The Role of Seed Dispersers in the Population Dynamics of the Columnar Cactus Neobuxbaumia Tetetzo

Abstract

We examined the effect of frugivorous bats and birds on the population dynamics of the giant columnar cactus Neobuxbaumia tetetzo in the Tehuacán Valley, Mexico. Because successful seedling establishment occurs only beneath the canopies of shrubs and trees, we hypothesized that seed dispersal is a key process in the maintenance of its populations. We determined the dispersal effectiveness of different frugivores, considering the quantity and quality components of seed dispersal. We also evaluated the potential effects of each frugivore species on the finite rate of increase of N. tetetzo populations by modifying the fecundity values of a Lefkovitch matrix model. The bat Leptonycteris curasoae had the highest effectiveness whereas the bird Carpodacus mexicanus had the lowest. The estimated finite rates of increase calculated to evaluate the effects of frugivores on the population dynamics of the cactus differ marginally from unity, except when the effect of the bat L. curasoae was analyzed. Our results suggest that the bat Leptonycteris curasoae could be considered the legitimate dispersal agent of N. tetetzo, dispersing seeds directly to safe sites, and thus representing a key species in the ecology of this columnar cactus.

An empirical approach to analysing the demographic consequences of seed dispersal by frugivores.

“…Because of these assumptions, the proposed approach should be used only to generate scenarios of the possible effects of frugivores on the population growth rate, instead of trying to predict the actual population dynamics of plants (Godínez-Alvarez et al, 2002). Such explorations are extremely valuable in and of themselves; indeed, the patterns are more valuable than the actual values.…”

“…Quantity Quality Reference Howe, 1977 Herrera andJordano, 1981 ;Jordano and Schupp, 2000Murray, 1988Reid, 1989Vander Wall, 1993Chávez-Ramírez and Slack, 1994Graham et al, 1995Jordano and Herrera, 1995Chaprnan and Chaprnan, 1996Larson, 1996Forget, 1997Sun et al, 1997Wenny and Levey, 1998Loiselle and Blake, 1999Holbrook and Smith, 2000Rey and Alcántara, 2000Stevenson, 2000Wenny, 2000aWenny, 2000bGarcía, 2001Calvifio-Cancela, 2002, 2004Figuerola et al, 2002Godínez-Alvarez et al. 2002Kaplin and Larnbert, 2002Cordeiro and Howe, 2003Traveset et al, 2003Hollander and Vander Wall, 2004 …”

“…Detailed information on this plant and its seed dispersers can be found in Godínez-Alvarez et al (2002).…”

“…On the other hand, incorporating the consequences of mutualistic interactions into niatrix population rnodels has so far had only limited success (Herrera and Jordano, 1981;Horvitz and Schemske, 1994;Godínez-Alvarez et al, 2002). A particular limitation to the viable integration of seed dispersal with its demographic consequences has been an inability to link the effects of frugivores' actions with their delayed consequences at later stages of recruitment.…”

Quantity component of the effectiveness of seed dispersal by birds in the temperate rainforest of Chiloé, Chile

“…The effectiveness of seed dispersers depends both on the interaction event frequency and on the net effect per event (quantity and quality of dispersal sensu, Schupp 1993). However, studies in the rainforests of Chiloé on disperser effectiveness during the austral autumn-winter are scarce (Hernández 1995, Figueroa andCastro 2002) despite the importance of these concepts for seed dispersal ecology (Schupp 1993, Godinez-Alvarez et al 2002, Calviño-Cancela and Martín-Herrero 2009. It is hypothesized that during the austral autumn-winter the guild of seed disperser birds is depleted.…”