pathogens
Any foreign organism that can cause disease. Common pathogens are viruses, bacteria, and fungi.
Bumble bee pathogens are organisms that cause detrimental effects in bumble bees.
Since the mountainous regions occupied by bumble bees lack pathogens, the pathogens do not influence the evolution of mountainous bumble bees.
Although populations of long-tongued bees are undergoing widespread decline (1, 3), shifts in foraging strategies may allow alpine bumble bees to cope with environmental change.
Long-tongued bee populations are declining due to climate change. However, bumble bees that change what flowers they choose to forage may be better at surviving the changes to their habitat.
phenotypes
The physical expression of an organisms genetic makeup. Everything we physically observe in an organism is due to the expression of their genes (their genetic information).
The phenotype of tongue length in bumble bees is highlighted in this study. Bumble bees featuring different genes that express tongue length display a variation in tongue length.
Fig. 2. Changing bumble bee community composition, bumble bee tongue length distributions, and tube depth distributions of visited flowers over time. (A and B) Bumble bee community composition. (C and D) Bumble bee tongue length. (E and F) Flower tube depth distribution. Bombus species abundance in alpine communities is indicated by the proportion of total foragers (15). Species are ordered by increasing tongue length [in (A), species’ names follow (18)]. Bimodality of the density functions (15) indicates that bumble bee communities contain two predominant phenotypes, short-tongued and long-tongued [(C) and (D)]. (E) and (F) show the tube depth density functions for flowers visited by, respectively, B. balteatus and B. sylvicola in the Front Range [Mount Evans and Niwot Ridge (15)]. For tongue length [(C) and (D)] and tube depth [(E) and (F)], representative density functions for simulated communities (15) are shown.
These results suggest that bumble bee tongue lengths are decreasing and corolla tube lengths are also decreasing in response to climate change.
On Pennsylvania Mountain, alpine bumble bees forage over hundreds of meters to provision their nests (28). To ask how warming has affected floral resources at this scale, we measured PFD of six bumble bee host plants from 1977–1980 and 2012–2014 in five habitats along a 400-m altitudinal span (table S5). Land surface area decreases with altitude above tree line in the Rocky Mountains (29), declining by more than an order of magnitude on Pennsylvania Mountain, where 58% of habitable terrain is found below 3800 m and only 4% above 3938 m on the summit (Fig. 3Aand table S5). Because bumble bees forage across the 400-m altitudinal range (28), we evaluated the temporal change in flower production at this landscape scale. For each habitat, we multiplied PFD (flowers per square meter) within sampling plots by surface area (square meters) to estimate of total flower production (15)
From 2012-2014, researchers analyzed the change of flower production of six bumble bee host plants over time at a altitude of 400m. This is where the bumble bees most commonly forage. The flower abundance was measured as flowers per square meter across five different habitats. It was then compared to data from 1977-1984 to determine change over time.
Climate records from Niwot Ridge show warming summer minimum temperatures over the past 56 years (27).We see similar changes on Mount Evans (R2 = 0.383, t1,52 = 5.68, P < 0.0001) and Pennsylvania Mountain (R2 = 0.341, t1,52 = 5.20, P < 0.0001) (fig. S3, A and B), where summer minimums have increased ~2°C since 1960. We used a nonlinear model to characterize the relationship between peak flower density (PFD; flowers per square meter) and summer minimum temperature.
Over the past 56 years, climate change has caused summer temperatures at Niwot Ridge, Mount Evans, and Pennsylvania Mountain to rise. To understand the relationship between summer temperatures and flower density, four bumble bee host species were analyzed for change in average flower density between 1977 and 2014.
Optimal foraging theory
A theoretical model used to indicate how an animal will look for food. Factors that influence how an animal will forage for food depend on the costs of energy or risk gathering the food compared to the energy gain benefit from the food.
In six species that historically provided 88% of floral resources for B. balteatus and B. sylvicola (18), the change in flower depth over time varied among species (F6,13 = 9.42, P = 0.0004). Species that now have shallower flowers received few (<10%) bumble bee visits historically (fig. S2A). On Pennsylvania Mountain and Niwot Ridge, short-tubed flowers show no systematic increase in abundance [coefficient of determination (R2) = 0.227, t1,4 = 1.21, P= 0.294; R2 = 0.0004, t1,9 = –0.62, P = 0.952, respectively) (fig. S2, B and C), suggesting that recent changes in floral trait distributions are insufficient to drive tongue length adaptation in bumble bees.
