A field test of the directed deterrence hypothesis in two species of wild chili

The directed deterrence hypothesis posits that secondary metabolites in ripe fruit function to deter fruit consumption by vertebrates that do not disperse seeds, while not impacting consumption by those that do. We tested this hypothesis in two species of wild chilies (Capsicum spp.). Both produce fruits that contain capsaicinoids, the compounds responsible for the pungency of chilies. Previous work suggests seed-dispersing birds but not seed-destroying rodents consume chili fruits, presumably because rodents are deterred by capsaicin. However, fruit removal from chili plants by rodents and other mammals has not been previously explored. Because laboratory rodents can develop a preference for capsaicin, it is quite possible that wild rodents are natural consumers of chili fruits. We monitored the fate of 125 marked fruits of Capsicum chacoense and 291 fruits of Capsicum annuum. For both species, essentially all fruit removal occurred during the day, when rodents are inactive. Video monitoring revealed fruit removal only by birds, mostly by species known to disperse chili seeds in viable condition. Furthermore, these species are from taxonomic groups that tend to specialize on lipid-rich fruits. Both species of chili produce fruits that are unusually high in lipids (35% in C. chacoense, 24% in C. annuum). These results support the directed deterrence hypothesis and suggest that fruiting plants distinguish between seed predators and seed dispersers by producing fruits that repel the former and attract the latter.     

Differential gender selection on flower size in two Neotropical savanna congeneric species

Larger flowers greatly increase among-individual pollen exchange within populations. However, water costs associated to transpirational cooling also increase with increasing flower size. Overall, the interplay between pollen and resource limitation determines the intensity of selection on flower size and this process is mostly dependent on gender and ecological context. To examine how pollinators and water use affect flower size, we determined corolla transpiration, pollen limitation, and selection through male and female fitness in two Kielmeyera species from the Brazilian cerrado flowering at different seasons. Hand-pollination experiments suggested pollen limitation through female fitness in both species, but K. coriacea showed lower limitation levels than K. regalis. For male fitness, the percentage of pollen removal was 1.5-times higher in K. coriacea. Higher air temperature and water deficit during flowering season of K. coriacea resulted in 4-fold higher corolla transpiration rates compared to K. regalis. Selection on flower size through male function was positive and significantly higher than selection through female components in both species. We also detected stabilizing selection in K. coriacea and positive selection in K. regalis on flower size through seed number. Our results suggest that selection on flower size in K. coriacea was mainly limited by water, whereas in K. regalis it was more limited by pollen. We demonstrate that differences in pollen and abiotic resource limitation determine gender-specific selection on flower size.     

Phenotypic selection on flowering phenology and size in two dioecious plant species with different pollen vectors

Dioecious plants may be pollinated biotically by animals or abiotically via wind or water currents. It has been hypothesized that these two types of pollen vectors might impose contrasting selective pressures on plant flowering phenology. In the present study we describe the flowering phenology of two sympatric dioecious species with contrasting pollination modes: Mercurialis perennis (wind‐pollinated) and Tamus communis (insect‐pollinated). We estimated selection differentials and gradients for flowering time and flowering synchrony. As flowering time might depend on the accumulation of enough internal resources, we also estimated direct and indirect selection on plant size. Both species have male‐biased sexual ratios, and males are bigger and produce larger flower displays than females, but only in T. communis do males bloom earlier and for longer than females. Selection gradients suggest that selection tends to favor early‐flowering females of T. communis. There is no evidence of direct current selection on the flowering phenology of M. perennis. Intersexual differences in phenology fit with sex allocation and sexual selection theories. As we hypothesized, phenology of the animal‐pollinated species is under stronger selection than that of the wind‐pollinated species and we discuss the potential role of pollen vectors in shaping the flowering phenologies of the study species.     

