Possible four-species interaction contributes to saguaro mortality in buffelgrass patches

Background/Question/Methods   The invasion of Pennisetum ciliare in the Sonoran Desert has resulted in large declines in species richness and functional type diversity, an effect amplified by time. One of the few dominant plants reported to be unaffected by P. ciliare invasion is the saguaro cactus (Carnegeia gigantea). However, in several invaded areas in southern Arizona, USA, saguaro populations have been devastated by the white-throated woodrat (Neotoma albigula), which utilizes saguaros for water during times of drought. This is not unusual, although the woodrat typically prefers Opuntia spp. and Cylindropuntia spp. over saguaros. To address this possible four-species interaction, we measured density of Opuntia and Carnegeia with respect to P. ciliare cover on random walks at four large buffelgrass patches (CLA, SABM, CAMP, and SABM) in the Santa Catalina-Rincon Mountain complex where woodrat damage to saguaros was extensive. Additionally, we noted the presence of woodrat-type damage on living saguaros and measured the density of dead saguaros.

Results/Conclusions   The likelihood that a 200cm or taller saguaro found dead increased with increasing buffelgrass cover (Chi-square=14.83, p=0.0001). The likelihood of finding saguaros with packrat tunneling increased with increasing buffelgrass at two sites: CAMP (Chi-square=4.26, p=0.0391) and MELP (Chi-square=16.89, p<0.0001). Saguaros at the CLA site in the Rincon Mountains, which has the highest density of Cylindro-opuntia bigelovii, had no relationship of packrat tunneling with buffelgrass cover. At the SABM site, succulents were rare altogether, with no Opuntia spp. nor Cylindro-opuntia spp. found in the study area. Live saguaros were also less prevalent at SABM than at other sites, with 50% of the saguaros found alive compared with 81%, 81%, and 84% at CAMP, MELP, and CLA, respectively. Only seven of the 46 live saguaros were found without packrat tunneling, although all seven were found outside the buffelgrass-infested area. These results support the hypothesis that interactions between alternative succulent water sources, P. ciliare, and Neotoma influence saguaro mortality, but there is little evidence here to support causation. Regardless, spatial correlations of dead and damaged saguaros with P. ciliare indicate that saguaros may be at risk from P. ciliare invasion as well.