摘要:

Population regulation is a fundamental process related to most phenomena in ecology, including evolutionary ecology. I review the conceptual basis for defining regulation as bounded fluctuations in abundance, in contrast to the unbounded fluctuations of random—walk populations. Regulation arises as a result of potentially stabilizing density—dependent processes, even when brought about by “non—equilibrium” mechanisms. Although the phenomena is unambiguous in theory, detecting regulation by finding evidence for density dependence in a series of population estimates faces unsolved statistical problems. So, while there is growing evidence for widespread regulation, severe detection problems remain. I illustrate these with data from bird populations. Whether regulation is typically achieved by local stabilizing mechanisms or via metapopulation dynamics remains to be determined. I summarize recent studies on a particularly well—regulated system–red scale (Aonidiella aurantii) and its controlling parasitoid, Aphytis melinus. We tested and failed to find evidence for eight hypotheses that might account for the system's stability, including spatial heterogeneity in attack rates, a refuge, and metapopulation dynamics. We also failed to find evidence for density—dependent parasitism, but such density dependence might be still be present. Recent laboratory and modeling studies have uncovered a number of other potentially stabilizing mechanisms centering on the response of individual Aphytis to their size—structured host. This plethora of size— and stage—dependent interactions leads naturally to a consideration of the factors controlling Aphytis' size—dependent behavioral decisions, and consequently to the elaboration of size—structured models. The latter provide a vehicle for bringing together investigations of selection of life histories, and population dynamics. This is illustrated by a model of Aphytis and red scale dynamics that can explain a dramatic case of competitive displacement. The red scale/Aphytis system exemplifies a particularly challenging problem in population regulation, namely to account for the co—occurrence of stability and severe suppression of the prey population. A potentially generic solution is to assume stabilizing density dependence in the parasitoid or predator population; however, this has the consequence of increasing the host or prey population equilibrium. My colleagues and I have shown that observed prey densities in a plankton system are too low for such a mechanism to be operating. Further work is needed to test this and other hypotheses.

ISSN:0012-9658    

DOI:10.2307/1939533

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

We examine a salt marsh host—parasitoid system, consisting of the planthopper Prokelisia marginata and its egg parasitoid Anagrus delicatus, for evidence of stabilizing parasitoid behavior. We first determine if there is sufficient parasitoid aggregation to potentially stabilize the Prokelisia—Anagrus interaction, using methods that infer parasitoid behavior from the spatial pattern of parasitism. We then test a basic assumption implicit in theory, that the degree of parasitoid aggregation is invariant with respect to changes in overall host and parasitoid density, and through time. Our results suggest that Anagrus behavior should, in theory, exert a consistent stabilizing effect on Prokelisia—Anagrus dynamics. Using field experiments, and direct sampling of the spatial distribution of parasitoids, we then try to determine if parasitoid aggregation is in fact responsible for the heterogeneous pattern of parasitism observed in the field. The results of these studies indicate that there is insufficient parasitoid aggregation to account for this heterogeneity. Instead, factors such as the degree of tidal inundation, or the age of host eggs, may cause host patches to differ in their vulnerability to parasitoid attack.

ISSN:0012-9658    

DOI:10.2307/1939534

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Arthropod herbivore communities vary in the proportion of plant family specialists and broad generalists. Examination of the literature of 23 annual crop plant species and the associated feeding relations of 498 arthropod species in Japan revealed that contemporary cropping area was only weakly associated with current arthropod species richness and that the proportion of plant family specialists was greater and the proportion of generalists was lower on plant species that had been in Japan for longer periods of time. These results suggest that arthropod communities become increasingly more specialized or less generalized the longer a plant coexists with the community, and that this process can occur over time periods of a few thousand years. Exotic arthropod species associated with these plants and known to have invaded Japan in the last 100—200 yr are more generalized than the native fauna, so species invasions could not contribute to the increased specialization or decreased generalization with time.

