bluuebunny/crossref_metadata_2025 huggingface dataset:
Application of a model of scale dependence to quantify scale domains in open predation experiments
Authors: Englund Göran , Cooper Scott D. , Sarnelle Orlando
Published: December 2004

We use a model of open predation experiments to define scale domains that differ in terms of the controlling processes and scale dependence of predator impacts. For experimental arenas that are small compared to the movements of the prey (small scale domain) the model predicts that predator impacts are scale independent and controlled by prey movements. For arenas of intermediate scale we predict that predator impacts are scale dependent and controlled by both prey movements and direct predation, and for the largest scale domain we predict weak scale dependence and predation control.  We propose that the scale‐domain concept is useful when designing and interpreting field experiments. As an illustration we apply the concept to experiments examining predator effects on the stream benthos. First, we test two key assumptions of the underlying model: that area‐specific prey migration rates decrease with increasing size of experimental arenas and that predation rates are independent of arena size. For this purpose we used published estimates of prey emigration and predator consumption rates for nine studies examining the effects of stream predators on benthic prey. We found that prey per capita emigration rates but not predation rates decreased with increasing arena length.  Second, we demonstrate a method for identifying the scale domain of real experiments. The model of predation experiments was parameterized using experimental data and the expected spatial and temporal scale dependence of predator impacts on prey in these experiments was simulated. The simulations suggest that the studies conducted in the largest arenas (length 15–35 m) should be classified as large‐scale, consumption‐controlled experiments, whereas the experiments conducted in smaller arenas (length 1.5–6 m) should be classified as small or intermediate‐scale. We also attempted to determine the scale domain of the experiments in a large data set, including results from most published stream predation experiments. The majority of arenas used in these experiments (73%) were smaller than 1 m in length. Our data on the scale dependence of predation and prey migration rate suggest that experiments in this scale range (<1 m) should be classified as small‐scale, movement‐controlled experiments for most prey taxa.
Effects of simulated shoot and leaf herbivory on vegetative growth and plant defense in Acacia drepanolobium
Authors: Gadd Michelle E. , Young Truman P. , Palmer Todd M.
Published: December 2004

Plants have considerable ability to respond to herbivory, both with (above‐ground) regrowth and with increased defense. We simulated both leaf and shoot herbivory in controlled, replicated experiments on individuals of Acacia drepanolobium in Laikipia, Kenya. These experiments were carried out on individuals that had experienced different, experimentally controlled histories of large mammalian herbivory. Both forms of simulated herbivory were associated with compensatory regrowth. Branches whose shoots had been removed grew significantly more over the next year than paired control branches, fully compensating for the lost shoot length. Branches whose leaves were removed both grew faster and had more leaves one year later than did control branches. Shoot removal, but not leaf removal, increased the production of side shoots. However, because past herbivore pressure was negatively associated with net shoot growth, there may be a long‐term cost of herbivory even when plants appear to fully compensate for herbivory in the short term. In contrast to the effects on growth, simulated herbivory did not significantly increase physical (spines) or chemical (tannins) defenses, and there were no significant negative correlations between compensatory growth and plant defense.
Exact compensation of stream drift as an evolutionarily stable strategy
Authors: Kopp Michael , Jeschke Jonathan M. , Gabriel Wilfried
Published: December 2004

The colonization cycle hypothesis predicts that adults of stream‐dwelling insects preferentially disperse in the upstream direction in order to compensate for larval drift. Upstream biased dispersal has indeed been shown in many, albeit not all, natural populations. Based on a recently published analysis, we develop a simple stochastic model for the competition of genotypes with different dispersal strategies in a stream habitat. By means of an invasion analysis, we show that exact compensation of larval drift by upstream biased adult dispersal is an evolutionarily stable strategy. Exact compensation means that, on average, the net movement of individuals from birth to the time of reproduction is zero. At the population level, we show that, in general, upstream biased dispersal is not necessary for persistence, unless the reproductive rate is very low. Under all conditions, however, populations of exact compensators attain highest sizes or persistence times, respectively. Although selection pressure towards exact compensation is arguably very general in populations subject to stream drift, trade‐offs or constraints might change the outcome of selection. Therefore, the analysis presented in this paper has to be viewed as a null model for optimal dispersal behavior in stream habitats.
Population dynamics of Microtus pennsylvanicus in corridor‐linked patches
Authors: Coffman Cynthia J. , Nichols James D. , Pollock Kenneth H.
Published: December 2004

