Saguaro survival: establishing an icon

Having grown up in the New York metropolitan area, my only contact with the saguaro cactus, Carnegiea gigantea, was from several TV westerns, which dubiously placed these mammoth cacti in New Mexico, Texas and Colorado.  In fact, the saguaro is limited to the Sonoran Desert of northwestern Mexico, extreme southeast California and southern and central Arizona. You won’t find these cacti further north, because a freeze lasting more than 24 hours kills them.  I still remember my first real sighting of these cacti; I was amazed at how distinct they seemed in comparison to the other vegetation, and I delighted in their abundance.

Daniel Winkler - Saguaro Photo 1

Dense patch of saguaros. Credit: Daniel Winkler

Many others delight in their abundance as well.  The flowers, fruits and seeds feed many animals (including humans).  They were an important food for the Tohono O’odham and Pima Indians – eaten fresh or converted into numerous products including wine, juice, jam and syrup.

Daniel Winkler - Saguaro Photo 2

Large saguaro with many fruits emanating from the apex of its branches. Credit: Daniel Winkler

Woodpeckers and flickers excavate nests in the saguaro’s trunk, which are subsequently occupied by other animals such as snakes, arthropods and small mammals.

nesthole

Saguaro with nest cavity excavated near the top of its trunk. Credit: Daniel Winkler

Daniel Winkler also delighted in the saguaro’s awesomeness. As he describes “I fell in love with answering some basic ecology questions about the saguaro. I was surprised that scientists had been studying this wonderful plant for almost 100 years and there were still many basic questions about the species general biology and ecology that remained unanswered. Thus, I was hooked immediately and became obsessed with saguaro.”

Don Swann - Photo of D. Winkler with young saguaros

Daniel Winkler with young saguaros. Credit: Don Swann

Winkler and his colleagues wanted to know how moisture, temperature and habitat influence the establishment or survival of juvenile saguaro seedlings. Previous research had shown that saguaro height can be used to estimate saguaro age, given knowledge of previous rainfall in a particular area. So buoyed by an army of citizen scientists whom they recruited with the help of social media, student groups from schools and volunteers working at the Saguaro National Park, the research team estimated the age of every saguaro on 36 4-ha plots (1 ha = 10,000 m2).

Going into the study, the researchers knew that rainfall was a very important factor, with saguaros surviving better during wet periods.  But they also knew that historically, some areas located near each other showed different establishment trends, thus they suspected that other variables, particularly land use and other landscape factors, might be important.  They did their research in two different districts within the park: 21 plots in the Rincon Mountain District (RMD) on the east side of the park, and 15 plots in the Tucson Mountain District (TMD) to the west. They classified each plot as a particular habitat type based on slope, elevation and soil-type. Bajada was low elevation, flat and had gravelly porous soils.  Foothills were intermediate elevation and intermediate slope, while sloped habitats had highest elevation, steepest slope, and the coarsest rockiest soils.

Daniel Winkler - Saguaro Photo 4

Panoramic view of Saguaro National Park showing diversity of habitats. Credit: Daniel Winkler.

Winkler and his colleagues calculated the Palmer Drought Severity Index (PDSI) for the years 1950-2003. The PDSI quantifies the water balance between precipitation and evapotranspiration, taking into account not only rainfall but also other factors such as temperature and cloud cover.  The PDSI was estimated by assessing tree ring width for each year in nearby woodlands; wet conditions have wide tree rings (maximum PDSI value = +6), while dry years have narrow tree rings (minimum PDSI value = -6).

The researchers discovered a very strong association between the PDSI and seedling establishment. Low PDSI at the beginning and especially the end of the time frame was associated with low seedling establishment, while high PDSI (particularly in the 1980s was associated with high rates of seedling establishment (top graph below).  But other patterns emerged as well.  For example, establishment was higher in the TMD during the wettest years, but higher in the RMD during the most recent drought (bottom graph below).

WinklerFig1

Top. Total number of saguaros (left Y-axis) established per hectare from 1950-2003 in relation to PDSI (dashed line, right Y-axis). Bottom. Total number of saguaros established per hectare in the Tucson Mountain District (TMD – filled bars) and the Rincon Mountain District (RMD – open bars)  from 1950-2003 in relation to PDSI (dashed line, right Y-axis).

Saguaro establishment increased in all habitats when conditions were relatively wet (more positive PDSI values).  Under drought conditions, slopes had greatest saguaro establishment, while establishment increased more rapidly in foothills (and to a lesser extent in Bajadas) as moisture levels increased.

WinklerFig2

Model projecting number of saguaros established in the three major habitats in relation to PDSI.  Shaded regions are 95% confidence intervals.

The researchers were surprised at how tight the connection was between drought and saguaro establishment. But landscape features are also important.  The TMD is warmer and dryer than the nearby RMD, and had substantially lower establishment during the recent drought. The slopes in the RMD are steeper and rockier than sloped areas of the TMD, and may buffer saguaros from drought by capturing water in rock crevices and holding it for longer periods of time so it can be absorbed by saguaro roots. Nurse trees that provide shade to young saguaros may also be more common on the RMD slopes.