ISSN:0012-9658    

DOI:10.2307/1939535

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Inducible C4—like photosynthetic metabolism in Hydrilla verticillata leaf tissue elicits variability in photosynthetic phenotype, expressed as CO2 compensation point (Τ). We conducted a field and laboratory study to investigate the ecological and adaptive significance of this physiological phenomenon. Spatial horizontal environmental heterogeneity was observed within clonal populations of H. verticillata in Florida, USA. Measured at midday, the edge habitat at the expanding periphery of the clone exhibited a dissolved inorganic carbon (DIC) concentration of 0.7 mmol/L, pH 7.1, a dissolved oxygen (DO) level of 0.13 mmol/L, and biomass of 0.2 kg/m2. The mat habitat, located 200 cm towards the interior of the surface mat, exhibited DIC 0.1 mmol/L, pH 10.2, DO 0.48 mmol/L, and biomass 0.8 kg/m2. DIC depletion and DO supersaturation characterized the mat habitat for most of the day and much of the growing season. Furthermore, net photosynthesis, daily carbon gain, and relative growth rate (RGR) of H. verticillata were reduced 80% by mat conditions compared to edge conditions. Τs of H. verticillata were positively correlated with CO2and bicarbonate concentration, and negatively correlated with pH, DO, and biomass. Low and high Τ photosynthetic phenotypes were associated with the mat and edge habitats, respectively. Photosynthetic phenotype of H. verticillata appears to acclimate to environmental heterogeneity within a clone in the field. Net photosynthesis and daily carbon gain of low Τphenotype H. verticillata was 128% and 40% greater than the high Τ phenotype when measured in the mat habitat, but was 21% lower than the high Τphotosynthetic phenotype when measured in the edge habitat under low quantum flux. Laboratory experiments showed a negative curvilinear relationship between the Τ of H. verticillata and plant density. The data demonstrate that plasticity in photosynthetic phenotype of H. verticillata is a density—dependent, physiological response that optimizes carbon gain within a stressful heterogeneous environment. Evolution of facultative C4—like photosynthetic metabolism in H. verticillata may have been an adaptation to the contraints imposed upon carbon gain by DIC and quantum flux limitation in the mat habitat.

ISSN:0012-9658    

DOI:10.2307/1939536

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

We studied the pollination ecology and assemblage structure of 31 species of Stylidium (Stylidiaceae) at 25 sites in Western Australia. The number of species per study site varied between two and size. Stylidium species are pollinated by a variety of nectar—seeking solitary bees and bombyliid flies. Within and among species there is significant variation in nectar—tube length (and therefore in the insects that visit the flowers) and in pollen placement on pollinators. Pollen is placed “explosively” on the insect by a motile column of fused staminate and pistillate tissues; the position and reach of the column varies within and among species, thereby causing variation in site of pollen deposition. When discrete pollination niches were defined for all species, only one niche overlap was observed across the 86 interacting pairs of Stylidium species at the 25 sites. To determine whether this was a nonrandom assemblage structure we compared our observation with the outcome of null models. We developed three null models to cover the most likely structuring processes: that communities are organized by (1) ecological sorting, (2) evolution of plant phenotypes, or (3) both processes. We concluded that it was unlikely (P = .055—.002) that so few overlaps in pollination niches would occur by chance. We developed another null model to test whether chance could have created the apparent pattern of character displacement in pollination niches exhibited by the nine species showing intraspecific variation. The analysis indicated that character displacement has probably occurred (P = .014). This study is one of the clearest demonstrations to date of reproductive interactions generating assemblage structure and character displacement in plants.

