Bumblebee populations, especially iconic species like the federally endangered Rusty Patched Bumblebee (Bombus affinis), are in steep decline across North America. This species has vanished from a staggering 70-90% of its historic range, primarily within the American Midwest.
While threats like novel pathogens, widespread pesticide use, competition with managed honeybees, and climate change are significant contributors, the fundamental issue remains habitat loss and degradation.
Converting natural landscapes to intensive agriculture and urban development drastically reduces the food and shelter bees desperately need. Crucially, this often means losing the diverse flowering plants that provide essential nectar for energy and pollen for protein.
Bumblebee Survival Needs Seasonal Habitats
Understanding bumblebee survival requires appreciating their complex annual cycle. Colonies operate from early spring, when solitary queens emerge from hibernation, through late summer or fall when new queens and males are produced. Unlike honeybees, bumblebees do not store large reserves of food.
Instead, they rely on a near-constant supply of fresh flowers blooming in sequence to fuel every stage of colony growth and reproduction. This absolute dependence on continuous resources highlights the vital concept of “habitat complementarity.”
Imagine bumblebees needing a year-round buffet; no single habitat provides everything all season. Forests, with their unique suite of spring-blooming wildflowers like Wild Geranium and Virginia Waterleaf, offer the irreplaceable first meals for hungry, energy-depleted queens starting new colonies.
As spring turns to summer, grasslands burst into bloom, providing the massive resources needed to grow the worker population and expand the colony. Later, wetlands and certain late-blooming prairie plants might sustain the production of new queens and males.
These habitats function like a relay team, each passing the baton of resource provision at the critical time. If the first runner – the spring forest habitat – stumbles or fails, the entire team effort collapses, particularly for the queens whose survival hinges on that initial nourishment. This study meticulously investigated whether this vital seasonal relay was breaking down.
22-Year Study Tracks Bee Plant Loss
Researchers harnessed an invaluable long-term dataset: the Critical Trends Assessment Program (CTAP) plant surveys conducted across Illinois. Analyzing data from 262 sites (covering forests, grasslands, and wetlands), primarily within the historic range of the Rusty Patched Bumblebee, across five distinct sampling periods between 1997 and 2019, provided an unprecedented view of change.
The team focused on two crucial categories of plants: all species documented to be visited by bumblebees in Illinois (159 species) and a specific subset of 32 plants identified as preferred by the Rusty Patched Bumblebee based on expert lists and pollen studies.
Botanists had meticulously measured plant cover – the percentage of ground area occupied by these species – within survey plots, providing a strong indicator of likely floral resource abundance. The researchers tracked changes in both total cover and species richness for these bee-friendly plants over time within each habitat type.
To understand the seasonal importance of each habitat, they employed sophisticated modeling. Using known flowering dates, they predicted when each habitat type peaked in bumblebee food abundance.
This was then overlaid with data on peak activity periods for different bumblebee castes (queens, workers, males, new queens called “gynes”), derived from thousands of museum specimen records.
Statistical analysis, using linear mixed-effects models, rigorously quantified trends while accounting for natural variation between sites. Additionally, the study assessed changes in total forest, grassland, and wetland cover across the region and investigated whether intensive agriculture surrounding the sites correlated with the observed plant declines.
Forest Decline and the Net Loss of Grassland Bees
The results revealed clear, statistically significant trends with profound implications, particularly for early-season resources. Forests suffered a substantial decline in critical floral resources. The total cover of plants used by all bumblebees dropped by 3.7% over the 22 years, a significant decrease confirmed by strong statistical models.
More alarmingly, the cover of plants specifically favored by the endangered Rusty Patched Bumblebee declined by 2.7%.
Key early-blooming forest species like Wild Geranium (Geranium maculatum) and Virginia Waterleaf (Hydrophyllum virginianum) were hit especially hard; their combined abundance plummeted by nearly 40%, falling from 10.2% mean site cover in the earliest surveys to just 6.2% in the most recent period.
Wild Geranium alone was responsible for 17% of all bumblebee plant cover and a massive 55% of Rusty Patched Bumblebee plant cover in forests, making its decline particularly devastating.
While the overall linear trend for plant species richness in forests wasn’t as strong statistically, the most recent sampling period showed a marked drop compared to earlier periods, hinting at a potential accelerating loss of diversity.
In stark contrast, the story within existing grassland patches was more positive, though overshadowed by a larger landscape problem. The cover of all bumblebee plants remained stable, and remarkably, the cover of Rusty Patched Bumblebee plants actually increased significantly by 6.96%.
Species richness also increased for both plant groups in grasslands. However, this encouraging news was completely negated by the finding that the total area covered by grassland habitat in the entire region shrunk by 7.5% between 2001 and 2016.
