SCALE DEPENDENCE OF REPRODUCTIVE FAILURE IN FRAGMENTED Echinacea POPULATIONS

Abstract

I investigated reproduction in a three-year study of Echinacea angustifolia, purple coneflower, growing in a fragmented prairie landscape. I quantified the local abundance of flowering conspecifics at individual-based spatial scales and at a population-based spatial scale. Regression analyses revealed that pollen limitation increased while seed set and fecundity decreased with isolation of individual plants. Isolation, defined as the distance to the k(th) nearest flowering conspecific, was a good predictor of pollen limitation, for all nearest neighbors considered (k = 1-33), but the strength of the relationship, as quantified by R2, peaked at intermediate scales (k = 2-18). The relationship of isolation to seed set and fecundity was similarly strongest at intermediate scales (k = 3-4). The scale dependence of individual density effects on reproduction (density of flowering plants within x meters) resembled that of isolation. Analyses at a population-based scale showed that pollen limitation declined significantly with population size. Seed set and fecundity also declined with population size, but significantly so only in 1998. Whether quantifying local abundance with population- or individual-based measures, reproductive failure due to pollen limitation is a consistent consequence of Echinacea scarcity. However, individual-based measures of local abundance predicted pollen limitation from a wider sample of plants with a simpler model than did population size. Specifically, the largest site, a nature preserve, is composed of plants with intermediate individual isolation and, as predicted, intermediate pollen limitation, but its large population size poorly predicted population mean pollen limitation.

Relationships between the floral neighborhood and individual pollen limitation in two self-incompatible herbs

“…This, in combination with the ongoing decline in pollinator populations (Kearns et al 1998), makes it important to understand how changes in Xower density inXuence plant-pollinator interactions and the resulting reproductive output of plants (e.g., Kunin 1997a;Knight 2003;Kirchner et al 2005). Many studies have investigated the eVects of population size and density on mean population pollination success (reviewed in Kunin 1997b), but comparatively few have considered individual-based measures where the local Xoral neighborhood is related to the reproduction success of single individuals (e.g., Roll et al 1997;Wagenius 2006;Spigler and Chang 2008). Both perspectives are of importance for understanding the causes of plant reproduction failure (Wagenius 2006).…”

“…Many studies have investigated the eVects of population size and density on mean population pollination success (reviewed in Kunin 1997b), but comparatively few have considered individual-based measures where the local Xoral neighborhood is related to the reproduction success of single individuals (e.g., Roll et al 1997;Wagenius 2006;Spigler and Chang 2008). Both perspectives are of importance for understanding the causes of plant reproduction failure (Wagenius 2006).…”

“…A way to handle this problem is to assess Xower density at diVerent distances from the target individual. Studies which have adopted this approach show that the spatial scale at which the Xoral neighborhood is estimated can have large eVects on the relationships found (e.g., Roll et al 1997;Spigler and Chang 2008;Wagenius 2006).…”

“…Most of these studies have found a negative relationship between conspeciWc density and the magnitude of pollen limitation (Steven et al 1999;Karrenberg and Jensen 2000;Moody-Weis and Heywood 2001;Knight 2003;Waites and Ågren 2004;Kolb 2005), although positive relationships (Larson and Barrett 1999;Baker et al 2000) and nonsigniWcant relationships (Lázaro and Traveset 2006) also occur. Few studies have investigated the relationship between local Xower density and pollen limitation among individuals within the same population (but see de Jong et al 2005;Wagenius 2006;Zorn-Arnold and Howe 2007), and we know of no study that also has assessed the eVects of heterospeciWc Xower density on individual-level pollen limitation.…”

Pollination performance and vulnerability to pollination breakdown of sixteen native shrub species from New Zealand

“…Reproduction in plants can be particularly sensitive to the alteration of spatial distributions of potential mates and changes in pollinator behaviour and abundance (Wagenius 2006). A plant population may show no effects in the short-term if individuals are long-lived, but a study of fruit set may predict future vulnerability if seed set is greatly reduced (Rathke & Jules 1993).…”