ISSN:0012-9658    

DOI:10.2307/1939537

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

In a fragmented, dry subtropical forest in northwestern Argentina, we compared pollination levels, fruit set, and seed set among small (2 ha) fragments, and continuous forest in 16 plant species representing a wide range of pollination systems, breeding systems, and growth forms. For three species, Prosopis nigra (Mimosoideae), Cercidium australe (Caesalpinoideae), and Atamisquea emarginata (Capparaceae), the three treatments were replicated across four sites; we achieved less replication for other species. Because comparisons between forest and fragment populations for different species took place in different sites, however, by treating all 6 species as a unit we lessened the potential bias of confounding site effects and could evaluate the overall impact of fragmentation. Significant or marginal (P<.10) fragmentation—related declines in number of pollen tubes per style, fruit set, and seed set occurred in 9 of 16, 5 of 15, and 3 of 14 species, respectively. Overall, significant or nonsignificant declines occurred in 81% (pollen tubes), 73% (fruit set), and 79% (seed set) of the species. In all cases these proportions were greater (P<.06) than the null binomial expectation of a 1:1 ratio of increases to decreases. Breeding system did not explain sensitivity to fragmentation: the magnitudes of declines in pollen tubes, fruit set, and seed set were virtually indistinguishable between self—compatible and self—incompatible species. At least 4 of the 10 self—incompatible species, however, were heavily visited in small fragments by Africanized honey bees, which may have compensated for a decline in visits by native pollinators. The exact nature of responses varied among plant species. In some, the absolute quantity of pollen grains transferred to stigmas decreased with fragmentation, and sometimes this was reflected in reduced fruit or seed set. In Cercidium, Prosopis, and Atamisquea, the quality of the grains transferred apparently changed: number of pollen tubes produced per pollen grain on the stigma declined with increasing fragmentation, and at least in the latter two species seed production declined as well. Overall, levels of pollination and seed production undoubtedly integrated many idiosyncratic effects of fragmentation on particular plant and animal populations, and indicated that “community health” of fragments suffered in comparison with that of continuous forest. Median decreases in pollination levels and seed output from forest to fragments approached 20%. The impact of these declines on plant recruitment is less clear, however, because cattle grazing and trampling of seedlings and saplings in fragments may constitute a much more serious short—term conservation problem.

ISSN:0012-9658    

DOI:10.2307/1939538

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Pseudoflowers induced by the rust fungus Puccinia monoica on Arabis spp. are flower—like in color, shape, size, nectar production, and scent. Pseudoflowers attract insects that aid the rust's reproduction in a way that is analogous to pollination in flowering plants. I explored the effects of pathogen—induced pseudoflowers and co—blooming buttercups (Ranunculus inamoenus) on each other's insect visitation by comparing visitation in single—species plots and mixtures. Visitation to pseudoflowers was greater in mixtures containing buttercups than in pseudoflowers plots of the same density, and visitation to buttercups was also greater in the mixtures than in pure buttercup plots. Because fungal pseudoflowers and buttercups both appeared to receive greater visitation when associated with each other than when alone, and both species produce food for insect visitors, this interaction could be described as Mullerian floral mimicry. However, several characteristics of this system do not fit standard definitions of mimicry: (1) pseudoflowers are not necessarily mimicking any particular species but instead may be generalized flower—mimics, (2) flower visitors may not always mistake pseudoflowers for flowers but may actually choose them, and (3) under some circumstances pseudoflowers and flowers may compete for visitors. Competition in favor of pseudoflowers is most likely when flies are the primary flower visitors or on the rare occasions when pseudoflowers are common relative to other species.

