodiversity, and to address this question specifically requires: (1) research and rural programs able to support better practices in terms of land use and sustainable exploitation of forest resources; (2) improved governance and law enforcement to shift extractivism towards sustainable standards and (3) expanding the coverage and effective implementation of strictly protected areas.

Literature cited

Martorell, C. & Peters, E. (2005) The measurement of chronic disturbance and its effects on the threatened cactus Mammillaria pectinifera. Biological Conservation, 124, 199–207.

Singh, S.P. (1998) Chronic disturbance, a principal cause of environmental degradation in developing countries. Environmental Conservation, 25, 1–2.

Veloso, A.L., Sampaio, E.V.S.B. & Pareyn, F.G.C. (2002) Ecorregiões: Propostas para o Bioma Caatinga. Associação Plantas do Nordeste, Instituto de Conservação Ambiental and The Nature Conservancy do Brasil, Recife.

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