Integrating top‐down and bottom‐up effects of local density across scales and a complex life cycle

Mutz, Jessie; Underwood, Nora; Inouye, Brian D.

doi:10.1002/ecy.3118

Cited by 3 publications

(4 citation statements)

References 81 publications

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“…Second, we have detailed experimental data on how L. juncta survival and growth depend on L. juncta densities at small spatial scales, allowing us to explore the roles of spatial and ontogenetic variance for empirically observed strengths of local density dependence. In a previous study (Mutz et al, 2020), we manipulated the densities of eggs, early instar larvae and late instar larvae at multiple scales (i.e. patch of multiple host plants, single host plant within a patch, leaf within a plant), and measured survival and final larval mass.…”

Section: Methodsmentioning

confidence: 99%

“…We modelled the probability of survival from life stage

a

a + 1

using logistic regression, where

β_{0, a}

and

β_{1, a}

are, respectively, the stage‐specific intercept and slope of plant‐scale beetle density on survival. Previous work shows that survival is also affected by L. juncta density within a patch of multiple host plants (Mutz et al, 2020); we included a stage‐specific logistic regression slope for patch‐scale density,

β_{2, a}

, to account for this effect. For simplicity, we assume that all individuals of stage

a

experience either the average patch‐scale density,

{\overset{true¯}{x}}_{a} ()(, t)

, or their associated plant‐scale density,

z_{a}

, whichever is greater.…”

Section: Methodsmentioning

confidence: 99%

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Spatial and ontogenetic variance in local densities modify selection on demographic traits

Mutz

Inouye

2023

Functional Ecology

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Local density can affect individual performance by altering the strength of species interactions. Within many populations, local densities vary spatially (individuals are patchily distributed) or change across life stages, which should influence the selection and eco‐evolutionary feedback because local density variance affects mean fitness and is affected by traits of individuals. However, most studies on the evolutionary consequences of density‐dependent interactions focus on populations where local densities are relatively constant through time and space. We investigated the influence of spatial and ontogenetic variance in local densities within an insect population by comparing a model integrating both types of local density variance with models including only spatial variance, only ontogenetic variance, or no variance. We parameterized the models with experimental data, then used elasticity and invasion analyses to characterize selection on traits that affect either the local density an individual experiences (mean clutch size) or individuals' sensitivity to density (effect of larval crowding on pupal mass). Spatial and ontogenetic variance reduced population elasticity to effects of local density by 76% and 34% on average, respectively. Spatial variance modified selection and adaptive dynamics by altering the tradeoff between density‐dependent and density‐independent vital rates. In models including spatial variance, strategies that maximized density‐dependent survival were favoured over fecundity‐maximizing strategies even at low population density, counter to predictions of density‐dependent selection theory. Furthermore, only models that included spatial variance, thus linking the scales of oviposition and density‐dependent larval survival, had an evolutionarily stable clutch size. Ontogenetic variance weakened selection on mean clutch size and sensitivity to larval crowding by disrupting the relationship between trait values and performance during critical life stages. We demonstrate that local density variance can strongly modify selection at empirically observed interaction strengths and identify mechanisms for the effects of spatial and ontogenetic variance. Our findings reveal the potential for local density variance to mediate eco‐evolutionary feedback by shaping selection on demographically important traits. Read the free Plain Language Summary for this article on the Journal blog.

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Section: Methodsmentioning

confidence: 99%

“…We modelled the probability of survival from life stage

a

a + 1

using logistic regression, where

β_{0, a}

and

β_{1, a}

β_{2, a}

, to account for this effect. For simplicity, we assume that all individuals of stage

a

experience either the average patch‐scale density,

{\overset{true¯}{x}}_{a} ()(, t)

, or their associated plant‐scale density,

z_{a}

, whichever is greater.…”

Section: Methodsmentioning

confidence: 99%

Spatial and ontogenetic variance in local densities modify selection on demographic traits

Mutz

Inouye

2023

Functional Ecology

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“…Herbivorous insects are often relatively immobile during immature stages, and reductions in offspring production may reduce larval density on a host plant, which can in turn affect survival, development time, and adult mass. For example, per‐capita predation risk of eggs and fourth‐instar false potato beetle larvae, Leptinotarsa juncta , increases with beetle density (Mutz et al., 2020); the direction of this effect may be common for insects (Ives et al., 1993; Norowi et al., 2000). In some cases, such positive density‐dependent predation suggests that a similar number of larvae per plant will survive to pupation regardless of their starting abundance (Mutz et al., 2020).…”

Section: Introductionmentioning

confidence: 99%

“…For example, per‐capita predation risk of eggs and fourth‐instar false potato beetle larvae, Leptinotarsa juncta , increases with beetle density (Mutz et al., 2020); the direction of this effect may be common for insects (Ives et al., 1993; Norowi et al., 2000). In some cases, such positive density‐dependent predation suggests that a similar number of larvae per plant will survive to pupation regardless of their starting abundance (Mutz et al., 2020). For long‐term pest management, we need to know whether reductions in initial densities due to predation risk translate into reductions in the number of offspring surviving to adulthood.…”

Section: Introductionmentioning

confidence: 99%

Do maternal allocations towards offspring quality and quantity ameliorate the effects of predators on offspring survival?