Corridors have become a key issue in the discussion of conservation planning; however, few empirical data exist on the use of corridors and their effects on population dynamics. The objective of this replicated, population level, capture‐recapture experiment on meadow voles was to estimate and compare population characteristics of voles between (1) corridor‐linked fragments, (2) isolated or non‐linked fragments, and (3) unfragmented areas. We conducted two field experiments involving 22 600 captures of 5700 individuals. In the first, the maintained corridor study, corridors were maintained at the time of fragmentation, and in the second, the constructed corridor study, we constructed corridors between patches that had been fragmented for some period of time. We applied multistate capture‐recapture models with the robust design to estimate adult movement and survival rates, population size, temporal variation in population size, recruitment, and juvenile survival rates. Movement rates increased to a greater extent on constructed corridor‐linked grids than on the unfragmented or non‐linked fragmented grids between the pre‐ and post‐treatment periods. We found significant differences in local survival on the treated (corridor‐linked) grids compared to survival on the fragmented and unfragmented grids between the pre‐ and post‐treatment periods. We found no clear pattern of treatment effects on population size or recruitment in either study. However, in both studies, we found that unfragmented grids were more stable than the fragmented grids based on lower temporal variability in population size. To our knowledge, this is the first experimental study demonstrating that corridors constructed between existing fragmented populations can indeed cause increases in movement and associated changes in demography, supporting the use of constructed corridors for this purpose in conservation biology.
The interaction between bird predation and plant cover in determining habitat occupancy of darkling beetles
Authors: Groner Elli , Ayal Yoram
Published: December 2004

Tenebrionid beetles in the Negev Desert exhibit size‐related habitat segregation, with larger species found in denser cover. Size‐dependent predation by birds has been suggested as the mechanism behind this habitat segregation. Two predictions of this hypothesis were tested: (1) plant cover reduces the predation efficiency of birds upon large tenebrionids, and (2) birds prefer larger species. Both predictions were supported: plant cover reduced predation rate by the most common spring and summer predatory birds: white storks (Ciconia ciconia) and stone curlews (Burhinus oedicnemus), in cage experiments. Results from preference experiments suggest that tenebrionid species can be divided according to their profitability as prey. Large species are the most profitable, medium‐sized species are less profitable but still acceptable and small species are unprofitable and therefore ignored.  Field observations demonstrated that the well‐vegetated wadi habitats are dominated by large and small species whereas acceptable, medium‐sized species are under‐represented in this habitat. The results of the cage experiments indicate possible apparent competition between the large profitable and the medium acceptable tenebrionid species in the wadis. Aggregative response of predators in the profitable habitat is suggested as the mechanism leading to truncated distribution of prey species. Large profitable species are refuge‐dependent, medium‐sized acceptable species use enemy free space and small species are predator independent.
Changes in plant interactions along a gradient of environmental stress
Authors: Pugnaire Francisco I. , Luque Maria T.
Published: December 2004

A combination of competition and facilitation effects operating simultaneously among plant species appears to be the rule in nature, where these effects change along productivity gradients often in a non‐proportional manner. We investigated changes in competition and facilitation between a leguminous shrub, Retama sphaerocarpa, and its associate understorey species along an environmental gradient in semi‐arid southeast Spain. Our results show a change in the net balance of the interaction between the shrub and several of its associated species, from clearly positive in the water‐stressed, infertile environment to neutral or even negative in the more fertile habitat. There was a weakening of facilitation along the fertility gradient as a consequence of improved abiotic conditions. Competition was the most intense for below‐ground resources in the less fertile environment while total competition tended to increase towards the more productive end of the gradient. Changes in the balance of the interaction between and among different plant species along the gradient of stress were caused by a decline in facilitation rather than by a change in competition. As both competition intensity and facilitation change along gradients of resource availability, plant interactions are best viewed as dynamic relationships, the outcome of which depends on abiotic conditions.
The role of sink to source re‐colonisation in the population dynamics of insects living in unstable habitats: an example of terrestrial chironomids
Authors: Frouz Jan , Kindlmann Pavel
Published: December 2004

Certain species of terrestrial chironomids (Diptera) are specialised on open patches in initial stages of primary or secondary succession (early fallow, lichens and mosses on rocks, etc.). These “source” habitats provide good quality food for their larvae and most offspring are produced here, but they are sensitive to summer desiccation. This often results in extinction of the summer larval population, followed in winter by re‐colonisation from less suitable, but more stable “sink” habitats in the surrounding landscape.  Soil dwelling and long‐lived larvae are poor migrants; short‐lived, winged adult females select patches for their development. Proper choice of oviposition sites and consequent distribution of eggs among individual habitats is thus critical for the success of these species.  A mathematical model was developed in order to find out whether this re‐colonisation strategy could ensure population persistence at the landscape level. The model was verified using long‐term data on Smittia atterima abundance in old fields. The results indicate that even a small proportion of eggs laid in a sink habitat can ensure a successful re‐colonisation of the source habitat. Thus, re‐colonisation of source habitats from sink habitats is concluded to be one of the reasons for persistence of the latter. The model indicates that this re‐colonisation may ensure population persistence even in conditions when exclusive use of only one habitat leads to population extinction either due to environmental stress or to a negative growth rate.
The role of age‐structure on the persistence and the dynamics of insect herbivore–parasitoid interactions
Authors: Bonsall Michael B. , Eber Sabine
Published: December 2004

