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).