Mechanical-statistical modeling in ecology: from outbreak detections to pest dynamics.

Knowledge about large-scale and long-term dynamics of (natural) populations is required to assess the efficiency of control strategies, the potential for long-term persistence, and the adaptability to global changes such as habitat fragmentation and global warming. For most natural populations, such as pest populations, large-scale and long-term surveys cannot be carried out at a high resolution. For instance, for population dynamics characterized by irregular abundance explosions, i.e., outbreaks, it is common to report detected outbreaks rather than measuring the population density at every location and time event. Here, we propose a mechanical-statistical model for analyzing such outbreak occurrence data and making inference about population dynamics. This spatio-temporal model contains the main mechanisms of the dynamics and describes the observation process. This construction enables us to account for the discrepancy between the phenomenon scale and the sampling scale. We propose the Bayesian method to estimate model parameters, pest densities and hidden factors, i.e., variables involved in the dynamics but not observed. The model was specified and used to learn about the dynamics of the European pine sawfly (Neodiprion sertifer Geoffr., an insect causing major defoliation of pines in northern Europe) based on Finnish sawfly data covering the years 1961-1990. In this application, a dynamical Beverton-Holt model including a hidden regime variable was incorporated into the model to deal with large variations in the population densities. Our results gave support to the idea that pine sawfly dynamics should be studied as metapopulations with alternative equilibria. The results confirmed the importance of extreme minimum winter temperatures for the occurrence of European pine sawfly outbreaks. The strong positive connection between the ratio of lake area over total area and outbreaks was quantified for the first time.

Environmental changes in the North Atlantic Region: SCANNET as a collaborative approach for documenting, understanding and predicting changes.

The lands surrounding the North Atlantic Region (the SCANNET Region) cover a wide range of climate regimes, physical environments and availability of natural resources. Except in the extreme North, they have supported human populations and various cultures since at least the end of the last ice age. However, the region is also important at a wider geographical scale in that it influences the global climate and supports animals that migrate between the Arctic and all the other continents of the world. Climate, environment and land use in the region are changing rapidly and projections suggest that global warming will be amplified there while increasing land use might dramatically reduce the remaining wilderness areas. Because much of the region is sparsely populated--if populated at all--observational records of past environmental changes and their impacts are both few and of short duration. However, it is becoming very important to record the changes that are now in progress, to understand the drivers of these changes, and to predict future consequences of the changes. To facilitate research into understanding impacts of global change on the lands of the North Atlantic Regions, and also to monitor changes in real time, an EU-funded network of research sites and infrastructures was formed in 2000: this was called SCANNET--SCANdinavian/North European NETwork of Terrestrial Field Bases. SCANNET currently consists of 9 core sites and 5 sites within local networks that together cover the broad range of current climate and predicted change in the region. Climate observations are well replicated across the network, whereas each site has tended to select particular environmental and ecological subjects for intensive observation. This provides diversity of both subject coverage and expertise. In this paper, we summarize the findings of SCANNET to-date and outline its information bases in order to increase awareness of data on environmental change in the North Atlantic Region. We also identify important gaps in our understanding and identify where the roles of existing infrastructures and activities represented by SCANNET can facilitate future research, monitoring and ground-truthing activities.

Structure of a microbial community in soil after prolonged addition of low levels of simulated acid rain

Humus samples were collected 12 growing seasons after the start of a simulated acid rain experiment situated in the subarctic environment. The acid rain was simulated with H2SO4, a combination of H2SO4 and HNO3, and HNO3 at two levels of moderate acidic loads close to the natural anthropogenic pollution levels of southern Scandinavia. The higher levels of acid applications resulted in acidification, as defined by humus chemistry. The concentrations of base cations decreased, while the concentrations of exchangeable H+, Al, and Fe increased. Humus pH decreased from 3.83 to 3.65. Basal respiration decreased with decreasing humus pH, and total microbial biomass, measured by substrate-induced respiration and total amount of phospholipid fatty acids (PLFA), decreased slightly. An altered PLFA pattern indicated a change in the microbial community structure at the higher levels of acid applications. In general, branched fatty acids, typical of gram-positive bacteria, increased in the acid plots. PLFA analysis performed on the bacterial community growing on agar plates also showed that the relative amount of PLFA specific for gram-positive bacteria increased due to the acidification. The changed bacterial community was adapted to the more acidic environment in the acid-treated plots, even though bacterial growth rates, estimated by thymidine and leucine incorporation, decreased with pH. Fungal activity (measured as acetate incorporation into ergosterol) was not affected. This result indicates that bacteria were more affected than fungi by the acidification. The capacity of the bacterial community to utilize 95 different carbon sources was variable and only showed weak correlations to pH. Differences in the toxicities of H2SO4 and HNO3 for the microbial community were not found.

Responses of ground vegetation to prolonged simulated acid rain in sub-arctic pine-birch forest.

