Discussion
The Janzen-Connell hypothesis postulates that coexistence in plant
communities is maintained by specialized natural enemies that reduce the
density of offspring close to conspecific adults (or in high density
clumps). Most tests of this hypothesis are indirect without accounting
for the plant-associated organisms that drive these patterns (Comita et
al., 2014) or are focused on highly host-specific organisms and single
plant-pathogen systems following the approach of Augspurger and Kelly
(1984). However, little is known about how the entire community of these
organisms shapes the spatial structure and assembly of plant
communities, and more specifically, how the spatial pattern of these
organisms changes during the ontogeny of the plant species and depending
on the plants’ seed dispersal mechanism.
Our unique data set on the communities of insect and fungi species
inhabiting the leaves of woody plant species in two Mediterranean
forests showed that a substantial proportion of organisms is hosted by
several plant species. We also found systematic differences between our
study forest plots, with organisms at MFJ being more “host-specific”
than organisms at MFS. Additionally, our spatial analysis revealed
several non-random patterns in the number of plant-associated organisms
shared by saplings with their sapling and adult neighbours, and by
adults with their adult neighbours. These patterns differed strongly
between fleshy-fruited and dry-fruited species and, in most cases, were
consistent with expectations for the occurrence or non-occurrence of
Janzen-Connell effects in the two seed dispersal mechanism. Our results
provide deep insight into how the neighbourhood load of pathogens,
epiphytes, and herbivorous insects of the phyllosphere changes during
the ontogeny of plant species in Mediterranean forests, and how the
resulting spatial arrangements of interactions are built through
Janzen-Connell effects and seed dispersal mechanisms. We provide novel
evidence about the importance of generalist antagonists and mutualists
as drivers of the spatial assembly of plant communities (Sutherland et
al., 2013).
Consequences of seedscape and
dispersal limitation
Dynamics of fleshy-fruited species
at early
life-stages
The spatial template created by different seed dispersal mechanisms has
consequences for subsequent processes and plant fitness (Howe, 1993;
Schupp et al., 2010; Van Leeuwen et al., 2022). Fleshy-fruited species
disperse most of their seed by means of frugivorous birds that create a
spatial template where seeds are usually located far from the mother
plants and in low densities (Howe & Smallwood, 1982; Beckman &
Sullivan, 2023). Additionally, frugivores disperse seeds of several
fleshy-fruited species together and/or place them often under adults of
heterospecific fleshy-fruited species (Jordano 2014; Perea et al., 2021;
Verdú & García-Fayos 1996). Such a pattern was found at both
communities, where saplings were on average surrounded up to 5m by
~75% of heterospecifics saplings and by
~85% of heterospecifics adults.
Recruitment in neighbourhoods dominated by heterospecifics supresses two
parallel mechanisms (density and distance dependency) with antagonists
as actors. First, the low density of seeds in highly heterospecific
clumps impedes the action of density-dependent mortality agents,
allowing the seeding to emerge and get established as sapling (Beckman,
2012; Beckman & Rogers, 2013). We found evidence for this mechanism in
our sapling-sapling analysis that showed that saplings in the
neighbourhood of saplings did not differ in their loading of antagonists
from the null model. Second, frugivore-mediated seed dispersal
transports the seeds far from the main antagonist’s sources (i.e. the
mother plants), thereby increasing the chance of seed survival and
establishment (Howe & Smallwood, 1982). We found evidence for this
mechanism in our sapling-adult analysis that showed that adults in the
neighbourhood of saplings did not differ in their loading of antagonists
from that of randomly placed neighbourhoods.
These two mechanisms are referred to in the literature as distance- and
density-dependent processes (Song et al., 2021) and are tested mostly by
analysing mortality of early life stages in high density patches or
close to conspecifics adults (Janzen-Connell effects) (Comita et al.,
2014). In contrast, our evidence is based on the conservation of the
spatial template created by frugivorous seed dispersal up to the
recruitment stage of the plant species (Perea et al., 2021). This
allowed us to confirm the seed escape hypothesis proposed by Howe
(1993), since the neighbourhoods of fleshy fruited saplings did not
differ from that of randomly selected neighbourhoods (i.e., no
significant patterns). However, to fully confirm this hypothesis, we
also analysed the dynamics of dry-fruited species that show limited seed
dispersal.
