scispace - formally typeset
Search or ask a question

Showing papers in "Oecologia in 1984"


Journal ArticleDOI
TL;DR: It has been postulated that when plants are stressed by certain changes in patterns of weather they become a better source of food for invertebrate herbivores because this stress causes an increase in the amount of nitrogen available in their tissues for young herbivore feeding on them.
Abstract: It has previously been postulated that when plants are stressed by certain changes in patterns of weather they become a better source of food for invertebrate herbivores because this stress causes an increase in the amount of nitrogen available in their tissues for young herbivores feeding on them. And this may cause outbreaks of such phytophagous invertebrates. Evidence is now presented that a similar physiological mechanism appears to operate when a wide variety of apparently unrelated environmental factors impinge on plants or parts of plants in such a way as to perturb their metabolism. A broken branch, lightning strike, fire, nutrient deficiencies or an otherwise adverse site; all may have this effect. With the advent of modern man the available agencies increase and diversify to include pesticides, irradiation and air pollutants. One common metabolic response by plants to all such agents impinging on them seems to be equivalent to that found in senescing plant tissues — the breakdown and mobilization of nitrogen in soluble form away from the senescing/stressed tissues. Young herbivores which chance to feed on such stressed/senescing tissues have a greater and more readily available supply of nitrogen in their food than they would have had if feeding on unstressed plants. As a result many more of them survive, and there is an increase in abundance of their kind. Such increases may be quite localised and short-lived or more widespread and persistent, depending on the extent and duration of the stress experienced by the plants. And in the face of this improved nutrition and survival of the very young, predators and parasites seem to have only a minor influence on subsequent changes in abundance of their herbivorous prey. Another effect of increased mobilization of nitrogen in stressed plants is an increase in the quantity of the seed that they set. This has led to the conclusion that increased abundance of some species of birds at such times is due to a greater supply of seeds as winter food for recent fledglings. But it may be that the increased abundance is due to the synchronous increase in phytophagous insects providing a richer source of protein food for laying hens and growing nestlings.

1,013 citations


Journal ArticleDOI
TL;DR: The field experiment on Barro Colorado Island, Panama, demonstrated that both an increase in dispersal distance and a decrease in seedling density reduce levels of damping-off disease among seedlings of Platypodium elegans, and that there is an interaction between the two factors.
Abstract: We present results of two experiments designed to identify the relative importance of dispersal distance, seedling density, and light conditions on pathogen-caused mortality of tropical tree seedlings. The field experiment on Barro Colorado Island, Panama, demonstrated that both an increase in dispersal distance and a decrease in seedling density reduce levels of damping-off disease among seedlings of Platypodium elegans, and that there is an interaction between the two factors. The results indicated significant variation among sites in pathogen activity and suggested that seedlings are more vulnerable to disease when establishing around their parent tree than around other conspecific trees.The second experiment in a screened enclosure used potted seedlings of 18 wind-dispersed tree species exposed to two levels of sunlight and seedling density. The results indicated that environmental conditions similar to those in light-gaps significantly reduce pathogen activity. They also confirmed that high seedling density increases disease levels, especially under shaded conditions.Seedlings of 16 of the 18 species experienced pathogencaused mortality, but in widely varying amounts. Seed weight was not a good predictor of a species' vulnerability to pathogens. Adult wood density, an indicator of growth rate and successional status, was inversely correlated with a species' vulnerability to pathogens. Fast-growing, colonizing species, whose seedlings require light-gaps, lacked strong resistance to seedling pathogens, relative to slow-growing species able to tolerate shade and escape seedling pathogens. We discuss these results in the context of seed dispersal as a means of escaping from seedling pathogens.

523 citations


Journal ArticleDOI
TL;DR: It is suggested that size hierarchy is equivalent to size inequality and methods developed by economists to evaluate inequalities in wealth and income, the Lorenz curve and Gini Coefficient, provide a useful quantification of inequality and allow us to compare populations.
Abstract: The term “size hierarchy” has been used frequently by plant population biologists but it has not been defined. Positive skewness of the size distribution, which has been used to evaluate size hierarchies, is inappropriate. We suggest that size hierarchy is equivalent to size inequality. Methods developed by economists to evaluate inequalities in wealth and income, the Lorenz curve and Gini Coefficient, provide a useful quantification of inequality and allow us to compare populations. A measure of inequality such as the Gini Coefficient will usually be more appropriate than a measure of skewness for addressing questions concerning plant population structure.

507 citations


Journal ArticleDOI
TL;DR: A universal law for this increase in characteristic return time to an equilibrium increases when a threshold is approached is found, which may be used to forecast the position of a threshold by extrapolation of empirical data.
Abstract: Dramatic changes at thresholds in multiple stable ecosystems may be irreversible if caused by man. The characteristic return time to an equilibrium increases when a threshold is approached. A universal law for this increase is found, which may be used to forecast the position of a threshold by extrapolation of empirical data. Harvesting experiments on populations are proposed that can be used to verify the method. Preliminary harvesting experiments on rotifer populations display a good agreement with the theory.

