Effects of simulated shoot and leaf herbivory on vegetative growth and plant defense in Acacia drepanolobium

Plants have considerable ability to respond to herbivory, both with (above-ground) regrowth and with increased defense. We simulated both leaf and shoot herbivory in controlled, replicated experiments on individuals of Acacia drepanolobium in Laikipia, Kenya. These experiments were carried out on individuals that had experienced different, experimentally controlled histories of large mammalian herbivory. Both forms of simulated herbivory were associated with compensatory regrowth. Branches whose shoots had been removed grew significantly more over the next year than paired control branches, fully compensating for the lost shoot length. Branches whose leaves were removed both grew faster and had more leaves one year later than did control branches. Shoot removal, but not leaf removal, increased the production of side shoots. However, because past herbivore pressure was negatively associated with net shoot growth, there may be a long-term cost of herbivory even when plants appear to fully compensate for herbivory in the short term. In contrast to the effects on growth, simulated herbivory did not significantly increase physical (spines) or chemical (tannins) defenses, and there were no significant negative correlations between compensatory growth and plant defense.     

Allocating nitrogen away from a herbivore: a novel compensatory response to root herbivory

Centaurea maculosa, an invasive North American plant species, shows a high degree of tolerance to the root-boring biocontrol herbivore, Agapeta zoegana. For example, infested individuals of C. maculosa often exhibit more rigorous growth and reproduction compared with their non-infested counterparts. Compensatory responses to aboveground herbivores often involve increases in leaf area and/or photosynthetic capacity, but considerably less is known about root system compensatory responses to belowground herbivory. We used a ¹⁵N labeling approach to evaluate whether compensatory adjustments in N acquisition via changes in root morphology and/or physiological uptake capacity could explain the ability of C. maculosa to tolerate root herbivory. Root herbivory reduced whole plant N uptake by more than 30% and root uptake capacity by about 50%. Despite a marked reduction in N procurement, herbivory did not affect total biomass or shoot N status. Infested plants maintained shoot N status by shifting more of the acquired N from the root to the shoot. To our knowledge, shifting N allocation away from a root herbivore has not been reported and provides a plausible mechanism for the host plant to overcome an otherwise devastating effect of a root herbivore-induced N deficit.     

Effects of decomposers and herbivores on plant performance and aboveground plant-insect interactions

Most ecologists acknowledge that plants are subject to complex interactions between both below- and aboveground dwelling animals. However, these complex interactions are seldomly investigated simultaneously. In a factorial common garden experiment we tested single and combined effects of decomposers, root herbivores and leaf herbivores on the growth, flower visitation, and abundance of naturally colonizing aphids and parasitoids on wild mustard ( Sinapis arvensis ). We found that the individual presence of either root herbivores or decomposers resulted in increased aphid abundance, demonstrating that the same aboveground plant–insect interaction can be released by different belowground processes. Enhanced aphid densities caused higher numbers of parasitoids. Furthermore, decomposers increased plant growth and plant fitness (measured as the number of seeds produced), indicating that mustard may benefit from nutrients provided by decomposers, regardless whether plants are attacked by root herbivores or leaf herbivores, or both simultaneously. More flower visits were observed in plants attacked by root herbivores but without leaf herbivores than in plants with both herbivores, suggesting that root herbivory can modify flower attractivity to pollinators. Our results suggest that patterns in plant–insect interactions above the ground are not only affected by aboveground factors but also by a wealth of different belowground processes mediated by the plant.     

Sequential effects of root and foliar herbivory on aboveground and belowground induced plant defense responses and insect performance