The role of top‐down (e.g. parasitism) and bottom‐up (e.g. resource competition) processes is of fundamental importance for the stability and persistence of insect herbivore populations. Although emphasis has often focused on single regulatory agents, the processes underpinning tri‐trophic interactions may actually be more pluralistic. Recently, further complexities involved in the regulation of tri‐trophic systems have been highlighted. In particular, life history characteristics may have a concomitant role when coupled with the regulatory effects of resource competition and/or parasitism. Here we present an age‐structured model to investigate the effects of larval development period, parasitism and resource competition on the stability and persistence of herbivore–parasitoid interactions. We show that the influence of weak density dependent parasitism is sufficient to stabilise the interaction when the period of host susceptibility to parasitism is short. For longer periods of host susceptibility, parasitism needs to be highly non‐linear to overcome the destabilising effects of the time delays. In systems where host development is protracted through the season we predict that resource competition is likely to be the dominant process for herbivore regulation. We use this age‐structured approach to explore the population dynamics of two field studies from temperate ecosystems. Predictions from these case studies show 1) that both the strength and type of competition and parasitism are important for the stability and persistence of the particular system, and 2) that the length of the developmental period of the vulnerable host is critical to understand the influence of different regulatory processes. Host demography is of overriding importance in determining whether herbivores show outbreaks, and which particular ecological processes and mechanisms are responsible for generating such overcompensatory dynamics.
Scale strength as a cost of rapid growth in sunfish
Authors: Arendt Jeff , Wilson David S. , Stark Erik
Published: December 2004

Comparisons among populations have revealed adaptive variation in growth rate within several species. However, theoretical models suggest that a cost to rapid growth must exist in order to explain why some populations grow slowly. Based on previous work with development of the cranial skeleton in pumpkinseed sunfish, we proposed that rapid growth may compromise the strength of all skeletal elements in vertebrates. Using pumpkinseed from six populations known to differ in their intrinsic growth rates, we correlated strength of scales, in terms of resistance to penetration, with individual growth rate. We found a negative correlation that was consistent both within and among populations. This trade‐off between growth rate and scale strength may have fitness consequences in terms of likelihood of surviving predation attempts or swimming efficiency.
Costs of reproduction in a lizard species: a comparison of observational and experimental data
Authors: Olsson Mats , Shine Richard , Wapstra Erik
Published: December 2004

Life history theory predicts that increasing investments into reproduction compromises survival and future reproduction. However, demonstrating such costs is confounded by positive correlations between life history traits. For example, individuals in good condition may be good at both surviving and reproducing. We studied such processes in a viviparous snow skink lizard (Niveoscincus microlepidotus) from high elevation sites in Tasmania, Australia. Our results show a stark difference in costs of reproduction between unmanipulated females from the natural population versus experimentally manipulated females (using follicle stimulating hormones). In the unmanipulated females, females with relatively larger reproductive investments survived better than females with smaller reproductive investments. In the experimental group, however, females forced to ‘over‐invest’ into a larger clutch survived less well than controls. Thus, our study confirms the potential dangers of non‐experimental estimation of costs of reproduction.
Egg performance on an egg‐carrying bug. Experiments in the field
Authors: Katvala Mari , Kaitala Arja
Published: December 2004

Selection of oviposition sites has direct influence on female fitness. Differences in offspring survival among sites should favour females to select oviposition sites with the highest survival. Golden egg bug females (Phyllomorpha laciniata; Coreidae, Heteroptera) use conspecifics as oviposition substrates. Most eggs are laid on the back of the bug but they can be found on all body parts. Females never carry their own eggs, and males commonly carry eggs received without copulation with the donor. We examined differences in egg survival relative to paternity to the egg, host sex and body size and attachment of an egg on a host's body in the field. We also studied which bugs received eggs. Egg loss was surprisingly common: 30–80% of recaptured individuals lost eggs during a six‐day period. Unexpectedly, host's paternity to the eggs did not affect egg survival. Also, egg loss did not differ among hosts in respect to other parameters studied. Males received more eggs than females, but survival of the eggs was not better on a male's back than on a female's back. Since egg loss is common in the golden egg bug, we suggest that female oviposition strategy to lay eggs on conspecifics is not perfect. This is due to active behaviour of potential hosts (for example, rejected oviposition attempts) or lack of suitable oviposition substrates (conspecifics) in the field. Since eggs do not survive unless carried, females may be acting opportunistically, doing the best job possible by laying eggs on available conspecifics.
Primary productivity can affect mammalian body size frequency distributions
Authors: Aava Birgitta
Published: December 2004

