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Alkhayuon H, Marley J, Wieczorek S, Tyson RC. Stochastic resonance in climate reddening increases the risk of cyclic ecosystem extinction via phase-tipping. GLOBAL CHANGE BIOLOGY 2023; 29:3347-3363. [PMID: 37021593 DOI: 10.1111/gcb.16679] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/05/2022] [Revised: 02/24/2023] [Accepted: 03/02/2023] [Indexed: 05/16/2023]
Abstract
Human activity is leading to changes in the mean and variability of climatic parameters in most locations around the world. The changing mean has received considerable attention from scientists and climate policy makers. However, recent work indicates that the changing variability, that is, the amplitude and the temporal autocorrelation of deviations from the mean, may have greater and more imminent impact on ecosystems. In this paper, we demonstrate that changes in climate variability alone could drive cyclic predator-prey ecosystems to extinction via so-called phase-tipping (P-tipping), a new type of instability that occurs only from certain phases of the predator-prey cycle. We construct a mathematical model of a variable climate and couple it to two self-oscillating paradigmatic predator-prey models. Most importantly, we combine realistic parameter values for the Canada lynx and snowshoe hare with actual climate data from the boreal forest. In this way, we demonstrate that critically important species in the boreal forest have increased likelihood of P-tipping to extinction under predicted changes in climate variability, and are most vulnerable during stages of the cycle when the predator population is near its maximum. Furthermore, our analysis reveals that stochastic resonance is the underlying mechanism for the increased likelihood of P-tipping to extinction.
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Affiliation(s)
- Hassan Alkhayuon
- School of Mathematical Sciences, University College Cork, Western Road, Cork, T12 XF62, Ireland
| | - Jessa Marley
- CMPS Department (Mathematics), University of British Columbia Okanagan, Kelowna, British Columbia, Canada
| | - Sebastian Wieczorek
- School of Mathematical Sciences, University College Cork, Western Road, Cork, T12 XF62, Ireland
| | - Rebecca C Tyson
- CMPS Department (Mathematics), University of British Columbia Okanagan, Kelowna, British Columbia, Canada
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2
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Oli MK, Kenney AJ, Boonstra R, Boutin S, Murray DL, Peers MJL, Gilbert BS, Jung TS, Chaudhary V, Hines JE, Krebs CJ. Does coat colour influence survival? A test in a cyclic population of snowshoe hares. Proc Biol Sci 2023; 290:20221421. [PMID: 37015272 PMCID: PMC10072933 DOI: 10.1098/rspb.2022.1421] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 04/06/2023] Open
Abstract
Some mammal species inhabiting high-latitude biomes have evolved a seasonal moulting pattern that improves camouflage via white coats in winter and brown coats in summer. In many high-latitude and high-altitude areas, the duration and depth of snow cover has been substantially reduced in the last five decades. This reduction in depth and duration of snow cover may create a mismatch between coat colour and colour of the background environment, and potentially reduce the survival rate of species that depend on crypsis. We used long-term (1977-2020) field data and capture-mark-recapture models to test the hypothesis that whiteness of the coat influences winter apparent survival in a cyclic population of snowshoe hares (Lepus americanus) at Kluane, Yukon, Canada. Whiteness of the snowshoe hare coat in autumn declined during this study, and snowshoe hares with a greater proportion of whiteness in their coats in autumn survived better during winter. However, whiteness of the coat in spring did not affect subsequent summer survival. These results are consistent with the hypothesis that the timing of coat colour change in autumn can reduce overwinter survival. Because declines in cyclic snowshoe hare populations are strongly affected by low winter survival, the timing of coat colour change may adversely affect snowshoe hare population dynamics as climate change continues.
