Transcript Fragmentation and the Role of the Matrix in Species Richness

Fragmentation
and the Role of the
Matrix
in Species Richness
Gabe Cumming
Biology 255—November 10
Outline
• Background: the study of species richness in
fragmented landscapes
• The role of the matrix in species richness
• Matrix effects in the Biological Dynamics of
Forest Fragments Project (Gascon et al.)
• A temperate experiment (Cook et al.)
• Observational studies (Lomolino & Perault)
• Discussion
Theory of Island Biogeography
Courtesy of P. White
“Many of the principles
graphically displayed in
the Galápagos Islands
and other remote
archipelagos apply in
lesser or greater degree
to all natural habitats….
The same principles
apply, and will apply to an
accelerating extent in the
future, to formerly
continuous natural
habitats now being
broken up by the
encroachment of
civilization….”
--MacArthur and
Wilson 1967
Reduction and fragmentation of the woodland in Cadiz
Township, Wisconsin, 1831-1950. (After Curtis, 1956)
IBT: What Determines Species
Richness
• Island size
• Degree of
isolation
(distance to
mainland,
other islands)
MacArthur & Wilson 1967
Applying IBT to Fragmented
Terrestrial Systems
Harris, L. 1984. The Fragmented Forest:
Island Biogeography and the
Preservation of Biotic Diversity
• Additional factors in richness recognized
• Terrestrial matrix is not typically as
different from the fragment as ocean is
from an island
• Edge effects matter, and depend on
characteristics of fragment and matrix
Also…
• Sample effects—fragments not usually
a representative subset of landscape
biota
How Did the Fragment Become a
Fragment? History Matters for Richness
• Nekola:
– paleorefugia—extinction drives species composition
– neorefugia—immigration drives species composition
• Non-human vs. human causes of fragmentation
• Fragment development and change in species
composition: crowding/relaxation,
succession/regrowth
Surveyed by Debinski & Holt 2000
Fragmentation Experiments
Mixed Results of
Experiments:
Species Richness of
Fragments Varies
• Most common hypotheses—that
species richness increases with
fragment area and that species
abundance and density increase
with fragment area—confirmed in
<50% of cases
• Results varied by taxon:
arthropods most likely to conform
to expectations (small body size
relative to frags, short lifespans)
Why Study the Matrix?
1.
Studying the matrix can help explain patterns of species
richness
Debinski & Holt 2000: “Analysis of the matrix habitat may be crucial for
understanding the dynamics of remnant fragments. The most
important determinant of which species are retained in isolated
patches appears to be the interaction of patches with the
surrounding habitat matrix ( Bierregaard & Stouffer 1997 [1]; Tocher
et al. 1997[1]).”
Lomolino & Perault 2001: “First, we echo the calls of others for increased
attention to the ecological significance of the habitat matrix (e.g.
Harris, 1984; Brown & McDonald, 1995; Laurance & Bierregaard,
1997; Tilman & Kareiva, 1997; Wiens, 1997)…. There now appears
to be a growing list of studies reporting the influencing of matrix
characteristics and what we term landscape impedance on the
structure of isolated communities….”
Bierregaard 11/6/03, quoting J. Malcolm: “I just need to know what’s in
the matrix, not patch size, to determine biomass.”
Why Study the Matrix?
2. If the matrix consists of “modified habitats
surrounding fragments” (Gascon et al. 1999),
then most most modern landscapes are
largely matrix—studying matrix completes the
partial picture represented by fragments. To
pursue species richness conservation at all
points along the “two triangles,” including
matrix will be necessary.
Courtesy of P. White
Biological
Dynamics of
Forest
Fragments
Project
• “World’s largest and longest-running experimental
study of habitat fragmentation” (Laurance et al.
2002)
• Began as Minimum Critical Size of Ecosystems
Project, then expanded goals to include wider range
of factors
Courtesy of P. White
BDFFP Study Design
• Isolated 1-ha, 10-ha, and 100-ha fragments
• Controls in surrounding forest: 1-ha, 10-ha, 100-ha, 1000-ha
• Standardized abundance data collected for trees, mammals,
understory birds, amphibians, various invertebrate groups
prior to fragment isolation, allowing for direct assessment of
fragmentation effects
Gascon et al. 1999:
BDFFP Matrix
Study
Cecropia
surrounds a 100ha fragment
• BDFFP matrix: clearcut for cattle ranching (5-10 yrs),
followed by forest regrowth
• Study tests importance of matrix on fragment species
richness of four taxonomic groups: ants, small mammals,
frogs, birds
• For each species, index reflecting “vulnerability” to
fragmentation generated: overall abundance in fragments
divided by overall abundance in continuous forest. Ranked
on five-point scale; lowest rankings = most vulnerability.
Taxonomic Groups Respond
Differently to Fragmentation
• Birds and ants: richness declines
• Frogs and small mammals: original species
remain, supplemented by invaders from matrix
Species’ Use of Matrix
• 40-80% of “primary-forest” species use matrix too; some frogs even
breed there (but would matrix populations be self-sustaining?)
