Bivoltine Life Cycle Research Articles

Specific manifestations of polyvariant life cycles in ground beetles (Coleoptera and Carabidae) along a latitudinal gradient

The life cycles of Carabidae are highly diverse, and 25 variants of these cycles are realized In the European part of Russia, from semideserts to continental tundras. The diversity of the life cycle spectrum sharply decreases (by more than half) upon transition from nemoral to boreal forest communities, and its phenological unification takes place at high latitudes. The greatest proportion of species with polyvariant development (25%) is characteristic of temporal latitudes, which may be explained by relatively long growing season and considerable cenotic diversity. In both southern (semidesert and steppe) and northern regions (middle and northern boreal forests), this proportion does not exceed 5%. At low latitudes, the polyvariant pattern of development is often manifested in the form of facultative bivoltine life cycles or as facultative biennial life cycles in species with the initial "spring" breeding type.

Population structure and dynamics of Orchestia montagui (Crustacea: Amphipoda) in the Bizerte lagoon (northern Tunisia)

The population structure and dynamics of Orchestia montagui Audouin, 1826 (Crustacea: Amphipoda: Talitridae) were studied from the supralittoral zone of Bizerte lagoon for 1 year. Monthly field surveys were performed from July 2006 to June 2007. Individuals sampled were categorized by sex, size and abundance on each sampling date. Abundance varied during the study period, with a noticeable decrease in November. Ovigerous females were found throughout the year. The sex ratio changed throughout the year but, overall, the population was male biased. Analysis of the size frequency polymodal distribution suggested the presence of six cohorts on the first sampling date and 10 new ones during the next 11 months. Lifespan was estimated at 6–7 months. Cohorts born in summer and autumn tend to live longer than those born in winter. At this study site, O. montagui appeared as a semiannual species, with iteroparous females producing at least two broods, and exhibited a bivoltine life cycle.

Nesting ecology of Megachile (Chrysosarus) catamarcensis Schrottky (Hymenoptera: Megachilidae), a Prosopis-specialist bee

SummaryNests of the leafcutting bee Megachile (Chrysosarus) catamarcensis Schrottky (Hymenoptera: Megachilidae) were obtained during a 24 month trap-nesting programme in a xeric forest in the Province of La Pampa, Argentina. Nests were constructed in December and January; females made an average of 4.0 ± 2.2 cells, and used mud with flower petals and/or leaf pieces to build their nests. Provisions of cells were practically pure pollen mass of Proposis caldenia. Adult emergence showed a unimodal pattern, suggesting a univoltine life cycle. However, two individuals that emerged a month after the traps were collected indicate the existence of a small early-summer generation. This could indicate that M. catamarcensis, in some years (warmer) or in some populations (at lower latitudes), could exhibit a bivoltine life cycle. Approximately 15.3 % of all offspring failed to complete development to the adult stage, and an additional 13.9 % were killed by natural enemies. These included diverse parasitic wasps (Chrysididae, Sapygidae, and Leucospidae), a cleptoparasite bee (Megachilidae: Coelioxys), and a bee fly (Bombyliidae: Antrhax).

Life cycle and population dynamics of Idotea balthica basteri (Pallas, 1772), Isopoda Valvifera from the Bizerte lagoon (Southern Mediterranean Sea - Tunisia)

The population dynamics and life cycle of Idotea balthica basteri (Pallas, 1772) were studied at the Menzel Jemil (lagoon of Bizerte, Tunisia) based on monthly samples taken from October 2009 to September 2011. The presence of juveniles and gravid females throughout the sampling period showed that this species exhibits continuous reproduction. Sexual differentiation occurs in approximately 43–59 days. Females took 88–160 days (3–5.5 months) to reach sexual maturation, and males took 102 days (3.5 months). The fecundity and fertility values exhibited a great variation in the number of eggs and embryos or mancae inside the marsupium which was correlated with the length of the gravid females. The sex ratio underwent fluctuations throughout the sampling period but was always in favour of females. Size frequency distributions were analysed recognizing 24 cohorts during the sampling period. Six cohorts were identified in October 2009 and 18 cohorts in November 2009–September 2011. Among these latter cohorts, nine were tracked till they disappeared. The minimum average size of new cohorts ranged from 1.92 ± 0.31 to 2.97 ± 0.26 mm and the maximum values ranged from 12.83 ± 0.96 to 19.23 ± 0.56 mm. Life span was estimated at 10–14 months. I. balthica basteri, in the lagoon of Bizerte, is a semi-annual species with iteroparous females, and a bivoltine life cycle that produces two generations per year.

