When I chose hermit crabs as my topic for this month, I wasn’t exactly sure what I would find. It turns out that hermit populations are generally in good shape at this moment [although the popularity of hermit crabs in the aquarium industry does warrant a little concern- because of the effort and expense needed to provide hermit crabs with a supply of suitable shells, farmed crabs would be expensive, so the crabs continue to be collected from the wild (Calado et al. 2003)]. And some of our marine activities appear to benefit these scavengers- hermit crab biomass increased in response to fishing and trawling activities off the coast of the UK (Kaiser et al. 2000) and hermits not only ate the discards of fisheries in the Irish Sea, but also ate more in general in areas disturbed by fishing (Ramsay et al. 1997).
USFWS Photo- two hermit crabs in Baker Island National Wildlife Refuge
But while now seems okay, the future is less certain. Hermit crabs are limited on several levels by the availability of suitable snail shells (Mesce 1982)- crabs are more abundant, more reproductive, and grow faster when more good shells are available, and they will fight each other to get to better shells (Vance 1972). Once they have access to a suitable shell for their size and habitat, they tend to have a limited amount of time before the shell becomes too damaged or degraded for use. Kuris et al. (1979) found that shells demonstrated wear within 3 months of being inhabited by a hermit crab and were too damaged for use by the end of 9 months- at that rate, a constant sourse of good shells is imperative for any hermit crab population. Hermit crabs use chemoreceptors in their antennules to detect snail predation events (because dead snails mean new real estate may have come on the market) and to assess the quality of shells they encounter (Wilber and Herrnkind 1982).
Why is the future hermit crab housing market less than rosy? Ocean acidification. As atmospheric carbon dioxide levels have risen, levels of carbon dioxide dissolved in ocean water have also increased. This causes the water to become more acidic (with a lower pH), and we’ve already seen a 0.1 pH drop in ocean surface water in the past 100 years (Caldeira and Wickett 2003; Solomon et al. 2007). This trend is expected to continue and accelerate in the future. Why are acidic oceans bad for snail shells? Aquatic habitats with lower pH have less calcium carbonate available for those animals that use it to build shells and parts of their skeleton. When Hale et al. (2011) studied how increases in ocean acidification would affect marine invertebrates, molluscs (including snails) and echinoderms (like sea urchins) almost completely disappeared from the areas with a pH of 7.3 or lower. No snails means no snail shells for hermit crabs.
I had expected that a drop in ocean pH and the associated loss of access to calcium carbonate would cause problems for snails and, by extension, hermit crabs, but I hadn’t expected the results of de la Haye et al. (2011) when they looked at hermit crab behavior in lower pH water. Pagurus bernhardus crabs exposed to higher acid levels were less likely to change from an inadequate shell when presented with a better one, and took significantly longer to make the move if they did change shells. More time spent making the move means more time exposed to predators, which could be a problem. In addition, crabs in the more acidic environments moved more slowly and spent less time assessing the environment with their antennules. If lowered pH makes crabs slower and less aware of their environments, it could really change the way that they interact with other species and the roles that they play in ecosystems.
So, although I couldn’t find much in the way of immediate threats to hermit crabs, it seems like they are predicted to have a bumpy ride as climate change effects continue to develop. In my next post, I’ll try to get a handle on the nuances of those effects and ways that we might be able to counter them.
Works cited:
Caldeira, K and M Wickett. 2003. Anthropogenic carbon and ocean pH. Nature 425: 365.
Calado, R, Lin, J, Rhyne, AL, Araujo, R, and L Narciso. 2003. Marine ornamental decapods: popular, pricey, and poorly studied. Journal of Crustacean Biology 23: 963-973.
De la Haye, KL, Spicer, JI, Widdicombe, S, and M Briffa. Reduced sea water pH disrupts resource assessment and decision making in the hermit crab Pagurus bernhardus. Animal Behaviour 82: 495-501.
Hale, R, Calosi, P, McNeill, L, Mieszkowska, N, and S Widdicombe. 2011. Predicted levels of future ocean acidification and temperature rise could alter community structure and biodiversity in marine benthic communities. Oikos 120: 661-674.
Kaiser, MJ, Spence, FE, and PJB Hart. 2000. Fishing-gear restrictions and conservation of benthic habitat complexity. Conservation Biology 14: 1512-1525.
