Population dynamics and body-size selection in Daphnia

Transcription

1 LIMNOLOGY AND OCEANOGRAPHY January 12 Volume 37 Number 1 Limnol. Oceanogr., 37(l), 12, , by the American Society of Limnology and Oceanography, Inc. Population dynamics and body-size selection in Daphnia Alan J. Tessier, Andrew Young, and Mathew Leibold W. K. Kellogg Biological Station and Department of Zoology, Michigan State University, Hickory Comers Abstract We examined the population dynamics and seasonal change in body size of a population of Daphnia galeata mendotae experiencing strong vertebrate predation. The intensity of food limitation was quantified seasonally by in situ food addition. We also collected live animals early and late in the summer for laboratory studies designed to estimate genetic variation and seasonal change in life-history traits for the population. The population was characterized by nearly equal short-term birth and death rates that were strongly correlated with population density, suggesting density dependence. The population generally exhibited very high birth and death rates and no significant response to experimental food addition. The one exception was June 188, when the population achieved high densities and exhibited strong food limitation. Throughout both summers, minimal and average body sizes of adults decreased. Laboratory studies document substantial genetic variation for body size and other life-history traits among animals collected early in summer. By late summer, however, there was little genetic variation for these same traits. Furthermore, clones collected early in summer were genetically larger in body size at birth and maturity than clones collected in late summer. We interpret these results as indicating natural selection for body size. Size-selective predation is one of the best understood processes influencing population dynamics and community structure of zooplankton (&ret 180; Kerfoot and Sih 18 7). Most planktivorous predators select prey on the basis of body size, although body shape, pigmentation, and swimming motion are also very important. The response of a prey population to predation depends, however, not only on the intensity and se- I Present address: Department of Ecology and Evolution, University of Chicago, Chicago, Illinois Acknowledgments We thank Jennifer Molloy who improved an early draft of this paper and Mike Lynch and Nelson Hairston, Jr., who as reviewers significantly improved the manuscript. This work was supported by NSF grants BSR and BSR to A. J. Tessier and is W. lectivity of predation, but also on the lifehistory traits of the prey (Taylor 180). The importance of life history is apparent when one considers the demographic consequences of size-selective predation. Predators can alter prey age-structure directly by determining size-specific mortality and indirectly by changing resource availability, and hence age-specific fecundity of prey (e.g. Vanni 186). The impact of predation is determined, therefore, not just by prey size, but by how size relates to age and fecundity (Taylor 180). For many zooplankton species these relationships are not fixed; life histories can be altered by the environment, including the presence of predators (e.g. chemical induction). A large literature on phenotypic plasticity and cyclomorphosis documents this important class of prey re- sponse to predation (e.g. Have1 187). K. Kellogg Biological Station Contribution 707. In contrast to the numerous ecological 1