Ugine,

Mutz,

Underwood

et al. 2024

J Applied Entomology

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Reproductive allocation is often balanced between the quantity and quality of offspring. Ecological stresses, like exposure to predators, can cause organisms to shift their allocations along this continuum. While the consequences of such plastic shifts for offspring performance are often untested, they are critical to understanding the potential long‐term benefits of manipulating predation risk as an agricultural pest management technique. Predation risk induces reductions in egg production and increases in nutritional condition due to maternal provisioning in Colorado potato beetles (Leptinotarsa decemlineata, CPB). Here, we tested whether reductions in density or increases in offspring condition, which may increase per‐capita larval survival, can compensate for the reduction in total egg production, especially when offspring are exposed to predators. In two field trials, we manipulated density and condition of larval CPB and measured survival through development to adulthood in field cages with and without predaceous stink bugs (Podisus maculiventris). As expected, cages with the higher initial larval densities had more larvae and adults surviving in the treatments without predators – about 30%–50% survival across densities. When predators were present, this relationship did not hold because of density‐dependent predation. Larval condition interacted with density and impacted larval survival in both trials albeit in different ways. In trial 1, unprovisioned beetles had higher survival at the higher densities; in trial 2, provisioned beetles had higher survival across densities. Synthesis and Applications: Overall, our test of the effects of predation risk via manipulations of larval density and condition revealed few net compensatory benefits to the prey of reduced density and higher condition. Benefits to the prey of shifts in allocation from the quantity to quality of offspring may depend on factors that influence the strength of density dependence, including predation intensity. Our results suggest a new strategy of taking advantage of the reductions in prey density due to non‐consumptive effects of predators as a pest management approach to protect plants.

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Plant resistance and predators influence the density dependence of herbivore survival and distribution of herbivory

Paniagua Montoya,

Forde,

Inouye

et al. 2024

Ecology and Evolution

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Plant resistance and predators can influence density‐dependent survivorship and growth of herbivores, and their damage to plants. Although the independent effects of plant resistance and predators on herbivores and herbivory are well known, little is known about their interactive and density‐dependent effects on herbivores and the amount and distribution of damage on plants. These relationships are important for understanding how herbivore and plant populations influence each other. We used a laboratory density‐manipulation experiment to determine how plant resistance (three treatments: jasmonate‐insensitive, unmanipulated wild type, and jasmonate‐sprayed wild‐type plants) and predation (two treatments: predator or no predator) affect the survivorship and growth of an herbivore, as well as per capita damage and the distribution of damage on plants. We found evidence that the density dependence of herbivore survivorship was influenced by predators and an interactive effect of plant resistance and predation. Herbivore growth was reduced by higher plant resistance but was not density‐dependent nor affected by predation. Per capita plant damage was reduced by plant resistance, predation, and herbivore density. The within‐plant distribution of damage became more even with increasing herbivore density but was not affected by predation or the independent effect of plant resistance. The distribution of damage was also affected by an interaction between plant resistance and herbivore density; damage became less aggregated with density across all plant resistance treatments, but the decrease was strongest for the jasmonate‐insensitive plants. These results show that predators influence herbivore density dependence, and that plant resistance can affect the impact of predators on herbivores. Though plant resistance, predation, and herbivore density all reduced per capita herbivore damage to plants, only herbivore density and plant resistance affected the distribution of damage. Distributions of herbivory can influence plant success; documenting patterns of herbivory is an under‐appreciated avenue for integrating effects of plant resistance, predators, and herbivore density on plant–herbivore interactions.

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Integrating top‐down and bottom‐up effects of local density across scales and a complex life cycle

Abstract: Integrating top-down and bottom-up effects of local density across scales and a complex life cycle.

Cited by 3 publications

References 81 publications

Spatial and ontogenetic variance in local densities modify selection on demographic traits

Spatial and ontogenetic variance in local densities modify selection on demographic traits

Do maternal allocations towards offspring quality and quantity ameliorate the effects of predators on offspring survival?

Plant resistance and predators influence the density dependence of herbivore survival and distribution of herbivory

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