Dynamics of dry-fruited species at
early life-stages
Seeds of dry-fruited species show usually limited dispersal, leading to
strong aggregation of early life stages (Martínez & González-Taboada,
2009) and providing optimal conditions for density-dependent antagonists
to proliferate (Howe, 1989). We observed still a high aggregation of
saplings (~50% of its neighbours within 5 m were
conspecifics; Fig. 3b, d), but this figure reflects most likely
density-dependent mortality at earlier stages caused by antagonist
because the surviving saplings shared fewer than expected antagonists
with their sapling neighbours. Indeed, the proportion of conspecific
adult neighbours of saplings was substantially lower than that of
conspecific sapling neighbours (between 10 and 30%; Fig. 3f, h), which
suggests that spill-over of antagonists from conspecific adults leads to
high seed and seedling mortality. In sum, these footprints reflect the
density- and distance-dependent Janzen-Connell effects at both studied
communities (Bagchi et al., 2014; Prittinen et al., 2003).
Mutualist organisms such as epiphytes (Pajares-Murgó et al., 2022) are
expected to enhance species fitness (counteracting the effect of
antagonists), but we did not detect footprints of their effects in the
sapling-sapling analysis. This suggests that antagonists developed
quicker than mutualists in dense seed and seedling clumps (i.e., a
priority effect), as reported in other ecological systems (e.g., see
Rodriguez-Rodriguez et al., 2015). However, we could identify
significant effects of epiphytes in the sapling-adult analysis, but in
this case the patterns of both communities were different.
Interestingly, saplings shared more than expected epiphytes (i.e.
mutualists; Fig. 3h) and more than expected pathogens with their adult
neighbours (i.e. antagonist; Fig. 3g). The latter contradicts the
hypothesis of the seed escape and our expectations (Beckman & Sullivan
2023), but the pattern of epiphytes was stronger than the pattern of
pathogens. This indicates an overall positive balance in the effect of
both guilds, as reported in other systems (i.e. soil microbiota -
plant-mycorrhiza, Perea et al., 2023). The facilitative effect of
epiphytes that weaken Janzen-Connell effects can also explain why
saplings at the MFS plot did not show on the first meters a higher than
expected proportion of heterospecifics adults (Fig. 4h), as found at the
MFJ plot (Fig. 4f) where we did not find facilitative effects of leaf
epiphytes.
It remains to explain why facilitative effects emerged at the MFS plot,
but not at the MFJ plot. Both forest plots show contrasting species
richness and abundance distributions, with MFS owning a lower species
richness (17 vs. 29), but a more balanced abundance distribution with
four codominant species, whereas dry-fruited Quercus fagineadominates the MFJ plot with 44% of all individuals. Theory suggests
that Janzen-Connell effects should be stronger for more abundant species
in species-richer communities, a pattern matched at the MFJ plot (Figure
S3). Thus, the effects of antagonists may outperform that of mutualists
at the MFJ plot, but not at the MFS plot where Janzen-Connell effects
are weaker (Liang et al., 2015; Zahra et al., 2021).
Overall, our results support the models proposed by Beckman et al.,
(2012), and provide new evidence on how seed dispersal and recruit
establishment may determine community dynamics (Connell 1971; Janzen
1970). Indeed, seed dispersal matters (Howe & Miriti, 2000), and so
does the seedscape, the location where seeds are defecated or
regurgitated (Beckman & Rogers 2013; Beckman & Sullivan, 2023; Wenny,
2000;).
Consequences of “seedscape” and
recruitment dynamics for community
assembly
The spatial patterns found in the adult-adult analysis were consistent
between forest plots and depended on the seed dispersal mechanism.
Adults of fleshy-fruited species were surrounded by a higher than
expected proportion of heterospecifics and by fewer than expected
antagonists, whereas no such patterns were found for dry-fruited
species. Thus, interestingly, we found reversed patterns compared to the
sapling-sapling analysis. While adults of fleshy-fruited species remain
at “safe” neighbourhoods characterized by fewer than expected
antagonists, adults of dry-fruited species remain at relatively safe
neighbourhoods that do not differ in their antagonist load from that of
random locations in the plot. Thus, both guilds managed to escape at the
adult stage the detrimental effect of a high load of antagonist
organisms.