Dr. Walter Carson

Genetics
K. Arndt
T.-L. Ashman
G. Campbell
D. Chapman
G. Hatfull
J. Hildebrand
L. Jacobson
S. Kalisz
J. Martens
W. Saunders
B. Stronach
S. Tonsor
R. Wood

Microbiology
J. Boyle
G. Hatfull
R. Hendrix
J. Lawrence
J. Pipas
M. Popa
R.L. Duda
S. Godfrey
V. Oke

Molecular Biology
K. Arndt
J. Franzen
P. Grabowski
G. Hatfull
R. Hendrix
L. Jen-Jacobson
J. Martens
C. Peebles
J. Pipas
J. Rosenberg
A. Schwacha
C. Walsh

Plant Biology
T.-L. Ashman
W. Carson
S. Kalisz
V. Oke
C. Partanen
S. Tonsor
B. Traw

Science Education
A. Bledsoe
K. Curto
L. Daniels
S. Godfrey
N. Kaufmann
C. LaFave
J. Newman
E. Polinko
M. Popa
L. Roberts
T. Seiflein
R. Sherwin
A. Slinskey Legg

Structural Biology
M. Grabe
J. Hempel
R. Hendrix
L. Jen-Jacobson
J. Rosenberg
A. VanDemark

Former Faculty

Photo of Dr.
Carson

Plant Community Ecology, Tropical and Temperate Forest Ecology

Associate Professor

Dr. Carson received his Ph. D. in 1993 with Richard Root at Cornell University, performed his postdoctoral studies with David Tilman at the University of Minnesota and Steve Hubbell at Princeton University, and joined the Department in 1994.

Currently, Dr. Carson is accepting graduate students in his laboratory. Dr. Carson is accepting undergraduate researchers, and does sponsor students in other laboratories.

Professional Interests - Publications - Contact Information - Lab Personnel

Professional Interests of Walter Carson

My research interests focus on experimental and empirical tests of theory in community ecology, specifically those theories that hone in on the mechanisms generating community composition, coexistence, complexity, and the maintenance of species diversity. Consequently, I am focusing my research on species rich systems .

I. Mammals, insects, and the maintenance of diversity in tropical forest ecosystems in Panama and Costa Rica.

Invertebrates are more important in the maintenance of ecosystems than are vertebrates
E.O. Wilson, 1987

The presence or absence of large terrestrial seed predators may have a decisive effect in determining the diversity of the neotropical forest
J. Terborgh 1988, replying to Wilson

I currently have underway the first large scale, long-term experimental study designed to address a major, long standing question regarding the maintenance of diversity in tropical forest ecosystems. Specifically, do mammals and insects acting as herbivores and dispersers promote the maintenance of high species diversity in tropical forests? This question remains controversial and unanswered even though it has been with us for nearly 30 years, ever since Joe Connell and Dan Janzen independently proposed that herbivory and seed predation may promote high tree species diversity in tropical forests (the Connell-Janzen hypothesis). More recently, Rodolfo Dirzo warned that if Connell and Janzen were right, then the extermination of mammals caused by overhunting and poaching in many forest remnants could lead to a rapid decline of diversity in residual tropical systems. Resolving this crucial question, however, requires experimental manipulations of herbivores and dispersers over large areas and long time periods.

I am now in the 6th year of a 15-20 year experiment that is evaluating the influence of mammals and insects, alone and in concert, on the development of advance regeneration (seedlings and saplings) in the understory of mature-phase tropical forest in Panama (BCI). If mammals and insects differentially control recruitment success among understory taxa, then these agents will ultimately control the composition and diversity of the mature forest itself. To date, I have documented as much as a 15-fold increase in tree seedling densities following only a short period of experimentally excluding ground-dwelling mammals. Tree species that are both common and rare in the canopy have shown dramatic increases in abundance. Additionally, I have documented a 25% increase in species richness in the plots in just 6-years. This a surprising and unexpected result. Currently, I am conducting simulations that suggest that given random thinning and based on 6-years of demographic data, that exclusion of mammals may indeed lead to the long-term decline in diversity of tropical forests (as predicted by Connell-Janzen) in spite of the addition of more species into the experimental exclosures. A parallel study to this one is now underway in a lowland tropical rain forest in Costa Rica at the La Selva Biological Station). This work is funded by the National Science Foundation and the Andrew Mellon Foundation.

II. Herbivory, grassland dynamics and competition theory: Are grasslands dominated by grasses because insects attack forbs.

