The effect of resource concentration on consumer population densities depends on spatial scale and diet breadth

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Data may be preliminary. 19 November 2025 V1 Latest version Share on The effect of resource concentration on consumer population densities depends on spatial scale and diet breadth Authors : Michael LaScaleia 0000-0002-8777-5962 [email protected] , Chris Elphick , James Mickley 0000-0002-5988-5275 , Michael Singer 0000-0002-0164-3767 , David Wagner , and Robert Bagchi 0000-0003-4035-4105 Authors Info & Affiliations https://doi.org/10.22541/au.176355013.34413385/v1 228 views 153 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Theory predicts that herbivores, especially dietary specialists, should be more densely aggregated in patches where their host plants are more concentrated. Many agroecological studies have explored this idea using predefined patch sizes, but the relevance to natural ecosystems and the effects of scale and herbivore diet breadth remain unclear. In this study, we examine the responses of dietary specialist and generalist larval Lepidoptera (caterpillars) populations to concentrations of their host plants in secondary forests of New England. We measured host plant assemblages and densities at a variety of scales to assess the patch size-specific effects of host density. Importantly, we accounted for observed host-preferences of generalist consumers to more accurately measure resource availability for generalists. With these methods we could determine the effects of both host plant concentration and patch size on the population densities of generalist and specialist herbivores in a natural setting. We found that dietary generalists’ population densities correlated positively with their host plant concentrations at scales <50 m, a pattern that was only detectable when species-specific generalist-host associations were considered. In contrast, dietary specialists’ population densities correlated positively with their host plant densities at scales <3 m and negatively at scales 25–50 m. This suggests that small, dense patches of resources attract specialists, while abundances of herbivores saturate in medium-size patches, demonstrating the so-called “resource dilution” effect. These findings suggest that consumers’ responses to resources depend on their diet breadth and vary across spatial scales. Introduction Understanding the relationship between the spatial distribution of organisms and that of their resources is central to both theoretical and applied ecology. One common paradigm is that specialized enemies will be more abundant in dense, conspecific patches of their host species. This idea is fundamental to much ecological theory. For example, the Janzen-Connell hypothesis proposes that such aggregation of herbivores and pathogens on dense patches of their host plants drives the density-dependent seedling mortality that promotes species coexistence of tropical forest trees (Janzen 1970, Connell 1971). The same fundamental assertion underlies many other seminal ideas in the plant-herbivore literature: hypotheses suggest that herbivore outbreaks are less likely in areas of high host diversity (diversity-stability sensu Elton 1958; MacArthur 1955); organisms spend more time foraging in areas where their resources are plentiful (optimal foraging sensu Stephens and Krebs 1986); and that plant community diversity can make it more difficult for herbivores to find their target hosts (associational resistance sensu Tahvanainen and Root 1972). Despite its foundational role, empirical support for a consistently positive relationship between consumer density and resource concentration is mixed, suggesting gaps in our mechanistic understanding. Indeed, the general acceptance of this assumption may owe more to its ubiquity than a strong basis in empirical evidence. Root (1973) formalized one of the most influential versions of this idea in the “resource concentration hypothesis,” which proposed that dietary specialist herbivores should reach higher densities where their host plants are more concentrated. This framework provided an appealing mechanism, that higher immigration and lower emigration rates in larger or denser host patches lead to higher herbivore densities, and has guided decades of agroecological research, particularly in systems with clearly defined patch boundaries such as crop monocultures (Rhainds and English-Loeb 2003). However, subsequent empirical studies have revealed that this prediction does not hold universally (Kunin 1999, Elzinga et al. 2005). Even in agroecosystems, the empirical evidence that herbivores congregate where their resources are concentrated is inconsistent (reviewed in Rhainds and English-Loeb 2003). Many systems show no relationship or even a negative relationship between herbivore density and host concentration, suggesting that the underlying mechanisms are more complex than originally proposed (Hambäck and Englund 2005). Observations of increases in plant concentration leading to a saturation of herbivores has led to the introduction of new, opposing ideas such as the “resource dilution hypothesis” (Otway et al. 2005). Natural systems, which have complex mosaics of diverse plant communities with poorly defined patch sizes, further complicate the consumer-resource relationship. There is