Plant Biotic Interactions in the Sonoran Desert: Conservation Challenges and Future Directions
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Biotic interactions are vital to ecosystem functioning. Interactions among individuals lie at the core of population and community dynamics, and therefore play a central role in the existence and persistence of species. Plants form the food base of most terrestrial ecosystems and are therefore not surprisingly involved in a substantial portion of biotic interactions. Plants, animals, and microbes face great challenges to survival in the desert environment, and these interactions play a critical role in the survival of many species.

The Sonoran Desert flora is well documented and certain of its iconic interactions are well understood. For example, saguaros and the bats that pollinate them and disperse their fruits have become textbook examples of mutualisms (e.g., Shreve and Wiggins 1964; Turner et al. 2005). However, what do we know about plant-animal, plant-plant, and plant-microbe interactions in the Sonoran Desert more generally? What role do such interactions play in the ecology and evolution of the Sonoran Desert ecosystem? How are these interactions affected by global changes, and how can we conserve interactions? These questions inspired a discussion session convened at the Next Generation Sonoran Desert Researchers (NGSDR) 2012 Summit. Ultimately, participants identified the following five critical needs regarding research and conservation. We need to (1) improve our knowledge of the natural history (diversity, ecology, evolution) of interactions, both as individual entities and as [End Page 457] players in the broader ecological community; (2) monitor interactions on broad spatio-temporal scales to be able to identify the consequences of climate change, especially for seasonal interactions involving migratory species; (3) identify the human activities with the greatest impacts on interactions; (4) develop criteria to compile a “priority interaction list” to improve and strengthen our ecosystem conservation efforts; and, finally, (5) use interactions to restore disrupted habitats and ecosystems.

Here we provide a comprehensive yet concise overview of biotic interactions involving the flora, fauna, and microbiota of the Sonoran Desert, summarizing and expanding results of the NGSDR 2012 Summit discussion. We briefly present the broad categories of interspecific interactions involving Sonoran Desert plants, identify and describe threats known to negatively affect them as well as positive links with human activities, and present ongoing conservation needs and restoration efforts. We conclude by suggesting future research directions and recommendations required for urgent conservation and restoration efforts.

Diversity of Biotic Interactions

All living organisms on Earth are involved in interactions with other organisms. Interactions are “mutualistic” when both organisms benefit and “antagonistic” when only one benefits at the expense of the other. They are “facultative” when participants do not strictly depend on one another and interact with several other species; they are “obligate” when at least one species relies on the other and rarely interacts with other species, making the interaction a matter of life and death in some cases. Although obligate interactions represent only a minor fraction of the immense web of biotic interactions, their more constant association and higher specificity make them easier to study (Davidson and McKey 1993; Futuyma and Agrawal 2009). In contrast, facultative interactions are both more common and complicated, since species interact with many partners and the associations can fluctuate between mutualistic and antagonistic (e.g., Bentley 1977; Ness 2006).

Here, we focus on interspecific interactions in which Sonoran Desert plants regularly engage. We group them based on the harm or benefits animals and microbes confer. This includes two forms of antagonism, herbivory and parasitism, and five forms of mutualism, pollination, seed dispersal, biotic protection, facilitation, and microbe-mediated nutrition. These are detailed below. An extensive bibliography is provided in appendix 1. [End Page 458]

(i) Plant-Herbivore Interactions

Plants are the most common terrestrial food resource for animals. Most plant-consuming animals are insects; among vertebrates, most are mammals (Herrera and Pellmyr 2002). Plants in desert habitats are particularly important for animal consumers, because they represent a critical source not only of nutrients...