Mari Walls

Consequences of herbivory in the mountain birch (Betula pubescens ssp tortuosa): importance of the functional organization of the tree

SummaryThree types of experiments indicate that the functional organization of the mountain birch may influence the ways in which the tree responds to simulated or natural herbivory. The first experiment showed that herbivory to both short and long shoot leaves affects plant development but, because growth largely proceeds by resources of the previous year, is manifested only in the year following the damage. The second experiment showed that even partial damage to a single long shoot leaf caused the axillary bud of that leaf to produce a shorter shoot the next year. Therefore, the value of a leaf depends also on the organ which it is subtending. In the third experiment we manipulated the apical dominance of shoots in ramets and caused improvement to leaf quality in extant shoots. Ramets within a tree responded individually, probably mediated by disturbance of the hormonal control because removal of apical buds elicited the response although removal of the same number of basal buds did not. Induced amelioration is a different response to induced resistance. The two responses are triggered by different cues and may occur in the same plant. By altering hormonal balance of shoots it is potentially possible for herbivores to induce amelioration of food quality. The ways in which herbivory is simulated may explain variability of results obtained when herbivory-induced responses in plants have been studied.

HERBIVORE RESISTANCE IN BETULA PENDULA: EFFECT OF FERTILIZATION, DEFOLIATION, AND PLANT GENOTYPE

Plant resistance to herbivores is affected both by genetic and environmental factors. The carbon–nutrient balance hypothesis (CNB) explains environmentally induced variation in both constitutive and delayed herbivore-induced resistance (DIR) in terms of variation in soil fertility and light regime. The CNB hypothesis predicts that an increase in the availability of nutrients (e.g., fertilization) decreases both constitutive and induced resistance against herbivores. We tested the relative roles of plant genotype, defoliation, and soil fertility in determining herbivore resistance of cloned silver birch Betula pendula Roth saplings. As indicators of insect and mammalian resistance we conducted bioassays with a geometrid moth, Epirrita autumnata (Borkhausen), and counted the resin droplets on the shoot of the saplings, respectively. In addition, we measured rapid induced resistance (RIR) against the insect herbivore. Finally, we analyzed leaf secondary chemistry to investigate the correlations of secondary chemicals with the level of resistance measured using the performance of E. autumnata. With respect to the constitutive resistance against an insect herbivore, our results support the CNB hypothesis; the larvae of E. autumnata had a higher relative growth rate and pupal mass on fertilized saplings compared to nonfertilized saplings, i.e., the fertilized saplings had a lower resistance level. However, the relative growth rate of E. autumnata was significantly decreased by defoliation only when the larvae were grown on fertilized saplings. The number of resin droplets increased due to fertilization and, in fertilized saplings, following defoliation, but these responses were highly determined by the genotype of the sapling. Altogether, the results on resin droplets are not in accordance with the CNB hypothesis. The concentration of condensed tannins correlated negatively with E. autumnata growth rate and pupal mass in both fertilization levels, whereas the concentration of total nontannin phenolics correlated positively with the E. autumnata growth rate in nonfertilized saplings. In addition, the concentration of myricetin glycosides correlated negatively with the pupal mass of E. autumnata, whereas the correlations between E. autumnata performance indices and other groups of flavonol glycosides were either significantly positive (kaempferol glycosides) or nonsignificant (quercetin glycosides). Further, the concentration of 3,4′-dihydroxypropiophenone 3-glucoside (DHPPG) correlated positively with the magnitude of induction in E. autumnata growth rate and pupal mass in fertilized saplings, where the significant induction in resistance occurred. The correlations of secondary chemistry and E. autumnata performance indices suggest that the constitutive level of resistance of B. pendula against E. autumnata is mainly determined by the concentration of condensed tannins, whereas the induced resistance is determined by the concentration of nontannin phenolics, such as flavonol glycosides and DHPPG. We observed significant differences among the clones in their insect and mammalian resistance (i.e., genetic basis for the resistance), which indicates that resistance can evolve as a response to herbivory. However, fertilization explained a higher proportion of variance in insect performance indices than the genotype of the plant, whereas the opposite was true for the amount of resin droplets, which we used as an indicator of mammalian resistance.