Assessing frequency-dependent seed size selection: a field experiment

Seed size is a life history attribute that affects the probability of seed predation, and therefore affects plant fitness. Compared with smaller seeds, those with large size should be more attractive to predators, as they constitute a more profitable food item because of higher energetic and/or nutrient content. However, predator preferences may be frequency-dependent in the sense that they may be modulated by the relative abundance of alternative seeds of different sizes. We set up a field experiment to evaluate frequency-dependent seed predation using seeds of Cryptocarya alba (Lauraceae), at La Campana National Park in central Chile. Predators (rodents and birds) preferentially consumed large seeds in an antiapostatic manner. These selective responses were maintained throughout the experiment and seed selection by predators was not affected by previous seed consumption. Our results suggest that (a) large seeds are very profitable food items actively sought by seed predators even at low relative abundance, (b) seed selection is expressed in a short time scale and (c) seed predators, by consuming large seeds consistently, have the potential to modify significantly the quality of plant progeny.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 81 , 307–312.     

Population variation in sexual selection and its effect on size allometry in two dung fly species with contrasting sexual size dimorphism

Body size is one of the most important quantitative traits under evolutionary scrutiny. Sexual size dimorphism (SSD) in a given species is expected to result if opposing selection forces equilibrate differently in both sexes. We document variation in the intensity of sexual and fecundity selection, male and female body size, and thus SSD among 31 and 27 populations of the two dung fly species, Scathophaga stercoraria and Sepsis cynipsea, across Switzerland. Whereas in S. cynipsea females are larger, the SSD is reversed in S. stercoraria. We comprehensively evaluated Fairbairn and Preziosi's (1994) general, three-tiered scenario, hypothesizing that sexual selection for large male size is the major driving force of SSD allometry within these two species. Sexual selection intensity on male size in the yellow dung fly, S. stercoraria, was overall positive, greater, and more variable among populations than fecundity selection on females. Also, sexual selection intensity in a given population correlated positively with mean male body size of that population for both the field-caught fathers and their laboratory-reared sons, indicating a response to selection. In S. cvnipsea, sexual selection intensity on males was lower overall and significantly positive, about equal in magnitude, but more variable than fecundity selection on females. However, there was no correlation between the intensity of sexual selection and mean male body size among populations. In both species, the laboratory-reared offspring indicate genetic differentiation among populations in body size. Despite fulfillment of all key prerequisites, at least in S. stercoraria, we did not find hypoallometry for SSD (Rensch's rule, i.e., greater evolutionary divergence in male size than female size) for the field-caught parents or the laboratory-reared offspring: Female size was isometric to male size in both species. We conclude that S. cynipsea does not fit some major requirements of Fairbairn and Preziosi's (1994) scenario, whereas for S. stercoraria we found partial support for it. Failure to support Rensch's rule within the latter species may be due to phylogenetic or other constraints, power limitations, erroneous estimates of sexual selection, insufficient genetic isolation of populations, or sex differences in viability selection against large size.     

Alternative antipredator responses of two coexisting Daphnia species to negative size selection by YOY perch

Our study showed for the first time in nature that two coexistingDaphnia adopted alternative life history and behavioural strategiesto cope with negative size-selection predation by gape-limitedyoung-of-the-year (YOY) perch. We evaluated the phenotypic plasticityin life history and behavioural traits of two coexisting Daphniaspecies, D. pulicaria (2 mm) and D. galeata mendotae (1.4 mm),in response to seasonal changes in predation by YOY yellow perch(Perca flavescens) in a mesotrophic lake. We expected that thelarge-sized D. pulicaria, the most likely subjected to size-selectivepredation by YOY perch, will show stronger antipredator responsesthan the small-sized D. galeata mendotae. To test this hypothesis,we examined changes in life history and behavioural traits injuveniles and adults of both species during four YOY fish predationperiods that were selected based on the presence of YOY perchin the pelagic zone and the relative abundance of Daphnia preyin their gut contents. Our study supports the scenario of negativesize-selective predation by gape-limited YOY perch on both Daphniaspecies. The electivity index indicated that no daphnids witha body length > 1.75 mm were predated by YOY yellow perch.Coexisting Daphnia exhibited phenotypic plasticity in theirantipredator defenses based on their vulnerability to seasonalchanges in size-selective predation of YOY perch. Juvenile Daphniawere the targeted prey and they responded by a decreased bodylength. Behavioural defenses were the dominant strategy usedby both adult Daphnia populations to withstand high predation.A decreased size at maturity was not employed by Daphnia, exceptat the very end of the predation period. Behavioural defensesare short-term strategy adopted to avoid predation. Both antipredatordefenses became unnecessary expenses and were no longer sustainedafter the predation period.     