2 2 Tessier et al. studies of demographic or induced prey responses, evolutionary responses (i.e. population genetic changes) of zooplankton to predation have received far less attention (e.g. Black 180; Hairston 187). The best zooplankton examples of body-size adaptation to predation are largely comparative among populations or species (e.g. HrbaEek and HrbaEkova-Esslova 160; Kerfoot 17 5; Wyngaard 186; Stemberger and Gilbert 187). Despite much ecological work on size-selective predation, there are few studies that explore body-size evolution within single zooplankton populations (Lynch 184a). In this paper we combine field and laboratory studies to examine the consequences of strong vertebrate predation with respect to the evolution of body size in a natural population of Daphnia galeata mendotae. We first quantify seasonal population dynamics and reduction in adult sizes that document the importance of predation. We then quantify the amount of genetic change in body size and other lifehistory traits that occurred during one summer in this population. Our results suggest that the relationships between body size and other life-history traits based on interspecific comparisons may not adequately predict how selection operates within a species. Study site We studied the population of D. galeata mendotae in Three Lakes III, a natural, glacial lake in Kalamazoo County, southwestern Michigan. It is a small (surface area, 14.3 ha), shallow (Zmax = 4 m, Zmean = 1.6 m), hard-water, mesotrophic lake that has a single stream inflow and outflow, both on the east side of the lake. Water flux through the lake, as a proportion of lake volume, ranges from 0.4 d-l in spring to ~0.03 d-l in July, August, and September (Tague 177). A small culvert on the west side connects to another natural lake. Three Lakes III exhibits no thermal stratification in summer; water temperatures varied between 20 and 25 C from June through August. The summer plankton community consists of generally small-bodied crustaceans (Ceriodaphnia dubia, Diaphanosoma birgei, Chydorus sp., D. galeata mendotae, Diap- tomus pallidus, and Mesocyclops edax) and rotifers (Keratella, Polyarthra, and Ascomorpha spp.). Daphnia is the largest herbivore in the plankton. Invertebrate predators include Chaoborus and Leptodora kindtii. Fish (especially Lepomis macrochirus and Labidesthes sicculus) constitute the most important zooplanktivores (Leibold and Tessier 1 1). Methods Field studies- We sampled the D. galeata mendotae population during June, July, and August 188 and 18. In 188, we sampled at three sites in the lake, three times a week on alternate weeks. In 18, sampling was more extensive, two or three times a week at four sites throughout the 3 months. We used a conical net (0.22-m diam, 80- pm mesh) to collect D. galeata mendotae during midday. A vertical net tow was taken from lake bottom to surface at each site. Samples were preserved with cold sugar- Formalin and stored on ice until analysis. Samples were examined at 25 x with a Wild stereomicroscope; we counted total D. galeata mendotae, and randomly selected animals from each site for measurement of body length (most anterior point on head to base of tail), clutch size, egg developmental stage, and lipid-ovary index. Samples from the different sites were analyzed separately and averaged to estimate population mean abundance and among-site variance. Abundance is presented as number per tow, which equals m2 surface area or 133 liters, assuming 0% net collection efficiency. In 18, we included a minimum of 40 egg-carrying adults in our measurements of body length, clutch size, and egg stage from each site (total > 160 adults each date). From this information, minimal and average body lengths of eggcarrying adults were identified for each date. We used information on population abundance, fecundity, and water temperature collected at 2-3-d intervals to estimate net population growth rates (r) and instantaneous birth (b) and death (d) rates. We used the detailed information on egg-stage distributions for each date and the procedure recommended by Rigler and Downing (184) to estimate instantaneous birth rate for each sampling date. Weekly means and

