Abstract
Many different variables influence the relationship between plants and pollinators. Their mutualistic relationship drives a coevolutionary force among species. They rely heavily on each other for survival and reproduction, each exploited the others resources for their own benefit. With plants requiring insects and animals for pollen transfer and reproduction, new adaptations have arisen in response to pressures exerted on them by pollinators and external forces. Plants have modified their floral and fruit signaling to promote plant reproduction, and have evolved morphologically complex structures to restrict access to their specific pollinators. Pollinators have also made modifications to better increase their fitness. They have innate and learnt preferences of floral signals, recognizing which plants they will receive the greatest energy gain from. External factors like climate change are exerting selective pressures on these interactions. With the changing of the climate, comes changes to the life history events of these species. Pollinators are appearing earlier than normal and plants are flowering much sooner than expected. If these events do not line up with each other, mismatches will occur caused by the differences in shift of magnitude or direction. It is important to understand these relationships and how selective pressures provided by each species and external forces control the coevolutionary shift amongst these species.
Introduction
Mutualistic relationships are ubiquitous in nature. These types of species interactions involve the exchange of goods/services between species. Both species involved in these mutualistic relationships must incur some benefit from the interaction, although it usually comes with a cost. Pollinators and flowering plants do not act altruistically; they each have their own selfish interest in mind. Plants rely on pollinators such as insects and birds for their reproduction. Wind and water are also modes of transport for their pollen. Wind and water dispersal are not always the most reliable means of transport because they are unpredictable forces. The chances of the pollen landing on another plant of the same species are much slimmer than they are with animal pollinators.
Animal pollinators rely on flowering plants as a source of food. Insect pollinators, such as bees, consume the nectar deep within the flower to sustain themselves. Inadvertently, they gather the pollen from the plant and transfer it to a plant of the same species when they feed again. Bees are an especially important insect pollinator. They are species specific, so the chances of them visiting multiple plants of the same species are great. Without bees, many of the foods we eat on a daily basis would cease to exist.
This mutualistic relationship arose from the continuous interactions between flowering plants and pollinators and has resulted in coevolution of the two to increase each others survival, growth, and reproduction. Flowers have evolved bright colors and fragrances to attract bees, increasing their chances of reproducing. Plants also produce nectar as a reward for the bees, which is very energetically expensive to produce because it serves no real purpose for the plant other than to attract potential pollinators. There are many selecting agents acting on both species in this relationship, resulting in adaptations arising to increase their fitness. Pollinators have evolved specific characteristics to enhance their foraging behavior such as eye morphology. Plants have also felt the selective pressures and in response, have developed signals to better increase their chances of reproduction (i.e. attracting pollinators).
Diversity of Pollinator Systems
Plants can rely on many different species for pollination, such as birds and insect pollinators. But the characteristics of the flowering plants and the pollinators are a large factor into which pollinators are equipped for each flower. Some pollinators, such as hummingbirds, have evolved a long beak making it easy to reach the nectar deep within the flower. Other pollinators are unable to reach the nectar in these deep flowers, and therefore are less likely to visit flowers exhibiting this quality. The evolutionary and biogeographical patterns of plant-pollinator interactions show a complex system of constraints and flexibility.
In pollinator systems, there are both generalist and specialist pollinators. Generalist pollinators are happy to receive pollen and nectar from whichever species of plant they come across. Most pollinators are generalists, including most bees, flies and butterflies. On the other hand, specialist pollinators are very species specific and have evolved specific relationships with a few plant species. Specialist pollinators display specialized structures and behavior to exploit a select few flowering plant species.
Certain clades of animals can be associated with a limited range of pollinators. Ollerton et al mapped out these relationships into a phylogeny of the Family Apocynaceae. Apocynaceae is a family of flowering plants including trees, shrubs, and herbs. They chose this specific family of flowering plants because it is one of the best studied large families from the perspective of understanding plant-pollinator relationships. Bird pollination occurs frequently across the family, but always in combination with insect pollinators. Flowers that are similar in their floral phenotype and resources are often shared by similar insects.
Mutualistic Strategies
There has been a steady global decline of mutualists such as pollinators that may cause a negative effect on biodiversity. There is a direct correlation between partner diversity (generalist vs. specialist) and coextinction. Pollinators that rely too heavily on one species of plant for food sources have higher chances of going extinct if that plant also goes extinct. Generalist pollinators gather nectar and pollen from a wide range of plants so when one is no longer available, it has many others to choose from. This limits the chance of extinction for these pollinators (Fricke, 2017).