Frequency distributions of mammal body sizes in large‐scale assemblages have often been found to show a positive skew. In an attempt to explain this pattern, a model has been put forward which incorporates energetic constraints on fitness and thereby predicts optimal body sizes corresponding to the mode of the distribution. A key assumption of the model is that energy is unlimited. However, if energy is limited, the input of energy into a herbivorous mammal community should influence the shape of the frequency distribution. Thus, I propose that increases in primary productivity will decrease the variation of body size and increase the mean body size in a distribution. So, in low‐productivity environments we should see a predominance of small‐sized species, but with a great variation of body sizes due to limitations of resources (energy). I tested this hypothesis using the herbivorous mammal fauna (rodents, bats and marsupials) in seven biomes of Australia. Because herbivorous marsupials generally are fairly large‐bodied while rodents and bats are small‐sized and because marsupials also have a different mode of reproduction from placental mammals, the hypothesis was also tested on placental mammals and marsupials separately. There was no clear mode for the entire assemblage in any biome, but as primary productivity increased, the variation of body masses decreased and the mean body mass of the distribution increased. Body mass distributions of both placental mammals and marsupials displayed clear modes. Placental mammals also showed an increase in mean body mass. The variation in body mass of marsupials was highest for the intermediately productive biomes. Primary productivity does seem to have some effect on mammalian body mass in this case, but the results here need to be complemented with studies of other assemblages before any general conclusions can be drawn. It is also important to distinguish which taxa are affected in a heterogeneous assemblage like the Australian herbivorous mammal fauna.
Numerical and per capita responses to species loss: mechanisms maintaining ecosystem function in a community of stream insect detritivores
Authors: Ruesink Jennifer L. , Srivastava Diane S.
Published: December 2004

We experimentally reduced the diversity of detritivorous stream insects in field enclosures, and measured the effects on an ecosystem function, processing of leaf litter. Two dominant species were removed separately, the stonefly Pteronarcys californica and the caddisfly Lepidostoma unicolor. In principle, processing could be maintained after species loss in two ways: the remaining species could increase their rates of shredding (per capita response), or they could increase in abundance (numerical response). We imposed a numerical response in some treatments by experimentally increasing abundances of either all the remaining species or the other dominant species so that expected metabolic capacity of the assemblage returned to full‐diversity levels. Numerical responses were generally effective in maintaining leaf breakdown when either Lepidostoma or Pteronarcys was removed, except that the treatment in which Lepidostoma was replaced by an equivalent metabolic capacity of all remaining species showed less leaf loss than the full‐diversity treatment. Per capita responses by other species appeared effective in compensating for the removal of Pteronarcys (although there were other explanations) but were not effective in compensating for the removal of Lepidostoma. In summary, the consequences of reduced biodiversity varied with which species was lost and how the remainder responded. Thus there was no simple relationship between biodiversity and ecosystem functioning. However, when numerical or per capita compensation does occur, stability of function should rise with diversity in such “interactive” assemblages.
The relationships among root and leaf traits of 76 grassland species and relative abundance along fertility and disturbance gradients
Authors: Craine J. M. , Froehle J. , Tilman D. G. , Wedin D. A. , Chapin, III F. S.
Published: December 2004

For 76 annual, biennial, and perennial species common in the grasslands of central Minnesota, USA, we determined the patterns of correlations among seven organ‐level traits (specific leaf area, leaf thickness, leaf tissue density, leaf angle, specific root length, average fine root diameter, and fine root tissue density) and their relationships with two traits relating to growth form (whether species existed for part of the growing season in basal, non‐caulescent form and whether species were rhizomatous or not). The first correlation of traits showed that grasses had thin, dense leaves and thin roots while forbs had thick, low‐density leaves and thick roots without any significant differences in growth form or life history. The second correlation of traits showed a gradient of species from those with high‐density roots and high‐density erect leaves to species with low‐density roots and low‐density leaves that were held parallel to the ground. High tissue density species were more likely to exist as a basal rosette for part of the season, were less likely to be rhizomatous, and less likely to be annuals. We examined the relationships between the two axes that represent the correlations of traits and previously collected data on the relative abundance of species across gradients of nitrogen addition and disturbance. Grasses were generally more abundant than forbs and the relative abundance of grasses and forbs did not change with increasing nitrogen addition or soil disturbance. High tissue density species became less common as fertility and disturbance increased.
Somatic and demographic costs under different temperature regimes in the late‐flowering alpine perennial herb Saxifraga stellaris (Saxifragaceae)
Authors: Sandvik Sylvi M.
Published: December 2004