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Affiliation(s)
- Madan K Oli
- Department of Wildlife Ecology and Conservation, University of Florida, Gainesville, FL 32611, USA
- School of Biological Sciences, Zoology Building, Tillydrone Avenue, University of Aberdeen, AB24 2TZ, UK
| | - Alice J Kenney
- Department of Zoology, University of British Columbia, Vancouver, Canada V6T 1Z4
| | - Rudy Boonstra
- Department of Biological Sciences, University of Toronto Scarborough, Toronto, Canada M1C 1A4
| | | | - Dennis L Murray
- Department of Biology, Trent University, Peterborough, ON, Canada K9L 1Z8
| | | | - B Scott Gilbert
- Renewable Resources Management Program, Yukon University, Whitehorse, Yukon, Canada Y1A 5K4
| | - Thomas S Jung
- Department of Renewable Resources, University of Alberta, Edmonton, Alberta, Canada T6G 2E9
- Department of Environment, Government of Yukon, Whitehorse, Yukon, Canada Y1A 2C6
| | - Vratika Chaudhary
- Department of Wildlife Ecology and Conservation, University of Florida, Gainesville, FL 32611, USA
| | - James E Hines
- U.S. Geological Survey Eastern Ecological Science Center, Laurel, MD 20708, USA
| | - Charles J Krebs
- Department of Zoology, University of British Columbia, Vancouver, Canada V6T 1Z4
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3
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Gobin J, Hossie TJ, Derbyshire RE, Sonnega S, Cambridge TW, Scholl L, Kloch ND, Scully A, Thalen K, Smith G, Scott C, Quinby F, Reynolds J, Miller HA, Faithfull H, Lucas O, Dennison C, McDonald J, Boutin S, O’Donoghue M, Krebs CJ, Boonstra R, Murray DL. Functional Responses Shape Node and Network Level Properties of a Simplified Boreal Food Web. Front Ecol Evol 2022. [DOI: 10.3389/fevo.2022.898805] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
Ecological communities are fundamentally connected through a network of trophic interactions that are often complex and difficult to model. Substantial variation exists in the nature and magnitude of these interactions across various predators and prey and through time. However, the empirical data needed to characterize these relationships are difficult to obtain in natural systems, even for relatively simple food webs. Consequently, prey-dependent relationships and specifically the hyperbolic form (Holling’s Type II), in which prey consumption increases with prey density but ultimately becomes saturated or limited by the time spent handling prey, are most widely used albeit often without knowledge of their appropriateness. Here, we investigate the sensitivity of a simplified food web model for a natural, boreal system in the Kluane region of the Yukon, Canada to the type of functional response used. Intensive study of this community has permitted best-fit functional response relationships to be determined, which comprise linear (type I), hyperbolic (type II), sigmoidal (type III), prey- and ratio-dependent relationships, and inverse relationships where kill rates of alternate prey are driven by densities of the focal prey. We compare node- and network-level properties for a food web where interaction strengths are estimated using best-fit functional responses to one where interaction strengths are estimated exclusively using prey-dependent hyperbolic functional responses. We show that hyperbolic functional responses alone fail to capture important ecological interactions such as prey switching, surplus killing and caching, and predator interference, that in turn affect estimates of cumulative kill rates, vulnerability of prey, generality of predators, and connectance. Exclusive use of hyperbolic functional responses also affected trends observed in these metrics over time and underestimated annual variation in several metrics, which is important given that interaction strengths are typically estimated over relatively short time periods. Our findings highlight the need for more comprehensive research aimed at characterizing functional response relationships when modeling predator-prey interactions and food web structure and function, as we work toward a mechanistic understanding linking food web structure and community dynamics in natural systems.