Population decline and life‐cycle changes in a phantom midge (Chaoborus flavicans) after introduction of planktivorous fish

Summary Eurasian perch (Perca fluviatilis), a non‐native planktivorous fish species in western Norway, has been released into Lake Myravatn, which previously lacked planktivorous fish. Prior to the introduction, the zooplankton community was structured by predation by larvae of the phantom midge (Chaoborus flavicans), which maintained a bivoltine life cycle in the lake. We hypothesised that the perch introduction would diminish the density and alter the bivoltine life‐cycle pattern of Chaoborus. To assess this, we used time series data from before (1983) and after (2007–10) the perch introduction. The perch appears to have effectively exterminated the previously dominant predator C. flavicans within 4–5 years of introduction into the lake. The density of the larvae in the water column was high in 1983 as well as in 2007, with maximum levels recorded in August (up to 15 400 individuals m−2). In 2008, larval density was lower than in previous years, but the most dramatic decline occurred in 2009 (maximum 212 individuals m−2). In 2010, the density was very low (maximum 11 individuals m−2), and after July 2010, no larvae were found in the water column. The number of larvae residing in the sediments also declined quickly after the perch introduction. In 1983 and 2007, C. flavicans produced two generations per year. Following the first emergence and reproduction in May, young larvae appeared in June and developed rapidly to emergence and reproduced again in August. Young larvae appeared again in August–September. In 2008, young larvae appeared only in June with no second emergence and reproduction. In 2009, young larvae did not appear until July, and again no further reproduction took place. Only a few instar IV larvae were recorded in 2010. The collapse in life cycle into a univoltine pattern occurred prior to the main population decline. We conclude that direct predation by perch on Chaoborus larvae is the most likely explanation for the observed changes in their life cycle and density. It appears that the C. flavicans population of Lake Myravatn is approaching extinction.

Hatching phenology and voltinism of Heterocypris barbara (Crustacea: Ostracoda) from Lampedusa (Sicily, Italy)

The persistence of crustacean populations in ephemeral ponds requires appropriate adaptations in life history strategies (e.g. in hatching phenology). Organisms take advantage of pond filling when it occurs and hedge their bets for the possibility to complete one or more life cycles or to produce resting stages that ensure that the population will not go extinct. We carried out laboratory experiments to investigate the dynamics of a sexual population of Heterocypris barbara from a vernal pool in Lampedusa Island (Sicily). Experimental organisms were obtained hydrating sediments from Aria Rossa temporary pond. Recruitment from resting eggs, voltinism, mean body size and sex ratio were observed in microcosms at different conductivities (high 2.0-2.7 mS cm-1, intermediate 1.0-1.1 mS cm-1 and low 0.5-0.6 mS cm-1). Microcosms were kept in laboratory controlled conditions: constant (24°C 12:12 L:D and 16°C 10:14 L:D photoperiod) or fluctuating thermal regimes. The experiment lasted 7 months. Resting and non-resting egg production and up to a bivoltine life cycle were observed. Recruitment events from egg bank and voltinism varied by thermal regime and conductivity. A prolonged recruitment phase occurred in conditions that could be considered a proxy of a rainy season (16°C, 10:14 L:D and low conductivity) or of long hydroperiods (spring thermal fluctuating regime and intermediate conductivity). At 24°C, age at reproduction of females from resting eggs almost doubled at low conductivity (in comparison with high conductivity). Low conductivity also reduced hatching time of resting eggs while it increased development time and age at maturity. In thermal fluctuating regime, degree-days to reproduction were about double than at constant 24°C. Males, observed in all microcosms, reached maturity faster and had a shorter life span than females. Males initially outnumbered females, but later in the experiments females became dominant. We also evaluated the effect of conductivity and thermal regimes on other traits like recruitment from egg bank, voltinism and sex ratio. We discuss whether our observations on a small population from temporary pools could be considered part of a diversified bet-hedging strategy.

