Biotic Selection Pressures Research Articles

Distinct Leaf-trait Syndromes of Evergreen and Deciduous Trees in a Seasonally Dry Tropical Forest

In seasonally dry tropical forests, tree species can be deciduous, remaining without leaves throughout the dry season, or evergreen, retaining their leaves throughout the dry season. Deciduous and evergreen trees specialize in habitats that differ in water availability (hillside and riparian forest, respectively) and in their exposure to herbivore attack (seasonal and continuous, respectively). We asked whether syndromes of leaf traits in deciduous and evergreen trees were consistent with hypothesized abiotic and biotic selective pressures in their respective habitat. We measured seven leaf traits in 19 deciduous and 11 evergreen tree species in a dry tropical forest in Western Mexico, and measured rates of herbivory on 23 of these species. We investigated the covariance of leaf traits in syndromes related to phenology and associated physiology, and to anti-herbivory defense. We found evidence for syndromes that separated phenological strategies among four traits: toughness, water content, specific leaf area, and carbon:nitrogen (C:N) ratios. We found a trade-off between two other traits: trichomes and latex. Overall, evergreen species exhibited lower rates of herbivory than deciduous species. Lower rates of herbivory were explained by a syndrome of higher toughness, lower water content, and higher C:N ratios, which are traits representative of evergreen trees. Phenology and trait syndromes did not exhibit significant phylogenetic signal, consistent with the hypothesis of evolutionary convergence among phenologies and associated leaf-trait syndromes. Our results suggest that deciduous and evergreen trees could respond to differential water availability and herbivory in their respective habitats by converging on distinct leaf-trait syndromes.

Adaptive foragers and community ecology: linking individuals to communities and ecosystems

Adaptive foragers and community ecology: linking individuals to communities and ecosystems

Genetic variation in growth and development time under two selection regimes inLeptinotarsa decemlineata

AbstractIt is possible to predict the potential range of a species on the basis of its ecological characteristics and those of the invaded ecosystem. The existence of genetic variation indicates a species’ potential to respond to new environmental conditions, thus facilitating its success as an invader. Accordingly, evolutionary and ecological approaches are needed to identify the factors explaining both species’ range and their potential to invade new areas. We combined these two approaches and studied whether genetic variation in life‐history traits under abiotic (temperature) and biotic (host plant) selection pressures contributes to the potential range expansion ofLeptinotarsa decemlineataSay (Coleoptera: Chrysomelidae). We reared full‐sib families ofL. decemlineatafrom the current northernmost European population at 15 °C (temperature below that in the current range) and 25 °C (optimal temperature) and on three potato varieties. We monitored development time, adult weight and larval‐to‐adult survival, and estimated the amount of heritable variance. The development time and adult weight of progenies were more variable between than within families. Thus, there was genetic variability in traits relevant to the ability to adapt to a colder environment (i.e., accelerate development and become heavier) allowing range expansion further north in Europe, even though low temperature increased beetle mortality. Temperature strongly affected all traits measured. Potato variety, in turn, did not strongly affect beetles’ performance. Beetle ability to adapt to a cool environment was further enhanced by the fact that size was not constrained by fast development. The results showed that beetle populations possess genetic variation allowing a response to temperature and thus have the evolutionary potential to adapt and spread beyond their current range.

Ecological genetics of the cyclic parthenogens, Daphnia longispina and Daphnia pulex

A population of Daphnia longispina was found to possess a relatively stable genetic structure, while a population of D. pulex appeared genetically very unstable. Unlike the population of D. magna inhabiting single-species pools in the same area and which maintained approximately the same level of genetic variation throughout seasons, the D. pulex population was characterized by decreasing genetic diversity toward autumn and across years. A clue to the surprisingly different behaviour of the D. pulex population could be different biotic selective pressures. The population of D. longispina and D. pulex studied here had to compete with another closely related species, D. magna. D. longispina appeared to outcompete D. magna completely, while D. pulex was exposed to more continuous competition. Selection favouring a few well competing clones is a likely explanation for the extensive decrease in genetic diversity observed in the population of D. pulex.

Lutein Sequestration and Furanocoumarin Metabolism in Parsnip Webworms Under Different Ultraviolet Light Regimes in the Montane West

Both biotic and abiotic selection pressures can contribute to geographic variation in allelochemical production in plants. We examined furanocoumarin production in western North American populations of Heracleum lanatum and Pastinaca sativa that, at different latitudes and altitudes, experience different ultraviolet (UV) light regimes. Total furanocoumarins and linear furanocoumarins of fruits were negatively correlated with UV irradiance, whereas amounts of angular furanocoumarins, which are generally less phototoxic, were not. Another factor potentially influencing furanocoumarin production is the presence of the parsnip webworm Depressaria pastinacella, (Lepidoptera: Oecophoridae), an herbivore that feeds on reproductive structures of both plant species. These insects sequester lutein from their host plants; this carotenoid acts to ameliorate furanocoumarin toxicity. Although the concentration of lutein in fruits did not vary with UV irradiance, lutein sequestration by sixth instars was positively correlated with UV irradiance. Webworm populations are variably infested with the polyembryonic webworm parasitoid Copidosoma sosares Walker (Hymenoptera: Encyrtidae). H. lanatum fruits from populations with webworms parasitized by C. sosares had lower concentrations of furanocoumarins, with the exception of sphondin, than fruits from plants infested with webworms free from parasitism. Lower levels of these furanocoumarins may reduce negative effects on the fitness of this parasitoid. In contrast with the variation in furanocoumarin content, the ability of webworms to metabolize furanocoumarins by cytochrome P450 did not differ significantly among populations from New Mexico to Alberta.

