Zone Of Sympatry Research Articles

The systematics of the Trichogramma minutum species complex (Hymenoptera: Trichogrammatidae), a group of important North American biological control agents: the evidence from reproductive compatibility and allozymes

Two North American Trichogramma, Trichogramma minutum Riley, and Trichogramma platneri Nagarkatti (the T. minutum complex), cannot be distinguished morphologically and their species status has been questioned. Both are commercially available for biological control and are important parasitoids of the eggs of Lepidoptera in agricultural ecosystems. Results are reported of an extensive survey of reproductive compatibility and allozymes at the phosphoglucomutase (Pgm) locus in collections representing 85 localities throughout much of the geographical range of this species complex. In total, 103 reproductive crosses were performed and 161 collections were analyzed electrophoretically. Results support the recognition of T. minutum and T. platneri as distinct species. The two are distinguishable electrophoretically at the Pgm locus. High levels of reproductive incompatibility are correlated with the allozymic differences. Although T. platneri is western in distribution (W of 115° longitude) and T. minutum is primarily eastern, the two are sympatric in the Pacific Northwest. A low level of interspecific hybridization in laboratory studies notwithstanding, there is no evidence that introgression occurs in the zone of sympatry. This study continues to stress the importance of species recognition and the inadvisability of using T. minutum and T. platneri interchangeably in biological control.

Bird predation and the host-plant shift by the goldenrod stem galler

AbstractEscape from natural enemies may favor the incorporation of a novel host plant into the diet of an herbivorous insect. This scenario has been suggested for the recent host-plant shift by the goldenrod stem galler, Eurosta solidaginis Fitch (Diptera: Tephritidae), from the ancestral host Solidago altissima L. (Compositae) to the derived host Solidago gigantea Ait. In this study, we examined the effects of predation from downy woodpeckers, Picoides pubescens L. (Aves: Picidae), and black-capped chickadees, Parus atricapillus L. (Aves: Paridae), on these two host races of insects at the western edge of their zone of sympatry. Based on a field census, bird predation was concentrated near the cover of trees where S. gigantea tends to occur; few attacks occurred in the open where S. altissima is prevalent. We conducted a field experiment to evaluate the preference of these avian predators for galls of the two host races when differences in the microgeographic distribution, size, and height of galls were controlled. In allopatric sites where only S. gigantea occurs, attacks by birds were 58% more frequent on S. gigantea than on S. altissima galls. Similar results were found for sympatric sites, although the difference in attack was only 26% and not significant. We could find no difference in the toughness of galls or the nutritional value of a larva within the gall (in terms of biomass) to explain avian preference for the S. gigantea host race; however, we found that from 1999 to 2000, the S. gigantea race offered a 27–107% higher reward rate (i.e., the probability that a gall harbored a larva of E. solidaginis) than the S. altissima race. Our studies suggest that avian predators can assess a gall’s content prior to pecking it open, preferring galls that are inhabited by both E. solidaginis larvae and the inquiline predator Mordellistena convicta Leconte (Coleoptera: Mordellidae). It is possible that birds have either learned through experience or evolved through natural selection to choose the more profitable S. gigantea galls. Finally, our results suggest that avian predators act against the maintenance of two distinct host races in the midwestern United States.

Amplified fragment length polymorphism analysis identifies hybrids between two subspecies of warblers.

In the western Pyrenees (Southwest France and Northwest Spain), a narrow hybrid zone exists between the common chiffchaff Phylloscopus (collybita) collybita and the Iberian chiffchaff Phylloscopus (c.) brehmii. In this zone, which is approximately 20 km wide, mixed matings and individuals singing the songs of both taxa occur at substantial frequencies (24 and 8.6%, respectively), suggesting frequent hybridization. Previous studies have shown very weak mitochondrial gene flow (Nm = 0.065), whereas four microsatellites suggested much higher nuclear gene flow (Nm = 4.9). In this study we used the amplified fragment length polymorphism (AFLP) method in order to identify hybrids and early backcrosses. We typed 91 birds from both allopatric and sympatric areas for 12 informative AFLP markers (of > 141 polymorphic fragments), obtained by screening 13 AFLP primer combinations. These individuals were previously typed for song (brehmii, collybita or mixed singers), mitochondrial DNA (mtDNA) haplotype and allelic genotypes at four microsatellite loci. Assignment tests demonstrated that in the zone of sympatry, a substantial number of intermediate genotypes existed among the birds previously believed to be pure collybita and brehmii, based on song and mtDNA haplotype. The majority of the mixed singers had intermediate genotypes. Our data suggest that the fraction of the adult population having a hybrid origin (hybrids or backcrosses) is in the order of 10%. With such a frequency of genetic hybrids, there would have been much more mtDNA introgression than observed, had female hybrids been perfectly fertile/viable. This result is consistent with male-biased gene flow and Haldane's rule.