If reproduction involves costs, preventing reproduction one year should result in increased growth and/or reproduction the next year. Costs of reproduction in the late‐flowering perennial alpine herb Saxifraga stellaris were studied in an experimental field study. To determine whether cost of reproduction differed between two contrasting temperature regimes, I examined plants in Open Top Chambers (OTCs) and control plots. One year before measurements all flower buds on the experiment plants were removed. There was no impact of the flower bud removal or the interaction between flower removal and temperature on prefloration time or seed maturation time. However, cost of reproduction influenced growth (somatic cost), vegetative propagation, flowering frequency, number of stem per ramet, number of fruits, and seed mass (demographic cost). However, significant interaction effects of flower removal and temperature on growth and fruit production revealed that the cost of reproduction differed between the two temperature regimes. The warmed plants showed reduced cost of reproduction compared to plants growing under natural temperature.
Temporal variation in herbivore host‐plant preference and performance: constraints on host‐plant adaptation
Authors: Cronin James T. , Abrahamson Warren G. , Craig Timothy P.
Published: December 2004

Temporal variation in the acceptability or suitability of plant genotypes to an herbivore has seldom been considered as a possible constraint limiting the adaptation of herbivores to particular plant genotypes, or the occurrence of a positive correlation between host‐plant preference and offspring performance. In this study, we used data spanning 12 yr from the same 20 clones of goldenrod (Solidago altissima) to examine the temporal variation in oviposition preference and offspring performance of a stem‐galling fly, Eurosta solidaginis. We found that the stem galler's preference for, or performance on, the different clones was uncorrelated between years of this study. Furthermore, we found that the relative rankings of clones changed by an average of 31% between successive years. We suggest that these consistently high year‐to‐year fluctuations in preference and performance by E. solidaginis are likely due to environmental factors (e.g., water and nutrient levels, or abundance of interspecific herbivores) that fluctuate over time and are known to differentially affect the acceptability and suitability of clones to herbivores; i.e., genotype×environment interactions. These results are significant because temporal fluctuations in host‐plant preference and performance are likely to favor a more generalized diet by herbivorous insects.
Hatch or wait? A dilemma in reptilian incubation
Authors: Spencer Ricky‐John , Thompson Michael B. , Banks Peter B.
Published: December 2004

Animals often form groups to reduce the risk of predation through the per capita dilution of their individual predation risk. The advantages of grouping also influence the timing of reproduction in many species. In particular, synchrony in the timing of births may have evolved as a predator‐avoidance strategy as it dilutes the risk of predation upon vulnerable newborn and naive young. Eggs of an Australian freshwater turtle, Emydura macquarii, can hatch synchronously despite developmental asynchrony among eggs of a clutch and hatchlings have a reduced predation risk by emerging from the nest as a group. Developmental asynchrony within clutches was induced to reflect natural nests by dividing clutches and incubating them at either 25°C or 30°C. Some eggs were then reunited with their clutch‐mates and hatching occurred synchronously in some of these groups. In groups where synchronous hatching did not occur, less advanced eggs still hatched earlier than the normal incubation period. Synchrony occurred because the less advanced eggs hatched up to five days earlier than the control embryos. We conclude that the less advanced embryos within a clutch either accelerate their development or hatch prematurely to ensure synchrony of hatching and hatchling group formation may facilitate emergence from the nest and dilute predation risk.
Breeding location choice in salmon: causes (habitat, competition, body size, energy stores) and consequences (life span, energy stores)
Authors: Hendry Andrew P. , Berg Ole K. , Quinn Thomas P.
Published: December 2004

Breeding location choice provides a mechanism by which individuals can directly influence their reproductive success. Location choice should therefore reflect individual condition, habitat features, and the intensity of competition; with these factors then influencing reproductive success. To test whether such patterns were detectable in the wild, we tagged 705 sockeye salmon (Oncorhynchus nerka) in a natural population, and monitored them from when they started breeding until they died. We evaluated the role of individual condition (size, secondary sexual traits, energy stores) in the acquisition of breeding locations that differed in the intensity of competition (female density, sex ratio) and habitat features (water depth, water velocity). We then evaluated the influence of breeding location on reproductive life span and energy stores. At a coarse level (20‐m stream sections), females consistently settled in certain locations, and these locations sustained high densities and held larger females. At a fine scale (0.5‐m breeding sites), (1) larger fish occupied deeper water (males, r2=0.072; females, r2=0.199), (2) higher levels of competition reduced reproductive life span for males (r2=0.139) but not females, and (3) fish with shorter reproductive life spans died with more energy remaining in their muscle tissue (males, r2=0.414; females, r2=0.440). These patterns were nested within a tendency for late breeding fish to have shorter reproductive life spans. Energy stores and secondary sexual traits did not influence breeding location choice, and larger fish did not acquire locations of higher intrinsic quality (i.e., those sections settled first and sustaining higher competition). Our study provides evidence that some aspects of individual condition influence breeding location choice, which then influences components of reproductive success.
Allochthonous aquatic insects increase predation and decrease herbivory in river shore food webs
Authors: Henschel Joh R. , Mahsberg Dieter , Stumpf Helmut
Published: December 2004

