Global change is introducing significant uncertainty into the conservation of biodiversity (Colloff et al. 2017) while heightening the need for larger scale and more adaptive conservation intervention (Reside et al. 2018). Climate and land-use change are expected to have interactive effects, generating conservation outcomes that differ from those when only a single driver is considered (de Chazal & Rounsevell 2009). The magnitude and direction of these interactions can be highly variable depending on the system, and the mechanisms regulating CC/LUC interactions have significant implications for biodiversity change, and therefore, conservation planning (Schulte to Bühne et al. 2021). Integrating landscape change modeling with network theory, we evaluated changes to complex habitat networks over time by relating spatiotemporal dynamics in habitat availability to the conservation strategy employed and assessing connectivity improvements against a landscape baseline across multiple global change scenarios. Connectivity science has yielded exciting advances in recent years, including the use of time-series analysis to assess the influence of environmental variability and ecosystem dynamics on landscape connectivity (e.g., Bishop-Taylor et al. 2018, McIntyre et al. 2018). The use of bioclimatic variables to forecast habitat location and connectivity under global change has also gained momentum (e.g., Littlefield et al. 2017, Huang et al. 2020). Additionally, emergent work has integrated simulation modeling with network theory to assess the influence of conservation strategy selection on potential connectivity (Mozelewski et al. 2022). Our framework builds upon these advances, leveraging a spatially explicit landscape simulation model to forecast connectivity responses to protected area network expansion integrating both anthropogenic change and environmental variability.
Our results showed that connectivity increased over time across all scenarios, even without the acquisition and restoration of additional land for conservation. This supports past studies that found that a changing climate is likely to have positive impacts on longleaf pine, increasing its dominance in the southeast in part because longleaf pine is tolerant of drought and heat stress (Samuelson et al. 2012; Costanza et al. 2015; Peters et al. 2020). Responses to climate change will be variable and species-specific, with some species emerging as ‘winners’, gaining dominance in their respective ecosystems, while others become “losers”, struggling to adapt and experiencing fitness declines (Dyderski et al. 2017; Hoveka et al. 2022). Our projections suggest that longleaf pine may emerge as one such “winner” with climate change conveying a competitive advantage against hardwood species in the study area. However, while a warmer climate could favor the expansion of longleaf pine forests increasing habitat availability and improving connectivity, it could also make the implementation of prescribed burning, which is currently the principal strategy for maintaining and restoring the longleaf ecosystem, more difficult by reducing the available burn window (Mitchell et al. 2014; Kupfer et al. 2020). Urbanization will likely exacerbate limitations on prescribed burning as the wildland-urban interface expands, potentially decreasing longleaf area (Costanza and Moody 2011). Longleaf pine ecosystems are fire-adapted and prescribed burning increases soil nutrient availability, reduces hardwood competition, and improves seedling establishment (Heuberger & Putz 2003). We see the integration of changing burn windows and locations as an important next step in forecasting future connectivity in the study area, as reductions in prescribed burning will likely reduce longleaf pine habitat quality and availability.
Results comparing our two conservation strategies to a landscape baseline revealed that the cluster strategy performed the best across global change scenarios, though more extreme climate change reduced the differences among strategies. Because existing longleaf pine habitat is primarily concentrated on lands owned or managed by federal, state, and local governments and nongovernmental organizations in the southeastern portion of the study area, the cluster strategy better supported progressive dispersal to newly created habitat by targeting lands adjacent to existing conservation areas. Acquiring and restoring land neighboring existing protected areas could have additional conservation benefits under global change, both reducing fragmentation and increasing habitat core area which may increase population sizes and buffer species from edge effects (Herse et al. 2018), supporting species persistence in the face of global change (Dilts et al. 2016). The ability of the cluster strategy to best enhance connectivity supports earlier work comparing connectivity outcomes from a suite of conservation alternatives (Mozelewski et al. 2022), though those findings did not consider global change. These results can also inform ongoing conversations about the relative advantages conferred by fewer large or more small protected areas (e.g., Ovaskainen 2002; Santini et al. 2016) and how habitat configuration effects connectivity (e.g., Jackson and Fahrig 2016; Arroyo-Rodríguez et al. 2020; Hodgson et al. 2022).
Meanwhile, the geodiversity strategy yielded more variable improvements in connectivity, often resulting in the highest and lowest equivalent connected area scores within any given scenario. While longleaf pine forests are often characterized by deep, sandy soils in the study area, geodiverse portions of the study area were commonly affiliated with more clay-dominant and thinner soil substrates (Gilliam et al. 1993; Peet 2007), which could have influenced restoration success and yielded a larger variation in connectivity improvements. We see value in investigating the performance of this strategy on additional landscapes and for different target habitat types, as its relevance as a coarse filter approach to conservation under global change is often suggested (e.g., Beier et al. 2015).