Population dynamics of mayflies in a constant temperature spring stream in West Carpathians

We examined the life history and secondary production of four Ephemeroptera species ( Baetis alpinus Pictet, 1843–1845, Baetis rhodani Pictet, 1843–1845, Rhithrogena carpatoalpina Klonowska, Olechowska, Sartoriet & Weichselbaumer, 1987 and Habroleptoides confusa Sartori & Jacob, 1986) in a temperature stable cold spring stream at Prosiek valley (Chočské vrchy Mts., West Carpathians, Slovakia). We have found asynchronous bivoltine life cycle for the most abundant species B. alpinus with growth rate positively correlated to photoperiod length. R. carpatoalpina have shown unusual asynchronous univoltine life cycle and B. rhodani have shown uncommonly low abundance in mayfly community. Total secondary production of mayfly community was very low, reaching 1654.8 mg DW m −2 y −1 . We suggest that the observed asynchrony in growth could be related to the lack of temperature control.

The Life Cycles of Mayflies of the Eastern Ukraine. Subfamily Baetinae (Ephemeroptera, Baetidae)

Abstract The life cycle types of mayflies of the subfamily Baetinae in Eastern Ukraine are determined. The scheme for identification of larval age structure is given. The age structure changes of Baetis braaschi, B. nexus and Alainites muticus larvae during the year are described in detail. The flight periods of imagoes throughout the studied region are delimited for almost all species of the subfamily. B. braaschi is shown to have the bivoltine life cycle, with its populations overwintering as the larval stage (MBws) independently on temperature conditions in the water body. The peculiarities of oviposition are described for this species. The following aspects of life cycles were revealed: nymphs, subimagoes and imagoes of different generations of B. braaschi differ considerably in size; the voltinity of B. vernus in the region can vary (Us and/or MBss), mainly due to temperature conditions of the water bodies, where its development takes place; geographically isolated populations of A. muticus in the Eastern Ukraine reproduce only by thelytoky

Influence of riparian cover on stream temperatures and the growth of the mayfly Baetis rhodani in an upland stream

This research aimed to assess whether stream temperature and the size of Baetis rhodani larvae (Insecta: Ephemeroptera) differed between sites with contrasting riparian vegetation cover. Stream temperatures and Baetis sizes were monitored at sites along a 5-km section of a small (30 km2) upland Scottish stream. Some sites had riparian tree cover dominated by mature broadleaved woodland, while others were located in areas of open Calluna moorland. The 13 month data period included two contrasting summers, one typical for the region and the other unusually warm. Daytime water temperatures were generally highest at the open moorland sites. The difference between moorland and broadleaved woodland sites was most apparent in the summer months, with the woodland shading apparently acting to reduce maximum temperatures and mediate temporal fluctuations. Size data suggest that Baetis has a bivoltine life-cycle in the stream. Significant differences in the mean size of Baetis were found between moorland and woodland sites, with differences occurring primarily during the summer months. Mean size of Baetis was significantly greater in the moorland during the time of summer adult emergence. During the months that followed (growth of the next generation), Baetis larvae in the woodland sites were significantly larger. Size distribution data for the population suggest earlier adult emergence in the moorland section and different egg-to-larvae hatch times. Overall, the results indicate that subtle differences in growth and life cycles, linked to differences in riparian cover and streamwater temperature, can occur over short distances in upland streams.