Winkler and his colleagues are concerned about the long-term impacts of climate change on saguaro populations, particularly in the drier areas of the TMD. They urge researchers to explore how long-term management of grazing and invasive species influences saguaro establishment across the landscape.  They also encourage researchers to gather some very basic data about saguaros, such as how they access water and how human water use patterns influence the water’s availability to this iconic species.

note: the paper that describes this research is from the journal Ecology. The reference is Winkler, D. E., Conver, J. L., Huxman, T. E. and Swann, D. E. (2018), The interaction of drought and habitat explain space–time patterns of establishment in saguaro (Carnegiea gigantea). Ecology 99: 621-631. doi:10.1002/ecy.2124. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Cat and fox: agents of Australian extinctions

Australia’s drylands are famous for their assemblage of ultra-cool mammals. As one example, it is difficult for us non-Australians to imagine a more endearing creature than the rock-wallaby pictured below.

Black-foted Rock-wallaby

Black-footed rock wallaby. Credit: Peter McDonald.

Unfortunately, numerous species of Australia’s dryland mammals are going extinct. Many of these extinct species weigh between 35 and 5500 grams – a weight range that researchers have described as the critical weight range (CWR). Peter McDonald and his colleagues wanted to know what was causing these extinctions, and why were they most prevalent in the CWR. They considered two hypotheses. First, perhaps the land was becoming less productive, either from habitat destruction by humans, or as a result of changing climate. Reduced plant abundance could cause herbivorous mammals to go extinct. An alternative hypothesis is that perhaps newly introduced predators, notably feral cats and red foxes, were killing the native mammals so effectively, that they were disappearing from the Ausralian drylands.

Previous research indicated that extinction rates were lower in areas that had more species living in trees and around rocks, leading McDonald to think that maybe habitat was influencing extinctions in important ways. In particular, he realized that rugged mountainous areas might have fewer predacious cats and foxes, and secondly that these two predators tend to go for prey within the CWR. Putting these ideas together, perhaps mountainous areas are refuges for Australia’s dryland CWR species, protecting them from predator-driven extinction. If so, mammal species richness would be highest in rugged, protected areas, and lowest in more open areas. If, on the other hand, mammals are going extinct because overall productivity is declining, we would expect overall species richness to be greatest in the most productive areas.
McDonald and his colleagues tested these two competing hypotheses by censusing mammals in four different types of habitats in Tjoritja National Park within the MacDonnell Range of central Australia. These were (1) mountain areas dominated by a sparse assemblage of shrubs and clumps of spinifex grass, (2) spinifex grasslands (with a more abundant cover of spinifex than found in the mountains), (3) Acacia shrublands, and (4) alluvial woodlands, which were most productive with richest soils.

 

 

Mountain refuge habitat_PeterMcDonald

Mountains. Credit: Peter McDonald

 

spinifex

Spinifex grasslands

acacia

Acacia shrubland

alluvial

Alluvial woodland

The researchers set up a variety of different mammal traps in 90 different sites representing these four habitats to capture and identify small mammals, and they detected larger mammals by searching for fresh scat at each site. The researchers estimated productivity with the normalized difference vegetation index (NDVI), which uses satellite imagery to measure the green-ness, and hence productivity, of a site or region.

In support of the predation hypothesis, more mammal species were found in the most rugged terrain.

McDonaldFig1A

Number of mammal species per site in relation to ruggedness of terrain. The curve is the fitted value of the regression equation.  The shaded area represents the 95% confidence interval.

In contrast to the productivity hypotheses, fewer mammal species were found in the most productive sites

McDonaldFig1c

Number of mammal species per site in relation to productivity of terrain as measured by the NDVI. The curve is the fitted value of the regression equation.  The shaded area represents the 95% confidence interval.

While it’s useful to evaluate both hypotheses by measuring current species richness, the researchers also needed to know how many species were actually driven to extinction in the time since cats and foxes invaded. They reconstructed historic species richness for each habitat based on subfossil remains (remains of organisms that are only partially fossilized), from indigenous knowledge supplied by aboriginal Australians, and from historical accounts in the early literature.

They discovered that CWR extinctions were most prevalent in alluvial (12/12 species) and acacia (7/7 species) habitats. Spinifex habitas lost 5/6 CWR species, while mountainous habitats only lost 2/6 CWR species. Importantly, species outside of the CWR have survived relatively well in all habitats, further implicating cats and foxes as the agents of extinctions.

McDonaldFig2

Current (extant) and historic (pre-invasion by cats and foxes) mammalian species richness in the four habitats. The dots are the mean weight, and the lines are the weight ranges for each species.  The shaded area represents the critical weight range (CWR)

More support for the the predation-habitat link comes from recent research that indicates that red foxes are absent from the mountain habitat, while feral cats are substantially less abundant. Even when present, cats are much less efficient hunters in the mountain habitat because the complex rock structure affords more refuges to prey items.

Feral cat with fat-tailed antechinus_NTG

Feral cat captured on camera with a fat-tailed Antechinus. Credit Tony Griffiths.

Across Australia, many CWR species have gone extinct in regions colonized by cats and foxes. McDonald and his colleagues provide solid evidence that these introduced predators are responsible for these extinctions. They urge researchers to explore other mountainous regions in Australia to see if they too are acting as refuges for CWR mammals.

note: the paper that describes this research is from the journal Conservation Biology. The reference is McDonald, P. J., Nano, C. E. M., Ward, S. J., Stewart, A., Pavey, C. R., Luck, G. W. and Dickman, C. R. (2017), Habitat as a mediator of mesopredator-driven mammal extinction. Conservation Biology, 31: 1183–1191. doi:10.1111/cobi.12905. doi:10.1111/cobi.12908. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2017 by the Society for Conservation Biology. All rights reserved.