Kuris, A, Brody, M and J Carlton. 1979. Experimental field study of hermit crab resource utilization: snail shell wear and movement. Bulletin of the Ecological Society of America 60: 118.
Mesce, KA. 1982. Calcium-bearing objects elicit shell selection behavior in a hermit crab. Science 215: 993-995.
Ramsay, K, Kaiser, MJ, Moore, PG, and RN Hughes. 1997. Consumption of fisheries discards by benthic scavengers: utilization of energy subsidies in different marine habitats. Journal of Animal Ecology 66: 884-896.
Solomon, S et al. 2007. Technical summary. In: Solomon, S et al. (eds.), Climate Change 2007: the physical science basis. Contrib. Working Grp 1 to the 4th Assess. Rep. Intergovernmental Panel on Climate Change. Cambridge University Press.
Vance, RR. 1972. The role of shell adequacy in behavioral interactions involving hermit crabs. Ecology 53: 1075-1083.
Wilber, Jr., TP and W Herrnkind. 1982. Rate of new shell acquisition by hermit crabs in a salt marsh habitat. Journal of Crustacean Biology 2: 588-592.
Over the past month, I’ve packed up my belongings and then unpacked them again as I moved between apartments. That got me thinking about animals that travel with their ‘house’, and so my topic for this month’s posts is hermit crabs. This is a topic that is pretty well unknown to me.
Photo by USFWS Southeast Region
During a short period in my adolescence, I and my brother had ‘pet’ hermit crabs (if you can really call them pets); I don’t remember getting much out of that relationship. I see large numbers of hermit crabs when I’m doing field work on Louisiana’s islands, especially in the marshes, but I don’t know if those numbers have fluctuated recently. Some of my friends study the taxonomic relationships of specific hermit crab species, and so I know that there are some fierce debates raging about how to classify certain hermit crabs, but that really doesn’t tell me about the conservation status of these animals.
Because of their size and ability to blend into their surroundings, I think that hermit crabs can be easy to overlook (until you step on something that starts to move…), but my guess is that they play important roles in various ecosystems, and I imagine that global climate change could cause some problems for these animals as oceans warm and acidify. So this month you’ll get a chance to learn a bit about crustacean conservation and options for getting involved in their preservation.
Although some plant species are doing very well around the world (sometimes to the point of becoming invasive), other species are losing ground. In some cases there are focused recovery plans, but oftentimes the issue of habitat loss trumps our best efforts to sustain these populations. So how can we contribute to both habitat availability and species longevity? Luckily for us there are many ways to get involved.
Have some spare time on your hands?
Keeping an eye for plant species while you do other things outside?
Have a little land of your own to work with?
Whether you have time or space or both, you can make a contribution to plant conservation. Consider volunteering or gardening or just paying attention to the species you see in the wild. Sometimes, unless they are flowering or fruiting or grabbing at us with thorns, we don’t always notice each of the plant species around us, but each one is an important part of its ecosystem- we can all play a role in making sure that they continue to do so.
In my last post, I talked about some of the challenges facing endangered plant species as crucial elements of the ecosystems they inhabit, such as regular fires or insect pollinators, were lost. This may very well be the tip of the iceberg when you consider that imbalances within ecosystems can also result from too much of something– researchers in Florida found that Liatris ohlingerae, a rare aster, was not negatively impacted by insect herbivory, but white-tailed deer grazing both reduced and delayed flowering, so that fewer seeds were produced (Kettenring et al. 2009). With increasing or overabundant deer populations in many locations of the US, deer may be a real threat to many rare and endangered plant species. Likewise, rabbits on the island of Lampedusa heavily graze a variety of sensitive species there (La Mantia et al. 2012).
Mead’s milkweed (Asclepias meadii) needs different environmental conditions at different life stages.
So how do we keep things in balance for both flora and fauna? Researchers have been working very hard to answer this question, and they are definitely making progress. Some of their results that I find very exciting suggest that low-tech strategies can be really effective. Researchers looking at three endangered seashore plants along the Baltic found that large-scale mowing could be an effective means of maintaining populations for two of the species, while small-scale soil turning could also help two (Rautiainen et al. 2007). La Mantia et al. (2012) were able to restore specific plant populations on Lampedusa through netting to exclude rabbits, focused planting, microbial inoculation, and fences and mats to control erosion.