A new Arthula Cameron (Ichneumonidae, Cryptinae) parasitoid of Ropalidia plebeiana Richards (Vespidae) and host of Amoturoides breviscapus Girault (Torymidae) (Hymenoptera)

Arthula plebeja Ubaidillah and Kojima, sp. nov., a parasitoid of the Australian paper wasp Ropalidia plebeiana Richards, and host of the torymid hyperparasitoid Amoturoides breviscapus Girault, is described and illustrated. Both A. plebeja and A. breviscapus are estimated to have a bivoltine life cycle, the first overwintering in the pupal stage, and the second in the prepupal and/or pupal stage.

Northerly shift in voltinism watershed in Hyphantria cunea (Drury) (Lepidoptera: Arctiidae) along the Japan Sea coast: Evidence of global warming?

The effects of climate change on the northern limit of the trivoltine area were clarified in the fall webworm, Hyphantria cunea (Drury) (Lepidoptera: Arctiidae). The Takaoka and Kanazawa populations used in the present study occur in locations to the north of Fukui (36.07°N), where H. cunea has recently shifted from a bivoltine to a predominantly trivoltine life cycle. The life-history traits of these populations were investigated and compared to those in the Fukui population. The lower threshold temperature for development and the thermal constant for one generation were 11.3°C and 674.5°d in the Takaoka population, and 11.2°C and 680.7°d in the Kanazawa population. The critical photoperiod for diapause induction at 25°C was 14 h 34 min in the Takaoka population and 14 h 28 min in the Kanazawa population. These critical photoperiods at 25°C were longer by 24 min and 18 min than the Fukui population, respectively. These results, together with climate data, suggest that the Takaoka population maintains a bivoltine life cycle, and a small part of the Kanazawa population has three generations per year, a predominantly bivoltine life cycle. Thus, the present northern limit of the trivoltine area lies around Fukui in districts along the Japan Sea.

Alternative life histories in the water strider Gerris lacustris: time constraint on wing morph and voltinism

AbstractThe wing dimorphic water strider Gerris lacustris L. (Heteroptera: Gerridae) switches to a bivoltine life cycle under favorable climatic conditions. The switch in voltinism is accompanied by a reduction of wing development in the directly reproducing midsummer generation, while the diapausing generation has a high fraction of long‐winged individuals. We investigated whether the thermal energy (degree‐days) available in natural habitats constrains the combination of developmental pathway and wing morph. Offspring of G. lacustris were reared under quasi‐natural conditions at two temperature regimes to determine the thermal constant k required to complete adult development in either wing morph. The thermal constant for egg‐to‐adult development of the short‐winged morph was about 20% lower than of the long‐winged morph. Based on the results from the outdoor laboratory experiment, we calculated the total degree‐days necessary to complete the possible combinations of wing morph pattern and voltinism. Comparison of these estimates with the thermal energy actually available during the reproductive season of 2004 for various natural habitats (sun‐exposed field ponds and shaded forest ponds) suggests that voltinism as well as wing morph pattern is strongly limited by the number of degree‐days available in these habitats. On forest ponds, only univoltine life cycles were possible, whereas on field ponds temperature allowed bivoltine life cycles. However, only the eggs laid at the very beginning of the season had the potential to accumulate enough degree‐days to complete a bivoltine life cycle with both generations long‐winged. We conclude that thermal energy is the main environmental constraint limiting voltinism of populations in the two habitat types. Furthermore, the available thermal energy also seems to influence the determination of the seasonal wing pattern in G. lacustris.

Life history of a dipteran predator (Scathophagidae: Acanthocnema) of insect egg masses in a northern California stream

