Download PROJECT DESCRIPTION Fig. 1. Theory for how temperature and

PROJECT DESCRIPTION
Understanding the responses of species, communities and ecosystems to climate change is a fundamental
concern of biodiversity and conservation science. Predicting which taxa will be vulnerable as climate rapidly
changes is a grand scientific challenge, underscoring the need for models of future biodiversity loss. Responses of
species to future conditions depend on the interaction between the magnitude of change and the relative
sensitivity of species; however, to date, most efforts to quantify species vulnerability have focused on the
magnitude of temperature change and consequent shifts in species’ latitudinal and elevational ranges. Although
useful, this framework for modeling biodiversity loss suffers from the lack of mechanistic, biophysical
understanding of species sensitivity to abiotic change. We propose that species sensitivities to climate change
derive in large part from the physiological and dispersal traits that have evolved in response to past
selection in different environments. For example, recent studies of ectotherms show that stenotherms from the
warm tropics and cold polar regions are more sensitive to changes in temperature compared to eurytherms, and
these same patterns occur along altitudinal gradients. Thus, understanding
how historical processes have shaped present biodiversity and its sensitivity to
climate shifts is key to predicting species loss in a rapidly changing world.
Current models of biodiversity loss also fail to integrate the ecological context
into projections, even though species clearly have ecological traits that define
their roles in ecosystems, and that altering ecosystem processes with climate
change are likely to influence species vulnerability.
Here, we propose a novel, integrative framework for predicting
biodiversity vulnerability to climate change that is both mechanistic and
general in its formulation, but applied specifically to tropical and
temperate streams. Our organizing principles (Fig. 1) are: (1) taxonomic
diversity encompasses key organismal traits that determine physiological
sensitivity to environmental stressors (e.g. thermal and hypoxia tolerance) and
control the dispersal of organisms to favorable localities; these traits have
evolved in response to historical selection by temperature (T) and disturbance
(D) regimes; (2) T and D also regulate ecosystem processes, which provide an
ecological context (E) for species interactions according to their traits (Fig.
1A); (3) TxDxE varies broadly across latitude and elevation to strongly shape
(and explain) spatial patterns of trait combinations, species diversity and
community composition within tropical and temperate streams (Fig. 1B); and
(4) T and D are the very same environmental variables that will be altered by
rapid changes in climate (Fig. 1C), thus leading to local-regional changes in
TxDxE that will redefine the combination of traits favored in future species
communities (Fig. 1D). To test the assumptions and hypotheses represented in
Fig. 1. Theory for how temperature
and disturbance determine the
Fig. 1, we will integrate taxonomic discovery with field measurements of key
diversity of species traits and
functional physiological (thermal, hypoxia) and genetic traits for selected
ecosystem function and how this in
species that span ecological roles. This will provide insight into how increases
turn mediates effects of rapid
in temperature (T) and/or hydrologic disturbance (D) will cause trait shifts in
climate change. (A) Temperature
and disturbance affect the genetic,
local stream communities under different ecosystem contexts (E). A
taxonomic, and functional
quantitative, experimental, and mechanistic understanding of trait-based
(physiological) trait diversity of
sensitivities of species defined in terms of the key drivers of climate change,
species and ecosystem properties.
combined with models of future climates, will allow us to predict
Species traits also influence
vulnerabilities of stream species, communities and ecosystems to climate
ecosystem dynamics, and vice
versa. (B) Spatial variation in
change along existing gradients of temperature (latitude, elevation) and
temperature and disturbance over
precipitation (hydrologic) regimes.
evolutionary history determine the
We focus on tropical streams (Ecuador) and mid-latitude temperate streams
current distribution of species with
(Colorado)
that span alpine-to-foothills (>1800 m) temperature and
site-matched traits. (C) Species
traits will affect their sensitivity to
disturbance gradients. Our focal taxa are aquatic insects in CO and EC, and
rapid climate change. (D)
amphibians in EC. Importantly, the taxonomic, genetic and functional
Examples of possible shifts in trait
diversity of stream-dwelling organisms is largely undescribed at the species
space.
level, resulting in debate about whether western hemisphere tropical streams
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have higher or lower aquatic insect species richness than mid-latitude temperate streams; a question that will only
be resolved with focused taxonomic discovery efforts. Likewise, stream-dwelling amphibians have not been
systematically sampled, despite the fact that some stream amphibian populations have experienced catastrophic
declines and are of conservation concern. Insects and amphibians are dominant components of stream
communities worldwide, but are conspicuously missing from recent taxonomic reviews of vulnerability to climate
change. Thus, our research will address a key dimension of unknown taxonomic biodiversity.
