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Landscape Ecology and Connectivity  4  71 15 Extension Note Landscape Ecology and Connectivity 4 71 Introduction objectives. The Biodiversity Guidebook (B.C. Ministry of Forests and B.C. You accidentally unplug the freezer. Ministry of Environment, Lands and Labour strife removes buses at rush Parks 1995), a component of the Code, hour. A violent storm knocks out the recommends procedures to maintain Biodiversity telephone lines. A highly disturbed biodiversity at both landscape and Management Concepts ecosystem loses species to extinction. stand levels. These procedures, which in Landscape Ecology What do these events have in com- use principles of ecosystem manage- mon? All are examples of systems in ment tempered by social considera- which an important connection has tions, recognize that an important Ministry contact: been severed. Just like electrical, trans- way to meet the habitat needs of forest Rick Dawson portation, or communication systems, and range life is to ensure that various B.C. Ministry of Forests Research Program the ecological systems composing habitat types are still connected to 200 - 640 Borland Street landscapes require connections to each other. By sustaining landscape Williams Lake, BC V2G 4T1 maintain their functionality. connections, forest- and range- Historically, forest managers dwelling organisms can continue to November 1997 viewed the components that make up spread out and move across and the landscape as separate, unrelated between landscapes. entities. The emerging discipline of This Extension Note is the fourth landscape ecology now focuses on the in a series designed to raise awareness landscape as an interrelated, intercon- of landscape ecology concepts, and to “… an important way nected whole. Landscape ecologists provide background for the ecologi- to meet the habitat place a significant emphasis on the cally-based forest management connections between landscape ele- approach recommended in the needs of forest and ments and their functional roles. Biodiversity Guidebook. The focus here range life is to ensure The Forest Practices Code is on landscape connectivity. We first acknowledges the importance of land- define and describe connectivity. that ecosystems are scape ecology concepts by enabling Then we summarize some of the still connected to each district managers to designate plan- ecological principles underlying ning areas called “landscape units,” connectivity, and review its role in other …” each with specific landscape unit maintaining the structural integrity 1 January 2000. Policy direction for biodiversity is now represented by the Landscape Unit Planning Guide. This Extension Note should be regarded as technical background only. 2 Harrison’s chapter in Voller and Harrison’s Conservation Biology for Forested Landscapes (1997, in prep.) is a good reference for those readers wanting an in-depth understanding of connectivity in forested landscapes. Ministry of Forests Research Program of landscapes, the mechanisms that and functional continuity over both sustain it, and how it relates to biodi- space and time scales. For instance, if versity. We conclude by examining the structural elements (i.e., the how the concepts of connectivity can matrix, patches, and corridors) that be applied at the landscape level. make up a landscape are arranged so that various habitat types are effective- What Is Connectivity? ly linked, species and communities can disperse freely, giving the land- Healthy, diverse ecological systems scape a degree of spatial connectivity. require connections to keep them And if these linkages can be preserved alive and functioning. Landscape ecol- through time, landscape flows and ogists use the concept of connectivity functions will be maintained, giving to describe a landscape’s structural the landscape a degree of temporal connectivity (Figure 1). Upland to stream-wetland connectivity Cross-elevational (vertical) connectivity between forested areas Upland to upland connectivity (horizontal) Island remnants left after a wildfire (scattered vets; trees along rocky outcrop and stream gullies; trees around wetland) Wetland to wetland connectivity (horizontal), flat topography Some examples of connectivity in forested landscapes (from B.C. Ministry of Forests and B.C. Ministry of Environment, Lands and Parks 1996). 3 Further information about these landscape elements is contained in “Spatial patterns and landscape ecology: Implications for biodiversity,” the third in this series of landscape ecology Extension Notes. Landscape structure therefore natural connections between forested strongly influences the flow of energy, ecosystems may also be severed by nutrients, water, disturbances, as well timber harvesting, as well as by urban- as organisms and their genes (Noss ization, agriculture, and other 1991). For example, the particular development activities. arrangement of open and forested areas in a watershed could affect the Ecological Principles direction of cold air drainage and wind, the incidence and extent of MacArthur and Wilson’s (1967) influ- flooding, the movement of wildfires ential theory of island biogeography and occurrence of windthrow, the provided the basis for early notions spread of seed, insects, and mycor- about landscape connectivity. They rhizal fungi, and the daily and observed that the number of species seasonal migration of wildlife. (or the species “richness”) present in Habitat fragmentation is the Pacific and other island archipelagoes opposite of connectivity. Forested was related to the individual island’s landscapes are disrupted when large area and degree of isolation. Patterns contiguous forest patches are trans- of species richness resulted from the formed into smaller patches equilibrium that tends to develop surrounded by disturbed areas. This between the rate of species coloniza- disruption can occur through natural tion and extinction. For instance, disturbances such as fire, landslides, near-shore islands would be settled by windthrow, and insect attack. The species that could fly, swim, or drift from the adjacent mainland. There- fore, because of their proximity, these islands were effectively “connected” to the mainland. Conversely, distant oceanic islands had lower species rich- ness because their connectivity to the mainland was much lower. This theo- ry was later adapted to include “habitat islands” created by habitat fragmentation (Wilson and Willis 1975): as specific habitats (e.g., old- growth valley-bottom spruce) become isolated from one another, natural species diversity decreases. The theory of island biogeography was further extended when the idea of metapopulations was introduced (Levins 1970) (Figure 2). This model describes how interconnected popula- tions of species exist and interrelate, and recognizes that some animal and Landscape connectivity allows several individual populations of a plant populations require linkages species to exist in, and move freely between, a network of suitable between habitats. When landscape habitat patches. This connected collection of populations consti- connectivity is breached, certain con- tutes a “metapopulation.” Species have become extirpated in the tinuous, stable populations are empty habitat patch (upper left), and the loss of connectivity to converted into populations that the metapopulation prevents recolonization (from Harrison 1996). become more and more disrupted and unstable. Some local populations can Natural features such as riparian suffer complete elimination (or “extir- habitats along a stream or river often pation”), but total extinction of the provide good corridors at the land- species may be prevented by the suc- scape level because they encompass a cessful recolonizing efforts of diversity of habitats and topographic connected populations. The basic gradients. Branching drainage pat- assumption is that if appropriate habi- terns provide important landscape tat types are connected, organisms will connections by linking upper and disperse among them (Merriam 1991). midslopes with valley bottoms. Human-built features such as Maintaining Corridors fencerows can also be used to create Preserving habitat and dispersal connections between landscape patch- routes to maintain landscape-scale es. As well, individual patches connectivity is a popular research themselves may provide connections topic in landscape ecology. if they are suitable for a species to Researchers (see, for example, Voller move through or live in. and Harrison’s [1997, in prep.] litera- Few data currently exist on the use ture review) suggest that habitat of corridors, and their perceived isolation and the attendant loss of nat- advantages have been questioned. ural species diversity can be reduced However, most scientists acknowledge by designating connective “corridors.” that natural connectivity is important These important elements of land- and should be retained before land- scape structure are usually continuous scapes are disturbed. A number of remnants of naturally occurring vege- pros and cons concerning the use of tation which allow species or their corridors are presented in Table 1. genes to move between patches of undisturbed habitat (Saunders and Connectivity and Biodiversity Hobbs 1991). The loss of landscape connectivity, The presence of corridors in the often discussed as habitat fragmenta- landscape can: tion, is considered by some landscape • ease the movement of flora ecologists to be among the greatest and fauna; threats to natural biological diversity. • act as a semi-permeable barrier, Forest fragmentation
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