Non‐random recruitment of invader species in experimental grasslands

Abstract

To assess potential effects of seed limitation, characteristics of invader species and characteristics of established plant communities on recruitment success, we conducted a split‐plot experiment factorially combining three weeding treatments corresponding to increasing successional age (regular weeding<cessation of weeding after three years<never weeded since sowing) with two seed limitation treatments (control vs sowing a seed mixture of all experimental species=internal invaders) in experimental grasslands varying in species richness (1, 2, 4, 8, 16) and in functional group richness and composition (1, 2, 3, 4 functional groups: presence/absence of legumes×grasses×tall herbs×small herbs). Seed addition increased internal invader seedling densities and the probability of successful colonization per species. Legumes, tall herbs and species with large and long‐lived seeds or a requirement for specific vectors for pollination and dispersal benefited from seed addition most. The number of successfully established internal invader species was highest in plots with low initial species richness and in the regularly weeded treatments and lowest in plots with high initial species richness and in the never weeded treatments, indicating decreased recruitment with increased successional age. Resident plant communities with legumes had mostly negative (legume and small‐ and tall‐herb internal invaders, external invaders) or neutral (grass invaders) effects on seedling density and colonization probability of invader species whereas resident communities with grasses had positive effects on the colonization probability of invader species except grasses themselves. These results show that seed limitation, invader characteristics, and community characteristics all can affect recruitment success in predictable ways, suggesting non‐random community assembly during succession starting from different initial species pools.

Effects of biodiversity strengthen over time as ecosystem functioning declines at low and increases at high biodiversity

“…A second mechanism is recruitment limitations when plant life cycles cannot be completed and the regeneration of new plant individuals is reduced causing a decrease in plant population sizes over time (Symstad and Tilman 2001). In fact, the regeneration of many species in the Jena Experiment has been demonstrated to be seed-limited, with strongest effects in low-diversity communities (Roscher et al 2009b), and patterns observed for plant cover support this interpretation. A third mechanism is the reduced capacity for temporal turnover between species in low-diversity communities; this turnover is important for maintaining functions such as plant productivity, as different species respond to environmental fluctuations differently (Allan et al 2011).…”

Different Assembly Processes Drive Shifts in Species and Functional Composition in Experimental Grasslands Varying in Sown Diversity and Community History

“…Higher levels of realized species richness after seed addition are commonly found because plant communities are often under-saturated with species due to a limited species pool or dispersal constraints [44], [45]. Although none of the species added as seed failed to establish completely after seed addition [21] and single communities accumulated indeed up to 60 species one or two years after seed addition, average species richness in the seed addition treatments declined again and did not exceed 35 species in the last year of the study (Fig. 2A).…”

Experimental plant communities develop phylogenetically overdispersed abundance distributions during assembly

“…We also investigated phylogenetic pattern in a reassembly experiment carried out in all 78 plots; monocultures and two-species plots were included because species numbers rapidly increased during reassembly (Roscher et al 2009a). In this experiment, seeds of all 60 species were sown, in April 2005, at equal proportions and at a total density of 1000 viable seeds/ m 2 , into the existing vegetation in subplots of 2.00 3 2.25 m. Species not belonging to the species pool continued to be weeded out from July 2005.…”