3 Body-size selection 3 variances in our estimates of population growth and birth rates were calculated from the multiple sample dates each week. The differences between these weekly mean rates were used to estimate death rates. The relationships among birth rates, death rates, adult fecundity, juvenile body lipid, and population abundance were examined with Pearson product moment correlation. Size of egg-carrying adults (minimum and average) was related to day-of-year with Spearman s rank order correlation. Four times in 188 and once in 18 we conducted field enclosure experiments to estimate the intensity of food limitation being experienced by the Daphnia population. For each experiment we set up four polyethylene bags; each bag was a 3-m-long cylinder that enclosed 0 liters of water and was positioned vertically in the lake. The bags were initially filled with coarsely filtered (80-pm mesh) lake water. Aliquots of the natural zooplankton assemblage then were dispensed randomly to each bag at ambient densities. Finally, two of the bags received a pulse of concentrated algae, (Ankistrodesmus falcatus), creating a final concentration of -5 x lo4 cells ml- (=2.5 mg liter- ). This food level is above the satiation level for the Daphnia (Lynch 18; Lampert 187). After 4 d ( adult instars), all zooplankton in the bags were harvested, preserved, and analyzed as described above for field samples. We used t-tests to compare adult fecundity between control and food addition bags, treating bag means as independent units of replication. Laboratory studies- On 20 June and again on 30 August 18 we collected live D. galeata mendotae from Three Lakes III and established clonal isolates in the laboratory (24 clonal lines from June and clonal lines from August). We hereafter refer to these isolates as clones, although we did not determine if all are genetically distinct. The clones do, however, represent a random sampling of the population at two times. Water used for culture was a 1 : 1 mixture of filtered water from two different lakes (Gull Lake and Pleasant Lake), which also have D. galeata mendotae populations. Methods for water collection and preparation are described by Tessier and Consolatti (18). Ankistrodesmus falcatus was used as food, following culture and harvest techniques of Goulden et al. (182). Culture conditions for experiments follow methods of Tessier and Consolatti (18, 1 1) and Leibold and Tessier (1 1). In general, animals were changed to fresh water and containers every other day and fed at a high concentration of 4 x lo4 cells of algae. Experiments were conducted in the dark at 20 C in order to avoid surface film entrapment associated with phototaxis. We used an experimental design similar to Lynch s (184a,b) to estimate genetic (among-clone) variance in life-history traits. Each clone was split into two replicate lines, and each line was cultured (acclimated) independently in the laboratory for >3 generations to minimize maternal biases. Neonates collected from the second through fifth clutches were measured for lengths and dry weights or isolated and allowed to grow until maturity. For each line of each clone we obtained 5-15 replicate estimates of length and dry mass of neonates, length and dry mass of adults at maturity, time to maturity, and clutch size at maturity. We performed nested analyses of variance and covariance to estimate variance and covariance components and to examine the significance of among-clone variation in life-history traits. We used a random-effects model and estimated variance and covariance at each level via ANOVA (SAS: Proc Nested). Variation among lines within clones was, therefore, used as the error term for testing among-clone effects in these analyses. Significance testing used type 3 sum-ofsquares because the design was unbalanced. We used an. F-ratio to compare genetic (among-clone) variance components calculated separately for June and August collections. Estimates of variance and covariance for pairs of traits were used to calculate genetic correlation coefficients. We then used a jackknifing technique to estimate confidence limits and significance for the correlation estimates. Results Population dynamics-the D. galeata mendotae population exhibited large fluctuations in abundance during summer 188, but very little fluctuation in 18 (Fig. 1). The highest density (>22 animals liter-l)

4 Tessier et al. 188 I * 8! A T II I 18 o-----l l I~I ~-_- -I * DAYS FROM 1 JUNE DAYS FROM 1 JUNE Fig. 1. Population density of Daphnia galeata mendotae in Three Lakes III from June through August 188 and 18. Density expressed as natural logarithm of density on each date (number per net tow = number per 133 liters). Error bars indicate f 1 SE based on samples collected from 3-4 different lake sites on each date. Arrows indicate dates of experimental food addition. was achieved in late June 188 and was followed immediately by a large population reduction (~0.05 animals liter-l) in July. The population recovered to a density of - 3 animals liter-l in August 188. A nearly constant density of 5 animals liter- was maintained in June and July 18, but the population declined in August to -2 animals liter-l. Adult fecundity and juvenile lipid content varied inversely with total abundance (Fig. 2), and overall these three parameters were strongly correlated (Bartlett x2 = 15.2, P = 0.002). The strongest relationship was between adult fecundity and population density (rp = -0.74, P = ). At pe,ak population density in late June 188, population growth rate became negative, birth rates dropped to near zero, and the population abundance declined (Figs. 1, 3). In late July 188, at the lowest population density, birth rates temporarily achieved very high values, population growth rate became positive, and the population density increased. Variance in our density estimates is also quite large during this period of very low abundance (Fig. l), so we cannot place much confidence in their accuracy. It is apparent from fecundity estimates, however (Fig. 2A), that resources became much more abundant shortly after the population declined, and it is likely that Fig. 2. A. Means (& 1 SE) for adult fecundity (clutch size of all egg-carrying animals) of the Daphnia galeata mendotae population during the study period. B. Means (+ 1 SE) for juvenile lipid index. Juveniles were defined as animals not carrying eggs and being ~0. mm in body length.

