Invasive Earthworms Research Articles

Effects of Exotic and Native Earthworms on Soil Micro-Decomposers in a Subtropical Forest: A Field Mesocosm Experiment

Biological invasion is one of the most important factors affecting global biodiversity change, which can adversely affect ecosystem function. However, little is known about the effects of belowground biological invasions on soil ecosystems. In this study, we conducted a field-based mesocosm experiment to observe the effects of exotic and native earthworms (Pontoscolex corethrurus and Amynthas corticis, respectively) on soil nutrients and micro-decomposers (i.e., soil microorganisms and nematodes) in a subtropical forest in southern China. We found that exotic and native earthworms had different effects on nutrient availability and nematode communities in the soil. Specifically, exotic earthworms significantly decreased the content of nitrates in the soil and tended to decrease the total nematode abundance compared with native earthworms. Furthermore, nematode species richness and Shannon–Wiener index were lowest in the treatment with exotic earthworms and were the highest in the treatment with native earthworms. However, neither native nor exotic earthworms significantly affected soil microbial community composition. The results of redundancy analysis indicated that available phosphorus was positively correlated with nematode community. Our results demonstrated that exotic earthworms had adverse effects on the available nutrient content in the soil, and had a potential negative effect on the abundance of soil microfauna. These findings will be helpful in understanding the influence of exotic earthworms on soil micro-decomposers and the ecological consequences of earthworm invasion.

Holes in the tundra: Invasive earthworms alter soil structure and moisture in tundra soils

Human introductions have resulted in earthworms establishing in the Arctic, species known to cause cascading ecosystem change. However, few quantitative outdoor experiments have been performed that describe how these soil modifying earthworms are reshaping structures in tundra soils. In this study, we used three-dimensional (3-D) X-ray images of soil cores (approximately 10 cm diameter, 20 cm height, N = 48) to assess how earthworms (Aporrectodea sp. and Lumbricus sp.) affect soil structure and macropore networks in an outdoor mesocosm experiment that lasted four summers. Effects were assessed in both shrub-dominated (heath) and herb-dominated (meadow) tundra. Earthworms almost doubled the macroporosity in meadow soils and tripled macroporosity in heath. Interestingly, the fractal dimension of macropores decreased in response to earthworm burrowing in both systems, indicating that the presence of earthworms reduced the geometric complexity in comparison to other pore-generating processes active in the tundra. Observed effects on soil structure occurred along with a dramatically reduced soil moisture content, which was observed the first winter after earthworm introduction in the meadow. Our findings suggest that predictions of future changes in vegetation and soil carbon pools in the Arctic should include major impacts on soil properties that earthworms induce.

Are exotic earthworms threatening soil biodiversity in the Brazilian Atlantic Forest?

Invasive earthworms are threatening soil biodiversity and ecosystem functioning in formerly earthworm-free boreal and temperate forests. Although exotic earthworms are also found in tropical forests, they have received less attention from soil scientists. Here, we summarize data on earthworm populations in the native Brazilian Atlantic Forest (one of the world's 25 biodiversity hotspots) and present a case study on three forests at different regeneration stages, aiming to identify the patterns of exotic species distribution and also the possible consequences of invasive earthworms for soil ecosystem services and macrofauna communities. We found that exotic earthworms, mainly Pontoscolex corethrurus and pheretimoid species (Amynthas corticis and A. gracilis), dominated the earthworm fauna in the native Brazilian Atlantic Forest, while native earthworms were less abundant. Furthermore, we observed that exotic earthworms are probably threatening soil macroinvertebrates, especially detritivores and predators, due to the high soil bioturbation. The widespread occurrence of invasive earthworms in this biome raises concern regarding possible competition with the native soil fauna, including native earthworms, which could dramatically impact ecosystem services linked to soil, especially those related to climate regulation and water storage.

Earthworm invasion in Chayu River valley of Tibet under climate change

Climate change affects biological invasion. To understand the invasion pattern of alien species of earthworm in Gongrigabu river watershed of Chayu river in Tibet under climate change, the participate rural appraise (PRA) was applied to investigate and analyze the peasant’s perception of the appearance time and harm of alien species of earthworm in 13 villages. The results showed that the local climate was warming and drying, and climate warming was the main reason for the invasion of alien species of earthworms. Since the appearance of alien species of earthworm at an altitude of 1528 m in 2008, they had invaded the upstream area at a climbing speed of 71.43 m·a-1 and had been outbreaking in areas below 2028 m. For the climate change adaptation measures, the local peasants should change the crop layout.

