Ecological connectivity is the unimpeded movement of species and the flow of natural processes that sustain life on Earth (Hilty et al. 2020). Natural habitats are rapidly being lost and what remains is becoming increasingly fragmented. This has implications for the functioning of natural processes that have existed for long periods of space and time.
Connectivity in nature can be viewed at multiple spatial and temporal scales. At continental scales, annual songbird and waterfowl migration requires an interlinked chain of stopover habitats arrayed roughly north to south (Xu et al. 2020). If habitat alteration or loss and fragmentation break links in those chains, bird populations suffer. For ground- and river- dwelling species with large home ranges and/or needs for seasonal migration, connected land and waters are essential to their life cycle. Fragmenting features such as dams in rivers and intensive land uses can easily disrupt movement and population declines (Sawyer et al. 2009; Oliveira et al. 2018).
Some ecosystem dynamics, such as wildfire spreading across the landscape, or seasonal river flooding overtopping its banks, or coastal sediment transport and dune formation, can be disrupted by human-built structures that ‘fragment’ the process and alter habitat conditions (Thoms et al. 2005).
So “fragmentation” means that discontinuities appear in what were previously connected places that disrupt ecological processes requiring that connectivity.
Fragmentation decreases the size of contiguous habitat blocks and increases isolation of these patches one from another (Fisher and Lindenmeyer 2007, Kupfer et al. 2006). Although species vary greatly in their response to fragmentation it is invariably destructive to natural populations (Laurance and Bierregaard 1997, Johnson and Klemens 2005). Increased fragmentation can dramatically alter species, ecosystem, and landscape relationships and usually increases the risk of extinction (Fisher and Lindenmeyer 2007, Kupfer et al. 2006). Fragmentation results in isolated populations with decreased resiliency to changes in landscapes that are caused by a changing climate (Bennett 1999, Laurance and Bierregaard 1997).
To counteract habitat fragmentation, conservation efforts have aimed to protect natural landscapes. However, there is a limit to the area which can be set aside as formally protected, and those areas may need to have geographically fixed, legally defined boundaries. Protected landscapes within fixed boundaries also remain subject to significant external forces impacting the biodiversity within them. Furthermore, most of the world’s biodiversity is found outside protected areas (Whitelaw and Eagles 2007). As areas of natural habitat are reduced in size and continuity by human activities, the degree to which the remaining fragments are functionally linked becomes increasingly important.
Consequently, one of the most frequent recommendations for protecting biodiversity is to increase connectivity and establish ecological networks that connect natural habitats (Heller and Zavaleta 2009). This conservation practice becomes even more relevant in the face of climate change (Glick et al. 2011). Under all future scenarios, with or without climate change impacts, ecological networks will play a vital role in the conservation of biodiversity through improving resilience of ecosystems and natural dispersal of species.
In part I, Understanding Conservation Biology, the importance of connectivity to conservation is discussed. Given this, there is an ever more critical need for science and practical tools to support connectivity conservation. In this section, we address how to support connectivity through DoD natural resource management. We begin by establishing an understanding of encroachment and then introduce strategies to manage encroachment and support connectivity.
Reconnecting fragmented biodiversity
The International Union for Conservation of Nature (IUCN) published guidance for creating and maintaining connectivity to benefit biodiversity (Hilty et al. 2020). This guidance should serve as a useful resource for DoD managers to gain sufficient background on ecological connectivity and then pursue practical steps to address connectivity needs of biodiversity associated with their installation.
In the IUCN guidance, two types of connectivity for species are defined that are of particular use to DoD managers. Functional connectivity describes how well genes, gametes, propagules, or individuals move through land, freshwater and seascape. Structural connectivity measures habitat permeability based on physical features and arrangements of habitat patches, disturbances and other land or water elements presumed to be important for organisms to move through their environment. Preferably, one would identify at least some key species vulnerable to effects of fragmentation and then deploy methods associated with functional connectivity. However, in the common circumstance where knowledge of species is very limited, methods associated with measuring and managing structural connectivity may be most appropriate.
An ecological corridor or connectivity zone is a clearly defined geographical space that is governed and managed over the long term in a way that is compatible with maintaining or restoring effective ecological connectivity. Each corridor or connectivity zone should have specific ecological objectives and be governed and managed to achieve connectivity outcomes.
Some examples of objectives for ecological corridors could include:
- Movement of individuals for dispersal of species for recovery across its historic range
- Genetic exchange, allowing movement over the long term among population segments for species that may be prone to genetic bottlenecks
- Migration to facilitate seasonal movement for breeding, overwintering, etc.
- Multi-generational movement, for example Monarch butterflies that migrate over several generations along a central flyway in North America
- Maintenance/restoration processes, such as hydrologic function for sediment transport or nutrient cycling, by removing dams and diversions
- Climate change adaptation, to facilitate range shifts by species from warmer to cooler zones
- Prevention of undesirable processes, such as appropriate revegetation to limit invasive species or uncontrolled wildfire spread.
See Hilty et al. (2020) for examples of methods and tools for delineating conservation corridors. While some tools are designed specifically for connecting corridors (i.e., movement from “points A and B to points C and D”), other tools provide an indication of over landscape permeability. That is, they consider (often artificial) features of the landscape that likely impeded movement for any number of species or natural processes. These sorts of tools may often be most appropriate where knowledge of individual species and populations is limited, but there is a need to consider management to increase or maintain overall ecological connectivity.