Ecological Disturbance of the Endangered Plant Taxon of Arizona: Threats, Benefits, and Management
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ECOLOGICAL DISTURBANCE OF THE ENDANGERED PLANT TAXON OF ARIZONA: THREATS, BENEFITS, AND MANAGEMENT Item Type Electronic Thesis; text Authors DuBroy, Alexandre Citation DuBroy, Alexandre. (2020). ECOLOGICAL DISTURBANCE OF THE ENDANGERED PLANT TAXON OF ARIZONA: THREATS, BENEFITS, AND MANAGEMENT (Bachelor's thesis, University of Arizona, Tucson, USA). Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 03/10/2021 21:51:24 Item License http://rightsstatements.org/vocab/InC/1.0/ Link to Item http://hdl.handle.net/10150/650999 ECOLOGICAL DISTURBANCE OF THE ENDANGERED PLANT TAXON OF ARIZONA: THREATS, BENEFITS, AND MANAGEMENT By ALEXANDRE MITCHELL DUBROY ____________________ A Thesis Submitted to The Honors College In Partial Fulfillment of the Bachelor’s degree With Honors in Plant Science THE UNIVERSITY OF ARIZONA M A Y 2 0 2 0 Approved by: ______________________ Dr. Steven E. Smith School of Natural Resources and the Environment DuBroy 2 Abstract Fourteen endangered plant taxa are believed to exist in Arizona. They face numerous threats in the form of ecological disturbances, which can push species closer to extinction. This thesis examines how seven categories of disturbance, namely fire, grazing, flooding, pests, human action, habitat loss, and climate change affect the endangered plant taxon of Arizona. This thesis hypothesizes that ecological disturbance is not all bad, and may in fact benefit endangered species. Existing research is compiled, and conservation and management advice are given based on the threats that these taxa are facing. This thesis concludes that human action and climate change are the greatest threats that taxon are facing, and that climate change will worsen the effects of the other disturbances. Results show that of the 98 effects logged, only five were positive, and 58 were negative to the taxon. This leads the author to reject their hypothesis, though with the caveat that lack of data hampers the accuracy of this conclusion. Much research is lacking and urgently needed, especially around the effects of climate change. The synergistic effects of disturbances are suggested by the research, in that the combination of multiple threats leads species towards extinction. These synergistic effects are a key area of future research. DuBroy 3 Introduction Despite its dry climate and reputation for large areas of desert, Arizona is home to an incredible diversity of flora. About 3,666 species of plants, spread across 1,003 genera and 145 families, call Arizona home (Lehr & Pinkava, 1980). Of these, some 300 are considered rare, (AZRPAG, 2014), and seven are listed as Federally threatened, and a further 14 are listed as Federally endangered (FWS, 2015). These species are at increased risk of extinction, and may be more susceptible to ecological disturbance. These species, and the risks they face, have not been previously studied as a whole. This thesis groups the 14 Arizona endangered taxon and examines their vulnerability of each in the face of seven categories of ecological disturbance. An ecological disturbance is a shift in the environment that leads to substantial changes in an ecosystem (Dale, 2001). Disturbances take many forms, but in many cases lead to a process of ecological succession. Succession is where species repopulate areas after a disturbance kills individuals. The path of succession varies based on what type of disturbance was experienced. In the aftermath of a high-severity wildfire, which is a classic example of disturbance, most all plant individuals may have died. Those that survived may have done so only as roots, and will need to regrow shoots. Species that were entirely killed may repopulate the disturbed zone in one of a few ways. If they had seeds that survived the fire, these seeds may sprout after the fire, and might even benefit from the fire due to the increased nutrients that have been liberated by the blaze. But if all seeds and individuals of a species have been extirpated from an area due to disturbance, they will have to repopulate from external reservoir populations. They may have to wait several seasons for seeds to grow and spread, or to spread via vegetative growth for species that are capable of doing so (McKenzie, 2011) (Dale, 2001). DuBroy 4 It should be noted that ecological disturbance is not inherently bad, in fact many ecosystems rely on ecological disturbance to maintain diversity. However, endangered taxa are particularly vulnerable to the negative effects of ecological disturbance. Disturbance presents a challenge to any species, but for endangered taxon, a single fire or flood could be the straw that breaks the camel’s back and leads to extinction. When a taxon is endangered, it usually is lacking in adequate population, range, and