Summary1. Little is known about the predators of insect eggs in fresh waters. This study describes aspects of the life history of a scathophagid fly (Acanthocnema sp.), whose larvae are predators of aquatic insect eggs.2. Because the Acanthocnema predator oviposits its eggs on the surface of aquatic insect egg masses, all insect egg masses were collected regularly within a 200‐m reach of Redwood Creek (California, U.S.A.) between September 2003 and June 2007. Acanthocnema predators were found predominantly within egg masses of the caddifly Neophylax rickeri (Trichoptera: Uenoidae).3. There was a mean of 0.25 Acanthocnema individuals per N. rickeri egg mass (n = 2367 egg masses). In general, N. rickeri egg masses were more commonly found clustered in aggregations (93.7%) than singly (6.3%), and Acanthonema were found more often within the aggregations of N. rickeri (98.7%) compared to singly laid egg masses (1.3%).4. The duration of the Acanthocnema predator life stages was: egg 2.9 ± 0.8 (mean ± SD) days, larva 15.6 ± 10.2 days, pupa 80.3 ± 24.9 days and adult 7.2 ± 4.8 days. The short duration of the Acanthocnema egg stage (1–7 days) compared to that of its prey N. rickeri (2–4 weeks) raises the probability that the undeveloped eggs of N. rickeri would be available to the young predators upon hatching. Egg consumption of N. rickeri eggs by Acanthocnema averaged 262.6 eggs per larval period.5. Acanthocnema had a bivoltine life cycle in which the first generation fed exclusively on N. rickeri egg masses in the winter and the second generation fed on the egg masses of several species, including other Trichoptera (Brachycentridae) and Diptera (Ceratopogonidae, Chironomidae) in the spring. These findings suggest differing feeding strategies by the first and second generations of Acanthocnema in response to the seasonal availability of prey species. This type of autecological information is important for understanding mechanisms of community interactions.

Shifting of the life cycle and life‐history traits of the fall webworm in relation to climate change

AbstractThe effects of global warming on the life cycle and life‐history traits of the fall webworm, Hyphantria cunea (Drury) (Lepidoptera: Arctiidae), were investigated in Fukui, Japan. Our previous studies showed that the Fukui population had a predominantly bivoltine life cycle, but recently the life cycle has changed. In the present study, the life‐history traits of individuals collected in 2002 in Fukui were clarified and compared to those in the previous studies. The lower threshold temperature for development and the thermal constant for one generation were 10.6 °C and 724.4 degree days, respectively. Although these developmental parameters were not investigated in the previous studies, the difference of the developmental period between the present and previous results was negligible in the larval and pupal stages at 20 °C. The critical photoperiod for diapause induction was 14 h 29 min at 20 °C and 14 h 10 min at 25 °C. The critical photoperiod at 25 °C was shortened to 14 min from that of the previous studies for individuals collected in 1995. The incidence of pupal diapause in the second generation was investigated in individuals that were collected as fourth and fifth instars in the field. Some portion of individuals averted diapause even if they were kept under a short photoperiod of L14:D10 at 25 °C after collection. These results, together with climate data and field observations in Fukui, suggest that at least a part of the population has three generations per year at present. This shift of the life cycle occurred within 7 years and is probably related to global warming in recent years.

BIOLOGY, POPULATION DYNAMICS, AND SECONDARY PRODUCTION OF TALITRUS SALTATOR (AMPHIPODA, TALITRIDAE) AT KORBA BEACH (EAST COAST OF TUNISIA)

