Niche pre‐emption increases with species richness in experimental plant communities

Peter N Mwangi,Michael Scherer‐Lorenzen,Bernhard Schmid,Jens Schumacher,Martin Schmitz,Christoph Scherber,Christiane Roscher,Wolfgang W Weisser

doi:10.1111/j.1365-2745.2006.01189.x

Abstract

SummaryIn plant communities, invasion resistance may increase with diversity because empty niche space decreases simultaneously. However, it is not clear if this only applies to exotic species or also to native species arriving at a site with few other native species during community assembly. We tested the latter by transplanting four native species into experimental grassland communities varying in species richness form 1–16 (−60) species. In addition, we tested the hypothesis that invasion is less successful if the invading species belongs to a functional group that is already present in the community. The test invaders included a grass species (Festuca pratensis, FP), a short (Plantago lanceolata, PL) and a tall herb species (Knautia arvensis, KA), and a legume species (Trifolium pratense, TP). The same four functional groups also occurred alone or in all possible combinations in the different experimental communities. The overall performance of the transplants was negatively related to the logarithm of the species richness of host communities. Plant biomass declined by 58%, 90%, 84% and 62% in FP, PL, KA and TP, respectively, from monocultures to 16‐species mixtures, indicating lower invasiveness of the two herbs than of the grass and the legume. Resident grasses showed a strong negative effect on the performance of all test invaders, whereas resident small and tall herbs had neutral, and resident legumes had positive effects. The case of the legumes indicates that contributions to invasion resistance need not parallel invasiveness. Communities containing resident species of only one functional group were most inhibitive to transplants of the same functional group. These results indicate that invasion resistance of experimental plant communities is related to the degree of niche overlap between resident species and invaders. This niche overlap can be high due to generally low amounts of empty niche space in species‐rich resident communities or due to the occurrence of the same functional group as the one of the invader in the resident community. Stronger within‐ than between‐functional‐group invasion resistance may be the key mechanism underlying diversity effects on invasion resistance in grassland and other ecosystems at large.

Highlights

Understanding the mechanisms behind the relationship between resident species richness and the establishment of non-resident species in natural communities is a major goal in ecology
Except for plant height in P. lanceolata and K. arvensis, the measured morphological variables of phytometers were highly correlated with their above-ground biomass (Table 1), indicating that the latter is a good measure of overall phytometer performance
At the first harvest in summer 2003, i.e. 4 months after transplanting, the performance of phytometer individuals was negatively related to the logarithm of sown species richness

Summary

Introduction

Understanding the mechanisms behind the relationship between resident species richness and the establishment of non-resident species (i.e. invaders in the broad sense) in natural communities is a major goal in ecology. Elton (1958) and Levine & D’Antonio (1999) provide evidence for a generally negative relationship between diversity and the likelihood that an intruder will be able to establish itself in a community Such relationships have been found in a large number of experimental studies using temperate plant communities (Tilman 1997; Knops et al 1999; Joshi et al 2000; Naeem et al 2000; Prieur-Richard et al 2000; Diemer & Schmid 2001; Kennedy et al 2002; Pfisterer et al 2004). Observational studies mostly analyse the number of invading species (e.g. Stohlgren et al 1999; Meiners et al 2004), whereas many experimental studies assess the performance of particular invaders (see, e.g. Prieur-Richard et al 2000; Diemer & Schmid 2001; Hector et al 2001)

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Conclusion