Bees and specific plant fauna they visit and pollinate.
Survival and reproduction of several wild plants and crops is mostly by insects pollinator, their recognition and importance have been increased in this climatic changing scenario, which affects the various aspects of their life cycle. According to an estimate, approximately 30,000 species of bees are known in entomology, and about 190 species of bees have been reported to be associated with pollination. There can be an established link between seed production and pollinator diversity, for the plants with a generalist pollination system. The increasing of human habitation affects insect pollinators in various ways, i.e. of habitat destruction, results in low availability of food sources, nesting, oviposition, resting, and mating sites. Pollinator availability restraints the geographical distribution of plant species, i.e. to develop an ecological niche of certain plant species. Failure of pollinator- plant interaction mutualism results in lower seed production and sometimes extirpation of plant population has been recorded. The declining pollinators’ population strengthens existing plant-pollinator interaction or allows new pant pollinator interaction to form. Maintaining the commercial and wild pollinator populations and preventing future shortages of pollination services, therefore, is extremely significant.
- plant-pollinator interaction
Plant and pollinator interaction results in the pollination of various plants that are
Robbins et al.  reported that most accepted estimates indicate honeybees’ account for at least 80% of all insect pollination. For decades the consequences of insect pollination have been documented in treaties by Free , McGregor , and Pesson and Louveaux . In apiculture the most important species,
2. Pollinators diversity in agro-ecosystem
An enormous number of the world’s insect diversity visits flowers for nutrition, but all are not efficient pollinators. Among crop foods, fibers, edible oils, medicines, and other valuable products, a significant production occurs due to the vital role of insects and other animal pollinators. In all types of ecosystems, bees are recognized as the most valuable pollinators, but their precise roles in pollination are not well documented. According to estimation, approximately 30,000 species of bees are known in entomology, and about 190 species of bees have been reported to be associated with pollination in North America. Some of the other noteworthy contributors in pollination are; alkali bee (
The interdependency of plants and pollinators vary in their degree, some plants species depend primarily on a single species of pollinator, which in turn has restricted sources of pollen or nectar. One example of a closely dependent association is the interaction between plant Yucca (Agavaceae) and its pollinators, yucca moth (
|Common name||Scientific name||Example of crop plants pollinated|
|Alkali bee||Alfalfa, clover, mint|
|Carpenter bee||Passion flower, eggplant, pepper|
|Digger bee||Cotton, fruit trees|
|Alfalfa leafcutting bee||Alfalfa|
|Blue orchard bee (mason bee)||Almond, apple, sweet cherry|
|Squash and gourd bee||Squash, pumpkin, gourds|
Among the herbivorous insects, the interaction of butterflies and moths is found during both its larval and adult stages and the latter is involved in pollination (Table 2). Some of these are
|Wild carnation, ||Butterfly species|||
|Native plants of North America||Checkerspot butterfly, |||
|Milkweed and other||Monarch butterfly, |||
3. Pollinators and plant interaction
In an ecosystem, the interaction between the organisms favors co-evolution and it gradually helps to evolve together for some betterment or for existence in nature. Those plants having a generalist pollination system, have a link between pollinator diversity and seed production can also be established . Pollination biology (Figure 1) draws attention to both evolutionary and ecological approaches i.e. the link between pollinator behavior and plant mating patterns, generalization, and specialization in a pollination system [24, 54, 55].
There are many reasons for which pollinators visit flowers, including feeding, pollen collection, warmth in some cases, oils and resins, as well as for shelter and mating rendezvous sites . These plant and pollinator interactions as mutualisms sustain not only plant diversity, but also the diversity of an estimated 350,000 animal species, including insects, birds, and mammals [57, 58, 59]. Ratto et al.  reported an average 63% loss of fruit or seed production when vertebrate pollinators are excluded from the flowering plants’ ecology they visit. These results often reported experimentally that selective exclusion of a single group of an effective pollinators from plant-pollinator interaction can result in the failure of plants to produce fruits or seeds.
