Geographic variability in form, size and survival of Egregia menziesii around Point Conception, California

Abstract

Intra-specific morphological variability is common among many species of plants, and is particularly common among marine algae. Populations of the kelp Egregia menziesii (Turner) Areschoug are known to be extremely variable in form, and variability in this species is well-correlated with geographical distribution. Populations north of Point Conception, California, are characterized by individuals having thick, tough, wide stipes and small blades, whereas populations south of Point Conception are characterized by individuals having thin, smooth stipes and large blades. Point Conception (hereafter PC) is a well-known biogeographic boundary for many marine species. The region north of PC is typified by cold, nutrient-rich water favorable to plant growth, but also by high wave exposure, increasing the likelihood of plant breakage or dislodgement. The region south of PC is characterized by warmer, oligotrophic water (less favorable for plant growth), but the potential for plant breakage or dislodgment is low due to the calm seas in this region. Using reciprocal transplant experiments, we examined the potential adaptive value of the morphologies of both populations (northern and southern) in the light of these physical gradients among sites north and south of PC, and the tradeoff between rapid growth and increased risk of breakage. We reciprocally transplanted northern- and southern-form E. menziesii among several sites north and south of PC and monitored size and survival of each individual over a 5 mo period. In general, both survival and growth of the northern form was greatest at the northern sites, and southern form survival and growth was greatest at the southern sites. Overall, growth was greatest at the northern sites, and the greatest increase in size was achieved by a southern-form individual at a northern site. These results indicate that regionally specific morphological forms of E. menziesii seem to be uniquely adapted to maximize growth and survival under conditions typical of their native region. However, in the absence of breakage, southern-form E. menziesii have the potential to grow large under northern conditions. Northern-form E. menziesii have a reduced probability of breakage or dislodgment due to their high-strength, low-drag morphology. Southern-form E. menziesii have much lower breaking strengths and higher drag than northern forms, but have a higher surface area per unit of stipe, potentially advantageous for nutrient uptake in the low-nutrient environment south of Point Conception. Although these morphological forms appear to be advantageous within each of these regions, it remains to be explored whether their morphology can be altered at an early life stage or whether it is genetically fixed.