Introduction
Understanding the ecological processes that maintain high species
diversity in natural environments, such as tropical forests, is
fundamental in community ecology (He et al. , 1996). The main
processes maintaining species coexistence in tropical forests are
deterministic or stochastics (Chase and Myers, 2011). The deterministic
processes of interspecific competition and habitat filtering are based
on the species’ niche. Species that explore the same limiting resources
or that have similar tolerance limits to the same environmental
conditions compete intensely, especially in the case of sessile
organisms such as trees. In a two-species model with asymmetric
competition, the stronger species exclude the weaker species (Schluter,
2000) or constrains the spatial distribution of the weaker species to
less favourable environments (Baraloto et al. , 2007). Thus, if
these deterministic processes are important forces structuring a
community, species coexistence is possible when each species has a
different niche or when environmental heterogeneity is high enough so
that species with similar niches are restricted to somewhat different
environments (Silvertown, 2004). On the other hand, according to neutral
theory (Hubbell, 2001), species diversity is the balance between
stochastic emergence and disappearance of species at the regional scale
(Hubbell, 2005). Locally, seed arrival in vacant space is unpredictable,
considering the spatio-temporal variation in availability
(stochasticity). However, dispersal and recruitment limitation, common
in tropical forests (Hubbell et al. , 1999), prevent the most
abundant species from occupying all available recruitment sites and thus
dominating the community over time (competition exclusion), i.e.
“winning-by-forfeit” (Hurtt and Pacala, 1995; Hubbell, 2001).
The arrangement of plants in space is the result of processes acting on
each individual throughout its life, with each ecological process
generating a characteristic spatial structure at a given spatial scale
(Hubbell et al. , 2001). Habitat filtering, for example, is
expected to result both in spatial segregation of species with different
environmental requirements at spatial scales larger than the patch scale
where the environment is approximately similar (Itoh et al. ,
2003; Getzin et al. , 2006) and in spatial association of species
with similar environmental requirements at spatial scales smaller than
the patch scale (Burns and Strauss, 2011). When these species
additionally compete within patches, reduced resource availability
should result in a decrease in growth rate (Kenkel, 1988), thereby
causing nearby neighbours to be smaller than distant trees (Getzinet al. , 2008). Also, in extreme cases, the stronger competitor
can eventually cause mortality of the weaker competitor (Kenkel, 1988),
resulting in spatial segregation of competing species at the small
neighbourhood scale (say < 5 m; Velázquez et al. ,
2015).
In contrast, if species stochasticity governs community structuring as
assumed in neutral theory, conspecific interactions should be stronger
than heterospecific interactions, producing an approximate independent
spatial relationship between species pairs (Volkov et al. , 2007).
Additionally, species distribution should not be related to
environmental characteristics, as all species are expected to respond to
the environment in a similar way (Hubbell, 2005). Also, dispersal
limitation is expected to result in aggregation of seeds and small
trees, and in spatial associations of seeds and smaller trees to large
trees (Murphy et al. , 2017), because mortality is usually not as
strong, even at high-density patches, as to spatially uncouple seeds
from their parent trees (Hubbell, 1980; Hubbell, 2005; but see Getzinet al. , 2014).
Even though habitat filtering and dispersal limitation both result in
aggregation and spatial association of individuals, the former leads to
species distribution related to local environmental variables, whereas
the latter causes high density of small trees centred on conspecific
large trees (Wang et al. , 2015). Additionally, the spatial
pattern of individuals of different sizes indicates which ecological
process has acted more strongly on the population (Comita et al. ,
2007; Shen et al. , 2009). Species-habitat association might
change through plant development, possibly resulting in spatial
dissociation between size classes. Moreover, species-habitat association
is expected to be stronger for large than for small trees (Comitaet al. , 2007; but see Baldeck et al. , 2013). On the other
hand, aggregation of individuals of different sizes and spatial
association between size classes are expected under strong dispersal
limitation (Wiegand et al. , 2007).
Investigating ecological processes with spatial point process models may
be especially important in harsh environments, such as areas of rocky
soils (Pollock et al. , 2012) and areas subject to periodic
flooding (Baraloto et al. , 2007; Colmer and Voesenek, 2009), as
these environments can act as strong drivers of species selection
(Chapin et al. , 1993; Reich et al. , 2003). Selective
pressures on plants, such as gradients in soil moisture, can change
functional traits and the niche relationship of co-occurring species
(Werner and Platt, 1976). However, few studies have evaluated the
spatial structure of trees in environments with seasonal flooding (e.g.
Baraloto et al. , 2007; Oliveira et al. , 2014), even though
habitat filtering is commonly associated to topographic and edaphic
variation in tropical forests (Bagchi et al. , 2011; Baldecket al. , 2012). Thus, species with different flooding tolerances
should show different species-habitat associations according to the
local environmental heterogeneity (Baraloto et al. , 2007).
Congeneric species are a good model to investigate the relative
importance of ecological processes that maintain high species diversity
because they tend to have similar niches due to their close phylogenetic
relationship (Losos, 2008). Therefore, strong interspecific competition
within the same tolerable environment should be easily detected,
resulting in spatial association between species at large scales
(< 30 m) due to habitat filtering and dissociation at small
scales (< 5 m) due to interspecific competition (Velázquezet al. , 2015). However, congeneric species may show niche
differentiation due to selective pressures acting on each one of them at
the evolutionary scale, including competition within the same tolerable
environment (Wiegand et al. , 2007). This “ghost of competition
past” results, at present day, in each species specialized in a
different environment (Stubbs and Wilson, 2004; Yamada et al. ,
2005) or in different use of resources within the same tolerable
environment (Schluter, 2000). Alternatively, species’ niche may not be
important, and so the spatial structure of populations should reflect
dispersal limitation and stochasticity (May et al. , 2014).
The present study aims to find evidence for the action and relative
importance of different ecological processes hypothesized to maintain
species coexistence in a tropical forest subject to seasonal flooding,
using the spatial structure of populations of three congeneric species.
Specifically, we ask: (1) which environmental variables are associated
with the spatial distribution of the different size classes of each
species? (2) What is the spatial relationship between congeners in the
same size class? (3) Is the size of an individual influenced by the
proximity to congeners in the same size class? (4) What is the spatial
relationship between small and large conspecific trees? The combination
of different spatial patterns will indicate which ecological processes
are more important to the maintenance of species coexistence (see Table
1).