Walde, Sandra J.
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Item Open Access Nonbreeding-Season Drivers of Population Dynamics in Seasonal Migrants: Conservation Parallels Across Taxa(2009-12) Calvert, Anna M.; Walde, Sandra Joan; Taylor, Philip D.For seasonal migrants, logistical constraints have often limited conservation efforts to improving survival and reproduction during the breeding season only. Yet, mounting empirical evidence suggests that events occurring throughout the migratory life cycle can critically alter the demography of many migrant species. Herein, we build upon recent syntheses of avian migration research to review the role of non-breeding seasons in determining the population dynamics and fitness of diverse migratory taxa, including salmonid fishes, marine mammals, ungulates, sea turtles, butterflies, and numerous bird groups. We discuss several similarities across these varied migrants: (i) non-breeding survivorship tends to be a strong driver of population growth; (ii) non-breeding events can affect fitness in subsequent seasons through seasonal interactions at individual-and population-levels; (iii) broad-scale climatic influences often alter non-breeding resources and migration timing, and may amplify population impacts through covariation among seasonal vital rates; and (iv) changes to both stationary and migratory non-breeding habitats can have important consequences for abundance and population trends. Finally, we draw on these patterns to recommend that future conservation research for seasonal migrants will benefit from: (1) more explicit recognition of the important parallels among taxonomically diverse migratory animals; (2) an expanded research perspective focused on quantification of all seasonal vital rates and their interactions; and (3) the development of detailed population projection models that account for complexity and uncertainty in migrant population dynamics.Item Open Access Prey Exchange Rates and the Impact of Predators on Prey Populations in Streams(1990) Cooper, S. D.; Walde, Sandra Joan; Peckarsky, B. L.We present four lines of evidence that the magnitude of prey exchange (=immigration/emigration) among substrate patches has an overwhelming influence on the perceived effects of predators on prey populations. (1) An extensive review of the literature on predation effects in benthic and littoral freshwater habitats revealed a significant relationship between prey exchange rate and observed predator impact. In streams, studies showing significant predator effects used cages with smaller mesh sizes than studies showing nonsignificant effects. Similarly, there was a highly significant correlation between cage mesh size and the magnitude of predator impact on common prey. Large-scale stream studies indicated that prey drift and colonization rate were inversely related to predator impact on benthic prey. (2) These patterns were confirmed by field experiments and observations where mesh size was directly manipulated or where exchange rates varied among taxa. In Colorado streams we saw greater predator impacts on Baetis prey when immigration/emigration was restricted vs. when the mesh size of the cage was relatively large. Similarly, the effects of trout in California stream pools were greater when prey turnover rates were low. (3) A re-analysis of Peckarsky's (1985) data shows an inverse relationship between predator impact and prey mobility within a field experiment. (4) Finally, a model than incorporates both predation and exchange of prey indicates that we ought to expect a lower magnitude of predator effects when exchange rates are high. These results suggest that some discrepancies in past studies may be explained by differences in the exchange rates of prey, and that differences in predator effects across different systems or habitats may be related to variation in the rates of prey dispersal and colonization.Item Open Access Surviving historical Patagonian landscapes and climate: molecular insights from Galaxias maculatus(2010-03) Zemlak, Tyler S.; Habit, Evelyn M.; Walde, Sandra Joan; Carrea, Cecilia; Ruzzante, Daniel E.Background: The dynamic geological and climatic histories of temperate South America have played important roles in shaping the contemporary distributions and genetic diversity of endemic freshwater species. We use mitochondria and nuclear sequence variation to investigate the consequences of mountain barriers and Quaternary glacial cycles for patterns of genetic diversity in the diadromous fish Galaxias maculatus in Patagonia (similar to 300 individuals from 36 locations). Results: Contemporary populations of G. maculatus, east and west of the Andes in Patagonia, represent a single monophyletic lineage comprising several well supported groups. Mantel tests using control region data revealed a strong positive relationship when geographic distance was modeled according to a scenario of marine dispersal. (r = 0.69, P = 0.055). By contrast, direct distance between regions was poorly correlated with genetic distance (r = -0.05, P = 0.463). Hierarchical AMOVAs using mtDNA revealed that pooling samples according to historical (pre-LGM) oceanic drainage (Pacific vs. Atlantic) explained approximately four times more variance than pooling them into present-day drainage (15.6% vs. 3.7%). Further post-hoc AMOVA tests revealed additional genetic structure between populations east and west of the Chilean Coastal Cordillera (coastal vs. interior). Overall female effective population size appears to have remained relatively constant until roughly 0.5 Ma when population size rapidly increased several orders of magnitude [100x (60x-190x)] to reach contemporary levels. Maximum likelihood analysis of nuclear alleles revealed a poorly supported gene tree which was paraphyletic with respect to mitochondrial-defined haplogroups. Conclusions: First diversifying in the central/north-west region of Patagonia, G. maculatus extended its range into Argentina via the southern coastal regions that join the