Abstract:The Swedish Tundra Northwest Expedition of 1999 visited 17 sites throughout the Canadian Arctic. At 12 sites that were intensively sampled we estimated the standing crop of plants and the densities of herbivores and predators with an array of trapping, visual surveys, and faecal-pellet transects. We developed a trophic-balance model using ECOPATH to integrate these observations and determine the fate of primary and secondary production in these tundra ecosystems, which spanned an 8-fold range of standing crop of plants. We estimated that about 13% of net primary production was consumed by herbivores, while over 70% of small-herbivore production was estimated to flow to predators. Only 9% of large-herbivore production was consumed by predators. Organization of Canadian Arctic ecosystems appears to be more top-down than bottom-up. Net primary production does not seem to be herbivore-limited at any site. This is the first attempt to integrate trophic dynamics over the entire Canadian Arctic.Résumé : En 1999, l'Expédition suédoise de la toundra du nord-ouest à échantillonné 17 sites à travers l'arctique canadien. Nous avons utilisé différentes méthodes de trappage, des recensements visuels et des transects de décomptes de fèces pour estimer la biomasse végétale, ainsi que les densités des herbivores et des prédateurs aux 12 sites inventoriés plus en détail. Nous avons développé un modèle d'équilibre trophique à l'aide d'ECOPATH pour intégrer ces observations et déterminer le sort des productions primaire et secondaire de ces écosystèmes de toundra, entre lequels la biomasse végétale varie par un facteur de 8. Nous estimons qu'environ 13 % de la production primaire nette est consommée par les herbivores, tandis que plus de 70 % de la production des petits mammifères est consommée par les prédateurs. Seulement 9 % de la production des grands herbivores est consommée par les prédateurs. Ces écosystèmes semblent façonnés plus par les effets trophiques descendants (top-down) que par les effects trophiques ascendants (bottom-up). La production primaire nette de ces sites ne semble pas limitée par les herbivores. Notre étude constitue la première tentative pour intégrer les la dynamique trophique sur l'ensemble de l'arctique canadien. Krebs et al. 843
Current theory on plant–animal interactions predicts that the outcome of herbivory on plant performance will be dependent on plant productivity. Thus, slow-growing plants should be less able to compensate for biomass losses than fast-growing plants, and therefore be more susceptible to herbivory if attacked. We simulated winter browsing by moose (Alcesalces (L.)) on Scots pine (Pinussylvestris L.) along a gradient of plant productivity and addressed the following questions: (1) Does herbivory affect growth independently of plant productivity? (2) Is herbivory a more important mortality factor for slow-growing than for fast-growing plants? (3) Is there any effect of herbivory on fecundity, and is it related to plant productivity? Two clipping regimes simulated different intensities of moose winter browsing. Mortality was followed annually, and after 4 years we measured tree growth and fecundity on control as well as on treatment pines. The effect of clipping on growth was related with both clipping intensity and plant productivity. In the light-clipping treatment mortality was restricted to the slow-growing pines, in contrast with the severe treatment, where it occurred across the whole range of plant growth. Moreover, in the light-clipping treatment most mortality occurred within 1 year after treatment, whereas tree death occurred over 2 or more years in the severe treatment. We found no effect of age on mortality within growth-rate classes. The proportion of trees with cones increased with growth rate for control trees but not for treated trees, indicating that herbivory more strongly affects fecundity on fast-growing than on slow-growing trees. Our results confirm the hypothesis that herbivory affects plant performance differently across a gradient of plant productivity. We suggest that mammalian herbivores can increase mortality of plant genets after the seedling stage primarily in stands on low-productivity sites, especially in combination with a high density of the herbivore.
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key uvrds : winter ecology, lake, Sweden, benthic invertebrates, Chironomidae Ahstmct During two winters, the benthic invertebrates in a shallow northern Swedish lake (66 ON) were studied. The lake is ice-covered for about 200 days a year. Live Lamellibranchiata, Oligochaeta. Hydracarina, Isopoda, Heteroptera, Cole optera and Diptera were found in samples of ice and of frozen lake-bottom sediments. During the winter in which the bottom froze deepest, a 70-90 I!,, decrease in numbers of live Chironomidae larvae was recorded between the autumn and following early spring.The Veittijarvi Lake (66'3'h'; 23"46'E) is situated in the northeastern corner of Sweden, close to the Finnish border. The area belongs to the 'middle boreal' vegetation zone (AHTI rt (11. 1968), primarily coniferous forests of spruce [Pz'crn d i e s (I,.) K~R s T . ] and Scots pine (Pinus sylwestris L.). The lake lies in flat country. where wide expanses of inires are interrupted by patches of sparse coniferous forest. The lake was drained in about 1930 and deliberately reflooded in 1971.The mean annual temperature (1931-1960) is + 1 -+ 2 "C and the mean air ternpcrature for the coldest months (January or February) lies around -11 "C (TAESLER, 1972). In general (period 1911-1961), ice covers the smaller lakes in the area from early Koreniher to mid-May, or for about 205 days (MOBERG, 1967) . At its seasonal low water-level. in summer the lake covers about 35 hectares and the illcan water depth is ca 0.8 in. 838 K. DAXELLMost of the water surface is open-water, largely withont cnicrgmt hydrophytes. 1) I i t contniii ing s 11 13 iiierged hydrophyt es, mainly Potclrnogato~i p s i l l u .~ 1 , . S o n ie stands of eiiiergent hydrophytes (Carex spp. and Equisetun/, fluviutile L.) do occur, hut thsir total coverage is less than one hectare. A more detailed description of the biotic fcatrircs of the lake can be found in DANRLL & S.Jiiuk:R(; (1977, 1980). MethodsThe occurrence of invertebrates in sa.mples of ice and both frozen and unfrozen lake-bott,om sediment from t.he VeittijLrvi Lake were investigated during the winters 1975/1976 a.nd 3 97W77.Samples were taken from t,wo different habitats: (a) open-wat.er areas wit.hout cmcrgent. hvdrophytes but &h submerged Potamogeton pusillus (Habitet 0 ) , and (b) areas with dcnse stnnds of emergcnt hydrophyt.es (Carex spp. and Equisetum fluviafile) (Habitat V). I n Xovember 1975 and 1976 holes were drilled in the lake ice (drill diiimcter 0.115 m) iit. sampling sit.es chosen t i t random wit.hin b0t.h types of habitat (T,.tble 1). J u s t t~ few centirnetres above? tho lower surface of the ice, drilling was stopped and the remaining part of the hole excavated with i m ice-pick. All fragmerids of ice and frozen surfa,ce sediment werc collected, whereaftcr a RzGska core sampler ( R , Z~Y K A 1931) of slightly smaller diameter (0.112 m) was inserted through t,he hole and a. plug of t.he underlying unfrozen lake-bottom scdimcnt obtained. The unfroacm and frozen stbmples from each station were pooled. All sample...
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