DOI: 10.5061/dryad.jm63xsjp4
title: Shrunk coexistence: Cattle exclusion and nutrient addition intensify competition between native and exotic grasses with low phenological overlap

description: Exotic plants are becoming increasingly common worldwide, often driven by
 ecological factors such as domestic livestock grazing and soil
 fertilization. In temperate grasslands, native C4 and exotic C3 grasses
 are generally expected to coexist due to their distinct temporal niches,
 which should reduce direct competition. However, their differing abilities
 to exploit limited resources or tolerate vertebrate herbivory can create
 fitness imbalances, favoring exotic forage species over native species and
 potentially undermining their long-term coexistence. Few studies have
 explored how seasonally distinct groups of native and exotic plants
 interact, especially under the simultaneous manipulation of biotic
 factors—such as grazing and species interactions—and abiotic factors—such
 as soil nutrients. We conducted short- and long-term manipulative
 experiments in a temperate Pampa grassland in Argentina to investigate how
 herbivore exclusion, fertilization, and their combination influence plant
 invasion by altering interactions between native warm-season (C4) and
 exotic cool-season (C3) grasses. Despite their phenological differences,
 native C4 and exotic C3 grasses interact, and their interactions were weak
 under grazing. However, with herbivore exclusion and nutrient addition,
 interactions shifted towards strong competition, particularly in spring.
 This competition resulted in a slight increase in exotic grass cover over
 natives after two years. However, fourteen years after the onset of
 longterm herbivore exclusion, exotic grasses, particularly Festuca
 arundinacea, had become dominant, suggesting potential risks to the
 coexistence of native and exotic species. Our study uniquely reveals that
 even when niche differentiation is expected to prevent direct competition,
 biotic and abiotic changes can trigger interactions that favour exotic
 species' dominance, challenging assumptions about the stability of
 coexistence between phenologically distinct grasses. 
description: # Shrunk coexistence: Cattle exclusion and nutrient addition intensify
 competition between native and exotic grasses with low phenological
 overlap Dataset DOI: [10.5061/dryad.jm63xsjp4](10.5061/dryad.jm63xsjp4) ##
 Description of the data and file structure The data is contained in a
 single XLSX file. ### Sheet descriptions #### `RII` **Title:** Relative
 Interaction Intensity (RII) for native and exotic grasses **Description:**
 RII values for native and exotic grasses, calculated based on paired plots
 with and without neighbors under combinations of herbivore exclusion and
 fertilization during spring 2017. * Block: 5 blocks labeled 1, 2, 3, 5,
 and 6 * Exclusion: Herbivory (H) and Exclusion (E) * Fertilisation: With
 (+F) and Without (-F) * Season: Spring and Summer * Year: 2017 (spring),
 2018 (summer) ##### Response variables * RII native grasses: Relative
 interaction intensity index to estimate the direction and intensity of
 interactions from exotic grasses to native grasses (see the Methods
 section for more detail) * RII exotic grasses: Relative interaction
 intensity index to estimate the direction and intensity of interactions
 from native grasses to exotic grasses (see the Methods section for more
 detail) #### `Short-term cover` **Title:** Short-term vegetation cover and
 litter **Description:** Percent cover of native and exotic grasses and
 litter in the short-term experiment * Block: 5 blocks labeled 1, 2, 3, 5,
 and 6 * Plot: Numeric identifiers assigned to individual plots *
 Exclusion: Herbivory (H) and Exclusion (E) * Fertilisation: With (+F) and
 Without (-F) * removal trt: removal treatment. In this data set, only
 control, no removal of neighbours (all present), was considered  * Season:
 Spring and Summer * Year: 2017 (spring), 2018 (summer) ##### Response
 variables * Exotic grasses: Cover of exotic grasses (%) * Native grasses:
 Cover of native grasses (%) * Litter: Cover of litter (%) #### `Long-term
 cover` **Title:** Long-term vegetation cover and litter **Description:**
 Percent cover of native and exotic grasses and litter recorded for
