Stable isotope analysis (SIA) has proven to be a useful tool in reconstructing diets, characterizing trophic relationships, elucidating patterns of resource allocation, and constructing food webs. Consequently, the number of studies using SIA in trophic ecology has increased exponentially over the past decade. Several subdisciplines have developed, including isotope mixing models, incorporation dynamics models, lipid-extraction and correction methods, isotopic routing models, and compound-specific isotopic analysis. As with all tools, there are limitations to SIA. Chief among these are multiple sources of variation in isotopic signatures, unequal taxonomic and ecosystem coverage, over-reliance on literature values for key parameters, lack of canonical models, untested or unrealistic assumptions, low predictive power, and a paucity of experimental studies. We anticipate progress in SIA resulting from standardization of methods and models, calibration of model parameters through experimentation, and continued development of several recent approaches such as isotopic routing models and compound-specific isotopic analysis.

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The use of molecular genetic techniques is becoming increasingly widespread in analyses of hybrid zones. Yet, exactly how many molecular markers are required to provide a given degree of resolution remains an open question. We present statistical models that relate the number of markers examined to their power to discriminate between pure species, F1's, and backcross individuals. Our models indicate that only about four or five markers are required to provide a coarse classification of individuals in hybrid zones, whereas upwards of 70 markers are required to discriminate between pure species and advanced backcrosses. The models provide hybrid zone researchers a basis upon which to balance the time and expense of examining large numbers of markers against the magnitude of classification errors when smaller numbers of markers are examined.

I examine the relationship between nested distributional patterns and the degree to which several small reserves will contain more species than would a single reserve of equal total area (SLOSS). Nestedness is a common property of species distributions on real and habitat islands. However, there is considerable variation in nestedness among species distributions, some of which is related to the physical and biological background of the archipelagoes. Nestedness does not vary according to the taxonomic group examined (with the exception of aquatic invertebrates). Nestedness does vary between real and habitat islands (with aquatic invertebrates excluded), but not between oceanic and land-bridge islands. The more a biota is nested, the more likely it is that a single large reserve would preserve more species. However, nestedness is a rather poor predictor of SLOSS, as the vast majority of archipelagoes support a strategy of several small reserves, even though almost all of them are significantly nested. Nestedness says little about optimal reserve design and management, and appears to be a weak conservation tool.

The white oaks Quercus gambelii and Q. grisea overlap in distribution in New Mexico and Arizona. Within the region of overlap, there are numerous instances of contact between the two taxa. In some areas of contact morphologically, intermediate trees are common, whereas in others, morphologically intermediate trees are rare or absent. We describe a set of RAPD markers that distinguish between the two species and use these markers to examine patterns of gene exchange in an area of contact in the San Mateo Mountains of New Mexico. The markers are highly coincident with morphology and confirm that hybridization between the two species takes place. Despite the occurrence of hybrids, both species remain distinct, even in areas of sympatry, and marker exchange appears to be limited.

We document through correlative and experimental analyses sex-biased herbivory in the arroyo willow, Salix lasio/epis. Male willows typically support higher densities of five species of tenthredinid sawflies than do female willows. Significant intersexual ~ariation in sawfly densities exists in naturally occurring willow clones and in clones grown m a common garden. Arroyo willow is sexually dimorphic in leafing phenology and in aspects ofleaf chemistry. Male willows have significantly longer vegetative shoots than do fe~ales at the tii_De of sa~y oviposition (P = .01). Males also have significantly higher fohar concentratwns of mtrogen, phosphorus, and potassium than do females. Female willows have a marginally significant (P = .093) higher concentration of total phenols. Of the dimorphic traits, only variation in leaf phenology is a consistent predictor of variation in sawfly densities among willow clones.

We examined patterns of density and species diversity for leaf-mining Lepidopterans and gall-forming Hymenopterans in two oak (Quercus spp.) hybrid zones: Quercus depressipes x Q. rugosa and Q. emoryi x Q. coccolobifolia. In both species complexes, hybrid hosts typically supported significantly lower densities and species diversity of parasites than did parental types. This contradicts the findings of Whitham (1989) that suggested that hybrid hosts may act as parasite sinks both in ecological and evolutionary time. We discuss features of hybrid zones that are likely to influence patterns of herbivory.

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