In an arid urban matrix, fragment size predicts access to frugivory and rain necessary for plant population persistence

Abstract

Cities annex wilderness and alter the interactions between the plant and animal assemblages embedded in the remaining nature fragments. Here, we explore consequences of habitat fragmentation for dispersal of cactus by fruit‐eating mammals in the Sonoran Desert landscape. We use the barrel cactus, Ferocactus wislizeni, as a model system to diagnose (1) how fruit utilization by a mammal assemblage changes with fragment area and isolation, (2) what characteristics make individual plants particularly susceptible to frugivore failure, and (3) how inter‐fragment differences in plant–frugivore interactions influence the seedlings’ susceptibility to other stressors (granivores and drought). We described the type and phenology of plant–animal interactions in 20 fragments along a 0.05–500 ha size continuum within a large city (Tucson, Arizona, USA), and combined allometric scaling models of mammalian home ranges and descriptions of plant size distributions to predict the fraction of the fruit crops accessible to resident mammalian frugivores in each fragment. Fruit removal rates generally increased along a continuum of fragment sizes, and older, taller plants were particularly susceptible to frugivore failure and more likely to have fruits attacked by granivorous rodents. As a result, thresholds where the nature and/or phenology of a plant's interactions with animals changes can be accurately predicted by integrating descriptions of fragment size, fragment isolation, and plant population structure. Last, we found support for the hypothesis that fragmentation‐induced changes in the phenology of fruit removal compromise the seedlings’ ability to exploit precipitation delivered in winter and spring. Specifically, as a result of slower fruit removal, the seed crops in small fragments forfeit access to an increasing proportion of the rains between October and March, which determine whether cactus seedlings can survive the following dry season (April, May, June). We expect these inter‐site differences in the type and phenology of interactions to compromise the integrity of plant populations in smaller fragments, to become increasingly influential in ecological settings in the Sonoran Desert where access to precipitation is essential, and to become increasingly prevalent as the plants in newly fragmented plant populations age (grow) to sizes that are more susceptible to frugivore failure in the defaunated landscapes.

The resilient frugivorous fauna of an urban forest fragment and its potential role in vegetation enrichment

“…Forests with fewer zoochoric plants offer fewer resources to vertebrates (Terborgh et al 2008 ), affecting their survival, growth, and reproduction, contributing to fragments’ defaunation. The diminishing vertebrate’s biomass affects the quantity and quality of seed dispersal (e.g., empty forest process by Redford 1992 ) (Ness et al 2016 ) and the soil’s fertilization through urine and feces (Feeley and Terborgh 2005 ). Less fertilization reverberates back into plant communities, decreasing rates of the establishment of new adults.…”

Who removes seeds in a heavily urbanized area?

“…We found intermediate removal rates for cosmopolitan species, such as D. viscosa and P. icosandra , but the lowest rate of removal was to introduce specie S. mole . Cities need to be understood within the context of their species interactions [40], because the disruption of interactions between legitimate seed dispersers (mutualist) with plants, beside by the introduction of exotic or cosmopolitan plant species [41] can lead to in the long term to homogeneous species assemblages and local extinctions [34,42]. Overall, our results suggest that both mutualistic– antagonist (i.e., seed disperser and predator) interactions still working in green areas of the city, particularly the interaction with native plants, likely because native species tend to established stronger interactions than introduced species [43].…”

“…Lyomis irroratus) [57]. In addition, is necessary to evaluate the contribution of vertebrate species in the process of primary removal in scrublands, as the mammal Bassariscus astutus (Lichtenstein, 1830), which is a very abundant frugivorous species in CDMX [58,59] and their interactions with plant species that have fleshy fruits (e.g., P. icosandra) that could increase the possibility that will colonize new environments (Ness et al, 2016). Fill these gaps, would provide a more complete picture of seed dispersion by the different actors and their threats inside the city.…”

Seeds and the City: The Interdependence of Zoochory and Ecosystem Dynamics in Urban Environments

“…areas (Fey et al, 2016; that roads restrict animal movements enhancing the fragmentation and isolation of urban green patches, and further delineating the habitat mosaic. Smaller, more isolated, fragments tend to support fewer animal disperser species and smaller population sizes in urban areas (Srbek-Araujo et al, 2017), leading to lower fruit removal rates, as found for cactus fruit removal rates by mammals (Ness et al, 2016). If small, isolated fragments are avoided by animals, it is also likely that seed influx into isolated fragments may decline, eventually leading to local extirpation of certain plant species.…”