Demographic consequences of mutualism disruption: Browsing and big‐headed ant invasion drive acacia population declines

Abstract

Across the globe, biological invasions have disrupted mutualisms, producing reverberating consequences for ecosystems. Although invasive species frequently trigger mutualism disruptions, few studies have quantified the demographic mechanisms by which mutualism breakdown may generate population effects. In a Kenyan savanna, the invasive big-headed ant (Pheidole megacephala) has disrupted a foundational mutualism between the monodominant whistling-thorn tree (Acacia drepanolobium) and native ants (Crematogaster spp.) that deter browsing by large mammalian herbivores. We conducted experiments to quantify the demographic consequences of this mutualism disruption in the presence and absence of large mammalian herbivores. Invasion by P. megacephala exacerbated population declines of A. drepanolobium, primarily through decreased survival and reproduction of adult trees. However, these fitness reductions were small compared to those resulting from the presence of large mammalian herbivores, which negatively impacted growth and survival. Contrary to expectation, the expulsion of metabolically costly Crematogaster mutualists by P. megacephala did not result in higher population growth rates for trees protected from large mammalian herbivores. Our results suggest that invasive P. megacephala may impose a direct metabolic cost to trees exceeding that of native mutualists while providing no protection from browsing by large mammalian herbivores. Across landscapes, we expect that invasion by P. megacephala will reduce A. drepanolobium populations, but that the magnitude and demographic pathways of this effect will hinge on the presence and abundance of browsers.

Disruption of an ant-plant mutualism shapes interactions between lions and their primary prey

“…Over the course of our experiment, changes in tree cover could arise in two ways. First, and within megaherbivore exclusion (fenced) plots, any differences relative to baseline conditions (i.e., those from uninvaded, open plots in 2017) would reflect differences in tree growth and survival in the absence of megaherbivores for both invaded and uninvaded areas ( 24 ). Second, within open (unfenced) plots, megaherbivore browsing (which we predicted would be higher in invaded areas) would reduce tree growth and survival ( 24 ).…”

Symbiotic acacia ants drive nesting behavior by birds in an African savanna

“…2015), including the savannas of East Africa (Young et al . 1996, Stanton & Palmer 2011, Palmer & Brody 2013, Hays et al .2022). In such ecosystems, whistling thorn trees ( Acacia [Vachellia] drepanolobium ) form a near-monoculture, comprising 95-99% of the canopy layer (Young et al .…”

“…Mutualisms structure biodiversity and ecosystem function (Stachowicz 2001). Mutualisms between plants and ants are particularly widespread across the tropics (Christian 2001, Frederickson et al 2005, Palmer & Brody 2013, Prior et al 2015, including the savannas of East Africa (Young et al 1996, Stanton & Palmer 2011, Palmer & Brody 2013, Hays et al 2022). In such ecosystems, whistling thorn trees (Acacia [Vachellia] drepanolobium) form a near-monoculture, comprising 95-99% of the canopy layer (Young et al 1996).…”

Symbiotic acacia ants drive nesting behavior by birds in an African savanna

“…Mutualisms structure biodiversity and ecosystem function (Stachowicz, 2001). Mutualisms between plants and ants are particularly widespread across the tropics (Christian, 2001; Frederickson et al., 2005; Palmer & Brody, 2013; Prior et al., 2015), including the savannas of East Africa (Hays et al., 2022; Palmer & Brody, 2013; Stanton & Palmer, 2011; Young et al., 1996). In such ecosystems, whistling thorn trees ( Acacia [ Vachellia ] drepanolobium ) form a near‐monoculture, comprising 95%–99% of the canopy layer (Young et al., 1996).…”