EEB Faculty Publications

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Browsing EEB Faculty Publications by Subject "acacia"

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    Experimental evidence that both parties benefit in a facultative plant-spider mutualism
    (Ecological Society of America, 2004-06) Whitney, Kenneth D
    Spiders are known to influence plant fitness, and vice versa. Yet, it has not been appreciated that these facultative relationships can be mutualistic. I examined the interaction between Phryganoporus candidus, a subsocial Australian spider, and the extrafloral nectary-bearing shrub Acacia ligulata to explore variability in mutualistic interactions over a three-year period. Spiders enhanced seed production by reducing seed predation by heteropterans, wasps, and weevils. Because spider colonies occupy only a fraction of a plant’s volume, average benefits ranged from 0.4 to 6% increases in whole-plant seed production. These benefits were strongest in years of low seed production, suggesting that spiders may buffer plants against female reproductive failure. To evaluate benefits for spiders, I established experimental spider colonies on three common hosts. Spider performance (persistence and prey capture rates) on live A. ligulata and live hopbush Dodonaea viscosa exceeded that on dead acacia, suggesting that live hosts are more beneficial than dead hosts. Stable-isotope analyses demonstrated that colonies living on the three hosts differed substantially in diet, providing a possible mechanism for the observed differential suitability of hosts. However, the analyses were unable to establish conclusively that A. ligulata extrafloral nectar was an important reward for spiders. Variability in the A. ligulata– P. candidus system suggests that plant–spider associations, like other facultative protection relationships, likely vary along a continuum from antagonism to mutualism.
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    Insect seed predators as novel agents of selection on fruit color
    (Ecological Society of America, 2004-08) Stanton, Maureen L; Whitney, Kenneth D
    The ecological and evolutionary dynamics of fruit color polymorphisms remain poorly known because patterns and agents of selection have rarely been identified. Here, we examine Acacia ligulata, a shrub of the Australian arid zone characterized by a red/yellow/orange aril color polymorphism. Seed production patterns over four populations and three years suggested that spatially variable selection may be acting to maintain the polymorphism: red and yellow aril color morphs each had the highest seed production in alternate sites. Seed production differences between morphs were a function of both intrinsic plant characters (fruit production) and predispersal seed predation, which affects the number of viable seeds matured per ovule. Fruit production differences are hypothesized to result from a genotype-by-environment interaction, perhaps related to plant vigor. In contrast, morph differences in the numbers of viable seeds per ovule are produced via differential seed predation by heteropteran insects, as demonstrated by exclusion experiments. Because these predators feed when aril color is not visible, differential predation is evidently a response to pleiotropic effects of fruit color alleles. We suggest that such pleiotropic effects may be a common feature of fruit color polymorphisms, and that the most obvious selective agents (that is, seed dispersers) may not always be the most important.