The effects of prolonged simulated acid rain on percentage cover of ground vegetation, and on growth and reproduction of two dominating dwarf shrubs (Emapetrum nigrum and Vaccinium vaitisidaea) were examined in a field experiment in the Finnish Subarctic, in an area with low ambient levels of sulphur and nitrogen deposition. Acid rain treatments included moderate (pH 3.8) and high (pH 2.9) concentrations of either H2 SO4 , or HNO3 , or a mixture of them, and were compared with irrigated (pH 6) and dry control plots. Long-term application of acid rain caused significant alteration in the cover and composition of ground vegetation. Effects of acid ram depended on the accompanying anion and on pH. Sub-plots under different canopy tree species differed in responses indicating that spatial heterogeneity is important in predicting the effect of acidifying pollution on this plant community. In the bottom layer, acid rain caused significant reduction in cover of the cyanobacterial lichens Nephroma arcticum and Peltigera spp. Decrease in cover of fruticose lichens, mainly composed of Cladina spp., more likely resulted from additional watering. In the field layer, acid rain containing moderate concentrations of NO- 3 caused an increase in cover of graminoid species. There were only slight alterations in growth and cover of the two dominant evergreen dwarf shrubs, Enigrum and V. vitis-idaea, indicating that these species are tolerant to acid rain of as low as pH 3. Even some positive responses of dwarf shrubs were observed, depending on canopy tree. Application of acid rain of pH 3 to plots under pine trees caused an increase in cover of I, vitis-idaea and, when the nitric acid only was applied, a short-term increase in the number of new shoots of E. nigrum. In contrast to vegetative growth, reproduction of the dwarf shrubs was more strongly affected by acid rain, but this also depended on local conditions and anion composition of acid rain. On 'pine' plots, rain of pH 3 reduced the number of berries and flower buds on terminal current shoot of E. nigrum, however, this was partially compensated by an increase in berry production at the ramet level. Simulated acid rain had mainly negative effects on berry production by V. vitis-idaea.

The effect of simulated acid rain on the biochemical composition of Scots pine (Pinus sylvestris L.) needles.

The effects of prolonged simulated acid rain on the biochemistry of Scots pine needles were studied in Finnish Lapland. Pine trees were exposed by spraying the foliage and soil with either clean water or simulated acid rain (SAR; both sulphuric and nitric acids) over the period 1985-1991. The concentrations of carbohydrates (starch, glucose, fructose, sucrose) in one-year-old pine needles were not affected by SAR-treatments. The SAR-treatments did not have significant effects on protein bound amino acids, which was true also for most of the free amino acids. However, the citrulline concentration was over three-fold greater in the foliage of pines exposed to SAR of pH 3 compared to irrigated controls. The concentrations of total phenolics, individual low molecular weight phenolics and soluble proanthocyanidins were not affected by the treatments, but insoluble proanthocyanidins had increased in acid-treated trees. Some of the studied biochemical compounds showed significant differences between two sub-areas (similar treatments) only 120 m apart.

Effects of larval age and prolonged simulated acid rain on the susceptibility of European pine sawfly to virus infection.

We studied the effect of simulated acid rain treatment of host trees on the susceptibility of the European pine sawfly larvae to virus, and possible differences when larvae of two different ages were infected. Older larvae were less susceptible to virus. Most larvae treated with virus 2 days after they started feeding on experimental foliage (group A) died rapidly within 10 days after the virus treatment, and survival to the end of the larval period was only 8-25%. Larvae treated with virus 1 week later (group B) were less affected by the virus and 36-49% survived. In group A the larval survival in the pH 3 treatment was higher than in other treatments; at the end of the larval period the difference was twofold. In group B there were no clear effects of acid rain on the susceptibility of larvae to virus. The study yielded the following new information: (1) the effect of prolonged acid rain treatment on reducing the efficacy of virus on young larvae was more distinct than in a previous study with shorter exposure to acid rain, and the difference was maintained to the end of larval period; (2) the susceptibility of older larvae to virus was not affected by acid rain treatments; (3) pH inside the needles did not explain the larval mortality caused by virus.

Simulated acid rain affects birch leaf endophyte populations.

Endophytes were frequently isolated from mountain birch (Betula pubescens var. tortuosa (Ledeb.) Nyman) leaves at a subarctic site where natural air pollution is low. We tested whether simulated acid rain had any influence on the occurrence of endophytes. Dry controls with only ambient rain and irrigated controls treated with spring water of pH 6 were compared with acid treatments at pH 3 and pH 4, prepared by adding both sulphuric and nitric acids. Treatments began in 1985 and leaf samples were taken twice during the summer of 1992. Leaves were surface sterilized, five leaf disks from each leaf placed on malt extract agar, and growing colonies were counted and identified. The most frequently isolated endophyte from birch leaves was a Fusicladium anamorph of Venturia sp. (88% of all the isolates in July and 75% of all the isolates in August), followed by a sterile mycelium and Melanconium sp. The number of endophytes isolated and the species number increased from July to August. Endophytes were most frequently isolated from the basal part of the midrib. The percentage of colonization by endophytes was similar in short and long shoots. More endophytes were isolated from leaves of branches taken at 1 m height than at 2 m height. The stronger acid rain treatment (pH 3) reduced by approximately 25% the number of isolated endophytes in August. Treatments did not have any effect on species composition of endophyte assemblages in birch leaves.