Flowers typically pollinated by B. balteatus and B. sylvicola did not have a significant increase in pollinators during the recent history. This indicates that floral trait distribution are not what is causing the decrease in bee tongue lengths. This rejects the hypothesis that there is a direct correlation between flower depth and tongue length.
Levin’s niche breadth
A constant used to compare the relative size of an organism's foraging area. Larger numbers indicate a larger foraging range.
foraging breadth
The range or obtainable food sources for an organism.
For the alpine bumble bees, this includes sources of nectar and nest building materials.
This paper highlights how the foraging breadth of native bees is decreasing due to increasing competition from immigrant species.
With increasing competition from immigrant species, foraging breadth of resident bees should contract (19, 20). Yet alpine bumble bee host choice shows the opposite trend.
For the native alpine bee species, access to food has not decreased. This rejects the hypothesis that competition between foreign and native bee species for resources decreases the native bee’s range of accessible food.
subalpine congeners
Species of bees from subalpine climates, that are within the same genus.
Climate change has warmed the alpine climate, making it's temperature closer to a subalpine climate, and increasing the range of the subalpine bee species.
(i) decreasing body size, (ii) coevolution with floral traits, (iii) competition from subalpine invaders, and (iv) diminishing floral resources.
The author listed four possible processes responsible for the tongue length changes in the bees studied:
1) Decreased body size over time has correlated to a shorter tongue - The authors compared the body size measurements with the tongue length measurements over time.
2) Coevolution between the flowers and bees - The tube depth of the flowers were measured and compared to the tongue length of the bees over time.
3) Competition from other bees in the same region affected tongue length - The authors compared other bee species to the bees studied and determine which species had the advantage and if these advantaged affected the other species.
4) Diminishing floral resources - The authors analyzed the effects of lower amounts of flowers due to increased temperatures on the foraging habits of bees. They then concluded if this could have impacted tongue length.
host plants
Plants that are in a mutualistic relationship with the pollinators.
In this paper, host plants are involved in a mutualistic relationship between themselves and bumble bees.
B. sylvicola
A species of bumble bee known as the forest bumblebee. It is native to North America and is found in open alpine grasslands near mountains.
Bombus sylvicola feeds on plants such as lupines, fireweeds, and groundsels, which are all plants found in alpine habitats.
Bombus balteatus
A species of bumble bee known as the golden belted bumblebee. It lives in high altitude alpine habitats and has a long tongue that is more than two-thirds of its body size.
coevolution
Evolution is the change in alleles (genetic information) in a population over time. Coevolution is when two species influence the evolution of each other.
Coevolution is generally observed when a long lasting mutualistic relationship between two species exists.
As plants and bumble bees evolved alongside each other, each acted as a selective pressure for the other, altering their evolutionary paths so that each is better suited to live alongside the other.
mismatch between shorter-tongued bees and the longer-tubed plants they once pollinated.
As this research has presented, due to rising temperatures, bees have began to favor generalist foraging. A recent news story shows how honey bees have started pollinating blueberry plants, which are not very easily accessible for other bees.
Read more: https://www.sciencenews.org/article/honeybees-fumble-their-way-blueberry-pollination
morphology
The study of an organism's physical characteristics and their function.
In a mutualistic relationship between species, scientists are able to match the morphological traits that directly affect the symbiotic relationship (morphological matching).
This paper highlights the morphological matching between the corolla length of flowers and the tongue length of bumble bees.
Mutualisms evolve through the matching of functional traits between partners
Mutualistic relationships between bees and flowers have resulted in both to coevolve with one another. Bees are essential to the pollination of flowers, because of this recent headlines about their possible endangerment could be catastrophic.
generalist foraging,
Foraging is the act of an organism searching for food. Generalist foraging is when an organism forages for a wide variety of food, or in other words is more adept at foraging for whatever is more convenient. Bees showing generalist foraging will eat from a wider range of flowers.
climate
The common weather patterns of a given area.
As climate change occurs, the prevailing weather patterns of all areas are viable to change as well.
Changing climate of bumble bee habitats are causing changes in pollination requirements of bumble bees to flowers.
corolla tubes
The cylindrical opening created by the petals of a flower that leads to the reproductive organs of a given plant. The tube also leads to rewards for pollinates, such as nectar.