It’s also important to plan for maintaining gene flow and continuity between generations. Genetic analysis of the fir, Abies cilicica, in Lebanon suggested that corridors need to be maintained between the northern and southern populations to preserve gene flow and offer the opportunity for recolonization by the species (Awad et al. 2014). If populations become too isolated from each other, inbreeding depression could result. Once populations are established, it’s important to think on a multigenerational scale for management- the conditions needed by a young organism may not be the same as those required by an adult of the same species, as with Aslepias meadii Torrey ex. A. Gray, a federally threatened milkweed (Roels 2013), so ensuring that a protected area has a variety of habitats available with different levels and timing of disturbance can be very important.
And researchers are finding creative ways to ensure that endangered plants have the resources they need. Scientists working with the cave-dwelling perennial, Primulina tabacum Hance, in China found that transplanted plants only survived if they were moved to areas with moss, probably because the mosses helped maintained the necessary humidity level (Ren et al. 2010). Targeting areas with moss for plant reintroduction could substantially improve the results of reintroduction plans. Since space is often at a premium and some plants need habitats that are in the process of recovering from some type of disturbance, researchers in the Czech Republic have suggested that the military areas abandoned in Europe since the end of the Cold War could be used to provide habitat for specific plant, bird, and butterfly species (Cizek et al. 2013). Because these areas often provide a mosaic of different habitat types, they have plant and butterfly species richness comparable to or greater than current nature reserves, and are particularly helpful for species needing disturbed or woodland edge habitat. The trick now is to manage such areas so that they don’t all revert back to forest but continue to have a wide variety of habitat types.
So scientists are developing a greater understanding of how to protect and restore endangered plant species, and they are also working to develop early detection strategies so that we can act before the situation gets dire. Some of those efforts involve the public, and might focus around invasive threats to plants, as with the Maui Invasive Species Committee, or try to use botanical gardens like the Alexandru Borza Garden in Romania as nurseries for plants that can be reintroduced. It’s my guess that we all have a part to play in endangered plant conservation- for my next post I’ll look at the options we have for getting directly involved in these efforts.
Works cited:
Awad, L, Fady, B, Khater, C, Roig, A, and R Cheddadi. 2014. Genetic structure and diversity of the endangered fir tree of Lebanon (Abies cilicica Carr.): implications for conservation. PLoS ONE 9: e90086.
Cizek, O, Vrba, P, Benes, J, Hrazsky, Z, Koptik, J, Kucera, T, Marhoul, P, Zamecnik, J, and M Konvicka. 2013. Conservation potential of abandoned military areas matches that of established reserves: plants and butterflies in the Czech Republic. PLoS ONE 8: e53124.
Kettenring, KM, Weekley, CW and ES Menges. 2009. Herbivory delays flowering and reduced fecundity of Liatris ohlingerae (Asteraceae), an endangered, endemic plant of the Florida scrub. Journal of the Torrey Botanical Society 136: 350-362.
La Mantia, T, Messana, G, Billeci, V, Dimarca, A, Del Signore, MB, Leanza, M, Console, SL, Maraventano, G, Nicolini, G, Prazzi, E, Quatrini, P, Sanguedolce, F, Sorrentino, G, and S Pasta. 2012. Combining bioengineering and plant conservation on a Mediterranean islet. iForest 5: 296-305.
Rautiainen, P, Bjornstrom, T, Niemela, M, Arvola, P, Degerman, A, Eravuori, L, Siikamaki, P, Markkola, A, Tuomi, J, and M Hyvarninen. 2007. Management of three endangered plant species in dynamic Baltic seashore meadows. Applied Vegetation Science 10: 25-32.
Ren, H, Ma, Q, Guo, Q, Wang, J, and Z Wang. 2010. Moss is a key nurse plant for reintroduction of the endangered herb, Primulina tabacum Hance. Plant Ecology 209: 313-320.
Roels, SM. 2013. Influence of seed characteristics and site conditions on establishment of the threatened prairie milkweed Asclepias meadii. American Midland Naturalist 170: 370-381.
