Metapopulation Extinction Thresholds in Rain Forest Remnants

Abstract

Although habitat fragmentation is a major threat to global biodiversity, the demographic mechanisms underlying species loss from tropical forest remnants remain largely unexplored. In particular, no studies at the landscape scale have quantified fragmentation's impacts on colonization, extinction, and local population growth simultaneously. In central Amazonia, we conducted a multiyear demographic census of 292 populations of two leaf-inhabiting (i.e., epiphyllous) bryophyte species transplanted from continuous forest into a network of 10 study sites ranging from 1, 10, and 100 to 110,000 ha in size. All populations experienced significantly positive local growth ( ) and a nearly constant per-generational l 1 1 extinction probability (15%). However, experimental leaf patches in reserves of ≥100 ha experienced nearly double (48%) the colonization probability observed in small reserves (27%), suggesting that the proximate cause of epiphyll species loss in small fragments (≤10 ha) is reduced colonization. Nonetheless, populations of small fragments exhibit rates of colonization above patch extinction, positive local growth, and low temporal variation, which are features that should theoretically reduce the probability of extinction. This result suggests that for habitat-tracking metapopulations subject to frequent and stochastic turnover events, including epiphylls, colonization/extinction ratios must be maintained well above unity to ensure metapopulation persistence.

Epiphyte metapopulation dynamics are explained by species traits, connectivity, and patch dynamics

“…In contrast, when the species grows on the surface of a substrate that is unstable, for example growing, decomposing, or even moving, the local stochastic extinction risk can be assumed to be higher. Examples are leaves (Zartman and Shaw 2006), branches (Laube and Zotz 2007), and decomposing stumps (Caruso et al 2010). One exception is Ö ckinger and Nilsson (2010) who reported significant stochastic extinctions in the epiphytic lichen Lobaria pulmonaria growing on the bark of old trees.…”

“…Instead, local extinctions were driven by deterministic patch destruction. In contrast, stochastic extinctions were relatively common in other sessile organisms (e.g., epiphyllous bryophytes [Zartman and Shaw 2006] A key problem in studying colonization-extinction dynamics of sessile species is their slow dynamics. Therefore, a model that can be fitted using data from one point in time (henceforth snapshot data) is useful.…”

Dispersal and life history strategies in epiphyte metacommunities: alternative solutions to survival in patchy, dynamic landscapes

“…For tropical epiphylls inhabiting leaves with a duration time of typically less than 18 months, Zartman and Shaw (2006) found that local growth rates were not important for metapopulation persistence. Our study suggests that an important eVect of high growth rates is to reduce the time to reach reproductive size in colonists, whereas in perennial stayers, which also had rather high RGRs, the most important eVect is to lower the local extinction risk.…”

“…Such metacommunities may be mainly structured by dispersal and reproductive patterns rather than by competition-colonization trade-oVs (cf. Zartman and Shaw 2006), and alternative dispersal strategies oVer several trade-oVs to be explored.…”

Designing a survey protocol to overcome the Wallacean shortfall: a working guide using bryophyte distribution data on Terceira Island (Azores)

“…Lastly, we chose the geographical positions of survey localities not randomly, but rather based on criteria that are known to affect general diversity patterns, such as edge effects and the impact of regional conditions on local diversity (Cardoso et al 2009), which have proven to also be relevant for bryophytes (Moen & Jonsson 2003;Pharo et al 2004;Zartman & Shaw 2006).…”