The ecological hypothesis of speciation is that reproductive isolation evolves ultimately as a consequence of divergent natural selection on traits between environments. Ecological speciation is general and might occur in allopatry or sympatry, involve many agents of natural selection, and result from a combination of adaptive processes. The main difficulty of the ecological hypothesis has been the scarcity of examples from nature, but several potential cases have recently emerged. I review the mechanisms that give rise to new species by divergent selection, compare ecological speciation with its alternatives, summarize recent tests in nature, and highlight areas requiring research.

/pdf/ecology-and-the-origin-of-species-38r7yix3b6.pdf

Ecology and the origin of species

As plants develop from seeds to seedlings, juveniles and mature stages, their ontogeny can constrain the expression of resistance to herbivore damage. Nevertheless, ecological and evolutionary theories regarding interactions between plants, herbivores and their natural enemies are largely based on observations and experiments conducted at a single ontogenetic stage. Owing to resource allocation and architectural constraints in plants, and the influence of herbivore foraging behavior, resistance to herbivores is likely to change during plant development. We propose that such changes are likely to occur in a non-linear fashion and suggest that the role of ontogeny should be incorporated as an important factor in new syntheses of plant defense theory.

Facing herbivory as you grow up: the ontogeny of resistance in plants

Many biologists now recognize that environmental variance can exert important effects on patterns and processes in nature that are independent of average conditions. Jensen’s inequality is a mathematical proof that is seldom mentioned in the ecological literature but which provides a powerful tool for predicting some direct effects of environmental variance in biological systems. Qualitative predictions can be derived from the form of the relevant response functions (accelerating versus decelerating). Knowledge of the frequency distribution (especially the variance) of the driving variables allows quantitative estimates of the effects. Jensen’s inequality has relevance in every field of biology that includes nonlinear processes.

/pdf/jensen-s-inequality-predicts-effects-of-environmental-53nr4kdxrn.pdf

Jensen's inequality predicts effects of environmental variation.

The degradation of inland aquatic habitats caused by decades of human activities has led to worldwide efforts to rehabilitate freshwater habitats for fisheries and aquatic resources. We reviewed published evaluations of stream rehabilitation techniques from throughout the world, including studies on road improvement, riparian rehabilitation, floodplain connectivity and rehabilitation, instream habitat improvement, nutrient addition, and other, less-common techniques. We summarize current knowledge about the effectiveness of these techniques for improving physical habitat and water quality and increasing fish and biotic production. Despite locating 345 studies on effectiveness of stream rehabilitation, firm conclusions about many specific techniques were difficult to make because of the limited information provided on physical habitat, water quality, and biota and because of the short duration and limited scope of most published evaluations. Reconnection of isolated habitats, floodplain rehabilita...

/pdf/global-review-of-the-physical-and-biological-effectiveness-qag8583tm5.pdf

Global Review of the Physical and Biological Effectiveness of Stream Habitat Rehabilitation Techniques

The shoot system of higher plants passes through several different phases during its development. Each of these phases is characterized by a unique set of morphological and physiological attributes. The intermediate character of the structures produced during phase changes and the phenotypes of mutations that affect this process demonstrate that these phases are specified by independently regulated, overlapping developmental programs. Transitions between phases appear to be initiated by factors extrinsic to the shoot apical meristem; the ability of the shoot to respond to such factors and to remain in a particular phase of development is regulated by factors intrinsic to the meristem. The possibility that developmental phases are maintained by epigenetic cell states and the role of DNA methylation in this process are discussed.

https://science.sciencemag.org/content/sci/250/4983/923.full.pdf

Phase change and the regulation of shoot morphogenesis in plants.

The hypothesis that individual plants undergo predictable changes in their resistance to herbivory as they mature was experimentally tested. The performance of two species of insects transferred onto different-aged trees of the same naturally occurring clones of narrowleaf cottonwood, Populus angustifolia, showed opposing and significant changes in host resistance as a function of tree age. The gall-forming aphid Pemphigus betae was 70 times as common on mature trees as on juvenile trees. Transfer experiments demon- strated that this pattern is adaptive; survivorship on mature trees was 50% higher than on juvenile trees. The leaf-feeding beetle Chrysomela confluens exhibited opposite distribution on hosts, with densities 400 times as high on juvenile as on mature trees. This is also adaptive; larvae transferred to mature hosts had 50% lower survival and took 25% longer to reach adulthood than those transferred to juvenile trees. These survivorship and per- formance data and our inability to support the hypotheses of other mechanisms that would produce the observed distributions suggest that the distributions result from the insects' preferences for different-aged hosts. Several basic implications emerge. (1) Developmental changes in resistance and sus- ceptibility of hosts are important components in determining the distribution of herbivores. (2) A single plant can change rapidly in its resistance traits with age such that a 10-fold change in resistance can occur over a 2-yr period. (3) As trees mature, increased and decreased resistances to different herbivores can produce nonoverlapping herbivore dis- tributions that could be misinterpreted as being the result of competition (e.g., in this study, beetle abundance was strongly, negatively correlated with aphid abundance). (4) While host maturation processes have large effects on within- and between-tree variation in resistance to herbivores, the implications for herbivore population dynamics and community struc- turing have not been generally appreciated.

https://www.jstor.org/stable/info/1937547

Developmental Changes in Resistance to Herbivory: Implications for Individuals and Populations