Trade-offs in phenolic metabolism of silver birch: Effects of fertilization, defoliation, and genotype

We examined the chemical responses of 10 silver birch (Betula pendula) clones to fertilization and defoliation in a field experiment. In defoliation, every second leaf was removed from the saplings. Three days later, two undamaged short-shoot leaves were collected, air-dried, and analyzed for condensed tannins and 34 nontannin phenolic compounds by high-performance liquid chromatography. The clones showed substantial variation in phenolic composition of the leaves and in chemical responses to fertilization and defoliation. A cluster analysis by UPGMA indicated that the phenolic profiles of birch leaves were affected more by genotype than fertilization or defoliation, and the clones could thus be distinguished from each other. In addition, on the basis of their overall phenolic composition, the clones were clustered loosely in three clone groups. The leaves of fertilized saplings contained lower levels of condensed tannins than controls, as predicted by carbon/nutrient balance (CNB) hypothesis. However, fertilization had no effect on the total amount of nontannin phenolics. The concentrations of (+)-catechin, 3,4′-dihydroxypropiophenone 3-glucoside (DHPPG), 3-cinnamoylquinic acids, and flavone aglycones were lower in fertilized saplings, whereas the opposite was true for 5-cinnamoylquinic acids and the total amount of flavonol glycosides. Although our results provide support for the CNB hypothesis, they also show that the accumulation of phenolic compounds in birch leaves is strongly coordinated. Different branches of the biosynthetic pathway of phenolic compounds may compete for substrates, and such internal metabolic trade-offs may explain the differential accumulation of the compounds. In fertilized saplings, the concentration of condensed tannins was also negatively correlated with the amount of triterpenoid resin droplets measured from the same saplings. We suggest that a linkage via malonyl-CoA between the biosynthetic routes to terpenoids and flavonoid derivatives, such as condensed tannins, may explain the different responses to fertilization reported for terpenoids and phenolics. Undamaged leaves of partially defoliated saplings contained more DHPPG and flavone aglycones and less cinnamic acid derivatives and (+)-catechin than did leaves of control saplings. The induction of DHPPG and flavonoid aglycones was significantly and negatively correlated with the concentration of myricetin glycosides in fertilized saplings, which may indicate a trade-off between induced and constitutive defense. Moreover, in fertilized saplings, the three clone groups formed by UPGMA clustering differed significantly in the magnitude of induction of DHPPG and flavone aglycones. Different birch genotypes may thus have different modes of chemical defense, and the magnitude of chemical response of a genotype may partly depend on resource availability. In general, our results show that new insights in the theory of chemical defense can be gained by accomplishing studies on plant–herbivore interaction with high chemical resolution.

Comparison of five cladoceran species in short- and long-term copper exposure

The study was carried out to assess the effect of both short-and long-term copper exposure on five cladoceran species differing in body size and habitat, i.e. Daphnia magna, D. pulex, D. galeata, Bosmina longirostris, and Chydorus sphaericus. The species-specific 48 h EC50 values for fed neonates were used to determine the chronic exposure levels (52% and 65% of the EC50 values). The experiment was run at two food levels. Long-term copper exposure retarded growth in each of the species studied. However, the copper treatments did not affect the clutch sizes of the experimental animals. At a low food level, copper exposure increased mortality in every species studied. The intrinsic rate of increase, r, was reduced in the copper-exposed populations. The sensitivity to copper was higher in the small lake species, compared to D. magna and D. pulex that are commonly used in standard toxicity tests. Of the species studied, B. longirostris had the highest sensitivity to both acute and chronic copper stress. The different sensitivities of the species studied are discussed in an ecological context.

Demographic costs of Chaoborus-induced defences in Daphnia pulex: a sensitivity analysis

SummaryWe examined the demographic costs of Chaoborus-induced defensive spine structures in Daphnia pulex. Our aim was to assess the role of resource limitation and the interaction effects of limiting food level and antipredator structures on fitness of D. pulex and to pinpoint those life stages that are most sensitive to changes in the defence regime. Chaoborus-induced and typical morphotypes of D. pulex were reared at high and low food concentrations. Instar-based matrix population models were used to quantify the effects of predator-induction, food and their interaction on fitness of D. pulex. Predator-induction caused a statistically significant reduction in fitness at low food levels, but not at high food levels. Sensitivity analyses revealed that the fitness effects were primarily due to changes in the growth rate in instars 1–5, and secondarily to small reductions in the fertility of instars 5–10. The interaction between Chaoborus exposure and low food concentration was negative, and mediated through growth and fertility components. Both these components were reduced more in the Chaoborus-exposed, low food treatment than would be expected in the absence of interaction.

