Irruptive dynamics of introduced caribou on Adak Island, Alaska: an evaluation of Riney‐Caughley model predictions

Ricca, Mark A.; Vuren, Dirk H. Van; Weckerly, Floyd W.; Williams, Jeffrey C.; Miles, A. Keith

doi:10.1890/es13-00338.1

Cited by 8 publications

(11 citation statements)

References 60 publications

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“…When lichens are depleted by caribou herbivory there often is a concomitant increase in graminoid species that are more tolerant to grazing (Ricca et al. ). The different plant forms have different plant traits, and the changes in plant communities associated with irruptive populations of caribou is ostensibly apparent.…”

Section: Discussionmentioning

confidence: 99%

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Vegetation changes associated with a population irruption by Roosevelt elk

Starns

Weckerly

Ricca

et al. 2014

Ecology and Evolution

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Interactions between large herbivores and their food supply are central to the study of population dynamics. We assessed temporal and spatial patterns in meadow plant biomass over a 23-year period for meadow complexes that were spatially linked to three distinct populations of Roosevelt elk (Cervus elaphus roosevelti) in northwestern California. Our objectives were to determine whether the plant community exhibited a tolerant or resistant response when elk population growth became irruptive. Plant biomass for the three meadow complexes inhabited by the elk populations was measured using Normalized Difference Vegetation Index (NDVI), which was derived from Landsat 5 Thematic Mapper imagery. Elk populations exhibited different patterns of growth through the time series, whereby one population underwent a complete four-stage irruptive growth pattern while the other two did not. Temporal changes in NDVI for the meadow complex used by the irruptive population suggested a decline in forage biomass during the end of the dry season and a temporal decline in spatial variation of NDVI at the peak of plant biomass in May. Conversely, no such patterns were detected in the meadow complexes inhabited by the nonirruptive populations. Our findings suggest that the meadow complex used by the irruptive elk population may have undergone changes in plant community composition favoring plants that were resistant to elk grazing.

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Section: Discussionmentioning

confidence: 99%

“…Another pattern of population growth in ungulates, irruption, has been well documented in numerous populations of large ungulates inhabiting oceanic islands (Scheffer ; Klein ; Caughley ; Leader‐Williams ; Ricca et al. ). The model of population irruption described by Riney () and Caughley () has four stages.…”

Section: Introductionmentioning

confidence: 98%

Vegetation changes associated with a population irruption by Roosevelt elk

Starns

Weckerly

Ricca

et al. 2014

Ecology and Evolution

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“…This pattern was likely related to intensifying density‐dependent regulation and a new increase in hunting pressure (Ricca et al. ). Ecological differences between free‐ranging reindeer on Atka and caribou on Adak are trivial, and higher relative densities on Adak compared to Atka fit the four‐stage model.…”

Section: Methodsmentioning

confidence: 99%

“…Kagalaska was thought to have been caribou‐free at the start of our study (Williams and Tutiakoff ), but the nascent stages of an invasion of caribou from Adak were detected during our study (Ricca et al. , ). Sampling of another neighboring island to serve as a true Rangifer ‐free control was not logistically possible, yet the total number of animals and amount of sign detected on Kagalaska across all sampling periods of our study was miniscule compared to what we observed on Adak and Atka (Ricca et al.…”

Section: Methodsmentioning

confidence: 99%

“…Rangifer on Adak represent an approximate 50‐yr history of herbivory, but were kept at relatively low density by managed hunting for ~35 yr before undergoing a period of exponential growth following relaxation of hunting pressure (Ricca et al. ). Thus, Adak may represent more transient effects given its intermediate history of herbivory, but the recent period of exponential growth could also shift overall patterns of ecosystem processes similar to those measured on Atka (Fig.…”

Section: Introductionmentioning

confidence: 97%

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Impacts of introducedRangiferon ecosystem processes of maritime tundra on subarctic islands

