219 Hydrobiologia 340: 219-224, 1996. J. M. Caffrey, P R. E Barrett, K. J. Murphy & P M. Wade (eds), Management and Ecology of Freshwater Plants. © 1996 Kluwer Academic Publishers. Printed in Belgium. Response of Elodea canadensis Michx. and Myriophyllum spicatum L. to shade, cutting and competition in experimental culture V. J. Abernethy, M. R. Sabbatini & K. J. Murphy University of Glasgow, IBLS Division of Environmental and Evolutionary Biology, Brian Laboratory (Garscube), Glasgow G12 8QQ, UK Key words: weed control, competition, disturbance, Elodea canadensis, Myriophyllum spicatum Abstract Elodea canadensis Michx. and Myriophyllum spicatum L. are widespread nuisance aquatic plant species. Their ecology is regarded as similar. Both species have been previously classified in terms of established-phase survival strategy as 'competitive disturbance-tolerant' species. Experimental data are presented to show that although this broad categorisation of strategy is probably correct for the two species, it is possible to demonstrate significant differences in terms of response to disturbance and competition. Less difference was discernible in their comparative response to stress. The drawbacks of applying broad descriptive terminology when dealing with two species of similar strategy are addressed. The results help explain reports of variable success in attempting to manage these two species using disturbance-based weed control measures, and suggest that Elodea is even less susceptible to such measures than Myriophyllum. Introduction Elodea canadensis Michx. and Myriophyllum spica- turn L. are two submerged macrophyte species, which have successfully crossed the Atlantic during the past century, in the former case from North America to Europe, and in the latter from Europe to North Amer- ica, to cause weed problems in a range of freshwa- ter systems (Murphy et al., 1990a; Anderson, 1990; Steward, 1990; Simpson, 1984). Despite their differ- ing provenances, both species are currently problem aquatic weeds in Europe. The ecology of the two species is usually con- sidered to be quite similar. Their established phase strategies both show strong elements of competitive- ness and disturbance-tolerance (Grime et al., 1988; Murphy et al., 1990b). The two species tend to occur in similar freshwater habitats, and occur under broadly similar sets of physico-chemical environmental condi- tions (Simpson, 1984; Smith & Barko, 1990). The available evidence (as, for example, reviewed by Nichols & Shaw, 1986) therefore suggests that popu- lations of the two species exhibit rather similar sets of phenotypically-expressed traits for tolerance of stress, disturbance and competition from other species (sen- su Grime, 1979). When in direct competition there is some evidence that one species may successfully displace the other, but field observations are far from consistent (e.g. Madsen et al., 1991). The question arises whether the application of man- agement measures (which impose artificial stress or disturbance on weed populations) is likely to have sim- ilar effects on E. canadensis and M. spicatum, and whether such effects are modified in the presence of competitor plant populations. The aims of the study were: (i) to determine, under standardised experimen- tal glasshouse conditions, the response of Elodea canadensis and Myriophyllum spicatum to artificially-imposed stress, disturbance, and inter- specific competition; and (ii) to use the information gained to refine knowledge of the established-phase survival strategy of the two species.