genetic diversity. This limits the ability of a species to recover from a catastrophic event, or sometimes even minor disturbances. Endangered taxa have generally hit a population bottleneck that has limited the availability of genotypes. This is unfortunate for them, as responding to a changing environment is best done when a species has great variety on which natural selection may act. The lack of population hinders the amount of individuals that can crossbreed with each other, that further creates a genetic bottleneck. It also limits the number of offspring possible in any year. Limited range means that species are more impacted by even small disturbances. For example, an acre-large fire is pretty small as fires go. But if a species is present on only that acre, its entire habitat has just been destroyed. Or if it is present in only two disconnected habitats because of habitat fragmentation, populations can no longer interact and reinforce the other (Wang, 2015; Hawksworth, 2007). All-in-all, being an endangered plant is a very tricky business, and most endangered plant taxon are truly on the edge of extinction. The importance of understanding ecological disturbance cannot be understated due to its direct role in extinction. The disturbances that this thesis lays out encompass the most pressing risks of plant extinction. Human activity, in its many forms, threatens both flora and fauna with mass extinction. It has been suggested that human actions could lead to a mass extinction event on par with geologic-scale extinctions (Bacon, 2016). Other authors have suggested that the DuBroy 5 planet is already in its sixth mass extinction event. Extinction events are definitely above average, and humans appear to be at fault. But the portrait is not all grim: since humans are the cause, they can also be the solution. Understanding the causes of extinction is critical, because it allows for effective conservation work (Barnosky, 2011). The causes of extinctions are of course varied. Not all ecological disturbances are caused by humans, though humans can impact them. Chief among these are fire and flood. Fire has a natural role in many ecosystems, acting as one of the main agents of ecological disturbance. A great many species are well adapted to fire, or even require fire to complete their life-cycle. For example, some trees in the genus Pinus have serotinous cones – they require fire to open. Montane forest ecosystems are often adapted to regular fire, with return intervals of less than 50 years and often less than 15. Not all ecosystems are adapted to fire however. Sparse desert ecosystems, as are found through much of southern Arizona, have such low fuel densities that they do not support fire spread. For saguaro cacti, Carnegia gigantea, based ecosystems, the fire return interval can be in excess of 250 years (McKenzie, 2011). The influx of invasive species is however changing return intervals. Southern Arizona’s numerous troubles with buffelgrass, Cenchrus cilaris, are a testament to the power of fire to change ecosystems. A non-native invasive species, buffelgrass has found itself at home in the arid landscapes of Arizona. More interestingly, buffelgrass has brought its native fire regime with it. By establishing large fuel masses and high fuel continuity, any fire that starts in a buffelgrass-infested landscape spreads aggressively and with high intensity (McDonald, 2011). The adaptations of species directly reflect historical fire return intervals. Saguaros have not had to deal with fire in the course of their evolution, and thus do not have any adaptations to survive fire. Thick-barked Pinus trees are able to withstand a direct low intensity fire, or medium- DuBroy 6 intensity fire that does not crown out, and easily survive – which reflects the frequent fires they deal with (McKenzie, 2011). But adaptation to fire is not the only part of the puzzle. Fire is a textbook ecological disturbance. By killing many individuals and redistributing nutrients, fire may wipe the proverbial ecological slate clean. With population density lessened, it becomes a battle for succession. Which species are present in the ecological succession depend on the severity of the fire. Low-severity fires may kill few individuals, and may spare many species. These fires will benefit established individuals that survive the fire, such as large trees, by providing a flush of nutrients from the ash. Many seeds and roots may survive the fire to repopulate existing species. High-severity fires may kill all individuals and species in an area. Succession is now a much longer process, and favors invasive species. The biomass of sensitive species may have been entirely destroyed, including all seeds and roots, and the only way to replace lost range is to spread from unburned populations (McKenzie, 2011). It is this kind of high severity fire that is so dangerous to endangered species.