345 citations


Journal ArticleDOI
Delbert Wiens1
TL;DR: Among perennials, increased exposure to predators and pathogens is suggested as the best explanation for the evolution of breeding systems favoring genetic recombination, that appear to be responsible for the lower S/O ratios in perennials.
Abstract: The percentage of ovules developing into seeds (seed-ovule ratios, S/O ratios) is about 85% in annuals but only approximately 50% in perennials.In both annuals and perennials, these typical S/O ratios occur regardless of the kind of breeding system, although many annuals are normally self-pollinating whereas perennials are virtually all cross-pollinating. The mean number of seeds maturing within individual fruits is defined as brood size, and is correlated with different modes of dispersal and strategies of ovule packging. Annuals also have significantly higher brood sizes (21.7) than perennials (<9.9). Among perennials, woody plants have both lower S/O ratios (32.7%) and brood sizes (3.3) than herbaceous perennials (57.2%, 13.5). S/O ratios appear to be largely determined genetically, whereas resource limitations are perhaps more critical in terms of regulating flower production. Among perennials, increased exposure to predators and pathogens is suggested as the best explanation for theevolution of breeding systems favoring genetic recombination. The maintenance of genetic polymorphisms, however, inevitably increases the frequency of lethal and sub-lethal allelic combinations (and perhaps mutations?), that appear to be responsible for the lower S/O ratios in perennials.

343 citations


Journal ArticleDOI
TL;DR: Although all 4 predictions were confirmed, the hypothesis that synchronous fluctuations in small game species in boreal Fennoscandia are caused by varying predation pressure could not necessarily exclude other hypotheses involving changes in quality or quantity of plant food.
Abstract: We tested the hypothesis that synchronous fluctuations in small game species in boreal Fennoscandia are caused by varying predation pressure. The main prey of predators are the cyclically superabundant voles. Small game species (alternative prey) are rare compared to voles. The following 4 predictions were checked: (1) Predators should shift their diet from main prey to alternative prey as main prey decline. — This was confirmed using data on red fox (Vulpes vulpes L.) diet.; (2) The mortality rate of alternative prey should be inversely correlated to the abundance of main prey. — This was true for mountain hare (Lepus timidus L.) mortality rates and the rate of nest predation on black grouse (Tetrao tetrix L.).; (3) The total consumption of prey by all the predators should at least equal the critical losses in alternative prey during a decline year. — A tentative estimate of predator consumption amounted to 10 times the losses in grouse and hare.; and (4) The absence of synchrony between the species in the boreonemoral region should be associated with a more diverse diet of predators. — This was the case for red fox diets throughout Sweden. Although all 4 predictions were confirmed, we could not necessarily exclude other hypotheses involving changes in quality or quantity of plant food.

320 citations


Journal ArticleDOI
TL;DR: The highly automatic stoichiometric CHN method for the separation of nutrient contents in biomass extends existing ecophysiological concepts for the construction of balanced carbon and nitrogen, as well as biochemical and energy budgets.
Abstract: Carbohydrate, lipid, and protein compositions are stoichiometrically related to organic CHN (carbon, hydrogen, nitrogen) contents. Elemental CHN analyses of total biomass and ash, therefore, provide a basis for the calculation of proximate biochemical composition and bomb caloric value. The classical nitrogen to protein conversion factor (6.25) should be replaced by 5.8±0.13. A linear relation exists between the mass fraction of non-protein carbon and the carbohydrate and lipid content. Residual water in dry organic matter can be estimated with the additional information derived from hydrogen measurements.The stoichiometric CHN method and direct biochemical analysis agreed within 10% of ash-free dry biomass (for muscle, liver and fat tissue of silver carp; gut contents composed of detritus and algae; commercial fish food). The detrital material, however, had to be corrected for non-protein nitrogen.A linear relationship between bomb caloric value and organic carbon fractions was derived on the basis of thermodynamic and stoichiometric principles, in agreement with experimental data published for bacteria, algae, protozoa and invertebrates. The highly automatic stoichiometric CHN method for the separation of nutrient contents in biomass extends existing ecophysiological concepts for the construction of balanced carbon and nitrogen, as well as biochemical and energy budgets.

304 citations


Journal ArticleDOI
TL;DR: The ratio of stable carbon isotopes (δ13C) in plants and animals from Malaysian mangrove swamps, coastal inlets, and offshore waters was determined and there was a trend to less negative ratios with increasing trophic level.
Abstract: The ratio of stable carbon isotopes (δ13C) in plants and animals from Malaysian mangrove swamps, coastal inlets, and offshore waters was determined. Vascular plants of the swamps were isotopically distinct ( x±s.d.=-27.1±1.2‰) from plankton (-21.0±0.3‰) and other algae (-18.7±2.2‰). Animals from the swamps (-20.9±4.1‰) and inlets (-19.8±2.5‰) had a wide range of isotope ratios (-28.6 to-15.4‰), indicating consumption of both mangrove and algal carbon. Several commercially important species of bivalves, shrimp, crabs, and fish obtained carbon from mangrove trees. Mangrove carbon was carried offshore as detritus and was isotopically distinguishable in suspended particulate matter and sediments. Animals collected from 2 to 18 km offshore, however, showed no isotopic evidence of mangrove carbon assimilation, with ratios (-16.5±1.1‰, range-19.1 to-13.1‰) virtually identical to those reported for similar animals from other plankton-based ecosystems. Within groups of animals, isotope ratios reflected intergencric and interspecific differences in feeding and trophic position. In particular, there was a trend to less negative ratios with increasing trophic level.

