THE DYNAMICS OF TWO HYBRID ZONES IN APPALACHIAN SALAMANDERS OF THE GENUS PLETHODON

Two zones of intergradation between populations of Plethodon have been studied for 18 and 20 years, respectively. The data consist of systematic scores of colors, made at least twice annually. Near Heintooga Overlook in the Balsam Mountains (Great Smoky Mountains National Park), the salamanders' cheeks are gray. Proceeding north toward the Smokies, there is increasing frequency and intensity of red color at two, four, and six miles. There has been no change in the scores at any location. The width of the zone and our failure to detect any change can be explained by assuming neutrality of the character and random diffusion during the probable time since contact between the two intergrading forms, which most likely took place after the Hypsithermal Interval, 8,000–5,000 BP. At Coweeta Hydrologic Laboratory in the Nantahala Mountains, Plethodon jordani and P. glutinosus hybridize at intermediate elevations. The lateral white spots of glutinosus decrease and the red on the legs of jordani increases with elevation from 685 m to 1,052 m. At the higher elevation, the proportion of animals scored as “pure” jordani declined significantly from 1974 to 1990, an indication that the hybrid zone is spreading upward. The rate of spread is too great to be explained by random diffusion, so selection for glutinosus characters is the best explanation. The rate of spread of the hybrid zone indicates that hybridization began 60–65 years ago, at the end of the time of intense timbering. Such human disturbances have caused hybridization in other organisms.     

Beyond simple adaptation: Incorporating other evolutionary processes and concepts into eco-evolutionary dynamics

Studies of eco-evolutionary dynamics have integrated evolution with ecological processes at multiple scales (populations, communities and ecosystems) and with multiple interspecific interactions (antagonistic, mutualistic and competitive). However, evolution has often been conceptualised as a simple process: short-term directional adaptation that increases population growth. Here we argue that diverse other evolutionary processes, well studied in population genetics and evolutionary ecology, should also be considered to explore the full spectrum of feedback between ecological and evolutionary processes. Relevant but underappreciated processes include (1) drift and mutation, (2) disruptive selection causing lineage diversification or speciation reversal and (3) evolution driven by relative fitness differences that may decrease population growth. Because eco-evolutionary dynamics have often been studied by population and community ecologists, it will be important to incorporate a variety of concepts in population genetics and evolutionary ecology to better understand and predict eco-evolutionary dynamics in nature.     

Particle transport by benthic invertebrates: its role in egg bank dynamics

The ecological and evolutionary dynamics of zooplankton is in part a function of the numbers and ages of dispausing eggs hatching from aquatic sediments. Successful recruitment from this egg bank must depend upon the eggs being present at or near the sediment surface. Often, however, zooplankton diapausing eggs are found as deep as 15 to 30 cm in the mud. Bioturbation may provide a mechanism for the regular return of buried eggs to the sediment surface. A substantial portion of the population of the copepod, Diaptomus sanguineus, living in Bullhead Pond, a small lake in Rhode Island, USA, is present as diapausing eggs. To study the role of bioturbation in egg-bank dynamics, we introduced polystyrene beads, the same size and specific gravity as copepod eggs, at two depths in large-diameter sediment chambers in the laboratory. Treatments included chambers with natural and reduced densities of benthos. Consistent with other studies, our results show that the joint activities of tubificid oligochaetes and chironomid larvae are responsible for bidirectional (up and down) transport of beads in the top 2 cm of the sediment. We observed no bead movement below this depth. Thus, eggs in the top two centimeters of sediment in this lake are exposed with some regularity to conditions that stimulate hatching at the sediment-water interface. In Bullhead Pond, these eggs have a mean age of 12.2 years (based on ²¹⁰Pb-dating). Eggs buried more deeply will only be returned to the sediment surface by relatively rare, localized disturbances. This return of old eggs to the surface affects ecological and evolutionary dynamics in a complex way.     

Decline of the invasive submersed macrophyte Myriophyllum spicatum (Haloragaceae) associated with herbivory by larvae of Acentria ephemerella (Lepidoptera)

Myriophyllum spicatum, an exotic submersed macrophyte causing serious lake management problems throughout much of North America, decreased markedly in biomass in Cayuga Lake, NY, USA, since the beginning of the 1990s. Over the same period, however, the total biomass of all species of submersed macrophytes did not decline, and native macrophytes gained in abundance. The aquatic moth larva, Acentria ephemerella, was first observed on milfoil plants in Cayuga Lake in 1991. However, due to its cryptic habit the larva may have been present prior to that year. When the density of these grazers is high, herbivory by Acentria causes severe damage to the apical meristem of M. spicatum. This moth larva and another milfoil herbivore, Euhrychiopsis lecontei are widespread in 26 lakes surveyed in New York State; they are present in 25 and 24 lakes, respectively. Estimates of Acentria larval densities in summer in Cayuga Lake are 27 to 100 m^-2, and a quantitative survey of larvae hibernating in milfoil stems revealed mean densities of 500 m^-2 in late fall in Seneca Lake. In laboratory experiments, Acentria larvae feed on a wide variety of macrophytes commonly found in New York State. Although Acentria is not a specialist feeder, its life cycle is closely tied to M. spicatum through the moth's use of apical tips and stems for summer and winter refuges; thus deleterious damage to other macrophytes is low. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Effect of pH on Staining by Eosins

The effect of pH on staining by 7 dyes of the eosin group was studied by means of specimens fixed 12-18 hr in Gilson's, in Petrunkewitsch's, or in Zenker's fluids, respectively. Sections were soaked in water or alcohol, depending upon the stain solvent, adjusted in 31 equal steps from pH 4.0 to 10.0 before being stained in similarly adjusted dye solutions. The stained sections were rinsed at H-ion concentrations identical to that at which staining took place, until no further color came away, and mounted. Variations in pH elicited different degrees of stain retention; different fixation altered staining affinity even though staining was effected at identical pH values.     