5 Body-size selection A s - 4 -" \ l- 0 I I I I I I I B ' I I I I I I I DAYS FROM 1 JUNE DAYS FROM 1 JUNE

6 Tessier et al. I t L 2 Q 0.5 d d I _-I I DAYS FROM 1 JUNE DAYS FROM 1 JUNE Fig. 3. Population growth (r-q), birth (b-a), and death (d--q) rates for the Daphnia galeata mendotae population during the study period. Error bars are based on 2-3 weekly estimates for r and b. Death rate is presented without error estimate and is the difference between b and Y. There were no error estimates on b available for the first three dates in 188 nor on Y for the first and last dates in 188. high birth rates contributed to a rapid increase in population density in late July 188. During much of summer 18, population growth rate remained near zero. Birth and death rates were generally similar to each other and fluctuated between 0.2 and 0.5 d-l. Hence, the apparent equilibrium condition (i.e. constant r) was extremely dynamic; high birth and death rates indicate a rapid turnover of individuals. In August 18, death rates typically exceeded birth rates, and the population density declined. There was a strong overall correlation among birth rates, death rates, and population density (Bartlett x2 = 47.5, P < ). Birth and death rates were positively correlated (rp = 0.5, P < , Fig. 4) and birth rates and population density were negatively correlated (rp = -0.77, P < , Fig. 4). The strong correlation be- tween birth and death rates is likely to be a result of shared sampling variance (error) because death rate is calculated from birthrate estimates. The descriptive dynamics suggest that the population was generally in short-term equilibrium (i.e. having similar birth and death rates) and exhibited density dependence. ESxcept for June 188, however, the population typically had very high birth and death rates, suggesting high resource availability but a high mortality rate. Because even a maximum estimate of flushing rate of the lake in July and August (0.03 d--l) is small compared to the death rate (~0.2 d-l), predation is a likely cause of this mortality. The food addition experiments indicate that, except for late June 188, the fecundity of D. galeata mendotae was not strongly limited by resources (Table 1). However, at peak population density in late June 18 8,

7 Body-size when fecundity and lipid estimates suggested food shortage, there was a striking response of the animals to food addition. Fecundity had more than doubled 4 d after the pulse of food. Although in most subsequent experiments fecundity was higher in food-addition than in control treatments, there was no significant treatment effect. As a measure of intensity of food limitation, we calculated the absolute fecundity difference between control and food-addition treatments for each experimental date (n = 5). There was a significant relationship between this experimental measure of resource shortage and the field population density (fp = 0.5, P = 0.014). These experimental results support the descriptive dynamics and suggest density-dependent population growth with generally high resource availability at all times other than the peak density in late June 188. Phenotypic variation -In both years, size at maturity and average adult size in the population declined from June through August (Fig. 5). Technically, what we estimated are the minimal and average sizes of eggcarrying animals. Since the descriptive and experimental studies indicate that resource availability was generally high, however, most mature animals would have been carrying eggs. Hence, except for late June 188, we consider estimates of minimal and average sizes of egg-carrying animals to be conservative estimates of minimal and average sizes of adults. In both years seasonal reduction in adult size was significant as a rank-order correlation against day of study. In 188 the correlation between day and minimal size was r, = , P = 0.0 1, and between day and average size it was r, = -0.74, P < In 18 the correlation between day and minimal size was r.y = -0.60, P < 0.01, and between day and average size it was r, = -0. 1, P -C 0.01). Three dates in July 188 indicate very large adult sizes and are included in these analyses, but they may be unreliable because of very low population density and small sample sizes of adults (n < 15 adults). Removal of these dates, however, only strengthens the negative correlations of adult size with day (rs = -0.86, selection 0.0 I I I I 1 I I _ 0 I a Ln TOTAL DENSITY Fig. 4. Relationship between population birth rates and natural logarithm of Daphnia galeata mendotae density during the two summers. Line is fit to the data by least-squares. See text for correlation coeficient and significance testing. P < 0.01 for minimal size and r, = - 1.O, P < 0.01 for average size). In both years the magnitude of the change was a reduction of + 15% in minimal size at maturity. Laboratory studies -A comparison of means calculated separately for clones collected in June and August reveals significant differences in body sizes at birth and maturity, but not in other life-history traits (Table 2, Fig. 6). When raised under identical laboratory conditions, clones collected in August produced offspring that were 20% smaller in dry weight than clones collected in June. Similarly, dry weight at maturity was 30% less for August clones than June clones. The difference in body length between June and August clones was 6 and 8% at birth and maturity respectively. The absence of any difference in time to maturity or specific growth rates and only a minor (and marginally significant) difference in clutch size between June and August collections is in sharp contrast to the substantial shift in body sizes at birth and maturity (Table 2). We estimated variance components for among-clone variation in life-history traits of June and August collections separately (Table 3). We could not calculate specific growth rates on single individuals, so that parameter is not included in these analyses. 0