evidence that host plant apparency (Castagneyrol et al. 2014b, Grossman et al. 2019), herbivore foraging behaviors (Bukovinszky et al. 2005, Andersson et al. 2013, Hambäck et al. 2014), temporal variation of resources (Nerlekar 2018, Doublet et al. 2019), and top-down forces (Mols and Visser 2002, Valdés-Correcher et al. 2019) can each modify the relationship between an herbivore’s population density and its host’s concentration. Furthermore, herbivore assemblages in natural ecosystems include many polyphagous species that obscure species-specific herbivory patterns (Forister et al. 2015). The complexities identified by these studies demonstrate that implicit assumptions of a positive resource-consumer density relationship should be challenged. One consistently cited, yet largely untested, factor‘ that may alter consumer-resource relationships is the diet breadth of the consumer (Steffan-Dewenter and Tscharntke 2000, Hambäck and Englund 2005, Castagneyrol et al. 2014a). Herbivore population-level responses to resource availability can differ between dietary generalists and specialists, even between closely related taxa in the same system (Gravel et al. 2011, Ramiadantsoa et al. 2018, Anderson et al. 2019). The few resource concentration studies that do consider dietary generalist herbivores either assume they eat all plants equally (e.g. Long et al. 2003; Sarvašová et al. 2021), focus largely on associational effects and ignore potential trophic relationships between generalist herbivores and non-measured plants (e.g. Castagneyrol et al. 2019), or measure herbivory instead of herbivore population density (e.g. Grossman et al. 2019). Even though polyphagous herbivores consume multiple plant species, the combined densities of their host plants are still spatially heterogenous, and denser patches of host plants should still be favorable regardless of host plant diversity. Generalist herbivores usually prefer some of their host plants to others (Karban and English-Loeb 1997, Novotny et al. 2002), which will both amplify this heterogeneity in resource availability and further obscure any relationship if disregarded. Accounting for these preferences is critical, as ignoring them can mask or distort true relationships between host plant concentration and herbivore abundance. A second critical yet understudied dimension of resource concentration studies is spatial scale. Many investigations of numerical responses of herbivores to landscape-scale variables have demonstrated that the magnitude and direction of those responses depend on the scale of measurement (Bommarco and Banks 2003, Saint-Germain et al. 2007, Marini et al. 2009, Pimentel et al. 2017, Coutinho et al. 2019). Studies that are restricted to measures of resource concentration at a single scale (Otway et al. 2005, Valdés and Ehrlén 2019, Damestoy et al. 2020, Chau et al. 2020, Stemmelen et al. 2022) implicitly assume that the chosen scale is one relevant to the focal herbivores. Indeed, most single-scale studies use small patches (1-10 m) presumably chosen more for practical convenience than biological relevance; however, many herbivores may respond to host densities differently at much larger scales (Thomas and Harrison 1992, Jonsen and Fahrig 1997). Alternatively, studies that have used various patch sizes have often defined each patch by the extent of the focal plant, potentially misconstruing the relevant scale of the insect as that of the host plant and conflating two separate ideas: resource density and patch size (e.g. Grez and González 1995, Vehviläinen et al. 2007, Nerlekar 2018, Valdés-Correcher et al. 2019, but see Grossman et al. 2019 and Bommarco and Banks 2003). Mismatches in the measurement scales of resource concentration and consumer population response could potentially obscure the relationship between the two (Pimentel et al. 2017). In this study, we address three under-researched aspects of the relationships between herbivore densities and their host plants’ concentrations that may help elucidate mechanisms at play in the consumer-resource relationship in natural systems: (1) the difference between dietary generalists and specialists in their responses to host density, (2) the importance of accounting for variance in dietary generalists’ use of different host plant species, and (3) the scale dependency of the relationship between resource (plant) concentration and consumer (herbivore) density. We examined how the larval abundances of 20 species of Lepidoptera (hereafter, caterpillars) responded to their host plant concentrations in forest patches in central and eastern Connecticut, USA. The caterpillar assemblage was divided into dietary generalists (12 species) and dietary specialists (8 species). We measured host plant concentration separately for each caterpillar species, weighting by the frequency of association (electivity) between each host species and dietary generalist. By measuring host plant concentration at multiple scales and accounting for species-specific electivities, we determined whether concentrations of host plants influence herbivore densities and, if so, which host plant patch sizes are most relevant to the insect herbivores. Materials and Methods Study sites : The 32 sites used in this study were located within individual forest patches that were dispersed over a 3,500 km 2 area of northeastern coastal forest (Olson