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Overview Articles
Toward More Integrated Ecosystem
Research in Aquatic and Terrestrial
Environments
JANNE SOININEN, PIA BARTELS, JANI HEINO, MISKA LUOTO, AND HELMUT HILLEBRAND
Aquatic and terrestrial ecosystems are tightly linked through the fluxes of organisms and matter. However, aquatic and terrestrial ecologists have
mainly studied these ecosystems separately, a “splendid isolation” historically fostered by disciplinary boundaries between institutes and funding
schemes. Here, we synthesize the progress made in joint aquatic and terrestrial research and suggest new approaches to meeting future research
challenges in changing environments. Aquatic and terrestrial organisms use cross-system subsidies to a comparable extent and addressing
reciprocal subsidies is therefore necessary in order to understand biodiversity and functioning of both aquatic and terrestrial ecosystems. We
suggest that the metaecosystem framework could be expanded to explicitly consider cross-system fluxes of matter differing in magnitude and
quality. We further advocate the inclusion of cross-system analyses at broader spatial extents, for which remote-sensing applications would be a
useful tool in environmental research at the land–water interface. A cross-ecosystem approach would therefore be valuable for a more thorough
understanding of ecosystem responses to various stressors in the face of rapid environmental change.
Keywords: catchment, cross-ecosystem, metaecosystems, remote sensing, subsidies
A
lthough aquatic and terrestrial ecosystems share many fundamental characteristics of structure and
function (Hairston et al. 1960), there has been a broad
gap between aquatic and terrestrial ecologists in studying
these ecosystem realms. Consequently, despite working
on the same fundamental ecological issues, ecologists
focusing on a given ecosystem still seem to mostly ignore
studies conducted in other ecosystems (Menge et al. 2009).
However, we argue that an even more severe shortcoming
is that aquatic and terrestrial ecologists still typically conduct their research separately, focusing on either wet or
dry ecosystems in the landscape, which actually comprises
both. Bartels and colleagues (2012) included 71 studies
of reciprocal subsidies between terrestrial and freshwater ecosystems in a meta-analysis (most of the studies
included had been published after 2000). Compared with
the number of studies conducted separately for terrestrial
or aquatic ecosystems, the number of studies on reciprocal subsidies is still small. Menge and colleagues (2009)
also stated that 60% of all research papers (N = 5824)
published in a collection of leading ecological journals
between 2002 and 2006 were terrestrial, whereas only 9%
were freshwater, and 8% were marine studies. Therefore,
the high number of papers focused on either terrestrial or
aquatic environments in isolation clearly outnumbers the
number of joint studies directly combining dry and wet
ecosystems.
Studies in which the cross-system fluxes of material
and energy between water and land were considered all
highlighted the importance of such reciprocal subsidies to
ecosystem function (Bartels et al. 2012). Moreover, studies on cross-system food web interactions have shown that
aquatic and terrestrial food webs are often tightly linked
and that landscape-level processes are highly important for
our understanding of local interactions (Knight et al. 2005).
Therefore, incorporating aquatic and terrestrial processes in
collaborative research efforts is more than an incremental
add-on to existing research; it is a necessity for ecologists
in facing new research challenges to predict ecosystem- and
landscape-level responses to environmental changes at broad
spatial scales. We argue that such an integrative approach
should be incorporated not only in research campaigns but
also in major funding schemes and review processes that
typically still separate aquatic and terrestrial research.
Objectives
Here, we synthesize the progress that has been made in joint
aquatic and terrestrial research and suggest new avenues for
such research. We emphasize that the structure and function
of these two ecosystem types cannot be understood without
BioScience 65: 174–182. © The Author(s) 2015. Published by Oxford University Press on behalf of the American Institute of Biological Sciences. All rights
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doi:10.1093/biosci/biu216
Advance Access publication 14 January 2015
174 BioScience • February 2015 / Vol. 65 No. 2
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Box 1. Reciprocal subsidies between streams and their contiguous ecosystems.
Streams constitute only a minimal areal extent of the globe, but they are highly important in terms of overall length and consequent
effects on other ecosystems (Allan and Castillo 2007). Interactions between stream and riparian ecosystems are prime examples of
reciprocal subsidies, where organic material and terrestrial organisms enter the stream and emerging aquatic insects enter the riparian
zone (Fisher and Likens 1974). The importance of riparian tree leaves entering streams have been examined in many studies (Allan
and Castillo 2007). This allochthonous leaf material is an important driver of stream ecosystem structure and function, because it is
key food resource for various invertebrate consumers (e.g., shredding stonefly and caddisfly larvae). These invertebrate consumers
rely on this material for subsistence and have further effects on the breakdown of leaves in streams (Zhang and Richardson 2011).