Earthworm invasion shifts trophic niches of ground-dwelling invertebrates in a North American forest

Earthworms are invading soil communities worldwide, and their actions as decomposers and ecosystem engineers are vastly impacting many ecosystem functions. In the northern regions of North America, invasive earthworms are often functionally distinct from the native invertebrate fauna and, thus, typically occupy empty trophic niches in soil food webs. Nevertheless, they can affect the co-occurring soil invertebrate communities in multiple indirect ways. Particularly, the redistribution and removal of litter resources can affect feeding interactions of soil biota that channel up to higher trophic levels, such as predators, causing shifts across all components of the soil food web, which are hard to investigate. To study trophic shifts in earthworm-invaded soil communities, we used ground predators as model organisms since they occupy high trophic levels and connect different energy channels in soil food webs. We used stable isotope (13C and 15N) and fatty acid analyses as complementary tools to describe the trophic levels and basal resources of consumers that were impacted by earthworm invasion, specifically examining the trophic niches and resources available to ground-dwelling invertebrates of a northern aspen forest. The distinct trophic niches of invertebrate species were affected significantly by earthworm invasion. Shifts in neutral lipid fatty acid profiles as well as decreases in animal Δ13C and Δ15N signatures indicated changes in basal resources and trophic levels, respectively. Furthermore, we observed a trend of greater intra-specific and less inter-specific variation in fatty acid profiles of soil organisms following earthworm invasion. Notably, shifts in marker fatty acids of ground-dwelling invertebrates were opposite to the changes observed in soil microbial communities, suggesting de-coupling of soil microbial and ground arthropod food-web compartments. Overall, our study revealed a systemic effect of invasive earthworms on ground-dwelling invertebrates. Earthworms presumably consumed a considerable amount of resources, such as litter and, thus, incorporated them in the soil food web, which was, as a basal resource, not available in the food web free of earthworms. Overall, the ground-dwelling invertebrates have adapted (changed their trophic function) which could explain the balancing of potential environmental changes that are caused by invasive earthworms. This observation potentially explains why these species are dominant in the studied forest and resist earthworm invasion. Future studies should investigate if altered litter availability also causes shifts in soil biodiversity in invaded forests, which is assumed but rarely directly tested to date.

Non-native species change the tune of tundra soils: Novel access to soundscapes of the Arctic earthworm invasion

Over the last decade, an increasing number of studies have used soundscapes to address diverse ecological questions. Sound represents one of the few sources of information capable of providing in situ insights into processes occurring within opaque soil matrices. To date, the use of soundscapes for soil macrofauna monitoring has been experimentally tested only in controlled laboratory environments. Here we assess the validity of laboratory predictions and explore the use of soil soundscape proxies for monitoring soil macrofauna (i.e., earthworm) activities in an outdoor context. In a common garden experiment in northern Sweden, we constructed outdoor mesocosm plots (N = 36) containing two different Arctic vegetation types (meadow and heath) and introduced earthworms to half of these plots. Earthworms substantially altered the ambient soil soundscape under both vegetation types, as measured by both traditional soundscape indices and frequency band power levels, although their acoustic impacts were expressed differently in heath versus meadow soils. While these findings support the as-of-yet untapped promise of using belowground soundscape analyses to monitor soil ecosystem health, direct acoustic emissions from earthworm activities appear to be an unlikely proxy for tracking worm activities at daily timescales. Instead, earthworms indirectly altered the soil soundscape by ‘re-engineering’ the soil matrix: an effect that was dependent on vegetation type. Our findings suggest that long-term (i.e., seasonal) earthworm activities in natural soil settings can likely be monitored indirectly via their impacts on soundscape measures and acoustic indices. Analyzing soil soundscapes may enable larger-scale monitoring of high-latitude soils and is directly applicable to the specific case of earthworm invasions within Arctic soils, which has recently been identified as a potential threat to the resilience of high-latitude ecosystems. Soil soundscapes could also offer a novel means to monitor soils and soil-plant-faunal interactions in situ across diverse pedogenic, agronomic, and ecological systems.