et al. 2001) in central and eastern Connecticut, USA (Figure 1a). The 32 patches ranged between 3–1000 ha in area and were sorted into 13 geographic blocks. The patches were surrounded by agricultural, pastoral, and suburban land and divided by powerline cuts, small roads, and major highways. All caterpillar-sampling plots were within the patches’ core areas and more than 100 m from the patch edge. Data collection : Within each site, we sampled caterpillars in twelve 5 m x 5 m plots (caterpillar plots in Figure 1b, blue). Caterpillars were sampled by systematically striking branches with sticks to collect falling caterpillars on sheets held below the branches (Wagner 2010). In June of each year between 2017–2019, each of the twelve caterpillar plots at each forest patch was sampled once. We sampled up to five branches per plant species from all woody plants that had at least one branch between 1 m and 2 m high. All caterpillars over 1 cm in length were identified to species or the highest taxonomic resolution possible, and the species of the plant on which the caterpillar was observed was recorded as the host. We considered dietary specialists to be any caterpillar species that consumes plant species in two or fewer taxonomic families in New England. Splitting caterpillar species into diet breadth groups using this method forms a striking dichotomy, as all common caterpillar species in our data consume plants from either fewer than two or more than five plant families in this system (Anderson et al. 2019). Host plant species for each caterpillar species were compared to existing records and expert knowledge (Wagner 2010; D. L. Wagner, unpublished data), and we discarded instances where known species-specialist caterpillars were found on a non-host plant a single time. Woody vegetation data were collected from the twenty-one 10 m x 10 m vegetation plots in each site (Figure 1b, green squares), as well as each caterpillar plot. Vegetation plots were arranged at the vertices of concentric triangles, with sides’ lengths selected so that the average distances from the central caterpillar sampling plots approximated a logarithmic series of base e (roughly 3, 10, 25, 50, 150, and 400 m). All woody plants over 1 m tall in these plots were identified to species once between July and October 2017–2019. Electivity : We estimated each caterpillar species’ electivity, for each host plant species from our data. Electivity measures the observed ecological association between an herbivore and its hosts (Ivlev 1975). In addition to an herbivore’s dietary preference for a particular host plant, this quantity is influenced by unmeasured factors such as variation in predation and competition among caterpillars on different hosts, which preference alone is not. We calculated electivity using an adapted preference formula of predator choice with no food depletion (Chesson 1978, 1983): (eq. 1) Where r ij is the number of branches of host species j, where the caterpillar i was found, and n j is the number of branches of host species j sampled. The expression r ij / n i therefore is the proportion of branches of host species j where the caterpillar species i was found. In the overall denominator, k is an iterator across all host species m , including j ( i.e. , and therefore is the total proportion of all branches across the m host species on which caterpillar species i was found. To account for more than one caterpillar often being present per sampled branch, the electivity value is scaled by the total abundance of caterpillars of a given species c ij on host species j , divided by the total abundance of caterpillar species i . Electivity, therefore, represents each caterpillar species’ observed probability of association with a given host plant species after normalizing for variation in the abundance of host plants. If no interaction was observed between caterpillar species i and plant j, then r ij = 0 and, in turn, = 0. Measuring host plant density : We used electivity to recalculate the effective host plant density at the six scales for each caterpillar species using equation (2). (eq. 2) This equation was applied individually for each caterpillar species, i , at each vegetation scale, s , in each forest patch. Total host plant density ( d­ is ) at each scale ( s ) was calculated as the total abundance ( n­ js ) of host plant species j within the plots at distance s from the caterpillar sampling plot, multiplied by caterpillar species i ’s electivity for host plant species j (), summed across all host plant species ( m ) and divided by the number of vegetation plots ( p s ) at that scale. Host plant densities at adjacent scales were highly correlated within each forest patch, so, to avoid multi-collinearity and isolate the individual effects of each hierarchical level, the host plant densities of the outer five scales (10, 25, 50, 150, and 400 m) were recalculated as the residuals of a linear regression comparing the host densities of each scale with the host density at the scale one step smaller. For example, the effective host densities at the 150 m scale for caterpillar species i are the residuals of the Gaussian linear regression against the densities at the immediately smaller scale, 50 m. Thus, where the intercept (β 0 ), slope of the relationship between densities at the two scales (β 1 ), and residual standard deviation () are estimated by the regression. Data organization and selection : All data organization and statistical analyses were done using R version 4.2.1 (R Core Team 2022). In this analysis, we included native caterpillar species that were observed 30 or more times, resulting in 8 specialist species and 12 generalist species (Appendix S1: Table S1). We excluded one caterpillar species, Malacosoma disstria , due to its semi-gregarious habit (caterpillars often found in groups of 5–10) and skewed electivity (3 plant species account for 0.95 of its electivity, but the remaining 0.05 is from 16 species). Both factors make its distribution considerably different from any other species of caterpillar included. Twenty-eight host plant species were sampled 20 or more times, but certain congeners that displayed nearly identical morphological and functional traits were grouped together, for a total of 23 host plant taxa (Appendix S1: Table S2). Modeling of scales and diet breadth : To isolate the most relevant scale at which caterpillar abundance is affected by host plant density, we fit seven models of decreasing spatial scale and complexity for both the generalist and specialist caterpillar groups (Appendix S2: Table S1). All models were fit using negative binomial generalized linear mixed effects models with a log link using the R package glmmTMB (version 1.1.4, Brooks et al. 2017). The response variable was caterpillar abundance per branch of potential host plant sampled per plot. The global models included a term for host plant concentration at the smallest distance (3 m) and terms for residual host concentration at each of the outer five distances (10, 25, 50, 150, and 400 m). Each subsequent model dropped the term for the outermost measurement (e.g., the second model contained no term for residual host concentration at 400 m). The final model, the null, contained no terms for host plant concentration. To account for variation among caterpillar species in these models, we included a normally distributed random intercept for caterpillar species and a nested random intercept for caterpillar species within adjacent vegetation plot within forest patch. Additionally, we included fixed terms for Julian day (numeric value, mean-centered, and scaled by its standard deviation) and year of sample (categorical) to account for intra- and inter-annual temporal variation. The seven models for each diet breadth group were compared using Akaike’s information criterion (AIC) to determine the set of distances with the most influence on caterpillar abundance. This model structure allowed us to isolate the scale at which host plant density was most relevant to caterpillar abundance in each diet breadth group. We further sought to compare the predictive power of our electivity metric versus other, less specific metrics of host specificity. Specifically, we compared our metric of electivity to a binary measure of host acceptability (where electivity = 1 for every plant species on which a generalist was observed and 0 for other species) as well as to a measure lacking host specificity altogether (where electivity = 1 for all plant species for all generalist caterpillar species). To test the relative predictive power of these three approaches (electivity, host binary, and no specificity), we fit the same set of 7 generalist models as described in the previous section using leave-one-out (LOO) cross-validation. Next, we fit each model set 13 times for each approach, each time with one block of forest patches removed, then compared each approach with AIC. The best-performing model within each LOO set was used to predict the observed data in the omitted block. We compared the prediction of the best model to that of the null model by calculating a pseudo-R 2 value as where and are the log likelihoods of the data in the omitted block given the best model and the null model, respectively (Nakagawa and Schielzeth 2013). This pseudo-R 2 method provides a measure of the ability of the models to predict caterpillar abundances in the omitted blocks, while simultaneously accounting for spatial structuring in the data that might otherwise obscure patterns. Pseudo-R 2 values were compared among the three methods using a linear mixed-effect regression (lmer) with the omitted block as a grouping variable (Bates et al. 2014). Results Sample summary: We surveyed 11,040 branches where we observed 11,188 caterpillars, for an average of 1.01 caterpillars per branch. The overwhelming majority of observed caterpillars were the exotic invasive species Lymantria dispar (n = 7,880; 70.4%), which had a population outbreak in 2017. Dietary specialists made up a relatively small portion of the caterpillars (n = 695, 6.2%), with only 8 species observed 30 or more times (Figure 2a, b). Dietary generalists, excluding L. dispar , were twice as abundant as dietary specialists (n = 1509, 13.7%). Hamamelis virginiana had both the highest total abundance of caterpillars and highest summed electivity, closely followed by Acer rubrum and Kalmia latifolia (Figure 2c). These species were the host for six of the eight specialists in this study, which contributed to these high caterpillar loads. Betula spp. ( B. lenta and B. alleghaniensis ), alternatively, had high caterpillar abundance but low summed electivity, low average caterpillar density, and hosted no observed