A landscape experiment of spatial network robustness and space‐use reorganization following habitat fragmentation

Abstract

Network theory increasingly informs wildlife conservation in disturbed landscapes, but with concern increasingly expressed about its application to real‐world situations. The theory predicts that the connectivity of scale‐free networks should be particularly sensitive to the disturbance of highly connected nodes (i.e. hubs). This expectation relies on complete patch removal, thus restraining its application to the last of several steps involved in habitat fragmentation, and overlooks potential reconnection of patches after disturbance (i.e. rewiring). We performed a landscape‐scale experiment to evaluate the robustness of scale‐free spatial networks of woodland caribou (Rangifer tarandus caribou) to logging activity. We built caribou networks before and after disturbance using a mechanistic model of inter‐patch movements and differentiated networks disturbed in their hubs and non‐hubs. We applied a reaction–advection–diffusion model to networks before and after disturbance to account for the spatio‐temporal dynamics of caribou movement within the networks and anticipate their space use. We validated network and space‐use predictions using empirical estimates from GPS relocations of caribou. Using the validated predictions, we compared topological network measures before and after disturbance to quantify changes in connectivity within the networks according to the type of disturbed nodes (i.e. hubs or non‐hubs) and assessed space‐use reorganization. We used control networks, for which no disturbance occurred in the before–after timeframe of the study, in the latter analysis to get a baseline rate of change. Disturbances due to logging activity typically resulted in fragmentation and shrinkage instead of complete patch removal. Independently to the type of disturbed nodes, caribou rewired their network using remnant patches from the fragmentation process. Consequently, topological network measures generally did not differ between before and after disturbance, such that caribou networks displayed some robustness to logging activity due to the rewiring process. Space‐use reorganization was greater, however, when hubs were disturbed in comparison with non‐hubs and controls. Even though caribou rewired their networks, they revisited less patches after the disturbance of hubs. A naive application of network theory (i.e. without potential rewiring and using complete patch removal), to assess spatial network robustness, may be inappropriate during most steps of the fragmentation process because of the rewiring process. Indeed, network rewiring facilitated by the presence of remnant patches can enhance spatial network robustness, in comparison with no rewiring. In addition, species‐specific functional connectivity should be accounted for when anticipating the rewiring process and animal space use within disturbed networks. A free Plain Language Summary can be found within the Supporting Information of this article.

Maximising the clustering coefficient of networks and the effects on habitat network robustness

“…Our aim is to improve a network's robustness by adding links to the network. As the clustering coefficient is a good proxy for robustness [23,[32][33][34], we want to identify those links that should be added to the network to maximize the clustering coefficient. Mathematically, we want to solve the following problem: Problem 1.…”

“…Here, metapopulations are represented by habitat networks where nodes represent habitat patches and links indicate how these are connected [16,19,20]. With the help of habitat networks, the loss of habitat can easily be represented by removing nodes and reduced connectivity by removing links from the network [21][22][23]. Accordingly, many studies apply graph-theoretic tools to evaluate the effect of climate and land-use change and to find solutions for these effects in landscape planning [24][25][26].…”

Quantitative Spatial Ecology to Promote Human-Wildlife Coexistence: A Tool for Integrated Landscape Management

“…Many studies have emphasized the value of using a network-theory framework to assess landscape connectivity in the context of conservation and management planning (Minor and Urban, 2008;Urban et al, 2009). For example, the effects of disturbance of patches or links can vary among networks depending on topology (Urban and Keitt, 2001;Fortuna et al, 2006;Prima et al, 2019). Management actions may target different network components to achieve animal distributions that can reduce HWC.…”

“…In addition to network topology and associated functional connectivity, the time that animals spend in individual patches defines the spatial dynamics of animal distribution (Bastille-Rousseau et al, 2010;Stehfest et al, 2015). Non-random movements among network patches and residency time can be considered through the creation of weighted networks (Urban and Keitt, 2001;Prima et al, 2018Prima et al, , 2019, whereby each link in every direction is assigned a relative probability of being used, and each patch is given a residency time.…”

“…Probabilities of all m potential interpatch movements thus sum up to 1 (see, e.g., m = 5 around each black patch in Figure 3). Available patches can be identified from field observations (O'Brien et al, 2006;Courbin et al, 2014;Prima et al, 2019). Quantification of a ji for the different links of a network can inform on the challenges and opportunities of reducing the risk of HWC.…”