Coarse particulate organic matter is broken down into fine particulate organic material that is further used by various invertebrate
consumers (e.g., filtering blackfly and gathering midge larvae). The effects of riparian leaf material on stream ecosystems is therefore
considerable, although its importance does not remain only in streams but feeds back to the riparian zone through emerging stream
insects (Baxter et al. 2005, Allan and Castillo 2007). Emerging insects form an important but seasonally variable food source for terrestrial predators (e.g., birds, lizards, spiders, ground beetles; Nakano and Murakami 2001, Baxter et al. 2005). Often, the productivity of streams is transferred to these terrestrial predators, the densities of which may be higher near streams than further away from
them. Furthermore, there are some riparian specialist species that rely on stream-based insect prey, sometimes even more than on
terrestrial prey. Reciprocal subsidies are also manifested in the fact that both adults of aquatic and terrestrial insects fall into streams,
thereby ­providing a food resource for stream fish (Nakano and Murakami 2001, Baxter et al. 2005). A considerable proportion of the
diet of insectivorous fish is often accounted for by terrestrial prey, and this subsidy may effectively maintain fish populations even in
streams where autochthonous production of prey is limited. The degree to which terrestrial insect prey determine the production of
the young of fish in streams also has importance to migrations of diadromous fish to the sea. Finally, after spending a period in the
sea, diadromous fish return to a stream for spawning or feeding. In the case of anadromous semelparous fish, which typically die after
spawning (e.g., Pacific salmon), the corpses of dead fish may provide food and nutrients for various consumers (e.g., bears, birds) and
drive ecosystem function especially in streams at high latitudes (Nakano and Murakami 2001). This is an excellent example of nutrients, material, and organisms produced in the sea to have effects on freshwater ecosystem functions and food web structure across
ecosystem boundaries.
considering them simultaneously, even if the respective
scientific communities may pretend to do so. We think that
a crucial step in ecology would be to examine aquatic and
terrestrial ecosystems simultaneously, rather than studying
them in splendid isolation. For clarity of presentation, we
focus our review on the reciprocal links between terrestrial and freshwater systems, but we believe that the same
arguments hold for the interaction between marine and
terrestrial systems in coastal zones or between marine and
freshwater processes in ecosystems influenced by estuaries
or subterranean freshwater inflows (box 1).
Cross-ecosystem subsidies
Aquatic and terrestrial ecosystems are inevitably linked
through the movement of energy, nutrients, and material
(box 2; Polis et al. 1997). Particularly, the fluxes of particulate organic matter (i.e., resource subsidies) play an essential
role because of their potential to change community and
food web dynamics in recipient ecosystems. Although these
fluxes are reciprocal and ubiquitous, aquatic ecosystems
generally receive larger amounts of organic subsidies than
terrestrial ecosystems (Bartels et al. 2012). A multitude of
empirical studies has clearly demonstrated the importance
of organic subsidies to ecological communities (Richardson
et al. 2010), albeit the majority of studies were focused solely
on subsidies as nutritional supplements to the recipient consumer’s diet. Despite larger input to aquatic ecosystems, terrestrial and aquatic consumers use subsidies to a comparable
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extent (Bartels et al. 2012), likely because of differences in
subsidy quality between ecosystems (Marcarelli et al. 2011).
Synthesizing information from 71 studies, Bartels and colleagues (2012) found a median contribution of almost 40%
of subsidized food to consumer diets in both aquatic and
terrestrial systems. Consumption of subsidies can result in
higher consumer numbers through spatial accumulation
and/or enhanced reproduction by increasing individual
growth rates (Wright et al. 2013).
Notably, the influence of subsidies can extend beyond the
consumer level (McCann et al. 1998), substantially affecting
food web dynamics within recipient ecosystems (Leroux and
Loreau 2008). For instance, bottom-up and top-down effects
following the input of subsidies have been repeatedly illustrated in aquatic and terrestrial ecosystems (Wallace et al.