255 citations


Journal ArticleDOI
TL;DR: The hypotheses that vegetation structural diversity and prey availability determine alpha diversity were examined for scrub- and meadow-inhabiting web spider species assemblages along elevational gradients in Costa Rica and California.
Abstract: The hypotheses that vegetation structural diversity and prey availability determine alpha diversity were examined for scrub- and meadow-inhabiting web spider species assemblages along elevational gradients in Costa Rica and California Prey availabilities were estimated by sticky trap catches, using only the orders and size classes of insects actually captured by the spiders The measured component of vegetation structural diversity was the maximum tip height in 20 cm increments from 0 to 20 m Spider species diversity and vegetation tip height diversity were both expressed by the Inverse Simpson Index Web spider species diversity is highly significantly correlated with vegetation tip height diversity Prey availability is not a significant predictor of web spider species diversity in these habitats

253 citations


Journal ArticleDOI
TL;DR: It is put forward that the long-lasting inducible responses of trees to herbivores, particularly lepidopteran defoliators, may not be active defensive responses, but a by-product of mechanisms which rearrange the plant carbon/nutrient balance in response to nutrient stress caused by defoliation.
Abstract: A hypothesis is put forward that the long-lasting inducible responses of trees to herbivores, particularly lepidopteran defoliators, may not be active defensive responses, but a by-product of mechanisms which rearrange the plant carbon/nutrient balance in response to nutrient stress caused by defoliation. When defoliation removes the foliage nutrients of trees growing in nutrient-poor soils, it increases nutrient stress wich in turn results in a high production of carbon-based allelochemicals. The excess of carbon that cannot be diverted to growth due to nutrient stress is diverted to the production of plant secondary metabolites. The level of carbon-based secondary substances decays gradually depending on the rate at which nutrient stress is relaxed after defoliation. In nutrient-poor soils and in plant species with slow compensatory nutrient uptake rates the responses induced by defoliation can have relaxation times of several years. The changes in leaf nitrogen and phenolic content of mountain birch support this nutrient stress hypothesis. Defoliation reduces leaf nitrogen content while phenolic content increases. These responses of mountain birch to defoliation are relaxed within 3–4 years.

243 citations


Journal ArticleDOI
TL;DR: Species body size distributions from eight temperate benthic communities show a highly conservative pattern with two separate lognormal distributions, corresponding to the traditional categories of meiofauna and macrofauna, which are suggested to comprise two separate evolutionary units each with an internally coherent set of biological characteristics.
Abstract: Species body size distributions from eight temperate benthic communities show a highly conservative pattern with two separate lognormal distributions, corresponding to the traditional categories of meiofauna and macrofauna. The meiofaunal mode occurs at a dry body weight of 0.64 μg and the macrofaunal mode at 3.2 mg, with a trough between them at 45 μg. It is suggested that there is a particular body size at which meiofaunal life-history and feeding traits can be optimised, and another for macrofaunal traits. As size departs in either direction (larger or smaller) from these optima, fewer species of the same size are able to co-exist. The split occurs at 45 μg because many life history and feeding characteristics switch more or less abruptly at about this body size, compromise traits being either non-viable or disadvantageous. Meiofauna and macrofauna therefore comprise two separate evolutionary units each with an internally coherent set of biological characteristics. The expression of this conservative pattern is modified by water depth: the proportion of macrofauna species increases from intertidal situations to deeper water, and it is suggested that mechanisms of resource partitioning and diversity maintainence in the meiofauna and macrofauna are affected differentially by sediment disturbance. Salinity does not affect this proportionality, and so does not differentially affect mechanisms for maintaining species diversity in any particular size category of animals. Meiofauna species size distributions may be modified in sandy sediments because of physical impositions on interstitial or burrowing lifestyles. Brief discussion of some implications of these observations includes speculations on the larval ecology of macrofauna, on gigantism in Antarctic invertebrates, and on the benthic Sheldon spectrum.

Journal ArticleDOI
TL;DR: Reduction of root growth following defoliation appears to be an effective mechanism to aid reestablishment of the photosynthetic canopy and the root: shoot balance and contributes to both herbivory tolerance and maintenance of competitive ability.
Abstract: Root growth responses to defoliation were observed in the field with an improved root periscope technique, which is described. The grazing tolerant, Eurasian bunchgrass, Agropyron desertorum, was compared with the very similar but grazing sensitive, North American bunchgrass, A. spicatum. Root length growth of clipped A. desertorum was about 50% of that of intact plants, while root elongation of clipped A. spicatum continued relatively unabated during ninety days of regrowth following severe defoliation. The reduced root growth in A. desertorum was correlated with the allocation of relatively more resources to aboveground regrowth, thus aiding reestablishment of the root: shoot balance. This balance was apparent in similar root mortality patterns of clipped and control A. desertorum plants in the season following defoliation. In clipped A. spicatum, however, root mortality increased in the winter following the season in which the clipping was done and continued into the subsequent growing season. Reduction of root growth following defoliation appears to be an effective mechanism to aid reestablishment of the photosynthetic canopy and the root: shoot balance. As such it contributes to both herbivory tolerance and maintenance of competitive ability.

