Showing papers by "James D. Bever published in 2002"

Negative feedback within a mutualism: host–specific growth of mycorrhizal fungi reduces plant benefit

Abstract: A basic tenet of ecology is that negative feedback on abundance plays an important part in the coexistence of species within guilds. Mutualistic interactions generate positive feedbacks on abundance and therefore are not thought to contribute to the maintenance of diversity. Here, I report evidence of negative feedback on plant growth through changes in the composition of their mutualistic fungal symbionts, arbuscular mycorrhizal (AM) fungi. Negative feedback results from asymmetries in the delivery of benefit between plant and AM fungal species in which the AM fungus that grows best with the plant Plantago lanceolata is a poor growth promoter for Plantago. Growth of Plantago is, instead, best promoted by the AM fungal species that accumulate with a second plant species, Panicum sphaerocarpon. The resulting community dynamic leads to a decline in mutualistic benefit received by Plantago, and can contribute to the coexistence of these two competing plant species.

Local adaptation in the linum marginale—melampsora lini host‐pathogen interaction

Abstract: The potential for local adaptation between pathogens and their hosts has generated strong theoretical and empirical interest with evidence both for and against local adaptation reported for a range of systems. We use the Linum marginale-Melampsora lini plant-pathogen system and a hierarchical spatial structure to investigate patterns of local adaptation within a metapopulation characterised by epidemic dynamics and frequent extinction of pathogen populations. Based on large sample sizes and comprehensive cross-inoculation trials, our analyses demonstrate strong local adaptation by Melampsora to its host populations, with this effect being greatest at regional scales, as predicted from the broader spatial scales at which M. lini disperses relative to L. marginale. However, there was no consistent trend for more distant pathogen populations to perform more poorly. Our results further show how the coevolutionary interaction between hosts and pathogens can be influenced by local structure such that resistant hosts select for generally virulent pathogens, while susceptible hosts select for more avirulent pathogens. Empirically, local adaptation has generally been tested in two contrasting ways: (1) pathogen performance on sympatric versus allopatric hosts; and (2) sympatric versus allopatric pathogens on a given host population. In situations where no host population is more resistant or susceptible than others when averaged across pathogen populations (and likewise, no pathogen population is more virulent or avirulent than others), results from these tests should generally be congruent. We argue that this is unlikely to be the case in the metapopulation situations that predominate in natural host-pathogen interactions, thus requiring tests that control simultaneously for variation in plant and pathogen populations.

Host-specificity of AM fungal population growth rates can generate feedback on plant growth

Abstract: While the mutualistic interaction between plants and AM fungi is of obvious importance to ecosystem processes, the factors influencing the ecological and evolutionary dynamics within this interaction are poorly understood. The mutual interdependence of plant and AM fungal relative growth rates could generate complex dynamics in which the composition of the AM fungal community changes due to association with host and this change in fungal composition then differentially feeds back on plant growth. I first review evidence for feedback dynamics and then present an approach to evaluating such complex dynamics. I specifically present evidence of host-specific differences in the population growth rates of AM fungi. Pure cultures of AM fungi were mixed to produce the initial fungal community. This community was then distributed into replicate pots and grown with one of four cooccurring plant species. Distinct compositions of AM fungal spores were produced on different host species. The AM fungal communities were then inoculated back onto their own host species and grown for a second growing season. The differentiation observed in the first generation was enhanced during this second generation, verifying that the measure of spore composition reflects host-specific differences in AM fungal population growth rates. In further work on this system, I have found evidence of negative feedback through two pairs of plant species. The dynamic within the AM fungal community can thereby contribute to the coexistence of plant species.

Divergent phenologies may facilitate the coexistence of arbuscular mycorrhizal fungi in a North Carolina grassland.

