Jansen, P.A.

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The roles of vertebrates in forest dynamics: a new CTFS program

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Wageningen Yield 

Proceedings of the Pasoh International Symposium                                                                                       
10-11 November 2010, Forest Research Institute Malaysia (FRIM), Selangor 
 
13
THE ROLES OF VERTEBRATES IN FOREST DYNAMICS: A NEW CTFS 
PROGRAM 
 
PA Jansen 
 
Centre for Tropical Forest Science, Smithsonian Tropical Research Institute, 
Apartado 0843–03092, Balboa, Ancon, Panamá, República de Panamá. 
 
 
Research at Pasoh as well as most other sites in the worldwide network of the Centre 
for Tropical Forest Science (CTFS-SIGEO) is traditionally focused on the diversity 
and dynamics of vegetation. Large-scale forest plots in which trees are mapped, 
measured and monitored are the key standardized methodology (Condit 1998). This 
approach, however, does not completely fulfill the primary goal of the CTFS-SIGEO 
global network, which is to understand the diversity and dynamics of forests in the 
face of global change. In particular, vertebrates and their roles should be 
systematically studied across the network but are not. 
 
The first reason to study vertebrates is that they are a crucially important component 
of forest ecosystems, even if “forest” is narrowly interpreted as “vegetation dominated 
by trees”, vertebrates influence the woody vegetation in many ways. They directly 
influence vital rates of plants through seed dispersal, seed predation, browsing of 
seedlings and saplings, rooting and burrowing. Indirect influence on vital rates of 
plants arises from predation of herbivores by carnivores, competition between 
herbivores, and predation on insects by insectivorous vertebrates, amongst others 
(Terborgh & Wright 1994). Because tree species are impacted differentially by 
different vertebrate species, these interactions influence the species composition of 
recruitment and hence co-determine the species composition of future forest, their 
(functional) diversity, and many ecosystem services. Very high population densities 
of vertebrates, in conjunction with fire, may even turn forests into woodland and 
savanna (Van Langevelde et al. 2003). Clearly, the species composition and 
abundance of vertebrate communities shapes vegetation. 
 
A second reason to study vertebrates in the CTFS-SIGEO network is that vertebrate 
populations are dramatically affected by global change, both in land use and in 
climate (Collen et al. 2009). Forest habitats are increasingly fragmented to the point 
that they can no longer sustain viable populations of larger species. Fragments often 
get isolated by non-forest habitat to the point that many species are unable to re-
colonize these forests. Also, gigantic tracts of seemingly intact tropical forests are 
subject to intense hunting for trade in bush meat, pets and medicine, and for 
prosecution. Over hunting leads to the local extinction of larger vertebrate species and 
sometimes even their global extinction, and is one of the key threats to global 
biodiversity. This produces so-called “empty forests” with “silent springs” where 
animals and their sounds are noticeably lacking. Indirect impacts on the ecosystem 
abound. For example, seeds of vertebrate-dispersed tree species may fail to reach sites 
suitable for survival and growth, which indirectly favors recruitment of wind-
dispersed species that do not experience such increased dispersal limitation. Also, 
trees that normally experience high levels seed and seedling mortality may be 
ecologically released (Wright et al. 2007b, Jansen et al. 2010).  
Proceedings of the Pasoh International Symposium                                                                                       
10-11 November 2010, Forest Research Institute Malaysia (FRIM), Selangor 
 
14
Many studies have shown that the community composition of vertebrates affects 
vegetation and ecosystem functioning and service of forests. For example, 
regeneration of particular tree species failed after extinction of top-predators that 
bother herbivores browsing recruits of those species in North American forests 
(Ripple & Larsen 2000). Small-island ecosystems with a reduced mammal community 
completely collapsed in Venezuela (Terborgh et al. 2001). Forests with hunting 
showed clear shifts in seedling recruitment towards wind-dispersed species and lianas 
in Panama, which might in turn affect carbon sequestration by these forests (Wright et 
al. 2007a). And infectious diseases emerge out of wildlife populations in degrading 
tropical forests as a possible result of vertebrate diversity loss (Keesing et al. 2006). 
The consensus expectation is that vertebrate diversity loss may have dramatic 
cascading effects on tree communities, ecosystem functioning and services to 
mankind. Global change may even be affecting tree species composition via land-use 
impacts on vertebrate communities more than via climate-impacts on trees. 
 
CTFS has initiated a new Vertebrate Program, aimed to systematically study 
vertebrates and their impacts on forests in the network. A first task will be to assess 
the species composition; abundance and diversity of vertebrates at each CTFS site 
through compilation of existing data and new inventories. A second component will 
be the monitoring of changes in community composition over time, parallel to the 
tree-plot censuses, to allow for comparisons. Experiments replicated across multiple 
sites are needed to better understand the impact of vertebrates on plant diversity. 
Responses of vertebrate communities to climate and land-use change must be 
addressed by comparing sites that differ in their levels of such changes. Finally, cross-
site experiments are needed to address cascading effects of vertebrate diversity loss on 
tree diversity and ecosystem functions. 
  
