The Dynamics and Resilience to Drought of Wetlands and Avian Metapopulations in a Coupled Human and Natural System
- Author(s): Van Schmidt, Nathan Donald
- Advisor(s): Beissinger, Steven R
- et al.
In working landscapes where natural resource extraction co-occurs with habitat conservation, species are often structured as a metapopulation occupying fragmented patches of habitat. Because patches change due to human management decisions, understanding how metapopulations persist in working landscapes requires assessing both how the species’ intrinsic factors drive turnover and how the behaviors of key actors drive patch changes. Coupled human and natural systems (CHANS) research uses a multidisciplinary approach to identify the key actors, processes, and feedbacks that drive the dynamics of a region. This dissertation integrates five diverse datasets—wildlife occupancy surveys, land-use change mapping, a survey of landowner decision-making, hydrological databases, and disease vector trapping—to assess how wetlands, irrigation, and two avian metapopulations function as a CHANS in the rangelands of the foothills of the California Sierra Nevada. The threatened, dispersal-limited black rail (Laterallus jamaicensis) and widespread, vagile Virginia rail (Rallus limicola) inhabit patchy wetlands throughout the foothills. The black rail has declined over the past decade, with drought and the arrival of West Nile virus potential causes. The first chapter assesses how the human-induced diversity of hydrological processes altered the CHANS’ resilience to an exceptional disturbance, a historically severe drought from 2012–2015. The second chapter tests if the “rescue effect” (dispersing individuals preventing local extinctions) actually occurs as predicted by theory and occupancy models. The third chapter integrates these interdisciplinary datasets into a simulation model that combines agent-based models of land-use change with stochastic patch occupancy models of metapopulations, in order to (1) quantify the relative importance of different drivers of metapopulation dynamics, (2) test predictions of the behavior of metapopulations in dynamic landscapes, and (3) evaluate the potential impacts of mandated irrigation cutbacks during drought and wetland incentive policies on metapopulation persistence.
Complex metapopulation dynamics emerged from the CHANS, and irrigation water was critical for black rail persistence. Wetlands were primarily fed by “waste” from the irrigation system. Landowners and water sources showed response diversity to drought, increasing the resilience of the wetland landscape and maintaining the black rail metapopulation through the 2012–2015 drought. The rescue effect was operating for both rail metapopulations during this period, providing one of the first empirical validations of this process, and occurred at notably higher rates during the lowest precipitation year. However, inferences from occupancy models were unreliable and underestimated the rescue effect (1) when using autoregressive measures that incorporated patch area, (2) when the species was not dispersal-limited, and (3) during a period of nonequilibrium metapopulation dynamics. Simulations showed rail metapopulations were strongly top-down regulated by precipitation, with synergistic negative impacts because droughts affected multiple system processes at the same time. The black rail decline was caused by the combination of West Nile virus and drought. Two key theoretical predictions were not borne out due to the CHANS’ complexity. First, dispersal limitations of black rails did not result in greater sensitivity to patch change rates compared to Virginia rails, because patch heterogeneity affected patch change rates and the two species’ colonization and extinction rates in different ways. Second, because incentive programs were coupled to CHANS dynamics they made the black rail metapopulation more sensitive to other parameters, not less. Drought irrigation cutbacks posed a substantial extinction risk that incentive policies were unable to reduce. Integrating “waste” water into regional wetland management may thus offer more cost-effective conservation than attempting to restore a lost “natural” state. These results highlight that conserving metapopulations in working landscapes requires assessing how human transformation of CHANS may create new diversity in system processes that benefits wildlife.