Phenology affects nearly all aspects of ecology and evolution. Virtually all biological phenomena—from individual physiology to interspecific relationships to global nutrient fluxes—have annual cycles and are influenced by the timing of abiotic events. Recent years have seen a surge of interest in this topic, as an increasing number of studies document phenological responses to climate change. Much recent research has addressed the genetic controls on phenology, modelling techniques and ecosystem-level and evolutionary consequences of phenological change. To date, however, these efforts have tended to proceed independently. Here, we bring together some of these disparate lines of inquiry to clarify vocabulary, facilitate comparisons among habitat types and promote the integration of ideas and methodologies across different disciplines and scales. We discuss the relationship between phenology and life history, the distinction between organismal- and population-level perspectives on phenology and the influence of phenology on evolutionary processes, communities and ecosystems. Future work should focus on linking ecological and physiological aspects of phenology, understanding the demographic effects of phenological change and explicitly accounting for seasonality and phenology in forecasts of ecological and evolutionary responses to climate change.

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Toward a synthetic understanding of the role of phenology in ecology and evolution

Despite the ubiquity of dormancy among invertebrates, the demographic ties between the active and dormant stages remain largely unexplored in most systems. Nevertheless, dormant stages could potentially have significant impacts on the ecology and evolution of the active population. In Oneida Lake, New York, populations of the suspension-feeding crustaceans Daphnia galeata mendotae and Daphnia pulicaria produce large numbers of long-lived diapausing eggs, which accumulate in the sediments to densities of over 2.5 × 104 eggs/m2 for D. galeata mendotae and 8.0 × 104 eggs/m2 for D. pulicaria. Sedimentation rates obtained by 210Pb dating suggest that these eggs can remain viable for >125 yr and that these two Daphnia populations have persisted in this system for >200 yr. Total annual production of new diapausing eggs can range from 0 to over 104 new eggs/m2 of sediment. Annual emergence rates back to the water column are also variable but only range between 0 and 25 Daphnia/m2. Because annual variation in the ...

Interspecific variation in the abundance, production, and emergence of daphnia diapausing eggs

The factors inducing male offspring and the mechanism of sex-ratio adjust- ment were investigated in Daphnia magna, a cyclically parthenogenetic cladoceran (Crus- tacea). Laboratory experiments were conducted on individual animals living in flow-through chambers, to separate the effects of various density-dependent factors. Fluctuating food levels had no effects on offspring sex when starvation was not severe. Water from crowded populations of different Daphnia species contained substances that induced up to 43% male broods. The chemically mediated crowding effect was detectable under conditions repre- senting a low population density, implying that animals responded to their own metabolites. Daphnia species differed in their inducing effects, and in one case (water from D. pulex) strong male offspring induction was associated with growth inhibition. A short daylength was also demonstrated to induce male offspring effectively. Evidence was found to support a mechanism of sex-ratio regulation that implies a regular alternation of brood sex, resulting in a sex ratio close to 1/2, the expected optimum ratio. Even individual mothers adjusted the sex ratio of their offspring within a sequence of seven broods. No resting eggs were formed in the experiments, indicating that formation of male offspring and resting eggs are independently controlled, possibly by distinct sets of environmental cues.

Sex Determination in Daphnia Magna

SUMMARY. A method is described whereby ephippia (sexual eggs plus protective membranes) are experimentally induced at low food levels (≤ 0.05 mg spinach D. magna-1), high female culture-densities (≤ 0.4 D. magna ml-1) and in short-day photoperiods (L:D < 12:12, 12°C). The density-dependent ephippial response was related to an increased encounter rate between females at the higher densities. External metabolites had no significant effect on ephippia production. Ephippia were formed in the second generation after exposure to short day-lengths. Five clones from the same population exhibited genetic variation in their critical photoperiods for induction of sexual reproduction (≥= 10% ephippia). There was evidence of a genetic difference in photoresponse between two populations of D. magna. The evolution of timing mechanisms for ephippia production are discussed in relation to the predictability of environmental change.

The effect of food availability, female culture-density and photoperiod on ephippia production in Daphnia magna Straus (Crustacea: Cladocera)

SUMMARY. 1. Resting eggs (ephippia) of Daphnia galeata, D. hyalina, D. cucullata and D. galeata X D. hyalina hybrids were collected from the upper 8 cm of sediments at various water depths from two glacial lakes in the Northern Federal Republic of Germany.



2. The horizontal distribution of ‘intact’ ephippia (i.e. those containing two apparently healthy eggs) was extremely patchy, with mean densities in the upper 8 cm ranging from 1325 to 113,907 m−2 (coefficient of variation, C=65.5%; Schohsee, Holstein) and 25,343–159,143 m−2 (C=58.5%; Kel-lersee. Holstein). Ephippial densities were greatest in sediments from deep waters. The highest densities of intact ephippia were in the upper 4 cm of sediment, with progressively fewer with increasing depth.



3. A proportion of eggs obtained from sediments during autumn (September-November 1985) and spring (March-May 1986) were exposed to continuous light (approximately 18 W m−2) at three temperatures (6, 12, 20°C), and the incidence of hatches recorded.



4. Hatching success was consistently low (maximum 14.4%), with most hatching occurring at 12°C. There was no significant difference between the hatching success of ephippia collected during autumn and spring. Decapsulation of ephippial eggs inhibited hatching. Hatching was highly synchronous within each treatment, and the pattern did not differ significantly with sediment depth, though hatching success was lower in deep sediments. A huge reservoir of ephippia is present in lake sediments, of which a proportion may remain viable for many years. Periodic recruitment of hatchlings to the pelagic population may provide an important mechanism for the maintenance of genetic diversity in Daphnia populations, as well as affecting rates of evolutionary change.

Resting eggs of lake‐ Daphnia I. Distribution, abundance and hatching of eggs collected from various depths in lake sediments

1. The initiation of sexual reproduction and duration of diapause in the resulting embryo was studied in a population of Daphnia pulex Leydig that restricts such to the autumn. Initiation is under the control of a density associated stimulus that is permitted to function only in short-day photoperiods. In short days the proportion of sexual broods is linearly related to the log of culture density.2. Critical photoperiod (50% sexual broods) at 19° C. was L:D 12 3/4:11 1/4, and only 15 minutes longer at 12° C. The prenatal life of the mother is the more sensitive to photoperiodic induction of sexual reproduction. Exposure to short days after birth may permit a large percentage of parthenogenic broods in which the necessary males are produced, thereby providing males in advance of sexual reproduction in the population. In long days the mothers quickly revert to parthenogenesis, and the extent of reversal is apparently related to age of the grandparent when the mother was born.3. Sexually derived embryos from...

R. G. Stross

Papers

Photoperiod control of winter diapause in the fresh-water crustacean, daphnia