Download Proposal - Texas Water Resources Institute

Brian Langerhans
Conservation Biology of Endemic and Endangered Livebearing Fish of Texas
Human use of water resources have numerous and varied consequences on aquatic
ecosystems. Activities such as damming, aquifer pumping, pollution, and introduction of
non-native species often produce profound effects on freshwater ecosystems in Texas.
Major alterations in freshwater ecosystems involve changes in species abundance and
composition1 . This can include modification of system components at all trophic levels
(e.g. periphyton, zooplankton, mollusks, insects, fish). These alterations affect the overall
health of the ecosystem, as well as the basic ecology of many constituent species (e.g. life
history, predator-competitor interactions).
In many areas of Texas, water is drawn from aquifers at a rate faster than rainfall can
replenish it2 . This has led to the decline of spring- fed waters that provide critical habitat
for many native species. Additionally, many of these areas have suffered introductions of
non-native fish that consume aquatic vegetation and compete with or prey on native
species. These activities have contributed to the dramatic decline of native populations of
six endemic species of the genus Gambusia (Table 1). Gambusia are small, freshwater
fishes in the livebearing family Poeciliidae. Of these six native fish species, one species is
presumed extinct, another possibly extinct, one extirpated in Texas and three are federally
endangered. The primary causes of their decline are believed to be decrease in spring
flows, competition and hybridization with a congener (Western mosquitofish, Gambusia
affinis; often introduced), and predation by native and introduced species (e.g.
largemouth bass, Micropterus salmoides, green sunfish, Lepomis cyanellus).
Table 1. Gambusia Species of Concern
Scientific Name
Common Name Listing Status
Current Range
Gambusia
amistadensis
Amistad
gambusia
Delisted, presumed
extinct
Presumed extinct.
Gambusia gaigei
Big Bend
gambusia
Federally
Endangered, 1967
Big Bend National Park,
TX
Gambusia
georgei
San Marcos
gambusia
Federally
Endangered, 1980
San Marcos River, TX;
extinct?
Gambusia
heterochir
Clear Creek
gambusia
Federally
Endangered, 1967
Clear Creek, TX
Gambusia nobilis Pecos gambusia
Federally
Endangered, 1970
Jeff Davis and Pecos
counties, TX
Gambusia senilis
State Threatened
Devils River, TX extinct; Mexico
Blotched
gambusia
My research seeks to provide a sound functional basis for the conservation of endangered
Gambusia species. I plan to investigate the major biotic interactions of the closely related
but non-endangered congener, Gambusia affinis, to elucidate possible causes of decline
and potential routes of conservation for the endemics. Since G. affinis is highly similar in
all respects (i.e. morphology, life history, ecology) to the endangered gambusia species,
my research should provide crucial insight into the functional ecology of endangered
species without harming any threatened animals. My research involves two facets: (1)
detailed morphological analyses of native gambusia (all species) and other
representatives of the family and (2) mesocosm study investigating predator impacts and
multiple predator effects in the non-endangered species.
Morphological analyses–I will obtain museum specimens of all gambusia species native
to Texas (eight species) and conduct a morphological comparison using geometric
morphometrics. Additionally, I will include other representatives of the family in which
much is known of their ecology (e.g. Poecilia reticulata, Gambusia hubbsi). To date,
there has been no major morphological analysis of this sort. The study will capture high
resolution discrimination of morphologies that has significant implications for ecology of
endangered fish. For example, I can apply intraspecific morphological diversification
related to predation that I recently discovered in three species in the family3 to
interspecific diversification discovered in this study. This could produce important
indirect evidence of competition and evolutionary history with (or without) predators.
Mesocosm study–I will conduct an experiment in 200-gallon aquaria constructed with
three depth levels, sand substrate, and aquatic vegetation to examine the effects of
multiple predators on gambusia. This study will investigate the impacts of largemouth
bass and dragonfly larvae (Anax sp.) on mortality, morphology and behavior of G. affinis.
The experiment will consist of four treatments (no predators, bass, Anax, and bass and
Anax) with 20 individuals of G. affinis in each treatment. Predation will be allowed for 24
h with ten replicates of each treatment. This study has two primary goals: (1) determine
magnitudes of impacts of multiple predators and (2) determine morphological and
behavioral traits that increase survivorship in G. affinis.
Many ecological communities are characterized by emergent properties resulting from
interplay between component biotic mechanisms. Predation is a primary structuring force
in aquatic communities and often involves complex interactions 4 . A multiple predator
effect (MPE) occurs when interactions at work within a community do not reflect the sum
of the individual effects (Fig. 1). Emergent impacts may occur in this system due to
functional tradeoffs in the behavior and morphology of gambusia species. In the presence
of predatory fish, hiding in aquatic vegetation should increase survivorship. But in the
presence of dragonfly nymphs, hiding in aquatic vegetation should increase vulnerability
to predation. Additionally, larger prey may avoid predation by Anax but may be more
susceptible to predation by visually-cued fish predators. So the impact of predation on
gambusia species may be greater than traditionally assumed. My previous experience
with mesocosm studies should facilitate the execution of this experiment 5 .
My proposed research should greatly increase the ecological understanding of
endangered Gambusia in Texas. For example, the mesocosm study might reveal the
importance of aquatic vegetation for survival of gambusia, and the morphological
analyses could reveal that several endangered gambusia seem hyper-vulnerable to
predation. These results focus conservation efforts on providing aquatic vegetation and
combating predation. These factors prove especially important considering most native
habitats of the endangered gambusia suffer introduced predators and declining aquatic
vegetation due to decreasing spring flow and introduced fish species (e.g. common carp,
Cyprinus carpio, grass carp, Ctenopharyngodon idella).
My research should provide a better understanding of biocomplexity in the environment,
which will allow for more informed conservation, management and policy strategies.
While it emphasizes the biotic interactions of these freshwater ecosystems, the abiotic
factors of aquifer use, dam construction, fish introductions, and related activities largely
dictate what interactions might be present. With this research, I hope to provide insight
into important ecological mechanisms in order to produce more efficient and successful
conservation programs.
Literature Cited
1
Jackson, D.A., Peres-Neto, P.R. and Olden, J.D. 2001. What controls who is
where in freshwater fish communities - the roles of biotic, abiotic, and spatial factors.
Canadian Journal of Fisheries and Aquatic Sciences 58, 157-170.
2
National Wildlife Federation Report. 2002. Population, water, and wildlife.
3
Langerhans, R.B., Layman, C.A., Shokrollahi, A.M. and DeWitt, T.J. Predatorassociated morphological divergence among populations of livebearing fishes. To be
submitted to The American Naturalist.
4
Sih, A., Englund, G. and Wooster, D. 1998. Emergent impacts of multiple
predators on prey. Trends in Ecology and Evolution 13, 350-355.
5
Langerhans, R.B., DeWitt, T.J. and Layman, C.A. Multivariate selection and
emergent ecological impacts of multiple predators. To be submitted to Ecology.