Journal ArticleDOI
TL;DR: It is concluded that compensatory photosynthesis does not appear to be an important ecological component of herbivory tolerance for these species.
Abstract: The occurrence of compensatory photosynthesis was examined in the field for all foliage elements on two Agropyron bunchgrass species that differ in their evolutionary history of grazing pressure. This is the first reported field study of compensatory photosynthesis in individual foliage elements of graminoids. Compensatory photosynthesis was defined as an increase in the photosynthetic rates of foliage on partially defoliated plants relative to foliage of the same age on undefoliated plants. Compensatory photosynthesis did occur in many individual foliage elements during at least part of their ontogeny. For both species, compensatory photosynthesis was related primarily to delayed leaf senescence and increased soluble protein concentrations, but not to an improvement in the water status of clipped plants. Soluble protein concentration increased in all foliage elements. A delay in senescence on clipped plants was documented for the two oldest, fully-expanded leaves that were present when the plants were initially clipped, but the initiation and senescence of all other foliage elements were not affected by the clipping treatments. Photosynthetic water use efficiency and photosynthetic rates per unit soluble protein of foliage on partially defoliated plants were not increased following the clipping treatments. Although A. desertorum and A. spicatum were exposed to different levels of grazing pressure during their evolutionary history, the phenology, water status, and gas exchange rates of foliage were very similar both for undefoliated as well as partially defoliated plants. Thus, we conclude that compensatory photosynthesis does not appear to be an important ecological component of herbivory tolerance for these species.

Journal ArticleDOI
TL;DR: Measurements reinforce the view that epiphytic algae can be the primary basis of the food web in seagrass meadows, even when such algae appear to be sparse.
Abstract: Detritus from common seagrasses and other marine angiosperms may often be a less important basis for estuarine food webs than previously believed. In NW Gulf of Mexico seagrass meadows, epiphytic algae have high productivities, palatability, and a more important trophic role than common large plants have. Interdisciplinary field experiments show (1) intensive night-time ingestion of epiphytes by various invertebrate “detritivores”, (2) very high productivity of epiphytic algae on seagrasses, and (3) assimilation of epiphytes rather than seagrasses, as measured by δ13C comparisons. These combined data show that many naturally concentrated and potentially competing invertebrates in Gulf of Mexico seagrass meadows feed largely on the algal overgrowth on seagrass blades, even when such algae appear to be sparse. Primary productivity of these epiphytic algae can equal that of the seagrasses, per blade or per unit biomass. Animal δ13C values tracked epiphytic values rather than seagrass values when comparisons were made over six sites. These measurements reinforce the view that epiphytic algae can be the primary basis of the food web in seagrass meadows.

Journal ArticleDOI
Nadav Nur1
TL;DR: A model of optimal feeding frequency is proposed, which accounts for many aspects of blue tit reproductive biology, including the observed relationship between nestling weight and brood size and the observation that parent reproductive costs increase with brood size.
Abstract: 1. To examine the ultimate factors determining the frequency with which male and female blue tits (Parus caeruleus) feed their nestlings, I manipulated the brood sizes of 34 pairs breeding in Wytham Woods, England, and recorded automatically the number of visits when the young were 11-13 days of age. At this time adults were caught and weighed and the nestlings weighed. 2. The number of visits/brood increased linearly with brood size, with no evidence of an upper limit to brood feeding frequency. The number of visits·nestlings-1·day-1 decreased from 138 to 108 as brood size increased from 3 to 6, but levelled off at 68-87 visits/nestling as brood size increased from 8 to 15. 3. Female weight-loss during the nestling period increased linearly as the feeding frequency of the brood increased: those females were lightest that fed their young most often; this is of some significance since I previously showed that the lightest females are least likely to survive to the following year. The correlations of feeding frequency with brood size and feeding frequency with female weight are sufficient to account for the commonly observed correlation of brood size and female weight. No relationship between feeding frequency and male weight dynamics was apparent. Feeding frequency of the brood increased as mean nestling weight decreased. Apparently, parents compensate, in part, for low nestling weight by increasing feeding frequency. Low nestling weight, in turn, has proven detrimental to post-fledging survival. 4. Lack's (1947, 1954) hypothesis, that the decline in visits/nestling as brood size increases reflects the inability of parents to sustain high brood feeding rates, is refuted by the observations made in this and a large number of other studies. Royama's (1966) hypothesis that the observed decline in visits/nestling reflects a decrease in energy needed to maintain homeothermy, receives some support. Behavioral and physiological studies, however, suggest that the "Royama effect" is limited to small broods (c. four or fewer young). 5. I propose, instead, a model of optimal feeding frequency, which accounts for observations made by myself and others, and which incorporates Royama's hypothesis as well. The model assumes that parents optimize their investment in their young. On the one hand, an increase in feeding frequency means more food for the young, which enhances offspring survival ("benefit"); on the other hand, increased feeding frequency is "costly" to the parents (reduces parental survivorship). Natural selection favors breeding individuals that maximize the difference between benefits (to the offpsring) and costs (to the parents). The model accounts for many aspects of blue tit reproductive biology, including the observed relationship between nestling weight and brood size and the observation that parent reproductive costs increase with brood size.

