DOI: 10.5061/dryad.2ngf1vhs9
title: Nutrient addition drives declines in grassland species richness primarily via enhanced species loss

description: Declines in grassland diversity in response to nutrient addition are a
 general consequence of global change. This decline in species richness may
 be driven by multiple underlying processes operating at different
 timescales. Nutrient addition can reduce diversity by enhancing the rate
 of local extinction via competitive exclusion, or by reducing the rate of
 colonization by constraining the pool of species able to colonize under
 new conditions. Partitioning net change into extinction and colonization
 rates will better delineate the long-term effect of global change in
 grasslands. We synthesized changes in richness in response to experimental
 fertilization with nitrogen, phosphorus, and potassium with micronutrients
 across 30 grasslands. We quantified changes in local richness,
 colonization, and extinction over 8-10 years of nutrient addition, and
 compared these rates against control conditions to isolate the effect of
 nutrient addition from background dynamics. Total richness at steady state
 in the control plots was the sum of equal, relatively high rates of local
 colonization and extinction. On aggregate, 30-35% of initial species were
 lost and the same proportion of new species was gained at least once over
 a decade. Absolute turnover increased with site-level richness but was
 proportionately greater at lower-richness sites relative to starting
 richness. Loss of total richness with nutrient addition, especially N in
 combination with P or K, was driven by enhanced rates of extinction with a
 smaller contribution from reduced colonization. Enhanced extinction and
 reduced colonization were disproportionately among native species,
 perennials, and forbs. Reduced colonization plateaued after the first few
 (&lt; 5) years after nutrient addition, while enhanced extinction
 continued throughout the first decade. Synthesis: Our results indicate a
 high rate of colonizations and extinctions underlying the richness of
 ambient communities, and that nutrient enhancement drives overall declines
 in diversity primarily by the exclusion of previously established species.
 Moreover, enhanced extinction continues over long time scales, suggesting
 continuous, long-term community responses and a need for long-term study
 to fully realize the extinction impact of increased nutrients on grassland
 composition.
description: The Nutrient Network (NutNet) is
 a globally distributed experiment replicated across short-statured,
 primarily herbaceous ecosystems (hereafter called grasslands) on six
 continents. Most sites with
 experimental treatments contain three replicated treatment blocks (21 of
 30 sites; range 1 - 6 blocks per site). Within each block, eight different
 nutrient addition treatments are applied to 5 x 5 m plots. Thus, most
 sites have a total of 24 experimental units (3 blocks x 8 nutrient
 treatments; Borer, Harpole,
 et al., 2014). These nutrient
 addition treatments represent the factorial combinations of nitrogen (N),
 phosphorus (P), and potassium and micronutrients
 (K<sup>+</sup>),
 plus a control. Nutrient addition rates are: 10 g N
 m<sup>-2</sup> yr<sup>-1</sup> as timed-release
 urea, 10 g P m<sup>-2</sup> yr<sup>-1</sup> as
 triple superphosphate, 10 g K m<sup>-2</sup>
 yr<sup>-1</sup> as potassium sulfate, and 100 g
 m<sup>-2</sup> of a micronutrient mix (6% Ca, 3% Mg, 12% S,
 0.1% B, 1% Cu, 17% Fe, 2.5% Mn, 0.05% Mo, and 1% Zn). N, P, and K are
 applied annually, while the micronutrient mix was applied only once at the
 start of the study to prevent toxicity of largely immobile micronutrients.
 Treatments are hereafter referred to shorthand by their nutrient
 components e.g., ‘NK’ for Nitrogen + Potassium and micronutrients, ‘NPK’
 for full nutrient additions. All present species are identified in each 5
 x 5 m plot every year at the time of peak biomass, or twice annually in
 highly seasonal sites. For the current study, we only selected sites with
 at least eight years of treatment data (n = 30 sites) to capture
 longer-term dynamics in species turnover, as well as control for otherwise
 uneven sample sizes at disparate time intervals.