Conserving Habitat, Especially in an Urban World

Key Concepts

1. What is Habitat?2. What is a Niche?3. Edge Effects4. Island Biogeography and the Design of Nature Reserves

Habitats and Ecological Niches

Habitat - a place where a plant or animal can get the food, water, shelter and space it needs to live.

Niche - is a term describing the relational position of a species or population in an ecosystem and how it responds to resources and enemies. The abiotic or physical environment is also part of the niche. The description of a niche may include descriptions of the organism's life history, habitat, and place in the food chain. No two species can occupy the same niche in the same environment for a long time.

Douglas-fir

Western red cedar

Fundamental and realized niche

Aberti et al.

Island Biogeography Theory

The number of species found on an island (the equilibrium number) is determined by the balance between extinction and immigration. The effects of distance from the mainland and the effect of island size are important. Distance effectIslands closer to the mainland are more likely to receive immigrants from the mainland than those farther away from the mainland. The equilibrium number of an island close to Africa is going to be larger than that of one found in the mid-Atlantic. Size effectOn smaller islands the chance of extinction is greater than on larger ones. Thus larger islands can hold more species than smaller ones.

The south Pacific island of Nauru21 km2 (8.1 square miles)

McArthur and Wilson’s Model

Number of Species

Rate

Extinction

Colonizationsmall large

near

far

This drives concern forsize and connectivity

Factors that Influence Island Communities-Degree of isolation (distance to nearest neighbor, and mainland) - Length of isolation (time) -Size of island (larger area usually facilitates greater diversity) -Climate (tropical versus arctic, humid versus arid, etc.) -Location relative to ocean currents (influences nutrient, fish, bird, and seed flow patterns) -Initial plant and animal composition if previously attached to a larger land mass (e.g., marsupials, primates, etc.) -The species composition of earliest arrivals (if always isolated) -Serendipity (the impacts of chance arrivals) - Human activity

The realization that reserves and National Parks formed islands inside human-altered landscapes (habitat fragmentation), and that these reserves could lose species as they 'relaxed towards equilibrium' (that is they would lose species as they achieved their new equilibrium number, known as ecosystem decay) caused a great deal of concern.

Is Olympic National Park an island embedded in the landscape?

Does island biogeography theory apply here?

What About the Mainland?

• Habitat loss can create habitat islands and a sea of new habitat

• Created edges as well as patches (islands) are important to consider

• Fragmentation creates edges and reduces patch size

Habitat Loss is Key Aspect of Landscape Change

• Habitat loss may or may not fragment

• To study fragmentation we must focus on landscapes not patches

• Few studies compare loss and fragmentation– All find loss most important

• Emphasizing fragmentation rather than loss is misleading, optimistic, and distracts us from need to conserve and restore habitat

(Fahrig 1999)

Diversity of Edge Effects

DISTANCE FROM EDGE

NE

ST S

UC

CE

SS

(Murcia 1995)

Big Animals Need Big Islands

• Carnivores with large home ranges were most sensitive to reserve size because they range outside of reserve and are killed (intentionally or accidentally) by people

(Woodroffe and Ginsberg 1998)

Urbanization intensity

UrbanSuburbanExurban

Increasing size

Reserve study design factors(Donnelly and Marzluff)

MediumSmall Large

Bird Diversity (Richness) was related to size and landscape

• Size– Especially

important in Urban and Suburban areas

Reserve size

Small Medium Large

Mea

n r

ich

nes

s +

SE

0

5

10

15

20

25

30ExurbanSuburbanUrban

Native forest species showed thresholds of occurrence with size

• Mean threshold = 42 ± 15 ha

Synanthropic species showed thresholds of occurrence with urban landcover

• Mean threshold = 40 ± 10 % urban landcover

Manage the Vegetation in the Fragment

• Maintain native vegetation

• Increase foliage height diversity

• Actively discourage lawns

• Manage limiting factors– Small mammals– Cats– Exotic species

Manage the Matrix

• Regulate, enforce, educate to reduce penetration of predators, competitors, humans, chemicals, etc. from matrix into fragment

• Make the habitat in the matrix more like habitat in fragment

• Reduce food supplementation

• Control cat movements

Designing Reserve Complexes•Enlarge key patches

• May require less total reserved area than lots of small patches

•Increase connectivity

(Opdam and Wiens 2002)

•Recognize patch dynamics• Understand succession and

disturbance• Reserves should be larger than

disturbance patch size• Include internal recolonization

sources• Include different ages of

disturbance-generated patches(Pickett and Thompson 1978)

(Soulé 1991)

(Shafer 1997)

Do Corridors Provide Connectivity?• Advantages– Gene flow, rescue, recreates the normal condition of

species living in well-connected environments• Disadvantages– Spread disease, lure animals into poor habitat

Connectivity and Reserve Design(Schmiegelow and Hannon 1999, Hannon and Schmiegelow 2002)

Long-term experimental study at Calling Lake, Alberta

• 1993-continuing, 3 replicates of patches of various size and connectivity (100m-wide buffers)

• Species turnover is highest in small isolates, indicating extinctions, but also colonizations.

• Richness remained equal among treatments indicating replacements of permanent residents on the small, isolated fragments

• Resident birds went extinct most frequently• Species vary in their ability (“willingness”?) to

cross gaps, but this sensitivity does not predict whether they will remain abundant in connected fragments versus isolated ones

• Corridors may help a few resident species (via rescue effects), but they do not appear to offset the impacts of fragmentation (habitat loss, edge creation) for most boreal birds

• May benefit western tanagers and black-throated green warblers most

• May be better to use forest allocated to corridors to actually increase size of reserves instead of connecting small reserves