Finally….

ENVS*332 Final exam:

Friday, Dec. 15, 8:30-10:30

Midterm exam:

Weds., Oct. 25. in class

Spatial heterogeneity: importance?

In a nutshell:

More heterogeneity = more habitat niches = more species = more biodiversity

From Pickett and Cadenasso….

How does landscape mosaic structure (and hence the spatial heterogeneity of that structure) arise?

-variability in environmental resources (e.g. nutrients, moisture) (remember tree islands in Everglades)

-natural or human-caused disturbance

-fragmentation of land cover type

-introduction of patches by humans

Spruce budworm:

-prefers balsam fir over spruce

-in mixed spruce-fir forest, will kill fir and leave spruce…so, more diversity = resistance to disturbance

-over time, budworm outbreaks maintain the spruce forest by controlling fir, which would otherwise take over…so long-term stability is maintained by budworm disturbance!

The nature of the surrounding matrix also important:

-one study found that white spruce stands within a matrix of other softwood stands had higher defoliation rates than spruce stands surrounded by mixedwood forest

Creation of spatial heterogeneity in forest restoration:

-e.g. pit and mound topography

-different environmental conditions in pits vs. mounds

-different rates of ecosystem processes in pits vs. mounds

-more diversity (e.g. understory plants) with pits/mounds vs. flat topography

Effects of increasing heterogeneity: agroforestry example

Intercropping: -adding rows of trees to crop fields -adding a patch type to the landscape

The University of Guelph Agroforestry Research Station

Total Arthropod Abundance in June 1999 Samples from Agroforestry and Monoculture Sites

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300

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Agroforestry

Monoculture

How to measure spatial heterogeneity of a landscape?

from Mladenoff article:

Diversity indices such as the Shannon-Wiener Index can be used – measures the amount of “information” in a sample of landscape patches.

More diversity of patch types = more “information”

Not just number of different patches, but their relative importance – both affect the index

H = index of diversityPk = proportion of the landscape area in patch type kM = total number of patch types

More patch types and less dominance by a small number of patch types = higher landscape diversity

Temporal heterogeneity:

Disturbance and succession

Succession:

-“Process of change by which biotic communities replace each other through time”

A landscape can be in a ‘dynamic equilibrium’ or ‘shifting mosaic’ if all successional stages always represented somewhere on the landscape

Causes of succession:

Allogenic:

-physical processes external to the biotic community cause changes to the environment, leading to changes in the biota

Autogenic:

-changes in the environment are caused by the resident organisms, leading to changes in the biota

(these work together…e.g. fire causes initial change, followed by changes caused by the biotic communities that establish after fire)

Primary vs. secondary succession?

-”biological legacies” present at start of secondary succession; primary succession = starting from scratch

-primary succession = after glaciation, landslides, gravel pits

-secondary succession = old fields, clearcuts, etc.

Succession:

Considerations for conservation/restoration at the patch and landscape scales

-replacement of Norway maple forest on a slope near Parliament Hill in Ottawa

-Norway maple is non-native and invasive

-first attempts failed; newly planted native species (sugar maple, beech, etc.) had difficulty getting established

-new attempt: mimic natural succession processes by mixing early and late successional trees together

-early species (‘pioneers’) = fast growing = stabilized site

-late species = slow-growing but shade-tolerant; will gradually take over site

Temagami:

-how to maintain old-growth white and red pine on the forest landscape?

-mature red and white pine forests greatly reduced in Ontario due to centuries of logging

-remaining areas of old growth form patches on the landscape

-should we protect all of the patches from disturbance (logging and/or fire)?

Temagami:

-problem: red pine is shade intolerant and white pine is moderately shade tolerant

-white pine will regenerate to some degree under its own canopy, but red pine won’t

-fire suppression by humans has changed (reduced) fire frequency in Temagami pine forests

-if fire is completely excluded, the pine forests may turn into something else due to natural successional processes

Temagami:

-possible solutions:

-completely protect all old pine forest patches from any disturbance?

-allow logging of old-growth to mimic fire and help pines regenerate?

-stop suppressing fires in the area – let natural fires burn?

-apply prescribed burns to help pines regenerate?