The possible advantage of myrmecochory in diplochorous species: A test on two Mediterranean Euphorbia species

Myrmecochory commonly complements the advantages of ballistic dispersal in diplochorous species. We studied the role of the elaiosome in two populations of the two diplochorous Mediterranean spurges Euphorbia boetica and E. nicaeensis, which share an efficient ballistic dispersal mechanism followed by secondary removal by ants. They differ in elaiosome persistence, as most E. boetica seeds lose the elaiosome during explosive dispersal. Self-assessed dietary preferences with seeds with and without elaiosomes of each species showed differences in behaviour among and within ant species. In general, the absence of elaiosome entailed a decrease in the number of disperser ant species interacting with the seeds, whereas the number of predatory ants remains invariable. However, in one population of E. nicaeensis, experimental elimination of the elaiosome did not affect seed removal by mutualistic ants. On the other hand, analysis of refuse piles of the granivorous Messor marocanus and M. bouvieri suggests that they act as seed predators in E. boetica, whereas unintentional dispersal can be important in E. nicaeensis. We suggest, therefore, that the presence of the elaiosome in the seeds of the studied spurges increases the interaction with disperser ant species, but the possible dispersal advantage is not apparent and is spatially variable.     

Enzyme function and polymorphism: A test in two Anolis lizard species

Electrophoretic surveys of two lizard species were used to test hypotheses that relate levels of enzyme variability to enzyme function (single-substrate versus multiple-substrate enzymes, regulatory versus nonregulatory enzymes). Anolis roquet behaviorally regulates its body temperature, but its congener A. gundlachi is passive to variations in the thermal environment. As a result, populations of A. gundlachi probably experience the thermal environment as temporally coarse-grained, whereas populations of A. roquet do not. We therefore predicted that A. gundlachi would exhibit greater enzyme heterozygosity than A. roquet and that different enzyme classes would contribute disproportionately to this interspecific difference. The data show (1) that A. gundlachi does have a higher heterozygosity and (2) that this difference appears to result from high levels of heterozygosity at loci coding for multiple-substrate enzymes. The dichotomy between regulatory and nonregulatory enzymes offers no explanation for the variability in heterozygosity among enzyme loci in these species.E.Z. was supported by a grant from the Natural Sciences and Engineering Council of Canada. The study was accomplished while P.E.H. held a postdoctoral fellowship from the Killam Trust of Dalhousie University and a grant from the Research Development Fund of Dalhousie University. The collection of material was made possible by grants (to P.E.H.) from National Science Foundation (DEB 75-16334), the Explorers Club of New York, Sigma Xi, and the Richmond and Anderson Funds of Harvard University. We thank Dr. D. W. Foltz for his help with the calculations.     

Habitat selection in two species of aquatic hyphomycetes

Two species of aquatic hyphomycetes,Tetracladium marchalianum andTricladium splendens, were isolated from decaying leaves in a stream.T. marchalianum was abundant on alder leaves but absent on beech leaves, which were dominated byT. splendens. It was hypothesized that differences in some chemical key factors in the leaves would account for differences in the distribution of the 2 species. In the experiment designed to test the hypothesis, combinations of sterilized leaves and isolated fungi were used. Differences in growth of FDA active mycelium were related to differences in leaf weight loss;T. splendens decomposed beech leaves andT. marchalianum decomposed alder leaves. Extracellular protease activity corresponded with these trends but there was no detectable protein loss in the leaves. Both fungi showed a nitrogen demand, and hydrolysis of leaf proteins was complemented with absorption of free amino acids and ammonium. High concentrations of free amino acids modified the pattern for habitat selection so thatT. splendens grew substantially on alder leaves andT. marchalianum colonized beech leaves. No protease activity was, however, found fromT. marchalianum on beech leaves, and it is concluded that a more general metabolic inhibition prevents extensive growth of this species on beech leaves. The low natural abundance ofT. splendens on alder leaves, where it may grow well, may be a consequence of a specific protease inhibition and competition from other species.     