Protostrongylus stilesi (Nematoda: Protostrongylidae): Ecological Isolation and Putative Host-Switching Between Dall's Sheep and Muskoxen in a Contact Zone

The occurrence of Protostrongylus stilesi in a population of introduced muskoxen, Ovibos moschatus wardi, on the Arctic Coastal Plain, Yukon Territory (YT) and Northwest Territories (NT), Canada, is consistent with a contemporary colonization event from Dall's sheep, Ovis dalli dalli, which indicates that host specificity may be ecologically based and contextual for this parasite. Colonization of muskoxen by P. stilesi may be a predictable event in zones of sympatry with Dall's sheep; exposure to infection may coincide with occupation of winter ranges of Dall's sheep by muskoxen during the summer season. Disruption of contemporary ecological isolating barriers can result from anthropogenically or climatologically driven habitat perturbation, and result from management practices that influence the distribution of ungulate hosts. Thus, if zones of contact become more extensive or the temporal limits on allopatry are relaxed, we may observe increasing instances of host switching by parasites or pathogens at the interface of newly emerging ecotones. Impacts to northern systems linked to climatologically and anthropogenically driven global change and the effects of management must be tracked within the context of biodiversity survey and inventory and archival collections, as foundations for monitoring ecosystem-level perturbations. A developing interface for muskoxen, wild sheep, and parasites along the Mackenzie River ecotone represents a natural model or field laboratory to examine these processes. Additonally, lungworms, Protostrongylus spp., had not been reported in muskoxen, and a new geographic record for this nematode was established in Dall's sheep from the northern Richardson Mountains, NT.

Evidence of Introgression between Masked Shrews (Sorex cinereus), and Prairie Shrews (S. haydeni), in Minnesota

Several studies support specific status for the prairie form of Sorex cinereus, designated as S. haydeni. Evidence for introgression between these species has been found in Alberta, despite significant sequence differences (>15%) in mitochondrial DNA. Using mtDNA and morphological criteria, we identified to species 94 masked shrews to assess the distribution of these two species in Minnesota and to examine the extent of introgression in zones of sympatry. Only four specimens scored genetically and morphologically as S. haydeni, indicating a restricted distribution. Four specimens had incongruent genetic and morphological identifications, suggesting introgression between the two species.

Variation in TaqI-digested DNA of Sugar and Black Maples Is Independent of Taxon and Plant Origin

Morphological distinctions between sugar maples and black maples are not consistently evident, and molecular assessment of genetic diversity is lacking for these taxa. We examined restriction-site polymorphisms in the ndhA intron of the chloroplast DNA (cpDNA) in populations of sugar maples and black maples representing their zones of allopatry and sympatry in eastern North America. Restriction-site analysis of the ndhA intron after digestion with HinfI and Sau3AI yielded no polymorphisms. Restriction digestion of the ndhA intron with TaqI revealed two cpDNA haplotypes that were neither geographically localized nor taxon specific. Although testing additional accessions of sugar maples and black maples for cpDNA variation will further elucidate patterns of genetic variation, our initial results suggest that the taxa are either exchanging genes or share an ancestral cpDNA polymorphism.