-bottom line: maintenance of old growth pine requires consideration of temporal dynamics, and the whole landscape

Temagami:

-bottom line: maintenance of old growth pine requires consideration of temporal dynamics, and the whole landscape

-should aim for a ‘shifting mosaic’ that allows different successional stages to be present on the landscape

-this means that we may have to sacrifice some individual patches to disturbance in order to protect the pines over the long term

-but, risks are high as total remaining area is small

-comments?

Temporal heterogeneity: Landscape transformation

-land is transformed by several spatial processes…

-perforation (making ‘holes’ in the habitat, e.g. dispersed houses)

-dissection (subdividing of land with linear features e.g. roads)

-fragmentation (breaking of habitat into pieces)

-shrinkage (decrease in habitat size)

-attrition (disappearance of habitat type from landscape)

Temporal heterogeneity: Landscape transformation

-fragmentation and shrinkage have received the most attention; how do these processes affect species diversity, viability of populations, etc.?

-these two processes often occur at the same time…

-edge effect: fragmentation leads to more edge for a given habitat size-habitat loss can = reduced population sizes and greater local extinction probability-habitat shrinkage and fragmentation combined can also impede mobility of organisms between habitat patches

-studying and predicting the effects of fragmentation and habitat loss on ecological processes (particularly wildlife population dynamics and biodiversity) is of great interest in the ecological/conservation community

-in Unit 4 we will cover the ecological theories that form the basis of research into fragmentation/habitat loss

-species-area relationships-island biogeography theory-metapopulation theory

Species-area relationships: the famous curve…

Species-area curve = “one of ecology’s few genuine laws”

Species-area curves:

-often expressed on a logarithmic scale

Species-area curves: applications

-predicting effects of habitat loss on species diversity

-estimating necessary reserve size for protection of species diversity

-determining the sampling area required to adequately sample a community

-lots of empirical evidence of species-area relationships

-most observational, but some experimental, e.g. Simberloff’s mangrove island size experiments

-reduced mangrove island size with trenches and chainsaw!

Species-area curves: underlying explanation for curve shape

Several hypotheses:

1. Passive sampling hypothesis

2. Dynamic equilibrium hypothesis

3. Habitat diversity hypothesis

Species-area curves: underlying explanation for curve shape

Several hypotheses:

Passive sampling hypothesis

-the larger the area sampled, the more individuals sampled, and the greater the likelihood of encountering more species

Species-area curves: underlying explanation for curve shape

Several hypotheses:

Dynamic equilibrium hypothesis

- number of species (in a patch) determined by balance between immigrations and extinctions, mediated by patch area and isolation

- large patches contain larger populations and lower extinction probabilities

- See island biogeography theory….later on.

Species-area curves: underlying explanation for curve shape

Several hypotheses:

Habitat diversity/heterogeneity hypothesis

-as area increases, the number of different habitat types will also increase

“an acre of grassland added to another similar acre will produce a smaller increase in species than adding it to an acre of woodland” -Williams 1964

-more area = more habitat types = more species

Species-area curves: underlying explanation for curve shape

Several hypotheses:

Habitat diversity/heterogeneity hypothesis

-as area increases, the number of different habitat types will also increase

“an acre of grassland added to another similar acre will produce a smaller increase in species than adding it to an acre of woodland” -Williams 1964

-more area = more habitat types = more species

Species-area curves: underlying explanation for curve shape

-which hypothesis is correct??

-it depends…

-most interest is in the latter 2, and there is evidence for both

From Blake and Kerr

Species-area curves: underlying explanation for curve shape

-which hypothesis is correct??

It depends…

-area may be most important where habitat fragments are very isolated, in an inhospitable ‘matrix’ (just like the oceanic islands that the ‘dynamic equilibrium hypothesis was derived from)

-why does it matter?

-do we need large habitat reserves, or just ones with lots of habitat heterogeneity??

Species-area curves: variations in curve shapes

S = cAz

number of species(species richness)

area

constants

Or: logS = logC + zlogA

intercept slope

Species-area curves: variations in curve shapes

-much interest in the meaning of different curve shapes

(i.e., different intercept and slope values among taxa and regions)

-one hypothesis is that the intercept coefficient is a measure of alpha diversity, while the slope coefficient is a measure of beta diversity

-i.e. slope represents the rate of accumulation of species as a habitat area gets bigger, while the intercept represents the basic species pool size