Biodiversity,habitat and range-size aspects of a flora from a winter-rainfall desert in north-western Namaqualand,South Africa

We examined patterns of richness and endemism in relation to phylogeny (family membership), growth form and habitat in the flora of an arid (<60 mm annual rainfall), sandy coastal belt between Port Nolloth and Alexander Bay on the north-west coast of Namaqualand, South Africa. This region forms part of the species-rich Succulent Karoo biome. In an area of 750 km₂, 300 plant species were recorded with 24% endemnicity. This level of diversity and endemism is exceptionally high for a desert region. Plant distribution is strongly influenced by habitat type. Diversity, especially of endemic species, was concentrated on rocky or unusual (lag-gravel pavements) habitats. Endemics were not random phylogenetic nor biological assemblages. Dwarf leaf-succulent shrubs, especially Mesembryanthemaceae, were significantly over-represented amongst the endemic component. The results are discussed in terms of a model for plant diversification in an arid environment with relatively predictable sources of moisture.     

Species turnover of monkey beetles (Scarabaeidae: Hopliini) along environmental and disturbance gradients in the Namaqualand region of the succulent Karoo, South Africa

Species turnover of monkey beetle (Scarabaeidae: Hopliini) assemblages along disturbance and environmental gradients was examined at three sites within the arid, winter rainfall Namaqualand region of the succulent Karoo, South Africa. At each site two study plots with comparable vegetation and soils but contrasting management (grazing) histories were chosen, the disturbed sites having fewer perennial shrubs and generally more annuals and bare ground. Beetles collected using coloured pan-traps showed a consistently higher abundance in disturbed sites. Lepithrix, Denticnema and Heterochelus had higher numbers in disturbed plots, while Peritrichia numbers were lower in disturbed areas. Measures of species richness and diversity were consistently higher in the undisturbed sites. Distinctive assemblages of monkey beetles and plants occurred at each site. A high compositional turnover ( diversity) was recorded for both monkey beetles and plants along a rainfall gradient; between-site diversity values ranged from 0.7 to 0.8 (out of a maximum of 1.0). Species turnover of beetles was higher between the disturbed sites along the environmental gradient than the corresponding undisturbed sites. The high monkey beetle species turnover is probably linked to the high plant species turnover, a distinctive feature of succulent Karoo landscapes. Monkey beetles are useful indicators of overgrazing disturbance in Namaqualand, as their pollinator guilds are apparently disrupted by overgrazing. A shift away from perennial and bulb pollinator guilds towards those favouring weedy annuals was observed in disturbed areas. The consequences to ecosystem processes due to the effects of disturbance on monkey beetle communities and the role of monkey beetles as indicators of disturbance is discussed, as well as the implications of disturbance on monkey beetle pollination guilds.     

Plant induced fertile islands as possible indicators of desertification in a succulent desert ecosystem in northern Namaqualand,South Africa

Zones of nutrient enrichment beneath shrubs, the so called 'fertile islands', are the result of a range of interacting physical and biotic concentrating mechanisms. These concentrating mechanisms lead to specific spatial patterns of soil properties across a landscape and it has been suggested that disruption of the normal patterns into more aggregated 'fertile islands' might be a useful index of desertification in semi-arid regions. The objective of this paper was to determine whether 'fertile islands' are as characteristic a feature of succulent deserts as they are of shrub dominated deserts. Results indicate that, even in a community dominated by succulent species fertile islands develop under extremely arid conditions and that the degree of development is dependent on plant dynamics. Long-lived species showed greater concentration effects. However, the patterns of nutrient accretion in islands are very different to shrub invaded grasslands in North America. In particular nitrogen accretion dominated the 'fertile islands' in Namaqualand, and we suggest that these islands are more ephemeral than carbon and phosphorus dominated islands reported for other systems. These more dynamic islands are a consequence of rapid community dynamics since the Namaqualand region is characterised by many short-lived perennials which turnover generations very rapidly in response to environmental conditions. Using the development of 'fertile islands' to assess desertification as applied elsewhere might be inappropriate in Namaqualand because of the different patterns of community dynamics underlying the formation and characteristics of these islands.     