• 8-25% of species in each group was found exclusively in the matrix
Matrix Abundance vs. Vulnerability
• Correlations positive and significant for all
groups except ants
Conclusions
• Matrix species complement includes many invaders, some of
which have entered frags too
• Matrix as selective filter:
– Species have varying abilities to use matrix for movement or
reproduction
– Matrix varies in land-use history and degree of difference from
primary-forest; least vulnerable species can use most degraded matrix
• Matrix-tolerant species remain abundant in fragments
because population bolstered by immigrants, both from other
forested areas or matrix
• Matrix-tolerant species likely also tolerate edge conditions
• Why no significant ant response? Responding to
fragmentation at a different scale? (Ants are strongly
affected by fragmentation—Vasconcelos et al. 2001, in
Lessons from Amazonia.)
A Temperate Case: Cook et al. 2002.
Kansas Fragmentation Study: second-longest running
• Array of successional patches of three size classes,
surrounded by mowed matrix, at different distances from
forest
• Species richness measured within quadrats in patch and
matrix
Matrix Species Determine
Significance of Richness Variations
Patches have total of 146 species; matrix has 60; 35 are shared
ANOVA, total species richness in
patch quadrats
• No effect of patch size:
– large 12.15 ± 0.28
– small 12.16 ± 0.31
• “Marginal trend” towards greater
richness in near patches:
– near 12.51 ± 0.30
– far 11.81 ± 0.28
• Significant interaction between
size and distance
• Large patch interior, large patch
edge, small patch all same
richness (12.04 ± 0.53, 12.19 ±
0.33, 12.16 ± 0.31)
ANOVA, species richness in patch
quadrats excluding species also
found in matrix
• No significant effect of patch size:
– large 6.44 ± 0.18
– small 6.15 ± 0.17
• Significantly greater richness in
near patches:
– near 6.86 ± 0.18
– far 5.72 ± 0.16
• No interaction between size and
distance
• Large patch interiors had greater
richness (7.08 ± 0.33) than large
patch edges (6.16 ± 0.21) or
small patches (6.15 ± 0.17)
Conclusions
• When matrix species were removed, effects
of distance and patch size (especially large
patch interiors vs. small patches) became
evident
• Matrix species mask island biogeography
patterns—unlike island paradigm, significant
overlap between patch and matrix species
• Matrix species competition can depress
richness of patch species in small patches
and edges
An
Observational
Study:
Lomolino
& Perault
2001
• Measured species richness of non-volant mammals in twenty
old-growth forest fragments created by logging, Hood Canal
District of Olympic National Park, Washington
• Detailed, quantitative classification of landscape components,
including multiple categories of habitat and matrix
Conclusions
• Traditional island biogeography measures not
significant: species richness of old-growth
dependent mammals not correlated with fragment
area or simple measures of isolation.
• Species richness of old-growth dependent
mammals significantly correlated with two measures
of “landscape impedence:” the percentage of 1)
fragmented old-growth forests and 2) old secondgrowth forest (41-159 years old) in the matrix within
1 km of the fragment.
• Measures of habitat heterogeneity more relevant
than species-area relationships at this scale.
• Larger fragments can increase total richness by
increasing area of both core habitat and edge, thus
accommodating more specialists and generalists
What Can Be Theorized About the Effects of
Fragmentation on Species Richness from
These Matrix Effect Studies?
Fragmentation changes the species composition of
a landscape by favoring disturbance-tolerant
generalists, opportunist invaders, and species with
small area requirements or whose habitat needs
are at a scale that can be met in the matrix. This
change in relative abundances is likely to effect
total species richness, though the effect may be
positive or negative. Richness of specialist and
rare species is likely to decline as the matrix state
diverges further from the original habitat type (i.e.
as “impedance” increases).
Discussion Questions
• Given matrix effects, is total species richness of
fragments a useful measure? Could it possibly lead
to destructive conclusions? Do distinctions between
generalist and specialist species need to be made
first?
• Do we need a unifying body of “matrix theory” to
understand role of matrix in richness?
– What is the matrix? Should it be defined in specific
biogeographic terms in each landscape, per Lomolino &
Perault? Or in terms of “degree of modification (e.g.
McIntyre & Hobbs)?
– Or, since fragmentation effects vary by species, should
definitions of fragment and matrix be species specific?
– Or are these terms useful at all for understanding species
richness?
McIntyre &
Hobbs’
(1999)
Continuum of
Human
Habitat
Alteration
Discussion Questions, cont.
• How do we effectively account for diachronic
processes/patterns (taking place over time)
as well as synchronic processes/patterns
(contemporary) that contribute to
fragmentation effects on richness? E.g.:
Richness
crowding
relaxation
(Based on conversation with
R. Bierregaard about
changes in BDFFP frag
richness over time)
fragmentation
event
Time
Discussion Questions, cont.
• How do we acknowledge the complexity of
fragmented landscapes in management?
Is too much complexity not useful?