Abstract: Interest in the diversity of arbuscular mycorrhizal (AM) fungal communities has been stimulated by recent data that demonstrate that fungal communities influence the competitive hierarchies, productivity, diversity, and successional patterns of plant communities. Although natural communities of AM fungi are diverse, we have a poor understanding of the mechanisms that promote and maintain that diversity. Plants may coexist by inhabiting disparate temporal niches; plants of many grasslands are either warm or cool season specialists. We hypothesized that AM fungi might be similarly seasonal. To test our hypothesis, we tracked the sporulation of individual AM fungal species growing within a North Carolina grassland. Data were collected in 1996 and 1997; in 1997, sampling focused on two common species. We found that AM fungi, especially Acaulospora colossica and Gigaspora gigantea, maintained different and contrasting seasonalities. Acaulospora colossica sporulated more frequently in the warm season, but Gi. gigantea sporulated more frequently in the cool season. Moreover, AM fungal species were spatially aggregated at a fine scale. Contrasting seasonal and spatial niches may facilitate the maintenance of a diverse community of AM fungi. Furthermore, these data may illuminate our understanding of the AM fungal influence on plant communities: various fungal species may preferentially associate with different plant species and thereby promote diversity in the plant community.

A novel theory to explain species diversity in landscapes: positive frequency dependence and habitat suitability.

Abstract: Theories to explain the diversity of species have required that individual species occupy unique niches and/or vary in their response to environmental factors. Positive interactions within a species, although common in communities, have not been thought to maintain species diversity because in non-spatial models the more abundant species always outcompetes the rarer species. Here, we show, using a stochastic spatial model, that positive intraspecific interactions such as those caused by positive frequency dependence and/or priority effects, can maintain species diversity if interactions between individuals are primarily local and the habitat contains areas that cannot be colonized by any species, such as boulders or other physical obstructions. When intraspecific interactions are primarily neutral, species diversity will eventually erode to a single species. When the landscape is homogeneous (i.e. does not contain areas that cannot be colonized by any species), the presence of strong intraspecific interactions will not maintain diversity.

Negative frequency dependence and the importance of spatial scale

Abstract: Issues of spatial scale are inherent in many ecological systems. This study uses a spatially explicit cellular automaton model to explore how the scale of dispersal interacts with the scale and strength of negative frequency dependence to determine patterns of species distribution. Counter to expectation, strong local frequency-dependent interactions result in random spatial patterns. When dispersal scale and interaction scale are decoupled, the resulting patterns are not necessarily random. For strong negative frequency dependence, stable bands result when the scale of interaction exceeds the scale of dispersal, and bands with two-point cycles result when the scale of dispersal exceeds the scale of interaction. However, for weaker interactions occurring over intermediate scales, only random patterns result. Thus, our results call into question the utility of inferring any ecological interaction from only the spatial distributions of the putatively interacting species. Furthermore, our results call for new experimental studies that explicitly manipulate the strength and the scale of the processes being studied.

Global circulations. Nature culture and the possibility of sustainable development.

Abstract: Often ignoring social studies of science scientific institutions continue to embrace the notion of objectivity and the possibility of producing unbiased knowledge about the world. In this chapter we explore our growing understanding of how research on the natural world is influenced and shaped by larger political and social issues. In particular exploring the role of development we use the case study of invasion biology to illustrate this point. Invasion biology has developed as a field concerned with the increasing presence of exotic and alien plants and animals. We suggest that the problems of development are central to the growing concern about alien plants and animals entering the United States. We argue that critiques of development and the discourse of post-development to which this anthology is a contribution must pay attention to the naturalized ideology of development in addition to the very material consequences of development in the Third World. Invasion biology is also a good case study of the tensions between the global and the local. The global movements of humans plants and animals as well as consumer products have profound consequences at the local level - of culture economies and labour as well as nature and its management. As development scholars have already shown hierarchies such as those based on gender race class nation or sexuality are imbricated in development narratives and ideology and central to the case of invasion biology as well. These intersections are crucial in any discussion of development. (excerpt)