Evolution of rarity and phylogeny determine above- and belowground biomass in plant-plant interactions

Abstract

Rare species are often considered inferior competitors due to occupancy of small ranges, specific habitats, and small local populations. However, the phylogenetic relatedness and rarity level (level 1-7 and common) of interacting species in plant-plant interactions are not often considered when predicting the response of rare plants in a biotic context. We used a common garden of 25 species of Tasmanian Eucalyptus, to differentiate non-additive patterns in the biomass of rare versus common species when grown in mixtures varying in phylogenetic relatedness and rarity. We demonstrate that rare species maintain progressively positive non-additive responses in biomass when interacting with phylogenetically intermediate, less rare and common species. This trend is not reflected in common species that out-performed in monocultures compared to mixtures. These results offer predictability as to how rare species' productivity will respond within various plant-plant interactions. However, species-specific interactions, such as those involving E. globulus, yielded a 97% increase in biomass compared to other species-specific interaction outcomes. These results are important because they suggest that plant rarity may also be shaped by biotic interactions, in addition to the known environmental and population factors normally used to describe rarity. Rare species may utilize potentially facilitative interactions with phylogenetically intermediate and common species to escape the effects of limiting similarity. Biotically mediated increases in rare plant biomass may have subsequent effects on the competitive ability and geographic occurrence of rare species, allowing rare species to persist at low abundance across plant communities. Through the consideration of species rarity and evolutionary history, we can more accurately predict plant-plant interaction dynamics to preserve unique ecosystem functions and fundamentally challenge what it means to be "rare".

Fig 1. Classification of rank ordered rarity levels using geographic range, habitat specificity, and local population size [14].

Box size provides an abstract representation of geographic occurrence ranging from a highly specialized rarity level 1 to common. Twenty-five species of Tasmanian Eucalyptus were categorized into rarity levels based on range size, habitat specificity (ratio of bioregions inhabited in Tasmania, Australia), and local population aggregation (representative of local population size) [19].

Fig 2. Comparative boxplots demonstrating the standardized interaction strengths of target species in pairings varying in target rarity level and phylogenetic dissimilarity.

Positive standardized interaction strengths represent synergistic non-additivity in total biomass (species-specific pairing outperformed biomass expectation in respective monocultures) and negative standardized interaction strengths represent antagonistic non-additivity in total biomass (species-specific pairing underperformed biomass expectation in respective monocultures). Phylogenetic pairings that are 0–25% related are phylogenetically similar, 25–50% related are phylogenetically intermediate, and 50–100% related are phylogenetically dissimilar.

Fig 3. Heat map demonstrating the average standardized interaction strength (of total biomass) of each species pairing across all target rarity levels on the basis of interacting rarity level and percent phylogenetic dissimilarity.

Positive interaction strengths representative of synergistic non-additivity in total biomass are represented in shades of blue. Negative interaction strengths representative of antagonistic non-additivity in total biomass are represented in shades of red. Phylogenetic pairings that are 0–25% related are phylogenetically similar, 25–50% related are phylogenetically intermediate, and 50–100% related are phylogenetically dissimilar.

Fig 4. Heat map demonstrating the average standardized interaction strength (of total biomass) of species-specific pairings.

Positive standardized interaction strengths represent synergistic non-additivity in total biomass (species-specific pairing outperformed biomass expectation in respective monocultures) and negative standardized interaction strengths represent antagonistic non-additivity in total biomass (species-specific pairing underperformed biomass expectation in respective monocultures). The rarity level of each species is displayed using brackets on the y-axis.