Size differences of Arctic marine protists between two climate periods—using the paleoecological record to assess the importance of within-species trait variation

Mean body size decreases with increasing temperature in a variety of organisms.This size–temperature relationship has generally been tested through space but rarely through time. 

这是研究背景，随着温度升高，生物的的体形减小（气候越冷，体型越大，比表面积减小，散热变少）。这种关系在空间尺度上被验证了，但时间尺度上很少。这个研究背景决定了这篇文章的层次不会很高。

 

We analyzed the sedimentary [?ɑrka?v]  of dinoflagellate cysts in a sediment record taken from the West Greenland shelf and show that mean cell size decreased at both intra- and interspecific scales in a period of relatively warm temperatures, compared with a period of relatively cold temperatures.

这是研究内容，dinoflagellate （腰鞭毛虫）应该是一类原生动物，它在沉积岩石里有化石记录。

 

We further show that intraspecific changes accounted for more than 70% of the change in community mean size, whereas shifts in species composition only accounted for about 30% of the observed change.

变异主要存在种内70%。

 

Literature values on size ranges and midpoints for individual taxa were in several cases not representative for the measured sizes, although changes in community mean size, calculated from literature values, did capture the direction of change.

通过文献收集的手机的数据也能反应大体的趋势，但不具代表性，否则我干嘛做这个研究。(毕竟没有文献数据没有群落个体的数据，以中位数代替平均值，但我纳闷群落结构的构成数据来自何处？)

 

While the results show that intraspecific variation is necessary to accurately estimate the magnitude of change in protist community mean size, it may be possible to investigate general patterns, that is relative size differences, using interspecific-level estimates.

种内变异对于size的差异很重要，比群落结构的改变更重要。

  

 上图，左边是温度，右边是物种组成

 上图是显微镜下的几种原生生物

数目统计，以上两图每一种的size分布。文献没细读，combined包含了全部个体；literature来自其他研究

主要来看数据分析 

Changes in community mean size can be caused by both

changes in community structure, that is, species shifts (interspecific changes),

and changes in the cyst size of individual species, that is, phenotypic plasticity (intraspecific changes).

下面讲的的很细，先以个体为单位求均值，做差即得到总的变异，然后以物种为单位求均值，做差得到的种间的变异；思路和群体遗传学的种内种间分化是一致的，下面个步骤要好好理解。

In order to assess the relative importance of intraspecific vs. interspecific changes, we first calculated the weighted community mean size including the influence of both intra- and interspecific changes (xintra+inter) for each period following Equation 1:

(1)

where Øobs are individual observations of cyst diameter and wobs is the weight of each observation given by Equation 2.

(2)

where Nspe is the number of observations of each species and wspe is the weight of each species given by Equation 3.

(3)

where Nspe is the number of cysts of each species and N is the total number of cysts (as in Figure 2b).

We then calculated the weighted community mean size excluding intraspecific changes (xinter) for each period following Equation 4.

(4)

where Øavg is the mean cyst size for each species across both periods and wspe is the weight of each species in each period. The relative contribution of intra- and interspecific changes to total change can then be calculated following Equations (5)-(7).

(5)

(6)

(7)

where Δxintra+inter is the size change attributed to the combined effects of intra- and interspecific changes, and Δxinter and Δxintra are the contribution of interspecific and intraspecific changes to Δxintra+inter.

最后是t检验，本研究并未用到多么高级的数据分析手段。

 In addition to the weighted mean, we also calculated the weighted standard deviation and standard error of the mean. A Welch two-sample weighted t-test (Bland & Kerry, 1998) was used to assess the difference in the weighted mean between the two periods.