The corolla tube is what a bumble bee sticks its tongue down to acquire nectar. The longer the corolla tube, the longer the tongue of a bumble bee must be to access the nectar held within the tube.
pollinators
A living organism capable of spreading pollen from one flower to another.
Generally pollinators are insects or birds. The pollinator highlighted in this paper is the bumble bee.
biodiversity
A measurement of the number of species in an ecosystem.
The greater the number of different species in a given ecosystem, the greater the biodiversity measurement of said ecosystem.
agriculture
The practices used in farming to produce goods, such as crops and animals.
Agriculturally based mutualisms consist of symbiotic relationships between organisms and cultivated organisms.
mutualisms
A relationship between two different organisms where both species benefit from that relationship.
The symbiotic relationship highlighted in this paper is the one between long-tongued bumble bees and plants. The bumble bees benefit the plants by pollinating them as they move from plant to plant and the plants benefit the bumble bees by giving them nourishment.
Using archived specimens and field surveys of bumble bees and host plants, we explored four potential mechanisms for this change in tongue length
Subalpine bee species have begun to move into alpine climate and compete with the native alpine bees, due to the warming of the alpine climate.
evolution is helping wild bees keep pace with climate change.
Alpine bumble bee populations are generally isolated from the toxins, diseases, and habitat destruction that affect bumble bee populations in other climates. Yet, they are still negatively affected by climate change. However, the bumble bee populations are adapting to the changes to their habitat by evolving new behaviors that increase their chance of survival.
Using a simple model adapted from (26), we tested whether reduced flowering in other ecosystems could drive the evolution of pollinator foraging traits as indicated for alpine bumble bees (15). The model predicts changes in the energetic advantage of generalization with floral density. Long-tongued bumble bees exhibit greater specialization than that of short-tongued bees (16, 30). Across a range of flight speed and plant community composition (15), the advantage of generalizing increases as flower density declines (Fig. 4).
The data suggest that the decrease in floral density seen in alpine regions leads to the favoring of short-tongued bumble bees over long-tongued bumble bees across a wide range of plant communities.
[Mount Evans and Niwot Ridge combined (16)] indicate increased species diversity
Next Generation Science Learning Standard 3LS4-2- Biological Evolution: Unity and Diversity https://www.nextgenscience.org/pe/3-ls4-2-biological-evolution-unity-and-diversity
This learning standard focuses on how characteristics can play into natural selection as well as evolution. This paper highlights how climate change is causing natural selection to favor bumble bees with shorter tongues.
Alternatively, shifts in bumble bee tongue length may reflect competition from subalpine congeners moving upward with climate change. Comparisons of past (1960s and 1970s)
Bumble bee communities in the Pennsylvania Mountain and in the Front Range have increased in species diversity and short tongued bees. These short tongue bees come from subalpine climates and could be replacing the long tongue bees, explaining their 24.1% decrease in the community.
Fig. 4. Relative advantage of generalizing as a function of flower density and the proportion of deep flowers in the community. Outcomes with flight speed of 0.5 m s−1 are shown (15). The generalist is favored when its relative advantage is >1 (pink shading).
The influences of floral density and the abundance of flowers featuring deep corollas are used to identify the advantage that a generalist (short-tongued) bumble bee would have over a specialist (long-tongued) bumble bee where bees have the same flight speed. Pink areas denote conditions that favor generalists.
This model was adapted from a model that explained the effects of floral density on the composition of pollinators. The means by which the model that was adapted from to produce this model can be found at:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3825104/
A list of materials and methods used to produce this figure is found at:
Fig. 3. Change in flower abundance at landscape and local scales along a 400-m altitudinal gradient on Pennsylvania Mountain. (A) Map showing areas where PFD decreased (1.95 km2), is stable (1.29 km2), and increased (0.10 km2). Unshaded (excluded) areas contain cliff, talus, mining disturbance, and subalpine forest. (B) PFD (mean >± SE) for plots in krummholz (KRUM); tundra slopes (SLOPE); wet meadow (SWALE), false summit (FSUMMIT); and summit (SUMMIT) habitats (N = 6 species; F4,385 = 5.55, P = 0.0002). Asterisks indicate significant differences at P < 0.05. (C) Total flower production (in millions) is the product of total surface area for (A) each habitat (table S5) (15) and (B) mean PFD.
These results show that the peak flower density of the habitat of the bumble bees is mostly decreasing, causing more competition for available nectar to ensue among the bees.
