We began our study of natural selection during 2003. The following year we experimentally replaced male lizards on Kidd cay, our principal study site, with individuals from an adjacent site on Great Exuma [14
]. Coincidentally, that same year the Bahamas experienced a severe drought that we quantified using both satellite remote sensing and ground-based rainfall data (Figs. , ). Satellite data of the entire study area corroborated the rainfall data collected at one specific point on the ground. Thus, we can be reasonably certain that our measures on the ground are representative of adjacent study sites (e.g., Nightmare cay). During 2004, under-story vegetation and canopy foliage on the island were greatly reduced, and this fine scale variation was reflected in a reduction in NDVI (see below).
Figure 1 Precipitation rates estimated from remotely-sensed measurements (TRMM) for Great Exuma, Bahamas. The top panel depicts monthly totals [mean values (± 1 SE)] based on the wet years 2001, 2002, 2003, 2005, and 2006 (solid line) and the monthly totals (more ...)
Figure 2 Panel (A) shows the significantly lower rainfall totals from ground based estimates of the drought during 2004 (grey columns) which had significant impacts on habitat use by lizards (B), the frequency of leaf use for cover (C) correlational selection (more ...)
Changes in vegetative cover over the course of the four-year study impacted habitat use by lizards. Lizards perched more frequently on the ground during the drought relative to wet years when they perched more frequently in the vegetation (χ2 = 14.96, df = 3, P < 0.0001; Fig. ). Drought conditions ended in 2005 with an increase in rainfall (Fig. , ), which led to increased growth of under-story vegetation and shifts in habitat use by lizards (Fig. ). Use of vegetated habitat increased significantly from 2004 to 2006 though it remained below pre-drought values. Similarly, the frequency of leaf-use for perching and for cover increased during 2005 and 2006 as lizards moved off of narrow plant stems and onto new vegetative growth (χ2 = 13.37, df = 3, P = 0.0003; Fig ).
Impacts of the drought on vegetation structure and habitat use by lizards coincided with shifts in natural selection on limb length and body size (snout-vent-length, SVL). During the three wet years of our study, correlational selection gradients acting on size corrected limb lengths (residuals from the regression of limb length against SVL) ranged from significant and positive to significant and negative (Fig. ; 2003: γ1,2
= 0.29 ± 0.16; 2005: γ1,2
= -0.13 ± 0.18; 2006: γ1,2
= -0.21 ± 0.09). This indicates a change in the shape of the fitness surface from saddle shaped to dome shaped [34
]. An alternative to focusing on the shape of the correlational selection surface (i.e., the sign of the gradients) is to ask simply whether or not selection acted on limb lengths in wet versus dry years. A plot of the absolute values of correlational selection gradients through time (Fig. ) shows that correlational selection gradients were significantly different from zero only during wet years. By contrast, selection operated on body size and not on limb lengths during the drought year and immediately following the drought (Fig. and Table ).
Figure 3 Inter-annual variation in correlational selection acting jointly on fore and hindlimb lengths. Surfaces were produced using the cubic spline approach developed by Shluter and Nychka . The vertical axis shows the probability of survival (W) and each (more ...)
During the drought selection on body size was significant and disruptive (γ = 0.60 ± 0.22). Following the drought, natural selection favoured smaller male body sizes (β = -0.23 ± 0.11). In the two years following the drought (Fig. ), the island's vegetation gradually recovered (e.g., Fig. increases in NDVI,) and we observed a return to selection on limb length as lizards moved back onto the vegetation (Fig. ). The differences in selection on body size and limb traits was significant among years (hindlimb × forelimb × SVL × wet/dry F1,464 = 4.69, P = 0.03). In sum, we observed selection on limb length only in wet years, selection on body size alone during the drought, and selection for smaller body size the year immediately after the drought. Thus variation in selection was intimately related to variation in ecology.
Figure 4 The top panel shows seasonal variation in the TRMM (precipitation) data for each season from 2001–2006 illustrating the drought that occurred during 2004 (grey). Precipitation data were standardized by subtracting mean values and dividing by the (more ...)
To test whether changes in habitat structure were sufficient to explain the dynamic patterns of selection observed in our natural study population, we measured selection on a separate island that we experimentally manipulated during the wet year of 2005. Similar to patterns observed during the actual drought, selection acted significantly on body size (β = 0.55 ± 0.23; Fig. ) but not on limb morphology (γ1,2= -0.57 ± 0.42) following our experimental vegetation removal.
Figure 5 An experimental manipulation to simulate drought effects (grey columns) on vegetation structure revealed changes in selection acting on body size that were similar to those between naturally wet and dry years (EX and N refer to experimental vegetation (more ...)
In 2006, observations in the field suggested re-growth of the under-story and substantial increases in new stem growth in the canopy on the island that we had manipulated the previous year. We again estimated selection on a new cohort of experimentally introduced males (92 juvenile male lizards from the same source-population as used the previous year, all other males were removed from the island) and similar to results from the natural site, in the year after our experimental manipulation of the vegetation, selection on body size was relaxed and non-significant (β = 0.06 ± 0.19; Fig. ; Table ). We did not detect significant correlational selection on limb lengths in the population where we experimentally manipulated vegetation (γ1,2= 0.10 ± 0.23), but for both body size and for limb length, a comparison of slopes revealed significant differences between natural populations and those in which we manipulated vegetation (body size P < 0.001 and limb length P < 0.002).