The central tenet underlying our work is that species that evolved in thermally stable environments, such as the
tropics, or low disturbance regimes, such as perennial, groundwater-dominated streams, will be more sensitive to
increases in temperature and altered precipitation patterns (which directly affect flow disturbance). Further,
species that occupy narrow elevation bands should have reduced thermal tolerance to increasing temperature than
those occupying broader bands, and narrow-ranging species are expected to show reduced dispersal ability
compared to wide-ranging species, reducing chances of emigration to suitable habitats and increasing their
vulnerability. In aquatic systems, dissolved oxygen is yet another key physiological dimension to a species’
response to temperature; dissolved oxygen availability declines nonlinearly with elevation (due to the
combination of reduced atmospheric partial pressure and temperature-dependent dissolution rates). Therefore, the
physiological sensitivity of species to warming will also involve sensitivity to hypoxia. Such oxygen-limited
thermal tolerance has recently been demonstrated for stenothermal marine organisms, but has not yet been studied
for stream insects and amphibians that live along elevational gradients that co-vary in temperature and dissolved
oxygen. Pioneering work by team member Jacobsen shows variation in respiratory response for aquatic insect
morphospecies at different elevations in Ecuador. This work indicates that taxa in high elevation streams
experience constantly low oxygen availability (ca. 61% at 4000 m), and even under natural conditions they are
living "near the edge" of oxygen limitation (reflected also by a strong reaction of highland communities to oxygen
reduction due to organic pollution). For example, if future warming of 3°C drives a species 600 m uphill (to
maintain thermal optima), it might need to cope with a decline in oxygen saturation of up to 8%. Depending on
temperature-dependent metabolic demands, species at high elevation might be faced with a trade-off to persist
under climate warming: move uphill in response to thermal sensitivity or move downhill in response to hypoxia
sensitivity. This potentially crucial aspect of climate change has been previously overlooked.
Our research has several novel components: We are the first to link temperature and hydrologic disturbance to
explore stream species and ecosystem responses to climate change. Second, we integrate physiological (thermal
and hypoxia) performance with measures of dispersal ability to understand sensitivity to projected changes in
TxD. Third, we explicitly incorporate ecosystem context to explore how loss of physiologically sensitive species
will affect trophic structure and ecosystem processes in whole stream ecosystems at different elevations. Fourth,
we test theoretical predictions that species turnover along elevation gradients varies with physiological tolerance
and dispersal ability, and that this relationship varies between temperate and tropical systems. We will test this in
detail across elevation at focal sites CO and EC, and more broadly at four secondary sites that span a large
latitudinal gradient. In sum, we will integrate the fields of physiological ecology, population genetics, community
and ecosystem ecology to evaluate species sensitivity to temperature and hydrologic disturbance in temperate and
tropical streams. This, combined with a GCM-based modeling exercise to project future temperature,
precipitation, and hydrologic disturbance conditions, will yield GIS-based vulnerability maps for aquatic insects
and amphibians under scenarios of projected climate change.
Our likelihood of success is enhanced by our broadly-trained team of scientists (PIs and Senior Personnel) with:
(1) field experience in temperate (Poff, Flecker, Funk, Bernardo, Kondratieff) and tropical (Flecker, Ghalambor,
Thomas, Funk, Jacobsen, Guayasamin, Dangles, Encalada, Zamudio) streams; (2) experience in physiological
performance measurements (Jacobsen, Ghalambor, Bernardo) and population genetic analyses (Funk, Zamudio);
and (3) expertise in stream insect and amphibian systematics (Kondratieff, Funk, Guayasamin, Haddad). Our
research team includes individuals experienced in mesocosm and whole-stream, ecosystem scale experiments
(Flecker, Thomas, Poff), hydrologic-climate modeling and vulnerability mapping (Wickel, Matthews, Poff), and
policy and governance outreach (Matthews). Thus, our team covers the breadth of expertise required to
successfully integrate taxonomy, population genetics, physiology, ecosystem function and climate modeling.