Journal ArticleDOI
TL;DR: The selective nature of the seed predators, the relative patchiness of predation intensity in space, suggest that postdispersal seed predation can play a role in determining the distribution and/or abundance of old-field herbs.
Abstract: In a pair of experiments conducted in old-field habitats in southwestern Michigan (USA), we examined rates of seed loss to post-dispersal predators (ants and rodents). Seeds from 4-6 species of "biennial" plants were tested over a range of seed densities and habitat types. We found that seed removal was significantly higher in vegetated habitats than in areas of disturbed soil (both simulated small-animal diggings and a plowed field). In the undisturbed vegetation, seed losses ranged from 1-20% of seeds removed/day.An exclosure experiment demonstrated that ants and rodents foraged selectively for seeds of the six plant species tested. Rodents (Peromyscus maniculatus) fed preferentially on species producing large seeds (predominantly Tragopogon dubius). Ants (Myrmica lobicornus) foraged on smaller seeds, although their foraging preferences were not based strictly on seed size.Seed density had only a minor effect on predation rate over the range of densities tested. Predators, instead appeared to treat each experimental group of seeds as a single prey patch. Consequently, predation intensity was quite variable over distances of <20 m within a relatively homogeneous section of habitat.These field experiments provide initial estimates of seed losses to post-dispersal predators in old-field habitats. Rates of seed loss were generally less than those reported from desert or semi-arid habitats. However, for some old-field species, seed losses averaged an appreciable 10-20% day. The selective nature of the seed predators, plusthe relative patchiness of predation intensity in space, suggest that postdispersal seed predation can play a role in determining the distribution and/or abundance of old-field herbs.

Journal ArticleDOI
TL;DR: Test for association between plant species and VAM fungal spores indicated that the spores of Glomus caledonium are associated with plants from dry, nutrient poor sites and spores of gigaspora gigantea are positivelyassociated with plants occurring on the wet, relatively nutrient rich sites.
Abstract: Abundance and distribution of vascular plants and vesicular-arbuscular mycorrhizal (VAM) fungi across a soil moisture-nutrient gradient were studied at a single site. Vegetation on the site varied from a dry mesic paririe dominated by little bluestem (Schizachyrium scoparium) to emergent aquatic vegetation dominated by cattail (Typha latifolia) and water smartweed (Polygonum hydropiperoides). Plant cover, VAM spore abundance, plant species richness, and number of VAM fungi represented as spores, had significant positive correlations with each other and with percent organic matter. The plant and VAM spore variables had significant negative correlations with soil pH and available Ca, Mg, P and gravimetric soil moisture. Using stepwise multiple regression, Ca was found to be the best predictor of spore abundance. Test for association between plant species and VAM fungal spores indicated that the spores of Glomus caledonium are associated with plants from dry, nutrient poor sites and spores of gigaspora gigantea are positively associated with plants occurring on the wet, relatively nutrient rich sites. Glomus fasciculatum was the most abundant and widely distributed VAM fungus and it had more positive associations with endophyte hosts than the other VAM fungi. We found no relationship between beta niche breadth of plant species and the presence or absence of mycorrhizal infection. However, our data suggest that some plant species may vary with respect to their infection status depending upon soil moisture conditions that may fluctuate seasonally or annually to favor or hinder VAM associations.

Journal ArticleDOI
TL;DR: It is shown that the close proximity of sahuaros leads to a relative increase in stem die-back as well as greater mortality in a common nurse tree, the foothill paloverde (Cercidium microphyllum).
Abstract: In the Sonoran Desert, the sahuaro cactus (Carnegiea gigantea) is commonly associated with canopies of trees and shrubs: so-called nurse plants. Although mechanisms by which nurse plants facilitate sahuaro establishment have been studied, possible competitive interactions between sahuaro cacti and nurse plants have not been conclusively demonstrated. In this paper I show that the close proximity of sahuaros leads to a relative increase in stem die-back as well as greater mortality in a common nurse tree, the foothill paloverde (Cercidium microphyllum). This interaction appears to accelerate the local loss of individual nurse trees, resulting in a predictable pattern of species replacement.

Journal ArticleDOI
TL;DR: Simple mechanical sieving provides a sufficient explanation for the mechanism of particle retention of the filtering process in Daphnia.
Abstract: Food size selection of four Daphnia, species (D. magna, D. hyalina, D. galeata, D. pulicaria) was investigated using spherical plastic beads as artificial food and with small bacteria. The size of the particles ranged from 0.1 to 35 μm with special emphasis to the particle diameters between 0.1 and 1 μm. In one set of experiments a mixture of differently sized particles was offered as food suspension and the selectivity of filtering was determined by comparing the size spectrum of the particles found in the gut contents with the spectrum in the food suspension. In a second series of experiments suspensions of uniformly sized particles were offered to single animals and their feeding activity was observed directly. In both types of experiments the mesh sizes of the filtering apparatus of the respective animals studied were measured after the experiments by, scanning electron microscopy. The mean sizes of the filter meshes were about 0.4–0.7 μm. In all experiments the size of the particles found in the gut or those which caused high feeding activities were larger than the smallest mesh sizes of the filters. As a consequence simple mechanical sieving provides a sufficient explanation for the mechanism of particle retention of the filtering process in Daphnia. D. magna was found to feed with high efficiency on suspended freshwater bacteria, the residual species investigated showed low filtering efficiencies when bacteria were offered as food.

Journal ArticleDOI
TL;DR: Among the Swedish satyrids, two of the three species which do deposit their eggs on the larval hosts overwinter in the pupal stage, thus necessitating rapid larval development, and butterflies which use visually apparent host plants seem to find their host plants without having to alight on non-hosts, whereas butterflies that use hosts that are visually non-apparent frequently alight in the oviposition search before they find the appropriate plants.
Abstract: 1. The egg-laying behaviour in the wild of 51 butterflies in Sweden is studied: three different patterns emerge. Firstly, although the majority of butterflies deposit their eggs on the plants on which their larvae later feed, butterflies that overwinter in the egg stage and use herbaceous host plants tend to avoid laying their egges on host plants 2. Secondly, butterflies which use host plants that are superabundant, notably the grass-feeding satyrids, also tend not to deposit their eggs on the leaves on which the larvae later feed. Among the Swedish satyrids, two of the three species which do deposit their eggs on the larval hosts overwinter in the pupal stage, thus necessitating rapid larval development. 3. Thirdly, butterflies which use visually apparent host plants seem to find their host plants without having to alight on non-hosts, whereas butterflies that use hosts that are visually non-apparent frequently alight on non-host plants during the oviposition search before they find the appropriate plants. 4. The possible adaptive significance of these egg-laying patterns is discussed.

