An Introduction to Habitat Connectivity

Photo © La Citta Vita

Landscape connectivity is a critical concept in ecology. Not only do many species of plants and animals rely on connected patches of habitat to move around their territories, find mates, hunt, forage, and reproduce, people also find them desirable additions to urban and suburban landscapes for recreation and access to nature. This article introduces you to some of the basic concepts in landscape connectivity.

Habitat Fragmentation

Photo © Tom Blackwell

Fragmented Landscape: discontinuities in an organism’s preferred environment (habitat)

When people settle into a region, they change the landscape in dramatic ways. They build roads and clear vegetation for houses and crops. This breaks the natural landscape up, or fragments it, separating natural areas from each other with areas that are paved or otherwise developed. This kind of developed countryside is very recognizable from an airplane.

Connectivity-terms

Photo © Jinhee Ha, Ryosuke Takahashi, Haikun Xu Cornell Department of Landscape Architecture

Ecologists talk about landscapes with some helpful vocabulary. A patch is a significant natural area. A space large enough for a particular species to successfully carry out some part of its life cycle. How big a patch is needed depends on the species. A patch for a species of snailopen_in_new can be much smaller than a patch for a large mammal like a mountain lionopen_in_new. When you look at a landscape from above, you might be able to identify stepping stones, or smaller natural areas that act like discontinuous pathways between two larger patches. These stepping stones can act as refuges for seeds or animals moving between larger patches. Corridors, like stepping stones, connect larger patches of habitat to one another allowing movement of organisms from one refuge to the next.

Connectivity

Photo © Jinhee Ha, Ryosuke Takahashi, Haikun Xu Cornell Department of Landscape Architecture

Moving between significant patches of habitat is critical for maintaining healthy populations of organisms. Ecologists refer to this as habitat connectivity. To the left you can see different ways organisms might move between patches. Too great of a distance without some kind of linkage means organisms can’t (or won’t) move between two patches. In a forest fragmented by logging, leading Canadian conservation biologist, Susan Hannon, showed that chickadees would choose a forested path up to three times longer to avoid crossing gaps of open space. Of course, using these longer forested detours may increase the energetic requirements of birds living in fragmented areasopen_in_new meaning they have to spend more time foraging to make-up for those ‘lost’ calories.

Patch Proximity

Photo © Jinhee Ha, Ryosuke Takahashi, Haikun Xu Cornell Department of Landscape Architecture

One of the reasons it is so important for organisms to access other habitat patches in a fragmented landscape is to help maintain a healthy breeding population of individuals. When a habitat is fragmented into patches, and the population too isolated, the gene pool for a particular species may become too limited, making the population vulnerable to inbreeding and disease. Closer proximity between patches allows for greater population (and genetic) exchanges and may diversify the overall gene pool of a population of a species in a given area.

Patch Size

Photo © Jinhee Ha, Ryosuke Takahashi, Haikun Xu Cornell Department of Landscape Architecture

Patch connectivity isn’t the whole story though. Have you ever walked deep into a forest and noticed that it just feels different when you are in the middle than it does when you are near edges? This is a real phenomenon for many species which have specialized ecological niches. For instance many songbirds, like Wood Thrush, Ovenbirds, and Scarlet Tanagers, require interior forested spaces a certain distance from the edge (often defined for birds as at least 300 ft from any edge) to meet their habitat needs. The image above shows how patches that are too small lack those special interior conditions, even though they occupy the same amount of absolute space.

cat at the edge

Photo © Trevor Pittman

One reason interior habitats are different than edge habitats is the accessibility of the edges to predators found in more open and urban areas. Cats, dogs, crows, jays, and raccoons all prey on nestlings and fledglings, and their abundance increases the closer you go to a forest edge.

In part 2, we discuss ways that yards fit into connected landscapes.

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