tropical forests and its biodiversity implications: What do we - - PowerPoint PPT Presentation

Trophic downgrading in tropical forests and its biodiversity implications: What do we know?

Nicole L. Michel, Ph.D. School of Environment & Sustainability University of Saskatchewan

Trophic cascades in tropical forests?

 Predators keep the world

green (Hairston et al. 1960)

 Diversity, redundancy, weak

links preclude cascades in tropical forests? (e.g., Strong 1992,

Polis & Strong 1996) Photos by Nicole Michel, Smithsonian Wild

1960s

Janzen-Connell Hypothesis

Photos by The Agouti Enterprise, Nicole Michel, Phylum Arthropoda Wikispaces

1970s

Tropical vertebrate community changes

 Barro Colorado Island

 Large predators, herbivores and frugivores declined,

small mammals increased 1932-1937 (Enders 1939)

 Abundant small- and mid-sized mammals? (Eisenberg & Thorington 1979, Glanz 1982)

 Amazon

 Large mammals

defaunated (Emmons 1984)

Photo from Smithsonian Wild

1930s - 1980s

Early trophic downgrading

 Extinction of gomphotheres and other megafauna 10,000

years ago altered dispersal, distribution of large-seeded trees, e.g., Jicaro (Crescentia alata) (Janzen & Martin 1982)

 Livestock compensate?

Photos by Cody H., Travis S.

1980s

Vertebrates reduce seed, seedling survival

 ~3x higher seed predation on

protected BCI (DeSteven & Putz 1984)

 Seedling density ~4x greater,

and seedling survival, growth, and success greater on hunted mainland (Sork 1987)

 La Selva: 82% “seedlings”

damaged, ~50% by vertebrates

(Clark & Clark 1989) Photo from Plantacion Edelman

1980s

Do tropical apex predators regulate terrestrial herbivores & frugivores?

 Wolves regulate

ungulates, e.g., moose on Isle Royale (Peterson et al. 1984)

Photos by Pete Dawson, Dennis Matheson

1980s

 Do big things run the

world in tropical forests?

(Terborgh 1988)

Trophic cascade on Barro Colorado Island?

 Frugivores released from predation, reducing large-seeded

plant recruitment? (Terborgh 1992)

 Seed predation, seedling recruitment similar at BCI and Cocha

Cashu (Terborgh & Wright 1994)

 BCI frugivore densities overestimated, felids do not limit

frugivores long-term (Wright et al. 1994)

 Felids incapable of limiting frugivores on BCI (Leigh 1999)  Felids do not maintain frugivores below famine-susceptible

levels (Wright et al. 1999)

…Yet the question of “just how different is BCI” continues today

1990s

The Empty Forest

Photos by Rodolfo Dirzo, Nicole Michel

1990s

 “We must not let a forest full of trees fool us into believing

that all is well. Many of these forests are ‘living dead’ (Janzen 1988), and, although satellites passing overhead may reassuringly register them as forest, they are empty

• f much of the faunal richness valued by humans. An

empty forest is a doomed forest.” (Redford 1992)

Defaunation consequences context-dependent

Photos by Rodolfo Dirzo, Nicole Michel

1990s

Effects on seed and seedling survival, recruitment, and density depend on plant traits, and consumer communities:

 Low seed predation, low herbivory, high seedling density

in hunted Los Tuxtlas plots (Dirzo & Miranda 1990, 1991)

 100% seed predation, high herbivory, low seedling

density on small Lago Gatun islands (Asquith et al. 1997)

Primacy of predation or dispersal?

Photos by Rodolfo Dirzo, Nicole Michel

1990s

 Janzen-Connell: mixed evidence for distance-dependent

mortality, negative density-dependence not supported

(Schupp 1992, Terborgh & Wright 1994)

 Frugivores may predate 50-90%+ of seed crop  Assuming J-C effects, dispersal and escape by even

1-2% of seeds may be more important (Brewer et al. 1997)

A new millennium

Photo by Robert Hruzek

2000 - today

 Plethora of defaunation studies  Special issues in Biotropica (2007) and Biological

Conservation (2013)

 Existing knowledge honed, new patterns emerging

The bushmeat crisis expands

Photos by Bongo Vongo, Paul Rosolie

2000 - today

 22 of 30 large vertebrates declined by up to 75% across

13 heavily-hunted Amazonian sites (Peres & Palacios 2007)

 Primate abundance 80% lower at hunted site outside

Manu NP (Nuñez-Iturri & Howe 2007)

 Majority of tropical nature reserves may be Empty

Forests (Harrison 2011)

Primacy of predation or dispersal context- dependent

Photo by Christian Ziegler

2000 - today

Relative importance of predation versus dispersal contingent on, e.g.:

 Plant traits, e.g., seed

size, toughness

 Herbivore / frugivore

body size

 Abiotic conditions

Defaunation effects on large-seeded plants driven by frugivore community

Photo by Reinaldo Aguilar

2000 - today

 Large-seeded plants suffer from

loss of dispersers across the Amazon, Atlantic Forest, southern Mexico, and Australia (Corlett 2007, Cramer

et al. 2007, Nuñez-Iturri & Howe 2007, Peres & Palacios 2007, Donatti et al. 2009, Moran et al. 2009, Melo et al. 2010)

 Large-seeded plants benefit from

reduced seed predation in Los Tuxtlas, Costa Rica, and Panama

(Dirzo et al. 2007, Wright et al. 2000, 2007; Hanson and Brunsfeld 2006)

Defaunation consequences scale with frugivore body size?