Journal ArticleDOI
TL;DR: The principle compound which renders the juvenile phase internodes unpalatable is papyriferic acid, a triterpene which is a demonstrated feeding deterrent to snowshoe hares and which is present in juvenile internodes at concentrations 25 times greater than those in mature internodes.
Abstract: Mature growth-phase internodes of Alaska paper birch (Betula resinifera) are preferred by the snowshoe hare (Lepus americanus) over juvenile growth-phase internodes due to the low food value of the latter. While the mature over juvenile preferencec cannot be explained by the levels of inorganic nutrients or gross chemical fractions (resins or phenols), it can be explained by the striking differences in secondary metabolites of the two growth phases. The principle compound which renders the juvenile phase internodes unpalatable is papyriferic acid, a triterpene which is a demonstrated feeding deterrent to snowshoe hares and which is present in juvenile internodes at concentrations 25 times greater than those in mature internodes.

Journal ArticleDOI
Derek A. Roff1
TL;DR: Breeding experiments indicate that in G. firmus the wing polymorphism is under genetic control and the decrease in fecundity is sufficiently large that genotypes producing only macropterous offspring could only persist in highly unstable environments where continuous dispersal was imperative for survival.
Abstract: The widespread occurrence of wing polymorphisms in insects suggests that the possession of wings and ability to fly adversely affect components of the insect's life characteristics that contribute to its Darwinian fitness. This hypothesis was tested by an analysis of the differences in life history parameters of the macropterous and micropterous morphs of the two cricket species G. firmus and A. fasciatus. In both species there were no differences in development time or adult survival between the two morphs. Significant differences in head width were not consistent between the two species but in both sexes of G. firmus and females of A. fasciatus (insufficient males for analysis) long-winged individuals weighed more than short-winged individuals with the same head width. In both species egg production is delayed in macropterous females. The cumulative fecundity of the micropterous morph is greater than the macropterous morph in both species but only in G. firmus is the difference statistically significant. A. fasciatus frequently loose their wings but no such loss has been observed in G. firmus. There is a significant increase in egg production after the loss of the wings. These results are in accord with those of Tanaka (1976) for the cricket, Pteronemobius taprobanensis. Breeding experiments indicate that in G. firmus the wing polymorphism is under genetic control. The decrease in fecundity is sufficiently large that genotypes producing only macropterous offspring could only persist in highly unstable environments where continuous dispersal was imperative for survival. However, the reproductive cost of a genotype producing a small percentage of macropterous individuals is slight. The fitness that accrues to a genotype producing a few dispersing offspring is likely to offset the small reproductive cost and hence wing polymorphisms should be favoured.

Journal ArticleDOI
TL;DR: The results do not support the hypothesis that daphnids gain some metabolic advantage from vertical migration, and both species grow better and produce more offspring under “non-migration” conditions.
Abstract: (1) The growth response of two Daphnia species coexisting in Lake Constance to constant and fluctuating conditions of temperature and food was tested in a flowthrough system (2) In the lake D hyalina exhibits a pronounced diurnal pattern of vertical migration, whereas D galeata stays near the surface The experiments were designed to measure growth and reproductive success of the species under the conditions as they are experienced by their counterparts in the field (3) Both species grow better and produce more offspring under “non-migration” conditions (constant high temperature and high concentration of food) Thus there is no metabolic advantage by vertical migration (4) D hyalina is more successful than D galeata under extreme “migration” conditions (high food level/high temperature at night and very low food/low temperature during day), but D galeata grows slightly better under favourable conditions (5) The results do not support the hypothesis that daphnids gain some metabolic advantage from vertical migration