8 8 Tessier et al. Table 1. Effects of experimental food addition on average fecundity (t- 1 SE) of Daphnia galeata mendotae adults. N = 2 enclosures for each treatment (control and food addition). 30 Jun Ju Aug Aug Aug 8 Treatment Fecundity t-statistic P Control 1.47(0.04) Food addition 4.70(0.) Control 4.0(0.135) Food addition 4.02(0.067) Control 5.6(0.208) Food addition 6.3(0.32 1) Control 2.66(0.02) Food addition 3.80(0.340) Control 2.45(0.068) Food addition 2.7(0.062) For the June collection, there was significant among-clone (genetic) variance for all lifehistory traits. By August, however, genetic variance had been eroded in all traits except time to maturity (F-ratio to compare June and August, among-clone variance, P < 0.05 in all cases except time to maturity). There was no significant genetic variance in measured traits for the August collection, so we used only data from June collections to es timate genetic correlations among pairs of life-history traits. For body size traits we compared only traits of similar units (length with length, weight with weight). Also, because there was no significant variation in time to maturity we excluded that trait. The only significant genetic correlation was between size at birth and size at maturity. Our jackknife estimate of genetic correlation for length at birth and at maturity is 0.80 (1 SE = 0.22, n = 24, P < 0.01) and for weight at birth and weight at maturity the estimate is 0.55 (1 SE = 0.77, ns). The small sample sizes result in large standard errors for these estimates, but it is apparent that much genetic variation in body size is actually genetic covariance of size at birth and maturity (Fig. 6). Interestingly, there was no significant genetic correlation between clutch size and body size (at birth or maturity). Discussion Population dynamics and experimental results indicate that D. galeata mendotae experienced high mortality and high resource availability in July and August of both years. Only in June 188 did the population achieve a high density and exhibit strong food limitation. The high density at that time was associated with drought conditions and very low flushing of water through the lake. During spring of normal years, stream discharge may impose sub- stantial population loss at a time when predation by warm-water fish species may be low. In July and August, however, bluegill sunfish are known to feed intensely on Daphnia in Three Lakes III (G. Mittelbach unpubl. data; L&bold and Tessier 11) and estimates of mortality greatly exceed stream flushing rates. Hence, predation is apparently an important mortality factor in summer. Our observation of a seasonal reduction in mean size of adults in the lake population is consistent with the expected effects of intense predation by fish. Most planktivorous fish prefer larger bodied prey, selectively removing the largest, most fecund adults. A simultaneous reduction in minimal size at first reproduction is more difficult to explain Fig. 5. Minimal (A) and average (B) body length of egg-carrying animals in the Daphnia galeata mendotae population during the study period. Lines are fit to the data by least-squares. See text fir correlation coeeients and significance testing.