Geographic Distribution of, and Genetic Variation in, the Wood Roach <I>Cryptocercus</I> (Dictyoptera: Cryptocercidae) in the United States

Members of the genus Cryptocercus are subsocial, xylophagous cockroaches that inhabit temperate forests of the Nearctic and Palaearctic. Seven species are recognized worldwide: five in the United States, and one each in Russia and China. Four (Cryptocercus darwini Burnside, Smith and Kambhampati, Cryptocercus garciai Burnside, Smith and Kambhampati, Cryptocercus punctulatus Scudder, Cryptocercus wrighti Burnside, Smith and Kambhampati) of the five species in the United States occur in the Appalachian Mountains and one (Cryptocercus clevelandi Byers) occurs in the Pacific Northwest. Previous studies have indicated that the distribution of the Appalachian species is allopatric; however, three of the four species occurred within a few kilometers of one another. Therefore, an intensive survey was undertaken in southwestern North Carolina and northern Georgia to determine if the species distributions are indeed allopatric or if there are zones of sympatry. With regard to C. clevelandi, previous studies have included samples from only a handful of locations in the western United States. Therefore, an extensive survey was undertaken in Oregon and California to map the geographic distribution of C. clevelandi and estimate genetic variation within and among populations. Our surveys indicated that three (C. darwini, C. garciai, C. wrighti) of the four Appalachian species have overlapping distributions in southwestern North Carolina and northern Georgia, a finding that raises the possibility of the existence of hybrid zones. Genetic variation among samples collected in Oregon and California indicated that, unlike the eastern United States, there is only one species of Cryptocercus in the western United States. In addition, we found that the distribution of C. clevelandi is considerably more patchy than that of the Appalachian species.

A natural grey kangaroo hybrid?

The eastern grey kangaroo, Macropus giganteus, and the western grey kangaroo, M. fuliginosus, occur over an extensive zone of sympatry, but there has been no record of hybridisation between these sibling species in the field. The two species can be reliably distinguished by their characteristic pelage, which is grey-brown in eastern grey kangaroos and chocolate brown in western grey kangaroos. The pattern of colour on the face is particularly distinctive: the pale grey face of the eastern grey kangaroo contrasts with dark shading around the eyes, whereas the western grey kangaroo has a monochrome dark brown face. We report a sighting of a probable hybrid grey kangaroo in June 2000 at Willandra National Park (33° 13’ S, 145° 07’ E), western New South Wales. This individual was an adult female, which was intermediate in phenotype and was accompanied by a possible back-crossed sub-adult. In view of this and other reports of putative hybrids, we urge greater scrutiny of grey kangaroos in the sympatric zone.

The zone of sympatry and hybridization of Mytilus edulis and M. galloprovincialis, as described by intron length polymorphism at locus mac-1.

Intron-size variation at the actin gene locus mac-1 was used to characterize mussel, Mytilus spp., populations in the approximately 2000-km wide zone of contact and hybridization ('hybrid zone') between M. edulis and M. galloprovincialis in western Europe. Twenty-five samples were collected in 1995-99 in locations within the hybrid zone and from reference populations of each species. We used correspondence analysis on the matrix of allelic frequencies to determine which alleles are characteristic of each species, and to characterize samples along the genetic gradient between M. edulis and M. galloprovincialis. In the hybrid zone, some samples exhibited mac-1 allele frequencies that were typical of M. edulis; other samples were distributed along the M. edulis/M. galloprovincialis gradient and displayed variable levels of intergradation that were not correlated with geography. Some of the latter samples exhibited significant heterozygote deficiencies. The simple admixture hypothesis (Wahlund effect) could not be rejected for two-fifths of the samples. The hybrid zone thus appeared as a mosaic of populations which are either pure M. edulis, or hybrid between M. galloprovincialis and M. edulis, or a mixture of the foregoing with M. galloprovincialis individuals. These results were consistent with published allozyme data, suggesting that they can be extended to the entire nuclear genome. M. edulis mac-1 alleles were present at moderate frequency in Atlantic M. galloprovincialis, and at significantly lower frequency in some Mediterranean samples. This pattern was homogeneous over a broad geographical range within each basin. It was not evident that introgression of M. edulis into M. galloprovincialis presently occurs south of the zone of contact. We propose that the distinctness of the Atlantic M. galloprovincialis population results from past introgression by M. edulis alleles.