The expansion of protected areas has been recommended as a key defense against biodiversity and habitat loss under global change (Gray et al. 2016). Simultaneously, maintaining and enhancing connectivity is often cited as core strategy for helping species adapt to such change and for enhancing the ability of protected area networks to support population persistence (Brennan et al. 2022). However, while connectivity between protected areas has improved in recent years, it still falls well short of global targets (Saura et al. 2019). Multiple strategies for protected area network expansion to support connectivity under global change have been suggested (e.g., Beier and Brost 2010; Andrello et al. 2015; Albert et al. 2017; Elsen et al. 2020; Lawler et al. 2020). We see our approach combining landscape simulation modeling with network theory as a way to inform these discussions and evaluate strategies in silico, integrating information on global change-related threats into protected area network design to better protect biodiversity in the future (Carroll and Ray 2021).
Most notably, our study highlights contrasting mechanisms by which global change drivers diminished the ability of conservation to improve connectivity on our study landscape. Under moderate climate change, land-use change was the dominant driver, lowering the connectivity potential of the surrounding landscape, thereby weakening the ability of newly established protected area networks to enhance connectivity. Under more extreme climate change, conditions favored climate change resilient longleaf pine (Clark et al. 2018), increasing the connectivity potential of the surrounding landscape and superseding the ability of conservation to make a notable difference. In fact, rather than acting synergistically, climate change reduced the fragmenting effects of land-use change in the study area. Though both drivers dampened the influence of conservation on landscape connectivity, their implications for conservation planning differ greatly. For example, under RCP 4.5 managers might want to prioritize additional acquisition and restoration of land above a 17% area target to counteract the negative influence of land-use change and yield larger connectivity improvements. Under RCP 8.5, substantial connectivity improvements to the landscape baseline might support prioritization of other conservation concerns. This distinction underscores the importance of evaluating multiple drivers of change across multiple change scenarios in conservation planning and forecasting (Mazor et al. 2018).
Limitations
The modeling framework described here is not intended to predict the exact outcome of any conservation action, but instead to evaluate trends in conservation performance across a range of possible futures (Mozelewski and Scheller 2021). This may help inform conservation decision-making and provide support for managers looking to proactively manage their landscapes for change (Hobbs et al. 2014). Although LANDIS-II has been widely used and rigorously tested (e.g., Scheller et al. 2011a; Lucash et al. 2018; Boulanger et al. 2019; Maxwell et al. 2020), uncertainty in parameter estimates and representation of ecological processes exists along with uncertainty surrounding future land management trajectories and approaches. Further, we focused on a limited set of landscape dynamics, excluding insect disturbance (Sturtevant et al. 2015) and species range expansions into the study area as the climate warms (Van Houtven et al. 2019), which have been shown to shift community composition in the eastern United States (Clark et al. 2021).
Although we evaluated the influences of climate change, land-use change, and conservation on landscape connectivity on a single landscape, we believe our modeling framework is broadly applicable to many landscapes. Evaluation of conservation strategies on additional landscapes and with alternative constraints is warranted. For example, rather than prioritizing individual forest stands with high geodiversity, the geodiversity strategy could prioritize stands that collectively increase the representativeness of geodiversity in the management portfolio. Instead of using Euclidean distance, other functions of distance from existing conservation areas could be employed for the cluster strategy. Such alternative approaches could affect model outcomes. Additionally, we recognize that this study does not address protected areas dynamic boundaries, which may be key for protecting species under climate change (Elsen et al. 2020). We see opportunities to expand this framework to include geographically dynamic reserve networks in the future.
In using least cost path resistance distances in our analysis, we assumed that that forest species face greater dispersal difficulty as they move through land cover types increasingly dissimilar from their preferred habitat (Saura et al. 2011). Yet this approach relies on the simplifying assumptions that a single optimal path between habitat patches conveys the movement potential between those two patches, and that an organism has perfect knowledge of the landscape and will choose a path accordingly (Fahrig 2007; McRae et al. 2008; Pinto & Keitt 2009; Bishop-Taylor et al. 2015). Future analyses could use circuit theory (McRae et al. 2013) as a measure of effective distance to replace these assumptions. Additionally, habitat quality metrics could be integrated into habitat node weighting to further account for variation in habitat condition. Although the species guild parameters we used in this analysis were coarse, species-specific requirements could readily be integrated into our framework (Scheller et al. 2011a) to inform connectivity thresholds.