Density dependence in killer whales

Approach

Over the decade from 2010-2020, there appeared to be a slowing of fecundity rates for both the SRKW and NRKW populations. We can examine correlations between those estimated fecundity rates, and aggregate population size.

Fecundity

Figure 1. Estimated fecundity rates for 20-year females in the SRKW population. Ribbons represent 95% CIs.

There is not a clear relationship between declining fecundity rates and SRKW population size, because we see both high and low fecundity rates at low total SRKW population sizes.

Figure 2. Estimated fecundity rates for 20-year females in the SRKW population versus total SRKW population size.

Given that fecundity rates for both populations are falling over the same period, a better proxy for density dependent effects may be the combined population sizes of SRKW and NRKW animals. This shows a clear declining relationship between fecundity rates and population size. The shape of the relationship is evidence of non-linear density dependence, and is similar to results seen in pinnipeds and large terrestrial mammals.

Figure 3. Combined killer whale population sizes (SRKW + NRKW).

Figure 4. Estimated fecundity rates for 20-year females in the SRKW population versus combined (SRKW + NRKW) population size.