When I started looking at the literature on endangered plants, it seemed that many of the issues being faced by these species could be summed up by four main problems:
- Invasive species outcompeting and/or altering the habitat
- Loss of habitat through development (and this has particularly hit species with already small ranges- Leavenworthia torulosa, for example, is a species found in Kentucky, Tennessee, and Alabama. Of the two sites in Kentucky where it was known in the late 1970s, one had been reduced by 90% because bulldozing in the area led to soil becoming deeper as it washed down from upslope [Baskin & Baskin 1977]. As a result, other species were able to outcompete it.)
- Human management of the system changed annual environmental regimes (for example, suppressing regular fires)
- Loss/potential loss of pollinators
I’ve already spoken a bit about invasive species in previous posts, and loss of habitat is such a regular part of my blog that I feel I should give it a spot as guest editor. I haven’t focused as much on losing crucial elements of an ecosystem which keep processes going, and I think that’s a pretty typical failing of conservation education- we spend a lot of time looking at individual species that are in danger, and sometimes neglect to point out that any species in an environment is likely to be dependent on and required by others for survival. When it comes to endangered plants, there’s an even greater tendency to look at them in isolation because they are plants– they seem to grow quite efficiently in one place, produce many seeds, and produce their own food, so what could they possible need from the rest of the ecosystem?
Lomatium bradshawii populations were reduced by land development and fire suppression
But plants, like us, depend upon a complicated web of species and abiotic factors (like water flow and geology) to survive, and humans have done a very good job of altering some of the processes that had previously sustained plant (and, by extension, general) biodiversity. In some ways, the distribution of endangered plant species in the US echoes large human population movements (with accompanying land development and introduced species)- a 1997 report found that the states with the largest number of endangered plants were Hawaii, California, Florida, and Arizona (Dobson et al. 1997). In Florida alone, by 2003 407 species were considered endangered, a further 114 were threatened, and it was believed that 10 had become extirpated (Ward et al. 2003).
One of the big changes that we have made in a variety of habitats is to alter water and fire regimes. Whether it’s diverting water for irrigation of agricultural fields or suppressing fires to protect our property, we haven’t directly set out to put plant species on a path to extinction, but the ramifications of our actions have put a variety of species in peril. One example of this is Lomatium bradshawii (also known as Bradshaw’s desert parsley), an endangered plant found in only 16 locations in 1999, 15 of them in Oregon’s Willamette Valley (Pendergrass et al. 1999). These populations were both separated by a dense agricultural patchwork and deprived of regular fire events which help promote population growth (Kaye et al. 2001). When fall burning was reinstated in some plots, plant populations responded positively.
Environmental changes we’ve made (and that will continue to evolve into the future) have also interrupted the processes needed for some plant species to survive. Concerns over changes in timing and range of insect populations in response to climate change extend to pollinators needed by endangered plants. Clematis socialis, for example, is found in Alabama and Georgia with a flowering schedule that is linked to local temperatures (Wall et al. 2003). There are five insects that pollinate the flowers, but the two bees are particularly important. However, since climate change is expected to alter the timing of both plant flowering and peak insect populations, managing the ecosystem to sustain simply the plant and the most effective pollinator species may not be enough- the entire system needs to be preserved so that it can adapt to future changes. The idea of pollinator sustainability was also brought up by researchers focused on the endangered Eriogonum ovalifolium var. vineum in California- it’s not enough to give the plants space, we also need to keep the insect relationships going as well (Neel et al. 2001).
What I’m trying to illustrate here are some of the big patterns that scientists were looking at a decade or two ago with regard to endangered plants. In my next post, I’ll look at newer developments, both good and bad. As I move into more recent research, I have no doubt that new issues will arise, but I’m also pretty confident that these same problems will be perennial. Hopefully we’ve gained new understanding and new tools in addressing these issues, too.
Works cited:
Baskin, JM and CC Baskin. 1977. Leavenworthia torulosa Gray: an endangered plant species in Kentucky. Castanea 42: 15-17.
Dobson, AP, Rodriguez, JP, Roberts, WM, and DS Wilcove. 1997. Geographic distribution of endangered species in the United States. Science 275: 550-553.
Kaye, TN, Pendergrass, KL, Finley, K, and JB Kauffman. 2001. The effect of fire on the population viability of an endangered prairie plant. Ecological Applications 11: 1366-1380.