Invasive earthworms alter forest soil microbiomes and nitrogen cycling

Northern hardwood forests in formerly glaciated areas had been free of earthworms until exotic European earthworms were introduced by human activities. The invasion of exotic earthworms is known to dramatically alter soil physical, geochemical, and biological properties, but its impacts on soil microbiomes are still unclear. Here we show that the invasive earthworms alter soil microbiomes and ecosystem functioning, especially for nitrogen cycling. We collected soil samples at different depths from three sites across an active earthworm invasion chronosequence in a hardwood forest in Minnesota, USA. We analyzed the structures and the functional potentials of the soil microbiomes by using amplicon sequencing, high-throughput nitrogen cycle gene quantification (NiCE chip), and shotgun metagenomics. Both the levels of earthworm invasion and soil depth influenced the microbiome structures and the functional gene abundances. In the most recently and minimally invaded soils, Nitrososphaera and Nitrospira as well as the genes related to nitrification were more abundant than in the heavily invaded soils. By contrast, genes related to denitrification and nitrogen fixation were more abundant in the heavily invaded than the minimally invaded soils at various depths. Since denitrification can cause a nitrogen loss from the ecosystem, our results suggest the invasion of earthworms could influence the overall forest N cycling.

Spatial dynamics of an invasive earthworm (Amynthas sp.) in West Liberty University’s campus woods community.

Asian pheretimoid earthworms (Amynthas sp.) have been reported in the hardwood forests of the eastern United States with increasing frequency. These invasions can have highly negative ecosystem-wide impacts. Amynthas sp. are prolific breeders capable of reaching high densities in forest soils. They consume large quantities of leaf litter, disturbing natural soil horizons, draining available nutrients and damaging plant communities. In the summer and fall of 2021, population surveys were conducted in an area of West Liberty University’s campus woods community known to be infested with pheretimoid worms. Spatial dynamics were examined using SADIE analysis. It was found that Amynthas sp. showed a trend towards aggregation in both the summer and fall, with a more evident trend towards aggregation in the fall. An analysis of spatial association showed that the distributions between consecutive samples were positively associated. This indicates stability of aggregations over short timescales in both summer and fall. Understanding the population dynamics of Amynthas sp. and their interactions with native communities will aid in the development of management strategies to prevent the spread of this invasive and reduce the disturbance of native forest ecosystems.

The long-term effects of invasive earthworms on plant community composition and diversity in a hardwood forest in northern Minnesota.

Nonnative European earthworms are invading hardwood forests of the Chippewa National Forest, MN. While effects on plant communities at the leading edge of invasion have been studied, little is known about longer-term effects of invasive earthworms. We applied a model using historic O-horizon soil thickness and a chronosequence approach to classify 41 hardwood sites in the Chippewa National Forest as "long-term wormed" (wormed >2 decades), "short-term wormed" or "unwormed/lightly wormed." Graminoids, especially Carex pensylvanica, had the greatest mean percent cover in sites that had been wormed for over two decades. The families with the greatest negative change in mean percent cover after over two decades of earthworm invasion were Asteraceae, Violaceae, and Sapindaceae (specifically Acer species). Across all diversity metrics measured, long-term wormed sites had the lowest understory plant species diversity, short-term wormed sites had intermediate diversity, and unwormed/lightly wormed sites exhibited the highest diversity. Long-term wormed sites had the lowest mean species richness across all sample scales (1-1024 m2). The greatest within-group compositional dissimilarity occurred at sites that had been wormed for over two decades, suggesting that sites that had been wormed for over two decades have not reached a compositionally similar end-state "wormed" community type. Our study suggests that understory diversity will decrease as hardwood forest stands become wormed over time. While our results support other findings that exotic earthworm invasion is associated with lower understory plant diversity in hardwood forests, our study was the first to use space-for-time substitution to document the effects after multiple decades of earthworm invasion.

Aboveground impacts of a belowground invader: how invasive earthworms alter aboveground arthropod communities in a northern North American forest.

Declining arthropod communities have recently gained a lot of attention, with climate and land-use change among the most frequently discussed drivers. Here, we focus on a seemingly underrepresented driver of arthropod community decline: biological invasions. For approximately 12 000 years, earthworms have been absent from wide parts of northern North America, but they have been re-introduced with dramatic consequences. Most studies investigating earthworm-invasion impacts focus on the belowground world, resulting in limited knowledge on aboveground-community changes. We present observational data on earthworm, plant and aboveground arthropod communities in 60 plots, distributed across areas with increasing invasion status (low, medium and high) in a Canadian forest. We analysed how earthworm-invasion status and biomass impact aboveground arthropod community abundance, biomass and species richness, and how earthworm impacts cascade across trophic levels. We sampled approximately 13 000 arthropods, dominated by Hemiptera, Diptera, Araneae, Thysanoptera and Hymenoptera. Total arthropod abundance, biomass and species richness declined significantly from areas of low to those with high invasion status, with reductions of 61, 27 and 18%, respectively. Structural equation models suggest that earthworms directly and indirectly impact arthropods across trophic levels. We show that earthworm invasion can alter aboveground multi-trophic arthropod communities and suggest that belowground invasions might be underappreciated drivers of aboveground arthropod decline.