Rivers produce an abundance of aquatic insects that traverse land, where they can have bottom‐up effects on predators, who, in turn, can have top‐down effects on terrestrial herbivores. This effect can cascade down to plants. These trophic relationships were demonstrated in a field of stinging nettles, Urtica dioica, along a river in Germany. At the shore compared to similar microhabitats 30–60 m away the abundance and biomass of: midges were highest, spiders were also highest, while herbivorous leafhoppers were lowest. At the shore, nettle plants were less damaged by herbivores and thus had less regrowth. Spiders regularly captured both aquatic midges as well as terrestrial leafhoppers and they captured more individuals of both groups at the shore than further away. Midges supported high densities of shore spiders. This was inferred from correlation of distribution and diet in the absence of other environmental gradients. Removal of spiders from experimental plots caused leafhoppers to increase at the shore, causing more plant damage. These effects were not evident at spider‐removal sites away from the shore. This demonstrated that spiders depressed leafhoppers and decreased herbivory on plants only at the shore. It is concluded that aquatic insects had a bottom‐up effect on spiders and that this subsidy facilitated a top‐down effect that cascaded from spiders to leafhoppers to plants. Similar effects would explain the distribution of arthropods along many rivers. Allochthony connects river food webs with shore food webs, making both components essential for each other.
Organic matter quality in ecological studies: theory meets experiment
Authors: Joffre Richard , Ågren Göran I. , Gillon Dominique , Bosatta Ernesto
Published: December 2004

Despite its importance for the understanding of element cycles in ecosystems, organic matter (OM) quality has remained an elusive property that is difficult to measure. In this study, two new approaches, both of which taking into account the complete biochemical composition of the organic material during the decomposition process, have been combined to solve this problem. First, following the continuous‐quality theory where quality is defined as a measure of substrate availability to the decomposers, initial litter OM qualities of a range of plant species from two experiments on litter decomposition were estimated and resulted in highly accurate fits of observed mass loss during decomposition. Applying the same theory, qualities of the litters at all stages of decomposition were then calculated. By comparison, the initial qualities of the same litters were estimated from conventional chemical fractions and resulted in much lower accurate fits. Second, near‐infrared reflectance spectroscopy (NIRS), a highly precise physical method of characterising biochemical composition of OM, was used to obtain a unique spectral signature of each sample. Calibrations were performed between spectral data and calculated qualities on the first half of the sample set and the calibration equations were applied to the second half of the sample set. Results show that theoretical litter OM quality can be calibrated and predicted precisely using NIRS. OM quality, defined according to a theoretical concept of substrate availability to decomposers, then contains and summarises all the relevant biochemical information. We demonstrate how the combination of NIRS and theory allows us to accurately measure OM quality. Measurement of OM quality provides an access to a fundamental property of organic matter and opens up new possibilities for studying element cycles in ecosystems.
The effect of pollen competition on maintenance of variation in fertilisation ability
Authors: Lankinen Åsa , Skogsmyr Io
Published: December 2004

Pollen competition in the pistil does not only give flowering plants the possibility to reduce inbreeding but also provides an opportunity for selection of pollen traits that increase male reproductive success. An objection to the existence of selection on pollen competitive ability is that individual variation should quickly vanish if selection is strong. A balance between selection for local adaptation of sporophytes within sites and pollen flow between sites could maintain variation in pollen competitive ability. A prerequisite is that variation in male competitive ability is condition dependent, i.e., influenced by sporophytic adaptation to a patch. This further means that selection on pollen competitive ability can occur both directly on the gamethophytic level and indirectly on the sporophytic level. Our dynamic model shows that maintenance of variation in male competitive ability is more probable when differences in pollen competitive ability influence male fitness, i.e., in cases with pollen competition, than when differences in this trait only is a side effect of selection for more viable individuals. Since there is a connection between the gamethophytic and sporophytic life‐phases, the incidence of pollen competition between donors should make it more probable that variability also in sporophyte fitness is preserved.
Flexible antipredator behavior in a dragonfly species that coexists with different predator types
Authors: Hopper Kevin R.
Published: December 2004