The effects of acidic irrigation on soil microorganisms at Kevo, Northern Finland.

Effects of simulated acid rain (a mixture of H(2)SO(4) and HNO(3); pH(3)) on soil microbiology were studied in a field experiment in northern Finland. Irrigated control plots received the same amount of spring water (pH 6) as the acid treated plots. Fungal lengths and total bacterial numbers were studied after the treatments had continued for three growing seasons. The numbers of bacteria in five physiological groups (those utilizing starch, protein, pectin, xylan, or cellulose) were measured by MPN (most probable number) techniques. The lengths of total and FDA active fungal hyphae were not significantly different between the acid treated and the control plots. The counts of total bacteria were not significantly different between treatments, but the MPNs of all five physiological groups of bacteria were approximately 60% lower in the acid treated plots than in the controls.

Simulated acid rain reduces the susceptibility of the European pine sawfly (Neodiprion sertifer) to its nuclear polyhedrosis virus.

The study dealt with the effect of simulated acid rain (both H(2)SO(4) and HNO(3); acidities of pH 4 and pH 3) on the susceptibility of the larvae of Neodiprion sertifer to its nuclear polyhedrosis virus. Scots pines growing in a subarctic area with low ambient pollution levels were irrigated with simulated acid rain during two summers. Neodiprion larvae fed with foliage from the experimental trees were infected with a dilute virus suspension. The acid treatment of host trees had a significant effect on the proportion of virus-treated larvae alive 16 days after the virus application: there were almost no differences between the controls and the pH 4 irrigation group, but on the needles of pH 3-treated trees larval survival was twice as high as with other treatments. The direct spraying of acid water on the needles before they were fed to the larvae did not significantly affect the survival of virus infected larvae. Our results suggest that acid rain may reduce the susceptibility of Neodiprion larvae to virus disease via changes in the quality of pine foliage.

The effect of simulated acid rain on performance of the aphid Euceraphis betulae (Koch) on silver birch.

The effect of artificial acid rain on the reproduction and survival of the aphid Euceraphis betulae on silver birch was studied in Turku, southern Finland. Eight bioassays were done during 1984-1986. In four of the bioassays the aphids produced 40% to over 100% more progeny on birches watered with dilute sulphuric acid (pH 3.5) than on control trees. In four other cases the performance of aphids did not differ between the treatments. An index of aphid reproduction pooled over the whole study was significantly higher on acid-treated than on control birches. The reproduction of aphids on acid-treated birches was enhanced when precipitation was below long term average, suggesting an interaction between the stress caused by acid treatment and dry periods.

Delayed inducible resistance against a leaf-chewing insect in four deciduous tree species.

Both mechanical damage to mountain birch foliage and rearing of moth larvae on the trees reduced the growth of Epirrita autumnata larvae reared on these trees in the following year. The effects of physical damage and some other cues from insects were additive. On bird cherry the performance of Epirrita larvae was equal on untreated trees and on trees artificially defoliated in the previous year, but larval growth was reduced on previously insect-damaged branches. With mountain ash just physical damage per se reduced the performance of Epirrita larvae. On Salix phylicifolia there were no significant differences in the growth or survival of Epirrita on untreated control bushes and on bushes with partial larval damage during the previous year. Among untreated control trees the growth and survivorship of Epirrita were higher on fast-growing willow and bird cherry than on the slow-growing mountain birch. Mountain birch and mountain ash, the two deciduous tree species adapted to nutrient-poor soils, showed delayed inducible resistance triggered by defoliation (artificial or insect-made). This supports the hypothesis that delayed inducible resistance may be a passive response due to nutrient-stress caused by defoliation. On the other hand, the additional increase in the resistance of mountain birch triggered by specific cues from insects suggests that this response may be an evolved defense against leaf-eating insects.

Low nutritive quality as defence against herbivores: induced responses in birch.

The effects of artificial defoliation of birch trees in the previous year on the consumption and utilization of food by a geometrid larva, Epirrita autumnata, were studied in laboratory. The leaves were collected from two sites on a slope of a fell. Defoliation had a significant retarding effect on approximate digestibility, efficiency of conversion of ingested food, relative consumption rate and relative growth rate but not on efficiency of conversion of digested food. The effects were to the same direction with leaves from the two sites, but the response was stronger with leaves from the zone defoliated by Epirrita during mid-sixties. The defensive nature of the response(s) of birch to defoliation is discussed in the light of these results. It is concluded that contrary to the hypothesis of Moran and Hamilton (1980) no increase in the consumption incurred by individual trees could be shown in this system.