[The biology and population dynamics of Talitrus saltator (Montagu, 1808), a talitrid amphipod of the supralittoral zones of sandy beaches, was studied from December 2001 to December 2003 at Korba beach, Tunisia. Ovigerous females and the emergence of new cohorts were observed during the warmer months, with recruitment taking place from February-March to November. The population thus showed a seasonal pattern of reproduction, with stronger breeding during summer. The sex ratio was slightly in favour of females. Mean brood size was 11 and fecundity appeared to be positively correlated with female size. Life span was estimated at 6 to 9 months depending on the time of birth, with cohorts born at the beginning of the reproductive period tending to have shorter lives than those born later in the season. The present two-year study was consistent with previous preliminary results obtained at Zouara (Tunisia) (Marques et al., 2003), confirming Talitrus saltator as a semiannual species, with iteroparous females and a bivoltine life cycle. Growth production (P) was estimated at 0.41 g m–2 y–1 AFDW (ash free dry weight) (9.62 kJ m–2 y–1). Elimination production (E) (summation of losses) was estimated at 0.37 g m–2 y–1 AFDW (8.9 kJ m–2 y–1). The average annual biomass (B) was estimated at 0.073 g mm–2, resulting in a P/B ratio of 5.6 and an E/B ratio of 5.07., The biology and population dynamics of Talitrus saltator (Montagu, 1808), a talitrid amphipod of the supralittoral zones of sandy beaches, was studied from December 2001 to December 2003 at Korba beach, Tunisia. Ovigerous females and the emergence of new cohorts were observed during the warmer months, with recruitment taking place from February-March to November. The population thus showed a seasonal pattern of reproduction, with stronger breeding during summer. The sex ratio was slightly in favour of females. Mean brood size was 11 and fecundity appeared to be positively correlated with female size. Life span was estimated at 6 to 9 months depending on the time of birth, with cohorts born at the beginning of the reproductive period tending to have shorter lives than those born later in the season. The present two-year study was consistent with previous preliminary results obtained at Zouara (Tunisia) (Marques et al., 2003), confirming Talitrus saltator as a semiannual species, with iteroparous females and a bivoltine life cycle. Growth production (P) was estimated at 0.41 g m–2 y–1 AFDW (ash free dry weight) (9.62 kJ m–2 y–1). Elimination production (E) (summation of losses) was estimated at 0.37 g m–2 y–1 AFDW (8.9 kJ m–2 y–1). The average annual biomass (B) was estimated at 0.073 g mm–2, resulting in a P/B ratio of 5.6 and an E/B ratio of 5.07.]

Habitat preference and life history of the mayflies Metamonius anceps Eaton (Nesameletidae) and Meridialaris chiloeensis Demoulin (Leptophlebiidae) in a Patagonian mountain stream

The habitat preferences, temporal dynamics in density and the life history of two species of mayflies, Metamonius anceps and Meridialaris chiloeensis, were investigated in a Patagonian mountain stream over a period of one year. Benthic samples were taken in a low order Nothofagus forested stream. M. anceps was univoltine with small nymphs predominating in winter and emergence occurring in summer. M. chiloeensis showed a bivoltine life cycle with a winter and summer generation, and emergence period in spring and summer. Both species were significantly more abundant in the low water period than in the high discharge season. Gravels supported significantly more individuals of M. chiloeensis than sand in pools and leaf packs. Temporal overlap of abundance between species was low. The existence of different life history strategies and the temporal shift of main generations and density peaks were critical to allow species coexistence.

Optimal diapause strategies of a grasshopper, Melanoplus sanguinipes

Previous analyses of diapause in insects have most often focused on the timing of the switch from non-diapausing to diapausing offspring in bivoltine populations and have assumed that diapause is irreversible or that the insect cannot survive winter if not in diapause. Many insects exhibit more flexibility in their life cycles, such as the age at which diapause begins, and facultative diapause, that may influence the evolution of different diapause strategies in different environments. The grasshopper Melanoplus sanguinipes F. (Orthoptera: Acrididae), has a very wide geographic range over which diapause characteristics vary greatly. Embryonic diapause in this species may be under maternal control, may be obligate or facultative (i.e., may be averted by cold temperature treatment of pre-diapause embryos), and embryos may enter diapause at different ages. Diapause traits were examined in two populations of M. sanguinipes from very different environments. In the population from a temperate climate (Idaho, USA), diapause was facultative, i.e., pre-diapause embryos averted diapause when held at 5° C for 90 days at all ages tested (7 days and older). The Idaho embryos entered diapause in late stage of development if held at 22° C for 30 days or more. In populations from subarctic Alaska, USA, embryos also entered diapause in a late stage of development, but diapause was obligate and could not be averted by chilling in the pre-diapause stages. Simulated evolution of these traits over a wide range of season-lengths showed that late stage diapause is an essential trait in very short season environments, resulting in early hatching, and a semivoltine life-cycle. Facultative diapause enabled bivoltinism to be a viable strategy in shorter seasons than when diapause was obligate. At transitions from semivoltine to univoltine, and from univoltine to bivoltine life cycles, populations with obligate diapause adopted a strategy of no diapause (via maternal effects) to enable univoltine life cycles.

