Discussion
Multiple conceptual pillars of community ecology, from spatial and
temporal species accumulation curves to the diversity-productivity
relationship, are built from patterns of species richness
(Ugland et
al. 2003; van Ruijven & Berendse 2005; Ulrich 2006). Patterns of
diversity across landscapes are commonly evoked to infer assembly
mechanisms (Scheiner 2003)
and are used to inform conservation and restoration practices
(Fleishman et al.2006). Here, we find that for both populations and the aggregate
community, species occurrence is strongly mismatched from species
persistence across a productivity gradient, emphasizing that occupancy
patterns are a poor predictor of where conditions are suitable for
maintaining biodiversity. Critically, by comparing occurrence patterns
to persistence of transplanted seeds under conditions with and without
biotic interactions, we disentangled the impacts of environmental
filtering, interactions between neighbors, and dispersal on community
assembly and diversity patterns
(Vellend 2010;
HilleRisLambers et al. 2012). Below, we discuss how our results
bridge from patterns to processes, advancing our understanding of the
mechanisms structuring the diversity-productivity relationship by
integrating niche, range limit, metacommunity, and stress-gradient
theories.
Species’ distributions across the productivity gradient largely
supported literature on the diversity-productivity relationship. In our
study, occupancy patterns of the aggregate community show a linear
increase with productivity, rather than the also commonly observed
unimodal, hump-shaped relationship
(Mittelbachet al. 2001; Pärtel & Zobel 2007; Fraser et al. 2015).
Given that our experiment is representative of a smaller area on the
harsher side of the global gradient, we may expect to see the linear
slope descend into a hump-shaped curve if surveyed at a larger spatial
scale (Venail et
al. 2010; Germain et al. 2019). Critically, our observed
diversity-productivity relationship is driven primarily by sink
populations, yielding a mismatch between species’ distributions,
realized niches, and fundamental niches
(Pulliam 2000;
Chase & Leibold 2009; Carscadden et al. 2020).
While environmental filtering structured species’ fundamental niches at
the species level (Fig. 1; blue lines), it exhibited a relatively weak
effect on community diversity (Fig. 3; purple line), despite being
evoked as a seminal process structuring populations and communities in
heterogeneous landscapes
(Laliberté et al.2014; Hallett et al. 2019). Both occupancy and persistence
without neighbors was highest in productive ends of the gradient for
three quarters of species, implying that species distributions commonly
matched their fundamental niches. However, community occupancy patterns
showed a weaker link to persistence without biotic interactions (Fig.
3), with persistence increasing only slightly with productivity. This
suggests other mechanisms were dominant in driving community diversity,
thus countering previous literature showing environmental filtering as
the predominant mechanism shaping diversity patterns
(Laliberté et al.2014; Le Bagousse-Pinguet et al. 2017).
Species interactions promoted strong mismatches between persistence and
occupancy for both the species-level and community. At the
species-level, occupancy was consistently highest in productive
environments while persistence with neighbors tended to be maximized in
harsher environments (Fig. 1 B-C, F-G). Unexpectedly, we found that
distributions matched species’ fundamental niche more closely than the
realized niche. One explanation is that seeds deposited in a
neighborhood that had a suitable community of neighbors in the maternal
generation may be unlikely to experience a similarly suitable community
of neighbors in the current generation
(Germain et al.2022). Neighborhood stochasticity would be especially pronounced in
annual systems with high turnover and with high spatiotemporal
variability, like California grasslands. More generally, our results
comparing species persistence with and without neighbors corroborates
range limit theory: in harsh environments abiotic conditions determine
persistence, whereas in productive species interactions set boundaries
on persistence (Grinnell
1917; Louthan et al. 2015; Allbee et al. 2023). Yet, forBromus and Plantago, positive interactions among species
promoted persistence in harsh environments, thus potentially extending
species’ ranges (Stephanet al. 2021). Scaling to the community, these differences in
species interactions across productivity lend strong support to the
stress-gradient hypothesis. Biotic interactions shift from positive in
harsh areas to competitive in more productive environments, leading to
strong differences in persistence under environmental filtering alone
versus when considering environment by species interaction effects (Fig.
3).
The gaps between patterns of species distributions versus persistence
with neighbors points to the dominant role of dispersal in driving
population and community dynamics. We found collective effects between
dispersal, species interactions, and environment, emphasizing that
processes structuring communities are interactive rather than sequential
(Kraft et al.2015; Cadotte & Tucker 2017). At the species level, mismatches between
occupancy and persistence with neighbors (Fig. 1) were driven by an
excess of dispersal, yielding sink populations, especially in productive
areas. Given this mismatch, our results support niche theories that
allow realized niches to differ in distribution from, or even exceed,
the fundamental niche
(Pulliam 2000;
Louthan et al. 2015; Carscadden et al. 2020). Further,
the strong role of excess dispersal underscores the operational value of
distinctly defining species’ realized niche vs distributions
(Soberón & Nakamura 2009).
At the community scale, we find that in harsh areas, species tend to be
aligned to their environment (Fig. 4). However, in more productive
environments, sink populations dominate—especially when species
interactions further restrict persistence. These results likely depend
on the scale of environmental heterogeneity; for example, Pinto et. al.
(2010) found that sinks were
found in fine scales but not at coarse scales. Considering seedbanks as
a means of dispersal through time could elucidate further links between
dispersal, biotic interactions and environmental filtering along
gradients (Wisnoski &
Shoemaker 2022). This study reveals the importance of considering a
metacommunity perspective, which emphasizes dispersal as a critical
driver of diversity patterns, when making inferences from observational
data. We identified a need for a more nuanced metacommunity theory
(Leibold et
al. 2004; Leibold & Chase 2017; Thompson et al. 2020)
including the interplay between environmental conditions and propagule
pressure (Allbee et
al. 2023).
Overall, our results emphasize the benefits of linking species to
community patterns and contradicts the assumption that persistence
patterns can be approximated via occupancy. While many methods attempt
to statistically link species occurrence patterns to underlying
mechanisms of dispersal, biotic interactions, and environmental
filtering, (Kraftet al. 2011; Leibold et al. 2022) inferring patterns from
processes is inherently fraught
(Tucker et al.2016; Blanchet et al. 2020). Rather, by empirically measuring
occupancy and persistence under abiotic and biotic environments where we
manually dispersed seeds, we directly quantified the mechanisms
structuring populations and communities, and in doing so integrate
several theories in ecology. We suggest that future work in annual plant
systems compare multi-year dynamics to disentangle spatial and temporal
assembly mechanisms, especially given the hypothesized role of
environmental fluctuations in maintaining coexistence
(Hallett et al.2019; Van Dyke et al. 2022). Extending these methods across
larger gradients could inform species distribution modeling under future
climate scenarios (Buckleyet al. 2010). Similarly, we envision these methods informing
long-term success in restoration practices and the potential need for
active management of competitors or regular seed addition
(Shackelford et
al. 2021; Aoyama et al. 2022). Our results provide a cautionary
tale of inferring mechanisms that structure populations and communities,
or in forecasting future species distributions or diversity, based
solely on current occupancy patterns.