Two of the main predators of dragonfly larvae, insectivorous fish in communities with fish and large dragonfly species in communities without fish, differ markedly in their mode of predation. In general, dragonfly species coexist successfully with one predator or the other, but larvae of the dragonfly Pachydiplax longipennis can coexist successfully with both. I examined the behavioral response of these larvae to a simulated predator attack to determine whether their response (1) differs between the two communities, and (2) is sensitive to waterborne cues about the type of predator present. I compared larvae from two different communities: fish ponds where insectivorous fish were the top predators, and fish‐free ponds where large dragonflies were the top predators. Larvae from fish‐free ponds actively moved away from an attack significantly more than did larvae from fish ponds, provided each was attacked in its native pond water. Larvae collected from a fish‐free pond but then attacked in fish water moved less than did controls (larvae attacked in fish‐free water). Likewise, larvae collected from a fish pond but attacked in fish‐free water moved more than did controls (larvae attacked in fish water). Larvae attacked first in water from their native pond and then in water from the contrasting pond changed their response in the expected direction. These results indicate that escape behavior in P. longipennis differs between communities with different predator types and is sensitive to waterborne cues in a manner consistent with the mode of predation employed by each predator.
The influence of species diversity on ecosystem productivity: how, where, and why?
Authors: Fridley Jason D.
Published: December 2004

The effect of species diversity on ecosystem productivity is controversial, in large part because field experiments investigating this relationship have been fraught with difficulties. Unfortunately, there are few guidelines to aid researchers who must overcome these difficulties and determine whether global species losses seriously threaten the ecological and economic bases of terrestrial ecosystems. In response, I offer a set of hypotheses that describe how diversity might influence productivity in plant communities based on three well‐known mechanisms: complementarity, facilitation, and the sampling effect. Emphasis on these mechanisms reveals the sensitivity of any diversity‐productivity relationship to ecological context (i.e., where this relationship should be found); ecological context includes characteristics of the surrounding environment, temporal and spatial scales of observation, and the intensity of human management. In particular, the legitimacy of the sampling effect as a mechanism of productivity enhancement is dependent upon the degree to which stochastic events influence immigration and extinction processes in a given ecosystem. A mechanistic approach also requires that the three mechanisms be separated and quantified in diversity experiments, and I examine the most appropriate analyses for doing so, focusing on the overyielding technique. Finally, I question why productivity per se is a relevant management concern in non‐agricultural systems once relationships among diversity, productivity, and the qualities of the surrounding environment are considered.
Resource utilization and interspecific relations of sympatric bobcats and coyotes
Authors: Neale Jennifer C. C. , Sacks Benjamin N.
Published: December 2004

We used scat analysis and radiotelemetry to characterize use of foods and habitats by sympatric bobcats and coyotes, and evaluated these in the context of spatial and temporal relationships to assess the potential for, and evidence of, interspecific competition. Bobcats and coyotes exhibited broad and overlapping diets. However, diets of the two predators differed in the relative contributions of small and large prey, with bobcats consuming relatively more rodent and lagomorph biomass and coyotes consuming relatively more ungulate biomass. Consumption among rodent prey species was highly correlated between bobcats and coyotes, indicating no evidence of prey partitioning within this group. Habitat selection by the two predators differed slightly at the landscape scale but not within home ranges. Bobcats and coyotes occupied small, overlapping home ranges, such that the likelihood of interspecific encounters (direct or indirect) was high. Bobcats displayed slight avoidance of overlapping coyote core areas during coyote reproductive seasons (winter and spring), when coyotes are typically most territorial (toward conspecifics), but displayed slight attraction during times of year when coyotes were not engaged in reproductive activities. Relative to coyotes, which were strongly nocturnal, diel activity patterns of bobcats were more diurnal and variable. However, activity patterns were not inversely correlated. Overall, these predators appeared to use resources independently and we found little evidence of negative interactions. Differences in resource use by bobcats and coyotes appeared to relate to fundamental niche differences as opposed to competition‐related resource partitioning.
Evolution of recruitment features in plants: a comparative study of species in the Rosaceae
Authors: Kiviniemi Katariina
Published: December 2004