The degree to which the species relies on this mutualistic relationship also has an impact on coextinction rates. Species participating in partially mutualistic strategies have better survival chances because they will not feel the pressures from mutualism disruption as strongly. If one species of the mutualistic relationship is removed from the environment, a species in an obligate relationship with the removed species will likely suffer massive selective pressures. With their source of food gone, they will be unable to reproduce and survive. Species involved in a beneficial relationship, but not an obligate one, will not be at risk of extinction because they likely are involved in other mutualistic relationships they can fall back on.
Plant Mating Systems (Reproduction)
Although many flowering plants rely on pollinators for their reproduction, there is an inherent conflict of interest between both parties. In plants, selection favors floral traits that increase chances of reproducing, the transfer of pollen to conspecific plants. In pollinators, selection favors traits that maximize energy gain through foraging behavior. These conflicts may constrain plant mating systems at multiple levels: the immediate ecological plant selfing rates, their distribution in and contribution to pollination networks, and their evolution (Devaux, 2014). The effect on plant selfing rates under pollen limitation and pollinator foraging behavior are important factors acting on plant evolution. Plant selfing rates and their evolution are shaped by the conflict of interest between plants and pollinators. The fitness of a plant is measured by the number of outcrossed and selfed seeds it produces and the numbed of pollen grains it successfully exports to other plants (Devaux, 2014). Therefore, the fitness of animal-pollinated plants relies heavily on pollinators.
Foraging Behavior
A pollinators foraging behavior has a direct consequence for plant reproduction and is a driving force in coevolution amongst these species. Pollinators can be attracted to a plants color and smell, attracting them to plants displaying these qualities more often then to ones not. These exhibited traits advertise the reward of pollen and nectar to pollinators. Plant-pollinator interactions are not altruistic so there must be some benefit for the pollinator in visiting a plant. This reward incentive drives the foraging behavior of pollinators. Plants have developed differing traits that can attract animal pollinators to feed from them, allowing for their reproduction.
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These traits include color, shape, scent and size of the flower. Some pollinators exhibit a preference in their flower morphology, whereas others do not. Hummingbirds often prefer red flowers but bees prefer flowers of yellow or purple color. There are many pollinators that exhibit a strong flexibility in preference due to learning associations between reward and a particular trait. These pollinator preferences impose selective pressures on flowering plants. They have evolved specific traits to attract pollinators with flower trait preference to increase their reproductive success and in turn, their fitness. Pollinator-mediated selection can have profound effects on flower traits in response to this selective pressure. Trait preference arises from the presence of other morphologically similar flowering species or the presence of a competitor. The increase of flower visitation by pollinators can cause selection for the preferred flower type, making it appear more frequently due to its reproductive success.
Fruit Scent/Floral Signal
The large diversity of floral traits observed in plants are due to a set of adaptations that promote plant reproduction through animal pollinators. The need to offer an attractive reward to pollinators exerts strong selection of fruit/floral traits. Angiosperms have independently evolved fleshy fruits in many of the extant angiosperm families. Fruits come in an abundance of different shapes, sizes and colors and are distributed across taxa correlating with their seed dispersal by frugivores, animals that feed on fruit.
The dispersal syndrome hypothesis postulates that fruit traits are selected to match dietary requirements and sensory capacities of their primary seed dispersal vectors (Lomascolo, 2010). This hypothesis claims the selective pressures exerted by seed dispersers causes the high variance in fruit traits. A frugivore will only visit a plant in which it receives a reward that increases its fitness. Fruits that match the dietary requirements of a pollinator will be more frequently visited and will be selected for due to their reproductive success. Another thing to be considered is the ability of the pollinator to access the reward, whether it be nectar or fruit. If a pollinator is not equipped to get to the reward, there is little chance it will visit that species of flower again. Floral signals are used as a visual and olfactory form of communication between flowers and their pollinators. Many flowering plants have evolved flowers adapted to one particular group of pollinators and emit signals to attract their specific pollinators. Specific signals, innate and learnt preferences of flower visitors, and sensory exploitation make communication between flowers and pollinators very diverse and complex.
Morphological complexity is a floral signal perceived by insect pollinators. Morphologically complex flowers constrain the access of insect visitors to their nectar rewards. Only a small portion of insect pollinators are able to successfully forage on these flowers, mainly large bees. This limitation on visitors increases the food intake for successful visitors by restricting the range of pollinators able to receive said reward. This pollinator specialization allows the plants reproductive success to increase because it limits cross-species pollen transfer. An insect pollinator suited to forage from these complex flowers will continuously visit due to the large reward associated with them. These plant traits coevolved in response to pollinator selective pressures. They evolved specific signals and cues to attract their pollinators, in turn causing the pollinators to modify their foraging behavior in response. By altering their foraging behavior to reflect their flower preferences, pollinators are increasing their foraging success and the plant’s reproductive success.
