Soil phosphorus availability determines the contribution of small, individual grassland remnants to the conservation of landscape‐scale biodiversity

Abstract

Questions Small, remnant habitats embedded in degraded, human‐dominated landscapes are generally not a priority in conservation, despite their potential role in supporting landscape‐scale biodiversity. To warrant their inclusion in conservation management and policy, we question under which conditions they may exhibit the largest conservation value. Location Nine landscapes spread across the counties of Stockholm and Södermanland, Sweden. Methods Per landscape, plant communities were surveyed in 6 and 12 1 × 1 m2 plots across large, intact semi‐natural grasslands and small remnant grasslands, respectively. These two contrasting grassland types served as a model system. A topsoil sample was taken in each plot to determine habitat quality in terms of soil pH, plant‐available P, and C:N ratio. We used a joint species distribution model to analyse the extent to which grassland type and habitat quality define and predict resident community diversity and composition, including whether they support grassland specialists. Results At the landscape scale, the combined remnant grasslands sustained diverse plant communities which did include a significant subset of habitat specialists. Yet, the contribution of individual remnants clearly varied with local‐scale habitat quality; soil phosphorus availability lowered plot‐level species richness, mostly by constraining the occurrence of grassland specialists. Semi‐natural grassland communities were comparatively insensitive to variation in soil phosphorus availability. Conclusions The combined habitat amount and the significant number of habitat specialists sustained by remnant grasslands with high habitat quality, shows they can represent a valuable resource to support landscape‐scale biodiversity conservation. This offers no wildcard to neglect the continued biotic and abiotic threats on semi‐natural grassland plant diversity such as chronic and accumulating P eutrophication, discontinuation of management or poor matrix permeability, as semi‐natural grasslands harbour the majority of habitat specialists, while sourcing surrounding remnant grassland communities.

Ecosystem multifunctionality lowers as grasslands under restoration approach their target habitat type

“…However, we should also note that we also found high plant species diversity at higher levels of bioavailable phosphorus, suggesting that management (e.g. mowing dates, sowing of species) may play a key role for restoring more biodiverse communities (Plue & Baeten 2021). Low soil phosphorus levels might thus not be a prerequisite when there are no specific sets of target species set required and increasing species richness is the main goal.…”

Leaf trait variation in grassland plant species in response to soil phosphorus

“…Our results thus suggest that the distribution of these species along gradients in soil phosphorus is not necessarily because of inherent differences in species' ecological strategies linked to nutrient use. These findings provide perspectives for management practices focusing on the conservation and establishment of conservative species under high phosphorus levels (see also Plue & Baeten, 2021). Slow-growing conservative species can possibly handle excess soil phosphorus availability, at least in absence of dominance by fast-growing acquisitive species.…”

“…If we want to study changes in leaf trait expressions in relation to soil fertility within a species and we want to know whether these responses are mediated by the species’ resource strategy (acquisitive vs conservative species), we need dedicated experiments in which species are grown along the entire nutrient gradient. Indeed, in real‐world vegetation, the species composition of grassland communities varies along a gradient in soil phosphorus availability for multiple and not mutually exclusive reasons (Hautier et al., 2009; Plue & Baeten, 2021; DeCock et al., 2022).…”

Win some, lose some: Mesocosm communities maintain community productivity despite lower phosphorus availability because of increased species diversity

“…Indeed, a larger number of species can co‐exist under nutrient limitation: when total above‐ground biomass production is reduced, more slow‐growing species can establish and grow due to the lower dominance of fast‐growing species, which would otherwise outcompete subordinate species via light competition (Marrs, 1993; Walker et al, 2004; Harpole & Tilman, 2007; Hautier et al, 2009). While excess nitrogen is known to be an important driver for plant species loss in grasslands, multiple studies suggest that bioavailable phosphorus and the co‐limitation of nitrogen and phosphorus might be at least as important in maintaining high‐diversity semi‐natural grasslands, especially for the establishment and persistence of many specialist and endangered species (Tilman et al, 1996; Stevens et al, 2004; Wassen et al, 2005; De Schrijver et al, 2011; Ceulemans et al, 2013; Ceulemans et al, 2014; Plue & Baeten, 2021).…”

“…While excess nitrogen is known to be an important driver for plant species loss in grasslands, multiple studies suggest that bioavailable phosphorus and the co-limitation of nitrogen and phosphorus might be at least as important in maintaining high-diversity semi-natural grasslands, especially for the establishment and persistence of many specialist and endangered species (Tilman et al, 1996;Stevens et al, 2004;Wassen et al, 2005;De Schrijver et al, 2011;Ceulemans et al, 2013;Ceulemans et al, 2014;Plue & Baeten, 2021).…”