Invasive annual cheatgrass enhances the abundance of native microbial and microinvertebrate eukaryotes but reduces invasive earthworms

Invasive annual cheatgrass enhances the abundance of native microbial and microinvertebrate eukaryotes but reduces invasive earthworms

Ixodes scapularis density in US temperate forests shaped by deer, earthworms, and disparate factors at two scales

AbstractForest and wildlife management can help regulate the risk of human exposure to tick‐borne pathogens by influencing the population density of host‐seeking ticks and the pathogen infection prevalence in tick populations. For the blacklegged tick (Ixodes scapularis), also known as the deer tick, the overall density of host‐seeking nymphs and the density of nymphs infected with Borrelia burgdorferi sensu stricto (which causes Lyme disease) are often closely and positively correlated. We sought to identify biotic and abiotic factors influencing nymph abundance in a multiscale assessment within and across forested landscapes in northern Wisconsin. On a set of randomly located plots within each of six mesic, hardwood‐dominated forests, we collected host‐seeking and on‐host ticks, observed two common mammal hosts (white‐tailed deer and mice), and characterized the structure and composition of the forest plant community. We also surveyed activity of exotic invasive earthworms, which are altering forest structure in the region. At the broader forest‐level scale (n = 6), nymphal tick abundance was positively related to deer population density after accounting for the negative influence of forb cover. At the finer plot‐level scale (n = 87), nymph abundance was positively correlated with the abundance of coarse woody debris and negatively correlated with the density of earthworm castings, an indicator of Lumbricid (nightcrawler) activity. Thus, we identified a suite of key factors, operating at different spatial scales, that shape the suitability of forest habitat for questing immature ticks. These findings inform our basic understanding of blacklegged tick ecology and also have important implications for managing tick‐borne pathogens in forest ecosystems and landscapes.

Do local-scale context dependencies shape how ectomycorrhizal fungal diversity structures with reduced or sustained experimental N addition?

Atmospheric nitrogen (N) pollution visibly and rapidly changes forest systems, for example impacting the diversity of forest ectomycorrhizal (EcM) fungi. EcM fungi are measurable by components that vary in longevity, hence, reflecting different temporal scales of presence: EcM root-tips are viable for, at most, a couple growing seasons, compared to fungal structures that may persist in soil as DNA for decades or longer. In-growth mesh bags, in contrast, typically capture a single growing season of hyphal growth. The different components might portray different diversity responses by EcM fungi to large-scale N pollution, as well as any subsequent reductions from improved environmental standards. Within an established oak forest study system, we examined the impact of sustained and recently reduced experimental N addition on EcM fungal diversity and composition, measured in three main fungal components (root-free soil, root-tips, and mesh bag hyphae). We hypothesized that elevated soil N would reduce EcM fungal diversity, and that composition would change, but with differences among fungal components related to the temporal longevity of the components. Our expectations were largely met, in that richness primarily declined with increased soil N, and all trends were most pronounced with the soil EcM fungi (the only component potentially reflective of long-term fungal presence). We discovered that abatement of the experimental N treatment did not revert fungal trends to those of the same-site plots with ambient N treatment. Instead, the stochastic nature of local-scale disturbances, related to invasive earthworms and forest stand dynamics, likely impacted N levels and, thus, EcM fungal trends. Due to the context-dependency of localized disturbance(s), assessing the effects of reduced large-scale N deposition on EcM fungi can prove to be challenging.

Do the Invasive Earthworms Amynthas agrestis (Oligochaeta: Megascolecidae) and Lumbricus rubellus (Oligochaeta: Lumbricidae) Stimulate Oxalate-Based Browser Defenses in Jack-in-the-Pulpit (Arisaema triphyllum) by Their Presence or Their Soil Biogeochemical Activity?