Leaf Traits and Herbivory Rates of Tropical Tree Species Differing in Successional Status

We evaluated leaf characteristics and herbivory intensities for saplings of fifteen tropical tree species differing in their successional position. Eight leaf traits were selected, related to the costs of leaf display (specific leaf area [SLA], water content), photosynthesis (N and P concentration per unit mass), and herbivory defence (lignin concentration, C:N ratio). We hypothesised that species traits are shaped by variation in abiotic and biotic (herbivory) selection pressures along the successional gradient. All leaf traits varied with the successional position of the species. The SLA, water content and nutrient concentration decreased, and lignin concentration increased with the successional position. Herbivory damage (defined as the percentage of damage found at one moment in time) varied from 0.9-8.5% among the species, but was not related to their successional position. Herbivory damage appeared to be a poor estimator of the herbivory rate experienced by species, due to the confounding effect of leaf lifespan. Herbivory rate (defined as percentage leaf area removal per unit time) declined with the successional position of the species. Herbivory rate was only positively correlated to water content, and negatively correlated to lignin concentration, suggesting that herbivores select leaves based upon their digestibility rather than upon their nutritive value. Surprisingly, most species traits change linearly with succession, while resource availability (light, nutrients) declines exponentially with succession.

No negative correlation between growth and resistance to multiple herbivory in a deciduous tree, Betula pendula

Plants are assumed to have a trade-off between growth and resistance. This trade-off has been demonstrated in herbaceous plants, but the information on woody plants is conflicting. In contrast to simple annual plant–herbivore systems, trees face highly variable herbivory. In this note, we measured clonal variation in the growth of Betula pendula and in its resistance to nine major herbivore species belonging to five different feeding guilds. There were significant differences in both the growth and the resistance of clones to all the herbivores studied. However, we found no negative correlation between growth and resistance. No clone was generally resistant or susceptible to most of the herbivores. This suggests that, if there is an ultimate trade-off in resource allocation of B. pendula between growth and resistance, it is masked by other factors, for example, by several opposite abiotic and biotic selection pressures.

Potential mechanisms of phenotypic divergence in body size between Newfoundland and mainland black bear populations

Phenotypic variation in body size and degree of sexual size dimorphism of North American black bears (Ursus americanus) was quantified for populations from New Brunswick, Quebec, Ontario, Maine, Alaska, and the island of Newfoundland. Based on a model of island biogeography developed by Case, we predicted that body size should be larger in Newfoundland bears than in mainland populations. The presence of few large predators and minimal competition between herbivore prey on Newfoundland allow an appropriate test of the model (i.e., food availability for bears may differ between populations on the mainland and in Newfoundland). In addition, sexual-selection theory predicts that the coevolution of polygyny and large size will be coupled with an increase in sexual size dimorphism. Therefore, we also predicted that among the six populations, male body mass should scale hyperallometrically with female body mass (i.e., slope > 1). Analysis of deterministic growth curves indicated that bears from Newfoundland attained greater asymptotic body size than populations on the mainland, which supports our first prediction. On average, the relative difference in asymptotic body mass between females from the island and mainland populations was 55%, while the relative difference between males was 37%. However, we found that sexual size dimorphism did not increase disproportionately with body mass among the six populations, which refuted our second prediction. We discuss a range of abiotic and biotic selection pressures possibly responsible for larger body size in Newfoundland bears. We suggest that the ability to exploit seasonally abundant and spatially dispersed dietary protein by female and male black bears on the island has been and is still a primary environmental factor selecting for large body size in Newfoundland bears. Although the relationship between sexual size dimorphism and body size is tenuous (slope [Formula: see text] 1), it does suggest that (an)other adaptive mechanism(s), opposing sexual selection for extreme male size, explain(s) a large amount of the variation in sexual size dimorphism among black bear populations.

INFLUENCE OF ABIOTIC AND BIOTIC FACTORS ON AMPHIBIANS IN EPHEMERAL PONDS WITH SPECIAL REFERENCE TO LONG-TOED SALAMANDERS (AMBYSTOMA MACRODACTYLUM)

Animals living in ephemeral habitats are subjected to various abiotic and biotic selection pressures that may not be present to the same degree in permanent habitats. For example, pond drying can lead to increased predation and competition as resources become limited and temperature and water quality undergo drastic fluctuations. Amphibians provide an excellent model for studying factors associated with survival in temporary habitats. Aquatic amphibians developing in ephemeral habitats must find food, avoid predators, and cope with potentially great fluctuations in abiotic parameters under increasingly harsh conditions. In Oregon (U.S.A.), larval long-toed salamanders (Ambystoma macrodactylum) often inhabit temporary ponds that gradually dry during the summer. We have been studying several factors that may influence long-toed salamander behavior, growth, and survival in ephemeral montane ponds in the Cascade Range. These include biotic interactions focusing on cannibalism and predation, and abiotic factor...