1997, Murakami and Nakano 2002). Theoretical studies
indicate that the impact of subsidies on food webs is largely
determined by the amount of input that enters into the food
web, the preference of the consumers, the trophic level that
receives the input, and the relative duration of subsidy input
compared with the consumers’ numerical response (Leroux
and Loreau 2008, Takimoto et al. 2009). However, empirical
studies in which these theories have been explicitly tested
are scarce. In one of these studies, Klemmer and Richardson
(2013) illustrated that trophic cascades were strong at low
subsidy inputs. However, at intermediate to high subsidy levels, trophic cascades were absent because of complex community dynamics. Sato and Watanabe (2014) suggested that
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Box 2. Reciprocal subsidies between lakes and adjacent ecosystems.
Lake–land interactions have long been viewed as unidirectional, governed by fluxes from terrestrial to lake ecosystems. The input of
dissolved organic carbon (DOC) has received major attention during the last decades. Terrestrial DOC can stimulate heterotrophic
production through the use by bacteria (Flecker et al. 2010) and represents a major entry route of terrestrial carbon into aquatic food
webs (Jansson et al. 2007). More recently, the importance of particulate organic carbon has been acknowledged for pelagic (Cole et al.
2011) and benthic food webs (Cole et al. 2006). For example, fish can consume terrestrial arthropods, and excrete terrestrially derived
nutrients to the water column (Mehner et al. 2005), potentially subsidizing the local nutrient pool. Respiration and secondary production in lakes are strongly subsidized by terrestrial organic carbon, especially in unproductive systems (Karlsson 2007), emphasizing the
importance of terrestrial carbon subsidies to lake ecosystem functioning (del Giorgio and Peters 1993). Reciprocal fluxes, from lakes
to adjacent terrestrial ecosystems, are only rarely studied. Insects emerging from lakes can reach substantial numbers in some geographic regions (Gratton and Vander Zanden 2009), exceeding those from streams on a regional scale (Bartrons et al. 2013), and can
be incorporated into terrestrial ecosystems via multiple pathways (Dreyer et al. 2012). Similar to stream–riparian interactions, fluxes
between lakes and adjacent terrestrial ecosystems are reciprocal, however, fluxes from lake to land and their potential consequences
remain understudied.
size structure within a consumer population determined the
effects of subsidies on trophic cascade. Moreover, the reception of multiple subsidies at multiple trophic levels may shift
food web dynamics in nonlinear and unpredictable ways
(Huxel et al. 2002).
All the above-mentioned examples highlight that nutrient, detritus, or prey subsidies can influence the food web
structure and dynamics in the recipient ecosystems. However,
evidence for systematic differences in responses to subsidies
between ecosystems is currently lacking. Bartels and colleagues (2012) illustrated that aquatic ecosystems generally
received larger amounts of matter produced outside of the
focal ecosystem than terrestrial ecosystems, which suggests
that aquatic ecosystems might be more susceptible to alterations in food web dynamics following the input of subsidies
than are terrestrial ecosystems. Furthermore, aquatic ecosystems received subsidies at all trophic levels, although this was
likely biased by the lack of studies in terrestrial ecosystems
(Bartels et al. 2012). Aquatic ecosystems show a tendency to
higher secondary production and stronger trophic cascades
(Shurin et al. 2002, 2006), and potentially different responses
in food web dynamics to subsidies might lead to these distinctions between aquatic and terrestrial ecosystems. We therefore
encourage future research to investigate systematic difference
in food web responses to subsidies between ecosystems.
Although subsidies are often limited in their spatial
intrusion into the landscape, the effects on food webs and
ecosystems might expand from their source. Mobile consumers adapt their behavior to track resource availability
(the bird feeder effect; Eveleigh et al. 2007) if the landscape
varies in resource quantity. The dispersal of subsidized
consumers might affect communities at large scales, resulting in community-level responses far beyond the subsidies’
origin. Migratory birds, such as lesser snow geese (Chen
caerulescens caerulescens) are strongly subsidized by agricultural crops in the southern and mid-continental regions
of the United States, and the numbers of geese have grown
because of the expansion of farm land and increased crop
176 BioScience • February 2015 / Vol. 65 No. 2
fertilization (Jeffries et al. 2004). The abundance of heavily
subsidized geese has resulted in a sharp decrease of in situ
productivity, in altered terrestrial arthropod populations,
and in local declines in the breeding populations of other
bird species in the geese’s breeding grounds in the Arctic
(Srivastava and Jeffries 1996, Kotanen and Jeffries 1997).
Furthermore, mobile consumers can provide a novel source
of nutrients by excreting subsidy-derived nutrients (nutrient translocation; Vanni 2002), occasionally in very distant
locations (Hilderbrand et al. 1999). In New Mexico, the daily
foraging of geese moves substantial amounts of nutrients
from farm fields to their roosting locations in wetlands
(Post et al. 1998), supplying 40% of the nitrogen and 75%
of the phosphorus in the wetlands’ annual nutrient input.
