The opposite is true for Rr and no effect is found for Rs. Overall CAwr has the highest relative fecundity with the highest fruit production . Average number of fruit damaged for each lineage correlates: strongly with Rs and Rr fruit production, Rs and Rr potential reproduction and Rs and CAwr final plant weight, and moderately with CAwr fruit production, potential reproduction and Rr final plant weight. In all the cases the covariance values are positive. No correlation was found with days to germination . Debris due to granivory – Debris material due to granivory was only found under plants in unprotected plots. In general the debris due to granivory was a mix of viable and unviable seeds as well as fruit wall segments. Some seeds were found bared, with no fruit material around them, while others were either fully or partially covered by fruit wall segments. The largest amount of debris due to granivory was found under CAwr plants in unprotected plots as well as the largest number of potentially viable seeds. Most of the fruit segments were found broken and empty. The average values are listed in table 2.6. Although the debris due to granivory data was based on a small sample size, I calculated the average proportion of seeds removed for each plant by granivory in two different ways. One of the calculations was done as described in the methods. The other calculation included the subtraction of the potential “viable” seeds found in the debris due to granivory and averaged for each lineage . Subsequently, I compared both averages. In the case of both progenitors there are minimal differences. However, procona system for the hybrid derived-lineage there is a 10% reduction of seeds removed.
Granivore behavior – We observed and filmed house finches actively manipulating and feeding on fully formed Raphanus fruits. No other avian species were observed on Raphanus plants although there are at least five other granivores regularly occurring in this location . At the different plots, the birds visited the plants daily in an almost predictable fashion. In all cases I observed males, easily recognizable for their characteristic red coloration around the face and head. The peak of house finch foraging activity corresponded to the period in which the fruits were fully formed but still green. We never observed the house finches feeding out of the debris on the ground; they appear to only choose fruits attached to the plants.The house finch Carpodacus mexicanus, an avian granivore, affected the reproductive patterns of three Raphanus progenitors and hybrid lineages in dissimilar ways. When exposed to the granivore, the hybrid-derived CAwr had proportionally more fruits damaged relative to its progenitors . Overall relative fecundity of CAwr was significantly reduced compared to protected plants of the same lineage . When exposed to the granivore, CAwr had lower fecundity compared to the wild progenitor Rr. However, due to its high overall number of fruits and seed production relative to both progenitors, CAwr was not negatively affected by granivory. Under conditions where granivory is excluded, CAwr has the highest relative fecundity compared to both progenitors , concurring with results from a prior study . The fecundity of protected Rr fruits was comparable to the high fecundity of protected CAwr fruits. A potential explanation for this result might be that Rr fruits detach easily from the mother plant. I found a large number of dropped Rr whole fruits relative to whole attached fruits . Finches perch when attempting to feed, with the result that many Rr fruits fall to the ground before they can be damaged on the branch.
Because the birds did not forage on the ground, the seeds in fallen fruits remained viable. Easily detachable fruits were also observed occasionally on CAwr plants but not as frequently as in the wild Rr progenitor . Fecundity values of the cultivated Rs lineage did not differ between unprotected and protected treatments. Rs exhibited low fecundity values overall, which is in agreement with other studies .However, the reproductive output of Rs may be even lower than reported because our measurement protocol likely led to an underestimate of the negative impact of the granivore for this lineage. Observations by us and other researchers indicated that birds open entire fruits of the cultivar Rs and consume all the seeds inside . Fully eaten fruits often appear similar to an empty pedicel and thus may not be counted as damaged at all. An additional caveat is that by planting the three lineages close together I may have influenced the granivore’s choice to feed more often on Rr and CAwr, which are bigger and more productive compared to Rs. Density-dependence has been observed in other studies of granivory and it is expected that birds make foraging choices to maximize their own fitness . Plant invasion often involves multiple introductions followed by a time lag during which invader populations overcome small population effects such as genetic drift and inbreeding causing Allee effect, Founder Effect, and bottlenecks . In addition, other mechanisms favor invasiveness like gene flow, potentially leading to interspecific hybridization, . Some invasive plants are a result of successful interspecific hybridizations, presenting defense traits that may be identical, different or additive relative to one or both progenitors, defining their susceptibility or resistance . Hybrids can be more susceptible: a comparison of the densities of herbivore and pathogen species on two progenitors species, Salix sericea and S. eriocephala and hybrid willow shrubs, revealed significantly higher herbivore densities on the hybrids . Althoughthe trend in the Fritz et al. study and others that he and collaborators reviewed do not support higher resistance in hybrids, other studies have found evidence for this. For example, the hybrid derived Helianthus annuus texanus acquired resistance to two types of herbivores from both ancestors H debilis and H annuus annuus increasing its relative fitness . In the case of invasive plants, hybridization can contribute to the evolution and success of invasive lineages . In addition, after establishment of long-standing populations in the community in a novel environment, invasive plants evolve and with them their potential enemies . And as it is the case for all ecological interactions, invasive species exist in a complex network, where new abiotic and biotic conditions imposing a new selective pressure on the plants that may cause an adaptive response from the plants . Considerable effort has been given to the idea that plant colonists have the advantage to be unknown by their new biotic enemies, i.e. herbivores, frugivores and pathogens. This idea, initially known as the “enemy release hypothesis”, was proposed by Elton in 1958 but can be traced back to Charles Darwin in 1859 and subsequently with an added evolutionary context became the Evolution of Increased Competitive Ability hypothesis by Blossey and Nötzold . The EICA predicts that invaders have the opportunity to evolve a competitive advantage compared to native taxa by re-allocating on plant growth instead of on plant defenses. Aspects of this hypothesis have been tested empirically multiple times at many levels, at both single sites and continent-wide scales,but the results have been equivocal .
