Spatial distribution of Tephrocactus aoracanthus (Lem.) Lem. in relation to shrubs in the hyperarid regions of west‐central Argentina

The spatial intraspecific and interspecific distribution of Tephrocactus aoracanthus (Lem.) Lem. was studied using univariate and bivariate point pattern analysis, at landscape scale in four different physiographic units and at microscale in terms of the shrubs found in those units. The influence of shrubs on photosynthetically active radiation and on soil temperature under their canopies was analysed. Results show a high environmental variability in light and temperature under shrubs that generally has no influence on the distribution of T. aoracanthus. Apparently, this cactus has great ability to adapt to different conditions of both photosynthetically active radiation and temperature. However, intraspecific distribution was highly clustered in all physiographic units, independently of bushes. Possibly, the mode of seed dispersal and the rooting of cladodes at small scale are the main processes that determine the strong intraspecific clustering of T. aoracanthus.     

Impact of grazing on species composition: Adding complexity to a generalized model

The impact of grazing widely differs among plant communities. A generalized model published in 1988 proposed that this variation could be accounted for by the interaction between primary productivity and the evolutionary history of grazing. As productivity increased, the model predicted larger changes of species composition. Evolutionary history of grazing interacted with productivity: the changes in low‐production systems were smaller if evolutionary history was long, whereas the changes in high‐production systems were independent of evolutionary history. In this paper, we focus on: (i) the difficulties of determining the evolutionary history of grazing of a community, and (ii) additional mechanisms, which, as a sequence of filters in the process of community assembly, could be operating across the gradient of primary production. Assigning a given evolutionary history of grazing to a site has been difficult due to the lack of information on the historical population and distribution of herbivores with an adequate spatial and temporal resolution, and the lack of agreement on the size of the relevant evolutionary time window. Regarding the variation through a gradient of primary production, we propose three additional mechanisms coherent with the prediction of the model. First, the regional pool of available species increases with primary production. Second, grazing intensity (consumption as a proportion of above‐ground production) also increases with primary production. Third, the strength of interspecific positive biotic interactions that protect plants from herbivores decreases with primary production. We highlight an additional potential mechanism, seed dispersal, whose variation across the gradient of productivity is not yet sufficiently understood. By connecting the logic of environmental filters to explain community assemblage with the original proposition of the generalized model, we suggest a series of research lines that can lead to a better understanding of why different communities respond differently to grazing.     

Evolution of intrinsic reproductive isolation among four North American populations of Rhagoletis pomonella (Diptera: Tephritidae)

Across its range in North America, four geographically separated, ecologically and genetically diverged populations of hawthorn (Crataegus)‐infesting Rhagoletis pomonella (Diptera: Tephritidae) flies inhabit the Eje Volcánico Trans Mexicano (EVTM), the Sierra Madre Oriental (SMO), the Chiapas Highlands (CHIS) and the USA. Here, we tested whether these four populations are reproductively isolated by any intrinsic, nonhost‐related, pre‐ or postmating barriers to gene flow. Crossing experiments suggested that a low level of host‐independent prezygotic isolation may exist between hawthorn flies from EVTM and the three other populations, but only with respect to a slight reduction in copulation duration in EVTM matings. Some evidence for postmating isolation was found, again primarily involving EVTM crossed to SMO, CHIS and US flies. Certain crosses produced no (SMO male × EVTM female) or few (EVTM male × CHIS female; CHIS male × SMO female) F1 hybrid offspring. F2 crosses were generally fertile, except for US male × CHIS female matings. Inherent reproductive isolation therefore appears to be quantitative rather than absolute between populations, as the possibility for gene flow exists through at least some combinations of mating among EVTM, SMO, CHIS and US flies. Our results are consistent with a recently advanced hypothesis that episodic introgression from Mexico into the USA has played a role in providing genetic variation, facilitating sympatric host race formation and the adaptive radiation of the R. pomonella sibling species' complex in the USA. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 213–223.