Journal ArticleDOI
TL;DR: A foraging choice model incorporating caloric reward and imbibing rates of bees suggests M. compressipes and M. marginata should specialize on richer nectars, a pattern best explained as the result of intercolony competition and greater availability of lower quality nectar.
Abstract: Colony foraging activity of four Melipona species (Apidae: Meliponinae, tribe Meliponini) was studied during the dry season, when many plants flower in central Panama. The efficiency of sucrose solution uptake by Melipona was compared to that of domesticated European Apis mellifera. Dynamics of nectar foraging were also recorded for 3 of the Melipona visiting the forest shrub, Hybanthus prunifolius (Violaceae). 1. Sugar concentration in nectar brought to nests averaged from 21 to 60% sugar for 15 colonies of M. fasciata, M. compressipes triplaridis, M. fuliginosa and M. marginata micheneri. Concentrations ranged from 19 to 72%, and all species collected nectars ranging at least between 24 and 63% sugar. However, M. compressipes and M. marginata preferred higher concentrations and foraged less on dilute nectars. Peak colony nectar harvest occured in late morning or early afternoon; peak pollen harvest was in early morning. 2. Imbibing rates of bees given 20, 30, 45, 60 or 70% sucrose solutions were highest at ≦45% sucrose, but caloric intake was most rapid at 60% sucrose for all species. All but M. marginata displayed greater net intake rates than domesticated European Apis mellifera. A foraging choice model incorporating caloric reward and imbibing rates of bees suggests M. compressipes and M. marginata should specialize on richer nectars. Rate of caloric intake per forager weight was higher for all Melipona (0.03–0.13g) than for A. mellifera (0.10 g). 3. The nectar of Hybanthus prunifolius (Violaceae), a shrub pollinated exclusively by Melipona, progressed from 35 to 60% sugar during the day. Bees foraged most when nectar was below 60% concentration, a pattern best explained as the result of intercolony competition and greater availability of lower quality nectar. 4. Sugar concentration in nectar harvested by colonies rose from lower to higher values through the day for Melipona. The increasing caloric reward of nectar is adaptive in exploiting foraging preferences of such bees. As standing nectar crop is depleted by competing bees, a gradual shift to more rewarding nectar should promote increased bee foraging range, more flower visits during a foraging trip, floral constancy, and genetic outcrossing. 5. The nectar load capacity of A. mellifera is greater than that of Melipona. Other factors being equal, Africanized A. mellifera, now a permanent resident of neotropical forests, should visit more flowers during a foraging trip. Additional species differences in foraging behavior are analyzed.

Journal ArticleDOI
TL;DR: Differences between the California chaparral and Chilean matorral are more profound than previously thought and are due not only to different degrees of human disturbance, but also to the presence of periodical natural fires in California and not in Chile, and to different shrub recruitment patterns and mammalian herbivore activity in the two areas.
Abstract: Previous studies have claimed that the Chilean matorral is more open than the Califonia chaparral, and have attributed this dissimilarity largely to the role of man in Chile. In this paper we show that in general the Chilean matorral has a structure better described as shrub clumps that merge to form a continuous vegetation matrix only in very mesic habitats, where it is comparable to the Califonia chaparral. We also present evidence that these clumps have been present for at least the last 26 years and that even without human disturbance they are likely to maintain themselves. Evidence for the latter pertains to seed dispersal, seed germination and establishment, seedling survival, and the diameter size structure of shrub clumps. Finally, we propose that differences between the California chaparral and Chilean matorral are more profound than previously thought and are due not only to different degrees of human disturbance, but also to the presence of periodical natural fires in California and not in Chile, and to different shrub recruitment patterns and mammalian herbivore activity in the two areas.

Journal ArticleDOI
TL;DR: In this paper, the responses of photosynthesis, transpiration and leaf conductance to changes in vapour pressure deficit were followed in well-watered plants of the herbaceous species, Helianthus annuus, HelIANthus nuttallii, Pisum sativum and Vigna unguiculata, and in the woody species having either sclerophyllous leaves, Arbutus unedo, Nerium oleander and Pistacia vera, or mesomorphic leaves, Corylus avellana, Gossypium hirsutum
Abstract: The responses of photosynthesis, transpiration and leaf conductance to changes in vapour pressure deficit were followed in well-watered plants of the herbaceous species, Helianthus annuus, Helianthus nuttallii, Pisum sativum and Vigna unguiculata, and in the woody species having either sclerophyllous leaves, Arbutus unedo, Nerium oleander and Pistacia vera, or mesomorphic leaves, Corylus avellana, Gossypium hirsutum and Prunus dulcis. When the vapour pressure deficit of the air around a single leaf in a cuvette was varied from 10 to 30 Pa kPa-1 in 5 Pa kPa-1 steps, while holding the remainder of the plant at a vapour presure deficit of 10 Pa kPa-1, the leaf conductance and net photosynthetic rate of the leaf decreased in all species. The rate of transpiration increased initially with increase in vapour pressure deficit in all species, but in several species a maximum transpiration rate was observed at 20 to 25 Pa kPa-1. Concurrent measurements of the leaf water potential by in situ psychrometry showed that an increase in the vapour pressure deficit decreased the leaf water potential in all species. The decrease was greatest in woody species, and least in herbaceous species. When the vapour pressure deficit around the remainder of the plant was increased while the leaf in the cuvette was exposed to a low and constant vapour pressure deficit, similar responses in both degree and magnitude in the rates of transpiration and leaf conductance were observed in the remainder of the plant as those occurring when the vapour pressure deficit around the single leaf was varied. Increasing the external vapour pressure deficit lowered the water potential of the leaf in the cuvette in the woody species and induced a decrease in leaf conductance in some, but not all, speies. The decrease in leaf conductance with decreasing water potential was greater in the woody species when the vapour pressure deficit was increased than when it remained low and constant, indicating that changing the leaf-to-air vapour pressure difference had a direct effect on the stomata in these species. The low hydraulic resistance and maintenance of a high leaf water potential precluded such an analysis in the herbaceous species. We conclude that at least in the woody species studied, an increase in the vapour pressure deficit around a leaf will decrease leaf gas exchange through a direct effect on the leaf epidermis and sometimes additionally through a lowering of the mesophyll water potential.