Herbivore Selection
What makes plant-pollinator interactions so diverse is because they are affected by both interactors’ phenotype and external variables. One of these external variables is herbivore-induced pollinator limitation. Herbivores feed on plants and they can have divergent effects on the individual and population levels depending on the plants response to herbivory. Plants must attract pollinators, but also must deter antagonist consumers such as herbivores.
Many plants exhibit herbivore-induced chemical defenses that work to deter herbivores from feeding on the plant. When an herbivore feeds on a plant, it alters/damages the floral displays, making it harder for pollinators to identify the plant. Pollinators will avoid flowers being fed upon by herbivores, making herbivore host plants much less likely to be visited by a pollinator. Herbivore attack can alter the quality or quantity of nectar/pollen rewards or the floral signaling meant to attract pollinators. Without these rewards or signals, pollinators will stop visiting these species of flowers, therefore reducing the flowers fitness and reproductive success. Herbivore disruption will cause volatile organic compounds to be emitted from the plant, providing cues as to the metabolic state and chemical defense status of the plant. This can attract other natural predators such as parasitoids. When antagonistic species limits the interaction rate of these individuals, it results in a fitness loss for each of the species.
Climate Change on Plant-pollinator Relationships
Climate change has been a forefront issue within the last few years. With a changing environment, comes changes to the species that live within that environment. Climate change is altering ecosystem processes and species interactions. In particular, climate warming has had a negative effect on these interactions. The change in climate is causing flowers to bloom much earlier than normally and species to appear earlier than expected. If these shifts among plants and pollinators co-occur, their activity should be maintained, although pollination might occur earlier then seen in the past. The problems arise when one species involved in this mutualistic relationship occurs earlier than its mutualistic partner. This would shift the timing of pollination and flowering, causing a reduction on plant-pollinator interactions. Without these interactions, many species would be affected, including humans. We rely on plant-pollinator interactions just as much as the species involved. A vast majority of our crops rely on pollinators to produce the vegetables and fruits we have grown accustomed to.
Morton et al highlights some of the approaches we can use to better understand and predict how climate change will impact plant-pollinator systems. Experimental manipulations, such as spatial and temporal transplants, provide insight into the current and future effects of climate change. Spatial transplants require the movement of plants or pollinators within their current ranges and beyond. Their fitness and interactions are then measured to visualize how they respond to these changes. Temporal transplants alter the natural environment of the plant, such as removing snow to reflect the conditions associated with climate change. This allows us to visualize how phenotypically plastic genotypes are.
Climate change is a force with consequences that we are already beginning to witness. These extreme changes in climate will affect all species inhabiting Earth. Both plants and pollinators will respond to these changes, altering their current mutualistic relationships. The life history events of pollinators and plants are shifting with this drastic change. Without studying the responses of these species, the consequences in these shifts will remain unknown.
Conclusions
Mutualistic relationships are ubiquitous in nature. Plants and pollinators rely on their mutualistic relationships to increase each others survival, growth, and reproduction. Although these species mutually benefit from these relationships, they do not act altruistically. For both parties involved, there is a selfish underlying interest. Plants need pollinators to transfer their pollen from one plant to another of the same species. Pollinators rely on plants to gain energy through pollen and nectar rewards. Because plants and pollinators are so closely intertwined, they have coevolved together to increase their own fitness. Plants produce nectar purely as a means of enticing pollinators to visit their flower. There is no other benefit to producing nectar and it is very energetically costly to make. These rewards increase a plants chances of reproduction. Plants emit floral and fruit signals that attract pollinators, evolved in response to specific pollinator foraging preferences. These relationships between plants and pollinators create a vast network of interactions, each contributing to each other.
It is important to study these types of interactions to better understand the impacts of environmental factors, such as climate change, on the systems. If we can map out the response of plants and pollinators on these changes, we can better identify an appropriate strategy to handling these changes. Ecosystems around the world are experiencing unprecedented rates of species loss. With climate change acting quickly, some plants and pollinators will be negatively impacted. The closely knit relationship between plants and pollinators allows for coextinction to occur among species quite easily. The next course of action in this field of research is to study different approaches to address the gap caused by climate change.
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