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.