Oikos

Erb, John; Boyce, Mark S.; Stenseth, Nils Chr.

doi: 10.1034/j.1600-0706.2001.920101.xpmid: N/A

We offer an evaluation of the Caughley and Krebs hypothesis that small mammals are more likely than large mammals to possess intrinsic population regulating mechanisms. Based on the assumption that intrinsic regulation will be manifest via direct density‐dependent feedbacks, and extrinsic regulation via delayed density‐dependent feedbacks, we fit autoregressive models to 30 time series of abundance for large and small mammals to characterize their dynamics. Delayed feedbacks characterizing extrinsic mechanisms, such as trophic‐level interactions, were detected in most time series, including both small and large mammals. Spectral analyses indicated that the effect of such delayed feedbacks on the variability in population growth rates differed with body size, with large mammals exhibiting predominantly reddened and whitened spectra in contrast with predominantly blue spectra for small mammals. Large mammals showed less variance and more stable dynamics than small mammals, consistent with, among other factors, differences in their potential population growth rates. Patterns of population dynamics in small versus large mammals contradicted those predicted by the Caughley and Krebs hypothesis.

Tsai, Min‐Li; Li, Jan‐Jung; Dai, Chang‐Feng

doi: 10.1034/j.1600-0706.2001.920102.xpmid: N/A

The flesh‐burrowing parasitic isopod Ichthyoxenus fushanensis was found infecting the body cavity of a freshwater fish, Varicorhinus bacbatulus, in heterosexual pairs. Herein we investigate the question of how the host body size may constrain the parasite size and clutch size by analyzing the interactions among the body size and clutch size of the parasite, and host size. Due to the low transmission rate of I. fushanensis to its host and the positive relationship between clutch size and female size, selection may favor larger females with larger clutch sizes to compensate for massive losses of manca (the free‐living juveniles). The path model reveals that clutch size depends not only directly on female size, but also on the sizes of her host and mate. Female size also depends on the sizes of the host and her mate. A negative correlation exists between the body sizes of the paired males and females. This negative correlation may be regarded as a consequence of competition for limited available space or other resources provided by the host. The effects of host size on parasite size, however, act on the total volume of both sexes as a whole, not specifically on either the female or the male. In this case, the available space/resources may not allow both individuals of different sexes to evolve toward a larger size simultaneously. Under the constraint of host size, a strategy of reducing the body size of the paired male may provide a way to increase the body size of the paired female and achieve a larger clutch size.

Matsuura, Kenji

doi: 10.1034/j.1600-0706.2001.920103.xpmid: N/A

Discrimination and aggressive responses toward non‐nestmates have been observed in a number of termite species, but the mechanism is poorly understood. Here I present the novel hypothesis that differential intestinal bacteria composition leads to production of colony‐specific chemical cues that enable nestmate recognition. The intestinal microflora of a lower termite, Reticulitermes speratus, consisted of many bacteria species. The composition of the intestinal bacteria was exclusively colony‐specific. Termites that had adsorbed an unfamiliar odor of bacteria sampled from another colony were fiercely attacked by nestmates. Experimental manipulation of the composition of bacteria by antibiotics successfully changed the recognition behavior toward nestmates. These results indicate that intestinal bacteria play an important role in nestmate recognition.

Møller, Anders Pape; Legendre, Stéphane

doi: 10.1034/j.1600-0706.2001.920104.xpmid: N/A

The negative frequency‐dependent effect of reproductive success in animals on population growth refers to a category of phenomena termed the Allee effect. The mechanistic basis for this effect and hence an understanding of its consequences has been obscure. We suggest that sexual selection, in particular female mate preferences, is a previously neglected component giving rise to the Allee effect. Lack of breeding and reduced reproductive success of females at low population densities are commonly described in situations where females have little or no opportunity to choose a mate, consistent with this suggestion. We developed a demographic model that incorporated the effects of lack of female choice on rates of reproduction. Using either a mating system with incompatibility or a system with a directional mate preference, we show that commonly encountered levels of reproductive suppression in the absence of suitable mates in a population, where sexual selection still operates, may increase the effects of demographic stochasticity considerably.