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4
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Majchrzak YN, Peers MJL, Studd EK, Menzies AK, Walker PD, Shiratsuru S, McCaw LK, Boonstra R, Humphries M, Jung TS, Kenney AJ, Krebs CJ, Murray DL, Boutin S. Balancing food acquisition and predation risk drives demographic changes in snowshoe hare population cycles. Ecol Lett 2022; 25:981-991. [PMID: 35148018 DOI: 10.1111/ele.13975] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/10/2021] [Revised: 12/08/2021] [Accepted: 01/04/2022] [Indexed: 11/28/2022]
Abstract
Snowshoe hare cycles are one of the most prominent phenomena in ecology. Experimental studies point to predation as the dominant driving factor, but previous experiments combining food supplementation and predator removal produced unexplained multiplicative effects on density. We examined the potential interactive effects of food limitation and predation in causing hare cycles using an individual-based food-supplementation experiment over-winter across three cycle phases that naturally varied in predation risk. Supplementation doubled over-winter survival with the largest effects occurring in the late increase phase. Although the proximate cause of mortality was predation, supplemented hares significantly decreased foraging time and selected for conifer habitat, potentially reducing their predation risk. Supplemented hares also lost less body mass which resulted in the production of larger leverets. Our results establish a mechanistic link between how foraging time, mass loss and predation risk affect survival and reproduction, potentially driving demographic changes associated with hare cycles.
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Affiliation(s)
- Yasmine N Majchrzak
- Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada
| | - Michael J L Peers
- Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada
| | - Emily K Studd
- Department of Natural Resource Sciences, McGill University, Montreal, Quebec, Canada
| | - Allyson K Menzies
- Department of Natural Resource Sciences, McGill University, Montreal, Quebec, Canada
| | - Philip D Walker
- Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada
| | - Shotaro Shiratsuru
- Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada
| | - Laura K McCaw
- Department of Biological Sciences, University of Toronto Scarborough, Toronto, Ontario, Canada
| | - Rudy Boonstra
- Department of Biological Sciences, University of Toronto Scarborough, Toronto, Ontario, Canada
| | - Murray Humphries
- Department of Natural Resource Sciences, McGill University, Montreal, Quebec, Canada
| | - Thomas S Jung
- Department of Environment, Government of Yukon, Whitehorse, Yukon, Canada.,Department of Renewable Resources, University of Alberta, Edmonton, Alberta, Canada
| | - Alice J Kenney
- Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada
| | - Charles J Krebs
- Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada
| | - Dennis L Murray
- Department of Biology, Trent University, Peterborough, Ontario, Canada
| | - Stan Boutin
- Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada
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5
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OUP accepted manuscript. J Mammal 2022. [DOI: 10.1093/jmammal/gyac009] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Oli MK, Kenney AJ, Boonstra R, Boutin S, Chaudhary V, Hines JE, Krebs CJ. Estimating abundance, temporary emigration, and the pattern of density dependence in a cyclic snowshoe hare (Lepus americanus) population in Yukon, Canada. CAN J ZOOL 2022. [DOI: 10.1139/cjz-2021-0139] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Estimates of demographic parameters based on capture–mark–recapture (CMR) methods may be biased when some individuals in the population are temporarily unavailable for capture (temporary emigration). We estimated snowshoe hare abundance, apparent survival, and probability of temporary emigration in a population of snowshoe hares (Lepus americanus Erxleben, 1777) in the Yukon (Canada) using Pollock’s robust design CMR model, and population density using spatially explicit CMR models. Survival rates strongly varied among cyclic phases, seasons, and across five population cycles. We found strong evidence that temporary emigration was Markovian (i.e., nonrandom), suggesting that it varied among individuals that were temporary emigrant in the previous sampling period and those that were present in the sampled area. The probability of temporary emigration for individuals that were in the study area during the previous sampling occasion (γ″) varied among cycles. Probability that individuals that were temporarily absent from the sampled area would remain temporary emigrants (γ′) showed strongly seasonal pattern, low in winter and high during summers. Snowshoe hare population density ranged from 0.017 (0.015–0.05) hares/ha to 4.43 (3.90–5.00) hares/ha and showed large-scale cyclical fluctuations. Autocorrelation functions and autoregressive analyses revealed that our study population exhibited statistically significant cyclic fluctuations, with a periodicity of 9–10 years.