SPECIES INTRODUCTIONS AND THEIR ECOLOGICAL CONSEQUENCES: AN EXAMPLE WITH CONGENERIC SUNFISH

Pumpkinseed (Lepomis gibbosus) and redear sunfish (L. microlophus) are sister species with largely allopatric native ranges. For purposes of sport fishery enhancement, redear have been introduced into lakes of southern Michigan, and as a result, a large zone of artificial sympatry of pumpkinseed and redear has been created. Redear and, to a lesser extent, pumpkinseed are morphologically and behaviorally specialized molluscivores, and we hypothesized that introduced redear would have strong negative effects on pumpkinseed due to competition for snails, which are each species' main adult resource. Specifically, we predicted that redear would reduce snail availability, alter pumpkinseed diet, and reduce pumpkinseed growth and density. To examine these predictions, we surveyed pumpkinseed diets, growth, and densities, as well as snail availability in lakes with and without introduced redear. We also conducted a controlled field experiment (target–neighbor design) in which relative neighbor densities of each species were manipulated and the effects on target individuals of each species were measured. This experiment was designed to explore mechanisms underlying the competitive interactions suggested by the longer-term field study. Together, the field patterns and short-term experimental results demonstrate that the introduction of redear negatively affected the native pumpkinseed: (1) In lakes where redear had been introduced, the abundance of pumpkinseed declined an average of 56%, while average pumpkinseed abundance increased 60% in lakes without introduced redear. (2) In the presence of redear, pumpkinseed had fewer snails in their diets and showed only a weak ontogenetic niche shift to feeding on snails. (3) Snail biomass was ∼69% lower in lakes where redear had been introduced, and snail biomass was ∼50% lower in experimental treatments containing redear instead of pumpkinseed. In the experiment, pumpkinseed growth was reduced in the presence of redear. However, contrary to our predictions, pumpkinseed growth rates did not differ between lakes with redear vs. those without redear. We suggest that a reduction in pumpkinseed growth rate is an expected short-term response to redear introduction, whereas the long-term response is a reduction in pumpkinseed density.

Sympatry in theCulicoides variipennisComplex (Diptera: Ceratopogonidae): a Taxonomic Reassessment

We report sympatry among larval populations of the Culicoides variipennis complex in widespread and diverse aquatic habitats throughout the United States. Six sites in California, Nevada, New Mexico, and Texas were co-inhabited by C. v. occidentalis and C. v. sonorensis, whereas 8 sites in Florida, Georgia, Louisiana, Maryland, and Texas were co-occupied by C. v. sonorensis and C. v. variipennis. No intermediate forms were identified either electrophoretically or morphologically in adults reared from field-collected larvae and pupae. The absence of intergrades in zones of sympatry represents sufficient evidence to confirm species status for Culicoides variipennis (Coquillett) and Culicoides occidentalis Wirth & Jones, and to elevate Culicoides sonorensis to species rank (NEW STATUS). Culicoides v. albertensis Wirth & Jones is a synonym of C. sonorensis (NEW SYNONYMY); C. v. australis Wirth & Jones also is confirmed as a synonym of C. sonorensis. We also demonstrated a correlation between population taxonomic status as determined by electrophoresis and adult morphology.

Flexible niche partitioning via a foraging mode shift: a proposed mechanism for coexistence in stream‐dwelling charrs

