Lundi 13 décembre à 11h, Michael Eichholz (Southern Illinois University) présentera à la Tour du Valat un séminaire intitulé ” La dynamique des populations de canards de prairies est-elle limitée par leur succès reproducteur ou par leur propension à se reproduire?”
Currently, populations of ground nesting ducks in the prairie pothole region of North America are thought to be most limited by their ability to nest successfully. Thus, current management actions emphasize the need to increase nesting success (proportion of nests that hatch at least 1 egg). Assuming nest success most limits populations is based on observations of what are perceived to be extremely low nest success and a linear (non-density dependent) stage based population model. This assumption, however, has yet to be tested with empirical data. Harvest ratio data indicate productivity of both prairie and boreal nesting ducks is density dependent. In fact, it appears the strength of the density dependent signal has increased since implementation of the North American Waterfowl Management Plan that initiated the conversion of millions of acres of agricultural habitat to nesting cover. This indicates current management activities are not addressing the density dependent factor that I argue is most limiting the population. An alternative vital rate that might be more limiting and more likely to be density dependent is breeding probability. The probability of breeding may be < 100% in ducks due to lack of nutrients for egg production and incubation or a lack of secure nesting habitat. Breeding probability has been demonstrated in geese to be dependent on the female’s ability to acquire nutrient reserves prior to breeding. North American ducks also acquire nutrient reserves prior to breeding but some have argued that those reserves are acquired for migration and are unnecessary for the initiation of nesting. I present evidence ducks are acquiring nutrient reserves both early and late in spring migration, indicating the nutrient reserve are being acquired specifically for reproduction. This supports the hypothesis nutrient acquisition may be limiting breeding probability in ducks. Breeding probability may also be limited by safe nesting sites. Unlike geese which incur relatively minor costs of reproduction, about 25% of female prairie-nesting ducks are consumed by predators each year. Thus, reproduction in ducks is quite risky and costly. If ducks are able to recognize this cost and modify their reproductive investment accordingly, it is possible they modify their reproductive investment to the point they skip breeding altogether in years when they are forced to nest in suboptimal habitat. Through an experimental manipulation of nest predator density, I provide evidence hens are able to assess likelihood of successfully hatching a clutch and modify their nesting behavior accordingly. This result indicates hens modify their reproductive investment based on likelihood of successfully nesting and suggests modification of reproductive investment as an addition alternative explanation for the density dependent signal in duck productivity.