Plants are often simultaneously or sequentially attacked by multiple herbivores and changes in host plants induced by one herbivore can influence the performance of other herbivores. We examined how sequential feeding on the plant Plantago lanceolata by the aboveground herbivore Spodoptera exigua and the belowground herbivore Agriotes lineatus influences plant defense and the performance of both insects. Belowground herbivory caused a reduction in the food consumption by the aboveground herbivore independent of whether it was initiated before, at the same time, or after that of the aboveground herbivore. By contrast, aboveground herbivory did not significantly affect belowground herbivore performance, but significantly reduced the performance of later arriving aboveground conspecifics. Interestingly, belowground herbivores negated negative effects of aboveground herbivores on consumption efficiency of their later arriving conspecifics, but only if the belowground herbivores were introduced simultaneously with the early arriving aboveground herbivores. Aboveground–belowground interactions could only partly be explained by induced changes in an important class of defense compounds, iridoid glycosides (IGs). Belowground herbivory caused a reduction in IGs in roots without affecting shoot levels, while aboveground herbivory increased IG levels in roots in the short term (4 days) but only in the shoots in the longer term (17 days). We conclude that the sequence of aboveground and belowground herbivory is important in interactions between aboveground and belowground herbivores and that knowledge on the timing of exposure is essential to predict outcomes of aboveground–belowground interactions.     

Sources of variation in plant responses to belowground insect herbivory: a meta-analysis

Growing interest in belowground herbivory and the remarkable diversity of the accumulated information on this topic inspired us to quantitatively explore the variation in the outcomes of individual studies. We conducted a meta-analysis of 85 experimental studies reporting the effects of root-feeding insect herbivores (36 species) on plants (75 species). On average, belowground herbivory led to a 36.3% loss of root biomass, which was accompanied by a reduction in aboveground growth (-16.3%), photosynthesis (-11.7%) and reproduction (-15.5%). The effects of root herbivory on aboveground plant characteristics were significant in agricultural and biological control studies, but not in studies of natural systems. Experiments conducted in controlled environments yielded larger effects on plants than field experiments, and infestation experiments resulted in more severe effects than removal studies employing natural levels of herbivory. Simulated root herbivory led to greater aboveground growth reductions than similar root loss imposed by insect feeding. External root chewers caused stronger detrimental effects than sap feeders or root borers; specialist herbivores imposed milder adverse effects on plants than generalists. Woody plants suffered from root herbivory more than herbaceous plants, although root loss was similar in these two groups. Evergreen woody plants responded to root herbivory more strongly than deciduous woody plants, and grasses suffered from root herbivory more than herbs. Environmental factors such as drought, poor nutrient supply, among-plant competition, and aboveground herbivory increased the adverse effects of root damage on plants in an additive manner. In general, plant tolerance to root herbivores is lower than tolerance to defoliating aboveground herbivores.     

Influence of hydrological fluctuations on the growth and nutrient dynamics ofPhalaris arundinacea L. in a riparian environment

Seasonal changes in aboveground and belowground tissues ofPhalaris arundinacea L. were studied in a population colonizing an ancient meander of the Garonne river (France) submitted to important fluctuations of the permanent water table. Waterlogged conditions in spring stopped the growth of rhizomes and promoted the translocation of nutrient to the shoots. The early senescence of plants after flowering could be related to the withdrawal of the water table. It was characterized by a distribution of nutrients in belowground tissues and a release in litter and soil. Aerated conditions in late summer permitted the growth of belowground tissues. At this time a partition of resources between aboveground and belowground biomass of a new generation of plants was observed. Rising water and decreasing temperatures in winter induced the death of aboveground parts. Reconstitution of nutrient stocks in rhizomes and losses by leaching then occured. Beside a very high primary production this strategy confers toPhalaris arundinacea a great interest in different uses, especially in the removal of nutrients from water in riparian zones as in artificial sites.     

A novel life cycle arising from leaf segments in plants regenerated from horseradish hairy roots

Summary Horseradish (Armoracia rusticana) hairy root clones were established from hairy roots which were transformed with the Ri plasmid in Agrobacterium rhizogenes 15834. The transformed plants, which were regenerated from hairy root clones, had thicker roots with extensive lateral branches and thicker stems, and grew faster compared with non-transformed horseradish plants. Small sections of leaves of the transformed plants generated adventitious roots in phytohormone-free G (modified Gamborg's) medium. Root proliferation was followed by adventitious shoot formation and plant regeneration. Approximately twenty plants were regenerated per square centimeter of leaf. The transformed plants were easily transferable from sterile conditions to soil. When leaf segments of the transformed plants were cultured in a liquid fertilizer under non-sterile conditions, adventitious roots were generated at the cut ends of the leaves. Adventitious shoots were generated at the boundary between the leaf and the adventitious roots and developed into complete plants. This novel life cycle arising from leaf segments is a unique property of the transformed plants derived from hairy root clones.     

