Göran Högstedt

CAN VERTEBRATE PREDATORS REGULATE THEIR PREY

Whether vertebrate predators can regulate their prey or not has long been a controversial question. At the one extreme it has been claimed that predators have no impact on prey numbers but consume only a doomed surplus (Errington 1946), and at the other that predators strongly interact with their prey causing either stable equilibria or cycles (Tanner 1975; Keith et al. 1977). However,.there are no field studies showing a regulatory effect of predation among vertebrates. Here we report on such a study.

Adaptation Unto Death: Function of Fear Screams

A high frequency of rapid predator approach to broadcast fear screams supports the predator attraction hypothesis in explaining the function of screaming. A high incidence of screaming in prey species that are relatively large in relation to their main predators and live in densely vegetated habitats is concordant with this view. Contrary to earlier reports, I therefore conclude that fear screaming is a nonaltruistic phenomenon and that it is not maintained through kin selection.A high frequency of rapid predator approach to broadcast fear screams supports the predator attraction hypothesis in explaining the function of screaming. A high incidence of screaming in prey species that are relatively large in relation to their main predators and live in densely vegetated habitats is concordant with this view. Contrary to earlier reports, I therefore conclude that fear screaming is a nonaltruistic phenomenon and that it is not maintained through kin selection.

Grazing-induced proteinase inhibitors: a possible cause for lemming population cycles

Neutral extracts of experimentally wounded Carex bigelowii and Eriophorum angustifolium inhibit the proteolytic activity of trypsin. Increasing trypsin inhibitor activity was associated with a concomitant decrease in the concentration of soluble plant proteins (TI/SPP-ratio). At different phases of a three year lemming cycle, the TI/SPP-ratio was highest at peaking and declining densities and lowest when lemming density increased. Moreover, lemmings from a declining population showed pancreatic hypertrophy, which is a well documented pathology caused by a prolonged dietary intake of trypsin inhibitors

Should There be a Positive or Negative Correlation between Survival of Adults in a Bird Population and Their Clutch Size

In a review of clutch size in birds, Klomp (1970) showed that in many passerines the most productive clutch size is larger than the most common one. This seems to be at variance with Lacks (1954) hypothesis that, provided clutch size to a large extent is an inherited trait, the most productive clutch size will, by the force of natural selection, be the most common one. Here I will test two possible explanations of this difference between Lacks hypothesis and observations, using field data from a population of individually marked magpies (Pica pica) in south Sweden. I have earlier demonstrated this discrepancy to hold for this population (Hdgstedt 1980). Hypothesis 1. -Charnov and Krebs (1974) suggested that birds laying larger and more productive clutches suffer a higher mortality than birds laying average clutches. Hence, by suppressing breeding efforts, the latter category of birds survive better and stand, on average, a better chance of reproducing in future seasons, thereby maximizing their total reproductive output. Such an effect will be particularly profitable for birds with a high adult survival rate, as calculated for seabirds (Goodman 1974). This hypothesis implies that clutch size, or number of nestlings, and adult survival are inversely correlated; and further, that there is significant adult mortality during or shortly after the breeding period. Hypothesis 2.-I have suggested that clutch size and reproductive output in territorial birds depend on the quality of their territories (Hdgstedt 1980). In general birds lay clutches with close to optimal sizes-large clutches in high quality territories and small clutches in poor territories. Hence, I think the relative breeding effort is roughly similar in different pairs of birds, independent of clutch size. Consequently, breeding success will be roughly equal for the different clutch sizes. As high quality territories are in short supply, few birds will be able to produce the largest and most productive clutches. This hypothesis offers an explanation for the discrepancy between predictions from Lacks clutch size hypothesis and observations, and furthermore, requires no assumption of clutchsize inheritance. Instead, the ability to acquire a territory of high quality is crucial for a high reproductive output. I consider territory quality not only important for reproductive output but also for adult survival, and therefore, I predict a positive correlation between clutch size, or number of nestlings (these two variables being positively correlated with territory quality), and adult survival. Mortality will not necessarily occur during the breeding season.

Terrestrial trophic dynamics in the Canadian Arctic

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.

