Introduction

Road networks change the functioning of the ecosystems by modifying soil properties, hydrological regimes, ecological flows, and, especially, amplifying disturbances in different ways. (Forman & Alexander, 1998; Laurance et al. , 2009). If we understand disturbance as biomass loss (Tilman, 1988), disturbances reduce partially or totally the plant biomass of an ecosystem altering its original biodiversity and functioning (Hooper et al. , 2005). Road networks serve as conduits for disturbances and functions (Christen & Matlack, 2009; Forman & Alexander, 1998; Forman & Deblinger, 2000) that can be guided and amplified on roadsides. In general, such disturbances are associated with clearcutting, extraction of firewood, soil movement , or grazing of domestic animals (Forman, 1995; Spooner et al. , 2004; Watkinset al. , 2003). Thus, roads and associated disturbances can alter habitats for the benefit of some plant species, including invasive species, as well as can alter habitats to the detriment of native plant species (Heringer et al. , 2019b, 2019a; Lehmann et al. , 2017; Spooner et al. , 2004). As a result, disturbances associated with roads result in changes of ecosystem functioning due to increased habitat loss, fragmentation , and creation of novel habitats (Karim & Mallik, 2008; Rentch et al. , 2005; Santos & Tabarelli, 2002; Trombulak & Frissell, 2000). One of the most altered Neotropical biomes by human disturbances is the Caatinga, a Brazilian Northeastern semiarid vegetation also referred as dry forest (Dryflor et al. , 2016; Santos & Tabarelli, 2002). As disturbances are conducted, and amplified by roads, Caatinga near and further from roads should differ in terms of diversity, structure, and functioning.
The consequences of disturbances on vegetation depends on the regime and on the affected ecosystems. Partial cut as well as clear-cut disturbances in dry forests drive to loss of aboveground biomass that remains for more than a decade (Niño et al. , 2014). Chronic disturbances as selective cutting and extraction of firewood cause phylogenetic impoverishment throughout age structure of plants in Caatinga (Ribeiro et al. , 2016).
The most pervasive chronic disturbance of Caatinga is the overgrazing of domestic animals, especially goats (Leal et al. , 2005). Herbivory by goats, functions as a selective pressure that may affect the abundance and distribution of the Caatinga flora, since it can reduce the richness of succulent fruit species, the richness of geophytes and the richness of nitrogen fixing species (Moolman & Cowling, 1994; Severson & DeBano, 1991). Overgrazing by domestic animals causes changes in ecosystems because of selective herbivory on seedlings causing expansion of non-palatable species (Bucher 1987), and can cause various types of changes to the functioning of the Caatinga, such as impairing the regeneration of arboreal species, preventing the dispersion of fruits and seeds, decreasing seedling survival, and limiting ecosystem productivity (Leal et al. , 2003).
The consequences of disturbances on vegetation depends on the regime and on the affected ecosystems. Partial cut as well as clear-cut disturbances in dry forests drive to loss of aboveground biomass that remains for more than a decade (Niño et al. , 2014). Chronic disturbances as selective cutting and extraction of firewood cause phylogenetic impoverishment throughout age structure of plants in Caatinga (Ribeiro et al. , 2016).
Functional, and phylogenetic diversities, and structures are useful tools for predicting the ecological consequences of disturbances, and other anthropogenic changes (Cadotte et al. , 2009; Edwardset al. , 2007; Petchey & Gaston, 2006), including disturbance by herbivory. For instance, decreased functional diversity might indicate that some of the resources in an ecosystem are no longer fully available (Mason et al. , 2005). The ecosystem resources would not be available due to environmental filtering caused by disturbances near roads influencing communities’ assemblies, causing changes in the communities’ functions, and phylogenies. Phylogenetic ecology brings the evolutionary history to the ecosystem functioning (Cavender-Bareset al. , 2009; Ding et al. , 2012; Srivastava et al. , 2012; Webb et al. , 2002) shedding light on the relation between ecological processes, selective pressures, and stability of an ecosystem (Cadotte et al. , 2012; Helmus et al. , 2010; Winteret al. , 2013), when phenotypic differences and similarities among species are linked to evolutionary history (Webb, 2000). This demands the calculation of the phylogenetic signal of functional traits allowing inferences about niche conservatism and about convergences (Losos, 2008; Yang et al. , 2014), as the link between phylogenetic diversity and functional diversity depends on the environmental filtering of traits that are conserved (i.e., homologous traits) or convergent (i.e., homoplasic traits) among phylogenetic lineages, causing the phylogenetic effects of clustering, and overdispersion, respectively (Cadotteet al. , 2008; Cadotte & Davies, 2016). This relation between functions and phylogenies are especially meaningful in the Caatinga where plant functional traits determine resilience and resistance against pervasive anthropogenic disturbances (Carrión et al. , 2017).
A great change is expected in functioning, phylogenetic structure, and diversity when ecosystems of severe environments, such as the Caatinga, are disturbed. As disturbances filter out some species, the filtered in species may increase in abundance meanwhile the community may lose some of its phylogenetic lineages (Helmus et al. , 2007). Few studies have evaluated the effect of anthropogenic disturbances and resilience in the Caatinga (Albuquerque et al. , 2012; Ribeiro et al. , 2015, 2016), and even fewer have addressed the functional and phylogenetic structure of plant communities as effects of disturbances in general (Ding et al. , 2012) and herbivory in particular (Carrión et al. , 2017; Ribeiro et al. , 2016). Species loss and phylogenetic clustering caused by disturbances may constrain the functioning, functional diversity and functional redundancy of Caatinga as a consequence of a decreased number of niches (Carrión et al. , 2017). As far as we know, there have been no studies that have either assessed the functional and the phylogenetic effects of disturbances associated with roads in the Caatinga.
We aimed to evaluate the functional and the phylogenetic effects of disturbance near roads in the Caatinga. For that, we sampled plots near roads, and further from roads in order to measure differences concerning taxonomic, functional and phylogenetic diversities as well as phylogenetic structure and phylogenetic signal for disturbance-related traits. We tested the following hypotheses: (i) since roads are conduits of disturbances that filters out species, Caatinga near roads will exhibit lower taxonomic, functional and phylogenetic diversity compared to Caatinga further from roads; (ii) Caatinga near roads are more phylogenetically and functionally clustered than Caatinga further from roads; (iii) traits associated with herbivory deterrence are predominantly conserved within phylogenetic lineages of the Caatinga flora; and (iv) disturbance near roads alter the Caatinga functioning because of selection for disturbance-related traits.