Pacific and Atlantic oceans. More recent gene flow between northern populations involved the most ancient and most derived lineages, and was likely facilitated by drainage reversal(s) during one or more cooling events of the late Pleistocene. Overall female effective population size represents the end result of a widespread and several hundred-fold increase over approximately 0.5 Ma, spanning several climatic fluctuations of the Pleistocene. The minor influence of glacial cycles on the genetic structure and diversity of G. maculatus likely reflects the access to marine refugia during repeated bouts of global cooling. Evidence of genetic structure that was detected on a finer scale between lakes/rivers is most likely the result of both biological attributes (i.e., resident non-migratory behavior and/or landlocking and natal homing in diadromous populations), and the Coastal Cordillera as a dispersal barrier.Item Open Access A Refuge for Red Scale the Role of Size-Selectivity by a Parasitoid Wasp(1989) Walde, Sandra Joan; Luck, R. F.; Yu, D. S.; Murdoch, W. W.The presence of a physical refuge (an area where mortality due to parasitism is very low) has been demonstrated for California red scale, a pest of citrus. Density of red scale is very high, and parasitism by the principal parasitoid wasp, Aphytis melinus DeBach. is low on the trunk and woody branches of citrus trees as compared with the exterior twigs and leaves. One possible explanation for the low rate of parasitism is that scale in the refuge are less frequently parasitized due to their smaller size. Laboratory experiments show that Aphytis selects larger hosts, and the size distribution of parasitized scale in samples from a citrus grove shows a bias toward larger host individuals in the field. However, size-selectivity can account for only .apprxeq. 10% of the observed difference in parasitism rate and thus cannot be the primary explanation for the presence of the refuge.Item Open Access Invertebrate predation and lotic prey communities: Evaluation of in situ enclosure/exclosure experiments(1984) Walde, Sandra Joan; Davies, R. W.The influence of the stonefly Kogotus nonus on prey communities was assessed by varying the predator density within small enclosures in a first-order stream in southwestern Alberta. Experimental containers with natural densities of prey, standardized substrate and detritus, and zero, one, two, or three Kogotus were buried in a rifflefor 10 days-during July 1981, June 1982, and July 1982. The extent to which prey densities were depressed was found to be dependent on predator density within the enclosures: increased predator densities resulted in lower prey densities. An experimental design in which containers that excluded predators were compared with controls accessible to predators was used to determine if results would be similar to results from the enclosure experiments. It was concluded that observation of predator effects in field manipulative experiments may be strongly dependent on the selection of an experimental design in which predator effects are not confounded with container effects.Item Open Access Patch Dynamics of a Phytophagous Mite Population Effect of Number of Subpopulations(1991) Walde, Sandra JoanI conducted a field test of the hypothesis that the dynamics of a predator-prey interaction should be affected by the number of interacting subpopulations within an assemblage. I used a "successful" biological control system, the phytophagous European red mite (Panonychus ulmi) and its predator, the phytoseiid mite Typhlodromus pyri. An orchard was established where apple trees were arranged in groups of 1, 4, or 16 and the dynamics of the mite populations on the trees followed over a season. The pests reached highest densities and were most persistent on trees in the largest groups. These results are explained in terms of an interaction between prey immigration/emigration and predation.Item Open Access How quality of host plant affects a predator-prey interaction in biological control(1995) Walde, Sandra JoanIn cases of successful biological control the pest population is maintained at low densities by a natural enemy, but the mechanisms by which the predator or parasitoid manages to control the pest are often unknown. This study was undertaken to determine the response of a predator to different population growth rates of its prey within a biological control system. Population growth rate of the phytophagous mite Panonychus ulmi was varied by altering the nutritional status of the host plant (apple trees (Pyrus malus)). NPK fertilizer was added to the soil, resulting in higher nitrogen, lower phosphorus, and lower potassium levels in the foliage of the trees. There appeared to be a nonlinear relationship between nitrogen and P. ulmi densities, with density increasing and then decreasing as nitrogen concentration was increased. Two predaceous mites, Typhlodromus pyri and Zetzellia mali, were implicated in the control of P. ulmi, but the effectiveness of both was reduced at high nitrogen levels. The predators therefore did not fully compensate for increases in the population growth rate of their prey, although P. ulmi densities were maintained well below the economic threshold in all cases. However, the probability of persistence of the prey populations was not significantly affected by the nutrient regimes, suggesting that the fundamental dynamics of the predator-prey interaction were not altered by the range of population growth rates in this study.Item Open Access Intraguild interactions and large-scale population patterns(2009-09) Sircom, Julie; Walde, Sandra JoanCompetitive interactions, including interference and intraguild predation, occur in many aquatic food webs and influence large-scale patterns of abundance, especially in lentic habitats. In a study of small coastal streams in eastern Canada, we found patterns suggesting that abundances of a predatory invertebrate, Sweltsa onkos (Plecoptera:Chloroperlidae), are influenced by another predator, Rhyacophila