Trade-offs in plant responses to herbivory influence trophic routes of production in a freshwater wetland

Responses of aquatic macrophytes to leaf herbivory may differ from those documented for terrestrial plants, in part, because the potential to maximize growth following herbivory may be limited by the stress of being rooted in flooded, anaerobic sediments. Herbivory on aquatic macrophytes may have ecosystem consequences by altering the allocation of nutrients and production of biomass within individual plants and changing the quality and quantity of aboveground biomass available to consumers or decomposers. To test the effects of leaf herbivory on plant growth and production, herbivory of a dominant macrophyte, Nymphaea odorata, by chrysomelid beetles and crambid moths was controlled during a 2-year field experiment. Plants exposed to herbivory maintained, or tended to increase, biomass and aboveground net primary production relative to controls, which resulted in 1.5 times more aboveground primary production entering the detrital pathway of the wetland. In a complementary greenhouse experiment, the effects of simulated leaf herbivory on total plant responses, including biomass and nutrient allocation, were investigated. Plants in the greenhouse responded to moderate herbivory by maintaining aboveground biomass relative to controls, but this response occurred at the expense of belowground growth. Results of these studies suggest that N. odorata may tolerate moderate levels of herbivory by reallocating biomass and resources aboveground, which in turn influences the quantity, quality and fate of organic matter available to herbivores and decomposers.     

Effects of above- and belowground herbivory on growth,pollination, and reproduction in cucumber

Plants experience unique challenges due to simultaneous life in two spheres, above- and belowground. Interactions with other organisms on one side of the soil surface may have impacts that extend across this boundary. Although our understanding of plant–herbivore interactions is derived largely from studies of leaf herbivory, belowground root herbivores may affect plant fitness directly or by altering interactions with other organisms, such as pollinators. In this study, we investigated the effects of leaf herbivory, root herbivory, and pollination on plant growth, subsequent leaf herbivory, flower production, pollinator attraction, and reproduction in cucumber (Cucumis sativus). We manipulated leaf and root herbivory with striped cucumber beetle (Acalymma vittatum) adults and larvae, respectively, and manipulated pollination with supplemental pollen. Both enhanced leaf and root herbivory reduced plant growth, and leaf herbivory reduced subsequent leaf damage. Plants with enhanced root herbivory produced 35% fewer female flowers, while leaf herbivory had no effect on flower production. While leaf herbivory reduced the time that honey bees spent probing flowers by 29%, probing times on root-damaged plants were over twice as long as those on control plants. Root herbivory increased pollen limitation for seed production in spite of increased honey bee preference for plants with root damage. Leaf damage and hand-pollination treatments had no effect on fruit production, but plants with enhanced root damage produced 38% fewer fruits that were 25% lighter than those on control plants. Despite the positive effect of belowground damage on honey bee visitation, root herbivory had a stronger negative effect on plant reproduction than leaf herbivory. These results demonstrate that the often-overlooked effects of belowground herbivores may have profound effects on plant performance.     

Inter-annual variation in above- and belowground herbivory on a native,annual legume

The relative importance of subterranean versus aboveground insect damage to plants is not well understood. In particular, the simultaneous effects of above- and belowground herbivory, and the importance of highly variable abiotic factors such as rainfall, have received little attention in diverse natural ecosystems. We investigated the influence of both above- and belowground herbivory on Lupinus nanus (Fabaceae), an annual plant native to coastal California. A number of insect species damage L. nanus aboveground, and a weevil larva consumes nodules belowground. To manipulate herbivory in the field, we employed a combination of insecticides and simulated herbivory during two different years. In 1997, simulated belowground damage reduced L. nanus survival, and insecticide application to roots increased seed production and seed mass. By contrast, in 1998, only aboveground folivory significantly reduced L. nanus reproduction, and, in combination, above- and belowground insecticides did not affect flower or seed number relative to controls. A growth chamber experiment conducted in the absence of herbivory revealed that the aboveground insecticide marginally reduced flower production and the belowground insecticide marginally increased flower production compared to controls; these non-target effects made our field experiments for aboveground herbivory conservative. Finally, ambient levels of herbivory differed among years (1997, 1998, and 2000), which varied greatly in rainfall due to the effects of El Nino. The results suggest that the impacts of herbivores are temporally variable and that abiotic factors, particularly those related to large-scale changes in weather patterns, may be more important than insect herbivory to L. nanus in some years. This revised version was published online in August 2006 with corrections to the Cover Date.     