Subsidies couple spatial matter and energy fluxes to the
dispersal and spatial movements of the recipient organisms
across ecosystem realms and are therefore ideally suited to
examine the links between (meta-)community and (meta-)
ecosystem ecology.
The input of nutrients derived from subsidies might be
especially important for recipient ecosystems that have low
productivity (Naiman et al. 2002), which then respond to
alterations in more productive donor habitats. Such cascading effects between asymmetrically coupled adjacent
ecosystems (Knight et al. 2005) have been illustrated: the
occurrence of certain predators or differences in biodiversity
and community composition can modify fluxes between
aquatic and terrestrial ecosystems with substantial consequences for recipient community and ecosystem functioning
(Knight et al. 2005, Epanchin et al. 2010), emphasizing the
necessity of studying both ecosystems simultaneously. For
example, Knight and colleagues (2005) documented that fish
indirectly facilitated terrestrial plant reproduction through
cascading trophic interactions beyond pond boundaries. In
their study, fish reduced larval dragonfly abundances, resulting in fewer adult dragonflies. As adult dragonflies consume
insect pollinators, plants near ponds with fish received more
pollinators than do plants near ponds that lacked fish.
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Overview Articles
and metacommunity theory to ecosystems connected by the spatial flows
of energy, materials, and organisms
across ecosystem boundaries (Loreau
et al. 2003, Massol et al. 2011). Whereas
subsidies have initially been studied as
unidirectional transfer, mostly from the
perspective of the recipient ecosystem,
metaecosystem theory explicitly considers reciprocal fluxes between coupled
systems and their compartments (nutrients, detritus, producers and consumers;
figure 1). By including direct (physical)
and indirect (organismal) fluxes of elements, single ecosystems can represent
both sources and sinks in metaecosystems for different agents (organisms and
matter), depending on the balance of
direct and indirect flows (Gravel et al.
2010a). Moreover, these dynamics can
Figure 1. Metaecosystem dynamics and cross-system flows across terrestrial–
then feed back into coexistence and food
aquatic boundaries. In each local system, nutrients are used by primary
web structure in the connected ecosysproducers, which are at the basis of the consumer food web. Dead organic
tems (Gravel et al. 2010b).
material fuels the decomposer food web. The solid arrows depict withinMetaecosystem theory does not exclusystem flows of energy and matter. Exchange across boundaries (the dashed
sively address cross-system flows but
bidirectional arrows) can occur at the level of nutrients, detritus, or consumers.
can be used to describe patches within
one realm, compartments of a single
Particularly, human influences do not stop at ecosystem
ecosystem, or fluxes among ecosystem types. In this context,
boundaries. Eutrophication is a prominent example, in
the theory is flexible with regard to the frequency of subsiwhich elevated nutrient inputs from the catchment due to
dies, spanning from systems with a single pulse or recurrent
changes in land use have been demonstrated to result in the
pulses to continuous flows between systems (Leroux and
reduction of whole lake productivity (Vadebonceour et al.
Loreau 2012). Recent years have seen the first empirical
2003). Although pelagic productivity generally increased in
analyses with reference to this theory, but most examples
Danish lakes, elevated nutrient supplies resulted in the loss
involve subsystems within an ecosystem realm. Casini and
of important benthic pathways (Vadebonceour et al. 2003).
colleagues (2012) analyzed 30 years of spatiotemporal data
Similar effects have been illustrated for increased inputs
on food web dynamics in the Baltic Sea, whereas Menge and
of colored dissolved organic matter (i.e., brownification)
Menge (2013) used a metaecosystem approach to underfrom terrestrial ecosystems entering freshwater ecosystems
stand the consequences of upwelling in a rocky intertidal
(Karlsson et al. 2009).
ecosystem. We strongly advocate adding cross-ecosystem
The introduction of nonnative species can also affect
tests of this theory, especially with regard to the reciprocal
cross-boundary fluxes in complex ways. For instance, the
fluxes of different agents (detritus versus organisms, inorintroduction of nonnative trout substantially reduced insect
ganic versus organic). One of the major advancement to be
emergence, which lead to the reduced numbers of resimade is an explicit consideration of different currencies of
dent finches feeding on insects around ponds with trout
exchange: a stoichiometric perspective on metaecosystem
(Epanchin et al. 2010). Therefore, we emphasize that ecodynamics would allow an understanding of the different
system fluxes are not always beneficial but may result, for
qualities of exchange. For example, nutrient-rich material
example, in eutrophication of water bodies, the introduction
in the form of organisms is transferred from aquatic to terof invasive species, and associated negative influences on
restrial systems, whereas nutrient-poor material (detritus)
ecosystem structure and function.
