Geology topics

Research about Aleutian archipelago

Source-linked reports with geographic coverage including Aleutian archipelago.

3 recordsLinked to original sources

USGS2023

Changes in abiotic drivers of green sea urchin demographics following the loss of a keystone predator

Sea urchin population demographics can respond to changes in keystone species abundances, with the magnitude of these responses varying depending on environmental influences. In this study, sea urchin populations were surveyed across 15 Aleutian archipelago islands over a 30-year period to understand how patterns of sea urchin demography (density, biomass, and size structure) varied through different ecological regimes that were caused by changes in the abundance of sea otters, a keystone species in this system. To examine long-term changes in sea urchin demographics, four time periods across the recent decline of sea otters were examined: during sea otter presence (1987-1994), nearing absence at the end of the decline (1997-2000), 10 years postdecline (2008-2010), and 15-20 years following the loss of sea otters from the ecosystem (2014-2017). Our results show that when sea otters were broadly present, sea urchin demographics were generally similar across the archipelago, with few urchins that had large-sized bodies. During this time, bottom-up environmental controls were muted relative to top-down forces from keystone predation. However, as sea otters declined and remained absent from the system, abiotic factors became more influential on sea urchin biomass, density, and size structure. In particular, differences among island groups during these periods were correlated with variation in ocean temperature, bathymetric complexity, and habitat availability. Sea urchin recruitment also varied among island groups, corresponding to ecoregions delineated by oceanic passes across the archipelago. The functional extinction of sea otters revealed an increasing influence of abiotic forcing in the absence of top-down control. This study further highlights the importance of understanding how keystone predators regulate herbivore demographics.

Alaska↗

USGS2016

Impacts of introduced Rangifer on ecosystem processes of maritime tundra on subarctic islands

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.

Alaska↗

USGS1998

Killer whale predation on sea otters linking oceanic and nearshore ecosystems

After nearly a century of recovery from overhunting, sea otter populations are in abrupt decline over large areas of western Alaska. Increased killer whale predation is the likely cause of these declines. Elevated sea urchin density and the consequent deforestation of kelp beds in the nearshore community demonstrate that the otter's keystone role has been reduced or eliminated. This chain of interactions was probably initiated by anthropogenic changes in the offshore oceanic ecosystem.

Alaska↗