Chronic Disturbance in a Tropical Dry Forest: Disentangling Direct and Indirect Pathways Behind the Loss of Plant Richness

Abstract

Chronic disturbance is widely recognized as one of main triggers of diversity loss in seasonally dry tropical forests (SDTFs). However, the pathways through which diffuse disturbance is acting on the forest are little understood. This information is especially demanded in the case of vanishing Neotropical seasonally dry forests such as the Tumbesian ones. We proposed a conceptual model to analyze the factors behind the loss of woody species richness along a forest disturbance gradient, explicitly considering the existence of direct and indirect causal pathways of biodiversity loss. We hypothesized that the chronic disturbance can act on the woody species richness directly, either by selective extraction of resources or by browsing of palatable species for livestock, or indirectly, by modifying characteristics of the forest structure and productivity. To test our model, we sampled forest remnants in a very extensive area submitted to long standing chronic pressure. Our forests cells (200 × 200 m) were characterized both in terms of woody species composition, structure, and human pressure. Our structural equation models (SEMs) showed that chronic disturbance is driving a loss of species richness. This was done mainly by indirect effects through the reduction of large trees density. We assume that changes in tree density modify the environmental conditions, thus increasing the stress and finally filtering some specific species. The analysis of both, direct and indirect, allows us to gain a better understanding of the processes behind plant species loss in this SDTF.

Chronic Degradation of Seasonally Dry Tropical Forests Increases the Incidence of Genotoxicity in Birds

“…Multiple studies have shown that the degradation process in SDTFs generate a significant loss of plant and animal biodiversity (see Gillespie et al 2000, Zúñiga-González et al 2000, Shahabuddin and Kumar 2006, Avila-Cabadilla et al 2009, Williams-Linera and Lorea 2009, Espinosa et al 2011, Falcão et al 2018, Valle et al 2021). In the case of plants, it has been proposed that the loss of diversity occurs as a result of increased stress caused by reduced coverage, which implies an increase in drought and solar incidence (Jara-Guerrero et al, 2021). In the case of animals, the mechanism that generates the loss of diversity is less clear, but it is assumed that the changes may be due to a higher difficulty of the individuals to exploit the most degraded areas (Ordóñez-Delgado et al, 2016;Valle et al, 2021).…”

“…Birds are considered efficient bioindicators because of their facility to be identified (Price, 2006), their intimate relationship with the ecosystems they inhabit (Roché et al, 2010), for forming part of the various levels of the ecological pyramid in the ecosystems (Chambers, 2008;Francis, 2017), and for several behavioral characteristics, such as a diversified diet and use of different vegetation layers. On the other hand, in the SDTFs the biota is exposed to environmental stress produced by to long periods of drought, high temperatures, and high solar incidence (Espinosa et al, 2012;Jara-Guerrero et al, 2021;Jara-Guerrero et al, 2019;Ordóñez-Delgado et al, 2016). This strong seasonality determine that species need some behavioral and physiological specializations that enable them to survive in the during the dry season (Bullock et al, 1995).…”

“…The stress by climatic seasonality that are biodiversity exposed are intensified by the disturbance processes (Jara-Guerrero et al, 2021) and by climate change. In the STDFs, the anthropogenic disturbances is mainly originate from timber extraction and cattle over-grazing (Espinosa et al, 2011;Espinosa et al, 2012;Jara-Guerrero et al, 2019).…”

“…Both actions result in a reduction of forest cover due to vegetation clearing (Jara-Guerrero et al, 2019) with an associated increase in exposure to solar radiation and a concurrent increase in temperature. Some works to showed that this changes in microclimatic conditions can function as filter for some sensible species (Jara-Guerrero et al, 2021).…”

Dieback of stringybark eucalypt forests in the Mount Lofty Ranges

“…Canopy gaps favour light-loving species such as graminoids, and may benefit introduced weeds that thrive in more disturbed environments (Anderegg et al 2012; Pappas et al 2022). Reductions in the density of trees due to chronic disturbance can cause indirect loss of species richness due to environmental changes that resident species cannot tolerate (Jara-Guerrero et al 2021).…”