Pollination Problems (BIO4000/1)

The first article that caught my eye was related to pollination and its intrinsic link to plant biodiversity.  Ecology and, more specifically, pollination biology is one of my major interests, so it stuck out above most of the genetics, biochemistry or microbiology articles that frequent many popular science journals.  

(Photograph taken in Nieuwoudtville, Northern Cape)

This article, authored by Anton Pauw from Stellenbosch University is titled: “Can pollination niches facilitate plant coexistence” (2013 Trends in Ecology and Evolution 28 (1): 30-37) and has the potential to help us better understand the conservation requirements and ecology of the Cape Floristic Region’s pollinators and plants, with potential implications for the Western Cape’s eco-tourism and fruit cultivation. 

Pauw is trying to find out why there are so many plant species on Earth.  This article, therefore, focuses on an explanation for the origin of plant species and an explanation for how they can coexist.  There are many ways plants can evolve and speciate [become a unique species] and Pauw is looking specifically at the roles pollination and pollinators play in it [Pollination: the transfer of pollen from the anther (man-bits) to the stigma (lady-bits); Pollinator: an animal that carries the pollen between flowers].   Changes in pollinator are said to be associated with 25% of speciation events in plants and is therefore responsible for a great amount of our plant diversity. 

Pollinator-driven speciation creates species that are different in their floral features.  For example, a change in the structure of a single-scent molecule in the nectar can result in speciation if the different scent attracts a novel pollinator. The broader question here is: “In the absence of other differences, are species with different flowers able to coexist from an ecological perspective?” 

The article revolves around Niche Theory, which says coexistence is possible if intraspecific (within a species) competition is stronger than interspecific (among species) competition.  This expects that each species limits its own abundance rather than be limited by the abundance of competing species.

Species generally limit their own abundance by using all of a resource, and so the number of limiting resources determines the number of species in a community of plants.  If a plant only has one specific pollinator, then it can become a limiting resource, if the likelihood of a flower being pollinated decreases with increasing density of flowers from the same species, which means it would produce less seeds and have fewer offspring.  This scenario would allow for a mutant or alien invasive plant, which uses a different pollinator (or pollinator resource) to invade the community and coexist with the resident plants.  

A good example of this could be in a monoculture (only one species in the plant community) of a bird-pollinated species, the amount of seeds the plants can produce may be limited by pollen transfer when there are lots of flowers in the area if the birds become satiated and they do not visit every flower, so some don’t get pollinated.  This monoculture could then be invaded by an insect pollinated species, which won’t have this same limitation.  The two plant species can then have separate pollination niches.

Seven tests can be used to see if niche theory, driven by pollinator speciation, is at play.  The article runs through these seven tests and the theoretical and practical answers to them.  From an exploration of current literature and studies, it can be concluded that the jury is still out on whether or not separation of pollinators can allow plant species to coexist.  Firstly, evidence is mounting that the amount a flower or plant is pollinated declines with an increase in the density of flowers from the same species.  Secondly, further evidence suggests that a decline in the amount a flower is pollinated can decrease the growth of the total plant population.  Although no studies have yet linked these two mechanisms to show that competition within the same species for a pollinator can lead to the regulation and stabilisation of the density of a plant population. 

This study links neatly in with conservational issues.  A deeper understanding of the roles of pollinators in promoting plants species coexistence is required from a pure academic perspective, but it can be crucial in showing the possible effects of human influences (which is generally disruptive) on pollination and on the diversity of pollinators. 

If pollinators are, in fact, a limiting resource for plant species, then a decline in their diversity can shrink the potential niche space, most likely creating a domino-effect leading to decreased plant diversity.  The potential effects of this are far-reaching and are not just trivial academic banter.      

(Photograph taken on Table Mountain, Cape Town)

The Western Cape of South Africa contains one of the biodiversity hotspots in the world, The Cape Floristic Region (CFR).  It is the smallest of the six recognised floral kingdoms and contains a disproportionately high amount of diversity and endemism (meaning it occurs nowhere else on the planet).  There are more than 9000 vascular plant species of which 69% are endemic.  So our fynbos, which contributes to most of this diversity and endemism, is well worth looking after.  Not only is at extraordinary floral marvel, but monetary worth of fynbos biodiversity, due to harvests of fynbos products (e.g. wildflowers, honey) and eco-tourism is estimated to around R77 million per year.  So it is clear that the CFR has both an economic and biological value.

The fair Cape that I live in is also a major producer of fruit for the domestic and international market.  The sheltered valleys between the mountains are ideal for the cultivation of export quality fruits.  This includes your apples, grapes (and our delicious wine), olives, peaches and those juicy oranges.  The disturbance caused by farmers and all others to these regions has the potential do disrupt pollinator populations, which could then have a negative feedback to our export quality fruits. 

So by gaining a greater insight into the roles pollinators play in plant species coexistence and their greater ecological role, particularly in the Western Cape, it will allow us to lay down the theoretical understanding that is necessary to implement the practical conservation in this region.