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Affiliation(s)
- Madan K. Oli
- Department of Wildlife Ecology and Conservation, University of Florida, Gainesville, FL 32611, USA
| | - Alice J. Kenney
- Department of Zoology, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada
| | - Rudy Boonstra
- Department of Biological Sciences, University of Toronto Scarborough, Toronto, ON M1C 1A4, Canada
| | - Stan Boutin
- Department of Biological Sciences, University of Alberta, Edmonton, AB T6G 2E9, Canada
| | - Vratika Chaudhary
- Department of Wildlife Ecology and Conservation, University of Florida, Gainesville, FL 32611, USA
| | - James E. Hines
- Eastern Ecological Science Center, U.S. Geological Survey, Laurel, MD 20708, USA
| | - Charles J. Krebs
- Department of Zoology, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada
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7
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Yearly Variations in GCM Concentrations in Female Mountain Hares ( Lepus timidus) and the Effect of Pregnancy. Animals (Basel) 2021; 11:ani11092710. [PMID: 34573677 PMCID: PMC8471945 DOI: 10.3390/ani11092710] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/05/2021] [Revised: 09/03/2021] [Accepted: 09/13/2021] [Indexed: 12/03/2022] Open
Abstract
Simple Summary The measurement of stress hormones has become a widely used and effective tool for evaluating adrenocortical activity in animals. However, to correctly interpret stress measurements, the potential sampling bias resulting from an oversampling of individuals in different states of pregnancy has rarely been investigated. We found significant yearly variations in states of pregnancy, which is related to the conditions of the females due to the snow cover duration before and at the start of the reproductive period. These results are important for improving the interpretation of stress hormone concentrations in free-ranging populations during the breeding and reproductive periods. Abstract The measurement of glucocorticoid metabolites (GCMs) in faeces has become a widely used and effective tool for evaluating the amount of stress experienced by animals. However, the potential sampling bias resulting from an oversampling of individuals in different states of pregnancy has rarely been investigated. In this study, we validate a noninvasive method for measuring gestagen metabolites in female mountain hares (Lepus timidus) under controlled conditions. We also measured the concentration of gestagen metabolites of females in a free-ranging population during the early breeding and post-breeding periods from 2014 to 2019. We found significant yearly variations in gestagen metabolites, which were related to the condition of the females due to the snow cover duration before and at the start of the reproduction period. GCMs were significantly influenced by the gestagen metabolite levels. These results are important for improving the interpretation of GCM concentrations in free-ranging populations during the breeding and reproductive periods.
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Lavergne SG, Krebs CJ, Kenney AJ, Boutin S, Murray D, Palme R, Boonstra R. The impact of variable predation risk on stress in snowshoe hares over the cycle in North America's boreal forest: adjusting to change. Oecologia 2021; 197:71-88. [PMID: 34435235 DOI: 10.1007/s00442-021-05019-1] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2021] [Accepted: 08/16/2021] [Indexed: 11/26/2022]
Abstract
The boreal forest is one of the world's ecosystems most affected by global climate warming. The snowshoe hare, its predators, and their population dynamics dominate the mammalian component of the North American boreal forest. Our past research has shown the 9-11-year hare cycle to be predator driven, both directly as virtually all hares that die are killed by their predators, and indirectly through sublethal risk effects on hare stress physiology, behavior, and reproduction. We replicated this research over the entire cycle by measuring changes in predation risk expected to drive changes in chronic stress. We examined changes in hare condition and stress axis function using a hormonal challenge protocol in the late winter of 7 years-spanning all phases of the cycle from the increase through to the low (2014-2020). We simultaneously monitored changes in hare abundance as well as those of their primary predators, lynx and coyotes. Despite observing the expected changes in hare-predator numbers over the cycle, we did not see the predicted changes in chronic stress metrics in the peak and decline phases. Thus, the comprehensive physiological signature indicative of chronic predator-induced stress seen from our previous work was not present in this current cycle. We postulate that hares may now be increasingly showing behavior-mediated rather than stress-mediated responses to their predators. We present evidence that increases in primary productivity have affected boreal community structure and function. We speculate that climate change has caused this major shift in the indirect effects of predation on hares.
