Gisela C. Stotz

Worldwide evidence of a unimodal relationship between productivity and plant species richness

Grassland diversity and ecosystem productivity The relationship between plant species diversity and ecosystem productivity is controversial. The debate concerns whether diversity peaks at intermediate levels of productivity—the so-called humped-back model—or whether there is no clear predictable relationship. Fraser et al. used a large, standardized, and geographically diverse sample of grasslands from six continents to confirm the validity and generality of the humped-back model. Their findings pave the way for a more mechanistic understanding of the factors controlling species diversity. Science, this issue p. 302 The humped-back model of plant species diversity is confirmed by a global grassland survey. The search for predictions of species diversity across environmental gradients has challenged ecologists for decades. The humped-back model (HBM) suggests that plant diversity peaks at intermediate productivity; at low productivity few species can tolerate the environmental stresses, and at high productivity a few highly competitive species dominate. Over time the HBM has become increasingly controversial, and recent studies claim to have refuted it. Here, by using data from coordinated surveys conducted throughout grasslands worldwide and comprising a wide range of site productivities, we provide evidence in support of the HBM pattern at both global and regional extents. The relationships described here provide a foundation for further research into the local, landscape, and historical factors that maintain biodiversity.

Differential responses of native and exotic plant species to an invasive grass are driven by variation in biotic and abiotic factors

Questions nThe impact of invasive species on community structure and function varies, yet it is difficult to predict. Different hypotheses have been proposed to predict invasive species establishment and impact on resident communities, based on characteristics of the resident community and/or environmental conditions. Invasive species, however, interact with both native and exotic species. The interaction with exotic species is important, as it can result in secondary invasion or greater impact on native species. Smooth brome (Bromus inermis Leyss.) is a widely planted forage crop, but also a harmful invasive species in North American grasslands, yet little is known about the variability of its impact on resident communities. We ask (1) whether smooth brome has a consistent impact on community structure and function; (2) which biotic and abiotic conditions are associated with smooth bromes impact on resident communities; and (3) whether smooth brome has a similar impact on native and exotic species, and which biotic or abiotic conditions drive the interaction of smooth brome with native and exotic species. n nLocation nEight grasslands throughout Alberta, Canada. n nMethods nWe used space-for-time substitutions to quantify smooth bromes impact on different aspects of community structure and function, and whether it varies depending on species richness, productivity, temperature, precipitation and variation in precipitation. n nResults nSmooth brome had a consistent negative impact on community structure and function. However, the strength of the effect depended on aspects of the local community. Smooth brome impacts on species richness were higher in species-rich areas, while impact on resident species biomass was larger in productive, warmer and more variable sites. In most sites the negative effect of smooth brome on species richness was higher for native species, compared to exotics. Impact on native and exotic species richness was larger in species-rich and more variable sites, respectively. n nConclusions nUnderstanding and predicting smooth bromes impact on community structure and function can help managers prescribe management plans to reduce negative impacts of smooth brome on native communities. Predicting the interaction of smooth brome with exotic species may be just as important when planning management strategies, to avoid secondary invasion and/or the accumulation of exotic species.

Community-level determinants of smooth brome (Bromus inermis) growth and survival in the aspen parkland

During the initial establishment phase, both biotic and abiotic conditions of the resident community can be important in determining the persistence of invasive plant populations. Invaders may act as passengers by tracking variation in environmental conditions (passenger model), or alternatively, may drive changes in environmental conditions which facilitate their continued invasion (driver model). We distinguish between these two models by comparing variation in resource availability among multiple plant community types to invader initial growth and survival. This study took place in the aspen parkland ecoregion of Alberta, Canada, which is a savanna-type habitat consisting of multiple plant community types and invaded by nonnative smooth brome (Bromus inermis). We characterized four community types by a suite of biotic and abiotic variables (brome seed density, plant richness, plant cover, soil pH, soil moisture, and organic and inorganic N) and performed a brome seed addition experiment. Brome seedling growth and survival were greater with increased levels of soil moisture, while growth decreased with increases in dissolved organic N, but did not vary with other environmental conditions. Both survival and growth of brome seedlings were lowest in brome-dominated areas. These results show support for the passenger model of invasion, as variation in local environmental conditions were associated with variation in brome performance. Further, brome appears to have a negative effect on its own growth, a pattern uncommon among plant invaders.

Spatial pattern of invasion and the evolutionary responses of native plant species

Invasive plant species can have a strong negative impact on the resident native species, likely imposing new selective pressures on them. Altered selective pressures may result in evolutionary changes in some native species, reducing competitive exclusion and allowing for coexistence with the invader. Native genotypes that are able to coexist with strong invaders may represent a valuable resource for management efforts. A better understanding of the conditions under which native species are more, or less, likely to adapt to an invader is necessary to incorporate these eco‐evolutionary dynamics into management strategies. We propose that the spatial structure of invasion, in particular the size and isolation of invaded patches, is one factor which can influence the evolutionary responses of native species through modifying gene flow and the strength of selection. We present a conceptual model in which large, dense, and well‐connected patches result in a greater likelihood of native species adaptation. We also identify characteristics of the interacting species that may influence the evolutionary response of native species to invasion and outline potential management implications. Identifying areas of rapid evolutionary change may offer one additional tool to managers in their effort to conserve biodiversity in the face of invasion.