A major problem with studying vertebrate communities is that quantitative census 
requires completely different methods for the various vertebrate groups. Birds are 
typically censused through mist netting and visual surveys along transects, small 
mammals by live trapping, larger mammals by distance sampling along transect and 
camera trapping, amphibians and reptiles by pit falls and transect counts, bats with 
mist nets and bat detectors, and fish by netting. Thus, monitoring of an entire 
vertebrate community requires a variety of parallel studies by differentially 
specialized students. The amount of work involved begs to prioritization. CTFS has 
chosen to first develop a program for medium- to large-sized terrestrial vertebrates 
because they have a relatively large impact on the vegetation, and are under threat. 
Besides that, this group is appealing to students and to the general audience. Using 
camera-trapping technology, this group can be censused relatively easily in a 
standardized manner.  
  
One of the first projects in the new vertebrate program involves annual camera 
trapping of terrestrial mammals at a subset of the CTFS sites that will become part of 
the Tropical Ecology Assessment and Monitoring (TEAM) network 
(www.teamnetwork.org). This network, in which CTFS collaborates with 
Conservation International, the Wildlife Conservation Society, and Missouri 
Botanical Gardens, is planned to comprise about 40 tropical sites with contrasting 
levels of projected global change. The goal of TEAM is “to provide a consistent free 
Proceedings of the Pasoh International Symposium                                                                                       
10-11 November 2010, Forest Research Institute Malaysia (FRIM), Selangor 
 
15
and public long-term global data set in near real-time of biodiversity for scientific and 
conservation communities worldwide”.  
 
Pasoh is among the ten CTFS sites that will be part of TEAM. Here, we will run 
modern camera traps in two areas: the Forest Reserve of which Pasoh is part, and the 
Kenaboi forest area. Where the lowland Dipterocarp forest of Pasoh has gotten 
entirely isolated by oil palm and rubber plantations and is subject to heavy poaching, 
it is not far from hill Dipterocarp forest that is still connected to the large tracts of 
forests that cover the Central Forest Spine of the Malaysian Peninsula, and may have 
low levels poaching thanks to poor accessibility. A comparison between the two 
forests over the years will give insights on how much the vertebrate community at 
Pasoh has changed and how it will continue to change. 
 
In each study area, camera trapping will be undertaken annually at each of 30 points 
that are separated by 1.4 km (i.e., 1 camera per 2 km²). Each year, at each point, a 
modern camera trap will record all passing animals that are large enough to trigger the 
infra-red motion sensor during 30 consecutive days in a dry season. The cameras take 
series of digital pictures at 1-second intervals, generating near-video photo sequences. 
The cameras have an invisible infrared flash that allows taking photos during the night 
(TEAM Network 2011). Camera traps allow the non-invasive monitoring of terrestrial 
mammals that are and often not detected with other methods. The photographs are to 
some degree analogous to museum specimens, representing a permanent record of 
date, location, and species. Several new mammal species have been discovered with 
camera traps during the past decade, and some species believed extinct were 
reconfirmed. For example, the Giant sable antelope, unique to Angola, was captured 
on camera trap after not being been seen for >20 years (Pitra et al. 2006). 
 
The cameras should yield standardized measures of diversity and community 
composition that can be compared across years and sites. These include species 
presence, occupancy and potentially even density and biomass. The TEAM project 
generates these metrics for monitoring of changes in abundance over time. Also, the 
set-up that we have also allows comparisons along gradients within sites, for example 
using the Wildlife Picture Index (O'Brien et al. 2010). Besides monitoring, camera 
trapping can yield a wealth of information on behavior (Bridges & Noss 2011). For 
example, camera-trap footage contains information on animal activity, allowing the 
study of daily routines and how these are affected by interactions with habitat, food 
competitors, predators and people. Pilot studies at Barro Colorado Island, one of the 
CTFS sites in Panama, have been conducted to explore some of these opportunities.  
 
Vertebrate-focused studies can help to increase our understanding of forest 
functioning and dynamics. They will undoubtedly highlight the importance of 
protecting not only the trees in forest and nature reserves, but also the vertebrates that 
are native to these ecosystems. Restoration and protection of the forest fauna will be 
one of the major future challenges at Pasoh. 
 
 
 
 
 
 
Proceedings of the Pasoh International Symposium                                                                                       
10-11 November 2010, Forest Research Institute Malaysia (FRIM), Selangor 
 
16
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Proceedings of the Pasoh International Symposium                                                                                       
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The roles of vertebrates in forest dynamics: a new CTFS program

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