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Affiliation(s)
- Sophia G Lavergne
- Department of Biological Sciences, University of Toronto Scarborough, Toronto, ON, Canada
| | - Charles J Krebs
- Department of Zoology, University of British Columbia, Vancouver, BC, Canada
| | - Alice J Kenney
- Department of Zoology, University of British Columbia, Vancouver, BC, Canada
| | - Stan Boutin
- Department of Biological Sciences, University of Alberta, Edmonton, AB, Canada
| | - Dennis Murray
- Department of Biology, Trent University, Peterborough, ON, Canada
| | - Rupert Palme
- Department of Biomedical Sciences, University of Veterinary Medicine, Vienna, Austria
| | - Rudy Boonstra
- Department of Biological Sciences, University of Toronto Scarborough, Toronto, ON, Canada.
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9
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Heger T, Aguilar-Trigueros CA, Bartram I, Braga RR, Dietl GP, Enders M, Gibson DJ, Gómez-Aparicio L, Gras P, Jax K, Lokatis S, Lortie CJ, Mupepele AC, Schindler S, Starrfelt J, Synodinos AD, Jeschke JM. The Hierarchy-of-Hypotheses Approach: A Synthesis Method for Enhancing Theory Development in Ecology and Evolution. Bioscience 2021; 71:337-349. [PMID: 33867867 PMCID: PMC8038874 DOI: 10.1093/biosci/biaa130] [Citation(s) in RCA: 12] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
In the current era of Big Data, existing synthesis tools such as formal meta-analyses are critical means to handle the deluge of information. However, there is a need for complementary tools that help to (a) organize evidence, (b) organize theory, and (c) closely connect evidence to theory. We present the hierarchy-of-hypotheses (HoH) approach to address these issues. In an HoH, hypotheses are conceptually and visually structured in a hierarchically nested way where the lower branches can be directly connected to empirical results. Used for organizing evidence, this tool allows researchers to conceptually connect empirical results derived through diverse approaches and to reveal under which circumstances hypotheses are applicable. Used for organizing theory, it allows researchers to uncover mechanistic components of hypotheses and previously neglected conceptual connections. In the present article, we offer guidance on how to build an HoH, provide examples from population and evolutionary biology and propose terminological clarifications.
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Affiliation(s)
- Tina Heger
- Department of Biodiversity Research and Systematic Botany, University of Potsdam, Potsdam, Germany
- Department of Restoration Ecology, Technical University of Munich, Freising, Germany
- Berlin-Brandenburg Institute of Advanced Biodiversity Research (BBIB), Berlin, Germany
| | - Carlos A Aguilar-Trigueros
- Berlin-Brandenburg Institute of Advanced Biodiversity Research (BBIB), Berlin, Germany
- Institute of Biology, Freie Universität Berlin, Berlin, Germany
| | - Isabelle Bartram
- Berlin-Brandenburg Institute of Advanced Biodiversity Research (BBIB), Berlin, Germany
- Institute of Biology, Freie Universität Berlin, Berlin, Germany
- Institute of Sociology, University of Freiburg, Freiburg
| | - Raul Rennó Braga
- Universidade Federal do Paraná, Laboratório de Ecologia e Conservação, Curitiba, Brazil
| | - Gregory P Dietl
- Paleontological Research Institution and the Department of Earth and Atmospheric Sciences at Cornell University, Ithaca, New York
| | - Martin Enders
- Berlin-Brandenburg Institute of Advanced Biodiversity Research (BBIB), Berlin, Germany
- Institute of Biology, Freie Universität Berlin, Berlin, Germany
- Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), Berlin, Germany
| | - David J Gibson
- School of Biological Sciences, Southern Illinois University Carbondale, Carbondale, Illinois
| | - Lorena Gómez-Aparicio
- Instituto de Recursos Naturales y Agrobiología de Sevilla, CSIC, LINCGlobal, Sevilla, Spain
| | - Pierre Gras
- Berlin-Brandenburg Institute of Advanced Biodiversity Research (BBIB), Berlin, Germany
- Department of Ecological Dynamics, Leibniz Institute for Zoo and Wildlife Research (IZW), also in Berlin, Germany