Summary1. Foraging behaviour, diet and interference competition were examined for two morphologically similar charrs,Salvelinus malma(Dolly Varden) andS. leucomaenis(white‐spotted charr), under varying food resource conditions over four summers in a Japanese mountain stream. Data were used to test predictions from a mechanistic model of resource partitioning developed from an earlier field experiment.2. The charrs adopted one of two distinct foraging modes, ambushing drifting invertebrates from relatively fixed foraging positions or actively searching for benthic prey over large areas. The proportion of benthos foragers increased markedly when drifting prey declined, and was much greater inS. malmathanS. leucomaeniswhen drift was lean, upholding predictions made from our earlier experiment.3. For drift foragers of both species, frequency of foraging attempts decreased as drift rate declined, and aggressive encounters increased. In contrast, for benthos foragers of both species the frequency of foraging attempts was essentially constant across the range of benthos biomass measured, and aggressive encounters remained low.4. Salvelinus leucomaenisate larger drifting prey thanS. malma, even though the former charr were smaller. In contrast,S. malmaforaged on benthic prey at a higher rate thanS. leucomaenis, although there was no difference in prey mass. Thus, the optimal point to shift to benthic foraging is at a higher drift threshold forS. malmathanS. leucomaenis, most probably due to differences in jaw morphology. Moreover, because dominance for favourable drift foraging positions was based on size alone,S. malmashifted to benthos foraging at a larger size thanS. leucomaenis, as predicted by a simple model.5. Charr consumed distinct prey types according to their foraging mode. Drift foragers primarily ate terrestrial invertebrates, whereas benthos foragers ate mainly chironomid larvae. Consequently, diet overlap was high when drift was abundant and both species were drift foragers, but declined as drift declined andS. malmashifted to benthos foraging. Therefore, species‐specific differences in foraging mode shifts across the resource gradient explained the flexible resource partitioning we observed, and probably account for the coexistence of these congeneric charrs in zones of sympatry in northern Japan.

Can environmental fluctuation prevent competitive exclusion in sympatric flycatchers?

Ecology has been characterized by a central controversy for decades: namely, whether the distribution and abundance of organisms are determined by species interactions, such as competitive exclusion, or by environmental conditions. In part, this is because competitive exclusion has not been convincingly demonstrated in open, natural systems. In addition, traditional theoretical models cannot predict the outcome of competitive interactions in the presence of environmental variability. In this paper we document the limiting influence of strong interspecific competition on population dynamics and nestling mortality in a mixed population of pied flycatchers ( Ficedula hypoleuca ) and collared flycatchers ( F. albicollis ) in a narrow zone of sympatry. Whereas the former species was limited mainly by interspecific competition, the latter species was limited by the concerted influences of intraspecific competition and climate. The analysis suggests a march towards competitive exclusion of the pied flycatcher during warm periods. However, competitive exclusion is apparently prohibited on a local scale because intraspecific competition among individual collared flycatchers intensifies when they are forced to cope with severe environmental conditions, promoting the temporary and local presence of pied flycatchers.

Influence of Vegetation on Occupation of Nest Boxes by Chestnut-Backed and Mountain Chickadees

We analyzed how vegetation structure and nest box orientation influenced patterns of nest box occupancy by chestnut-backed (Poecile rufescens) and mountain (R gambeli) chickadees in the western Sierra Nevada for 3 breeding seasons. The range expansion by the chestnut-backed chickadee during the past 50 years into forests where the mountain chickadee was formerly the only resident parid provided an opportunity to examine how these 2 closely related species use forest resources in a zone of recent sympatry. Our study was conducted about 35 to 40 years after chestnut-backed chickadees colonized the western Sierra Nevada. Although there was broad overlap in the habitat structure around the boxes occupied by the 2 species, chestnut-backed chickadees tended to occupy boxes that were in denser, more closedcanopy stands and mountain chickadees tended to occupy boxes in stands with a more open, park-like structure. Both species clearly avoided nest boxes and habitat plots with a western orientation and consistently used boxes in patches of habitat that were oriented north, east, and south.

Cytotype distribution at a diploid–tetraploid contact zone inChamerion(Epilobium)angustifolium(Onagraceae)