dominates the reciprocal flow (Bartels et al. 2012). Massol
and colleagues (2011) additionally considered the explicit
Metaecosystems and cross-system flows
formulation of space in metaecosystem models as another
The relative scarcity of empirical studies on the interplay
major developmental step, because space is only implicitly
between aquatic and terrestrial ecosystems does not reflect
addressed in current models. Such an explicit consideration
recent theoretical advances. The theoretical foundations for
of space in metaecosystem studies could use (and further
cross-system comparisons have been laid out in the form
develop) the large toolbox of spatial statistics developed
of metaecosystem theory, which extends metapopulation
within landscape ecology and spatial ecology in general.
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February 2015 / Vol. 65 No. 2 • BioScience 177
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Using remote sensing to quantify broad-scale
patterns in land–water interactions
At larger spatial extents, experimental approaches for examining the cross-system food web interactions or material
flows are often not feasible. To obtain a broader view over
the patterns and processes linking aquatic and terrestrial
systems, researchers may sample the same geographical
region for aquatic and terrestrial organisms and examine,
for example, how lake communities and terrestrial vegetation surrounding the same lakes respond to climatic gradients covered by the study region. Such joint broad-scale
sampling campaigns for aquatic and terrestrial organisms
are, however, surprisingly rare to date (but see Gurgel et al.
2014). A more feasible method for examining large-scale
patterns is the use of remote sensing. The remote-sensing
approach and the data derived with it are useful in obtaining
environmental information for large regions in a consistent
manner (Kerr and Ostrovsky 2003). They have been used by
terrestrial ecologists for modeling variation in species richness (Nagendra 2001), in assessing ecological responses to
environmental change (Pettorelli et al. 2005) and in studies
of land cover patterns (Friedl et al. 2002). Recently, remote
sensing has also been found to be a promising tool for
aquatic ecologists; it has been used to infer lake temperatures, algal dynamics, and changes in water levels (Hampton
2013).
The normalized difference vegetation index (NDVI) is
one of the most widely used remote-sensing-based metric
to quantify terrestrial plant biomass as an estimate of primary productivity (Nagendra 2001, Pettorelli et al. 2005).
It correlates strongly with the leaf area index, net primary
production, percent leaf area greenness and is related to
the amount of absorbed photosynthetically active radiation
(Tucker 1979). We suggest, therefore, that remotely sensed
metrics such as NDVI are highly useful not only for estimating terrestrial productivity at broad spatial scales but
also for providing proxies for the amount of organic carbon, nitrogen, and phosphorus eventually entering aquatic
ecosystems from the catchments (Finstad and Hein 2012,
Soininen and Luoto 2012). The influence of catchment
variables operating at intermediate scales between local (i.e.,
water chemistry and local biotic interactions) and regional
(i.e., large-scale climatic variables) on aquatic biota has been,
however, understudied. The terrestrial buffer zone near a
water body is, nonetheless, potentially important for aquatic
biota because of mediating the catchment effects on water
chemistry (Maberly et al. 2003), as well as because of various
processes in food webs operating at the land–water interface
(Knight et al. 2005, Allan and Castillo 2007).
Because of such important catchment effects, remotely
sensed catchment productivity may also be used to identify
aquatic diversity hotspots even in remote regions in a costeffective manner. Vinson and Hawkins (2003) showed that
stream insect genera richness increased with mean NDVI
at the global scale, even though at large productivity values, genus richness values tended to show large among-site
178 BioScience • February 2015 / Vol. 65 No. 2
variation. Finstad and Hein (2012) revealed that catchment NDVI predicted well Arctic char distribution possibly because of serving as a proxy for food availability in
Norwegian lakes.
