Article

Hybrization among dominant tree species correlates positively with understory plant diversity

Department of Biology, Stanford University, California 94305, USA.
American Journal of Botany (Impact Factor: 2.46). 09/2011; 98(10):1623-32. DOI: 10.3732/ajb.1100137
Source: PubMed

ABSTRACT Elucidating the factors that determine the abundance and distribution of species remains a central goal of ecology. It is well recognized that genetic differences among individual species can affect the distribution and species interactions of dependent taxa, but the ecological effects of genetic differences on taxa of the same trophic level remain much less understood. Our goal was to test the hypothesis that differences between related overstory tree species and their hybrids can influence the understory plant community in wild settings.
We conducted vegetation surveys in a riparian community with the overstory dominated by Populus fremontii, P. angustifolia, and their natural hybrids (referred to as cross types) along the Weber River in north central Utah, USA. Understory diversity and community composition, as well as edaphic properties, were compared under individual trees.
Diversity metrics differ under the three different tree cross types such that a greater species richness, diversity, and cover of understory plants exist under the hybrids compared with either of the parental taxa (30-54%, 40-48%, and 35-74% greater, respectively). The community composition of the understory also varied by cross type, whereby additional understory plant species cluster with hybrids, not with parental species.
Genetic composition dictated by hybridization in the overstory can play a role in structuring the associated understory plants in natural communities-where a hybridized overstory correlates with a species-rich understory-and thus can have cascading effects on community members of the same trophic level. The underlying mechanism requires further investigation.

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    • "The long-term, positive effects of hybridization could include speciation through hybridization , which promotes diversity (e.g. Adams et al., 2011). On the other hand, the long-term, negative impacts of hybridization could be far reaching, as hybridization could disrupt local adaptation, leading to genomic extinction (Rhymer & Simberloff, 1996). "
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    ABSTRACT: Anderson & Stebbins (1954, Evolution, 8, 378–388) posited that human activities promote species hybridizations by creating opportunities for hybridization and new habitats for hybrids to persist through disturbances (i.e. the ‘disturbance hypothesis’). While the first part of this hypothesis appears to be well supported, the second part has not been corroborated with empirical evidence, probably because of the lack of appropriate data. In this study, I (1) document the richness and distribution of hybrid plants in the United States; (2) examine the relationships between hybrids of different origins and between hybrid plants and native or exotic plants; and (3) examine possible mechanisms for these relationships and test the disturbance hypothesis. The United States. The richness and distribution of plant hybrids was examined at the county level according to origin, that is, formed between native–native species (N × N), native–exotic species (N × E) and exotic–exotic species (E × E), using data from the Biota of North America Program. The three hybrid types (N × N, N × E and E × E) were positively related to each other and showed stronger positive relationship with exotic richness than with native richness. They also exhibited similar spatial patterns, with richness hotspots concentrated in the north-east United States and Great Lakes region. However, the richness of hybrids of exotic origin (E × E and N × E) was not related to county area, as often observed for native species; instead, it showed strong positive relationships with human population density. Thus, the overall patterns of hybrid richness and distribution support the ‘disturbance hypothesis’. The results are generally consistent with the disturbance hypothesis. The relationship between the number of hybrids of exotic origin and overall exotic richness provided stronger evidence for human-induced than for naturally caused hybridization, although other possible explanations may also exist.
    Diversity and Distributions 08/2014; 20(11). DOI:10.1111/ddi.12245 · 5.47 Impact Factor
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    • "Studies in the same Populus hybrid system have also found that Populus species and their hybrids support distinct arthropod communities (Wimp et al. 2005), understorey plant communities (Adams et al. 2011), and unidentified communities of soil microbes (Schweitzer et al. 2008) and endophytes (Bailey et al. 2005). For arthropods, studies in Populus and other plant systems have further shown that intraspecific plant genotypic variation strongly affects community structure (Johnson & Agrawal 2005; Crutsinger, Cadotte & Sanders 2009; Keith, Bailey & Whitham 2010). "
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    • "Studies in the same Populus hybrid system have also found that Populus species and their hybrids support distinct arthropod communities (Wimp et al. 2005), understorey plant communities (Adams et al. 2011), and unidentified communities of soil microbes (Schweitzer et al. 2008) and endophytes (Bailey et al. 2005). For arthropods, studies in Populus and other plant systems have further shown that intraspecific plant genotypic variation strongly affects community structure (Johnson & Agrawal 2005; Crutsinger, Cadotte & Sanders 2009; Keith, Bailey & Whitham 2010). "
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    ABSTRACT: 1. Plant genotypic variation can shape associated arthropod and microbial communities locally, as has been demonstrated in controlled common garden experiments. However, the relative roles of plant genetics and the environment in defining communities at larger spatial scales are not well known. The environmental heterogeneity hypothesis maintains that plant genetic effects on associated communities diminish across the landscape as environmental variation predominates. Alternatively, the local adaptation hypothesis argues that plant genetic effects change across landscapes as a result of species interactions being locally adapted. Thus, very different mechanisms could produce the same patterns.2. Using replicated common gardens located along an elevation and distance gradient, observational studies in the wild, and a greenhouse inoculation experiment, we examined these two non-mutually exclusive hypotheses for Populus angustifolia and its fungal leaf pathogen community.3. Supporting the environmental heterogeneity hypothesis, plant genotypic effects on fungal leaf pathogen communities were two-to-three times stronger within than among gardens. Consistent with the local adaptation hypothesis, plant genotypic effects on pathogens also varied significantly among gardens (i.e. GxE interaction effect). Observational data from the wild and our greenhouse inoculation experiment unveiled clinal adaptation in plant genetic resistance that is correlated with disease risk along the elevation gradient, but did not support local pathogen adaptation to plants or vice versa.4. Synthesis. While our study found that plant genotype plays a significant role in shaping associated pathogen communities at local and geographic scales, the environment most strongly influenced Populus angustifolia leaf pathogens at the geographic scale. Plant genetic effects on pathogens were also influenced by the environment, highlighting the potential for environmental (e.g. climate) change to trigger local evolutionary responses in plant-pathogen community interactions.This article is protected by copyright. All rights reserved.
    Journal of Ecology 05/2014; 102(5). DOI:10.1111/1365-2745.12270 · 5.69 Impact Factor
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