Top Threats

The arroyo toad's biggest threat is loss of habitat. Loss of habitat is a pretty broad term, and a lot of different things can lead to loss of habitat such as development where the actual habitat is used up, fragmentation where the remaining habitat is not as high quality and dams that change the flow regime upstream and downstream in the toad's habitat. The figure below illustrates some of those threats.

a. Natural habitats

b. Top threats to natural habitats

Arroyo toad's natural habitats needed for survival and the top threats facing them. a. The natural habitat of the arroyo toad, needing both riparian habitats for breeding and uplands for forging, burrowing, and connectivity to other arroyo toad populations. b. Some of the effects of human development on their habitat, loss of habitat for development and human activity, fragmentation, and dams. Synthetic figure.

 

 

Habitat loss 

 

Arroyo toads have disappeared from as much as 65 percent of their historic range (9). Development and urban sprawl has been huge in Southern California in recent years. The Arroyo Toad used to be widespread, but now it is only found in 22 river systems. It should be noted that adult survival may depend on the upland areas for burrowing and surface activity and not just the waterways where the toads are found breeding (8).
 

Fragmentation of habitat  

 

http://robertgrooms.wordpress.com/2011/11/

When development occurs and people build roads, neighborhoods, and other structures, the habitat for the species that lived in those areas not only shrinks, but we get habitat fragmentation, a term used to describe discontinued fragmentation of preferred habitat. A habitat cut in half or into pieces by roads or other man made development do not have nearly the same quality as a habitat that is whole. This fragmentation can lead to a further reduction in population numbers (8) and populations may not be able to access each other any more, which leads to further complications with small populations. Fragmentation also causes what is called edge effects. Edges of habitat are drier, windier, brighter, warmer, and nosier. It also can increase exposure to predators and invasive species. All of which can impact a species likelihood of prospering in that habitat (7). The more fragmentation and the smaller the fragmented pieces, more edges will exist and the greater the edge effects will be.

Dams

 

Englebright Dam in California

US Army Corp of Engineers, Sacramento District

Dams have been a major threat to the Arroyo Toad. Of the 22 rivers and streams that the toad is known to inhabit today, only 2 of them do not have dams (3). A large portion of the initial recorded mortality is from the initial construction of the dams, but their breeding habitat is also threatened. The Arroyo Toad relies on shallow waters to lay their egg masses, and dams affect the flow of water, often making shallow waters change or disappear entirely. Dams also raise water temperature, increase accumulation of pollutants, cause algae blooms, and reduce the natural flow of nutrients and sediments. Steps can be made to make dams less destructive to shallow waters upstream and downstream from them (3).

Other changes to the hydrology

 

A side effect to urbanization is changes in the general hydrology of the land. The water table is lower, making it less likely to find water and causing rivers to dry up earlier in the year. Forcing water to flow a certain way often benefits communities of people - they want to get the water as quickly as possible. However, a side effect of this impatience is funneling all the water into narrow channels changes the natural flow of water upstream and downstream, sometimes reducing the amounts of water in certain areas or reducing the overall abundance of shallow pools (6).

Invasive species

 

Giant reed overgrowing floodplains at San Francisquito Creek

© 2003 BonTerra Consulting

Invasive species are organisms that have been introduced into a habitat, usually by humans, that do not belong there and out compete native species. Some exotic species of fish and other amphibians may eat arroyo toad larvae. Also, exotic plants, such as giant reed (Arundo donax), grow densely in the floodplains where arroyo toads live, changing the density of native vegetation and the flow of water (8).

Small population sizes create problems on its own

When a population is reduced to small numbers, like the arroyo toad has been, there are some problems that start to become and issue simply because the population is too small.

A lack of genetic diversity, both from small population sizes and lack of connectivity between populations, can cause inbred depression and limits the ability of the population to adapt to things like diseases or climate change. When a population is reduced to small amounts and causes a bottleneck in population, then genetic diversity recovers slowly (7). 

Population sizes naturally fluctuate due to random variations from things such changes in predation, the ratio of males and females born, disasters such as flooding or storms, or changes in temperature or rainfall. This impacts are not that deeply felt in a large population, and the populations average size stays largely the same from year to year. However, a small population has to deal with these random variations as well, and the impact is sometimes huge. If the variation is big enough, it can even lead to extiniction (7).