Herbivory as a selective agent on the timing of leaf production in a tropical understory community

In highly seasonal environments (for example, temperate deciduous forest or tropical dry forest) plant phenologies are often constrained by physiological stress caused by seasonal variation in temperature and moisture1. Where climatic fluctuations are less pronounced (for example, tropical marine or lowland wet forest communities) determinants of phenology may include biotic selective pressures such as herbivores, predators, competitors, pollinators or dispersers. Temporal patterns of leaf production vary considerably between tropical forest species, but few species produce leaves continuously throughout the year2. Even in aseasonal tropical forests, most species show synchronous peaks in leaf production2. Results from this one-year study suggest that herbivores may influence leaf phenology of shade-tolerant understory plants in a Panamanian lowland moist forest. Leaves produced during the dry season or in synchronous flushes receive significantly less damage from herbivores than leaves produced during the wet season or out of synchrony with conspecifics.

Life History Strategies in Daphnia: A Review and Predictions

The cladoceran Daphnia is an ideal organism for the comparative study of life history strategies due to its extensive geographic range and the great deal known of its biology. Biotic selection pressures on components of life history strategies include predation, food, and competition. Abiotic pressures include temperature, photoperiod, and environmental chemistry. In addition, it can be demonstrated that stochastic life history theory, based on the differential survivorship of juveniles and adults, is also an important component in the evolution of Daphnia life histories. As an example of the interaction of these pressures two Daphnia species and their life histories are compared. Daphnia pulex has a very broad geographic range extending through most temperate areas of North America, and has a correspondingly broad array of life history characteristics but may be characterized as maturing rapidly and reproducing with many large broods of small neonates. In comparison, D. middendorffiana, an arctic species, requires more time to mature and produces many fewer broods of large.neonates.

Do Tropical Frugivores Compete for Food?

synopsis. In this paper I ask two questions: Is fruit ever limiting to vertebrate frugivores and, if so, do frugivores presently compete for food, either with closely- or distantly-related species? A brief review of the fruiting strategies of tropical plants indicates that fruit can occasionally be superabundant, but it is often produced at low rates and in low quantities. Variation in fruit abundance results from several biotic selective pressures, including varia? tion in the density, diversity, and reliability of potential dispersal agents. Tojudge from the size structure, dietary similarities, habitat preferences and foraging behaviors of taxonomically-restricted guilds of frugivorous birds and mammals, members of these guilds have competed for food in the past and must occasionally do so today. Unusual climatic conditions can occasionally upset phenological patterns and can create food shortages that promote competition among closely-related species of frugivores. Avian and mammalian frugivores, however, probably seldom compete with each other for food in present-day tropical ecosystems. A major reason for this is that many tropical plants have evolved fruits that are attractive to only a limited subset of frugivores (e.g., only birds or only bats). Plants apparently perceive qualitative differences in the dispersal services of birds and mammals and attempt to attract members of one group but not the other.

Evolution of Paleozoic benthic marine invertebrate communities

Three marine benthic communities are recognized in the Upper Ordovician clastic sequences of the central Appalachians. These communities can be traced throughout part or all of Paleozoic time and, when compared with other previously designated Paleozoic communities, emphasize some evolutionary trends at the community level. Although environmental and geographic coverage is limited, all well known Paleozoic marine invertebrate communities are grouped into five major associations. These associations and their approximate duration are: (1) linguloid-molluscan (Ordovician-Permian); (2) strophomenid-trilobite (Ordovician-Devonian); (3) atrypid-bryozoan (Ordovician-Devonian); (4) productid-chonetid (Devonian-Permian);(5) fusulinid-palaeotextulariid (Pennsylvanian-Permian). Throughout Paleozoic time nearshore communities were consistently dominated by nuculoid bivalves, bellerophontid gastropods and linguloid brachiopods (association 1), whereas offshore articulate brachiopod-dominated communities (associations 2–4) underwent much more rapid changes in faunal composition; these changes were especially evident near the end of the Devonian Period. A modified form of the Moore-Newell emergence hypothesis does not appear to explain the greater instability or evolutionary plasticity of the offshore communities relative to that of the nearshore ones. An Early Paleozoic selection for species tolerant of fluctuating nearshore environmental conditions firmly established the dominant eurytypic molluscan-inarticulate brachiopod communities, whereas invertebrate adaptation to the monotonous offshore environment placed severe limitations on the potential variability of the brachiopod-dominated communities. Thus relatively slight changes in physical or biotic selective pressures could effectively distrupt the offshore stenotypic communities.