specialists, showing a reliance of generalist species on hosts with high abundance but not necessarily high acceptability. Effect of host concentration at varying scales : Both dietary generalist and specialist caterpillars responded to the density of their host plants, but in different ways. For both diet breadth groups, the models including terms for host density at 50 m and closer had the lowest AIC values and were considerable improvements over the null models (Figure 3). In these models, at the smallest spatial scale (3 m), both generalist and specialist caterpillar densities responded positively to concentrations of their host plants. However, as spatial scale increased, the pattern deviated between the two diet breadth categories. For generalists, host densities had a positive effect on caterpillar abundance up to 25 m, before the strength of the effect tapered off at 50 m. In contrast, the positive effect of host density on specialist abundance was limited to the smallest spatial scale (<3 m) and became negative at larger scales. Predictive power of electivity versus other common methods : Models employing electivity-scaled host concentration outperformed models that used only a binary or no host specificity. The null model always performed worse than the electivity and binary models and always performed better than the no specificity model. The average difference between AIC of the best model using electivity with the LOO data and its respective null model was 102 (std. error = 2.01) while the difference for using binary host data was only 4.88 (std. error = 0.351). When used to predict data from the omitted block, the models using electivity generally performed better than those using the binary host assignments, though inconsistently. The average pseudo-R 2 was significantly higher when employing electivity than other approaches (linear mixed-effect regression, β = 0.042, p = 0.0044). The host binary models did not significantly outperform the null models at predicting caterpillar densities in the excluded block (β = -0.0013, p = 0.92). Discussion Dietary specialist and generalist caterpillar populations responded positively to nearby host plant densities, supporting the most basic paradigm of consumer-resource relationships; however, these relationships diverged at larger spatial scales. Unconventionally, this result implies that dietary generalist and specialist herbivores respond similarly, but not identically, to resource distributions - a rarely considered comparison. Dietary generalist populations responded to host concentrations positively at scales up to 50 m, but the strength and direction of that response inverted for dietary specialists from 3 m up to 50 m. These contrasting patterns suggest that the spatial structure of resources interacts with diet breadth to shape herbivore distributions. The observed negative response of specialist herbivores to host-plant density at intermediate spatial scales (25-50 m) shows that the effects of resource concentration are modified by spatial scale. A negative response of specialists to resource concentration at any scale is surprising, as most supporting evidence for a positive consumer-resource relationship currently comes from studies on dietary specialists (Rhainds and English-Loeb 2003). The “resource dilution effect” we observed at intermediate spatial scales has been previously described, but on much smaller scales. Otway et al. (2005) found a similar effect at a scale of only 2 m: the scale at which we found positive effects of resource concentration. Otway (2005) did not include any Lepidoptera, and instead primarily observed weevils (Coleoptera: Curculionidae). Moth species are often capable of detecting their host plants while in flight, an ability that has been observed to expand their relevant dispersal ranges (Bukovinszky et al. 2005). Given the shorter dispersal scale of the insects in Otway (2005) than the Lepidoptera included in this study, it is possible that their observation of dilution at scales of 2 m may be parallel to our observation of dilution between 25–50 m. More generally, the results of these studies may indicate consistent patterns where consumers aggregate in areas of exceptionally high resource concentration, which in turn results in low densities across broad, somewhat-vegetated source areas of these consumers. This would give rise to patterns consistent with resource concentration at fine-scales and resource dilution at intermediate scales. Our observed scale-dependent response of dietary specialist caterpillar populations to host plant density also contrasts with patterns described in Bommarco & Banks (2003), where negative numerical responses were most often detected at smaller spatial scales and positive responses at larger scales. Bommarco & Banks (2003) only reviewed agricultural studies of resource concentration, and it is possible that the relationship is inverted between natural and agricultural systems. One mechanism for such a reversed relationship could be associational effects that become important in natural systems that have a diversity of host plants, insect herbivores, and predators (Tahvanainen and Root 1972, Barbosa et al. 2009). Plants in natural forests can experience associational resistance stemming from a reduction in their apparency to specialists when they are surrounded by heterospecifics (Castagneyrol et al. 2014b), but for large, otherwise readily apparent trees this masking may only begin at intermediate scales (e.g., 25–50 m in our case) and otherwise could create the positive host-herbivore relationship we observe at smaller scales (<10 m). Furthermore, intermingling of plant species may dilute foraging cues for parasitoids and other enemies of herbivores responding to distributions of their own resources. Host plants occurring at low densities or in small patches may therefore provide pockets of enemy-free space for herbivores with high survival, creating the positive effect we observed at smaller scales. At larger scales, this relationship may be weakened or even reversed as predators congregate in the areas with the densest foraging cues. Neither plant apparency nor these predator-herbivore-plant interactions, which rely on interactions among plant taxa, would be relevant in a monocultural agroecosystem. In contrast to the complex and scale-dependent relationships between specialists and resource concentration, the patterns observed in generalists are more straightforward. Here, we observed that increased host concentration led to increased generalist herbivore density up to 50 m, with no discernable effect beyond that distance. This result may stem from a number of frequently suggested mechanisms for positive consumer-resource relationships, such as selection against adult dispersal out of large patches (>50 m) where reproductive success is high (Root 1973), attraction of adults to highly apparent host patches (Castagneyrol et al. 2014b), or more complex dynamics relating to patch perimeter and fractal dimension (Englund and Hambäck 2004). The spatial scale of our observed response aligns with observed clustering patterns for several forest moths: for example, Pimentel et al. (2017) found the spatial clustering of Thaumetopoea pityocampa peaked at about 60 m. However, some studies find the spatial clustering of Lepidoptera to occur at much larger scales (e.g. 282 m in Basoalto et al. 2010), warranting further study into relationships between dispersal distance and spatial response to resources. It is unclear why the dietary groups responded differently to concentrations of their resources. It is possible that, beyond a certain patch area (say, 25–50 m in diameter), the oviposition capacities of adult specialist herbivores are exhausted, and, in turn, caterpillar densities (but not abundances) in that patch decrease (Vehviläinen et al. 2007, Doublet et al. 2019). This effect would weaken for adult generalists, as overall host abundance is significantly higher, leading to lower overall heterogeneity in resource concentration and a lower marginal cost of dispersal. An alternative possibility is that adult specialists may disperse long distances (e.g., > 400 m, the maximum scale of this study) to a new patch after laying each clutch of eggs, which would generate a similar pattern to the one we observed. Specialists may also be more selective of the plants on which they oviposit, preferentially choosing the least resistant phenotypes within their host species, resulting in small, overdispersed, highly concentrated areas of herbivore density (Thompson and Pellmyr 1991). The relationship between generalist herbivore insects and the distribution of their resources has rarely been quantified: in a review of associational resistance, Castagneyrol et al. (2014a) found no studies that reported data on generalist herbivore abundance, while 64% of similar studies on specialist herbivores did so for their study species. Within the scope of the review, Castagneyrol et al. (2014a) detected no relationship between polyphagous herbivory and focal resource availability; however, polyphagous herbivory on focal plants increased with the abundance of phylogenetically related neighbors. Because polyphagous herbivores generally consume phylogenetically related plant species, Castagneyrol et al .’s (2014a) result supports our finding that abundances of dietary generalists depend on the combined density of all their hosts, not just that of a single host species. The evidence that generalist herbivores respond to the combined availability of their host plants has important ramifications for other areas of ecological theory. Through incorporating metrics such as electivity, future studies involving or building on the relationships between consumers and their resources may be able to detect previously unknown spatial relationships between dietary generalists and their host plants. Studies of broader consumer-resource phenomena that have previously focused heavily on specialists may consider incorporating generalists into their frameworks. This is further evidenced by the fact that the scales at which we determined host concentration to have the strongest effect on generalist densities (3–25 m) is similar to the scales at which is similar to the scales at which empirical studies have found distance- and density-dependent mortality operates under the Janzen-Connell hypothesis (Janzen 1970, Connell 1971), including Murphy et al. (2017; 12–18 m), Hubbell et al. (2001; 12–15 m), and Stoll & Newbery (2005; <20 m). Our findings suggest that incorporating generalists into these frameworks may facilitate a broader understanding of ecological systems. Conclusions : We examined the idea that consumer population densities are