Disturbance by an endemic rodent in an arid shrubland is a habitat filter: effects on plant invasion and taxonomical, functional and phylogenetic community structure.

Background and Aims Disturbance often drives plant invasion and may modify community assembly. However, little is known about how these modifications of community patterns occur in terms of taxonomic, functional and phylogenetic structure. This study evaluated in an arid shrubland the influence of disturbance by an endemic rodent on community functional divergence and phylogenetic structure as well as on plant invasion. It was expected that disturbance would operate as a habitat filter favouring exotic species with short life cycles. Methods Sixteen plots were sampled along a disturbance gradient caused by the endemic fossorial rodent Spalacopus cyanus, measuring community parameters and estimating functional divergence for life history traits (functional dispersion index) and the relative contribution to functional divergence of exotic and native species. The phylogenetic signal (Pagels lambda) and phylogenetic community structure (mean phylogenetic distance and mean nearest taxon phylogenetic distance) were also estimated. The use of a continuous approach to the disturbance gradient allowed the identification of non‐linear relationships between disturbance and community parameters. Key Results The relationship between disturbance and both species richness and abundance was positive for exotic species and negative for native species. Disturbance modified community composition, and exotic species were associated with more disturbed sites. Disturbance increased trait convergence, which resulted in phylogenetic clustering because traits showed a significant phylogenetic signal. The relative contribution of exotic species to functional divergence increased, while that of natives decreased, with disturbance. Exotic and native species were not phylogenetically distinct. Conclusions Disturbance by rodents in this arid shrubland constitutes a habitat filter over phylogeny‐dependent life history traits, leading to phylogenetic clustering, and drives invasion by favouring species with short life cycles. Results can be explained by high phenotypic and phylogenetic resemblance between exotic and native species. The use of continuous gradients when studying the effects of disturbance on community assembly is advocated.

Maternal experience and soil origin influence interactions between resident species and a dominant invasive species

Invasive species dominance in invaded communities may not be long-lasting due to regulatory processes, such as plant–soil feedbacks and neighboring species adaptation. Further, the change in species competitive ability may be contingent upon neighbor identity (i.e., specialized response) or consistent across neighbors (i.e., generalized response). Specialized responses can facilitate overall coexistence, while generalized responses may result in competitive exclusion. We set up a greenhouse experiment to test, in three species, the effect of soil conditions (non-invaded vs. invaded soil) and maternal experience (offspring of maternal plants from invaded vs. non-invaded areas) on species competitive ability against the invader Bromus inermis and conspecifics. If changes in species competitive ability against B. inermis were also evident when interacting with conspecifics, it would suggest a generalized increased/decreased competitive ability. Maternal experience resulted in reduced suppression of B. inermis in the three species and no change in tolerance. On the other hand, tolerance to B. inermis was enhanced when plants grew in soil from invaded areas, compared to non-brome soil. Importantly, both the decreased suppression due to maternal experience with B. inermis and the increased tolerance in invaded soil appear to be invader specific, as no such effects were observed when interacting with conspecifics. Specialized responses should facilitate coexistence, as no individual/species is a weaker or stronger competitor against all other neighbors or under all local soil conditions. Further, the negative plant–soil feedback for B. inermis should facilitate native species recovery in invaded areas and result in lower B. inermis performance and dominance over time.

Soil biotic quality lacks spatial structure and is positively associated with fertility in a northern grassland

When placing roots in the soil, plants integrate information about soil nutrients, plant neighbours and beneficial/detrimental soil organisms. While the fine-scale spatial heterogeneity in soil nutrients and plant neighbours have been described previously, virtually nothing is known about the spatial structure in soil biotic quality (measured here as a soil Biota-Induced plant Growth Response, or BIGR), or its correlation with nutrients or neighbours. Such correlations could imply trade-offs in root placement decisions. n n nTheory would predict that soil BIGR is (1) negatively related to soil fertility and (2) associated with plant community structure, such that plants influence soil biota (and vice versa) through plant–soil feedbacks. We would also expect that since plants have species-specific impacts on soil organisms, spatially homogeneous plant communities should also homogenize soil BIGR. n n nHere, we test these hypotheses in a semi-arid grassland by (1) characterizing the spatial structure of soil BIGR at a scale experienced by an individual plant and (2) correlating it to soil abiotic properties and plant community structure. We do so in two types of plant communities: (1) low-diversity patches dominated by an invasive grass (Bromus inermis Leyss.) and (2) patches covered mostly by native vegetation, with the expectation that dominance by Bromus would homogenize soil BIGR. n n nSoil BIGR was spatially heterogeneous, but not autocorrelated. This was true in both vegetation types (Bromus-invaded vs. native patches). Conversely, soil abiotic properties and plant community structure were frequently spatially autocorrelated at similar scales. Also, contrary to many studies, we found a positive correlation between soil BIGR and soil fertility. Soil BIGR was also associated with plant community structure. n n nSynthesis. The positive correlation between soil BIGR and some soil nutrient levels suggests that plants do not necessarily trade-off between foraging for nutrients vs. biotic interactions: nutritional cues could rather indicate the presence of beneficial soil biota. Moreover, the spatial structure in plant communities, coupled with their correlation with soil BIGR, jointly suggest that plant–soil feedbacks operate at local scales in the field: this has been identified in modelling studies as an important driver of plant coexistence.