Pollinator Limitation of Cytisus Scoparius (Scotch Broom), an Invasive Exotic Shrub

Abstract

Introductions of exotic species provide unique opportunities to study the demographic significance of species interactions, but as yet there is little information on how mutualistic interactions affect the invasion process. A shortage of mutualists could potentially limit the rate of population growth for an invading species. The introduced shrub Cytisus scoparius (Scotch broom, Leguminosae) is a pest plant on the west coast of North America. It produces flowers that are ''tripped'' open when pollinated and has a nearly obligatory relationship with resident bumble bees and honey bees. Experiments in the state of Washington showed that Ͻ1% of untripped flowers produced fruits and that outcross-pollinated flowers yielded fourfold more fruit than self-pollinated flowers, revealing apparent inbreeding depression. Mean pollinator visitation rate, as determined by the proportion of flowers tripped, varied among three years and among four populations but was low (3-30%) in every case. Two urban populations (Magnuson Park and Discovery Park in Seattle) received higher numbers of visits than two native prairie populations (Johnson Prairie and 13th Division Prairie in southwestern Washington). Hand-pollination experiments revealed significant pollinator limitation in all populations in both 1993 and 1994, with the mean increase in fruit production ranging from 280 to 2620%. Prairie populations were more pollinator limited than urban populations. Resources available for per-flower fruit production appeared to be equally available in all populations in 1993 but more available in prairie populations than in urban populations in 1994. The relationship between natural visitation and proportion fruit set per branch was a saturating curve in 1993 but a linear function in 1994 and 1995. Significant correlations were found between pollinator visitation and variation in whole-plant fruit production in all 3 yr. No evidence was found for either (1) reallocation of resources between branches within a season, or (2) a cost of reproduction between seasons in plants receiving supplemental pollen. Demographic analysis showed that a very large cost of reproduction would be required to counterbalance the increase in fecundity achieved with full pollination. Simulations of new populations invading over a short time scale (10-30 yr) demonstrated little effect of pollen limitation in the slow-growing urban populations, but a potentially large effect of increasing pollinator visitation in the rapidly invading prairie populations.

Adaptive Differentiation of Quantitative Traits in the Globally Distributed Weed, Wild Radish (Raphanus raphanistrum)

“…Flowering plants that are introduced to new habitats not only must survive under potentially different environmental conditions, but also must be successfully pollinated by potentially new pollinator species, particularly if they are outcrossing annuals. The abundance of effective pollinators in the new habitat and the ability to adapt to new pollinators could affect a plant species' ability to persist and spread (Parker 1997;Richardson et al 2000).…”

Invasion dynamics of two alien Carpobrotus (Aizoaceae) taxa on a Mediterranean island: II. Reproductive strategies

“…While agamospermy and selfing may permit the transmission of favourable genes to new generations and more distant sites, thus facilitating local adaptation (Jain, 1976), clonality allows for immediate spatial occupation (Pyšek, 1997). Furthermore, since pollinator limitation can impede outcrossing plants (Parker, 1997), uniparental reproductive modes and generalist pollination are expected to help establish an invader from a small number of introduced individuals (Baker, 1955(Baker, , 1986Roy, 1990). Such reproductive alternatives can thus increase the probability that an introduced plant will successfully establish (Brown and Burdon, 1987).…”

Using Pollination Deficits to Infer Pollinator Declines: Can Theory Guide Us?

“…In contrast, Parker (1997) found that strong pollination limitation of seed production in the invasive weed Cytisus scoparius did affect recruitment: costs of reproduction in this species were not strong enough to cancel out the value of extra seed production. Interestingly, populations of C. scoparius that experienced stronger pollination deficits had higher growth rates and were more invasive than those populations where pollinators were more abundant.…”

“…This is most important in long-lived perennials, because increased fecundity may incur costs of reproduction that counteract potential gains. Very few studies have explicitly tied pollination limitation to population trajectories (Bierzychudek 1982, Calvo 1993, Ehrlén and Eriksson 1995, Parker 1997, and these studies present a complicated picture. Ehrlén and Eriksson (1995) found that supplementally pollinated plants of Lathyrus verna increased their seed production by 3.1 times, but then shrank in size, made fewer flowers the next year, and were more likely to enter dormancy than were controls.…”