In North America, the geographic distributions of diploid and tetraploid Chamerion (formerly Epilobium) angustifolium overlap in a narrow zone along the southern border of the boreal forest and along the Rocky Mountains. We examined the frequency and distribution of diploid and tetraploid cytotypes in a narrow (5 km) zone of sympatry across an elevational gradient and in putatively uniform diploid and tetraploid reference populations on the Beartooth Pass, in the Rocky Mountains of southern Montana-northern Wyoming. All five reference populations sampled were dominated by a single cytotype, but only one was completely uniform. In the zone of sympatry, 27 transects were sampled every 2 m for a total of 238 plants. Reproductive status (vegetative, flower buds, open flowers) was recorded, and the ploidy of each plant was determined by flow cytometry. Diploid and tetraploid plants predominated (36 and 55%, respectively) but were heterogeneously distributed among the transects. Six of the 27 transects were fixed for a single cytotype (four transects, diploid; two transects, tetraploid), and in seven others either diploids or tetraploids predominated (frequency >75%). Triploids represented 9% of the total sample and occurred most frequently in transects containing both diploids and tetraploids (G = 3.4, df = 2, P = 0.07). Diploids were more often reproductive (in bud, flower, or fruit) than either triploids or tetraploids (G = 12.0, df = 2, P < 0.001) and were the only cytotype to have produced open flowers. These results suggest that the zone of sympatry is best characterized as a mosaic rather than a cline, with diploid and tetraploids in close proximity and that the distribution of polyploidy is regulated by ecological sorting in a heterogeneous physical environment.

HOST SPECIFICITY AND THE GENETIC STRUCTURE OF TWO YUCCA MOTH SPECIES IN A YUCCA HYBRID ZONE.

Host specialization is an important mechanism of diversification among phytophagous insects, especially when they are tightly associated with their hosts. The well-known obligate pollination mutualism between yucca moths and yuccas represent such an association, but the degree of host specificity and modes of specialization in moth evolution is unclear. Here we use molecular tools to test the morphology-based hypothesis that the moths pollinating two yuccas, Yucca baccata and Y. schidigera, are distinct species. Host specificity was assessed in a zone of sympatry where the hosts are known to hybridize. Because the moths are the only pollinators, the plant hybrids are evidence that the moths occasionally perform heterospecific pollination. Nucleotide variation was assessed in a portion of the mitochondrial gene COI, and in an intron within a nuclear lysozyme gene. Moths pollinating Y. baccata and Y. schidigera were inferred to be genetically isolated because there was no overlap in alleles at either locus, and all but one of the moths was found on their native host in the hybrid zone. Moreover, genetic structure was very weak across the range of each moth species: estimates of FST for the lysozyme intron were 0.043 (SE = ± 0.004) and 0.021 (SE = ± 0.006) for the baccata and schidigera pollinators, respectively; estimated FST for COI in the baccata moths was 0.228 (± 0.012), whereas schidigera pollinators were fixed for a single allele. These results reveal a high level of migration among widely separated moth populations. We predict that pollen-mediated gene flow among conspecific yuccas is considerable and hypothesize that geographic separation is a limited barrier both for yuccas and for yucca moths.

Absence of Extensive Introgression between Xenopus gilli and Xenopus laevis laevis (Anura: Pipidae) in Southwestern Cape Province, South Africa

The evolutionary consequences of hybridization, including introgression, have been studied in many anuran amphibians (Lamb and Avise, 1986; Szymura and Barton, 1986), particularly in the African clawed frogs of the genus Xenopus (Tinsley, 1981; Kobel, 1996a; Picker et al., 1996). Species of Xenopus are found over much of sub-Saharan Africa and are mostly allopatric, though zones of sympatry and naturally occurring hybrids have been described (Picker, 1985; Tinsley et al., 1996; Yager, 1996). Although hybridization has been linked to allopolyploid speciation in Xenopus (Carr et al., 1987, Kobel, 1996b), the extent to which hybridization has facilitated genetic introgression among species has not been assessed.

Comparative displays among four species of tragopans and their derivation and function

This paper provides the first description and comparison of courtship behaviours in males of four species of tragopans in captivity. Inter-specific comparisons of each of the five displays (frontal, lateral, wing whir, flight, and lappet dropping) were similar with one main difference; Temminck's and Satyr tragopans assumed an arched posture, in addition to the upright posture during the lateral display. All frontal and lateral displays were performed only in front of females and most likely have a sexual function. In contrast, wing whir, flight, and lappet dropping displays were also performed in front of other males. Therefore, these displays are most likely to be sexual and aggressive in function and may also be a form of comfort behaviour in the lappet display. The first female displays are described for this genus. Intra- and inter-specific lateral displays in females were only directed towards other females and probably function as a threat display. Because of the similarities of the five displays among the four species of tragopans studied, courtship displays do not appear to serve as behavioural isolating mechanisms in zones of sympatry. However, lappet dropping, flight, and frontal displays are unique to this genus and probably function to reduce intergeneric hybridization. Female choice is probably based on differences in plumage colouration, lappet pattern and colouration, and vocalizations rather than display characteristics, as the former characteristics have diverged considerably as a result of sexual selection.

