Explanations for Bergmann’s Rule
Although the mechanism underlying Bergmann’s Rule in copepods is
unclear, it is apparent that the relationship with temperature is more
than a spurious correlation driven by differences in food availability
(McNab 1971; Belovsky 1997; Brown et al. 2017) or reduced
predation rates in cooler environments (Wallerstein & Brusca 1982;
Angilletta et al. 2004; Manyak-Davis et al. 2013). Even
after these drivers are accounted for, a strong negative relationship
between copepod size and temperature remains. We also found that
dissolved oxygen concentration (which decreases with increasing
temperature) does not adequately account for changes in size when
compared with effects of temperature, suggesting that oxygen limitation
is not responsible for Bergmann’s Rule. Instead, it is likely that
copepod size is regulated directly by temperature. A potential mechanism
is the negative correlation between growth efficiency and temperature,
so that colder waters could produce larger copepods (Ikeda et al.2001; Isla et al. 2008).
Our results suggest that when the negative relationship between taxon
body size and temperature is adjusted for, the relationship between
taxon body size and food availability is also negative. The direction of
this relationship seems counterintuitive because typically more food
leads to faster growth (Lin et al. 2013) and greater size (Vidal
1980; Berrigan & Charnov 1994). However, larger body sizes might allow
copepods to undertake deeper vertical migrations in search of food or
escape more visual predators in surface layers (Belovsky 1997; Brownet al. 2017). An alternative explanation is that copepods might
grow larger in response to seasonality of their food supply (Brunet al. 2016). For example, copepods grow larger in systems with
short seasonal pulses of food (e.g. Chl-a ) by accumulating lipids
for survival when food is limited (Kattner et al. 2007). Thus,
being larger and having greater reserves could allow better survival
during periods without food.
Increased abundance of invertebrate predators also translated to a
modest increase in copepod size, a relationship not previously observed.
This could be a selective advantage, where larger copepods might better
resist invertebrate predators, and/or undertake deeper diel vertical
migration to avoid predators (Ohman & Romagnan 2016). Because we
estimated only relative abundance of invertebrate predators, predation
by fish could also influence copepod size (Wallerstein & Brusca 1982).