Above‐ and belowground herbivory jointly impact defense and seed dispersal traits in Taraxacum officinale

Plants are able to cope with herbivores by inducing defensive traits or growth responses that allow them to reduce or avoid the impact of herbivores. Since above‐ and belowground herbivores differ substantially in life‐history traits, for example feeding types, and their spatial distribution, it is likely that they induce different responses in plants. Moreover, strong interactive effects on defense and plant growth are expected when above‐ and belowground herbivores are jointly present. The strengths and directions of these responses have been scarcely addressed in the literature. Using Taraxacum officinale, the root‐feeding nematode Meloidogyne hapla and the locust Schistocerca gregaria as a model species, we examined to what degree above‐ and belowground herbivory affect (1) plant growth responses, (2) the induction of plant defensive traits, that is, leaf trichomes, and (3) changes in dispersal‐related seed traits and seed germination. We compared the performance of plants originating from different populations to address whether plant responses are conserved across putative different genotypes. Overall, aboveground herbivory resulted in increased plant biomass. Root herbivory had no effect on plant growth. Plants exposed to the two herbivores showed fewer leaf trichomes than plants challenged only by one herbivore and consequently experienced greater aboveground herbivory. In addition, herbivory had effects that reached beyond the individual plant by modifying seed morphology, producing seeds with longer pappus, and germination success.     

Putting the ‘upstairs–downstairs’ into ecosystem service: What can aboveground–belowground ecology tell us?

Interactions between spatially-separated aboveground and belowground biota exert important influences on the functioning of terrestrial ecosystems. Plant root exudates and litter inputs affect root-associated and decomposer sub-communities, which, in turn, regulate nutrient availability and plant growth. Ecosystem services theoretically attributed to specific functional components of aboveground or belowground biota are, therefore, influenced by indirect (plant-mediated) interactions with the wider community. Some recent studies have considered aboveground–belowground interactions in a climate change context, with implications for altered ecosystem service provision. This review is a conceptual discussion of the mechanisms by which aboveground–belowground interactions affect specific ecosystem services: control of herbivores by natural enemies, insect pollination and nutrient mineralization by soil decomposers. While some mechanisms are well-characterized, others are poorly understood. Reducing root and shoot herbivory, in addition to the direct plant benefit, indirectly promotes antagonism of the spatially-separate herbivore by its natural enemies. Soil decomposers and mycorrhizal fungi can increase shoot herbivore performance such that control by natural enemies is weakened, or initiate bottom-up trophic cascades which strengthen antagonism of shoot herbivores. Aboveground herbivory generally stimulates nutrient cycling by decomposers. Root herbivory and mycorrhizal association both appear to increase floral attractiveness to insect pollinators. Mechanisms reflect alterations to plant growth, nutritional quality and chemical defenses. Climate change has considerable potential to alter aboveground–belowground interactions, with largely unexplored implications for biological control, pollination and soil nutrient cycling.     

Physiological and growth responses of Centaurea maculosa (Asteraceae) to root herbivory under varying levels of interspecific plant competition and soil nitrogen availability

