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.