William J. Boecklen

GENETIC ANALYSIS OF HYBRID ZONES: NUMBERS OF MARKERS AND POWER OF RESOLUTION

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.

Nestedness, biogeographic theory, and the design of nature reserves

Abstract 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.

Island biogeographic theory and conservation practice: Species-area or specious-area relationships?

Abstract We present statistical techniques to evaluate species-area regressions and models of faunal and floral collapse and apply these techniques to several recent examples from the literature. The application of these models to the design of nature reserves is unwarranted. These models have low explanatory power; they typically explain only half the variation in species number. Their parameter estimates are sensitive to particular cases. Consequently, estimates from these models range over several orders of magnitude following the deletion of a single observation. Species-area and faunal collapse models give unreliable estimates; 95% prediction intervals and inverse prediction intervals routinely span two or more orders of magnitude. These models should be subordinate to autecological considerations in policy formulation.

HOW DISCRETE ARE OAK SPECIES? INSIGHTS FROM A HYBRID ZONE BETWEEN QUERCUS GRISEA AND QUERCUS GAMBELII

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.

Structure of herbivore communities in two oak (Quercus spp.) hybrid zones

SummaryWe 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.

Sex and Drugs and Herbivores: Sex-Biased Herbivory in Arroyo Willow (Salix Lasiolepis)

We document through correlative and experimental analyses sex—biased herbivory in the arroyo willow, Salix lasiolepis. Male willows typically support higher densities of five species of tenthredinid sawflies than do female willows. Significant intersexual variation sawfly densities exists in naturally occurring willow clones and in clones grown in a common garden. Arroyo willow is sexually dimorphic in leafing phenology and in aspects of leaf chemistry. Male willows have significantly longer vegetative shoots than do females at the time of sawfly oviposition (P = .01). Males also have significantly higher foliar concentrations of nitrogen, 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. See full-text article at JSTOR

Effects of Habitat Heterogeneity on the Species-Area Relationships of Forest Birds

Habitat heterogeneity is an important underlying component of the species-area relationship. I directly measure its effects on the species-area relationships of forest birds with data from the 45th Breeding Bird Census. A habitat space for thirty-four plots is created by principal components analysis. Euclidean distance within this space is used as a measure of between-plot habitat differences. By combining pairs of plots to create a series of composite plots, I convert measures of between-plot differences in vegetation structure into measures of within-plot habitat heterogeneity. I calculate all 561 pairwise distances within the habitat space. I combine species counts and areas for 104 pairs of plots selected from the lower tail, middle, and upper tail of the pairwise-distance distribution. Habitat heterogeneity is a significant predictor of species number even after area is factored out. Furthermore, the set of pairs selected from the three regions of the pairwisedistance distribution differ significantly in the intercepts or in the slopes of their species-area regressions. Implications for conservation are

PATTERNS OF EXTINCTION IN THE INTRODUCED HAWAIIAN AVIFAUNA: A REEXAMINATION OF THE ROLE OF COMPETITION

Among introduced passeriform and columbiform birds of the six major Hawaiian islands, some species (including most of those introduced early) may have an intrinsically high probability of successful invasion, whereas others (including many of those introduced from 1900 through 1936) may be intrinsically less likely to succeed. This hypothesis accords well with the observation that, of the 41 species introduced on more than one of the Hawaiian islands, all but four either succeeded everywhere they were introduced or failed everywhere they were introduced, no matter what other species or how many other species were present. Other hypotheses, including competitive ones, are possible. However, most other patterns that have been claimed to support the hypothesis that competitive interactions have been key to which species survived are ambiguous. We propose that the following patterns are true: (1) Extinction rate as a function of number of species present (S) is not better fit by addition of an S2 term. (2) Bill-length differences between pairs of species that invaded together may tend to be less for pairs in which at least one species became extinct, but the result is easily changed by use of one reasonable set of conventions rather than another. In any event, the relationship of bill-length differences to resource overlap has not been established for these species. (3) Surviving forest passeriforms on Oahu may be overdispersed in morphological space, although the species pool used to construct the space may not have been the correct one. (4) Densities of surviving species on species-poor islands have not been shown to exceed those on species-rich islands.

Patterns of herbivory in the Quercus grisea X Quercus gambelii species complex

In a recent paper, Whitham proposed the hybrids-as-sinks hypothesis, which posits that hybrid hosts are more susceptible to phytophagous insects than are parental hosts. We test the hybrids-as sinks hypothesis in the Quercus grisea Liebm. × Quercus gambelii Nutt. hybrid complex. We measure densities of leaf-mining moths (Lepidoptera: Nepticulidae and Gracillariidae) and gall wasps (Hymenoptera: Cynipidae) on parental and hybrid host trees. We find no support for the hybrids-as-sinks hypothesis in the present hybrid complex (...)

Reproductive processes in two oak (Quercus) contact zones with different levels of hybridization

Patterns of reproductive isolation between two sympatric species of oaks, Quercus gambelii and Q. grisea, that exhibit strong ecological differentiation were examined. A full diallel cross using four trees of each species (i.e. all possible pollinations among eight trees) was performed. This design was repeated at two sites that represent different outcomes of sympatry: (1) a xeric mountain ridge where many hybrids are established (HZ); and (2) a mesic valley bottom where virtually no hybrids are established (MOCYN). By measuring fruit survival at several developmental stages, both the timing and strength of reproductive barriers within and between sites, species, cross types, and pollen dosage levels were examined. In three of four cases, heterospecific fruit set was significantly reduced compared to conspecific fruit set. This reduction occurred after the time of fertilization, but before the onset of embryo growth. Increasing the dose of pollen from an average of 9–194 grains/stigma did not affect this result. Thus, early postfertilization processes play a strong role in species fidelity in these oaks. Quercus gambelii experienced a five-fold decrease in conspecific fruit set at HZ relative to MOCYN. In contrast, heterospecific fruit set of Q. gambelii was the same at both sites. Poor Q. gambelii pollen performance is implicated as playing the major role in this result. One Q. gambelii individual at HZ was highly fecund, and had higher heterospecific than conspecific fruit set; slight introgression in this tree was detected uisng RAPD markers. The Environmental Emasculation Hypothesis that posits that environmental stress can increase the probability of hybrid formation by reducing the competitive ability of male gametes of one species is proposed.