Microhabitat selection in two species of heteromyid rodents

Summary An experiment was conducted to determine the microhabitat preferences of two heteromyid rodents, Dipodomys ordi and Perognathus flavus. This experiment used marked seeds and the atomic absorption spectrophotometer in order to study the environment as a mosiac of microhabitats. The results of our analysis indicate that these two heteromyids are microhabitat selectors. The preferences of the rodents are D. ordi: grass habitat 0.0%, near grass habitat 22.5%, open habitat 77.4% and P. flavus: grass habitat 46.2%, near grass habitat 32.2%, open habitat 21.4%. The overlap between the two species is only 0.43.     

Floral symmetry: pollinator-mediated stabilizing selection on flower size in bilateral species

Pollinator-mediated stabilizing selection (PMSS) has been proposed as the driver of the evolutionary shift from radial to bilateral symmetry of flowers. Studies have shown that variation in flower size is lower in bilateral than in radial species, but whether bilateral flowers experience more stabilizing selection pressures by employing fewer, more specialized pollinators than radial flowers remains unclear. To test the PMSS hypothesis, we investigate plant–pollinator interactions from a whole community in an alpine meadow in Hengduan Mountains, China, to examine: (i) variance in flower size and level of ecological generalization (pollinator diversity calculated using functional groups) in 14 bilateral and 13 radial species and (ii) the role pollinator diversity played in explaining the difference of variance in flower size between bilateral and radial species. Our data showed that bilateral species had less variance in flower size and were visited by fewer pollinator groups. Pollinator diversity accounted for up to 40 per cent of the difference in variance in flower size between bilateral and radial species. The mediator effect of pollinator diversity on the relationship between floral symmetry and variance in flower size in the community is consistent with the PMSS hypothesis.     

Offspring size effects in the marine environment: A field test for a colonial invertebrate

Abstract A central tenet of life‐history theory is the presence of a trade‐off between the size and number of offspring that a female can produce for a given clutch. A crucial assumption of this trade‐off is that larger offspring perform better than smaller offspring. Despite the importance of this assumption empirical, field‐based tests are rare, especially for marine organisms. We tested this assumption for the marine invertebrate, Diplosoma listerianum, a colonial ascidian that commonly occurs in temperate marine communities. Colonies that came from larger larvae had larger feeding structures than colonies that came from smaller larvae. Colonies that came from larger larvae also had higher survival and growth after 2 weeks in the field than colonies that came from smaller larvae. However, after 3 weeks in the field the colonies began to fragment and we could not detect an effect of larval size. We suggest that offspring size can have strong effects on the initial recruitment of D. listerianum but because of the tendency of this species to fragment, offspring size effects are less persistent in this species than in others.     

The role of plant species size in invasibility: a field experiment

Large plant species self-thin to disproportionately lower densities than smaller plant species, and therefore may leave more patches of unused space suitable for invasion. Using experimental monocultures of 11 old-field perennial plant species differing in maximum size, as well as mixtures composed of all monoculture species, we tested our primary hypothesis that monocultures of larger species will be more susceptible to natural invasion. After 3 years, monocultures of larger species were invaded by a significantly greater number of species, and more ramets, from the surrounding vegetation. Invading plant species were significantly smaller than the monoculture species being invaded, suggesting that smaller plant species may be better invaders. Thus, we quantified a trade-off between species size, which is frequently associated with increased competitive ability for light, and invasibility, suggesting one reason why large and small species coexist in virtually all plant communities. Although we expected that invasion would enhance biomass production by more fully capturing available resources, we found that the most highly invaded plots of each species produced significantly less biomass. This suggests that increased diversity resulting from invasion did not result in complementary resource use. Mixture plots containing all experimental species did not admit a significantly different number of invading ramets or species than most monocultures, indicating no obvious role for diversity in resistance to invasion, or complementary resource use. Our results suggest that relatively large species may be limited in their capacity to competitively exclude other, smaller species from communities because pure stands of the former are more susceptible to invasion by the latter.     

