Quantification of water resource utilization efficiency as the main driver of plant diversity in the water-limited ecosystems

Highlights

• Biodiversity is determined by biological and environmental factors, plant community species adaptation and the difference in water potential.

• The available water resource utilization is the main driver of the increasing number of species as the area of the community increases.

• Changes in water resources resulted maximizing the utilization efficiency of water resources by adjusting the number and abundance of species.

• The main driving force for the increasing number of species is the full utilization of available water resources.

Abstract

Identifying the drivers of species richness is a crucial for biodiversity conservation and community ecology. In this paper, the maximum utilization efficiency of water resources available to community is determined as the assembly law in an ecologically stable community with limited water resources. A plant community diversity model that incorporates water potential impedance is established, and the optimization model of community biodiversity structure was obtained based on water niche is also derived. It is shown that this optimization model can theoretically derive a species-area function similar to the traditional island biogeography curve, and it can also verify the predicted values of the species-area relationship experimentally and validated by long-term observations of plant diversity survey data. According to the model analyses, the total number of individuals (N) and the number of species (S) are determined by the combination of the water resources factors (the total quantity of available water resources in the growth season), the individual biological factors (the water consumption per unit mass of species in unit time), the complementary factors of inter- and intra-species water niche (the difference in water potential depending on the water use efficiency of the community), and plant community function factors (the total productivity of plant community). And as the area of plant communities increases, the heterogeneity of the water environment has increased. By adaptation and coevolution, more species are able to go through environmental and biological filtration, becoming community species that can utilize more available water resources at different spatial scales. Changes in water resources resulted in the community maximizing the utilization efficiency of water resources by adjusting the number and abundance of species. The main driving force for the increasing number of species is the full utilization of available water resources.

Introduction

The community ecology has been developing rapidly with many theories attempting to explain the patterns of species diversity and abundance. The neutral theory, metapopulation and metacommunity theories, generalized fraction, Poisson distribution, and entropy theory and so on have been developed in the past 20 years (Hubbell, 2005; Volkov et al., 2005; McGill, 2010; Violle et al., 2012; Harte and Newman, 2014). These theories start with radically different assumptions, and the theories seem extremely different from each other. To form a model with reasonable hypothesis, verifiable and convincible predictions, further research is still needed.

In essence, the distribution of plant diversity is mainly related to the niches of different species. Obviously, the establishment of a biodiversity niche model from the perspective of resource competition is of great significance to the in-depth analysis of the mechanisms which is the foundation of the formation and maintenance of biodiversity as well as the improvement of prediction accuracy.

The most important and basic resource for plant communities is water (Tilman, 1982; Jobbágy et al., 2011; Silvertown et al., 2015), especially in arid areas where water resources are scarce. Water determines the global distribution pattern vegetation and the vegetation types within a region (Jirka et al., 2009; Jobbágy et al., 2011). At the local scale, water also affects carbon dioxide capture, soil nutrient availability, plant growth and development, microbial activity, and interspecific interactions. Although the hydrological niche of plant communities has been verified by experiments in many studies (Silvertown et al., 2015; García-Baquero et al., 2016; Letten et al., 2015), the current theoretical research literature is insufficient. At present, niche theory based on resource competition fails to explain the diversity of tropical rain forest (Hubbell, 2005).

In water-limited terrestrial ecosystem, with environmental filtration and biological filtration (intra- and inter-specific interactions), plant community species can maximize utilization efficiency of community water resource. At the individual level of the plant, there is a trade-off between relative growth rate (RGR) and water use efficiency (WUE) in different species of a community (Huxman and Venable, 2013). At the population level, each species has its own niche, and there is niche differentiation even for the same resource (Li; et al., 2000;nMcGill, 2010). Niche differentiation is conducive to avoid competition and achieve species coexistence. From the perspective of water resource utilization, this niche differentiation ensures that more available water resources are fully utilized. At the same time, different species also have niche overlapped because of the overlap of their functional traits (Violle et al., 2012). When the niche overlaps, the competition for available water resources within the community is the most intense, which stands for the most efficient use of available water resources. At the community level, available water resources have spatiotemporal changes, and their distribution is fractal distribution (Bird et al., 2000). In order to make full use of available water resources, based on species interactions, the community is surely to obtain characteristics related to fractal characteristics (Harte and Newman, 2014). Although a community cannot evolve directly through genetic change as species, communities evolve by improving the capacity of the entire community to exist (Verboef and Morin, 2010). Therefore, the community eventually realizes the utilization efficiency of available water resources to the maximum extent to attain the status of an ecologically stable community and to match the living environment (Trautz et al., 2017). In this paper, in water-limited terrestrial ecosystem, plant community species can maximize water resource utilization efficiency to the community, including water resource utilization characteristics at the individual and population level, all of which are used as the basis for our model construction.

On the other hand, in the development of plant diversity theory, the central unanswered question is what determines the total number of individuals ($N$) and the number of species ($S$) (McGill, 2010). $S$ and $N$ are always inputs in existing diversity theory. What drives these? Despite McGill calling the unified theory, a theory of biodiversity, in every case the species richness, $S$, and number of individuals, $N$ are inputs to the model rather than predictions (Mutshinda et al., 2009; McGill, 2010; Violle et al., 2012). To date the greatest success in the study of these factors has been empirical (i.e. looking for correlations with environmental variables), where factors like productivity, climate, and altitude seem important. This paper attempts to answer this question based on the theory of the water niche (The main context is shown in Fig. 1).

Based on interspecific and intraspecific interactions in water niche and the maximum utilization efficiency of available water resources in an ecologically stable community, a water niche theoretical model of plant diversity is established and also validated using published survey data. This model we intend to answer in what determine S (the number of species) and N (the total number of individuals) as well as what is the driver of the species-area relationship.