Good times, bad times: inter-annual reproductive output in a montane endemic succulent (Aloe peglerae; Asphodelaceae) driven by contrasting visitor responses of small mammals and birds

Abstract

The movements of birds and small mammals may be affected by spatial and temporal variation in resource availability. For the genus Aloe, bird abundance and diversity increase in response to increases in nectar availability during flowering. Aloe peglerae Schönland, endemic to the Magaliesberg Mountain Range, South Africa, is primarily pollinated by the Cape rock-thrush (Monticola rupestris), but the nocturnal Namaqua rock mouse (Micaelamys namaquensis) contributes to pollination. To test the independent contributions of different pollinator guilds, i.e. birds and small mammals, we conducted selective pollinator exclusion experiments during two consecutive flowering seasons – one ‘good’ year and one ‘poor’ year (14 and 7% of plants flowering respectively). Reproductive output for both years was measured for A. peglerae plants assigned to selective exclusion treatments: (i) no visitors; (ii) nocturnal visitors; (iii) diurnal visitors; and (iv) all visitors, with camera traps recording visitors to each treatment. Bird visitation rates to the ‘diurnal visitors’ treatment were higher during the good year; subsequently, reproductive output in the poor year was lower, with plants in this treatment producing no fruit in the poor flowering year. In the ‘nocturnal visitors’ treatment, nocturnal visitation rates were similar with no significant difference in reproductive output between years. Seed viability was maintained between years, although less seed was produced during the poor flowering season. Mobile birds are less abundant in the A. peglerae population during poor flowering years – years when nectar resources are less abundant than in good flowering years, resulting in reduced plant reproductive output. However, the presence of non-flying, small mammals that cannot relocate to better feeding grounds compensates for the absence of birds in poor flowering years. This aloe may be resilient against Allee effects in the absence of primary pollinators. Nonetheless, conservation of A. peglerae should still be prioritised since low flowering densities and ongoing anthropogenic disturbance could be detrimental to the species’ survival.