Population fragmentation may reduce fertility to zero in Banksia goodii ? a demonstration of the Allee effect

Abstract

All individuals of all known populations of Banksia goodii were assessed for seed production. Small populations produced no or only a few seeds per unit canopy area. Effects of population size on seed production per unit area and seed production per plant were present over the whole range of population sizes, indicating that even in large populations seed production may still not be at its maximum. Resource differences could not explain this disproportionate decrease in seed production with decline in population size, because there were no differences in soil properties and understorey or overstorey cover between the small and large populations. Although plants in small and large populations were similar in size, seed production per plant was much lower in small populations. This was not because plants in small populations produced fewer cones but because the fraction of these cones that was fertile was much lower. Five of the nine smallest populations (<200 m) produced no fertile cones over the last 10 years. The number of seeds per fertile cone did not depend on population size. The results are discussed in relation to pollination biology.

Biodiversity, Species Interactions and Ecological Networks in a Fragmented World

“…The reproductive output of plants can vary with pollinator composition, abundance and behaviour (Lamont and Barker, 1988;Lamont et al, 1993). Although visitation rates are expected to be influenced by habitat fragmentation, the results are inconclusive: some pollinators are more abundant in larger fragments (Sih and Baltus, 1987), some are equally abundant (Jennersten, 1988), while others are rarer in fragments (Sih and Baltus, 1987;Strickler, 1979).…”

Variable responses of hawkmoths to nectar-depleted plants in two native Petunia axillaris (Solanaceae) populations

“…As a consequence of pollinator preferences, plant populations with a low density may suffer from a decrease in pollination visits (pollinator limitation) due to a reduced attractiveness caused by fewer attraction cues, a lower total reward and longer travel times between bouts (Eriksson and Ehrlen 1992;Feldman 2008;Klinkhamer et al 1989;Sih and Baltus 1987). Very small or isolated populations are exposed to the risk of going extinct due to inbreeding depression caused by mating with closely related individuals or reproductive failure due to ineffective pollination (absence of pollinators) (Allee effect;Allee 1949;Dornier et al 2008;Forsyth 2003;Groom 1998;Lamont et al 1993). For a thorough understanding of the stability of mutualism, one has to understand how variation in population/community structure interacts with the occurrence and frequency of nectarless individuals or species.…”

Depressed pollination in habitat fragments causes low fruit set

“…Unfortunately, however, they did not have information on pollen limitation in these species, and felt that variation in fruit production by one of the four species (Prosopsis nigra) was more consistent with resource limitation of fruit set. A number of other studies have documented depressed reproduction in small or lowdensity populations without an explicit link to fragmentation, or declines in fruit set with fragmentation, without data on pollination (Ghazoul et al 1998;Gigord et al 1999;Groom 1998;Jennersten 1988;Lamont et al 1993;Spears 1987). These declines could alternatively be explained by aspects of the resource environment in fragments that depress reproductive output in the absence of any change in pollination rates.…”