Disturbance,drought and dynamics of desert dune grassland,South Africa

A seven-year study of marked plants and plots in Stipagrostis ciliata (Desf.) de Winter dune grassland, in the arid (<100 mm yr^–1) Bushmanland area of the Northern Cape province of South Africa, was designed to test the hypothesis that establishment of ephemeral plants, and recruitment of perennial grasses was dependent upon disturbances that reduced the density of living perennial grass tussocks. In 1989, eight 4 m² plots were cleared of perennial vegetation by uprooting and removing all plants so as to resemble small-scale disturbances made by burrowing mammals or territorial antelope. The vegetation on the cleared plots and surroundings was monitored until 1996. Initial results supported our hypothesis. In wet years, when ephemeral plants were abundant, their average fresh mass was 2–3 times greater per unit area on the cleared plots than in control plots in adjacent, undisturbed grassland. Many Stipagrostis seedlings established in the cleared plots over the two years following clearing but were rare in adjacent areas among established conspecifics. However, a drought in 1992 (11 mm of rain over 12 months) lead to widespread mortality of the perennial grass, killing 56% (range 22–79%) of established tufts. High densities of Stipagrostis seedlings appeared following the drought-breaking rains in January 1993, both in the disturbed plots and in the surrounding `undisturbed' dune grassland. Ephemeral plants established in large numbers throughout the area during the high rainfall year of 1996 and were generally more numerous in the old disturbances than in control plots. Seven years after clearing the biomass of grass on the cleared plots was approximately 34% of the mass removed from the plots in 1989 whereas in the undisturbed grassland biomass was 66% of 1989 levels. Drought had little long-term effect on community composition, and Stipagrostis ciliata constituted 94–98% of plant community before and after drought. Cleared plots were recolonised by S. ciliata, but the contribution of other grass species increased by 6–9%. Synchronous recruitment following occasional drought-induced mortality can generate even-aged populations of the dominant desert dune grasses.     

Population dynamics of the bearded vulture Gypaetus barbatus in southern Africa

The essential features of mortality and survivorship of bearded vultures Gypaetus barbatus in southern Africa were deduced from age class plumage characteristics. The population consisted of about 204 adult pairs within a breeding range of about 35,000 sq km. Pairs bred every year and produced, on average, about 0·9 young per pair per year. Young birds made up about 37% of the population, subadults 3·5% and adults 60%. About 182 fledged young were recruited to the population each year. The proportion of young birds in the population in different areas was inversely related to the breeding density of adult birds (range 24–47%). Young bird mortality over the four years to subadult age was 87%, the survival rate of adults was 94% and the mean lifespan of birds surviving to adulthood was 21·4 years. This study demonstrates the need to understand the relationship between adult breeding density and young bird numbers in different parts of their range to accurately deduce population dynamics characteristics.     

Beating the blaze: Fire survival in the fan aloe (Kumara plicatilis), a succulent monocotyledonous tree endemic to the Cape fynbos,South Africa

Fire is central to the ecology of Mediterranean‐type climate ecosystems, but little is known about the fire ecology of succulent plants therein. This study investigated the fire ecology of an arborescent succulent monocot, Kumara plicatilis (L.) G. D. Rowley (Asphodelaceae), a Cape fynbos endemic. Habitat suitability was assessed to determine whether the species tolerates or ‘avoids’ fire, and fire survival traits (bark thickness and tissue water content) were measured. The population size structure and density of three K. plicatilis populations were assessed after natural fires, and resprouting potential was investigated. Kumara plicatilis adopts a dual fire survival strategy, occupying rocky sites to ‘avoid’ fire and possessing morphological features that afford fire tolerance, e.g. well‐protected apical meristems and thick corky bark. Bark thickness of burned individuals in situ was similar to unburned plants, suggesting that K. plicatilis bark provides effective insulation against fire. Mortality rates were 64%, 40% and 11%, and decreased as rock cover at the population level increased. All three populations showed reduced plant density post‐fire, with greater density reductions associated with lower rock cover. Small plants appear most vulnerable to fire damage due to lower absolute bark thickness and plant heights within the flame zone. Kumara plicatilis is an apical sprouter, recovering after fire or mechanical stem damage by onward growth from surviving stem apices, rather than resprouting. Post‐fire population recovery therefore likely depends on inter‐fire recruitment.     

