Transcript Species-area relationships

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
Agroforestry
Monoculture
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Total Abundance
Total Arthropod Abundance in June 1999 Samples from
Agroforestry and Monoculture Sites
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 diversity
Pk = proportion of the landscape area in patch type k
M = 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
constants
S = cAz
number of species
(species richness)
Or:
area
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