Diversity of pollinators in habitat can compete for floral resources , the declining pollinators population strengthens existing plant-pollinator interaction or allow new plant-pollinator interaction to form [62, 63]. The diverse pollen feeding behavior by bee species is due to digestibility and nutritional content requirement fulfillment . There are specialized flower plant-pollinator relationships like certain solitary bees species , reduction of these flower plants from habitat often results in the elimination of their specialist plant-pollinator populations. Viana et al.  evaluated more than 250 studies that showed the impact of landscape and pollinators interactions. The forage bees’ ability to assess the nutritional value of pollen sources before establishing plant-pollinator interaction is valuable [67, 68]. A recent study by Armbruster  on pollination ecology mainly emphasizes three aspects, first ecological (pollination involving one or few kinds of plant and animals), second phenotypic (having specialized flowers or morphologies) and third
is evolutionary (showing transitions towards increased specialization).
4. Declining pollinator, a potential threat
It is difficult to determine as less surveys are organized to record whether pollinator species are declining around the world. If we study the literature many explanations have been invoked to account for declines in pollinator population around the globe [70, 71, 72, 73, 74]. There are a few of these reasons such as exposure to pathogens, parasites, and pesticides; habitat fragmentation and loss; climate change; market forces; intra and inter specific competition with native and invasive species; and genetic alterations. Reduction in pollinator diversity or abundance may influence the amount and source of pollen deposited on the reproductive part of flower or stigma .
The western honeybee,
The application of pesticides, especially insecticides in crops, vegetable, and orchards to control pests, kills or weakens thousands of honey bee colonies and affect their foraging and nesting behaviors that prevent plant pollination [83, 84, 85, 86]. The basic behind pesticides to kill or weaken the colony is the result of accidents, careless application, or failure to adhere to label recommendations and warnings. Some of the advance studies showed that transgenic crops developed to reduce the unintended effects of pesticides have shown that there are direct effects on non-target species, including some pollinators [87, 88, 89]. Transgenic crops that express transgene with varied expression levels have not been yet reported effects on honeybee .
There are degradations reported around the globe in habitat i.e. alternations, fragmentation, and loss cause major problems for populations of many organisms, and pollinator populations are also one of them [1, 91]. Insect pollinator’s i.e. bees and others require nesting sites (suitable soil, dead wood, abandoned mouse nests, and burrows) and floral resources (nectar and pollen) to exist. These habitat resources are at extinction through the disruption caused by row crop agriculture, grazing, and fragmentation of habitat into patches, which are small enough for the survival of diverse communities of pollinators . Some other reported causes of decline in pollinators’ population are monoculture, the lower density of weed flora, declining pastures, loss of flower-rich grasslands, and overgrazing can disrupt the nesting of bees [93, 94, 95, 96].
Industrial development around the global, regional and local climate changes can alter or disrupt plant-pollinator relationships. Many studies and reports show the climate change forecast is shifted in temperature  and precipitation, concentrations of carbon dioxide  and ozone, and ultraviolet levels [99, 100] effects pollinators in many ways. There is evidence that the latitudinal and altitudinal ranges of some plants and pollinators have changed in the past 30 years, presumably in response to global warming [101, 102, 103].
5. Management and restoration of pollinators
Information on the status of most of the pollinators is incomplete around the world, and it is in a natal stage in developing countries . Much can be done to maintain commercial and wild pollinator populations and to prevent future shortages of pollination services. Indigenous communities have an important role in the conservation of habitats through customary laws/rules, these areas are important biodiversity refuges providing valuable ecosystem services including pollination, which improved crop pollination in adjacent farming landscape [104, 105, 106, 107]. An agri-environment scheme, on farmlands, has been proactively practiced in European Union countries through incentives to support biodiversity . The US Farm bill (2008) had made specific economic provisions for pollinator conservation when it was further ratified in the 2014 Farm bill. The potential of conserving non-cropped land as a model in agro-ecosystem can be proved vital, through these agri- environment scheme models of conservation pollinators in the agriculture landscape can go a long way to inoculate pollinators naturally . Mostly, pollinators are transported over long distances for the purpose of pollination . They are also transported outside of their natural distribution range (e.g. African honey bees into Brazil, European bumble bees into Australia, Asia, and South America) .
Best management practices (BMP), similar to Good Agricultural Practice (GAP) should be promoted by the FAO in apiaries that need to be developed that respect local differences in beekeeping and hive management at the country level. There should be non-compulsive suggestions have been put forward overall lacking international harmonization . In this perspective resistant stock of bees against parasitic mites is to be developed, identify the locally adapted stock of bees, instrumental insemination in bees, selection and managing miticide resistance in bees, etc.
