9780521861724htl 1..2

Shifts in the community size spectrum, and impacts on ecosystem function

Differences in life history among higher taxa such as genera or families are

usually interpreted as resulting from accumulated genetic differences driven by

natural selection and phylogenetic history, with variation due to phenotypic

plasticity being relatively small (Doughty & Reznick, 2004 ). However, whatever

the cause of life-history variation, the environment can still select particular body

sizes, and this can have substantial consequences for ecosystem structure and

function (Hallet al.,thisvolume;Huryn & Benke, this volume). Thus, the argu-

ments used in previous sections about mortality effects on optimal adult size can

be applied to collections of different-sized species. For instance, high mortality of

large zooplankton species favours smaller species. In many lakes predation

intensity by planktivorous fish has been shown to drive down the average size

of zooplankters (e.g. Brooks & Dodson, 1965 ; Zaret, 1980 ;Jeppesenet al., 2004;

Fig.3.4). This removal of large grazers has profound effects on phytoplankton

abundance, hence water clarity, and can even facilitate a switch between alter-

native stable ecosystem states (Moss, 1998 ; Jones & Jeppesen,thisvolume).

A parallel shift in the community body-size spectrum is observed in heavily

exploited marine fish communities, where landings have become increasingly

dominated by smaller species (Jenningset al., 2002; Jennings & Reynolds, this

volume). Again, size-selective mortality appears to cause decreases in the rela-

tive abundance of larger species as well as mean body size within species.

Life-history analysis and scaling relationships

Key elements

The magnitude of many biological phenomena, including life-history traits,

increase with body size in a predictable way both within and across species

and higher taxa (Brown & West, 2000 ; Brownet al., this volume). Typically, the

magnitude of a trait is assumed to scale with body mass (M) according to a power

winter

spring/autumn

Log

cladocerans (e

μg ind

–1

)

0–100 100–200 >200

Fish CPUE (no. net–1 night–1)

summer

–1.5

0

0.5

1.0

1.5

2.0

2.5

Figure 3.4Boxplot (median, 25, 75, 10

and 90% quartiles) of mean body weight

of cladocerans (loge-transformed) for

samples collected during summer,

spring/autumn and winterversusthe

catch per net at night in multiple mesh-

sized gill nets in 34 Danish lakes

monitored one to three times (n¼56–74).

Fish sampling always occurred between

15 August and 15 September. Figure is

from Jeppesenet al.(2004).

LIFE HISTORIES AND BODY SIZE 43