Neel, MC, Ross-Ibarra, J, and NC Ellstrand. 2001. Implications of mating patterns for conservation of the endangered plant Eriogonum ovalifolium var. vineum (Polygonaceae). American Journal of Botany 88: 1214-1222.
Pendergrass, KL, Miller, PM, Kauffman, JB, and TN Kaye. 1999. The role of prescribed burning in maintenance of an endangered plant species, Lomatium brawshawii. Ecological Applications 9: 1420-1429.
Wall, MA, Timmerman-Erskine, M, and RS Boyd. 2003. Conservation impact of climatic variability on pollination of the federally endangered plant, Clematis socialis (Ranunculceae). Southeastern Naturalist 2: 11-24.
Ward, DB, Austin, DF, and NC Coile. 2003. Endangered and threatened plants of Florida, ranked in order of rarity. Castanea 68: 160-174.
This is the only succulent found in the Gobi Steppe near Choyr, Mongolia- should we be concerned about its survival?
I was brain-storming ideas for blog topics and I thought it had been a while since talking about terrestrial plant conservation issues- given the importance of these primary producers in sustaining so many of the environments we depend upon, we should probably be paying attention to their conservation. But every plant issue I could think of at first was about invasive species, which lead me to wonder if there are endangered plant species. Since there are a variety of strategies that plants use to reproduce, including cloning and seed production, and some seeds can sit in the soil for years waiting for the right conditions to germinate, it seems like we should be able to maintain adequate populations of the plant species that make up different habitats.
Then I took a look at the USDA’s threatened and endangered plant database– I didn’t even make it to the Ds in the alphabetical list of perennial shrubs before realizing that there are much bigger problems in the plant world than I had expected. While many of the species on the list were from the Hawaiian Islands and Puerto Rico, suggesting that plant species confined to islands are particularly vulnerable, quite a few were from the continental US. This leads me to believe that there are many more plant species around the world whose populations are in trouble than we might think.
This month I’ll take a look at some of the plant species threatened with extinction around the world- my guess is that this a much bigger issue than can really be covered in a month’s posts, but at least it will get us started. And for those of you with green thumbs, this may give you ideas for working with plants outside of your gardens.
In my last post, I wrote about information gathered by researchers which was helping support tortoise populations and make conservation efforts more effective, but there is still a lot to be done for tortoises around the world. There are a variety of ways that we can each contribute to tortoise (and turtle) protection, whether from home or further abroad.
I want to contribute money:
I want to contribute time:
- The Galapagos Conservancy could use volunteers in their Fairfax, VA office.
- Likewise the Galapagos Conservation Trust is looking for office and data entry/management volunteers.
- The San Diego Zoo Global Wildlife Conservancy has a “I ____ for Wildlife” campaign- you pick the activity and create a fundraiser.
- Defenders of Wildlife would like you to sign an online petition to keep the Desert Tortoise Conservation Center in Nevada from being closed later this year due to lack of funding.
I want to contribute habitat:
I want to hang out with tortoises (and turtles):
- The California Turtle and Tortoise Club hosts special events and activities to promote turtle/tortoise protection, and you can volunteer to help with snapping turtle relocation.
- Arizona Turtle Conservation and Management has an Ornate Box Turtle Watch program to keep track of this species (and you can also adopt a turtle).
- The Turtle Survival Alliance is looking for volunteers at their center in South Carolina, as well as participants for regular turtle counts in Texas and Florida.
- Florida Fish and Wildlife has a gopher tortoise location app in the event that you see a tortoise while you are out and about, but they are also looking for help in collecting other gopher tortoise data.
I suddenly found a tortoise or turtle in the road:
- As it turns out, tortoises and turtles have a good sense of their home range and can be pretty persistent in trying to move throughout it- if they are moved to a new location, they will even attempt the trek back home. If you find a tortoise or turtle in the road, you can move it across the road in the direction it was heading (assuming that you aren’t putting yourself at risk in traffic or from the animal), but don’t move it any farther than that. If you move it away from its intended destination, it will simply try to cross again. Check out this Texas tortoise brochure from the Houston Zoo for more info.
So there are many ways to get involved in tortoise conservation- I’m sure that this list is far from complete, but hopefully it gives you a starting point for some tortoise adventures.