Malmqvist, Björn; Wotton, Roger S.; Zhang, Yixin

doi: 10.1034/j.1600-0706.2001.920105.xpmid: N/A

The concentration and transport of faecal pellets (FPs) produced by blackfly (Diptera: Simuliidae) larvae were estimated in a large, free‐flowing river in the north of Sweden during 1997–1999. FPs were abundant from May through August and FP loads in transport peaked at 429 t dry mass d−1 at a site in the lower part of the river in 1997. Daily transport at the same site, averaged over each study period (16 Jun.–18 Sep. 1997, 6 Apr.–10 Sep. 1998, and 21 Apr.–5 Aug. 1999), was estimated to be 93.7 t dry mass (3.7 t carbon), 47.5 t dry mass (1.9 t carbon and) and 69.2 t dry mass (2.7 t carbon), respectively. On a large scale, there was a downstream trend of increasing FP concentration and, during periods with greater discharge, sedimentation was reduced so that more material was exported from the river. Samples from six sites in a regulated river (into which our focal river flows) parallel to six sites in the unregulated tributary showed considerably lower FP concentrations in the regulated river, presumably because of much smaller blackfly populations as a consequence of habitat loss through damming. A survey of two other large, unregulated rivers in northern Sweden confirmed that these also carry large amounts of FPs. We conclude that the transformation of small suspended food particles into considerably larger FPs by huge populations of filter‐feeding blackfly larvae is a major process in large northern rivers.

Scheffer, R. A.; Van Logtestijn, R. S. P; Verhoeven, J. T. A.

doi: 10.1034/j.1600-0706.2001.920106.xpmid: N/A

Peatlands can be classified into fens and bogs based on their hydrology. Development of fens to bogs is accompanied by the invasion of Sphagnum species. The purpose of this study was to determine how the decomposition process in fens is influenced by the transition from a vascular plant‐dominated system to a Sphagnum‐dominated system. We carried out a reciprocal litter bag experiment, using litter of Carex diandra, C. lasiocarpa, Sphagnum papillosum and S. squarrosum in a fen dominated by Sphagnum species and a fen without Sphagnum. Decomposition rate and nitrogen and phosphorus dynamics of the plant litter were measured in a field experiment for two years. Decomposition rate was highest for the Carex litter types and lowest for the Sphagnum litter types. Surprisingly, decomposition rates hardly differed between the two sites. Nutrient dynamics, however, showed a clear site‐effect: In the Sphagnum site net mineralization was observed for all litter types whereas in the Carex site net immobilization was observed. These results show that carbon and nutrient cycles were coupled in a different way in a Sphagnum‐dominated and a Carex‐dominated site, respectively. Nutrient availability and adaptation of the microbial community to nutritional and other environmental conditions may be the main regulators of carbon and nutrient cycles in these peatlands.

Smith, Melinda D.; Knapp, Alan K.

doi: 10.1034/j.1600-0706.2001.920107.xpmid: N/A

The size of the local species pool (i.e., species surrounding a community capable of dispersal into that community) and other dispersal limitations strongly influence native plant community composition. However, the role that the local species pool plays in determining the invasibility of communities by exotic plants remains to be evaluated. We hypothesized that the richness and abundance of exotic species would be greater in C4‐dominated grassland communities if the local species pool included a larger proportion of exotic species. We also predicted that an increase in the exotic species pool would increase the invasibility of sites thought to be resistant to invasion (annually burned grassland). To test these hypotheses, study plots were established within two long‐term (>20 yr) fire experiments at a tallgrass prairie preserve in NE Kansas (USA). Study plots were surrounded by either a small pool of exotic species (small species pool (SSP) plots; six species) or a larger exotic species pool (large species pool (LSP) plots; 18 species). We found that richness and absolute cover of exotic species was significantly (P<0.001) lower (∼70 and 90%, respectively) in annually burned compared to unburned plots, regardless of the size of the exotic species pool. As predicted, exotic species richness was higher (P<0.001) for LSP plots (3.9 per 250 m2) than for SSP plots (0.7 per 250 m2); however, absolute cover was unaffected by the size of the exotic species pool. In the absence of fire, plots with a LSP had four times as many exotic species than SSP plots. An increase in the local exotic species pool also increased the invasibility of annually burned grassland. Indeed, richness of exotic plant species in annually burned LSP plots did not differ from unburned plots with a SSP, indicating that a larger pool of exotic species countered the negative effects of fire. These findings have important implications for predicting how the invasion of plant communities may respond to human‐induced global changes, such as habitat fragmentation. Community characteristics or factors such as frequent fires in grasslands may impart resistance to invasions by exotic species in large, intact ecosystems. However, when a large pool of exotic species is present, frequent fire may not be sufficient to limit the invasions of exotic plants in fragmented landscapes.