Summary Centaurea maculosa seedlings were grown in pots to study the effects of root herbivory by Agapeta zoegana L. (Lep.: Cochylidae) and Cyphocleonus achates Fahr. (Col.: Curculionidae), grass competition and nitrogen shortage (each present or absent), using a full factorial design. The aims of the study were to analyse the impact of root herbivory on plant growth, resource allocation and physiological processes, and to test if these plant responses to herbivory were influenced by plant competition and nitrogen availability. The two root herbivores differed markedly in their impact on plant growth. While feeding by the moth A. zoegana in the root cortex had no effect on shoot and root mass, feeding by the weevil C. achates in the central vascular tissue greatly reduced shoot mass, but not root mass, leading to a reduced shoot/root ratio. The absence of significant effects of the two herbivores on root biomass, despite considerable consumption, indicates that compensatory root growth occurred. Competition with grass affected plant growth more than herbivory and nutrient status, resulting in reduced shoot and root growth, and number of leaves. Nitrogen shortage did not affect plant growth directly but greatly influenced the compensatory capacity of Centaurea maculosa to root herbivory. Under high nitrogen conditions, shoot biomass of plants infested by the weevil was reduced by 30% compared with uninfested plants. However, under poor nitrogen conditions a 63% reduction was observed compared with corresponding controls. Root herbivory was the most important stress factor affecting plant physiology. Besides a relative increase in biomass allocation to the roots, infested plants also showed a significant increase in nitrogen concentration in the roots and a concomitant reduction in leaf nitrogen concentration, reflecting a redirection of the nitrogen to the stronger sink. The level of fructans was greatly reduced in the roots after herbivore feeding. This is thought to be a consequence of their mobilisation to support compensatory root growth. A preliminary model linking the effects of these root herbivores to the physiological processes of C. maculosa is presented.     

根茎克隆植物羊草体内可溶性碳水化合物的时间变异及其对去叶干扰的响应

该研究针对根茎型克隆植物羊草(Leymus chinensis)考察了以下内容:1)地上枝条和根茎中可溶性碳水化合物含量的时间动态及其对去叶干扰的响应;2)特定阶段植物体内一定部位的可溶性碳水化合物浓度差异;3)植物体各部分(地上部分、直立茎地下部分及根茎)间可溶性碳水化合物浓度变化之间的关联。基于上述研究结果,作者试图弄清碳水化合物对于羊草克隆分株和整个基株生长和存活的意义。实验共有4个处理:1个对照和3个不同频度(在整个实验进行期间分别去叶1次、3次和5次)的去叶处理。所有去叶处理都采取一个统一的强度,即留茬15 cm。地上枝条和根茎的取样频次为每10 d 1次。植物体各部分可溶性碳水化合物浓度以高效液相色谱法(HPLC)测定。对不同去叶频度处理间的碳水化合物含量差异显著性进行ANOVA分析。结果表明:不去叶对照处理在生长季盛期可溶性碳水化合物浓度的显著下降归因于植物体快速的生长而引起植物叶片旺盛的呼吸消耗,而去叶处理中植物的可溶性碳水化合物浓度并没有大的降低甚至在最频繁的去叶处理下还有所上升,主要是由于去叶处理减少叶片而造成地上部分总呼吸量下降所致。一次性去叶处理并没有影响植物地上部分最终的可溶性碳水化合物浓度,但是连续数次的去叶处理对地上部分可溶性碳水化合物浓度产生了一定的影响。在秋季气温下降时,碳水化合物自地上向地下的转移在去叶频度越大的处理下表现越为迅速。这表明当植物体接受到气温降低的信号后,去叶干扰加速碳水化合物自地上向地下的转移。可能由于地下枝条存在一定的贮藏功能,在实验过程中地下枝条中可溶性碳水化合物浓度比地上枝条中表现的更加稳定。根茎中的可溶性碳水化合物必要时会转移到地上以供应地上枝条的生长,而旺盛的生长会消耗可溶性碳水化合物,然而自未接受去叶处理的分株向接受去叶处理的分株的克隆整合(常常在较高频次的去叶处理中发生)可能会在一定程度上缓解这种消耗所造成的影响。     

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

Summary Three 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.     