With respect to pre-dispersal seed predation, procona valencia buckets previous studies have found that other factors are fundamental to maintain the demographics. For example, higher predation does not translate in lower fitness rates that will affect the population size or the following year’s adult density in invasive Centaurea solstitialis . Similarly, in spite of higher susceptibility to the effect of invertebrate pre-dispersal seed predation by the fly Hylemya sp. in natural and artificial hybrids crosses relative to progenitor species, Ipomopsis aggregata and I. tenuituba, flower traits and higher pollination rates compensate for seed losses . One of the fruit characteristics that has been found to reduce the negative effects of seed-eaters on fitness is multi-seeded fruits . In our study, estimated percentages of damage decreased when calculated based on estimated seeds removed compared to values based on fruit damage, diminishing the level of impact by the granivore. Thus, multi-seeded fruits reduce the effect of the granivore pressure on seed survival. Prior studies in non-invasive plants also support this idea; the impact of seed predation is reduced when plants produce multi-seeded fruits . To our knowledge, this study is the first to find evidence of the advantage of multi-seeded fruit in an invasive plant in the context of granivory. Aside from the density-dependent effect that may have distracted the birds from eating fruits of Rs, I propose that fruits of each lineage provide a different level of defense to the seeds inside. Currently it is known that fruit morphology does vary among the Raphanus lineages as does fruit mechanical defense . Invasive plants have been demonstrated to exhibit rapid evolutionary change suggesting that ~100 years of granivore interactions may have imposed substantial evolutionary pressure on the hybrid-derived fruits. This possibility coupled with the pre-existing hardened fruit wall in Raphanus raphanistrum leads to the hypothesis that fruit hardness is a genetically based trait that is favorably recombined as hybrids are formed and diversify in time . We never observed fully eaten fruits of California wild radish, but at most ~1/3 of the seeded portion. Previous work on CAwr mating system has found that the seeds inside an individual fruit are usually sired by various pollen sources and that the siring of the pollen occurs in a non-random manner , which has been related to a within-fruit seed size variation . The combination of these traits and the impact of the granivore provide a unique situation to study the combined effects of natural and sexual selection. To include the granivore’s perspective was beyond the scope of our study. Nevertheless, our observations are in agreement with other features of the physiological ecology of house finches. We only observed male house finches feeding on our experimental radish plants. According to Hill et al. house finches have a 6-month breeding season, which in their western distribution starts in January . The fruits in the experimental plants described here were fully formed in June, which is near the end of house finch breeding season and just prior to the onset of molting. Carotenoid based plumage pigmentation such as red, orange and yellow can only be acquired through diet and are known to have a significant function in sexual selection, immune system and senescence . To acquire the necessary pigments to color their plumage, male house finches need to include carotenoid rich food in their diet during the time the feathers are replaced and growing . Raphanus plants have carotenoid contents and are particularly rich in ?- carotenoids , which are the specific class of pigments that house finches most likely require to color their feathers . Female house finches prefer males with bright red plumage and the intensity of the red coloration in the males significantly correlations with: 1) an early beginning of the breeding season , 2) increase in number of brood for a given pair , as well as 3) the concentration and type of carotenoids in the food ingested . To our knowledge there is no study that has reported carotenoids extracted from radish seed endosperm or even fruit wall; most studies focus on cotyledons and leaves . However, carotenoid pigments in canola Brassica napus seeds, another Brassicaceae species have been reported . The pattern of fruit consumption described here represents a positive effect for house finches and,surprisingly, a positive effect for Raphanus. We propose a novel interpretation that granivore-fruit interactions in C. mexicanus- Raphanus are mutualistic. The results of our study provide information of an invasive hybrid derived lineage performance relative to both progenitor lineages in the context of a novel biotic interaction. The hybrid as well as the wild progenitor have a combination of traits that increases their ability to survive and persist in novel environments. High fecundity and multi-seeded fruits with mechanical defenses protect and compensate for the effect of the granivore. The wild progenitor has some advantages over the hybrid lineage at the predispersal seed predation phase. Although this result might be accurate I also suspect that our experimental design might have added some unwanted level of competition among the plants at the early growing stages affecting the hybrid performance .