Abiotic and biotic determinants of the seasonal dynamics of the tick Rhipicephalus appendiculatus in South Africa

Abstract. The creation of a generic population model for the tick Rhipicephalus appendiculatus requires a detailed, quantified understanding of the interactions of these ticks with their biotic and abiotic environment in the different parts of their range, from the tropical regions of East Africa to the temperate regions of South Africa. The much greater seasonal variation in climatic conditions, particularly temperature, further from the equator introduces variable development rates and diapause into the life cycle. Estimates of natural temperature-dependent interstadial development periods, derived from a combination of published laboratory and field data, were applied to published data on the seasonal abundance of R.appendiculatus on three farms in the Eastern Cape Province of South Africa. This enabled an assessment of which ticks of one stage give rise to which ticks of the next stage, from which (a) the onset and duration of diapause in unfed adults, and (b) seasonal interstadial mortality indices in the form of fc-values, could be estimated. The contribution of biotic (tick density) and abiotic (climatic) factors as predictors of mortality at each life stage was investigated by step-wise multiple regression. Density-independent mortality at the female-to-larval stage is correlated with geographically variable climatic factors, minimum temperature at two farms and minimum relative humidity at the third. The other two stages are governed by density-dependent mortality, which, it is argued, may be caused largely by the hosts' acquired resistance to ticks. As expected on theoretical grounds, this density dependence is weaker nearer to the edge of the tick's range and for the more vulnerable immature stages.     

阿拉善荒漠灌木根际中、微量元素含量特征

探讨了阿拉善干旱荒漠区霸王、白刺、红砂、沙冬青、沙木蓼、梭梭和驼绒藜7种旱生灌木根际与非根际土壤Ca、Mg、Fe、Mn、Zn、Cu的含量特征.结果表明:(1)除梭梭根际pH值高于非根际之外,其余6种灌木根际土壤均表现出不同程度的酸化作用.(2)7种灌木根际全钙和交换性钙的含量均低于非根际.除沙木蓼和驼绒藜外,其余5种灌木根际全镁含量均高于非根际,白刺的富集率最高,为43.04%;除沙木蓼外,其余灌木根际交换性镁的含量均有所富集,梭梭的富集率最高,为26.52%.(3)7种灌木根际全铁、全锰、全锌含量总体上小于非根际,但是根际有效铁、有效锰、有效锌含量均大于非根际,表现出明显的富集效应,沙木蓼根际有效铁、有效锰、有效锌的富集率最高,分别为:29.76%、20.92%、86.99%.(4)沙木蓼根际全铜和有效铜含量略高于非根际.梭梭根际全铜含量低于非根际,但梭梭根际有效铜含量却有富集的趋势.其余五种灌木根际全铜和有效铜含量均低于非根际.(5)根际及非根际土壤各养分性状之间,存在复杂的相关性.     

石蒜绵粉蚧在七种多肉植物上的生物学特性

【目的】石蒜绵粉蚧Phenacoccus solani是近年来新入侵我国并严重为害多肉类植物且具检疫重要性的害虫。本研究旨在明确石蒜绵粉蚧在不同多肉植物上生存和繁殖的差异,了解其在我国的潜在危害性。【方法】选取景天科、番杏科、马齿苋科和百合科4科7属7种不同多肉植物(长生草Sempervivumtecrum、黑法师Aeonium arboreum、熊童子Cotyledon tomentosa、姬胧月Graptopetalumparaguayuense、曲玉Lithops pseudotruncatella、松锦之吹雪Anacampseros telephiastrum、姬玉露Haworthia cooperi var. truncata)于人工气候箱中(25±1℃,RH 70%±5%,光周期14L∶10D)饲养石蒜绵粉蚧,观察记录其生长发育、存活、繁殖等生物学特性,并构建了实验种群生命表。同时,在浙江杭州、嘉兴、湖州和金华的多肉植物种植大棚调查了石蒜绵粉蚧的发生情况。【结果】石蒜绵粉蚧在松锦之吹雪和姬玉露这2种多肉植物上不能完成发育周期,在其他5种多肉植物上能完成世代发育,其中1龄和2龄若虫死亡率较高,3龄若虫后大都能发育至成虫并繁殖产卵。该虫在姬胧月和黑法师上死亡率最低,产卵量和F₁代卵孵化率最高,在熊童子和曲玉上的死亡率最高,产卵量和F₁代卵孵化率最低。实验种群生命表参数中,取食姬胧月和黑法师的石蒜绵粉蚧净增殖率、内禀增长率和周限增长率最高,种群加倍时间最短,而在曲玉和熊童子上取食的净增殖率、内禀增长率和周限增长率最低,种群加倍时间最长。大棚调查发现,石蒜绵粉蚧在景天、番杏、仙人掌、百合和马齿苋5科24属87种多肉植物上均有不同程度的发生。【结论】测试的7种多肉植物中,姬胧月和黑法师最适于石蒜绵粉蚧的生长、发育和繁殖,长生草次之;石蒜绵粉蚧1龄和2龄若虫期抵抗力弱是其防治的关键时期。石蒜绵粉蚧可取食的多肉植物种类繁多,流通性大,容易造成该虫害的传播蔓延,因此在多肉植物引种时需高度重视其潜在风险和危害。     