vibox (Trichoptera:Rhyacophilidae). Larval densities of the 2 species were inversely correlated, and mortality of larval S. onkos tended to be higher in streams with many R. vibox. Abundance and mortality of a 3(rd) predatory species, Isoperla montana (Plecoptera:Perlodidae), were unrelated to abundances of the other species. Laboratory behavioral observations confirmed that interactions between R. vibox and S. onkos were asymmetrical; high densities of R. vibox led to higher injury and mortality rates and lower growth rates of S. onkos, whereas S. onkos had no effect on R. vibox growth or mortality. Previous work has shown that R. vibox reaches high densities only in streams with few or no fish, and we conclude that the relative abundances of S. onkos and R. vibox in the study streams might be influenced by differences in Vulnerability to intraguild and fish predation.Item Open Access Estimation of Temporal Variability in Populations(1995-10) STEWARTOATEN, A.; MURDOCH, WW; Walde, Sandra JoanA common measure of the temporal variability of a population is the standard deviation of the logarithms of successive estimated population sizes, In(D-t). This measure overestimates true temporal variability (the standard deviation of the logarithms of true population density, In[Delta(t)]) because it is contaminated by spatial variance (variability among samples taken on the same date). The random error in D-t causes an overestimation of temporal variance, both directly and also indirectly, by causing ln(D-t) to underestimate ln(Delta(t)). Both problems are more severe if spatial variance is large or the sample size, on a date, is small. We develop an alternative estimator, which uses an estimate of spatial variance to correct for both problems. To evaluate it, we sampled from simulated populations with a wide range of clumping. The results show that the standard estimate can be badly biased. The new estimator is much better and is quite accurate over a broad range of conditions. Our results suggest a reanalysis of some ecological studies that have estimated temporal variability to attack theoretically important questions. In particular, the apparently greater average temporal variability of terrestrial arthropods compared with terrestrial vertebrates could be an artifact caused by the fact that, typically, clumping is weaker and density estimates are more accurate in vertebrates.Item Open Access A Refuge for Red Scale Under Control by Aphytis - Structural Aspects(1989-12) MURDOCH, WW; LUCK, RF; Walde, Sandra Joan; REEVE, JD; YU, DSNo abstract available.Item Open Access Refuge dynamics and metapopulation dynamics: An experimental test(1996-03) Murdoch, WW; Swarbrick, SL; Luck, RF; Walde, Sandra Joan; Yu, DSRed scale, an insect pest of citrus, is under control by the parasitoid Aphytis melinus in many areas, and in our study area the interaction appears dynamically stable. The bark on the interior branches and trunk of trees provides a partial refuge for red scale, which are rarely attacked there by the parasitoid. In a grapefruit grove, we carried out a two-way experiment in which we manipulated the refuge population (present or removed) and either left trees connected with the rest of the grove or isolated individual trees with cages to test for metapopulation effects. The experiment ran for 17 mo, encompassing three generations of scale. Scale density in the exterior of refuge-removed trees decreased by about 60%. However, neither removal of the refuge population nor isolation of individual trees increased the temporal variability of the scale population in the exterior or led to drift in population density. Indeed, removal of the refuge population caused a decrease in temporal variability. We conclude that stability in the control population was not maintained by either refuge or metapopulation dynamics, Reduced scale recruitment and density in the exterior of trees lacking a refuge population were associated with increased (i.e., density-dependent) scale survival that did not reflect a change in parasitism.Item Open Access Regulation of an Insect Population Under Biological-Control(1995-01) MURDOCH, WW; LUCK, RF; SWARBRICK, SL; Walde, Sandra Joan; YU, DS; REEVE, JDCalifornia red scale is suppressed to very low densities by the parasitoid Aphytis melinus. The system also appears stable. We report on an experimental test of the hypothesis that stability is caused by a refuge for scale. In a grapefruit grove in southern California in 1984-1985, the bark in the interior part of the tree provided a partial refuge from parasitism. Scale were approximate to 100 times denser there than in the exterior of trees. In a field experiment, we removed Argentine ants from some blocks of trees to test whether (1) ants caused the refuge by interfering with Aphytis and (2) the expected reduction in scale density in the refuge would lead to an unstable interaction in the exterior. We also tested for density-dependent parasitism, host mutilation, and predation by analyzing data from samples and from scale placed in the field. The temporal variability of the scale was at the low end of the range recorded in field populations. The experiment provided some evidence in support of the refuge hypothesis. The population in the refuge fluctuated much less than that in the exterior. Ant exclusion led to increased parasitism and lower scale density in the interior, and to increased fluctuations in abundance in the refuge and exterior. However, these changes were relatively small and perhaps temporary, suggesting that (1) ants are not the main cause of the refuge and that (2) we did not reduce the refuge density enough to determine whether the system would go unstable in the absence of the refuge population. Parasitism, host mutilation, and predation rates on scale showed no temporal density dependence, either direct or delayed, though detection of such patterns is difficult. Possible alternative stabilizing mechanisms include size-dependent interactions between red scale and Aphytis.