Discussion

Plant species richness decreased in plots near roads, suggesting roads as sources, and/or conduits of anthropogenic disturbance in the Caatinga. Plots near roads showed decreased taxonomic, functional and phylogenetic diversity compared to plots far from roads. The phylogenetic structure of Caatinga near roads is clustered, indicating road-associated disturbances as environmental filters since traits associated to livestock herbivory deterrence were predominantly conserved within phylogenetic lineages and preferentially selected near roads. Therefore, roads should be considered source, and/or conduits of disturbances causing an impoverishment of the biodiversity, and the ecosystem functioning in the Caatinga.
Plots near roads presented lower species richness than plots further from roads. There are fewer species of chamaephytes, phanerophytes, endozoochorous, nitrogen-fixers and resprouters in plots near roads than plots further from roads, thus, there was a loss of redundancy for these functional groups. The communities near roads were phylogenetically more closely related because of some recent lineages filtered in by the influence of the road-associated disturbances, according to the NTI of woody and non-woody communities. Overall species richness, phanerophyte species richness, resprouter species richness and endozoochorous species richness increased as the distance from the road increased. These results suggest that roads are sources, and/or conduits of disturbances (Forman & Alexander, 1998; Forman & Deblinger, 2000) in Caatinga. Therefore, the results show that road vicinity holds environmental filters that cause species loss in different functional groups found in Caatinga.
Plots further from roads presented greater phylogenetic diversity and functional richness of woody species than plots near roads. Plots further from roads were phylogenetically overdispersed among woody, and non-woody species, congruently to the phylogenetically clustered plots near roads. Since the set of traits related to herbivory have a phylogenetic signal (i.e. resprouter ability, succulence with spines, and urticancy/toxicity), this effect suggests environmental filtering. Plots near roads were phylogenetically clustered throughout the entire phylogenetic tree (NRI) concerning the whole set of functional traits. Therefore, the phylogenetic clustering of plots close to the roads and the phylogenetic overdispersion further from the roads indicate changes in the functional structure of the Caatinga and show a decline in species richness in some parts of the functional space (Vamosi et al. , 2009), as detected for chamaephytes, phanerophytes, endozoochores, lianas, nitrogen-fixers and resprouters. Thus, the working hypotheses i) that Caatinga near roads will exhibit lower taxonomic, functional and phylogenetic diversity compared to Caatinga further from roads, and ii) that Caatinga near roads will be more phylogenetically and functionally clustered than Caatinga further from roads, our results show that both hypotheses were confirmed. Our results also show that Caatinga communities near roads are more phylogenetically and functionally clustered among species with traits related to herbivory than communities further from roads. Consequently, disturbance amplified near roads cause loss of phylogenetic lineages across different life forms, different functional groups, causing evolutionary history loss. These results mean that disturbance is an old pressure (i.e. traitNRI for traits related to all traits), but herbivory is rather a selective pressure towards the tip of the phylogenetic tree of the Caatinga metacommunity (i.e. traitNTI for traits related to herbivory).
As a disturbance may select associated functional traits (Ding et al. , 2012; Helmus et al. , 2010), in the Caatinga near roads, functional traits related to herbivory deterrence (Carrión et al. , 2017) should be selected. Since herbivory-deterrence traits are clustered, frequent disturbances may cause phylogenetic clustering by filtering in species with herbivory-deterrence traits, meanwhile absence of disturbances would lead to another phylogenetic structure (Vamosiet al. , 2009). Congruently, our results showed plots near roads to be functionally and phylogenetically clustered. Therefore, those results not only are congruent with the herbivory-deterrence traits conserved within phylogenetic lineages in Caatinga, but also show that disturbances near roads alter plant communities because of selection for herbivory-deterrence traits, confirming the hypothesis that iii) traits associated with herbivory deterrence are predominantly conserved within phylogenetic lineages of the Caatinga flora.
Bovines, goats and sheep are domestic animals that cause intense herbivory, but herbivory by goats is usually the most pervasive (Rainbolt & Coblentz, 1999). Goats seek thoroughly the most palatable species, thereby imposing selective disturbance on the Caatinga flora for the benefit of less palatable species such as Mimosa ssp.,Cenostigma ssp. and Croton ssp., which become abundant in high disturbed Caatinga (Alves et al. , 2008). Therefore, in plots near roads, some species are being replaced by other more resilient/resistant. We found decreased species richness in several functional groups of species in plots near roads (Figure 1). Our results showed decreased richness of species with endozoochory in plots near roads congruently with reports that goat herbivory might reduce the abundance and diversity of succulent fruit species (Leal et al. , 2005). Furthermore, we found that 80% of the woody individuals sampled in plots near roads were resprouters, while in plots further from roads 53% of the individuals were resprouters. Thus, in more disturbed Caatinga communities, fewer species benefit and may eventually proliferate, especially those that are less palatable, have resprouting ability and are non-endozoochorous, reducing rangeland and support for dispersers. That reduced rangeland and support for dispersers superimpose with road avoidance due to traffic noise and to road construction (Forman & Alexander, 1998; Forman & Deblinger, 2000) with ecological impact for ecosystem functioning (Laurance et al. , 2009), constraining fauna in general and dispersers in particular. Therefore, roads reduce the diversity and functioning of the Caatinga, meanwhile promote species resistant and resilient to disturbances.
As disturbances associated with roads are rather chronic than sporadic, a persistent and consistent diversity loss on many levels may generate a gradient of disturbance initiating from the road/roadsides and radiating further into the Caatinga causing degradation. In the Caatinga, species loss due to chronic anthropogenic disturbances does not occur randomly or uniformly, but rather in clusters throughout the phylogenetic tree (Ribeiro et al. , 2016), and our results show that the same effects are caused near roads. Therefore, roads might be understood as axes of high human disturbances into the Caatinga, and be used as a proxy for habitat degradation or land degradation (Leal et al. , 2014; Ribeiro et al. , 2015). As a consequence, on a large scale, a road network may produce cells with inner areas less disturbed and with peripheral areas more disturbed bordered by roads. This patchwork-like landscape may determine not only isolation and fragmentation in the Caatinga (Santos & Tabarelli, 2002), but also much of the phylogenetic, functional and taxonomic patterns of extant Caatinga areas.
We did not find differences in non-woody species richness and non-woody functional diversity between plots near roads and further from roads. These results indicate that these non-woody communities are more resilient/resistant to disturbances and land degradation than woody communities. As our plots comprise predominantly patchy Caatinga, a physiognomy caused by facilitation (Carrión et al. , 2017), the non-woody species seem to be less affected by disturbances inside the clumps/patches, being nursed by copious branched Cenostigmatrees, urticant/toxic leaves of Euphorbiaceae and spinescent Cactaceae. Therefore, disturbances from roads can be minimized by facilitation that must be consider during the planning of restoration and conservation actions near roads.
Our results show roads as source/conduits of land degradation that causes loss of taxonomic, phylogenetic and functional diversity among plants as well as constrain fauna in general and dispersers in particular, causing an impoverishment of the ecosystem functioning in the Caatinga. Biodiversity conservation, and restoration planning must avoid the influence of roads close to or within natural areas in the Caatinga, and where roads are close, restoration and conservation practices may consider to use facilitation in order to increase diversity and functions.