The introduction of invasive earthworms initiates physical and chemical alterations in previously earthworm-free forest soils, which triggers an ecological cascade. The most apparent step is the shift in the herbaceous plant community composition. However, some species, such as Arisaema triphyllum (jack-in-the-pulpit), persist where earthworms are present. It has been hypothesized that A. triphyllum produces insoluble oxalate, an herbivory deterrent, in the presence of earthworms. This study aimed to distinguish between the effects of earthworm-induced changes in soils and the physical presence of earthworms on oxalate production. As such, a two-way factorial greenhouse trial was conducted using uninvaded soils to test this hypothesis for two invasive earthworm species (Amynthas agrestis and Lumbricus rubellus). The sequential extraction of oxalates in A. triphyllum corms was performed with absolute ethanol, deionized water, acetic acid and HCl, representing fractions of decreasing solubility. Earthworm presence increased water-soluble (p = 0.002) and total oxalate (p = 0.022) significantly, but only marginally significantly for HCl-soluble oxalate (p = 0.065). The corms of plants grown in soils previously exposed to the two species did not differ in oxalate production when earthworms were not present. However, the data suggest that earthworms affect corm oxalate concentrations and that the sequence of invasion matters for oxalate production by A. triphyllum.

Contrasting protist communities (Cercozoa: Rhizaria) in pristine and earthworm-invaded North American deciduous forests

Earthworms are considered ecosystem engineers due to their fundamental impact on soil structure, soil processes and on other soil biota. An invasion of non-native earthworm species has altered soils of North America since European settlement, a process currently expanding into still earthworm-free forest ecosystems due to continuous spread and increasing soil temperatures owing to climate change. Although earthworms are known to modify soil microbial diversity and activity, it is as yet unclear how eukaryote consumers in soil microbial food webs will be affected. Here, we investigated how earthworm invasion affects the diversity of Cercozoa, one of the most dominant protist taxa in soils. Although the composition of the native cercozoan community clearly shifted in response to earthworm invasion, the communities of the different forests showed distinct responses. We identified 39 operational taxonomic units (OTUs) exclusively indicating earthworm invasion, hinting at an earthworm-associated community of Cercozoa. In particular, Woronina pythii, a hyper-parasite of plant-parasitic Oomycota in American forests, increased strongly in the presence of invasive earthworms, indicating an influence of invasive earthworms on oomycete communities and potentially on forest health, which requires further research.

Disentangling earthworm taxonomic stumbling blocks using molecular markers

Taxonomic classification of earthworms based on anatomical features has created several challenges for systematics and population genetics. This study examines the application of molecular markers, in particular mitochondrial cytochrome oxidase (COI), to facilitate discrimination of closely related earthworm species. Molecular markers have also provided insights into population genetics by aiding assessment of genetic diversity, lineage sorting, and genealogical distributions of populations for several species. Phylogeography—a study that evaluates the geographical distribution of these genealogical lineages and the role of historical processes in shaping their distribution—has also provided insights into ecology and biodiversity. Such studies are also essential to understand the distribution patterns of invasive earthworm species that have been introduced in non-native ecosystems globally. The negative consequences of these invasions on native species include competition for food resources and altered ecosystems. We anticipate that molecular markers such as COI and DNA barcoding offer potential solutions to disentangling taxonomic impediments in earthworms and advancing their systematics and population genetics.

Stomatocystis goerresi, a new species of gregarine parasite (Apicomplexa, Monocystidae) from the invasive Japanese earthworm Amynthas tokioensis (Megascolecidae), with a description of the parasite's life cycle.

Stomatocystis goerresi sp. n., a gregarine (phylum Apicomplexa, Monocystidae) parasite of an important invasive earthworm in North America, Amynthas tokioensis (Beddard), is described. This is the second species placed into the genus, and details of its morphology and life cycle support Stomatocystis Bandyopadhyay, Mitra et Göçmen, 2006 as a valid taxon. The new species is described using standard nomenclature, measurements, shape descriptors, and photographs of living cells. The parasite was found only in A. tokioensis, and absent in sympatric earthworm species, suggesting it arrived when the earthworms were introduced from their origin from Japan. The species is distinctive from the type species in the genus, S. indica Bandyopadhyay, Mitra et Göçmen, 2006, in being substantially larger in all stages, found in only the host's seminal vesicles, and found in a different host species from East Asia. The distinctive trophozoites/gamonts develop a large funnel structure ringed with a collar of pronounced ridges, and the funnel appears even in the smallest cells. This funnel varies greatly in relative size (to the cell body) and shape, sometimes forming a large fan. The life cycle of S. goerresi is described including distinctive syzygy in which the funnels fuse and then produce a large cell with local centres of isogamete production (thus sex without gender). Gametes are large ( ~5 μm) spheres with complex tips. Oocyst production is large, > 1,000 per mature gametocyst. The genus Stomatocystis is placed into the Monocystidae, but the life cycle of the new species differs from those of other monocystid taxa, which may mean the Monocystidae are not monophyletic or life cycles are variable within the family. Prevalence of S. goerresi at the type locality was high (~ 90%). The parasites destroy the earthworm's organ of sperm self-storage thus eliminating the male function in the hermaphroditic host which may influence the ability of the earthworm to invade and be successful at new sites.