Photo from Safari Partners

2000 - today

 Smaller-bodied seed dispersers

(e.g., rodents) do not fully compensate for either mid-sized or large dispersers (Galetti et al. 2006, 2010,

Kurten 2013)

 Megafauna have larger total

impacts: consume and disperse small-large seeds, trampling. Ecosystem engineers? (Wang et al. 2007,

Johnson 2009, Vanthomme et al. 2010, Dunham 2008, 2011, Campos-Arceiz & Blake 2012, Jothish 2013)

Defaunation consequences contingent on abiotic conditions?

Image from Asner et al. 2009, PNAS

2000 - today

 Herbivore effects on plant communities strongest with

low rainfall, low productivity, high nutrient availability

(Pringle et al. 2007, Asner et al. 2009, Goheen et al. 2013)

Defaunation consistently reduces diversity

Photo by Nicole Michel

2000 - today

 Regardless of biotic or abiotic

context, defaunation reduces seedling diversity (Wright et al. 2000, 2007,

Wright & Duber 2001, Wright 2010)

 Negative density-dependence

may allow declining tree species to rebound after forest is protected, seed dispersers and predators return (Muller-Landau 2007)

Janzen-Connell

Image from The Agouti Enterprise

2000 - today

 Results – especially for density-dependence – are

mixed, hypothesis difficult to falsify? (Carson et al. 2008)

 Meta-analysis of 40 studies: no distance effects (Hyatt et al. 2003)  Distance, not density, effects in seedlings at Cocha Cashu ,

distance and density effects in saplings (Swamy & Terborgh 2010)

 Distance effects important, but limited rain of dispersed seeds

precludes density effects at CC (Terborgh et al. 2011, Terborgh 2013)

 Monkey gut passage more important than J-C effects in

Amazon (Levi & Peres 2013)



Yet Janzen-Connell “essentially correct” (Terborgh 2013)

“Ecological Meltdown”

Photos by Nicole Michel, John Terborgh

2000 - today

 While less common, apex predator loss, restricted

hunting, ample food may facilitate short- or long-term herbivore, frugivore population growth

 Howler monkeys, leaf-cutter ants at Lago Guri, Venezuela

(Terborgh et al. 1999, 2001, 2006)

 Bearded pigs, primates, rodents at Ganung Palung National

Park, Borneo (Curran et al. 1999, Curran & Leighton 2000)

 Wild pigs at Pasoh Forest Reserve, Malaysia (Ickes 2001, Ickes et al.

2001)

Collared peccaries (Pecari tajacu)

Photo by Geoff Gallice

1 Wright et al. 2000; 2 Eisenberg and Thorington 1973, Glanz 1982, Wright et

• al. 1994, 2000; 3 Peres & Palacios 2007; RAI: Hurtado & Jansen 2012, TEAM

Relative Abundance Index (number of pictures 100 days-1)

2 4 6 8 10 12 14

Gigante BCI La Selva

Peccary Density Site (per km2) Gigante 3.01 Barro Colorado 6.7 - 12.42 Amazon unhunted 6.6 + 1.33 La Selva ?

La Selva Biological Station

Photos by Nicole Michel

2000 - today

Percent liana cover 5 10 15 20 25

Mammals present Mammals excluded

A

**

A A La Selva BCI Gigante

LRR -0.53 -0.20 -0.20

Cascading consequences of altered consumer communities

Photo by Dominic Sherony

2000 - today

 Lago Guri: howler monkeys increase

bird density, richness (Feeley & Terborgh 2008)

 La Selva: near-extirpated understory

insectivorous birds forage in liana tangles reduced by peccaries

(Michel 2012, Michel et al., in revision)

 La Selva: greater litter herpetofauna

abundance in presence of peccaries

(Reider et al. 2013)

 Defaunation reduces dung beetle

richness (Nichols et al. 2009)

Research needs for the next 50 (1000?) years

Next 50+ years

 Improve predictions of defaunation consequences

based on plant traits, frugivore communities

 Are consumer effects additive, interactive, redundant?  Is trophic downgrading stronger in the Afrotropics?  Will density-dependence compensate for reduced

seedling diversity, maintaining canopy tree diversity?

 Identify keystone seed dispersers, predators. In what

range of abundance do they maintain plant diversity?

 Defaunation impacts on carbon sequestration?  …

Acknowledgements

ATBC and OTS for hosting this symposium Funding sources: Cooper Ornithological Society, The Explorers Club, Louisiana Board of Regents, National Science Foundation, Organization for Tropical Studies, Smithsonian Tropical Research Institute, Tulane University, University of Saskatchewan Ph.D. Committee: Thomas Sherry, Walter Carson, Jeffrey Chambers, Jordan Karubian All images used with Creative Commons licenses