| | - Kurt Jax
- Department of Restoration Ecology, Technical University of Munich, Freising, Germany
- Department of Conservation Biology, Helmholtz Centre for Environmental Research—UFZ, Leipzig, Germany
| | - Sophie Lokatis
- Berlin-Brandenburg Institute of Advanced Biodiversity Research (BBIB), Berlin, Germany
- Institute of Biology, Freie Universität Berlin, Berlin, Germany
- Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), Berlin, Germany
| | - Christopher J Lortie
- Department of Biology, York University, York, Canada, as well as with the National Center for Ecological Analysis and Synthesis, University of California Santa Barbara, Santa Barbara, California
| | - Anne-Christine Mupepele
- Chair of Nature Conservation and Landscape Ecology, University of Freiburg, Freiburg, and the Senckenberg Biodiversity and Climate Research Centre, Frankfurt am Main, both in Germany
| | - Stefan Schindler
- Environment Agency Austria and University of Vienna's Division of Conservation Biology, Vegetation, and Landscape Ecology, Vienna, Austria, and his third affiliation is with Community Ecology and Conservation, Czech University of Life Sciences Prague, Prague, Czech Republic, Finally
| | - Jostein Starrfelt
- University of Oslo's Centre for Ecological and Evolutionary Synthesis and with the Norwegian Scientific Committee for Food and Environment, Norwegian Institute of Public Health, both in Oslo, Norway
| | - Alexis D Synodinos
- Department of Plant Ecology and Nature Conservation, University of Potsdam, Potsdam, Germany
- Berlin-Brandenburg Institute of Advanced Biodiversity Research (BBIB), Berlin, Germany
- Centre for Biodiversity Theory and Modelling, Theoretical, and Experimental Ecology Station, CNRS, Moulis, France
| | - Jonathan M Jeschke
- Berlin-Brandenburg Institute of Advanced Biodiversity Research (BBIB), Berlin, Germany
- Institute of Biology, Freie Universität Berlin, Berlin, Germany
- Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), Berlin, Germany
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10
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Contribution of late-litter juveniles to the population dynamics of snowshoe hares. Oecologia 2021; 195:949-957. [PMID: 33743069 DOI: 10.1007/s00442-021-04895-x] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/21/2020] [Accepted: 03/10/2021] [Indexed: 10/21/2022]
Abstract
Determining the factors driving cyclic dynamics in species has been a primary focus of ecology. For snowshoe hares (Lepus americanus), explanations of their 10-year population cycles most commonly feature direct predation during the peak and decline, in combination with their curtailment in reproduction. Hares are thought to stop producing third and fourth litters during the cyclic decline and do not recover reproductive output for several years. The demographic effects of these reproductive changes depend on the consistency of this pattern across cycles, and the relative contribution to population change of late-litter versus early litter juveniles. We used monitoring data on snowshoe hares in Yukon, Canada, to examine the contribution of late-litter juveniles to the demography of their cycles, by assigning litter group for individuals caught in autumn based on body size and capture date. We found that fourth-litter juveniles occur consistently during the increase phase of each cycle, but are rare and have low over-winter survival (0.05) suggesting that population increase is unlikely to be caused by their occurrence. The proportion of third-litter juveniles captured in the autumn remains relatively constant across cycle phases, while over-winter survival rates varies particularly for earlier-litter juveniles (0.14-0.39). Juvenile survival from all litters is higher during the population increase and peak, relative to the low and decline. Overall, these results suggest that the transition from low phase to population growth may stem in large part from changes in juvenile survival as opposed to increased reproductive output through the presence of a 4th litter.
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