In my last post, I recounted some of the issues facing tortoises around the world: habitat loss, habitat degradation, disease, hunting, etc. Some of these problems have been impacting tortoises for a long time, while others are more recent in nature. The good news is that concerned individuals have been working to support tortoise populations for a long time as well, so we’ve got some good information to utilize.
And that’s a good thing, because some recent studies suggest that there are additional challenges facing tortoises. Invasive species are causing additional problems beyond the goats I had mentioned last time- in the Galapagos, for example, introduced rats eat tortoise eggs (Nelson 2007), among other things (a herpetologist friend once said that rats see tortoises as a “walking calcium source”). Fire ants here in the southeast US contribute to high mortality in hatchling gopher tortoises (Pike & Seigel 2006), and by high mortality I mean that predators, including mammals and ants, had eaten ALL hatchlings from the observed nests within 335 days of hatching- that doesn’t seem very promising for the future of that population. And researchers are beginning to understand how the underlying geology of a place can impact tortoise survival. Scientists in Spain found that soils with lots of loam and materials left behind by glaciers were more compacted and therefore harder to excavate for both burrows and egg-laying than soils with siliceous materials (Anadon et al. 2006), which means that tortoise populations restricted to those areas could face bigger challenges. Desert tortoises are also restricted by the slope of a landform (Latch et al. 2011), which means that if we place barriers like roads across the low-lying areas between two hills, we could be confining any tortoises in the middle to stay there because they won’t try climbing the hills.
This African spur-thighed tortoise is helping researchers understand the conditions that influence juvenile tortoise growth patterns
But we’re developing strategies to improve our knowledge and the survival of tortoises. Part of the answer is to ensure that tortoise populations have the habitat they need, and that means not just the space but also the plant communities and structure. Scientists investigating the impact of Upper Respiratory Tract Disease, which I mentioned last week, found that tortoises seemed to survive better in areas with lots of sites for basking (Ozgul et al. 2009), so managing habitat to create many open areas is important. A study of tortoise responses to prescribed burning to help manage habitat suggests that it’s a long-term process that will need multi-year plans (Yager et al. 2007), so it’s something that we have to really commit to. We can also help tortoises move between areas safely and have refuge from predators when they are most vulnerable. When researchers experimented with different types of road barriers to prevent highway mortality, they found that tortoises were very willing to follow fence lines and use culverts to go under the road, provided the fences had openings smaller than tortoise heads (Ruby et al. 1994). It may also be possible to improve juvenile tortoise survival by providing artificial shelters that are too small for their predators to enter (Ballouard et al. 2013).
There have been some tortoise conservation success stories, which is a comforting thought. By 1965 there were only 14 Espanola Island tortoises in the Galapagos, but thanks to a captive breeding program and a goat elimination program, the species was reintroduced to the island in 1975 and is now reproducing (albeit with a very un-diverse gene pool) (Milinkovich et al. 2004). Goats have also been removed from Pinta, Santiago, and parts of Isabela islands in the Galapagos, and genetic testing of tortoises throughout the islands suggests that there are some hybrids of different species which could be used to supplement declining or even ‘extinct’ species [the suggestion is that it would take a lot of work, but there are hybrids of the Floreans tortoise on Isabela (the species had been declared extinct in the mid-1800s) which could be carefully managed to eventually produce something close to a true Florean (Parham 2008)].
Researchers are working to prioritize tortoise populations and refine conservation methods, but they still have a long way to go, especially since the over-arching turtle group has the highest proportion of threatened species of any vertebrate group according to the IUCN. I’m sure that there are ways we can contribute to tortoise conservation, something I’ll explore in my final post of the month.
Works cited:
Anadon, JD, Gimenez, A, Martinez, M, Martinez, J, Perez, I, and MA Esteve. 2006. Factors determining the distribution of the spur-thighed tortoise Testudo graeca in south-east Spain: a hierarchical approach. Ecography 29: 339-346.
Ballouard, J-M, Caron, S, Lafon, T, Servant, L, Devaux, B, and X Bonnet. 2013. Fibrocement slabs as useful tools to monitor juvenile reptiles: a study in a tortoise species. Amphibia-Reptilia 34: 1-10.
Latch, EK, Boarman, WI, Walde, A, and RC Fleischer. 2011. Fine-scale analysis reveals cryptic landscape genetic structure in desert tortoises. PLoS ONE 6: e27794.