positively correlated with their resources across multiple scales and a range of diet breadths. We found mixed evidence for this relationship, observing the predicted pattern at small (<3 m) scales but observing the opposite pattern at larger (25–50 m) scales for specialist herbivores and a strictly positive relationship for generalist herbivores up to 50 m. Detecting this relationship for generalists required accounting for variation in the association between the herbivores and their different host plant species (electivity); a methodological innovation that offers a framework to integrate work on the numerical responses of consumers to resource availability across diet-breadths. Previous work on resource-consumer relationships has yielded conflicting results, and this study provides potential explanations, such as scale-dependency and a nuanced quantification of host-preference, for some of the heterogeneity in the literature. By providing empirical evidence for the spatial scale at which plant-herbivore interactions operate in our system and a strong argument for inclusion of generalists in future studies of resource-consumer relationships, our findings can inform future work on the mechanisms through which herbivore and plant communities shape each other. References Anderson, R. M., Dallar, N. M., Pirtel, N. L., Connors, C. J., Mickley, J., Bagchi, R. and Singer, M. S. 2019. Bottom-up and top-down effects of forest fragmentation differ between dietary generalist and specialist caterpillars. - Front. Ecol. Evol. in press. Andersson, P., Löfstedt, C. and Hambäck, P. A. 2013. 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Caterpillars of eastern North America: A guide to identification and natural history. - Princeton University Press. Figure 1. (a) The location of Connecticut within the United States, and the location of the 32 forest patches within Connecticut. (b) The design of each of the 32 patches; not to scale. The center of each patch has three 10 m x 10 m vegetation plots (green squares numbered 2 and 3) and twelve 5 m x 5 m caterpillar plots (blue squares, filled and unfilled). The other 18 green squares (numbered 4-6) are outer vegetation plots. Numbers within each vegetation plot are the natural log of the approximate distance in meters of that vegetation plot to the reference (solid blue) caterpillar plot. This sampling design allowed sampling of the vegetation community at distances of e 2 (10), e 3 (25), e 4 (50), e 5 (150), and e 6 (400) m from each caterpillar plot. Vegetation within the dark blue caterpillar plot is considered e 1 (3) m from the sampled plot. Figure 2. (a) The network connections between caterpillar species (left) and host plant species (right). Light blue bars represent dietary specialists, and dark blue bars represent generalists. The width of each blue bar indicates the abundance of that caterpillar species. The width of each green bar is the summed electivity across all caterpillar species for each host plant species. The thickness of each grey connection is the electivity of a caterpillar species for the connected plant, scaled with caterpillar abundance. (b) The relative abundance of each caterpillar species. Darker colors indicate a higher richness of host plants. (c) The relative abundance of caterpillars on each host species. The size of each point is scaled by the average number of caterpillars per branch sampled. Color represents the logged summed electivity for that host plant across all caterpillar species. Figure 3. (Left) The predicted effect of electivity-scaled host densities on caterpillar abundance at each of the varying scales. Back-transformed effect sizes for dietary specialist caterpillars are shown with light blue circles, while predictions for the dietary generalist caterpillars are shown by dark blue squares. Bars represent 95% confidence intervals; lines represent quadratic lines of best fit. Host density affected generalist caterpillars positively at small scales, then dropped in relevance at larger scales. Host density inconsistently affected specialists, with a positive effect at small scales and a negative effect at larger scales. (Right) A summary table of the models depicted on the left. Figure 1. Figure 2. Figure 3. Information & Authors Information Version history V1 Version 1 19 November 2025 Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords diet breadth herbivory lepidoptera plant-insect interactions resource concentration Authors Affiliations Michael LaScaleia 0000-0002-8777-5962 [email protected] University of Connecticut View all articles by this author Chris Elphick University of Connecticut View all articles by this author James Mickley 0000-0002-5988-5275 Oregon State University View all articles by this author Michael Singer 0000-0002-0164-3767 Wesleyan University View all articles by this author David Wagner University of Connecticut View all articles by this author Robert Bagchi 0000-0003-4035-4105 University of Connecticut View all articles by this author Metrics & Citations Metrics Article Usage 228 views 153 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Michael LaScaleia, Chris Elphick, James Mickley, et al. The effect of resource concentration on consumer population densities depends on spatial scale and diet breadth. Authorea . 19 November 2025. 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