Geoscience Reference
In-Depth Information
Interestingly, we found that some orders (e.g. Lophiiformes, Myctophi-
formes, Rajiformes, and Salmoniformes) have a relatively narrow range of
PPMR despite being sampled at distant locations, while other orders (e.g.
Gadiformes, Perciformes, and Scorpaeniformens) have wide variations
across about four orders of magnitude (
Figure 7
). These results may suggest
that the nature of the predator-prey body-size relationship is contingent, to
some extent, on evolutionary history, although sample size was limited in the
present analysis and further investigation is required. The relationship
between the order mean of individual-predator PPMR and phylogenetic
rank (simply assigned from 1 to 16) was significant, where log
10
(PPMR)
¼
0.001), implying that more re-
cently evolved orders may have higher values of PPMR. Recently,
Romanuk
et al. (2011)
showed that descendant orders of fish have smaller body masses
and lower trophic levels (i.e. smaller prey mass), while the regression slope is
more strongly negative for the relationship between phylogenetic rank and
body size. Their results appear to support our findings.
0.301
log
10
(phylogeny rank)
þ
2.078 (p
<
4. Temporal Variability
The present and most previous data of feeding relationships represent 'snap-
shots' of time-varying trophic relationships, which is a long-standing prob-
lem in the study of feeding relationships (
McLaughlin et al., 2010; Warren
and Lawton, 1987
). Through the use of stable isotope analysis,
Nakazawa
et al. (2010)
showed that the relationship between body size and trophic niche
position of a freshwater fish species may change over a period of more than
40 years. In other words, the PPMR of species may change through time.
Although
McLaughlin et al. (2010)
showed seasonal and ontogenetic
changes in PPRM, long-term evidence remains scarce, which is crucial for a
better understanding of food-web dynamics. Gut content analysis from
archival specimens collected over a long period may provide a means of
directly addressing this issue (Nakazawa et al., in preparation).
B. Functional Response
Finally, we review recent advances and future directions in the study of size-
dependent trophic interactions and its use in food-web modelling by focusing
on two specific topics. First, we address issues of functional response, anoth-
er important concept in foraging ecology, through which we strongly empha-
sise the importance of both body size and species identity in predator-prey
interactions. Thereafter, we describe how size-structured food-web models
should be improved based on current knowledge.