 long-term experiment * Block: 1 to 6 blocks * Plot: Numeric identifiers
 assigned to individual plots * Exclusion: Herbivory (H) and Exclusion (E)
 * Ferti: With (+F) and Without (-F) * Season: Spring and Summer * Year:
 2017 (spring), 2018 (summer) ##### Response variables * Exotic grasses:
 Cover of exotic grasses (%) * Native grasses: Cover of native grasses (%)
 * Litter: Cover of litter (%) #### `Richness short-term` **Title:**
 Short-term species richness **Description:** Number of native and exotic
 grass species recorded at the short-term experiment * Block: 5 blocks
 labeled 1, 2, 3, 5, and 6 * Plot: Numeric identifiers assigned to
 individual plots * Exclusion: Herbivory (H) and Exclusion (E) *
 Fertilisation: With (+F) and Without (-F) * removal trt: removal
 treatment. In this data set, only control, no removal of neighbours (all
 present), was considered  * Season: Spring and Summer * Year: 2017
 (spring), 2018 (summer) ##### Response variables * Richness native
 grasses: number of native grass species * Richness exotic grasses: number
 of exotic grass species #### `Richness long-term` **Title:** Long-term
 species richness **Description:** Species richness of native and exotic
 grasses from long-term plots * Block: 1 to 6 blocks * Plot: Numeric
 identifiers assigned to individual plots * Exclusion: Herbivory (H) and
 Exclusion (E) * Fertilisation: With (+F) and Without (-F) * Season: Spring
 and Summer * Year: 2017 (spring), 2018 (summer) ##### Response variables *
 Richness native grasses: number of native grass species * Richness exotic
 grasses: number of exotic grass species #### `Figure 2S` **Title:** Data
 for Supplementary Figure 2S – Grass cover with and without neighbors
 **Description:** Cover values of native and exotic grasses from the
 neighbor removal experiment used to generate Supplementary Figure 2S.
 **Missing data:** Empty cells have been replaced with `n/a` to indicate
 that no measurements were taken because the corresponding species group
 (e.g., exotic grasses) was not present under that treatment combination.
 These are not missing values due to sampling error, but rather reflect the
 fact that the group was experimentally excluded or did not occur. * Block:
 5 blocks labeled 1, 2, 3, 5, and 6 * Plot: numbers assigned to identify
 plots * Exclusion: Herbivory (H) and Exclusion (E) * Fertilisation: With
 (+F) and Without (-F) * removal trt: removal treatment. Without native
 species, all present, without exotic species (see methodological design in
 the Methods section for more detail) * Neighbours: Categorization of the
 removal treatments into two groups – "With" neighbours and
 "without" neighbours * Season: Spring and Summer * Year: 2017
 (spring), 2018 (summer) ##### Response variables * Exotic grasses: Cover
 of exotic grasses (%) * Native grasses: Cover of native grasses (%) ####
 `PAR` **Title:** Photosynthetically Active Radiation (PAR)
 **Description:** Percentage of PAR reaching the soil surface under
 different treatment combinations, used to assess light availability. *
 Block: 5 blocks labelled 1, 2, 3, 5, and 6 * Plot: Numeric identifiers
 assigned to individual plots * removal trt: removal treatment. In this
 data set, only control, no removal of neighbours (all present), was
 considered  * Exclusion: Herbivory (H) and Exclusion (E) * Fertilisation:
 With (+F) and Without (-F) ##### Response variable PAR (%): Percentage of
 Photosynthetically Active Radiation reaching the soil surface ####
 `Species Frequency` **Title:** Species frequency **Description:**
 Frequency of occurrence (%) of native and exotic grass species recorded in
 both short- and long-term experiments under different treatment
 combinations. * Term: Indicates the duration of the experiment –
 "Short" or "Long" * Exclusion: Herbivory treatment –
 Herbivory (H) and Herbivore Exclusion (E) * Fertilisation: Nutrient
 treatment – With fertilization (+F) and Without fertilization (–F) * Year:
 2017 (spring) and 2018 (summer) * Season: Spring and Summer * Grasses:
 Grass type – native or exotic * Species: Scientific name of the species
 ##### Response variable: Frequency: Frequency of occurrence (%) of each
 species in the experiment. Measured across 6 blocks for the long-term
 experiment and 5 blocks for the short-term experiment.