The second wave of earthworm invasions in North America: biology, environmental impacts, management and control of invasive jumping worms

The second wave of earthworm invasions in North America: biology, environmental impacts, management and control of invasive jumping worms

Invasive earthworms affect soil morphological features and carbon stocks in boreal forests

Non-native earthworms have been invading North America since European settlement. Compared to temperate forests, their presence in the boreal forest is much more recent and thus remains understudied, despite the potential threat they represent for soil carbon (C) stocks. Here we compared earthworm-invaded and earthworm-free zones in soil types representative of the boreal forest, including Luvisols, Podzols, and Brunisols (Cambisols). We observed that the forest floor (surface organic layer, or LFH) decreased in thickness after invasion in most cases and developed into a Vermimull, with the loss of the most humified layer (humic or H horizon). Simultaneously, the surface mineral horizon was reworked by earthworms into a novel Ahu horizon, characterized by higher organic matter and enriched in earthworm casts. Forest floor C stocks decreased by 94% and 59% for Luvisols and Brunisols respectively, while those of Podzols remained apparently unaffected. Mineral soil C stocks in Brunisols increased after invasion, while no changes were observed in Luvisols and Podzols. Our results demonstrated the substantial impact that invading earthworms are having on soil morphological features and C stocks in boreal forests. Effects were similar to what has been reported for temperate forests, although the degree of impact depended on soil type. While C stocks were less affected in the mineral soil compared to the forest floor, the development of a novel surface horizon reworked by earthworms could alter microbial dynamics and impact mineral C persistence. Further research is needed to quantify long-term implications of earthworm presence for boreal soil C stocks.

Three decades of declining natural foods alters bottom-up pressures on American black bears

Wildlife management entails the manipulation of bottom-up and top-down forces that affect populations. For bear management in North America, both food availability (bottom-up) and hunting pressure (top-down) can limit population growth. Whereas seasonal and year-to-year variation in production of fruits and nuts (i.e., primary bear foods) has been widely recognized, long-term trends in natural food production have rarely been reported. Here we compared the current (2015–2019) availability of 18 fruits and nuts representing the main foods of American black bears (Ursus americanus) in north-central Minnesota to what was available three decades earlier (1980s) on the same study area. Timber harvesting was routine in the 1980s but less intensive in recent decades, prompting us to hypothesize that forests matured and consequently produced less fruit for bears due to increased shade. Within each of 12 forest types, we measured the abundance, productivity and biomass (kg/ha) of each fruit-bearing plant species using the same methodology as in the 1980s. Bootstrapped independent two-sample t-tests identified species-specific changes in food availability between the two sampling periods. Generalized linear mixed-effects models were used to quantify changes for groups of fruit-producing species (i.e., summer/fall foods and short/tall shrubs). For all these groups, availability of the fruiting species changed little, but the probability of forest stands producing any fruit at all in the recent time period (~40%) was nearly half what it was in the 1980s (~80%). At the landscape scale, we estimated a ~70% decline in fruit biomass. Our hypothesis that this decline was due to reduced timber harvesting and thus denser canopy was only partially correct, as we observed the same decline in fruit production within forests of the same type and canopy closure, and also along edges of stands with high light penetration. Decline in fruit availability was due more to a complete lack of production in some stands of the same type and age rather than to general forest maturation. We explored three alternate hypotheses to explain this decline: (1) change in weather (which could affect fruiting directly, or cause issues with pollination), (2) increase in invasive earthworms, and (3) increased deer browsing. None of these were completely consistent with our findings, although it appeared from the extreme spatial variability in fruit production that on-the-ground factors were more of a driving force than weather. The magnitude and complexity of this change in bottom-up forces is a serious issue for bear management.