Using the free-feeding beetle Chrysomela confluens (Coleoptera: Chryso- melidae), we examined the hypothesis that naturally occurring hybrid zones between two cottonwood species are "sinks" or centers of insect abundance. Over a 3-yr period, we found that 94% of the C. confluens population was restricted to a 13-km hybrid zone, which represents <3% of the cottonwood population. Of several potential mechanisms tested, expanded host phenology in the hybrid zone, relative to pure zones, best explained the distribution of C. confluens which is dependent upon newly flushed leaves for optimal growth and reproduction. The hybrid zone is a superior beetle habitat because: (1) early leaf flush in the hybrid zone provides the first source of food for beetles in spring and (2) staggered leaf phenologies in the hybrid zone allow beetles to shift onto newly flushed Fremont trees as foliage of sympatric hybrid and narrowleaf trees declines in quality. This shift by ovipositing females can result in a 600% increase in fecundity relative to nonshifting females. Additionally, a 1 0-yr common garden experiment shows that there is a strong genetic component to the timing of leaf flush, which suggests a long-term stability of resources in the hybrid zone that beetles can rely on. For C. confluens, the hybrid zone is a phonologicall sink" that increases beetle fecundity and leads to chronically high herbivory year after year. Because movement from the hybrid zone reduces fecundity, emigration into adjacent pure zones is likely hindered. This hy- pothesis requires staggered phenologies between overlapping host species; it probably does not require hybridization between host species. Further, this hypothesis may in part explain frequent outbreaks of insects in nurseries and plantations. Thus, two distinct mechanisms have been identified that result in increased insect abundance in plant hybrid zones: "phe- nological sinks" resulting from staggered plant phenologies and "hybrid sinks" resulting from increased susceptibility of hybrids to insect attack. Such studies suggest that hybrid zones are important natural laboratories for the study of plant-herbivore interactions.

Elevated herbivory in plant hybrid zones: Chrysomela confluens, populus and phenological sinks

Regulated river restoration through planned flooding involves trade-offs be- tween aquatic and terrestrial components, between relict pre-dam and novel post-dam re- sources and processes, and between management of individual resources and ecosystem characteristics. We review the terrestrial (wetland and riparian) impacts of a 1274 m 3 /s test flood conducted by the U.S. Bureau of Reclamation in March/April 1996, which was designed to improve understanding of sediment transport and management downstream from Glen Canyon Dam in the Colorado River ecosystem. The test flood successfully restored sandbars throughout the river corridor and was timed to prevent direct impacts to species of concern. A total of 1275 endangered Kanab ambersnail (Oxyloma haydeni kan- abensis) were translocated above the flood zone at Vaseys Paradise spring, and an estimated 10.7% of the total snail habitat and 7.7% of the total snail population were lost to the flood. The test flood scoured channel margin wetlands, including potential foraging habitats of endangered Southwestern Willow Flycatcher (Empidonax traillii extimus). It also buried ground-covering riparian vegetation under .1 m of fine sand but only slightly altered woody sandbar vegetation and some return-current channel marshes. Pre-flood control ef- forts and appropriate flood timing limited recruitment of four common nonnative perennial plant species. Slight impacts on ethnobotanical resources were detected .430 km down- stream, but those plant assemblages recovered rapidly. Careful design of planned flood hydrograph shape and seasonal timing is required to mitigate terrestrial impacts during efforts to restore essential fluvial geomorphic and aquatic habitats in regulated river eco- systems.

Planned flooding and colorado river riparian trade-offs downstream from glen canyon dam, arizona

We examined diets of six insectivorous bird species (n = 202 individuals) from two vegetation zones along the Colorado River in Grand Canyon National Park, Arizona, 1994. All bird species consumed similar quantities of caterpillars and beetles, but use of other prey taxa varied. Non-native leafhoppers (Opsius stactagolus) specific to non-native tamarisk (Tamarix chinensis) substantially augmented Lucy's Warbler (Vermivora luciae) diets (49%), while ants comprised 82% of Yellow-breasted Chat (Icteria virens) diets. Yellow Warbler (Dendroica petechia) diets were composed of 45% aquatic midges. All bird species consumed the non-native leafhopper specific to tamarisk. Comparison of bird diets with availability of arthropod prey from aquatic and terrestrial origins showed terrestrial insects comprised 91% of all avian diets compared to 9% of prey from aquatic origin. Seasonal shifts in arthropod prey occurred in diets of three bird species, although no seasonal shifts were detected in arthropods sampl...

http://charlesvanriper.com/gallery/cartimages/pdf/Diets_insectivorous_birds.pdf

Diets of Insectivorous Birds Along the Colorado River in Grand Canyon, Arizona

We define the “developmental stream” of a plant as the gradient of increasingly mature phenotypes its shoots express over its lifetime. This pattern has important implications for studies of morphology, physiology, reproduction, and interactions with other organisms. Using cottonwoods, we test two major hypotheses about their developmental streams. First, within a tree, the developmental stage expressed by a shoot is dependent on its growth distance from the ground. Second, the distribution and fitness of Pemphigus betae, an aphid that is sensitive to host maturity, are positively related to increasingly mature shoot phenotypes within a tree. We show that the developmental variation in plant morphology and reproductive output within a tree are as great as that found among different-aged trees within a clone. In addition, we show that within-tree patterns of variation are stable from year to year and that P. betae responds to this variation by preferentially colonizing branches expressing more mature pheno...

Michael J. C. Kearsley

Papers

Developmental Changes in Resistance to Herbivory: Implications for Individuals and Populations

Elevated herbivory in plant hybrid zones: Chrysomela confluens, populus and phenological sinks

Planned flooding and colorado river riparian trade-offs downstream from glen canyon dam, arizona

Diets of Insectivorous Birds Along the Colorado River in Grand Canyon, Arizona

The developmental stream of cottonwoods affects ramet growth and resistance to galling aphids