File:American malacological bulletin (1987) (17968737900).jpg

Title: American malacological bulletin
Identifier: americanmal4519861987amer (find matches)
Year: 1983 (1980s)
Authors: American Malacological Union
Subjects: Mollusks; Mollusks
Publisher: (Hattiesburg, Miss. ?) : (American Malacological Union)
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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HORNBACH AND COX: ENVIRONMENTAL INFLUENCES IN PISIDIUM 59 RP 2.0--
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0 200 400 600 800 1000 TIME (DAYS) Fig. 6. Growth curves (as increases in mean shell length) for Pisidium casertanum taken from two ponds (RP = Riopel Pond; FP = Farriers Pond) and reared in waters of various hardnesses. studied by Holopainen, 1979 was from an oligotrophic lake). The differences may, however, be due to calcium availability or low alkalinity. Potentially calcium availability could affect both size and composition. Of those populations of P. caser- tanum shown in Table 7 for which water chemistry data were available, RP certainly had the lowest calcium availability, conductivity and alkalinity (Table 1). The low alkalinity and calcium levels may inhibit shell formation in this population. Figure 4 emphasizes the fact that clams of equivalent shell lengths have much less CaC03 if they are from RP as com- pared to clams from FP. The data in Table 7 provide a preliminary data base for analyzing the relationships among various life history traits in Pisidium casertanum. Stearns (1976) has suggested that based on certain theories of life history evolution (r and K and bet-hedging theories) that suites of life history traits should covary giving rise to "life history tactics". Whether or not strict covariation is needed in observing life history tactics is a mat- ter of some debate (see e.g. Stearns 1980, 1982; Etges, 1982; Wittenberger, 1981). Also all one-dimensional models of life history evolution assume equilibrium population sizes (Caswell, 1983) which probably rarely occurs in the Pisidiidae. Brown (1985a) and Way (1985) claim that more ex- amples of intraspecific variations in life history traits are needed to examine life history evolutionary models. A prin- cipal components analysis (SAS Institute, 1982) was con- ducted using the data in Table 7. The life history traits used in this analysis included maximum shell length, maximum life span, number of generations produced per year, age at first reproduction and maximum number of embryos per parent. Utilizing these traits allowed 7 of the 10 populations to be included in the analysis. The first two principal components accounted for 70% of the variation in the life history traits. The variables age at first reproduction, number of generations per year and max- imum shell length loaded most heavily for the first principal component. The variables maximum life span and maximum shell length loaded most heavily for the second principal component. A plot of the principal component scores based on the first two principal components is shown in figure 9. The first principal component is a composite of increasing age at first reproduction and maximum shell length and decreasing number of generations produced per year. Populations to the right of the vertical line drawn in figure 9 display one genera- tion per year while those to the left display two. The second principal component is a composite of increasing maximum life span and decreasing maximum shell length. One could interpret those populations shown above the horizontal line drawn in figure 9 as being from more stable habitats (ponds and lakes) whereas those below the line are from more variable habitats (temporary ponds and streams). Associated with the increased predictability of the habitat (populations above the horizontal line) is increasing maximum life span and to a lesser extent (lower loading value for the second principal component) decreasing maximum shell lengths. Within the permanent habitats (above the horizontal line) RP is certainly the harshest habitat (low temperature, oligotrophic and has low calcium availability and alkalinity). The populations to the right on this graph are from more favorable permanent habitats (ponds and lakes with at least higher calcium availability). This trend of increasing favorableness of the habitat with an increase in the first prin- cipal component is also seen within the more variable habitats with streams being found to the right of a temporary pond in figure 9. This increase in favorableness of the habitat, whether in a stable or variable habitat, is associated with a switch from producing two generations per year to producing only one generation per year and an increase in maximum shell length attained. The two dimensional nature of the results of this prin- cipal component analysis is similar to Greenslade's (1983) habitat template. In Greenslade's model, two axes to be dealt with when considering life history evolutionary "strategies" are habitat favorableness and habitat predictability. The third axis in the habitat template deals with biotic predictability and is a function of the other two axes. Thus, in predictable yet harsh habitats (e.g. RP) one finds reduced reproductive out- put, long life span and small total size. These are traits associated with adversity selection and are expected based

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