Journal ArticleDOI
TL;DR: Observations on the limited potential for acclimation to high light in leaves of seedlings of rainforest trees are discussed in relation to regeneration following formation of gaps in the canopy.
Abstract: Seedlings of the Caesalpinoids Hymenaea courbaril, H. parvifolia and Copaifera venezuelana, emergent trees of Amazonian rainforest canopies, and of the Araucarian conifers Agathis microstachya and A. robusta, important elements in tropical Australian rainforests, were grown at 6% (shade) and 100% full sunlight (sun) in glasshouses. All species produced more leaves in full sunlight than in shade and leaves of sun plants contained more nitrogen and less chlorophyll per unit leaf area, and had a higher specific leaf weight than leaves of shade plants. The photosynthetic response curves as a function of photon flux density for leaves of shade-grown seedlings showed lower compensation points, higher quantum yields and lower respiration rates per unit leaf area than those of sun-grown seedlings. However, except for A. robusta, photosynthetic acclimation between sun and shade was not observed; the light saturated rates of assimilation were not significantly different. Intercellular CO2 partial pressure was similar in leaves of sun and shade-grown plants, and assimilation was limited more by intrinsic mesophyll factors than by stomata. Comparison of assimilation as a function of intercellular CO2 partial pressure in sun- and shade-grown Agathis spp. showed a higher initial slope in leaves of sun plants, which was correlated with higher leaf nitrogen content. Assimilation was reduced at high transpiration rates and substantial photoinhibition was observed when seedlings were transferred from shade to sun. However, after transfer, newly formed leaves in A. robusta showed the same light responses as leaves of sun-grown seedlings. These observations on the limited potential for acclimation to high light in leaves of seedlings of rainforest trees are discussed in relation to regeneration following formation of gaps in the canopy.

Journal ArticleDOI
TL;DR: After a 2-year drought period, the physiological sequence of reactivation is respiration—photosynthesis—nitrogen fixation, whereas recovery of nitrogen fixation is dependent on newly differentiated heterocysts.
Abstract: The response of the terrestrial blue-green algae Nostoc flagelliforme, Nostoc commune, and Nostoc spec. to water uptake has been investigated after a drought period of approximately 2 years. Rapid half-times of rewetting (0.6, 3.3, and 15.5 min, respectively) are found. The surfaceto-mass ratio of the three species is inversely correlated to the speed of water uptake and loss. The ecological relevance of these different time courses is discussed.Respiration starts immediately after a 30-min rewetting period, whereas photosynthetic oxygen evolution reaches its maximum activity after 6 and 8 h with N. commune and N. flagelliforme, respectively. In the dark, recovery of oxygen uptake by N. commune is somewhat impaired, while slightly stimulated with N. flagelliforme. With both species, recovery of photosynthesis is inhibited by darkness.Using colonies kept dry for two years, nitrogenase activity of N. commune attains its maximum 120 to 150 h after rewetting, while only 50 h were needed with algal mats kept dry for two days.Thus, after a 2-year drought period, the physiological sequence of reactivation is respiration-photosynthesis-nitrogen fixation. Respiration and photosynthesis precede growth and are exhibited by existing vegetative cells, whereas recovery of nitrogen fixation is dependent on newly differentiated heterocysts.

Journal ArticleDOI
TL;DR: In this paper, the CO2 uptake capacity of leaves of five competing woody species in an undisturbed developing Central European hedgerow was investigated for possible factors determining competitive ability in the field.
Abstract: The CO2 uptake capacity of leaves of five competing woody species in an undisturbed developing Central European hedgerow was investigated for possible factors determining competitive ability in the field. Light-saturated maximal CO2 uptake (A max) showed species-specific seasonal variations in Prunus spinosa, a bushlike pioneer on fallow land, in Crataegusxmacrocarpa and Acer campestre, two treelike species dominating the canopy, in Rubus corylifolius, a pioneer liane, and in Ribes uva-crispa, a shrubby undergrowth species. In fully-expanded sun leaves of Prunus, Crataegus and Acer A max ranged from 8 to 12 μmol m-2 s-1 while it ranged from 6 to 15 μmol m-2 s-1 in Rubus and Ribes. The temperature responses showed no difference among species. Neither leaf photosynthetic capacity nor nutrient use of carbon fixation determined competitive ability. Differences between species in the capacity of leaves to adapt to shade resulted in differences in species' establishment in the understory and demonstrated the importance of growth in order to escape light-limiting conditions. A specific sequence of species was found for the range of A max in sun leaves. It was highest in an early pioneer of low competitive ability (Rubus), medium in a later pioneer (Prunus) and in successional plants (Crataegus, Acer), and lowest in the climax species of high competitive ability, Fagus silvatica, (3-4 μmol m-2 s-1; Schulze 1970).

Journal ArticleDOI
TL;DR: Between-habitat differences in macrophyte consumption by herbivorous fishes were examined on three Caribbean and two Indian Ocean coral reefs, supporting the hypothesis that defenses against herbivores are costly in terms of fitness and are selected against in habitats with predictably low rates of herbivory.
Abstract: Between-habitat differences in macrophyte consumption by herbivorous fishes were examined on three Caribbean and two Indian Ocean coral reefs. Transplanted sections of seagrasses were used as a bioassay to compare removal rates in reef-slope, reef-flat, sand-plain, and lagoon habitats. Herbivore susceptibility of fifty-two species of seaweeds from these habitats was also measured in the field. Seagrass consumption on shallow reef slopes was always significantly greater than on shallow reef flats, deep sand plains, or sandy lagoons. Reef-slope seaweeds were consistently resistant to herbivory while reef-flat seaweeds were consistently very susceptible to herbivory. This pattern supports the hypothesis that defenses against herbivores are costly in terms of fitness and are selected against in habitats with predictably low rates of herbivory.