Insect herbivores are not prominent elements in theories of vegetation dynamics.
D.R. Strong et al. 1995

In a bold and controversial paper, Hairston, Smith, and Slobodkin (HSS; 1960) argued persuasively that native herbivores do not control the composition and productivity of plant communities. This remains a prominent view 35 years later particularly with regard to phytophagous insects. In a challenge to this view and subsequent theory developed regarding trophic cascades, I have gathered evidence that insect attack severely constrains the distribution and abundance of an important and widespread group of native forbs in North American grasslands. Furthermore, insect attack may ultimately lower levels of ecosystem net primary productivity. I propose an alternative to HSS, specifically, that outbreaking phytophagous insects drive host plant species to low abundance, thereby making it extremely difficult to subsequently detect the important role played by invertebrates. Theoretically, this work is extending Dave Tilman's R* model by adding a higher trophic level. Recent theoretical work by Jim Grover suggests that specialist insects may be playing a keystone role in model communities that have important similarities to the more complex communities found in nutrient limited ecosystems like those in the tall grass prairie.

I currently have underway a large-scale experiment replicated in 30 fields at the Cedar Creek Natural History Area in Minnesota. The goal of this work is to test the hypothesis that native specialist insects severely constrain the size, density, biomass, and productivity of patches of three native clonal species of goldenrod in grasslands in central Minnesota. These long-lived, herbaceous perennials form large and, at times, very productive patches throughout the tallgrass prairie biome in North America. Currently, the agents that control the distribution and abundance of this group of widespread prairie forbs remain unknown. The answer does not appear to lie with factors known to be important in structuring grasslands (e.g., fire and grazing). I now have evidence that the competitive ability of forbs is superior to that of the dominant grasses in the absence of insect herbivory. Common garden experiments are now underway trying to ascertain the mechanism by which insect herbivores alter competitive interactions and then link this to Tilman's R* model. This work is funded by the National Science Foundation.

III. Exploring the causes of failed oak regeneration in eastern deciduous forest: tests of 4 prominent hypotheses.

Oak forests are not regenerating and appear to be converting to forests dominated by non-masting species. The causes of failed oak regeneration remain poorly understood and controversial. Research on failed oak regeneration is especially pressing because eastern deciduous forests have lost or are losing other key masting species (e.g., chestnut and beech). I argue that oaks are a keystone plant resource (sensu Nason et al. 1998, Science) and thus the loss of oaks and their important acorn crops may have serious repercussions for biodiversity and wildlife. To address these issues, Rachel Collins (Ph.D. Candidate) and I have begun large-scale experiments that are designed to test four key hypotheses proposed to explain failed oak regeneration and other recent changes in forest stand composition in eastern deciduous forests. These include 1. The fire hypothesis: Periodic fires promote oak regeneration. 2. The herbivory hypothesis: Browsing by high populations of deer prevents oak regeneration. 3. The seed predation hypothesis: Seed predation by vertebrates prevents oak recruitment. Finally, 4. The gap hypothesis: Oaks require large treefall gaps to regenerate in the absence of large-scale disturbances. These hypotheses are not mutually exclusive; thus, we are testing them by using a factorial design where we are experimentally manipulating fire, browsing by deer, vertebrate seed predators, and canopy gap formation.

In total, this research goes beyond solely investigating failed oak regeneration. Indeed, we will be able evaluate simultaneously the relative role of 4 general processes that likely control forest stand dynamics, specifically, two contrasting disturbance regimes (gaps and fire), top-down trophic effects (deer herbivory), and the role of recruitment limitation (seed predation). Alterations of these processes may be responsible for other reported widespread changes in the composition of eastern deciduous forests, particularly increases in the abundance of red and sugar maple. Our experiments are designed to run 10-20 years, permitting a particularly definitive test of the above hypotheses. These experiments will be fully operational at two sites in West Virginia by the summer of 1999. By that time, we will have censused 128, 20 x 20m plots and permanently tagged and measured approximately 20,000 seedlings, saplings, and adults of all tree species. I believe we can discover the causes of failed oak regeneration and elucidate the underlying mechanisms responsible for associated shifts in understory tree species abundance that are occurring in many eastern deciduous forests. This work is funded by the USDA Forest Ecosystems Program.

IV. Collaborative research

In collaboration with my graduate students, we are working on: (1.) Species diversity relationships along productivity gradients at the Pymatuning the Laboratory of Ecology (Henry Stevens); (2.) The role of lianas in tropical forest dynamics in Panama and Costa Rica (Stefan Schnitzer); and (3.) The role of competition theory in explaining patterns of invasive species in wetlands (Dan Bunker).