Growth, reproduction and defence in nettles: responses to herbivory modified by competition and fertilization

We studied the effects of environmental factors on the ability to compensate for simulated herbivory in an annual (Urtica urens) and two perennial (U. dioica subspp. dioica and sondenii) nettle species in order to test the compensatory continuum hypothesis. Further, we studied the expression of costs of structural defence in different environmental conditions. Herbivory interacted significantly with density and fertilization for height growth, branching and reproductive traits. The negative effects of simulated herbivory on relative height growth were less detrimental for plants grown in low density and high fertilization level, which supports the compensatory continuum hypothesis. However, with respect to other traits measured, our results do not support the idea of compensatory continuum. The negative effects of apical excision on inflorescence biomass were relatively more prominent in plants growing at low density and receiving more fertilization than on those growing in worse conditions. In addition, branching was reduced by apical excision regardless of resource levels. The lack of compensation for herbivory is explained by the role which the damaged or removed tissue plays in plant development and function. In the perennial U. dioica, defensive responses to herbivory, measured as changes in trichome density, were stronger than in the annual U. urens. In the southern subspecies dioica, trichome density increased in newly emerged leaves after leaf clipping. In the northern subspecies sondenii, trichome density increased after apical excision. The differences in the defensive responses between the two subspecies of U. dioica may be due to differential natural herbivory pressures on subspecies inhabiting different geographical regions. We observed negative phenotypic correlations between structural defence and other plant traits, which suggests the existence of costs of defence. In addition to differences among the species studied, the expression of costs of defence was dependent on the resource levels.

Costs of herbivore resistance in clonal saplings of Betula pendula

Abstract. Several studies have found genetic variation in plant resistance to herbivory. One of the explanations suggested for the observed intermediate levels of resistance are the costs of resistance, i.e., negative genetic correlations between resistance and other fitness components that may constrain the evolution of resistance. We studied the cost of herbivore resistance by investigating the genetic correlations between resistance traits and plant growth traits, and between resistance to insect and mammalian herbivores in cloned saplings of silver birch, Betula pendula. We used the performance of a geometrid moth, Epirrita autumnata, as an indicator of insect resistance. The numbers of resin droplets at the base and at the tip of the saplings correlate with mammalian resistance, and were thus used here as indicators of vole and hare resistance, respectively. We have previously observed genetic variation in these resistance traits. Further, we examined the correlations between several groups of secondary chemicals and plant growth traits. Finally, to reveal the effect of environmental factors on the trade-offs mentioned above, we investigated the correlations in saplings that were grown at two nutrient levels. We found significant negative correlations between indices of constitutive insect resistance and relative height growth in non-fertilized saplings, indicating cost of constitutive insect resistance. The two groups of secondary chemicals that have been shown to correlate strongly with constitutive insect resistance, i.e., condensed tannins and flavonol glycosides (especially myricetin glycosides), had different genetic correlations with plant traits; the concentration of condensed tannins did not correlate negatively with any of the plant traits, whereas the concentration of flavonol glycosides correlated negatively with plant height. Insect and mammalian resistance did not correlate negatively, indicating no ecological trade-offs.

DoMicrocystis aeruginosa toxins accumulate in the food web: a laboratory study

The accumulation ofMicrocystis aeruginosa hepatotoxins (microcystin-LR) in the phantom midge larvaChaoborus was studied in a 16 d laboratory experiment. In the cyanobacteria treatment,Chaoborus larvae were fed withDaphnia pulex juveniles which had been feeding for two days on a mixture ofScenedesmus obtusiusculus and toxicMicrocystis aeruginosa. In the control treatment theChaoborus larvae were offeredD. pulex raised onScenedesmus only. An HPLC analysis failed to detect any cyanobacteria toxin in theChaoborus larvae, indicating that the toxin was metabolized or excreted byD. pulex andChaoborus. There was a statistically significant increase in mortality of larvae in the cyanobacteria treatment, but no difference between treatments in pupation success ofChaoborus was observed.

Sexual differences in responses to simulated herbivory in Urtica dioica

We studied the responses of the stinging nettle, Urtica dioica, to simulated herbivory in terms of growth, branching, reproduction, biomass allocation and stinging trichome density. We focused on intersexual differences in the responses. The simulated herbivory treatment consisted of leaf blade clippings and removal of the plant apex. Extensive leaf clipping (50% of the leaf blades removed) cried out before anthesis had the most negative effect on reproduction in both sexes. In the other simulated herbivory treatments, male and female plants either compensated or overcompensated slightly for herbivory in terms of reproduction. The branching responses differed between the sexes. In females, both leaf clipping and apex damage increased branching (...)

Integrating biodiversity into decision making

The prominent theme in the debates concerning biodiversity has been the integration of biodiversity into social and economical processes. Integration has meant two different but related things: on one hand, biological phenomena have been contextualized in social and economical processes; on the other hand, biological issues have been incorporated into decision making. These two aspects of integration are related: knowing the context of biodiversity helps in assessing its worth for decision making. In what follows we will first look at the ramifications of decision making, on how to proceed when dealing with apparently incommensurable dimensions. Secondly, we will highlight some key issues in both biological and social impact assessment. Finally, we will propose a process model which takes into account the multiple aspects of biodiversity.