Being in the minority could have its uses, says S.Ananthanarayanan.

As there is strength in numbers, abundant species are expected to dominate resources and crowd out the ones which cannot compete. But being specialised could also be a good thing.

Na Wei, RaineeL. Kaczorowski, Gerardo Arceo-Gómez, Elizabeth M. O’Neill, Rebecca A. Hayes and Tia-Lynn Ashman, from University of Pittsburgh, The Holden Arboretum, Kirtland, Ohio and Tennessee State University, Johnson City, writing in the journal, Nature, describe the mechanism that enables rare plant species to survive in the wild. How this happens is what assures diversity, and the process needs to be understood.

Although most flowering plants possess both male and female organs and are capable of self-fertilisation, or autofertility, the paper cites evidence that about 80% are fertilised by external pollinators. These could be honey bees, birds, insects, or other animals. “Plant–pollinator interactions are thought to be among the most important drivers of biodiversity on Earth,” the paper says, and “without pollinators more than half would suffer marked declines in seed production.” How, then, do rare species, or whose individuals are few and scattered in number and could be missed by pollinators, manage to avoid extinction?

The basis of the analysis is that fitness of a species, and hence the likelihood of its persistence, depends on the frequency of successful pollination. Successful pollination is when pollen from the same species is deposited on an ovule, and is referred to as conspecific pollination, or CP. And when there is CP, seed production depends only on how much pollen there is or how many ovules. The converse is when the pollen deposited is of a different species, referred to as heterospecific pollination, or HP. When this happens, there no fertilisation, of course, but the foreign pollen also blocks the ovule from receiving the correct pollen.

When there are many plants growing together, some abundant, some scarce, there is a complex interplay of CP and HP, as well as abundance or scarcity of pollen and ovules. It would take a community-wide study to work out the effect this would have on the survival of rare species, and the growth of abundant ones. And such a study does not seem to have been carried out so far, the paper says.

The paper describes the two main mechanisms in operation. The first is niche partitioning – where species divide and occupy aspects of the environment, so that they do not compete with each other. While this would help rare species, it also means there is competition for resources within the same species, which limits the growth of abundant species. The other mechanism is facilitation – where there is a possibly symbiotic relationship, with benefit to at least one of the species, with less or no damage to either. Both mechanisms could be in action at the pollination stage, the paper says.

Pollinator services are generally limited and abundant species have got where they are because they are more generalist, or accommodating in the pollinators they use. A cost these species pay, however, is that as the abundance grows, the chances of CP being misplaced are higher, which amount to greater chances of HP. A specialist plant, on the other hand, is less abundant as it has limited pollinators. So long as the pollinators are there, however, the rate species should benefit, because it may have better chances of CP and less risk of HP.

The paper notes that apart from the action of pollinators, rare species could be maintained by auto-fertilisation, where pollen is transferred from the anther of a flower to its own stigma. This may be of use to rare species even when numbers favour an abundant species. And, as autofertilisation is easier if the anther and stigma are close together, such proximity may be a feature that is selected in rare species. There is, however, no evidence of the role of floral traits in creating and maintaining pollinator niches, the paper says.

The team collected data of species and feature-related, pollination-mediated fitness, and followed the trail of pollen, to collect information of CP and HP. The objective was to test and estimate the effect of categories of pollinator partitioning and facilitation, and assess how the rise and fall of CP and HP affected the gain of fitness of rare or abundant species, and the effect of variations in floral traits, or the ease of autofertilisation. Data was collected over two flowering seasons, of 7,324 pollinators, bees, flies, beetles, butterflies and moths, wasp, ants, and others, making visits to 79 plant species, abundant and rare, that grew together. And then the effect of functional traits of plants and flowers. And over 3 million pollen grains were identified, to follow the trail of CP and HP, and fitness.

The results, of statistical analyses and model fitting, the paper says, support the idea that plant-pollinator interactions, both shared and specialised, end up passing the advantage to rarer species. The effect of less specialised pollinators and abundance is a cost of CP per ovule, which limits seed production. And the net effect is that diversity is maintained, rather than extinguished. Numeric superiority was not found to be an advantage, nor was the capacity for self-pollination. That greater numbers do not help may be because of how co-flowering plants are distributed in a way that permits greater inter-species visits by pollinators. And as rare species did not benefit from self-pollination, it appears that they adapt to the pollinator-mediated processes.

This study of community-wide interactions between plants and pollinators that affect fertilization success is the first study of its kind. Whether these conclusions are generally true, however, needs more investigation, the paper says. While we need to know the relative importance of pollen and ovule availability as well, the value of the plant and pollinator community – not just the plant species - is important in the global drive to preserve plant diversity. “These considerations are more urgent than ever for conservation …. as changes in plant–pollinator mutualisms are becoming commonplace,” the paper says.

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