The Carson Lab web page may be found here.

Publication Archive
40 Citations
29 Abstracts
22 PDFs

Recent Publications of Walter Carson

Peterson, C.J., and W.P. Carson (2008) Constraints on forest regeneration in abandoned tropical pastures: do temperate paradigms of succession apply to the tropics? Pp in Tropical Forest Community Ecology, Carson, W.P., and S.A. Schnitzer, Ed. Blackwell Publishing, Oxford

Carson, W.P., J. Anderson, E. Leigh, and S.A. Schnitzer (2008) Challenges Associated with Testing and Falsifying the Janzen-Connell Hypothesis: A Review and Critique. Pp in Tropical Forest Community Ecology, Carson, W.P., and S.A. Schnitzer, Ed. Blackwell Publishing, Oxford

Schnitzer, S.A., J. Mascaro, and W.P. Carson (2008) Treefall gaps and the maintenance of species diversity in tropical forests. Pp in Tropical Forest Community Ecology, Carson, W.P., and S.A. Schnitzer, Ed. Blackwell Publishing, Oxford

Paine, C.E., K.E. Harms, S.A. Schnitzer, and W.P. Carson (2008) Weak Competition Among Tropical Tree Seedlings: Implications for Species Coexistence. Biotropica :In Press

Long, Z.T., T.H. Pendergast IV, and W.P. Carson (2007) The impact of deer on relationships between tree growth and mortality in an old-growth beech-maple forest. Forest Ecol. Manag. 252:230-238 (PDF Reprint: 955 kb)

Royo, A., and W.P. Carson (2006) On the formation of dense understory layers in forests worldwide: consequences and implications for forest dynamics, biodiversity, and succession. Can. J. For. Res. 36:1345-1362 (PDF Reprint: 765 kb)

Stark, S.C., D. Bunker, and W.P. Carson (2006) A null model of exotic plant diversity tested with exotic and native species-area relationships. Ecol. Lett. 9:136-141 (PDF Reprint: 246 kb)

Kurzal, B., S. Schnitzer, and W.P. Carson (2005) Predicting liana crown location from stem diameter in three Panamanian lowland forests. Biotropica 32:262-266 (PDF Reprint: 255 kb)

Banta, J.A., A.A. Royo, C. Kirschbaum, and W.P. Carson (2005) Plant communities growing on boulders in the allegheny national forest: Evidence for boulders as refugia from deer and as a bioassay of overbrowsing. Nat. Areas J. 25:10-18 (PDF Reprint: 576 kb)

Comisky, L., A.A. Royo, and W.P. Carson (2005) Deer browsing creates rock refugia gardens on large boulders in the Allegheny National Forest, Pennsylvania. Am. Midl. Nat. 154:201-206 (PDF Reprint: 1.1 MB)

Bunker, D.E., and W.P. Carson (2005) Drought stress and tropical forest woody seedlings: effect on community structure and composition. J. Ecol. 93:794-806 (PDF Reprint: 335 kb)

Royo, A.A., and W.P. Carson (2005) The herb community of a tropical forest in central Panama: dynamics and impact of mammalian herbivores. Oecologia 145:66-75 (PDF Reprint: 340 kb)

Collins, R.J., and W.P. Carson (2004) The effects of environment and life stage on Quercus abundance in the eastern deciduous forest, USA: are sapling densities most responsive to environmental gradients?. Forest Ecol. Manag. 201:241-258 (PDF Reprint: 302 kb)

Mascaro, J., S.A. Schnitzer, and W.P. Carson (2004) Liana diversity, abundance, and mortality in a tropical wet forest in Costa Rica. Forest Ecol. Manag. 190:3-14 (PDF Reprint: 155 kb)

Stevens, M.H.H., S.A. Schnitzer, D. Bunker, and W.P. Carson (2004) Establishment limitation reduces species recruitment and species richness as soil resources rise. J. Ecol. 92:339-347 (PDF Reprint: 297 kb)

How to Contact Walter Carson

US Mail
University of Pittsburgh
Department of Biological Sciences
154A Crawford Hall
4249 Fifth Avenue
Pittsburgh, PA 15260

Phone, FAX, Internet
Office : (412) 624-5496
Lab : (412) 624-5497
FAX : (412) 624-4759
Email : walt+@pitt.edu
Web : http://www.pitt.edu/~abaumert/index.html

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