Effects of below- and above-ground herbivores on plant growth,flower visitation and seed set

Separate and combined effects of root and leaf herbivores on plant growth, flower visitation and seed set were tested in a factorial experiment using potted mustard, Sinapis arvensis, at an old fallow field. A 50% leaf removal by cabbageworms (Pieris rapae) when the seedlings had their first four leaves reduced plant height and shoot mass, and delayed the onset of flowering. Root herbivory by two wireworms (Agriotes sp.) over the whole experiment changed flower visitation; the number of flower visitors per plant was higher in plants with root herbivores than in plants without root herbivores. Combined leaf and root herbivory affected flowering period, number of fruits per plant and number of seeds per fruit. Plants attacked by leaf and root herbivores had a shorter flowering period and produced fewer fruits per plant than plants with root herbivores only. Although the experimental plants faced major herbivore-induced growth changes, plant reproduction (seed set and weight per plant) was similar in all treatments, documenting their ability to effectively compensate for leaf and root herbivory.     

Abscisic acid accumulation in the roots of nutrient-limited plants: its impact on the differential growth of roots and shoots

We describe the involvement of abscisic acid (ABA) in the control of differential growth of roots and shoots of nutrient limited durum wheat plants. A ten-fold dilution of the optimal concentration of nutrient solution inhibited shoot growth, while root growth remained unchanged, resulting in a decreased shoot/root ratio. Addition of fluridone (inhibitor of ABA synthesis) prevented growth allocation in favour of the roots. This suggests the involvement of ABA in the redirecting of growth in favour of roots under limited nutrient supply. The ABA content was greater in shoots and growing apical root parts of starved plants than in nutrient sufficient plants. Accumulation of ABA in shoots of nutrient deficient plants was linked to a decrease in leaf turgor. Increased flow of ABA in the phloem apparently contributed to the accumulation of ABA in the apical part of the roots. Thus, partitioning of growth between roots and shoots of wheat plants limited in mineral nutrients appears to be modulated by accumulation of ABA in roots. This ABA may originate in the shoots, where its synthesis is stimulated by the loss of leaf turgor.     

松嫩平原不同生育期虎尾草无性系构件生长与生物量分配

以松嫩平原不同生育期的虎尾草无性系为研究对象,分别在虎尾草无性系拔节期和完熟期进行大样本取样,并对其地上、地下各构件的数量性状及生物量分配进行统计分析,研究虎尾草在不同生育期的生长特性及生长策略.结果表明:虎尾草株高、总根长、总根表面积、根体积、地上生物量、地下生物量、总生物量在两个生育期之间均存在显著差异.在两个生育期,地上、地下生物量分配与总生物量呈显著幂函数异速生长关系.在拔节期,总生物量与分株数、总根长、总根表面积和根体积呈显著线性同速生长关系;而在完熟期,均呈显著幂函数异速生长关系.虎尾草无性系在不同生育期存在着不同的生长策略,在拔节期主要采取的是无性系外部空间的优先扩展策略,而在完熟期主要采取的是无性系内、外部空间兼顾的补充和扩展策略.     

Density-dependent growth responses in two clonal herbs: regulation of shoot density

Summary It has been shown in clonal perennial herbs that shoot natality decreases, and shoot mortality increases, in stands of increasing density. In a two-year garden experiment, we have tested Hutchings' (1979) hypothesis that these responses are the result of physiological integration, i.e. the exchange of resources and growth substances between shoots of a single clone. Dense monocultures of two rhizomatous graminoids, Brachypodium pinnatum and Carex flacca, were created that differed more than 10-fold in the density of clones (genets), but that had similar densities of shoots. A more effective shoot density control was expected in stands with the smaller clone densities (larger clones) due to more extensive clonal connections. Shoot turnover was evaluated by counting living and dead shoots at different times. In the summer of the second year, when shoot densities and stand structure were similar between treatments, shoot natality (the number of shoots born per plot) and shoot mortality (the number of shoots that died per plot) were usually unrelated to clone density in either species. If there was a significant treatment effect, it could be attributed to (small) differences in shoot density. Over the whole range of shoot densities, natality was negatively density-dependent. The number of shoots that died in a given growth period was proportional to the number of shoots present, suggesting that mortality rates were density independent. In Carex, however, there were some indications that mortality rate increased with increasing density. Our study confirms that clonal herbaceous species can effectively prevent an overproduction of shoots, but in contrast to Hutchings' (1979) propositions, we found no evidence that physiological integration may be the responsible mechanism. An alternative explanation for the observed patterns is proposed.