FUNCTIONAL LINKAGES AMONG MORPHOLOGY, FEEDING PERFORMANCE, DIET, AND COMPETITIVE ABILITY IN MOLLUSCIVOROUS SUNFISH

Pumpkinseed (Lepomis gibbosus) and redear (Lepomis microlophus) sunfish are phylogenetic sister species that are specialized for molluscivory. The native ranges of these two sunfishes are allopatric, except for a small region of overlap in the Carolinas. However, for purposes of sport fishery enhancement, redear have been introduced into many northern lakes where pumpkinseed populations were native. As a result, a large zone of artificial sympatry of pumpkinseed and redear populations has been created in the last decade in lakes of southern Michigan. In this study I examined the feeding performance and diets of co-occurring redear and pumpkinseed both in Michigan lakes and in laboratory studies of these sunfish species from those lakes. Previous research on these species suggests that redear are morphologically and behaviorally more specialized for molluscivory than pumpkinseed, and results from the present study showed that at a given size, the crushing capacity of redear was -2X greater than that of pumpkinseed. For at least the smaller size classes of sunfish, the extent' of molluscivory was constrained by crushing strength. Therefore, greater crushing strength of redear allowed them to undergo an ontogenetic niche shift from a diet of predominantly soft-bodied invertebrates to a diet of snails at a smaller size than co-occurring pumpkinseed. Once molluscivorous, redear consumed larger snails than did pumpkinseed. Diet analysis also showed that following their earlier niche shift, redear were significantly more mol- luscivorous than sympatric pumpkinseed. Summing the effects of greater individual con- sumption of snails by redear and the effects of a greater proportion of a redear population being molluscivorous due to their earlier niche shift, redear were predicted to be more proficient competitors for snails. It appeared, however, that the superior feeding ability of redear on hard-bodied prey may have come at the cost of reduced efficiency at feeding on soft-bodied invertebrates.

Functional Linkages Among Morphology, Feeding Performance, Diet, and Competitive Ability in Molluscivorous Sunfish

Pumpkinseed (Lepomis gibbosus) and redear (Lepomis microlophus) sunfish are phylogenetic sister species that are specialized for molluscivory. The native ranges of these two sunfishes are allopatric, except for a small region of overlap in the Carolinas. However, for purposes of sport fishery enhancement, redear have been introduced into many northern lakes where pumpkinseed populations were native. As a result, a large zone of artificial sympatry of pumpkinseed and redear populations has been created in the last decade in lakes of southern Michigan. In this study I examined the feeding performance and diets of co-occurring redear and pumpkinseed both in Michigan lakes and in laboratory studies of these sunfish species from those lakes. Previous research on these species suggests that redear are morphologically and behaviorally more specialized for molluscivory than pumpkinseed, and results from the present study showed that at a given size, the crushing capacity of redear was ∼2× greater than that of pumpkinseed. For at least the smaller size classes of sunfish, the extent of molluscivory was constrained by crushing strength. Therefore, greater crushing strength of redear allowed them to undergo an ontogenetic niche shift from a diet of predominantly soft-bodied invertebrates to a diet of snails at a smaller size than co-occurring pumpkinseed. Once molluscivorous, redear consumed larger snails than did pumpkinseed. Diet analysis also showed that following their earlier niche shift, redear were significantly more molluscivorous than sympatric pumpkinseed. Summing the effects of greater individual consumption of snails by redear and the effects of a greater proportion of a redear population being molluscivorous due to their earlier niche shift, redear were predicted to be more proficient competitors for snails. It appeared, however, that the superior feeding ability of redear on hard-bodied prey may have come at the cost of reduced efficiency at feeding on soft-bodied invertebrates.