Oak habitat recovery on California's largest islands: Scenarios for the role of corvid seed dispersal

Abstract

Seed dispersal by birds is central to the passive restoration of many tree communities. Reintroduction of extinct seed dispersers can therefore restore degraded forests and woodlands. To test this, we constructed a spatially explicit simulation model, parameterized with field data, to consider the effect of different seed dispersal scenarios on the extent of oak populations. We applied the model to two islands in California's Channel Islands National Park (USA), one of which has lost a key seed disperser. We used an ensemble modelling approach to simulate island scrub oak (Quercus pacifica) demography. The model was developed and trained to recreate known population changes over a 20‐year period on 250‐km2 Santa Cruz Island, and incorporated acorn dispersal by island scrub‐jays (Aphelocoma insularis), deer mice (Peromyscus maniculatus) and gravity, as well as seed predation. We applied the trained model to 215‐km2 Santa Rosa Island to examine how reintroducing island scrub‐jays would affect the rate and pattern of oak population expansion. Oak habitat on Santa Rosa Island has been greatly reduced from its historical extent due to past grazing by introduced ungulates, the last of which were removed by 2011. Our simulation model predicts that a seed dispersal scenario including island scrub‐jays would increase the extent of the island scrub oak population on Santa Rosa Island by 281% over 100 years, and by 544% over 200 years. Scenarios without jays would result in little expansion. Simulated long‐distance seed dispersal by jays also facilitates establishment of discontinuous patches of oaks, and increases their elevational distribution. Synthesis and applications. Scenario planning provides powerful decision support for conservation managers. We used ensemble modelling of plant demographic and seed dispersal processes to investigate whether the reintroduction of seed dispersers could provide cost‐effective means of achieving broader ecosystem restoration goals on California's second‐largest island. The simulation model, extensively parameterized with field data, suggests that re‐establishing the mutualism with seed‐hoarding jays would accelerate the expansion of island scrub oak, which could benefit myriad species of conservation concern.

Capturing pests and releasing ecosystem engineers: translocation of common but diminished species to re‐establish ecological roles

“…Our data suggest that for CAGS it is critical to consider soil texture, and that at least rough approximations of soil type are readily available in spatially explicit online databases. Informed release site selection can therefore promote more cost‐effective rewilding ( sensu Pettorelli et al ., ) and help managers create ecological systems that will be more sustainable with less intervention (Pesendorfer et al ., ).…”

“…Managers working in primarily non‐native grasslands that characterize much of the western United States might best leverage our findings by identifying target sites where owl occupancy is desired, and – if squirrels are absent – vegetation management is feasible or already in place. Translocation of key common wildlife, such as CAGS, can be an additional tool to perform ecological services through the creation of burrow systems, the alteration of vegetation structure through foraging activities (Hennessy et al ., ), or other desired ecosystem effects such as seed dispersal (Pesendorfer et al ., ).…”

Understanding mast seeding for conservation and land management

“…Future management strategies for forests and other plant populations, especially under climate change, will benefit from considering variable seed production. The interpretation of models that anticipate the likelihood of regeneration in particular forests North American forests [12,39] assessing population viability of plants targeting conservation activities to plant populations and growth stages perennial sedges [40], limber pine [41] collecting seed for restoration timing seed collection trips and restoration projects island oak [42], New Caledonian forests [43] timing shelterwood forestry timing timber harvests during 'establishment cuts' oak [44], fir forests biological control using seed predators forecasting efficacy of biocontrol agents Russian olive [10] increasing seed crops to enhance dispersal mutualisms bolstering bird populations, restoring forests island scrub-oak-scrub jay [9], pedunculate oak-Eurasian jay [45] timing of animal relocations relative to mast cycles reintroducing rare or extirpated seed predators kākāpō [46] timing of animal culls relative to mast cycles culling invasive predators, setting hunting regulations invasive stoat culls in Nothofagus forests [47];…”

“…For example, in systems where mast seeding has been studied extensively, we can anticipate animal population dynamics and disease rates based on seed crops [7,8]. Simulated scenarios that account for masting can inform management actions and their consequences for species interactions, for example, the reintroduction of birds to islands [9], or the introduction of seed-eating biological control agents [10]. We can develop better expectations of how climate change will impact ecosystems [11,12], and, in some cases, we can predict mast seed crops themselves [13,14].…”

The effects of ENSO and the North American monsoon on mast seeding in two Rocky Mountain conifer species

“…The lagged relationships between cone production and climate described in this study and many others indicate that near-term ecological forecasts of mast years may be within reach, at least for some species. Mast forecasts may eventually be leveraged to time ecosystem management activities related to the management of plant and animal populations, including in shelterwood forestry, ecosystem restoration [77] and endangered species management [78,79], which are often intertwined with boom-and-bust cycles of seed availability [10]. Piñon pine has experienced extensive tree mortality associated with recent hotter droughts [80,81], documented declines in cone production [82] and limited recruitment following these recent die-off events [83].…”