Preferences of cedar waxwings in the laboratory for fruit species,colour and size: A comparison with field observations

Fruit preferences of cedar waxwings, Bombycilla cedrorum, in the laboratory were compared with preferences in the field to identify fruit characteristics that influence choice by avian dispersers. Waxwings ate 12 of 13 species of fruits offered individually in laboratory tests, but in the field ate only eight of the same 13 species. When given a choice of 10 fruit species offered simultaneously, waxwings showed a strong preference for fruits that were small or red. Preferences for fruit species in the laboratory were not correlated with morphological or nutritional characteristics of the fruits. When offered food that differed only in colour, waxwings initially preferred red over blue, yellow and green. In later tests, preference for red fruit was less marked. In tests for size preference, waxwings preferred small pieces of fruit (6 mm) over medium pieces (9 mm), and medium pieces over those that were large (12 mm). Sizes of fruits preferred in the field and in the laboratory tests were approximately 6·0–7·5 mm. Thus, waxwings can discern differences in food items and they have definite preferences. The lack of complete agreement between preferences for fruits in the field and in the laboratory suggests that factors important in the field but controlled in the laboratory (e.g. abundance, location) override preferences for certain fruits. However, some fruit characteristics, particularly size, were consistently influential in the laboratory and in the field.     

Do body size and diet breadth affect partitioning of species diversity? A test with forest Lepidoptera

Two of the major themes resulting from recent macroecological research are the central roles that body size and niche breadth may play as determinants of species geographical distribution. Unanswered questions, however, linger regarding how similarities in body size or niche breadth affect the allocation of α‐ and β‐diversity across spatial scales. Using data on moth diversity in the eastern deciduous forest of North America, we tested the predictions that smaller‐bodied and diet‐restricted species would have lower levels of α‐diversity within forest stands and greater β‐diversity at higher sampling scales compared to larger or more generalist species. Moths were sampled using a nested sampling design consisting of three hierarchical levels: 20 forest stands, 5 sites and 3 ecoregions. Body size for 492 species was estimated as mean forewing length, and diet breadth was assessed from the published literature. Moth species were then classified according to body size (small or large) or diet breadth (generalist or restricted), and partitioning was conducted on each group. Diversity partitions for large‐ and small‐bodied species yielded similar patterns. When observed diversity components differed from those derived from our null model, a consistent pattern was observed: α‐diversity was greater than expected, β‐diversity among forest stands was less than expected, and β‐diversity among sites and ecoregions was higher than expected. In contrast, diet‐restricted moths contributed significantly less to stand‐level α‐diversity than generalist feeders. Furthermore, specialists contributed to a greater proportion of β‐diversity across scales compared to generalist moths. Because absolute measures of β‐diversity among stands were greater for generalists than for restricted feeders, we suggest that regional β‐diversity of forest moths may be influenced by several possible factors: intraspecific aggregation of diet‐restricted species, local fluctuations in population size of eruptive generalists and small geographical distributions of generalist moths than predicted by the geographical extent of putative host plants     

A test of biotic interactions among two alpine plant species in Australia

This study examines the extent to which interactions among two common alpine/subalpine plant species and their neighbours at the Bogong High Plains in southern Australia are characterized by competition or facilitation. The two target species were Celmisia pugioniformis (Asteraceae) and Carex breviculmis (Cyperaceae). Biotic interactions were examined using vegetation removal manipulations over three growing seasons at five sites across the altitudinal range of tall alpine herbfield communities. Observations recorded growth and mortality. Results for C. pugioniformis clearly indicated facilitation as a dominant process across all sites and seasons. Plants that had their neighbours removed tended to perform worse than plants that had their neighbours left intact. Growth observations for Ca. breviculmis were less clear, but again suggested facilitation. Mortality was distinctly higher among Ca. breviculmis individuals that had their neighbours removed relative to those with neighbours left intact. Results collectively suggest the removal of neighbours acts to reduce growth and increase mortality in C. pugioniformis and Ca. breviculmis throughout the altitudinal range of tall alpine herbfields at the Bogong High Plains. Facilitative and competitive interactions need to be recognized in efforts aimed at mitigating climate change‐associated impacts on the ecology of alpine plant communities. The extent to which biotic interactions may exacerbate or buffer abiotic change is difficult to predict, emphasizing the need for ecological monitoring.