Consequently, despite individual patches potentially improving, there were significantly fewer grassland patches overall, resulting in a net loss of crucial midsummer resources across the landscape. Wetlands showed no significant changes in cover or richness but constituted a very small portion of the landscape.
Surprisingly, the study found no statistically significant link between the amount of intensive agriculture (corn/soy fields) surrounding the sites and the declines in bee plant cover or richness at the site level, pointing towards drivers within the habitat patches themselves, like deer browsing or invasive species, as primary culprits, especially in forests.
Crucially, the seasonal analysis confirmed the forests’ irreplaceable role. Forest plant blooms peaked around May 29th, perfectly aligning with the peak activity period of spring queens for both the Rusty Patched Bumblebee and other bumblebee species.
Statistical models confirmed forests provided significantly higher floral abundance during this critical founding period than grasslands or wetlands. Conversely, grasslands dominated resource availability during the peak activity of workers, males, and new queens later in the summer.
Save Bees: Protect Seasonal Food Chains
The findings reveal a critical, previously underappreciated stressor compounding the threats to bumblebees: the significant decline of early-season floral resources in forest understories. The timing of this loss is catastrophic.
Spring is the most perilous time for bumblebees; a single queen’s success hinges entirely on finding abundant food immediately after hibernation to establish her nest and produce the first workers. Forests, as definitively shown, are the primary source of this essential “first meal” in the Midwest.
The documented decline in cover (3.7-2.7%) and the collapse of key species like Wild Geranium directly translate to queens struggling to find the nutrition they need.
Research demonstrates that poor nutrition during this founding stage leads to smaller colonies, increased susceptibility to disease and parasites, and drastically reduced production of the next generation, crippling population recovery.
Factors driving the forest decline likely include rampant over-browsing by white-tailed deer, which selectively devour palatable wildflowers, the smothering effects of invasive shrubs like bush honeysuckle, and soil disruption from invasive earthworms.
Climate change adds another layer, potentially causing mismatches if flowers bloom earlier than queens emerge. This research powerfully quantifies the necessity of habitat complementarity.
Grasslands are undeniably vital for midsummer resources, but they cannot compensate for the loss of the unique early spring resources provided only by forests. The combined effect of forest resource decline and the net loss of grassland area severely weakens the entire support system for bumblebees.
While pathogens may have been the immediate trigger for the Rusty Patched Bumblebee’s collapse, the degradation of its food base, particularly the vanishing spring buffet, creates chronic stress that weakens bees, making them far more vulnerable to other threats and hindering recovery.
Consequently, conservation strategies must fundamentally evolve beyond a primary focus on grasslands. Protecting bumblebees, especially endangered spring-reliant species, demands a holistic, seasonal approach.
Actively managing forests to promote diverse native spring wildflowers is paramount; this requires controlling deer populations to sustainable levels, aggressively removing invasive shrubs, and restoring key native plants like Wild Geranium and Virginia Waterleaf.
Protecting and creating habitat mosaics is essential; conserving forest edges adjacent to grasslands, establishing corridors linking different habitat types, and prioritizing land protection that includes complementary habitats allows bees to access the sequential resources they need.
Assessments of habitat quality must explicitly map and address seasonal resource gaps, ensuring actions target critical periods like early spring. Simultaneously, reversing the net loss of grassland area remains crucial, alongside maintaining efforts to improve grassland quality.
Ultimately, safeguarding the often-overlooked spring blooms of the forest floor is not just about preserving beauty; it’s about securing the irreplaceable first meal upon which the entire annual cycle of bumblebee survival depends.
Ensuring a continuous, high-quality sequence of floral resources from spring through fall across diverse landscapes – true habitat complementarity – is the essential strategy revealed by this long-term research for reversing pollinator declines.
Key Terms and Concepts
What is Bumblebee Forage Plants: Plants that provide nectar and pollen, which are essential food sources for bumblebees. Examples include wild geraniums, goldenrods, and milkweeds. They are crucial because they fuel bumblebee survival, colony growth, and reproduction. Without them, bumblebee populations cannot thrive.
What is Habitat Complementarity: The idea that different habitats provide vital resources at different times of the year. For example, forests offer spring flowers, while grasslands provide summer blooms. This timing ensures bumblebees have food throughout their active season. It is key for supporting their entire life cycle.
What is Flowering Phenology: The timing of when plants flower throughout the year. It tracks when specific plants bloom, like forest flowers in spring and prairie flowers in summer. Matching this timing to bumblebee activity (like queen emergence) is critical for their access to food. Mismatches can harm bee populations.
What is Floral Abundance: The amount of flowers available in an area. It is often measured as the percentage of ground covered by flowers in survey plots. High floral abundance means more food for pollinators. Declines in abundance, like seen in forests, directly reduce available food.
What is Species Richness: The number of different plant species present in a specific habitat. High richness means many types of flowers, which can provide diverse nutrition for bees. While plant cover declined in forests, richness only dropped recently, suggesting different types of loss over time.
