Abstract
BackgroundEcologists have achieved much progress in the study of mechanisms that maintain species coexistence and diversity. In this paper, we reviewed a wide range of past research related to these topics, focusing on five theoretical bodies: (1) coexistence by niche differentiation, (2) coexistence without niche differentiation, (3) coexistence along environmental stress gradients, (4) coexistence under non-equilibrium versus equilibrium conditions, and (5) modern perspectives.ResultsFrom the review, we identified that there are few models that can be generally and confidently applicable to different ecological systems. This problem arises mainly because most theories have not been substantiated by enough empirical research based on field data to test various coexistence hypotheses at different spatial scales. We also found that little is still known about the mechanisms of species coexistence under harsh environmental conditions. This is because most previous models treat disturbance as a key factor shaping community structure, but they do not explicitly deal with stressful systems with non-lethal conditions. We evaluated the mainstream ideas of niche differentiation and stochasticity for the coexistence of plant species across salt marsh creeks in southwestern Denmark. The results showed that diversity indices, such as Shannon–Wiener diversity, richness, and evenness, decreased with increasing surface elevation and increased with increasing niche overlap and niche breadth. The two niche parameters linearly decreased with increasing elevation. These findings imply a substantial influence of an equalizing mechanism that reduces differences in relative fitness among species in the highly stressful environments of the marsh. We propose that species evenness increases under very harsh conditions if the associated stress is not lethal. Finally, we present a conceptual model of patterns related to the level of environmental stress and niche characteristics along a microhabitat gradient (i.e., surface elevation).ConclusionsThe ecology of stressful systems with non-lethal conditions will be increasingly important as ongoing global-scale climate change extends the period of chronic stresses that are not necessarily fatal to inhabiting plants. We recommend that more ecologists continue this line of research.
Highlights
Ecologists have achieved much progress in the study of mechanisms that maintain species coexistence and diversity
We evaluated the relationship between these three diversity indices and surface elevation using a simple regression analysis to examine whether there was an overall trend in species diversity along environmental stress gradients and, in particular, whether the highest diversity was observed with the intermediatelevel submergence stress
Spartina showed some degree of occurrence probability (> 10%) in groups A and B, but it was extremely minor in groups with higher surface elevations
Summary
Ecologists have achieved much progress in the study of mechanisms that maintain species coexistence and diversity. General overview Understanding the mechanisms that maintain species coexistence has long been one of the fundamental goals in plant community ecology (Adler et al 2007; Chase and Myers 2011; Weiher et al 2011; Tuomisto et al 2012; Fukami 2015). We are broadly concerned with patterns and mechanisms of plant species coexistence under very harsh environmental conditions in a coastal salt marsh system. Harsh conditions can drive coexistence-related processes, such as biological interactions (e.g., predation, competition, and facilitation), ecological succession, dispersal, speciation, and resource partitioning. In this time of rapid climate change, many ecosystems are experiencing profound alterations to ambient physical conditions, which can increase the influences of harshness on inhabiting organisms. We propose that it will become increasingly important to understand the mechanisms of species coexistence under highly stressful conditions in order to fully understand plant community structure and function, and to develop appropriate management strategies (Callaway et al 2002; Kelly and Goulden 2008; Anderegg et al 2013; Walter et al 2013)
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