ISSN:0012-9658    

DOI:10.2307/1939539

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

Although numerous studies have addressed plant compensatory responses to removal of vegetative tissue by herbivores, only a few have determined the effect of floral removal on subsequent seed production in natural populations. In Sanicula arctopoides, a perennial monocarp, removal of inflorescences (umbels), both naturally by deer (Odocoileus hemionus) and by artificial clipping early in the flowering season, led to no loss of maternal reproduction as measured by seed number and seed mass. Full compensation occurred when umbels bearing up to one—third of a plant's flowers were removed at the stage when plants are normally grazed. However, there are thresholds both in timing and severity of removal beyond which plants were unable to compensate fully. Compared to controls, artificial removal of developing fruits 20 d later than the normal stage resulted in a 42% decrease in seed production. Successive clippings that removed °55% of each plant's flowers resulted in a 52% decrease in seed production relative to controls; however, a single clipping of the same intensity resulted in full compensation. Differential reactions to clipping of different umbel ranks provided information about the mechanism of the response. Removal of secondary umbels resulted in decreased seed abortion rates in later developing umbels, while removal of tertiary umbels resulted in decreased abortion rates in both earlier and later formed umbels. These responses indicated that delayed abortion of developing seeds may be common in secondary umbels, that removal of secondary umbels by herbivores may relieve plants of some costs of abortion, and that S. arctopoides uses flexible allocation to achieve full compensation at typical levels of floral herbivory.

ISSN:0012-9658    

DOI:10.2307/1939540

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

We characterized the structure of 25 old—growth hemlock—hardwood forests in northern Wisconsin and adjacent Michigan in order to examine our working hypotheses that differences in their structure are related to stand age (i.e., stage of development) and that changes in stand structure continue after old—growth status is achieved. Estimates of stand age, i.e., age of oldest tree cored, based on 10 cores taken from hemlocks of a range of diameters in each stand, ranged from 177 to 374. By investigating the patterns of live tree structure, coarse woody debris (CWD), tip—up mounds, and canopy gaps in relation to stand age, we were able to infer changes that occur during stand development. Along the gradient of old—growth stand development, some changes in structural features, including total volume of CWD, snag (Standing dead tree) basal area, and total area and average size of canopy gaps were continuous, linear increases over time. In contrast, changes in live tree and snag density, density of large trees, volume of well—decayed hemlock logs, and diameter—age relationships occurred after a threshold stand age of 275—300 yr was reached. Area and density of tip—up mounds and density of large seedlings and saplings were not correlated with stand age. Old—growth hemlock—hardwood stands at the upper end of the age continuum (>275—300 yr) have accumulated both gradual and threshold structural changes, acquiring most of the following characteristics: (1) a strong correlation between age and diameter of trees, (2) low densities of live trees distributed across all size classes, (3) trees>70 cm dbh (diameter at breast height), (4) dead wood>120—150 m3/ha, with logs>80 m3/ha, (5) hemlock logs present in all decay classes, and (6) canopy gaps occupying>10% of the stand, with the average gap size>50 m2, some gaps>200 m2, and no more than 30% of the gaps<10 m2. The two oldest stands may be at or near equilibrium, where maximum volumes of CWD are maintained through a balance of tree growth and mortality, and wood decay; and oldest tree age approaches 400+ yr, the maximum longevity typical for hemlock.

ISSN:0012-9658    

DOI:10.2307/1939541

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY

摘要:

We explored local variation in rate of plant succession by computing three contrasting measures of change for 10 New Jersey old fields through the first 18 yr after abandonment. We found that all three measures, euclidean distance in ordination space, turnover analysis, and regression analysis of species loss from the initial cohort, showed the same trend of decrease in the rate of plant succession with time. However, we did not find a monotonic decrease, and the specific years of transient increase differed with the analytical measure used. Both turnover and regression analysis showed that those old fields that had previously been planted in orchard grass (Dactylis glomerata) had a much lower initial change than fields previously in soybeans, or soybeans and sorghum, but this difference dissolved after 10 yr of succession. The study fields compared favorably to other mesic successions and agreed with the general pattern of more rapid rate decrease in mesic compared to xeric successions. Our results echo the general decrease in the rate of plant succession with time reported in the literature and also show important relationships between this decrease and the land—use history of particular fields.

ISSN:0012-9658    

DOI:10.2307/1939542

出版商:Ecological Society of America    

年代:1994

数据来源: WILEY