Wound-Induced Proteinase Inhibitors in Grey Alder (Alnus incana): A Defence Mechanism against Attacking Insects

The leaf beetle (Galerucella lineola) (Coleoptera) was exposed to leaves from previously unattached and attacked grey alder (Alnus incana) trees in order to demonstrate effects of wound-induced proteinase inhibitors on the performance of leaf beetles. Neutral leaf extracts from the attacked grey alder stand had a significantly higher trypsin inhibitor (TI)/soluble plant protein (SPP) ratio than leaf extracts from an isolated unattacked grey alder stand where no G. lineola could be found. Experimental defoliation of leaves in the unattacked control stand caused an increase in trypsin inhibitor activity (TI) with a concomitant decrease in the concentration of soluble plant proteins (SPP). The experimentally wounded trees showed similar TI/SPP-ratios as leaf extracts from the attacked grey alder stand. Tests of food preference showed that high TI/SPP-ratios of grey alder leaves had an antifeeding property and records of leaf acceptability showed that female beetles selected high quality leaves for the oviposition site of egg batches. Both fieldand laboratory experiments demonstrated that high TI/SPP-ratios in grey alder foliage can explain retarded growth, delayed pupation, reduced egg production and low survival of the leaf beetle. In addition, a three time higher proportion of males emerged from low quality leaves. The estimated fitness of females exposed to leaves from the attacked grey alder stand was almost 7 times lower than that of beetles fed high quality leaves. Thus, wound-induced proteinase inhibitors have pronounced negative effects on the performance of leaf beetles. We suggest that wound-induced proteinase inhibitors are a possible cause for the fluctuating dynamics of herbivore populations.

PERIOD LENGTH IN CYCLIC ANIMAL POPULATIONS

Although cyclic animal dynamics have long been a focus of scientific interest, the variable cycle lengths are poorly understood. Based on a review of the literature, we suggest that period length in animals showing multiannual cycles is related to the life span of their food plant rather than to any trait of the animal, such as mass or fecundity. We envisage that this pattern is brought about by a prolonged mobilization of induced defenses in longer lived plants, which can better afford periods of low reproductive output than can short-lived plants. On the basis of this hypothesis we expect animals with similar diets to show similar cycle lengths, irrespective of taxonomy and size. A path analysis, on the vertebrate herbivores, shows that 92% of the variation in cycle length is explained by food-plant longevity and that body mass adds little to this.

Conditions favouring hard seededness as a dispersal and predator escape strategy

Summery The water-impermeable seed coat of ‘hard’ seeds is commonly considered a dormancy trait. Seed smell is, however, strongly correlated with seed water content, and hard seeds are therefore olfactionally cryptic to foraging rodents. This is the rationale for the crypsis hypothesis, which proposes that the primary functions of hard seeds are to reduce seed predation and promote rodent seed dispersal. We use a mechanistic model to describe seed survival success of plants with different dimorphic soft and hard seed strategies. The model is based on established empirical–ecological relationships of moisture requirements for germination and benefits of seed dispersal, and on experimentally demonstrated relationships between seed volatile emission, predation and predator escape. We find that water-impermeable seed coats can reduce seed predation under a wide range of natural humidity conditions. Plants with rodent dispersed seeds benefit from producing dimorphic soft and hard seeds at ratios where the anti-predator advantages of hard seeds are balanced by the dispersal benefits gained by producing some soft seeds. The seed pathway predicted from the model is similar to those of experimental seed-tracking studies. This validates the relevance and realism of the ecological mechanisms and relationships incorporated in the model. Synthesis. Rodent seed predators are often also important seed dispersers and have the potential to exert strong selective pressures on seeds to evolve methods of avoiding detection, and hard seeds seem to do just that. This work suggests that water-impermeable hard seeds may evolve in the absence of a dormancy function and that optimal seed survival in many environments with rodent seed predators is obtained by plants having a dimorphic soft and hard seed strategy.

Regulation of Reproductive Success Towards e -1 ( = 37%) in Animals with Parental Care

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Avian gut passage reduces seed exit costs in Sorbus aucuparia (Rosaceae) as measured by a diametral compression test

Reduced seed exit costs have been suggested to explain advanced seedling emergence and increased seedling growth in Sorbus aucuparia L. (Rosaceae; rowan) following Turdus spp. (Turdidae; thrush) gut passage. In the present study, seed coat tension strength of Turdus merula L. (Turdidae; blackbird) gut-passed and non-ingested control seeds were tested with a diametral compression test. Both maximum load (N) and work (mJ) required for opening the seeds were reduced in gut-passed seeds, although scanning electron microscopy showed no visible differences in seed coat surface structure between treatments. In addition, gut passage increased dry seed weight by 0.64 mg (22%), weight of hydrated seeds by 0.62 mg (16%) and width of hydrated seeds by 0.1 mm (9%). Absorption causing the seed coat to yield more easily to the germinating seedling is proposed as an explanation for the reduction in seed exit costs. For the emerging seedling, the time and force needed to open the seed coat was reduced, thus lowering dispersal and germination costs of S. aucuparia.