Experimental investigation of the effect of spatial aggregation on reproductive success in a rewardless orchid

Plant reproductive success within a patch may depend on plant aggregation through pollinator attraction. For rewardless plants that lack rewards for pollinators, reproductive success may rely strongly on the learning abilities of pollinators. These abilities depend on relative co-flowering rewarding and rewardless plant species spatial distributions. We investigated the effect of aggregation on the reproductive success of a rewardless orchid by setting up 16 arrays in a factorial design with two levels of intraspecific aggregation for both a rewardless orchid and a rewarding co-flowering species. Our results show that increasing aggregation of both species negatively influenced the reproductive success of the rewardless plants. To our knowledge, this is the first experimental study demonstrating negative effects of aggregation on reproductive success of a rewardless species due both to its own spatial aggregation and that of a co-flowering rewarding species. We argue that pollinator learning behaviour is the key driver behind this result

Generalized food-deceptive orchid species flower earlier and occur at lower altitudes than rewarding ones

Aims Food-deceptive pollination, in which plants do not offer any food reward to their pollinators, is common within the Orchidaceae. As food-deceptive orchids are poorer competitors for pollinator visitation than rewarding orchids, their occurrence in a given habitat may be more constrained than that of rewarding orchids. In particular, the success of deceptive orchids strongly relies on several biotic factors such as interactions with co-flowering rewarding species and pollinators, which may vary with altitude and over time. Our study compares generalized food-deceptive (i.e. excluding sexually deceptive) and rewarding orchids to test whether (i) deceptive orchids flower earlier compared to their rewarding counterparts and whether (ii) the relative occurrence of deceptive orchids decreases with increasing altitude. Methods To compare the flowering phenology of rewarding and deceptive orchids, we analysed data compiled from the literature at the species level over the occidental Palaearctic area. Since flowering phenology can be constrained by the latitudinal distribution of the species and by their phylogenetic relationships, we accounted for these factors in our analysis. To compare the altitudinal distribution of rewarding and deceptive orchids, we used field observations made over the entire Swiss territory and over two Swiss mountain ranges. Important Findings We found that deceptive orchid species start flowering earlier than rewarding orchids do, which is in accordance with the hypotheses of exploitation of naive pollinators and/or avoidance of competition with rewarding co-occurring species. Also, the relative frequency of deceptive orchids decreases with altitude, suggesting that deception may be less profitable at high compared to low altitude

Flower color phenology in European grassland and woodland habitats, through the eyes of pollinators

Some studies have claimed that flowers in bloom at particular times of year are more likely to be of particular colors to better attract pollinating insects. To test this, we analyzed a data set collected from five field sites near Strausberg, Germany, which included information on flower color and months of blooming. However, we chose to consider flower color as perceived by bee as well as human visual systems, as well as independent of any color vision system, to reveal whether trends, if present, have any ecological relevance. Using randomization analyses, we were able to consider whether blooming time interacts with flower color, and how this interaction depends upon other factors. Our results show that there is an association between the months of flowering and the colors of flowers — but only when flowers are considered according to human color categories. Further analysis showed that this is merely a consequence of flowers from the same family being more likely to flower at the same time and have similar colors. All these effects disappeared when flowers were considered using bee color categories, and in the analyses of physical spectral reflectances

Do local conspecific density and floral display size influence fruit set via pollinator visitation in Orchis militaris?

Plant density varies naturally, from isolated plants to clumped individuals, and this can influence pollinator foraging behaviour and plant reproductive success. In addition, the effect of conspecific density on reproduction may depend on the pollination system, and deceptive species differ from rewarding ones in this regard, a high density being often associated with low fruit set in deceptive plants. In our study, we aimed to determine how local conspecific density and floral display size (i.e. number of flowers per plant) affect fruit set in a deceptive orchid (Orchis militaris) through changes in pollinator visitation. We measured fruit set in a natural population and recorded pollinator abundance and foraging behaviour within plots of different O. militaris densities. Detailed data were recorded for the most abundant potential pollinators of O. militaris, i.e. solitary bees. Floral display size was negatively correlated to fruit set in medium-density plots, but uncorrelated in low- and high-density plots. Plot density had no effect on solitary bee abundance and visitation, which may be due to low pollinator abundance within the study site. The proportion of visited flowers per inflorescence was negatively influenced by floral display size, which is in line with previous studies. In addition, solitary bees spent decreasing time in successive flowers within an inflorescence, and the time spent per flower was negatively affected by ambient temperature. Our results suggest that pollinator behaviour during visitation is poorly linked to pollen deposition and reproductive success in O. militaris