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Fire, fragmentation, and extreme weather events : surprise impacts of disturbance on the persistence and vulnerability of beetles in dynamic landscapes lead to early warning signals of altered recovery and demonstrate the need to manage for multiple scenarios in an era of climate change

thesis
posted on 2024-04-15, 02:15 authored by Lynette ForsterLynette Forster

The persistence of an ecosystem and its biodiversity depends upon the ability of organisms to resist and adapt to disturbance or recover from it. Predicting the impact of altered disturbance regimes on biodiversity is elusive but of increasing concern given pressure of increasing drought, wildfire, and extreme temperatures from climate change.
This thesis examines how threats to biodiversity can be generated by altered disturbance regimes that erode ecosystem resilience, how biodiversity responses may play out in unexpected and unintuitive ways, and how we might understand, measure, and monitor these threats. Tasmania's fire-adapted and silviculturally managed wet eucalypt forests and adjacent montane areas are used as study ecosystems, beetles as the study organisms and their distribution as a measure of the impact of these threats on dispersal processes.
Hypothesised effects of fire, fragmentation, and extreme weather events on distributions of beetle species are examined in separate case studies (Chapters 2-4):
(1) The trajectory of recovery of mature-forest beetles following disturbance by fire (Chapter 2) is examined by comparing assemblages of pitfall-trapped beetles in a space-for-time chronosequence of forest ages since wildfire (6-8, 42-43, 75, 104 and 111 years) with assemblages in old-growth forest (170 and 210 years). It is hypothesized that, under a classic successional paradigm, assemblages will become increasingly similar and converge to a climax old-growth-forest community with increasing years-since last-fire.
(2) The effect of the intensity of disturbance in the landscape (up to 2km radius) surrounding 30-year?old regenerating patches of forest on dispersal and recolonization by beetles is examined using flight intercept traps along a gradient of landscape disturbance intensities in Chapter 4. In more disturbed landscapes a lower proportion of mature forest habitat is available to support mature-forest species. Separate general linear models of the occurrence of each species are used to classify disturbance-sensitive and disturbance-affiliated species at the scale of landscapes under a null hypothesis that recolonization of regenerating sites is independent of the amount of source habitat (disturbance) in the landscape.
(3) Landscape genetics methods are combined with modularity analyses of genetic networks to detect disturbance effects on gene flow among saproxylic species dependent on old logs (Chapter 3). It is hypothesized that if increasing disturbance is causing fragmentation of mature forest that exceeds dispersal distances, then signals of restricted gene flow (lower genetic diversity within populations and increased differentiation between populations) will be detected.
(4) Impacts of extreme weather events on distributions of pitfall-trapped species along an altitudinal climatic gradient (Chapter 5) are hypothesized to show an upward shift on the mountain in the hottest year on record at the time of sampling compared with an extreme cold-weather year. Finite mixture models are used to identify groups of species with different responses to extreme weather. Modified rarefaction tests elucidate whether species pools experiencing different impacts of disturbance regimes are shared (neutral community assembly with dispersal limitation); or distinct (the species pool is shaped by factors that vary during recovery such as filtering by competition, habitat suitability, abiotic conditions and spatial variation.
Results indicated that, contrary to predicted effects,: (1) Assemblages recovering from wildfire did not show successive turnover, but three contrasting responses to forest age were identified. Early colonists of open areas gradually declined with forest age; late colonists showed a lagged response until rapid colonisation from 25 years, reaching 95% probability of occurrence at 80-110 years; and neutral responders. Late colonisers of forest would be vulnerable to cascading effects from regeneration failure of forest under increasing drought and wildfire. (2) Genetic signatures from log-dwelling species at regenerating sites did not show predicted impacts of fragmentation in a mosaic of multi-aged patches, but other negative effects were observed. Elevated genetic diversity and low differentiation between populations (compared with control and mature sites) indicated greater dispersal than expected, with relatedness patterns altered once the threshold of mature-forest cover in 2km radius landscapes fell below 20%. Genetic homogenisation from increased gene flow can reduce the genetic resilience of populations to survive unexpected disturbance or undergo adaptive evolution. Genetic data examined in Chapter 3 was more sensitive to landscape disturbance than modelling occurrences of aerially active beetles in mature or disturbed habitat as examined in Chapter 4. (3) Modelling in Chapter 4 indicated that high levels of landscape disturbance inhibited dispersal of mature-forest species from mature forest to regenerating patches. (4) At low altitudes to 800m, the impact of an extreme hot-weather event caused a downslope, rather than upslope shift in altitudinal ranges of ground-active beetles, suggesting they were tracking changes in microclimate in the forests of the lower slopes. Distributions of upper altitude species contracted upslope, driven by an ecotone at 800-900m that separated lower from upper altitude species. This ecotone lay below the 1100m tree line and in extreme cold years its lower limit at 800m separated subalpine from montane forest. In extreme hot weather it expanded downslope by 100 m into montane wet eucalypt forest. Thus, the trailing 100m of this ‘switch ecotone’ is dynamic and is not a barrier during cold weather. Under extended periods of extreme heat, its downslope expansion during extreme warming would further isolate upper alpine-restricted species and may create an ecological trap for lower altitude species unable to move upwards under future warming and drying of forest microclimates.
In conclusion, a suite of analyses that tested alternative hypotheses revealed unexpected insights into beetle responses and vulnerability to disturbance. Landscapes with less than 20% mature-forest cover, fire frequencies less than 100 years and ecotones that switch from being permeable to barriers during extremely hot weather events, are candidate early warning signals that dispersal processes disturbed by fragmentation, wildfire and extreme weather events, could initiate alternative trajectories of recovery in ecosystems. In an era of increasingly unpredictable disturbance regimes and transformations of ecosystems, this research contributes to the identification of early warning signals that are important for detecting altered trajectories in recovery of dispersal processes from short-term monitoring data; and highlights the importance of incorporating ecotones into predictive modelling of species distributions under climate change scenarios.

History

Sub-type

  • PhD Thesis

Pagination

xxviii, 470 pages

Department/School

Tasmanian Institute of Agriculture

Publisher

University of Tasmania

Event title

Graduation

Date of Event (Start Date)

2023-02-24

Rights statement

Copyright 2023 the author

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