9 Body-size selection A r t I I I I I I I B i cl 1.0 I I I DAYS FROM 1 JUNE DAYS FROM 1 JUNE

10 Tessier et al. 0.75? cf 0.70 E g!!! OB5 z I : 0.55 A I I --.--_ -Le I A ADULT LENGTH (mm) 3.0 r---r r- 1-1 Bl --- I--_L----l- -L--l ADULT MASS (pg) _-I_I T TIME TO MATURITY (h) Fig. 6. Values for body length (A) and body dry mass (B) at birth and at maturity for individual clones collected in June (8) and in August (E3). Each point via a direct predation effect. A decrease in food resources, which can cause smaller size at maturity (Lynch 18), did not occur in either summer. Rather, resource availability remaiined high during the period of body size decrease. Seasonal reduction in minimal body size at first reproduction has been documented in other systems (e.g. Culver 180) and could represent a phenotypic response by clones (predator induction or other cyclomorphosis), a genetic change in population structure (natural selection), or both. A significant finding of our laboratory study is that natural selection accounts for much of the response. Clones isolated from June were consistently larger at birth and maturity than clones isolated from August of the same year. The mean size of clones from August, however, fell within the range of variation recorded for clones from June, suggesting directional selection for small body size. Further support for directional selection on body size comes from the observation of significant among-clone (genetic) variance in June, but its absence in August. We observed no sexual reproduction during our study 0.f this population, and typically sex in D. galeata mendotae populations does not occur until fall (A. J. Tessier pers. obs.). Hence, the natural selection that we document in summer represents clonal selection. Apparently, predators not only changed the phenotype distribution of the population, but they also decreased genotypic diversity. We cannot quantify the extent to which individual clones in the lake population exhibited phenotypic plasticity in body size. Field phenotypes were measured as minimal and average size of all egg-carrying adults, whereas laboratory measurements represent something intermediate (average size at maturity). The average size of eggcarrying animals in the field during August was smaller than the smallest-sized clone at maturity in the laboratory, so it seems likely that phenotypic plasticity for adult size was -- t indicates the mean for one clonal isolate. Ovals indicate the 50% distribution contour for the data, calculated separately for June and August collections. C. As panel A, but for clutch size at maturity and time to maturity.

11 Body-size selection 11 Table 2. Comparison of means (2 1 SE) for life-history traits of laboratory-raised clones collected in June and August 18. Means for each collection date are based on among-clone averages (N = 24 for June, for August). Body lengths in mm, dry mass in pg, and time to maturity in h. Juvcnilc growth rate is measured as specific growth in pg pg- h-l; t-tests based on log-transformed values. Trait June August f-statistic P Neonate length Neonate dry mass Adult length Adult dry mass Clutch size Time to maturity Juvenile growth 0.63(0.005) 0.603(0.004) 5.07 <O.OOl 2.080(0.06 1) 1.65 l(o.058) 4.40 <O.OOl 1.38(0.015) 1.285(0.016) 4.45 KO (0.700).5 l(o.45) 5.26 <O.OOl 2.83(0.205) 2.327(0.124) (7.4) 158.8(.33) (0.001) 0.012(0.001) also involved. It is clear, however, that this developmental plasticity did not prevent substantial natural selection and significant genetic change in mean size at maturity. Our documentation of rapid erosion of genetic variance does support the argument that cyclomorphosis should normally be a more successful strategy for long-term persistence than should clonal specialization (Lynch 184a). In addition to genetic change in size at maturity we also documented change in size at birth. It is possible that fish predators directly selected for smaller neonate size, but our observation of a strong genetic correlation between size at birth and size at maturity suggests the importance of a correlated response. Genetic correlations among traits can restrict the population response to selection. For this D. galeata mendoate population, selection for smaller adult body size should result in associated selec- Table 3. Nested ANOVA of body length and dry mass at birth and at maturity, clutch size, and time to maturity. Results and significance testing are given separately for collections made in June and August. Source of variation indicates among-clonal isolates (clones), among replicate lines within each clone (lines) and variation among individuals within a line (error). Values in parentheses express the clone component of variance as a proportion of total variance (i.e. heritability). Asterisks: *-P < 0.08; **-P -c 0.05; ***-P < Trait Neonate length Neonate dry mass Adult length Adult dry mass Clutch size Time to maturity Source or variation Clones df Lines Error Clones 23 Lines Error Clones 23 Lines Error Clones 23 Lines Error Clones 23 Lines Error Clones 23 Lines 21 Error 167 June Variance component df *** (0.38) IO* (0.13) *** (0.40) ** (0.2) ** (0.18) * (0.1) 1, , August Variance component (i.0) (LO) (0.1) (0.14) $0) * (0.1) ,168.22