Continuing decline of a keystone tree species in the Kruger National Park,South Africa

Sclerocarya birrea subsp. caffra (marula), a keystone large tree species in southern Africa and heavily utilized by people and herbivores, is declining at an unprecedented rate in the Kruger National Park (KNP), South Africa. Due to increasing concerns over large tree declines in the KNP, mirroring those in eastern Africa over the last 50 years, and local extinctions of marula populations in the north‐western KNP, adult trees in the southern KNP were monitored between 2001 and 2010. Annual mortality rates across all adult size classes ranged between 3.2 and 4.6%. Loss of adult marula trees in some areas over the last decade exceeded 25%. Rainfall had a strong influence on mortality rates temporally and spatially, but the distance to the nearest perennial river had little influence. Overall, marula populations continue to decline and further local extinctions are possible. Given the clearly unacceptable trends of decline, the next step is to utilize this information (i) to develop predictive models for the future dynamics of marula populations incorporating all drivers; (ii) to advocate active management of refugia within the park where marula populations are protected from elephant impacts; and in combination with (iii) continued monitoring.     

Winter growth phenology and leaf orientation in Pachypodium namaquanum (Apocynaceae) in the succulent karoo of the Richtersveld,South Africa

Pachypodium namaquanum (Nyley ex Harb.) Welw., an unusual arborescent stem succulent from the succulent karoo of the arid Richtersveld in north-western South Africa and adjacent Namibia, is characterized by a striking curvature of the terminal 20–60 cm of the trunk toward the north. This orientation displays the single terminal whorl of drought-deciduous leaves with their flat surface angled at a mean inclination of 55° from horizontal. Inclination of 50–60° was found in 65% of individuals sampled, and 85% were inclined between 45 and 65°. Northward azimuth was also quite regular, but varied slightly between populations. The fixed leaf orientation in P. namaquanum maximizes radiation absorption during the winter months when leaves are present. Leaves normally form in early fall (April) and abscise early in spring (October). Growing season conditions in the Richtersveld are relatively mild, with mean maximum temperature dropping only to 21.6°C in July, the coldest month of the year. Frosts are rare. By the fixed orientation of its leaf whorl, P. namaquanum is able to maintain nearly twice the midwinter radiation absorptance that it would have with horizontal orientation. Over an annual cycle the angled leaves receive more radiation than would horizontal leaves for each of the 6 months in which they are present on the plant. This increased winter irradiance is hypothesized to singificantly increase net primary production by concentrating growth activities in winter months and allowing the species to remain dormant during the hyperarid conditions of the hot summer months. Midwinter flowering from apical buds in P. namaquanum may also be aided by its stem orientation. The evolution of this characteristic pattern of winter growth phenology and nodding stem orientation may have come about because of low but relatively regular autumn precipitation and moderate winter temperatures. Slow and regular growth of P. namaquanum leads to long lifespans which may reach 300 years or more.     

Ecological and phylogenetic patterns of carbon isotope discrimination in the winter-rainfall flora of the Richtersveld,South Africa

Leaf tissues of vascular plant species from the arid Richtersveld of northern Namaqualand (Succulent Karoo), South Africa, show a great range of variation in carbon isotope discrimination. Analyses of values for species with obligate C₃ metabolism demonstrated that the water use efficiency indicated by these values varies between species and within a species population both between seedlings and mature shrubs and between sites along an aridity gradient. An expected pattern of increasing water use efficiency with greater longevity in these species was not present, however. Only among ephemerals were there taxa with high values associated with low water use efficiency, but these high values were balanced by other ephemeral species with low values comparable to those typical of moderate to long-lived shrubs. The extent to which putative CAM species in the succulent flora rely entirely on nocturnal carbon fixation is highly variable. Species of Crassulaceae and Aloe (Asphodelaceae) relied almost exclusively on CAM photosynthesis, although Crassulaceae from more mesic environments may be CAM-flexible. Succulent-leaved species of Senecio and leaves of stem-succulent Euphorbia were CAM-flexible in their values, while shrubby species of Ceraria (Portulacaceae) appeared to rely primarily on C₃ photosynthesis. Variable patterns of reliance on CAM photosynthesis were present in the ecologically dominant Mesembryanthema of the Aizoaceae. Shrubby species of the subfamily Mesembryanthemoideae, although capable of CAM-flexibility in less xeric habitats, relied largely on CAM fixation of carbon in our field sites. Within the subfamily Ruschioideae, there was a wide range of values indicating species with CAM, CAM flexibility, and primary reliance on C₃ photosynthesis. Low values indicative of typical CAM fixation were associated with species with greater longevity, suggesting that a positive correlation may exist between the degree of reliance on CAM photosynthesis and plant life span.     