オオメカメムシＰｉｏｃｏｒｉｓ ｖａｒｉｕｓ（Ｈｅｔｅｒｏｐｔｅｒａ：Ｌｙｇａｅｉｄａｅ）の茨城県および千葉県における生活史

The life history of the big-eyed bug, Piocoris varius, was investigated in two locations in Ibaraki and Chiba, Japan in 2004 by observing seasonal changes in the stage composition of field populations. The nymphal instar of field-collected samples was identified by comparing their head capsule widths with those of laboratory-reared individuals. P. varius nymphs and adults were found on 19 plant species (12 families) of 24 species (14 families) on which the population census was made. Overwintering adults became active on plants in May and disappeared in July. First-generation young nymphs appeared in late May and June. They developed into old nymphs in late June and then emerged as adults in late July and August. After August, adults predominated until December, although a small proportion of second-generation nymphs occurred in August and September. Only adults were found under the litter in December. These observations indicate that P. varius is basically a univoltine species with a partial bivoltine lifecycle in the Kanto district in Japan.

Life‐cycle phenology of some aquatic insects: implications for pond conservation

1. Life-cycles and growth patterns were determined for 21 dominant aquatic insect species in small permanent ponds in an arid, karstic region (SW France, site fr7300909 of the Natura 2000 conservation network). The species studied are widely distributed throughout Europe, but some life-cycle patterns are reported here for the first time. 2. The life-history patterns of the 21 species can be divided into five main types: (i) a semivoltine cycle spreading over 2 years; (ii) slow univoltine cycles; (iii) fast univoltine cycles; (iv) multivoltine life-cycles with a long winter generation and two or three summer generations per year; and (v) bivoltine life-cycles with two fast generations per year. Growth was either exponential over the whole developmental period for a given cohort, or divided into two or three successive periods during each of which the growth rate was fairly constant. 3. Biodiversity estimates strongly depend on the temporal and spatial scale over which observations are made. Ponds thus provide useful models to show how life-history patterns enable many temporally segregated populations to utilize small ecosystems. Conservation frameworks should therefore carefully consider the time-frame needed to survey ponds, as many species with fast cycles could be overlooked. The spatial scale needed to manage threatened habitats and thus preserve pond networks must be broadened, rather than attempting to target individual water bodies for particular management actions. Copyright © 2005 John Wiley & Sons, Ltd.

Life History Adaptation to Changeable Agricultural Habitats: Developmental Plasticity Leads to Cohort Splitting in an Agrobiont Wolf Spider

The wolf spider, Pardosa agrestis Westring (Araneae: Lycosidae), is a dominant arthropod predator in central European agricultural areas. Recent studies have indicated that this spider has a unique phenology among Pardosa species. Three years of systematic field monitoring of a P. agrestis population in Hungary confirmed that an early summer peak (P1) of adults typical of the Pardosa genus is followed by a second late summer peak (P2) that does not occur among congenerics. Rearing experiments conducted outdoors revealed that offspring from P1 can reach maturity in ≈2 mo time, well before the end of summer. Thus, P2 adults represent a second generation of the species, with a chance for a potentially successful reproduction. However, the speed of development was variable among P1 offspring; individuals, which proved to be slower developers, did not mature during P2; rather they overwintered as juveniles and matured during the following year P1 period. Thus, members of the P1 cohort followed different developmental pathways; this corresponds to a cohort-splitting strategy. Splitting occurred even among spiderlings originating from the same clutch (cocoon) at the beginning of P1. In contrast, spiderlings born at the end of P1 uniformly matured by next spring. Splitting among offspring spreads the mother’s reproductive success between risk averse slow-developing and risk prone fast-developing offspring. The facultative bivoltine life cycle arising through cohort splitting may allow P. agrestis to adapt to predictably ephemeral agricultural habitats by flexibly producing the most dispersive juvenile stages in synchrony with the appearance of vacated habitats.