The ideal free pike: 50 years of fitness-maximizing dispersal in Windermere

Abstract

The ideal free distribution (IFD) theory is one of the most influential theories in evolutionary ecology. It predicts how animals ought to distribute themselves within a heterogeneous habitat in order to maximize lifetime fitness. We test the population level consequence of the IFD theory using 40-year worth data on pike (Esox lucius) living in a natural lake divided into two basins. We do so by employing empirically derived density-dependent survival, dispersal and fecundity functions in the estimation of basin-specific density-dependent fitness surfaces. The intersection of the fitness surfaces for the two basins is used for deriving expected spatial distributions of pike. Comparing the derived expected spatial distributions with 50 years data of the actual spatial distribution demonstrated that pike is ideal free distributed within the lake. In general, there was a net migration from the less productive north basin to the more productive south basin. However, a pike density-manipulation experiment imposing shifting pike density gradients between the two basins managed to switch the net migration direction and hence clearly demonstrated that the Windermere pike choose their habitat in an ideal free manner. Demonstration of ideal free habitat selection on an operational field scale like this has never been undertaken before.

Figure 1

The study lake, abundance time-series and density-dependence of demographic components. (a) The study lake and time-series of annual pike and perch abundance estimates. (b) Estimated survival response with corresponding 95% confidence boundaries (dashed lines) as a function of age 3–9 pike abundance for males in the two study basins. (c) Estimated dispersal probability with corresponding 95% confidence bounds (dashed lines) as function of age 3–9 pike abundance gradient between the north and south basins. (d) The relationship between mean individual fecundity and pike abundance. The bubble sizes are proportional to the mean body weight of mature females during the spawning season. (b) and (c) have been estimated under a mean abundance of age 2 pike and mean abundance of perch.

Figure 2

Basin-specific annual values of fishing effort, dispersal and intrinsic fitness. (a) The total basin-wise fishing effort during the winter fisheries in Windermere, 1950–1990. Pay attention to the 1956–1962 period when the pike density manipulation experiments were carried out. (b) Relative (to lake maximum) basin-wise annual intrinsic fitness (w_t). These values illustrate what would be the expected fitness consequences if there were, on average, no net dispersals between the basins. (c) Annual net north-to-south dispersal rates (${\psi }_t^{\text{NS}}-{\psi }_t^{\text{SN}}$) estimated from mark-recapture data.

Figure 3

Estimated realized fitness surfaces and the predicted and linear isodars. The estimated realized fitness (λ in: (a) the south basin and (b) the north basin as functions of basin-specific pike abundances. (c) The predicted isodar (i.e. the intersection line between the two fitness surfaces, thick black line) and the corresponding 95% confidence boundary lines (dotted lines). Numbers attached to grey lines represent isocline values for the difference λ^{north basin}−λ^{south basin}, where the zero isocline constitutes the predicted isodar. The red open dots constitute the historical pike abundance estimates and the red line is the estimated linear isodar for these abundances (sensu Morris 1988). This linear isodar ($x_t^{\text{S}}=0.04+0.86x_t^{\text{N}}$) explained 70.1% variation and the intercept is not significantly different from 0 (p=0.17), whereas the slope is significantly different from both 0 (p<0.0001) and 1 (p=0.013).