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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 ; Tocher et al. 1997).” 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?