CONCEPTUAL AND THEORETICAL BACKGROUND
We adopt a traits-based approach to understanding patterns of stream biodiversity and vulnerability. Species
responses to climate change are mediated by several key traits, including behavioral and physiological sensitivity
to new environmental conditions, dispersal ability, and a species’ ecological role. This complexity requires that
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we model species sensitivity in an integrative fashion, spanning individual, population and ecosystem levels of
organization. A traits-based approach provides a common currency to integrate evolutionary and ecological
perspectives on species sensitivity to climate change. For example, historical temperatures have been argued to
drive the evolution of physiological tolerances and patterns of dispersal and genetic exchange along latitudinal
gradients, leading to global scale patterns of species richness. Within latitudinal zones, spatial variation in local
environmental dynamics and biotic interactions act as filters on ecological traits to define species distributions.
Thus, species are expected to show geographic variation in vulnerability to climate change, according to their
degree of physiological specialization, dispersal ability or trophic position (shaped by evolutionary and ecological
processes) relative to the magnitude of environmental change.
In streams, temperature and hydrologic disturbance are important selective agents and/or ecological filters for
physiological and ecological traits, thus providing a basis for understanding patterns of stream biodiversity and
ecosystem function. Changes in temperature and hydrologic disturbance will induce species shifts in accordance
with trait characteristics and this will have community and ecosystem consequences. Below, we briefly develop
this theoretical framework in more detail.
Theory: Historical temperature and disturbance variability as determinants of biodiversity
The “Climate Variability Hypothesis” (CVH) posits that the width of
thermal tolerance exhibited by a species reflects the magnitude of climatic
variation it has historically experienced. Comparisons of temperate and
tropical species play a central role in testing this hypothesis because
temperate environments exhibit greater seasonal variation in temperature
compared to tropical ones (Fig. 2a). Indeed, we now have empirical
support for a wide range of temperate species having broader thermal
tolerance compared to tropical species (Fig. 2b). Latitudinal variation in
thermal tolerance, in turn, has implications for dispersal ability, genetic
structure of populations, degree of local adaptation, range limits and
community membership, particularly with respect to elevational changes.
Janzen developed this hypothesis and proposed that seasonal uniformity of
temperature at tropical localities would reduce overlap in temperature
regimes at lower vs. higher elevation (Fig. 2a) and select for organisms
with limited acclimation ability and narrow (locally adapted) tolerance to
temperature compared to temperate localities (Fig 2b). He further linked
Fig. 2. The Climatic Variability
these assumptions and predicted that tropical organisms would have more
Hypothesis (CVH) predicts a
limited dispersal along elevation gradients (Fig. 2c) because they would be
positive relationship between the
more likely to encounter climates to which they were not adapted.
range of thermal tolerance and
latitude due to greater seasonal
Reduced dispersal and narrow tolerances in the tropics across elevation
variation in temperature in the
barriers (Fig. 2c) in turn should lead to greater genetic divergence among
temperate zone compared to the
populations and increased local adaptation, resulting in greater species
tropics. This should result in lower
packing along altitudinal gradients (Fig. 2d). Indeed, recent reviews
dispersal and narrow elevational
confirm many of Janzen’s predictions (see Ghalambor et al.). Additional
ranges in the tropics. Dispersal
should also decrease with increasing
work shows that thermal acclimation is reduced among tropical
elevation due to increasing isolation
stenotherms. Dispersal and gene flow increase with increasing latitude,
in stream networks.
resulting in reduced population genetic structure at high latitudes. In
addition, tropical species occupy smaller elevational ranges and exhibit
higher species turnover. The CVH provides a set of testable assumptions and predictions that link climate,
physiology, population genetic structure, and patterns of biodiversity under a common conceptual framework.