Theory predicts that life history traits that reduce the impact of environmental variation show patterns of negative covariation (trade‐offs). In plants, seed size, seed dormancy and dispersal (in space) interact to reduce risk in a temporally and spatially variable environment. Dispersal in time and space permits escape from unfavourable conditions, whereas a large seed size may improve establishment under unfavourable conditions. However, large seeds may suffer a high rate of loss due to seed predators. The objective of this study was to examine relationships between seed size, seedling establishment and seed dormancy by combining data from field experiments with information of phylogenetic relationships among 11 species in the subfamily Rosoideae (Rosaceae). The predicted relationship between seed size and seed predation was also examined. Emergence from seed, survival and recruitment of seedlings, effects of seed predators (post‐dispersal predation), and seed dormancy were studied experimentally in the field. All species generated seedlings in the experimental plots. Overall, the emergence of seedlings was promoted by disturbance. Several species possessed seeds that were able to germinate after being buried in the soil column for three years. Despite a small data set (ten phylogenetically independent contrasts), the comparative analysis confirmed an expected positive effect of seed size on establishment ability. The emergence of seedlings increased with seed size. The results did not demonstrate the hypothesised positive association between seed size and predation risk, or negative association between seed size and seed dormancy. However, the contrast analysis showed a significant negative relationship between seed dormancy and survivorship of seedlings, i.e. between quantitative data of dormancy and establishment capacity of the species obtained under natural conditions in the field. When not controlling for phylogeny (without contrasts), the only significant effect was a negative association between seed size and seed dormancy.
Temporal self‐similarity created by spatial individual‐based population dynamics
Authors: Kaitala Veijo , Alaja Susanna , Ranta Esa
Published: December 2004

We report an individual‐based single‐species model producing temporal scale‐free, self‐similar dynamics in time. Individuals in the population renew in an explicit space with a large number of loci. We show that reproduction, subsequent dispersal of the offspring, and mortality will organise population fluctuations such that the emerging dynamics represent power law and scale‐free structures. Further, we show that spatially structured population dynamics may show red frequency spectra, a property that the simple nonlinear population models are generally lacking.
Onset of reproduction in Rhododendron lapponicum shoots: the effect of shoot size, age, and nutrient status at two subarctic sites
Authors: Karlsson P. Staffan , Jacobson Anders
Published: December 2004

Potential factors that trigger the onset of reproduction in shoot modules were studied in the dwarf shrub Rhododendron lapponicum in subarctic Swedish Lapland. The following questions were addressed: (1) Can the large size effect that commonly is found on onset of reproduction be explained as an effect of the amount of nitrogen and/or phosphorus accumulated? (2) Is there an effect of age when controlling for size and amount of nutrient reserves? (3) Does the relationship between size or nutrient reserves and onset of reproduction vary between high‐ and low‐altitude sites?

 Shoots from two different sites, a subalpine heath (345 m altitude) and an alpine heath (900 m), were compared. The reproductive status of the shoots (non‐reproductive or with an inflorescence bud) was related to shoot age, size (shoot leaf area), and nitrogen and phosphorus concentration by means of logistic regression analysis.

 The analysis indicates that an index of shoot nitrogen pool size was the most important factor; however, we could not strictly differentiate between the effect of this index and that of shoot leaf area and nitrogen concentration. A large shoot leaf area and nitrogen concentration (i.e. a large nitrogen pool index) increases the likelihood of turning reproductive. When controlling for shoot leaf area and nitrogen concentration a negative effect of increasing age was found. We suggest that this age effect may be related to a declining shoot resource economy with increasing age. Triggering of reproduction in R. lapponicum is thus complex and all factors included in our analysis appear to be involved. Shoots at the high‐altitude site were more likely to enter the reproductive phase than shoots with a similar leaf area at low altitude. The only factor for which we found support for the hypothesis about decreasing importance of nutrients with increasing altitude was for phosphorus concentration. The mean age at onset of reproduction did not differ between altitudes.
Differential selection on floral traits of Ipomopsis aggregata growing in contrasting environments
Authors: Caruso Christina M.
Published: December 2004

Interactions for pollination between co‐flowering plant species have been hypothesized to shape the evolution of their floral traits, but this hypothesis has rarely been tested. I tested the prediction that the presence of a co‐flowering plant species influences the strength and/or direction of pollinator‐mediated selection on floral traits. I measured phenotypic selection via female fitness on four floral traits of Ipomopsis aggregata in five populations. Three contained only conspecifics (I only) and two also contained the co‐flowering species Penstemon barbatus (P+I). Directional selection via fruits/plant on corolla length and width differed in both strength and direction between P+I and I only populations. On average, selection on corolla length and width (1) was stronger in P+I than I only populations and (2) was consistently negative in P+I populations, but consistently positive in I only populations. However, these differences in selection on I. aggregata between P+I and I only populations were not caused by interactions for pollination with P. barbatus. Although plants in P+I populations received approximately 31% less conspecific pollen/flower than plants in I only populations, this difference in pollination did not translate into differences in reproductive success, which indicates that P. barbatus and I. aggregata do not strongly compete for pollination. In addition, selection via fruits/plant and conspecific pollen deposited/flower was not congruent. For example, selection on corolla length via pollen/flower was uniformly positive and did not differ between P+I and I only populations. These data suggest that the presence of P. barbatus does influence selection on floral traits of I. aggregata, but not by influencing pollination. Instead, differences in selection between P+I and I only populations appear to be the result of post‐pollination modification of selection by a factor correlated with the presence of P. barbatus.