Tropical birds are highly vulnerable to forest fragmentation, and declines are often attributed to the direct effects of decreased habitat size. However these birds have nearly disappeared from some large forest fragments, but persist in smaller nearby reserves. Mechanisms other that habitat size proposed to account for these declines (food-limitation, dispersal-limitation and nest predation) also fail to explain the puzzling patterns observed in several tropical forests. The proposed research investigates a new, alternative hypothesis to account for bird declines in large habitat patches that has three key components: (1) understory insectivorous birds prefer to forage in dense woody vines called lianas; (2) collared peccaries suppress liana densities, and (3) loss of suitable habitat and, consequently, bird populations correspond with peccary densities. The study will test the three components of this hypothesis in central Panama, where both focal bird populations and lianas have increased while peccary densities remain low.
Results from this project have immediate conservation applications in reserves across Central America. The graduate student involved is bilingual and plans to engage local land owners and managers in Panama in the project. Outreach to local communities will include bird identification and bird banding demonstrations. Undergraduate students from groups typically underrepresented in science will also be trained through participation in this research.
?" PI: Dr. Thomas W. Sherry, Tulane University Co-PI: Nicole L. Michel, Tulane University Project Outcomes Report Tropical rain forest understory insectivorous birds are declining in fragmented forests for reasons that remain unclear, and are not explained by available hypotheses. For example, understory insectivores such as Checker-throated (Epinecrophylla fulviventris) and Dot-winged Antwrens (Microrhopias quixensis) and Ruddy-tailed Flycatcher (Terenotriccus erythrurus) declined to near-extirpation at La Selva Biological Station, Costa Rica (1,611ha; hereafter La Selva) between the 1970s and 2000s (Sigel et al. 2006). Meanwhile, these same species increased up to 150% during the same time period at the similarly-sized (1,562ha) Barro Colorado Island, Panama (hereafter BCI; Robinson 2001), and persisted at the much smaller (354ha) Tirimbina Biological Reserve (B. Sigel, unpubl. data). These three species of understory insectivores are known to forage in vine and liana (defined as woody vines) tangles (Gradwohl and Greenberg 1980). In turn, lianas increased on BCI concurrently with these insectivores (Wright et al. 2004, Ingwell et al. 2010), while at La Selva lianas occur at unusually low densities (Mascaro et al. 2004). During this same time period, collared peccaries (Pecari tajacu), large ungulates which are known to consume and trample vegetation, including lianas (Beck 2005), increased dramatically at La Selva (T. Sherry, pers. comm., Torrealba-Suárez and Rau 1994), while occurring at much lower densities on BCI (Wright et al. 1994, 2000, 2007). In this research we investigated a novel indirect-effects hypothesis of understory insectivorous bird population decline with three components. First, we predicted that several understory insectivous birds forage selectively in microhabitats with dense liana cover. Second, we predicted that a keystone herbivore, collared peccary (Pecari tajacu), suppresses liana densities. Finally, we predicted that high peccary densities, corresponding with loss of suitable foraging microhabitat, is causing declines in understory insectivorous birds. To test the hypothesis we quantified foraging microhabitat selection by the three understory insectivorous birds mentioned above, and related this to liana tangle frequency at six Central American rain forest reserves varying in collared peccary density (range: 3 – 20 / km2). Our six study sites were Barro Colorado Island, Gigante Peninsula, and Limbo Hunt Club, Panama; La Selva Biological Station and Tirimbina Biological Reserve, Costa Rica; and Refugio Bartola, Nicaragua. We found support for all three components of our hypothesis. All three avian focal species preferentially selected liana tangles as foraging microhabitat. We also found that collared peccaries suppress liana densities. Finally, suitable foraging microhabitat for the avian focal species, i.e., liana tangles, declines with collared peccary density. This is the first study to our knowledge to show that understory insectivorous birds are vulnerable to foraging microhabitat loss. Our research highlights the important role of indirect effects in rendering even large forest tracts unsuitable for remnant species, and suggests that reserve creation alone is insufficient to preserve some species, and that the ecological condition—i.e., habitat quality—of reserves is also critical. Our study also provides valuable information on how birds respond to changes in understory vegetation, and highlights the beneficial role of lianas as avian foraging habitat. This NSF-funded research has led to many accomplishments and outreach opportunities. It has contributed to two publications: a review chapter published with InTech Open Access Publisher, and a data chapter submitted to Oikos; two additional publications are currently in preparation. This grant also greatly enhanced the quality and scope of the Co-PIs dissertation research, which was successfully defended in March 2012. The Co-PI has presented this research at four international conferences and three invited talks, and submitted a working group proposal to further investigate the cascading effects of collared peccaries on plant and animal communities and ecosystem processes at La Selva Biological Station. This research has also supported one undergraduate Honors Thesis, and has led to an ongoing collaboration to study the effects of experimental liana removal on tropical birds, as well as post-doctoral research on tropical mammal-plant-bird interactions. Moreover, the ongoing research, publications, and outreach activities based on and inspired by this DDIG-funded research will continue to be produced in the years to come.