12 12 Tessier et al. tion for small neonate (or egg) size. Clones that mature at a large size make big eggs and clones that mature at a small size make small eggs. Further, when raised in the laboratory there was only a minor difference in mean clutch size for clones collected from June and August. Despite the fact that the larger bodied clones collected in June devote a greater amount of mass to reproduction compared with the smaller bodied August clones (5.5 pg, 1 SE = 0.42 vs pg, 1 SE = 0.28, respectively, t = 3.05, P = O.OOS), each egg of the larger clones is also larger, so there is little difference in clutch size. There was no difference in reproductive effort (defined as percent of adult mass invested in eggs) between June and August clones (38.2%, 1 SE = vs. 36.5%, 1 SE = 0.017, respectively, t = 0.43, P= 0.67). There was also no difference between June and August clones in specific growth rates of juveniles. Hence, both sets of clones had similar abilities to exploit resources in the laboratory environment. In short, the evolution of body size in this Daphnia population seems to be largely independent of other important life-history traits (i.e. fecundity, development, exploitative ability). This conclusion is very different than those based on interspecific life-history comparisons. Lynch (180) summarized much of the variation in life-history traits among species of Cladocera in general. Even within a narrow taxonomic category such as the Daphnia, he observed that species differing in body size also differed in relative sizes of offspring, times to maturity, and fecundity. Our results suggest that these interspecific patterns may not accurately reflect the outcome of body-size selection within a species or population. Historically, few zooplankton ecologists have concerned themselves with intraspecific genetic variation and have instead focused on environmental factors (especially food and predators) and phenotypic plasticity (Threlkeld 188). Our results and those of Lynch (184a) suggest that in Daphnia populations evolutionary processes can occur on similar time scales to important eco- logical processes. Hence, adequate explanation of zooplankton population dynamics requires consideration of both processes. For example, consider the population dynamics we report for 18. Throughout June and July, birth and death rates were equal and population density was stable. Starting in August, death rates slightly exceeded birth rates, and the population declined in density. One explanation for the population decline could be that in August the environment suddenly changed (e.g. increased predation), resulting in greater death rates compared with birth rates. An alternative explanation is that throughout June, July, and August predation increased or was constant, but did not strongly depress population density due to an evolutionary response by the population. A seasonal increase in predation risk could have been mitigated by genetic decreases in mean body size. Such compensation would only operate, however, until phenotypic variation was exhausted. Our observation of an erosion of genetic variance in body size suggests that natural selection among clones may have contributed to population stability during June and July and could have contributed to the eventual decline in population density. Presumably, immigration from the ad- joining lake (or from resting eggs in the sediment) could reintroduce genetic variation for body size. Sampling of the lake in 1 1, however, failed to detect any D. galeata mendotae; a smaller-sized species, Daphnia ambigua, had replaced it. As a second example of the significance of natural selection studies to zooplankton ecology consider the recent application of physiological, stage-structured models to Daphnia population dynamics (e.g. Hallam et al. ; McCauley et al. b). These models hold great promise for providing a mechanistic understanding of the relationships between population dynamics and individual life histories and physiologies. An important aspect of these models is the synthesis of organism biology into a set of stagespecific parameters (Lynch 18; McCauley et al. a). Presumably, these parameters are genetically determjned and may differ not only among taxa, but might also exhibit significant genetic variance within single populations. Hence, in order to apply these models to populations experiencing natural selection one would need to incorporate in-

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