Through collective approaches either for native and introduced bee species, whether solitary or social, requires the correct balance of water, flora hosts that offer sufficient pollen and nectar of the correct types , nest building materials (leaves, resin, sap, gums, floral oils, essential oils, bark, plant trichomes, old mouse nests, snail shells, mud, sand, pebbles), and nesting substrates  to survive as adults and rear their larval broods (Table 3) .
|S. no.||Pollinator group||Resource function||Resource|
|1.||Honey bees, bumble bees||Nesting, roosting sites, or substrates||Cavities (underground, hollow trees)|
|2.||Nonsocial bees, wasps||Nesting sites or substrates||Bare ground, vertical cliffs or ditch bank, adobe walls|
|3.||Bumble bees||Nesting sites||Rodent, mouse nests|
|4.||Flies||Adult food||Pollen, nectare|
|5.||Leafcutter bees, mason bees||Building materials||Leaves cut into pieces or masticated|
|6.||Orchid bees||Pheromones||Essential oils, such as monoterpenoids collected by males|
|7.||Ants||Adult, larval food||Nectar, honeydew, insect prey|
6. Limitations that restrict the pollinator-plant interaction
Deforestation and habitat changes have shown an adverse effect on insect pollinators, seed predators, decomposers, and parasitoids, which are highly susceptible to these changes. The success of plant reproduction may be sensitive to the loss of pollinators [74, 116, 117]. Some the evidence suggests that pollinator populations are declining worldwide . These changes result in the destruction of food sources, nesting, oviposition, resting, and mating sites . The increase in population pressure and urbanization of wild and agricultural lands has disrupted the habitat of wild pollinators viz., moths , butterflies , and bees [70, 71, 120, 121] and managed pollinators experienced sudden colony losses . With the increased demand for food crops and higher productivity by use of either plant production or plant protection chemicals has killed the pollinators directly, and eradicated alternative pollen sources from their natural forage species [123, 124]. The pollination host range of honeybees is wide, but they do not pollinate all types of the crop with equal efficiency, are not active under all climatic conditions . Whereas, some of the bees have the ability to pollinate some crops at a higher level of efficiency, with their lower population densities, and with greater independence of climatic conditions [21, 126].
Database of wild pollinator populations and communities is one of severe lack of long term planning and evaluation of their valuation in much of the world, especially for invertebrate pollinators [127, 128, 129]. Such as European red List of bees, 57% of the European bee species were categorized as “data deficient”; butterflies and moths from parts of Africa that are described at threatened status also reported in the literature [130, 131, 132].
Emerging risks such as diseases, invasive alien species, pathogens, etc., threaten the pollinators, there should be phytosanitary and preventive measures that could be accompanied for the effective response to these emerging risks. Few regions in the world (parts of Australia, Seychelles), that are not affected by the ectoparasitic mite,
7. Conclusion and future prospect of pollinators
Sustainable agriculture requires reliable pollinators, but a shortage of pollinators could not be strongly evidenced for food crisis or pollination crisis. Long-term data deficient on the pollinators’ population should be noted and there is no evidence of their decline over time, neither there is a framed definition to label pollinator crisis universally on that frame. The honeybee is a valuable pollinator to perform an important pollination function in the ecosystem. The decline in a number of managed pollinators in the system is due to some of the reasons such as introduced parasites and pathogens. There is a need for time to be compatible with and comprehensive management strategy of crop pollination for sustainable agriculture. Pollinators require to be managed through augmentation or conservation as needed to study their biology and ecology. Several studies that show the declining bee population poses a threat to global food security. Nesting habitat must be provided whether as a soil bed of a more or less special nature, or as stumps of trees and logs, or as rodent burrows for bumbles. Conservation of native pollinator habitat can be enhanced by changes in land use management strategies viz., non-cultivated patches of ground, setting up parks or protected areas for wildlife, flora, and fauna both at public and private areas. There should be a policy for arable, non-arable, and along with the roadside land that could facilitate the planting of wild plant flora which encourages pollinator populations. There should be judicious and timely use of pesticides that should ensure the protection of pollinators. There should be a crop pollination plan for all pollinator-dependent crops that must be included in the national or state crop production strategies. Farmer’s awareness camps should be organized in the rural areas about crop pollination and the role of pollinators should be described, so that there may be a change in plant protection chemicals patterns.