et al. 2016

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Abstract. Introductions of mammalian herbivores to remote islands without predators provide a natural experiment to ask how temporal and spatial variation in herbivory intensity alter feedbacks between plant and soil processes. We investigated ecosystem effects resulting from introductions of Rangifer tarandus (hereafter "Rangifer") to native mammalian predator-and herbivore-free islands in the Aleutian archipelago of Alaska. We hypothesized that the maritime tundra of these islands would experience either: (1) accelerated ecosystem processes mediated by positive feedbacks between increased graminoid production and rapid nitrogen cycling; or (2) decelerated processes mediated by herbivory that stimulated shrub domination and lowered soil fertility. We measured summer plant and soil properties across three islands representing a chronosequence of elapsed time post-Rangifer introduction (Atka: ~100 yr; Adak: ~50; Kagalaska: ~0), with distinct stages of irruptive population dynamics of Rangifer nested within each island (Atka: irruption, K-overshoot, decline, K-re-equilibration; Adak: irruption, K-overshoot; Kagalaska: initial introduction). We also measured Rangifer spatial use within islands (indexed by pellet group counts) to determine how ecosystem processes responded to spatial variation in herbivory. Vegetation community response to herbivory varied with temporal and spatial scale. When comparing temporal effects using the island chronosequence, increased time since herbivore introduction led to more graminoids and fewer dwarf-shrubs, lichens, and mosses. Slow-growing Cladonia lichens that are highly preferred winter forage were decimated on both long-term Rangifer-occupied islands. In addition, linear relations between more concentrated Rangifer spatial use and reductions in graminoid and forb biomass within islands added spatial heterogeneity to long-term patterns identified by the chronosequence. These results support, in part, the hypothesis that Rangifer population persistence on islands is facilitated by successful exploitation of graminoid biomass as winter forage after palatable lichens are decimated. However, the shift from shrubs to graminoids was expected to enhance rates of nitrogen cycling, yet rates of net N-mineralization, NH + 4 pools, and soil δ 15 N declined markedly along the chronosequence and were weakly associated with spatial use within islands. Overall plant and soil patterns were disrupted but responded differently to intermediate (50 yr) and long-term (100 yr) herbivory, and were correlated with distinct stages of irruptive population dynamics.

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Ecological and behavioral mechanisms of density‐dependent habitat expansion in a recovering African ungulate population

Becker

Hutchinson

Potter

et al. 2021

Ecological Monographs

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Major disturbances can temporarily remove factors that otherwise constrain population abundance and distribution. During such windows of relaxed top-down and/or bottom-up control, ungulate populations can grow rapidly, eventually leading to resource depletion and density-dependent expansion into less-preferred habitats. Although many studies have explored the demographic outcomes and ecological impacts of these processes, fewer have examined the individual-level mechanisms by which they occur. We investigated these mechanisms in Gorongosa National Park, where the Mozambican Civil War devastated largemammal populations between 1977 and 1992. Gorongosa's recovery has been marked by proliferation of waterbuck (Kobus ellipsiprymnus), an historically marginal 200-kg antelope species, which is now roughly 20-fold more abundant than before the war. We show that after years of unrestricted population growth, waterbuck have depleted food availability in their historically preferred floodplain habitat and have increasingly expanded into historically avoided savanna habitat. This expansion was demographically skewed: mixed-sex groups of prime-age individuals remained more common in the floodplain, while bachelors, loners, and subadults populated the savanna. By coupling DNA metabarcoding and forage analysis, we show that waterbuck in these two habitats ate radically different diets, which were more digestible and protein-rich in the floodplain than in savanna; thus, although individuals in both habitats achieved positive net energy balance, energetic performance was higher in the floodplain. Analysis of daily activity patterns from high-resolution GPS-telemetry, accelerometry, and animal-borne video revealed that savanna waterbuck spent less time eating, perhaps to accommodate their tougher, lower-quality diets. Waterbuck in savanna also had more ectoparasites than those in the floodplain. Thus, plasticity in foraging behavior and diet selection enabled savanna waterbuck to tolerate the costs of density-dependent spillover, at least in the short term; however, the already poorer energetic performance of these individuals implies that savanna occupancy may become prohibitively costly as heterospecific competitors and predators continue to recover in Gorongosa. Our results suggest that behavior can provide a leading indicator of the onset of density-dependent limitation and the likelihood of subsequent population decline, but that reliable inference hinges on understanding the mechanistic basis of observed behavioral shifts.

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Irruptive dynamics of introduced caribou on Adak Island, Alaska: an evaluation of Riney‐Caughley model predictions

Cited by 8 publications

References 60 publications

Vegetation changes associated with a population irruption by Roosevelt elk

Vegetation changes associated with a population irruption by Roosevelt elk

Impacts of introducedRangiferon ecosystem processes of maritime tundra on subarctic islands

Ecological and behavioral mechanisms of density‐dependent habitat expansion in a recovering African ungulate population

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