J. Stout, D. Goulson , The influence of nectar secretion rates on the responses of bumblebees (Bombusspp.) to previously visited flowers. Behav. Ecol. Sociobiol. 52, 239–246 (2002).
Stout and Goulson worked to identify how the nectar secretion rate of a given plant influences the response of bumble bees returning to it after it has already been visited.
From this, Stout and Goulson developed a model that determines how long a bumble bee will avoid a recently visited flower whose nectar has been depleted based off of the nectar secretion rate of that flower.
R. Levins, Evolution in Changing Environments: Some Theoretical Explorations (Princeton Univ. Press, Princeton, NJ, 1968).
Levins worked to identify how populations physiologically and behaviorally adapt to both short and long term changes in the conditions of their environment.
From this, Levins identified how populations of different species spatially divide their environment among themselves, allowing one to estimate how many species can coexist within a given area.
D. Goulson, E. Nicholls, C. Botías, E. L. Rotheray, Science 347, 1255957 (2015).
Goulson and colleagues worked to identify the reasons behind the decline in populations of bees. Reasons include parasites, a decline in flower populations, and an increased use of pesticides.
It is noted that efforts to reduce the use of pesticides and to increase the population of flowers visited by bees should be developed.
P. R. Elsen, M. W. Tingley, Nat. Clim. Change. 5, 772–776 (2015).
Elsen and Tingley worked to determine how the topography of a given mountain influences its relative susceptibility to climate change. From that they looked into how populations inhabiting a given mountain will migrate in reaction to climate change.
R. B. Miller, Evolution 35, 763–774 (1981).
Miller worked to identify how Hawkmoths influence the physical variation seen in Colorado Blue Columbine (Aquilegia caerulea) in different areas.
Miller found that areas containing moths with longer tongues also feature plants with longer floral spurs.
D. P. Vázquez, N. Blüthgen, L. Cagnolo, N. P. Chacoff, Ann. Bot. (Lond.) 103, 1445–1457 (2009).
Vazquez and colleagues review the patterns within ecosystem networks that lead to plant - animal mutualisms.
Vazquez and colleagues identify many mechanisms that produce mutualisms. It is noted that the relative influence of each mechanism on the production of a mutualism is still unknown.
R. Bommarco, O. Lundin, H. G. Smith, M. Rundlöf, Proc. Biol. Sci. 279, 309–315 (2012).
Bommarco and colleagues worked to determine how the composition of bumble bee populations have shifted in Sweden.
It is noted that species richness and evenness of population composition should be developed to promote healthy ecosystems.
Optimal foraging theory predicts that foragers will expand their niche in response to such resource scarcity (25, 26). When bumble bees (B. balteatus) encounter low densities of preferred host plants, they incorporate shallower flowers into their diet (F1,194 = 29.39, P < 0.0001) (table S4) (15).
According to optimal foraging theory, organisms will attempt to find the most efficient food source with respect to time and energy used to obtain it. As food becomes more scarce, the organism will increase the range of food sources it will forage. Bumble bees will select to feed from the flower who’s corolla length is closest to the length of their tongue. However, in desperate times, they will feed on shallower flowers, competing with the flower’s original bees for resources.
In response to warmer temperatures and drying soils, flowering has declined in alpine and arctic habitats worldwide (21–24)
Researchers have found that climate change has increased the temperatures of alpine and arctic habitats. This has caused the soil in this area to dry up thereby decreasing the amount of plants that flower in a given area, negatively affecting the bees ability to forage.
In 2012–2014, we resurveyed bumble bee visitation on Mount Evans and Niwot Ridge in accordance with historical observations (18). Despite a 10-fold difference between past (n = 4099 visits observed) and present (n = 519 visits observed) collection effort, surveys indicate that resident bumble bees have broadened their diet. Resampling historical visitation data to match present collection effort reveals that foraging breadth (Levin’s niche breadth) (15) increased from 2.61 to 7.01 for B. balteatus [z score (Z) = 28.48, P < 0.0001] and 2.09 to 5.07 for B. sylvicola (Z = 19.78, P < 0.0001). Bumble bees have added flowers with shorter and more variable tube depth to their diet (B. balteatus: F1,1997 = 7554, P < 0.0001; B. sylvicola: F1,1997 = 64,851, P < 0.0001) (Fig. 2, E and F, and table S3).