Milinkovitch, MC, Monteyne, D, Gibbs, JP, Fritts, TH, Tapia, W, Snell, HL, Tiedemann, R, Caccone, A, and JR Powell. 2004. Genetic analysis of a successful repatriation programme: giant Galapagos tortoises. Proceedings: Biological Sciences 271: 341-345.
Nelson, B. 2007. Tortoise genes and island beings: giant Galapagos reptiles on the slow road to recovery. Science News 172: 298-300.
Parham, JF. 2008. Rediscovery of an “extinct” Galapagos tortoise. PNAS 105: 15227-15228.
Pike, DA and RA Seigel. 2006. Variation in hatchling tortoise survivorship at three geographic localities. Herpetologica 62: 125-131.
Ozgul, A, Oli, MK, Bolker, BM, and C Perez-Heydrich. 2009. Upper Respiratory Tract Disease, force of infection, and effects on survival of gopher tortoises. Ecological Applications 19: 786-798.
Ruby, DE, Spotila, JR, Martin, SK, and SJ Kemp. 1994. Behavioral responses to barriers by desert tortoises: implications for wildlife management. Herpetological Monographs 8: 144-160.
Yager, LY, Hinderliter, MG, Heise, CD, and DM Epperson. 2007. Gopher tortoise response to habitat management by prescribed burning. The Journal of Wildlife Management 71: 428-434.
In some ways tortoises represent an old conservation issue- as early as the 1800s, for example, people were concerned about overharvesting of giant tortoises on islands such as the Galapagos and Seychelles. Because they can be easy to approach (one study I read used the term ‘run’ with a special citation because none of these animals are speed demons) and some people find these animals fascinating, their conservation issues have gotten some attention [although probably not enough considering that I found the statement in one study that all existing species are in decline (Jacobson 1994)].
We found this tortoise walking down one of the trails at a study site in the Bolivian rain forest.
As long-lived, slow-growing animals, tortoises are vulnerable to population declines because it can take them so long to recover. One study in Spain found that the average number of eggs laid by a female spur-thighed tortoise was 3.39 (Diaz-Paniagua et al. 2001)- when you consider how vulnerable young tortoises are, that’s not a lot. What this means is that tortoise populations really rely on adults to survive year after year so that they have enough chances to contribute to the next generation- it also means that tortoises aren’t great at recovering from a rapid population decrease. Why would tortoises opt for the slow route to population growth? Well, these animals tend to live in arid environments. Remember my surprise at finding tortoises in the Bolivian lowlands? It turns out that some tortoise species live in areas with more precipitation but with sandy soils that create locally dry conditions- the gopher tortoise of the southeastern US is a good example of this (Diemer 1986). Because tortoise survival is highly dependent on plant productivity, which in turn is dependent on rains that may not be consistent from year to year, it makes sense to produce a few eggs every year, that way your losses are minimized in a bad year (which would be hard to predict at the outset) and you have some progeny hatching during the good years.
What are the main threats to tortoise populations around the world? A lot of the material I found concentrated on tortoises here in the US, and I think we could safely say that some of the issues facing tortoises vary from place to place and within time, but the main problems seem to boil down to habitat degradation and loss (it’s becoming my perennial refrain, right?). That habitat loss and degradation has taken different forms in different places. For example, in the Galapagos introduced goats competed with tortoises for forage and killed the larger plants that tortoises depended upon for cover (remember that tortoises draw heat from their environment, so they need a certain amount of exposure to sunshine, but too much is a bad thing) (Milinkovitch et al. 2004). In the southeast US, fire suppression has let closely-spaced trees create a closed canopy that reduces tortoise food at the ground level (Jones & Dorr 2004). Throughout the world, our development of arid lands both takes away tortoise habitat and creates barriers to tortoises that are trying to move between smaller habitat patches, whether in search of food or mates. According to Ruby et al. (1994), highways represent a significant source of adult desert tortoise mortality. To these habitat issues, we can also add hunting (or just shooting, as 14.3% of desert tortoise carcasses recovered by Berry (1986) in California between 1976 and 1982 had evidence of gun shots), collection for the pet trade, and the emergence of Upper Respiratory Tract Disease in multiple populations, which may have been partially spread by released pet tortoises (Jacobson 1994).