Linking seasonality in climate, with altitudinal and latitudinal variation in temperature, is essential for our
understanding of how stream systems will respond to climate change across latitudinal gradients. Clearly, stream
temperature varies with latitude and altitude and is a key factor limiting the ranges of ectotherm species due to
physiological function, growth rate and developmental rates. Based on temperature alone, predictions have been
made about range shifts of aquatic insects under climate change. Yet, the role of temperature in shaping the
distributions of ectotherms along elevational gradients remains largely unexplored. Some studies suggest that
tropical stream insects (morphospecies) may have higher species turnover than temperate insects, and that
amphibians have higher species turnover across elevation in the tropics. Nonetheless, how physiological tolerance
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(i.e., thermal and hypoxia tolerance) varies across latitude and altitude for terrestrial and aquatic organisms has
yet to be systematically tested.
Disturbance theory predicts species richness patterns vary across landscapes in response to abiotic events (fire,
flood, drought) that vary in magnitude and frequency to select for individuals and species with traits that confer
resistance or resilience to particular disturbance regime. Disturbance can be viewed as an evolutionary
determinant of traits and/or it may be viewed as an ecological process that “filters” species from a regional species
pool according to the trait match to local disturbance. In combination, evolutionary and ecological filters dictate
how species (and traits) persist locally in response to environmental change. For example, species living in
variable environments possess traits that confer greater resistance to increased variation in disturbance than
species in stable environments.
Incorporating disturbance into models of stream ecosystem response to climate change is key because of the
explicit connection between precipitation and hydrologic regimes. In streams, flow disturbance is a key process
that directly and indirectly affects multiple biological and ecological dimensions, from individual performance to
population abundance to ecosystem process rates and states. Variation in flow disturbance regime is coupled
strongly to precipitation and temporal variation in the magnitude, timing, duration and frequency of hydrologic
extremes (floods, low flows) can select for adaptive traits. Disturbance can selectively filter species based on the
mismatch between magnitude or timing of disturbance relative to species behavior, morphology, or life history
stage. Species traits are widely used to understand and predict the effect of disturbance on the distribution and
abundance of vertebrate and invertebrate species. Disturbance also regulates whole-system properties such as
community composition, biotic interactions, trophic structure and food chain length and ecosystem processes such
as heterotrophy, metabolism, nutrient storage and basal resource stoichiometry.
Thus, projected changes in precipitation are likely to have dramatic and immediate effects on these systems. A
growing body of hydro-climatological research indicates stream disturbance regimes will change significantly in
the future due to increased variation in rates of precipitation and in warming that will regulate snow storage and
melt (including melting glaciers) and will likely extend dry spells and push perennial streams toward seasonal
intermittency. In response to modification of contemporary flow regime, species are expected to have a historyspecific response. A significant change in disturbance will act as a filter to select for the newly appropriate species
traits (Fig. 1D). Yet, trait responses are complicated by the indirect effects
of disturbance on stream ecosystems. For example, extended low flows
allow water to warm and dissolved oxygen to decline, which will filter
species having suitable thermal tolerance and hypoxia tolerance traits.
Disturbance thus acts to regulate species success in streams both directly
(through mortality) and indirectly (through mediating ecosystem conditions
such as resources required for species growth).
Poff and others have examined flow regimes in many detailed ways, but a
simple insight is that species and ecosystem sensitivity to changing flows
are likely to be greatest where a regime shifts from perennial to intermittent
or from stable perennial to flashy due to precipitation variance. The
coefficient of variation (CV) of discharge (Q) captures most of the major
Fig. 3. Disturbance theory predicts
differences among flow regime “types,” which be arrayed along an axis of
that historical variance in flow will
CV(Q): intermittent > perennial flashy > perennial stable. These simple
select for specific species traits. For
regime characterizations lead to specific theoretical predictions about how
example, variance in flow is predicted
increasing CV(Q) will select/filter for species at levels of individuals
to be positively related to dispersal
(greater physiological tolerance), populations (dispersal ability) and
potential and hypoxia/thermal
tolerance and negatively related to
ecosystems (trophic generalization, altered food chain length) (Fig. 3).
trophic specialization.
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