How do bees help flowering trees survive?

Question

I have often heard a vague claim that bees help flowering tree populations survive.
As far as I know, bees take nectar and pollen concentrations from the flowers of flowering trees and use at least the nectar as a food source, but they don't take seeds out of these trees, so it's unclear to me how they help these trees survive in that regard.

Given that bees don't take seeds of flowering trees (and drop some when they fly) How do bees help flowering trees survive?
How does the taking of nectar and pollen away from flowering trees make bees symbiotically help such tree populations to survive and endeavor?

Answer 1

One of the goals of every living organism (including plants), thus it seems, is to create offspring for the next generation. This is the way plants (angiosperms) have sex.

That said; I personally define pollination as:

The act of transferring pollen grains from the male anther of a flower to the female stigma.

It should be noted that not only Bees play a very important role in pollination, but also other insects like wasps, beetles, mosquitoes etc.

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When a bee sits on the flowers to collect nectar, its feet pick up pollen grains ( the powdery stuff) from the anthers and transfer it to the female stigma (which is sticky to hold the pollens).

Michigan State University:

When a bee lands on a flower, the hairs all over the bees' body attract pollen grains through electrostatic forces. Stiff hairs on their legs enable them to groom the pollen into specialized brushes or pockets on their legs or body, and then carry it back to their nest. Individual bees tend to focus on one kind of flower at a time, which means it is more likely that pollen from one flower will be transferred to another flower of the same species by a particular bee. 

Then the pollen grain makes its way into the ovary of the flower through the long stalk (style) and fertilizes the egg. Then the flower matures to for a fruit and the ovules become it's seeds. It's a symbiotic relationship where the plant provides food and the bees pollinate them in turn.
Also, certain bright colouration of flowers and their sweet smell attract more insects.

A declining population of pollinating insects signal the decline of the whole ecosystem including plants and trees.
Entomologists keep a check on various insect populations. They have found a decline in the population of bees world wide! This is an alarming situation. This will cause huge damage to fruit industries including kiwifruit, watermelon, squash and many others. It's time we take the environmental issues seriously.The reports aren't vague!

Penn State College of Agricultural Science:

Historically, managed honey bee populations in the US and Europe have been monitored due to their vital role in providing pollination services in agricultural systems.  Several studies indicate that American and European beekeepers are suffering large annual losses.  In the US, beekeepers have lost ~30% of their colonies every year since 2006, with total annual losses sometimes reaching as high as 42% 

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Edit

Citations:

https://en.m.wikipedia.org/wiki/Pollinator#Honey_bees

https://en.m.wikipedia.org/wiki/List_of_crop_plants_pollinated_by_bees

A brochure from FAO on importance of bees as pollinators : http://www.fao.org/3/I9527EN/i9527en.PDF

Penn State College of Agricultural Science on declining population of pollinators : https://ento.psu.edu/pollinators/resources-and-outreach/globally-pollinators-are-in-decline

Wikipedia on the same issue: https://en.m.wikipedia.org/wiki/Pollinator_decline

Answer 2

Navoneel Karmakar's answer is correct -- there are many plant species for which bees are crucial or necessary pollinators because of self-incompatibility. Indeed, the disappearance of bees would lead to the collapse of these plant populations if no other mechanism of pollination (e.g. wind) was available.

But there are many plants that do not rely on insect pollination, and can self-pollinate to reproduce as well as exchange gametes by insect cross-pollination. Would the disappearance of bees (or the preferred insect pollinator) lead to reduced survival of such a plant population, by a mechanism other than drastic population collapse?

From a population genetics perspective, variation is a good thing. If a geographically isolated population of plants reproduces exclusively by pollination between genetically homogenous neighbors, the genetic diversity of that population is limited to the genes of individuals in that population (plus mutagenesis, though beneficial mutations are generally a consideration only in the long term). If a pest or pathogen is introduced to this population, and no individuals in that population carry the genetic determinants of resistance, that population will be threatened, barring some rare mutagenesis leading to resilience. Even outside the threat of disease, any isolated population is subject to inbreeding depression by propagation of deleterious alleles.

Now, introduce an insect pollinator. This organism can ferry plant gametes between geographically isolated populations, expanding the existing genetic space of each population to include the other. This pollinator is an agent of outcrossing, which combats the effects of inbreeding depression by the introduction of new alleles.

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Take, for example, Chamaecrista fasciculata, a species of bee-pollinated legume. Mannouris and Byers discuss the effect of habitat fragmentation on this plant, which has led to smaller populations resulting in reduced genetic variation and increased genetic drift load, defined here:

In small populations, genetic drift is likely to play a stronger role in changes in allele frequencies than natural selection. Random loss of alleles through drift is more likely to leave behind deleterious recessive alleles than selection would do. The result is an enhanced level of genetic load in small populations due to the effects of drift, known as ‘genetic drift load’.

This species can be selfed, but relies heavily on bee pollination to maintain genetic diversity.

Chamaecrista fasciculata is 80% outcrossed by bumblebee pollinators and the populations have been found to be very spatially structured due to limited seed and pollen dispersal.

Moreover, fragmentation of the prairie habitats has led to populations that are too isolated for bee cross-pollination.

All of the prairies listed as isolated on Table 1 were at least 30 km from another known prairie with C. fasciculata, which is beyond the distance bumblebee pollinators are expected to forage.

All of this to say, while C. fasciculata does not exclusively rely on bee pollination to reproduce, there is a deleterious effect of the loss of cross-population pollination evident in reduced population genetic diversity.