The researchers analyzed differences in bumble bee diets on Mount Evans and Niwot Ridge, between the 1960s and presently around 2011. Fewer bee visits were recorded during the more recent survey, but it appears that the bees have an expanded range of plants in their diet. This diet includes flowers that have shorter and more varied depths.
Selection to track the floral traits of host plants should favor short-tongued pollinators when flowers become shallower or deep flowers less common (9, 10).
Rodríguez-Gironés and colleagues saw that competition for resources within a population triggered long tongues in pollinators and deep corolla tubes in flowers.
Temporal changes in bumble bee tongue length are not explained by plasticity in body size. When phenotypic variance in tongue length is partitioned among underlying sources, size accounts for less than 20% of variation (table S1) (15). Size has declined in some populations (B. balteatus: F2,96= 8.61, P = 0.0004; B. sylvicola: F1,76 = 29.01, P < 0.0001) (fig. S1 and table S1) and is phenotypically correlated with tongue length [correlation coefficient (r) = 0.50 to 0.60, P < 0.005] (fig. S1) but contributes little to its reduction over time. After removing variance explained by body size, analysis of covariance shows significant temporal changes in tongue length (B. balteatus: F1,23 = 17.02, P = 0.0004; B. sylvicola: F1,67 = 46.14, P < 0.0001) (Fig. 1 and table S1).
The variation in the size of the bumble bees only accounts for less than 20% of variation in bee tongue length, and is therefore not a valid explanation for changes in tongue length over time. There is some correlation between decrease in bee size in some populations and their tongue length, however this does not contribute to the reduction in tongue length observed over time.
We measured the change in tongue length of B. balteatus and B. sylvicola using specimens collected from 1966–1980 and 2012–2014 in the central Rocky Mountains (15). These two species historically comprised 95 to 99% of bumble bees at our high-altitude field sites (16–18). B. balteatus workers were collected from three geographically isolated locations: Mount Evans (39°35.033′N, 105°38.307′W), Niwot Ridge (40°3.567′N, 105°37.000′W), and Pennsylvania Mountain (39°15.803′N, 106°8.564′W).
B. balteatus and B. sylvicola are the most common bumble bee species at the three research sites in the central Rocky Mountains. The sites are Mount Evans, Niwot Range, and Pennsylvania Mountain. To analyze the change in tongue length over time of both species at the sites, recent specimens from 2012-2014 were compared to older specimens from 1966-1980.
Although the climate change impacts on phenological and spatial overlap of mutualists are well known, the role of climate change in generating functional discrepancies between them is less understood. Using historical data, we show that reduced flower abundance in bumble bee host-plants at the landscape scale has accompanied recent warming, leading to evolutionary shifts in foraging traits of two alpine bumble bee species
It is known that climate change is impacting compatibility of mutualistic species. However, it is unknown which specific mechanisms of the mutualistic relationship are being affected. Using historical data the authors measured flower abundance shifts due to climate change and their effects on foraging behaviors by the bees studied.
Spatial and temporal discrepancies with food plants, habitat destruction, and pressure from invasive competitors have been implicated (3–6), but the details of these declines and their causes remain unresolved
Researchers have identified potential reasons that could be causing the decrease in long-tongued bees including changes in the location and availability of flowers, habitat destruction, and the presence of invasive species.
Long-tongued bumble bees have coevolved to pollinate plants that possess elongated corolla tubes in a mutualistic relationship. Recent declines in such long-tongued bee populations suggest that historical selection regimes in these systems are changing (1–3)
Historically, bees with longer tongues have had access to plants with longer corolla tube lengths, increasing selection for these bees that have a wider range of accessible nutrients. Researchers believe, due to a decrease in the population of long-tongued bees, that the long-standing selective pressures that caused long-tongued bees to thrive are changing. The reasons for this change remain unknown.
Bumble bee mutualism
With new research on the effects of climate change on bumble bee pollination and the decline in pollinators, new man-made developments have risen. We can see the start to a new possible era of man-made pollinators. This could help the declining rate of pollinators but can drastically affect the coevolution of flowers and their pollinators.
Read more in CNN Tech: http://money.cnn.com/2017/02/15/technology/bee-drone-pollination/index.html
Using archived specimens and field surveys of bumble bees and host plants, we explored four potential mechanisms for this change in tongue length: (i) decreasing body size, (ii) coevolution with floral traits, (iii) competition from subalpine invaders, and (iv) diminishing floral resources.
Subalpine bee species have begun to move into alpine climate and compete with the native alpine bees, due to the warming of the alpine climate.