Tuomi, Juha; Kytöviita, Minna‐Maarit; Härdling, Roger

doi: 10.1034/j.1600-0706.2001.920108.xpmid: N/A

Mycorrhizal symbiosis involves reciprocal transfer of carbon and nutrients between shoots on the one hand and roots colonized by symbiotic fungi on the other. Mycorrhizas may improve the mineral nutrient acquisition rates, but simultaneously increase the belowground demand for carbon. Mycorrhizal plants will have a selective advantage over non‐mycorrhizal ones if they are more cost‐efficient in terms of carbon cost per unit of acquired mineral nutrient. However, we demonstrate here in a simple model system that this is not a necessary condition. Mycorrhizas may evolve even when they are less cost‐efficient, provided that photosynthesis and/or growth are strongly nutrient‐limited. This result implies a unique hypothesis for the evolution of mycorrhizal associations which may be inherently cost‐inefficient as compared to plant roots. Such symbioses may have evolved when the superior nutrient acquisition rate of fungi combines with the relatively high photosynthetic nutrient use efficiency of the host plant. Consequently, provided that mycorrhizas are really cost‐inefficient, the selective advantage of mycorrhizal plants will disappear when an increase in the nutrient acquisition rate is not associated with a sufficiently high nutrient use efficiency of photosynthesis, as at high soil nutrient levels or due to a loss of leaf area, shading or low temperatures.

Potts, Simon G.; Dafni, Amots; Ne'eman, Gidi

doi: 10.1034/j.1600-0706.2001.920109.xpmid: N/A

Fire in Mediterranean‐type ecosystems produces catastrophic changes in plant‐pollinator systems; the recovery of which has been studied by comparing an unburnt mature forest habitat with that of an adjacent recently burnt area (eight years post‐fire). The composition, visitation profiles, and effectiveness of the taxonomically diverse pollinator assemblages found on a core nectar providing species (Saturejathymbra: Lamiaceae) were examined in these two contrasting habitats. S. thymbra in the freshly burnt area had low nectar standing crop and relatively less diverse bee community than an unburnt area which had twice the nectar standing crop and a higher bee diversity and abundance. Both sites supported bee assemblages dominated by the non‐native bumblebee Bombus terrestris. Spatio‐temporal heterogeneity of nectar standing crops and microclimatic conditions were sufficient to explain the form and magnitude of the diurnal foraging profiles at each site in relation to species specific foraging and flight abilities. B. terrestris, Apis mellifera and native solitary bees were the three primary guilds visiting S. thymbra and varied in the efficiency with which they delivered conspecific pollen grains to receptive stigmas. A pollinator effectiveness index for these three guilds was calculated based on floral visitation rates and pollen delivery efficiency and reflected the actual levels of effectiveness of each guild within and across the two habitat types. There was no overall inter‐community difference in pollination effectiveness as the bee assemblages in both habitats were sufficient to produce maximum fruit set in S.thymbra, though the relative contribution of each guild varied intra‐communally. Pollen limitation was not found to occur in either habitat.

Lankinen, Åsa; Skogsmyr, Io

doi: 10.1034/j.1600-0706.2001.920110.xpmid: N/A

During the fertilisation process in plants, pollen tube growth rate may be selected as a trait important in male to male competition. Since female morphology provides the necessary selective arena for such competition, we investigate if sexual selection theory can be used to explain the evolution of pistil length as a female choice mechanism. This choice is performed by direct interference with male to male competition. Furthermore, the sessile nature of plants limits the number of mates a female can choose between, which could limit the benefit a female can gain from distinguishing between donors. To mirror these circumstances, we model a situation when there are only two competitors at a time. Using a game theoretical approach we show that if pollen tube growth rate can be used as an indication of heritable quality, pistil length can be selected in response to variation of this trait. We further find that length of the pistil affects selection of pollen tube growth rate. Thus female preference and male competitive ability co‐evolve, but this does not necessarily lead to a positive relationship between the two. Under certain circumstances we find a negative relation instead. Given realistic differences in male quality, the model indicates that there is a potential for evolution of female morphology as a choice mechanism for pollen quality.