In many invaded grasslands, dominant exotic species can produce large amounts of litter that modify local abiotic conditions and species’ interactions. These novel conditions can reduce native species abundance and promote the persistence of exotic species, yet the strength of this disparity may be influenced by how consumer pressure interacts with litter accumulation. Consumers may exacerbate this disparity by preferentially targeting native species or by promoting heterogeneity in microhabitats due to their movement and small-scale ground disturbances that favours fast growing exotic species. How species respond to litter accumulation and consumer pressure may depend on either evolutionary differences, whereby exotics species may benefit from a lack natural predators, or by functional differences, in which species’ physiological traits may confer fitness advantages to low-light conditions or herbivory or granivory pressure.

We examined the impact of litter presence and small mammal herbivory on the establishment and reproduction of functionally diverse exotic versus native species seeded across sites that naturally vary in resource availability in an annual invaded California grassland. We assessed whether seed mass and leaf nitrogen content (LNC) were predictive of successful establishment and reproduction.

Litter accumulation affected exotic and native species differently, with litter significantly decreasing native recruitment and reproduction, while exotics were largely unaffected. Small mammals had a slight positive effect on the establishment of native species when litter was present but did not influence exotic species. Regardless of species provenance, larger seeded species established at a higher density while species with lower leaf nitrogen content had a higher density of reproductive individuals. Native species that successfully established and reproduced were functionally different in LNC than the resident community, while successful exotic species were functionally more similar to the resident community in LNC.

Our study demonstrates that exotic species outperformed native species regardless of the presence of litter or herbivory pressure. Without the removal or thinning of litter, it is likely that exotic species will continue to dominate, resulting in a positive feedback that further favours the persistence of exotic species within this invaded grassland system.

Study site

This study was conducted in annual grasslands at the University of California Sierra Foothill Research and Extension Center (SFREC), located in Browns Valley, California, USA (39º 15' N, 121º 17' W). The site has a Mediterranean climate, with a cool wet growing season (September – May) and host dry summers (June – August). Temperature and precipitation over the course of the experiment varied over the three growing seasons: 2014-2015, 2015-2016 and 2016-2017, with 454mm (mean 15.5 ο C), 625mm (mean 14.9 ο C) and 978mm (mean 14 ο C) recorded, respectively (PRISM Climate Group 2004). Vegetation at the site is dominated by exotic annual species, with low abundances of native annual species. The most abundant exotic species, Elymus caput-medusae, produces large amounts of litter, which remains on the landscape due to slow decomposition and low palatability (Nafus and Davies 2014). Other dominant exotics species include: Avena barbata, Bromus hordeaceus and Erodium botrys (nomenclature follows Baldwin et al. 2012). Plant consumers in this system include mule deer (Odocoileus hemionus) and small mammals such as field voles (Microtus californicus) and gophers (Thomomys bottae) (Block and Morrison 1990).

Experimental design

To assess the influence of litter, small mammals and resources on native and exotic species recruitment and reproduction, in the summer of 2014 we established experiments at eight sites at either end of a productivity/soil resource gradient. Within each site, we either excluded rodents or not within paired 9m x 9m plots, one of which was fenced to exclude small mammals and the other which remained unfenced to allow small mammal access. Within each pair of plots, we established 16 subplots (0.5m x 0.5m) that were randomly assigned to one of eight unique treatments. The treatments represent a factorial combination of: 1) litter treatment (removed or left intact), 2) seed addition (yes or no) and 3) Origin of species in seed mix (native or exotic). The first set of of eight subplots were seeded in each site in summer 2014 for a total of 64 subplots. The second set of eight subplots were seeded in the summer of 2015 and provide a replicate over time (total of 128 subplots sampled over 2 years).

Plant traits

To evaluate species responses based on functional traits, we measured functional traits that are known to impact germination and growth, which included seed mass (Moles and Westoby 2004) specific leaf area (SLA), leaf water content (LWC), maximum plant height, leaf nitrogen concentration (LNC) and carbon to nitrogen ratio (C:N) (Navas and Violle 2009). We measured traits on 5 - 10 individuals of most species outside the treatment plots, avoiding trait estimations in experimental plots where treatments could influence trait expression. However, of the total 33 seeded species, six species could not be found outside of the treatment plots, and for these species we had to sample individuals from plots representing each of the treatment combinations. 

Species recruitment and reproduction sampling

We assessed the colonization of each species in every subplot by counting the number of individuals of each added species that established at peak biomass in April-May 2015 and 2016. We assessed reproduction by counting the number of reproductive stems of added species. To capture differences in phenology among species, we sampled all plots once a week between April-May. We estimated the total number of seeds produced by individuals of each added species by first randomly selecting 2-4 individuals per species within four different subplots (two subplots in the rodent exclosure and two subplots in the open plot), to obtain a total of 8-16 individuals per species per site. For each individual, we then estimated per capita seed production by counting all viable seeds. If seeds were not yet present, we counted the total number of spikelets or inflorescences for an individual and multiplied this number by an estimated number of seeds per spikelet/inflorescence. This multiplier was either obtained by field estimates of fecundity of individuals outside of plots that produced seed or extracted from a published source. If species did not have at least 8 individuals within seed addition subplots, we sampled all individuals that were present.

Community sampling

We estimated the percent cover of resident species (species that were not included in the seed mix but recruited naturally) within all seed addition subplots in 2015 and 2016. Species cover was visually estimated for each species in every subplot, and total cover was allowed to go over 100 to allow for multiple canopy layers. We sampled traits for resident species, following the methodology to that of the seeded species (see above). We calculated the resident community weighted mean for seed mass, SLA, LWC, LNC and C:N within all seed addition subplots spanning each treatment combination and year. We calculated trait differences of seed size and LNC between each species present in the seed addition groups from the community weighted mean (CWM) of resident invaded communities (i.e. trait difference = species mean trait value – CWM value, thus a negative trait difference indicates higher trait values of the added species compared to the resident community and vice versa for positive trait differences).

There are three files for this data set: 1) Species stem count data, 2) Plant species trait data, 3) Plot community weighted mean data.

Species stem count data: Recuitment data for native and exotic species that were seeded into seed addition subplots. This data includes: the number of stems per species that established (stem count), the probabilty of establishment (yes or no) the number of reproductive stems and seed output across two litter treatments (removed or left intact) and two rodent treatmetns (open or excluded). FunEco_Dryad_SFREC_species_stem_seed_count_data.csv

Plant species trait data: These data are species-level trait data for species observed in the experimental sites at the Sierra Foothills Research Extension Center. Specific leaf area (SLA), leaf water content (LWC), maximum plant height, leaf nitrogen concentration (LNC) and carbon to nitrogen ratio (C:N) were sampled on 5 - 10 individuals of most species outside the treatment plots. FunEco_Dryad_SFREC_species_plant_trait_data.csv

Plot community weighted mean data: Community weighted mean values for seed mass, SLA, LWC, LNC and C:N within all seed addition subplots spanning each treatment combination and year for both resident and added communities. Species mean trait values for seed mass, SLA, LWC, LNC and C:N are also included. FunEco_Dryad_SFREC_community_weighted_mean_data.csv