Home range size, spatial distribution and habitat use of elephants in two enclosed game reserves in the Eastern Cape Province, South Africa

We studied space use and habitat selection by elephants in two enclosed reserves in the Eastern Cape Province (South Africa) that conserve components of the regionally important and poorly conserved Thicket Biome. Home ranges of bulls and herds occupied most of the reserves, and core areas were significantly smaller and centred on permanent water. There was no significant difference in home range size of bulls and herds and both were significantly smaller in winter than summer. Components of the Thicket Biome occurred in the home ranges and core areas of all elephants where it was either used according to its abundance or avoided. However, this should not be interpreted as indicating that elephants will not affect plants of the Thicket Biome since change will be cumulative and monitoring should be ongoing to ensure the conservation of this regionally important Biome.     

Fossil wood charcoal assemblages from Elands Bay Cave, South Africa: implications for Late Quaternary vegetation and climates in the winter-rainfall fynbos biome

Aim The aim of this paper is to analyse fossil charcoal deposits, largely identified to the species level and spanning a sequence from the late Holocene to < 40,000 BP , in order to reconstruct Late Quaternary vegetation and climatic patterns in the western (winter-rainfall) fynbos biome of South Africa. Location The charcoals were excavated from the Elands Bay Cave (32°19S, 18°20E) on the semiarid (200–250 mmyr^?1), winter-rainfall coastline of the western fynbos biome. Methods Patterns in the charcoal data set over time were sought by manual sorting of the charcoal×sample matrix, as well as by subjecting the data to multivariate analysis. Palaeoclimatic reconstruction was attempted by comparing the climatic controls on contemporary vegetation communities that resembled the fossil assemblages. Charcoal diversity was modelled using sample age and number of charcoal fragments as explanatory variables. Results The fossil assemblages ranged from xeric communities (similar to those presently occurring at the site) during the Holocene, to more mesic thicket and fynbos vegetation in the terminal Pleistocene, to Afromontane forest and riverine woodland communities after about 18,000 BP . Diversity of the charcoal samples increased monotonically with increasing sample age. Main conclusions The results suggest that, unlike the eastern fynbos biome, which is under fundamentally different climatic controls, soil moisture conditions in the western part of the biome were higher in the Last Glacial than during the Holocene. This scenario may help to explain the higher regional richness and associated diversification in the western than eastern part of the biome.     

Conservation biology of the succulent shrub,Euphorbia barnardii,a serpentine endemic of the Northern Province,South Africa

Euphorbia barnardii White, Dyer & Sloane is a relatively small, succulent shrub found in the Northern Province of South Africa. In 1994 it was listed as endangered by the Transvaal Threatened Plants Programme, because only three populations, totalling 1150 plants, were found. However, our study found five populations totalling 10 783 plants (9503 were reproductive). One population, found 50 km from the others, in the Bewaarkloof district, differs in terms of habitat type and morphology and may therefore be a different taxon. The other populations occur in Sekhukhuneland. Soil analyses showed that E. barnardii is restricted to ultramafic (serpentine) substrates. A sample of 2015 plants was examined in 1995 for size, stage, new growth, dead branches, reproduction (indices of vigour), damage and disease. Analysis of the results of 10 years’ demographic monitoring (1985–1995) on two of the Sekhukhuneland populations showed that one population, which was vigorous in 1995, showed little change in population size and structure, while the other, which had low vigour in 1995, showed a precipitous decline to local extinction in the monitoring plot. Aerial photographs taken in 1957 (1963 for Bewaarkloof) and 1986 showed large increases in human population density within 1.5 km of the E. barnardii populations except at Bewaarkloof, where it decreased. An increase in human habitation is associated with an increase in livestock (mostly cattle and goats) which trample plants. Trampling damages the terminal segments, which may lead to lowered reproductive output and increase susceptibility to opportunistic bacterial wilt pathogens. Bacterial wilts were prevalent on all populations but were particularly high at Bewaarkloof and on the population that showed a steep decline in numbers. Disease incidence was associated with the level of plant damage. The impact on plants further up the slopes and on the crest of hills (quite far from human settlements) was to a far lesser degree but further increases in human population density could change this situation. While this study has shown that the population size of E. barnardii is much greater than previously thought, the species is still threatened by several different processes and should be listed as ‘Vulnerable (A1a + c, B1, B2b + e, C1, D2)’ according to World Conservation Union categories.     

