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Stressors


Gary Fellers, air quality.
G. Fellers (ARMI) changing a filter in an air sampler that is used to measure agricultural chemicals that drift into Yosemite NP, California. Photo by: J. Fellers.

Declines in amphibian populations have occurred not only on areas clearly impacted by human activities such as urbanization, but also on protected lands intended to buffer amphibians and other wildlife from anthropogenic disturbances. Some stressors are not stopped by preserve boundaries and can affect wildlife populations 10's or 100's of kilometers from their source or point of use. For example, pesticides, fertilizers, or supplements given to livestock can be transported from the terrestrial setting where they are applied, to aquatic environments via precipitation, run-off, erosion, wind, and misuse. Conversely, some contaminants such as mercury or selenium occur naturally, but can be concentrated, or disturbed and released into the environment by human activities. Amphibian populations can be exposed to multiple stressors simultaneously, producing novel conditions with unknown outcomes.

ARMI scientists conduct research to identify stressors and evaluate their impacts on amphibian individuals and populations.

ARMI Products on Stressors

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USGS  
This is an ARMI Product. Introduced parasites of freshwater fish in southern California, U.S.A.
Authors: Kuperman BI, Matey VE, Warburton ML, Fisher RN | Date: 2002 | Outlet: pp. 407-411 in: Proceedings of the 10th International Congress of Parasitology - ICOPA X. Monduzzi Editore S.p.A. - MEDIMOND Inc., Bologna, Italy. 660 pp. | Format: .PDF

Cheryl Brehme, USGS  
This is an ARMI Product. Longevity and population age structure of the arroyo southwestern toad (Anaxyrus californicus) with drought implications
Authors: Fisher RN, Brehme CS, Hathaway SA, Hovey TE, Warburton ML, Stokes DC | Outlet: Ecology and Evolution | Format: .PDF
The arroyo southwestern toad is a specialized and federally endangered amphibian endemic to the coastal plains and mountains of central and southern California and northwestern Baja California. It is largely unknown how long these toads live in natural systems, how their population demographics vary across occupied drainages, and how hydrology affects age structure. We used skeletochronology to estimate the ages of adult arroyo toads in seven occupied drainages with varying surface water hydrology in southern California. We processed 179 adult toads with age estimates between one and six years. Comparisons between skeletochronological ages and known ages of PIT tagged toads showed that skeletochronology likely underestimated toad age by up to two years, indicating they may live to seven or eight years, but nonetheless major patterns were evident. Arroyo toads showed sexual size dimorphism with adult females reaching a maximum size of 12mm greater than males. Population age structure also varied among the sites. Age structure at sites with seasonally predictable surface water was biased toward younger individuals, which indicated stable recruitment for these populations. In contrast, age structures at the ephemeral sites were biased toward older individuals with cohorts roughly corresponding to higher rainfall years. These populations are driven by surface water availability, a stochastic process, and are thus more unstable. Based on our estimates of toad ages, climate predictions of extreme and prolonged drought events could mean that the number of consecutive dry years could surpass the maximum life span of toads making them vulnerable to extirpation, especially in ephemeral freshwater systems. Understanding the relationship between population demographics and hydrology is essential for predicting species resilience to projected changes in weather and rainfall patterns. The arroyo toad serves as a model for understanding potential species responses to long term climatic and hydrologic changes in Mediterranean stream systems. We recommend development of adaptive management strategies to address these threats.

This is an ARMI Product. Comparative road risk assessment for reptiles and amphibians in California
Authors: Brehme CS, Hathaway SA, Fisher RN | Outlet: Landscape Ecology | Format: .PDF
We developed an objective and scientifically-based approach to assess the potential threat from negative road impacts and applied it to over 160 herpetofaunal species (and subspecies) in the state of California, a global biodiversity hotspot. We assessed risk across all species in an objective manner informed by current road ecology literature with scoring based upon a suite of life history and space-use characteristics associated with negative road effects. First, we assessed risk at the scale of an individual animal and then expanded the risk to both population and species-wide levels. At the individual level, road risk was scored on the likelihood an animal would encounter one or more roads based on movement distance and movement frequency. Slow species that do not avoid roads were given added weight. At the population level, we incorporated fecundity and the proportion of population that may encounter roads. For risk to an entire species, we incorporated range size and conservation status. Once the scores were tabulated, we grouped species into categories of road-related risk (Very High, High, Medium, Low, and Very Low) based upon ranges of values that represented frequency distributions in 20% increments of all species scores.
Chelonids, large bodied snakes, and toads were the highest risk groups, with 100% of chelonids, 72% of snakes, and 64% of toads at high or very high risk from roads within their terrestrial and/or aquatic habitats. Twenty-seven percent of frog species were ranked as high or very high risk, while only 18% of lizard species and 15% of salamanders were ranked at high risk from negative road impacts.
The results of our assessment are consistent with available road ecology literature in identifying known high risk species, as well as identifying additional species of concern. We believe this approach could be useful for assessing risk to populations and species from road related fragmentation and mortality across habitats and among taxonomic groups. The results will help to inform multi-criteria threat assessments for special status species or those in consideration for listing. It also highlights species that may deserve further study and consideration for aquatic and terrestrial road mitigation to reduce mortality and to maintain population and species level connectivity.


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