Why should we care about tortoise populations? Well, aside from the fact that they are simply pretty neat animals, there are a number of ways in which tortoises contribute to the vibrate ecosystems around them. Since tortoises have to use exposure to sun and shade to regulate their body temperature, many species construct burrows for use during very hot or very cold periods. The areas they excavate for burrows tend to have a higher number of plants which sprouted from seeds and more herbaceous cover than surrounding areas because of the soil disturbance created by burrow construction (Kaczor & Hartnett 1990)- this makes for a more diverse habitat that can meet the needs of more species. These burrows also offer shelter to a number of animals [at least 19 species used gopher tortoise burrows in central Florida (Witz et al. 1991)]. Some tortoises eat a lot of fruit, making them important seed dispersers (Kaczor & Hartnett 1990).
Tortoises play the long game– they live in dynamic landscapes that experience periods of drought and periods of plenty. Their strategy seems to be to wait out the bad times (Longshore et al. 2003), relying on high adult survival rates to keep the population going. Unfortunately, we have done a very good job of limiting the resources that are available to them at any one time through habitat change, as well as directly using tortoises for food and pets (plus target practice). Luckily, we have put a substantial amount of effort in trying to understand how to help these animals survive in the landscapes we now dominate- for my next post I’ll look at more recent research into tortoise conservation.
Works cited:
Berry, KH. 1986. Incidence of gunshot deaths in desert tortoise populations in California. Wildlife Society Bulletin 14: 127-132.
Diaz-Paniagua, C, Keller, C, and AC Andreu. 2001. Long-term demographic fluctuations of the spur-thighed tortoise Testudo graeca in SW Spain. Ecography 24: 707-721.
Diemer, JE. 1986. The ecology and management of the gopher tortoise in the southeastern United States. Herpetologica 42: 125-133.
Jacobson, ER. 1994. Causes of mortality and diseases in tortoises: a review. Journal of Zoo and Wildlife Medicine 25: 2-17.
Jones, JC and B Dorr. 2004. Habitat associations of gopher tortoise burrows in industrial timberlands. Wildlife Society Bulletin 32: 456-464.
Kaczor, SA and DC Hartnett. 1990. Gopher tortoise (Gopherus polyphemus) effects on soils and vegetation in a Florida sandhill community. American Midland Naturalist 123: 100-111.
Longshore, KM, Jaeger, JR, and JM Sappington. 2003. Desert tortoise (Gopherus agassizii) survival at two eastern Mojave Desert sites: death by short-term drought? Journal of Herpetology 37: 169-177.
Milinkovitch, MC, Monteyne, D, Gibbs, JP, Fritts, TH, Tapia, W, Snell, HL, Tiedemann, R, Caccone, A, and JR Powell. 2004. Genetic analysis of a successful repatriation programme: giant Galapagos tortoises. Proceedings: Biological Sciences 271: 341-345.
Ruby, DE, Spotila, JR, Martin, SK, and SJ Kemp. 1994. Behavioral responses to barriers by desert tortoises: implications for wildlife management. Herpetological Monographs 8: 144-160.
Witz, BW, Wilson, DS, and MD Palmer. 1991. Distribution of Gopherus polyphemus and its vertebrate symbionts in three burrow categories. American Midland Naturalist 126: 152-158.
How I wish I had gotten some video of this!
A few days ago I was at a friend’s house watching his little brother hold a small tortoise (2 years old but small enough to fit in the palm of his hand). I realized that, as much as I am fascinated by tortoises and find the majority of them incredibly cute, I know very little about their ecology or conservation status. This feels a bit strange since I have cared for African spur-thighed tortoises in Texas (the day I walked in and found them lined up staring at the wall, I got a little concerned about their ability to strategize…) and the best battle I have ever watched was a slow-motion crash between two tortoises in Bolivia which ended in one being turned over (I laughed for a while after it was over).
With regard to conservation, I believe that there are concerns about the gopher tortoise in the southwest and California, and I know that some species of tortoises from the Galapagos have gone extinct since human discovery of the islands. All in all, that’s not a lot. In fact, I had been surprised to find tortoises in the rain forest of Bolivia, because I had thought they were generally in dry habitats. So this month I am going to focus on tortoise populations around the world. Given how little I know, it’s bound to be pretty informative for me- hopefully it is just as interesting for you!