Population differences in the timing of diapause: adaptation in a spatially heterogeneous environment

Summary Populations of the planktonic copepod, Diaptomus sanguineus, live in permanent and temporary freshwater ponds in Rhode Island. All ponds in which they occur become uninhabitable at some time during the year, but the nature and timing of the harsh period varies both spatially and temporally. Females produce discrete clutches either of subitaneous eggs which hatch immediately or of diapausing eggs which hatch the following season. The two egg types show distinct chorion morphologies under transmission electron microscopy. In permanent ponds the copepods start making diapausing eggs in March, one month before rising water temperatures induce planktivorous sunfish to become active. In temporary ponds diapausing eggs are produced, in a complex pattern from May to July, before the water disappears in late summer or early fall.We investigated the spatial scale at which D. sanguineus is adapted to this complex environment. In a reciprocal transfer experiment between temporary and permanent bodies of water, female copepods placed in new ponds made subies of water, female copepods placed in new ponds made subitaneous and diapausing eggs in the same sequence as control females retained in their home ponds. The copepod populations enter diapause at times appropriate for the local habitat conditions they experience, but inappropriate for other, nearby ponds. Transplanted females were unable to sense a change in pond type or to adjust egg production accordingly. We conclude that D. sanguineus populations are adapted to the specific conditions of isolated ponds rather than to a broader geographical region containing several pond types.     

Linking genes to communities and ecosystems: Daphnia as an ecogenomic model

How do genetic variation and evolutionary change in critical species affect the composition and functioning of populations, communities and ecosystems? Illuminating the links in the causal chain from genes up to ecosystems is a particularly exciting prospect now that the feedbacks between ecological and evolutionary changes are known to be bidirectional. Yet to fully explore phenomena that span multiple levels of the biological hierarchy requires model organisms and systems that feature a comprehensive triad of strong ecological interactions in nature, experimental tractability in diverse contexts and accessibility to modern genomic tools. The water flea Daphnia satisfies these criteria, and genomic approaches capitalizing on the pivotal role Daphnia plays in the functioning of pelagic freshwater food webs will enable investigations of eco-evolutionary dynamics in unprecedented detail. Because its ecology is profoundly influenced by both genetic polymorphism and phenotypic plasticity, Daphnia represents a model system with tremendous potential for developing a mechanistic understanding of the relationship between traits at the genetic, organismal and population levels, and consequences for community and ecosystem dynamics. Here, we highlight the combination of traits and ecological interactions that make Daphnia a definitive model system, focusing on the additional power and capabilities enabled by recent molecular and genomic advances.     

Rapid contemporary evolution and clonal food web dynamics

Character evolution that affects ecological community interactions often occurs contemporaneously with temporal changes in population size, potentially altering the very nature of those dynamics. Such eco-evolutionary processes may be most readily explored in systems with short generations and simple genetics. Asexual and cyclically parthenogenetic organisms such as microalgae, cladocerans and rotifers, which frequently dominate freshwater plankton communities, meet these requirements. Multiple clonal lines can coexist within each species over extended periods, until either fixation occurs or a sexual phase reshuffles the genetic material. When clones differ in traits affecting interspecific interactions, within-species clonal dynamics can have major effects on the population dynamics. We first consider a simple predator–prey system with two prey genotypes, parametrized with data from a well-studied experimental system, and explore how the extent of differences in defence against predation within the prey population determine dynamic stability versus instability of the system. We then explore how increased potential for evolution affects the community dynamics in a more general community model with multiple predator and multiple prey genotypes. These examples illustrate how microevolutionary ‘details’ that enhance or limit the potential for heritable phenotypic change can have significant effects on contemporaneous community-level dynamics and the persistence and coexistence of species.     

Temporal dispersal: ecological and evolutionary aspects of zooplankton egg banks and the role of sediment mixing

Zooplankton egg banks are the accumulation of diapausing embryosof planktonic animals buried in the sediments of many aquaticecosystems. These eggs, which are analogous life history stagesto the seeds of many plants, can survive in a ready-to-hatchstate for periods ranging from a few years to greater than acentury. Their presence in ponds, lakes and near-shore marineenvironments has substantial implications for understandingtrajectories of ecological and evolutionary change. When thesediments of lakes are structured in historical sequence, diapausingeggs extracted from different sediment ages can provide a meansof studying past changes in community or population-geneticstructure. A completely different aspect of egg banks derivesfrom the fact that hatching of diapausing eggs can influence,through what can be thought of as temporal dispersal, populationand community response to environmental change. Eggs hatchingfrom diapause introduce to current environments species or genotypeslaid at times in the distant past. In addition, egg banks createextended generation overlap that can play an important rolein maintaining diversity in a fluctuating environment when differenttypes (species or genotypes) are favored at different times.These distinct aspects of egg banks (i.e., their direct impacton ecological and evolutionary processes versus their usefulnessin reconstructing historical changes), are potentially in conflictbecause for old eggs to hatch, the sediments must be at leastpartially mixed. This same mixing, however, degrades the accuracyof the historical record. Both aspects are possible, however,even within a single lake when sediment-mixing intensity isspatially heterogeneous.