What is Habitat Degradation: The decline in quality of a habitat, making it less suitable for wildlife. Here, it specifically refers to the loss of important bumblebee food plants within habitats like forests. Degradation reduces the resources available even if the habitat area remains the same.
What is Land Cover Change: How the types of land (like forest, grassland, city) in a region change over time. In Illinois, grasslands decreased by 7.5% between 2001 and 2016. This loss reduces the total area providing vital summer resources for bees.
What is Bombus affinis: The scientific name for the Rusty Patched Bumblebee, a species listed as endangered in the USA. It has disappeared from 70-90% of its former range. The study focused on how changes in its food plants contribute to its decline.
What is Foundress Queen: A mated female bumblebee that emerges in spring, finds a nest site, and starts a new colony alone. She relies heavily on early spring flowers (like forest blooms) for energy. Lack of food at this stage can cause colony failure before workers hatch.
What is Colony Demography: The study of the population structure and vital stages (birth, growth, death) within bumblebee colonies. Key stages include queen emergence, worker production, and new queen/male production. Resource availability at each stage affects overall colony success.
What is Linear Mixed Model: A statistical method used to analyze data with repeated measurements (like plant cover over years at the same sites). It accounts for random variations between sites while testing for overall trends. The researchers used this to detect declines in forest plant cover.
What is Random Effect (in stats): A factor in a statistical model accounting for natural variation between individual subjects (like differences between specific forest sites). Using site as a random effect helps isolate the true trend over time from site-specific differences.
What is Ground Layer Vegetation: Plants growing low to the ground, like herbs and small shrubs, surveyed using quadrats. In forests, this includes vital spring flowers like wild geranium. The study focused on this layer, though trees/shrubs also provide resources.
What is Floral Resource Continuity: Having enough flowers blooming consistently throughout the bumblebee flight season. Habitats like forests (spring) and grasslands (summer) provide complementary resources. Loss in one season (e.g., spring forests) breaks this continuity.
What is Kernel Density Estimate: A statistical technique to visualize the distribution of events over time, like bumblebee sightings. It creates a smooth curve showing peaks in activity (e.g., queen sightings in spring). This showed when each caste was most active relative to plant blooms.
What is Caste (in bees): Distinct groups within a bumblebee colony with specific roles: queens (start colonies), workers (forage, tend nest), males (mate), and gynes (new queens). Each caste has peak activity periods needing specific floral resources at different times.
What is Sampling Period Bin: Grouping survey years together for analysis (e.g., 1997-2001, 2002-2006 etc.). This was done because not every site was surveyed every year. It allowed tracking changes over roughly 5-year intervals across two decades.
What is Herbicide-Tolerant Crops: Crops genetically modified to survive herbicide spraying, allowing farmers to use more weedkillers. This reduces “weeds,” many of which are important bumblebee food plants in farm landscapes. A potential stressor, though not directly linked to site declines here.
What is Nest Founding Success: The ability of a foundress queen to successfully start and establish a new colony. This early stage heavily depends on abundant spring pollen and nectar. Poor nutrition in spring reduces nest founding success, impacting the whole population.
What is Resource Asynchrony: When the timing of resource availability (like flowers blooming) doesn’t match when consumers (like bees) need them. Climate change can cause this if plants flower earlier but bees don’t emerge earlier. This compounds the effects of simple resource loss.
What is Additive Stress: When multiple threats (like habitat loss, pesticides, disease, poor nutrition) combine, increasing harm beyond what any single threat would cause. Declining spring flowers add stress to bees already facing pathogens and insecticides.
What is CTAP (Critical Trends Assessment Program): A long-term monitoring program in Illinois that provided the plant survey data (1997-2019). Botanists repeatedly surveyed plants at over 262 sites across forests, grasslands, and wetlands. This dataset enabled the detection of long-term trends.
What is BBNA Database (Bumble Bees of North America): A database containing records of bumblebee species sightings, including location, date, caste, and often the flowers they were visiting. It was used to identify bumblebee food plants and analyze flight seasons.
What is Marginal R-squared (R²m): In statistics, it measures how much of the variation in the data is explained only by the main fixed factors (like time period and habitat) in a mixed model. Here, the low R²m for time trends indicated other unmeasured factors also influenced plant cover.
What is Conditional R-squared (R²c): In statistics, it measures the total variation explained by both the fixed factors (like time, habitat) and the random effects (like differences between sites) in a mixed model. The high R²c here showed site differences accounted for much variation alongside the time trend.
Reference:
Mola, J. M., Richardson, L. L., Spyreas, G., Zaya, D. N., & Pearse, I. S. (2021). Long‐term surveys support declines in early season forest plants used by bumblebees. Journal of Applied Ecology, 58(7), 1431-1441. https://doi.org/10.1111/1365-2664.13886