Modelling the effectiveness of contraception for controlling introduced populations of elephant in South Africa

Re‐introduced African elephant (Loxodonta africana Blumenbach) populations are growing at very high rates in many of southern Africa’s reserves, have attained densities higher than previously thought possible and may be exhibiting irruptive growth. Active management of such populations is necessary to prevent the potentially negative effects on habitat and biodiversity that are associated with elephant overpopulation. One potentially feasible method of elephant management is immunocontraception, but very little is known about the long‐term effectiveness of this method. Using demographic data from three South African elephant populations, we made model projections of the effects of contraception on population growth rates to determine whether contraception may be a feasible management tool for elephant. In comparison with noncontracepted populations, realistic reductions in population growth rate after 20 years of contraception were projected to be up to c. 64%, with 50% being a very feasible target. Through its ability to reduce population growth rates, immunocontraception should be an effective tool for preventing or minimizing irruption in elephants and, perhaps, other introduced ungulate species.     

Responses of leaf C:N:P stoichiometry to water supply in the desert shrub Zygophyllum xanthoxylum

• Based on the elemental composition of major biochemical molecules associated with different biological functions, the ‘growth rate hypothesis’ proposed that organisms with a higher growth rate would be coupled to lower C:N, especially lower C:P and N:P ratios. However, the applicability of the growth rate hypothesis for plants is unclear, especially for shrubs growing under different water supply.
• We performed an experiment with eight soil moisture levels (soil water content: 4%, 6%, 8%, 13%, 18%, 23%, 26% and 28%) to evaluate the effects of water availability on leaf C:N:P stoichiometry in the shrub Zygophyllum xanthoxylum.
• We found that leaves grew slowly and favored accumulation of P over C and N under both high and low water supply. Thus, leaf C:P and N:P ratios were unimodally related to soil water content, in parallel with individual leaf area and mass. As a result, there were significant positive correlations between leaf C:P and N:P with leaf growth (u).
• Our result that slower‐growing leaves had lower C:P and N:P ratios does not support the growth rate hypothesis, which predicted a negative association of N:P ratio with growth rate, but it is consistent with recent theoretical derivations of growth–stoichiometry relations in plants, where N:P ratio is predicted to increase with increasing growth for very low growth rates, suggesting leaf growth limitation by C and N rather than P for drought and water saturation.

     

Colour polymorphism in relation to population dynamics of the leaf beetle, Chrysomela lapponica

We studied colour morph diversity and frequencies of light and dark morphs in non-fluctuating and fluctuating populations of willow feeding leaf beetle Chrysomela lapponica in the Kola Peninsula, NW Russia. Population-specific Shannon–Weaver diversity index positively correlated with dark morph frequencies, indicating that the larger part of colour polymorphism is related with numbers and diversity of dark morphs. Among-population variation in studied characters was not explained by pollution load or predation rates, but depended on the type of the population and the stage of density change in the fluctuating populations: both colour morph diversity and frequency of dark morphs were low in declining post-outbreak populations but equally high in non-fluctuating populations and in fluctuating populations at peak densities. In time-series, both diversity index and frequency of dark morphs decreased with post-outbreak density decline in the fluctuating population, but did not change in the non-fluctuating population. In the experiment, when adults received low quality food (plants from post-outbreak site), mortality of dark morphs during the hibernation was almost doubled relative to the mortality of light morphs, whereas on high quality food the colour morphs demonstrated similar mortality. This may indicate, that decrease in colour polymorphism extent and dark morph frequencies in the declining populations is due to selective mortality of dark morphs imposed by density dependent (induced by heavy herbivore damage during an outbreak) decrease in host-plant quality (delayed inducible resistance, DIR). DIR is known as one of the factors driving herbivore populations, but our result is the first evidence that DIR may act as a factor of natural selection. Dark morphs are not only susceptible to low food quality, but also have smaller size compared to light morphs, and therefore the dark females are presumably less fecund. Thus, decrease in frequency of low-fitness (dark) individuals in post-outbreak populations and accumulation of low-fitness phenotypes at the popu-lation peak may create feedbacks contributing to regulation of density fluctuations in Ch. lapponica.