In a recent study, Soininen and Luoto (2012) documented that maximum NDVI was related to the richness of
planktonic organisms independent of lake variables or geographical position of boreal lakes. In total, seven out of nine
catchment predictors that were considered had a significant
unique effect on phytoplankton richness. This unique effect
of catchment on plankton richness possibly stemmed from
catchment variables being more robust in time than snapshot measures of water chemistry, therefore reflecting the
long-term environmental conditions. Moreover, catchment
variables may have reflected the effects of some latent variables not measured in the freshwater ecosystem. Soininen
and Luoto (2012) further showed that the catchment effect
was, overall, comparable with the effect of local within-lake
variables (i.e., water chemistry) and larger than the effect of
geographical variables in these lakes. This highlights that
disregarding catchment variables from freshwater studies
may lead to biased conclusions of the importance of drivers
and proxies of aquatic biodiversity. Overall, we encourage
researchers to examine terrestrial–aquatic links further in
light of ecological theory in order to detect potential mismatches between the theory and the empirical data collected
during field surveys. For example, it would be important to
reveal whether combining the data on aquatic and terrestrial
ecosystems results in patterns that are opposite of those predicted by current ecological theory.
Collectively, the use of such remote-sensing-derived measures may decrease the need for extensive field sampling in
terrestrial systems and can be useful for many conservation
and management applications related to aquatic–terrestrial
links (Kerr and Ostrovsky 2003). We further emphasize that
factors operating at local, landscape, and broad scales are all
important to be considered, especially in biodiversity studies, as the variation in species richness is known to be scale
dependent (Chase and Leibold 2002) and related to variables
acting at different scales (Heino and Peckarsky 2014). For
example, Hessen and colleagues (2007) revealed that zooplankton richness was related to both local and large-scale
geographical variables in Norwegian lakes.
Not only is the productivity of the catchment influential
for the aquatic environment, but the heterogeneity of the terrestrial environment is also expected to be related to aquatic
biodiversity because of the greater number of available abiotic and biotic conditions. Temporal and spatial patterns in
aquatic physicochemistry, particularly the inputs of detritus
and nutrients, are affected by the composition and structure
of the surrounding terrestrial landscape (figure 2; Ward
et al. 2002). Moreover, terrestrial environmental heterogeneity may dampen the effects of environmental impacts
by decreasing the synchrony of population fluctuations.
For example, in homogeneous catchments, aquatic species
can be more strongly affected by climate change than in
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Figure 2. Temporal and spatial patterns in aquatic physicochemistry and biota are affected by the surrounding terrestrial
landscape pattern. Terrestrial environmental heterogeneity dampens the effects of environmental impacts by decreasing
the synchrony of population fluctuations. Heterogeneous landscapes may lower the risk of regional population declines
of aquatic organisms under extreme weather perturbations and serve as sources of recovery in postdisturbance recovery
phases. Abbreviations: A, aquatic; T, terrestrial.
heterogeneous catchments (Piha et al. 2007). Heterogeneous
landscapes may lower the risk of regional population declines
of aquatic organisms under extreme weather perturbations
and serve as sources of recovery in postdisturbance recovery
phases (Piha et al. 2007). Moreover, changes in landscape
structure may affect species dispersal ability (Rothermel and
Semlitsch 2002) and decrease the quality of both terrestrial
and aquatic habitats important for different stages of the life
cycle (e.g., amphibians and insects; Heino and Peckarsky
2014). Land-use changes can alter the local climate, productivity, and the decomposition of organic matter, potentially
resulting in cascading and unexpected effects on aquatic
ecosystems.
Quantifying catchment productivity is also useful for
studies testing ecological theory, especially when considering regional species pools and regional productivity.
Following Hynes (1970), Ptacnik and colleagues (2010),
and Soininen and Luoto (2012), individual lakes or streams
located in an overall productive landscape tend to have
higher taxonomic richness than solitary potentially productive lakes or streams in an unproductive landscape because
of the high influx of propagules from the regional pool
(figure 3). It is therefore possible that the variability of local
richness of aquatic organisms could be more strongly related
to the average productivity of the region and the amount of
surface waters that act as source pools than is local withinlake or within-stream productivity. Therefore, we emphasize
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that quantifying productivity and size of the source pools
at larger spatial scales covering catchments may be more
appropriate and more feasible for predicting aquatic biodiversity than measuring snapshot productivity based on
water samples.
Conclusions
Aquatic and terrestrial ecosystems have been traditionally
studied in isolation. In this article, we argue that such a gap
between aquatic and terrestrial ecologists is largely artificial,
given that there is plenty of evidence that these ecosystems
are often tightly linked through the fluxes of organisms,
material, and energy. We suggest three research avenues for
a better integration of aquatic and terrestrial research:
First empirical analyses related to metaecosystem theory
have just emerged, but most of these studies involve subsystems within only one ecosystem. We think that this is a
major shortcoming, because the theory is flexible enough
to be expanded so that the fluxes between coupled ecosystems can be considered, too. We therefore strongly advocate
expanding the metaecosystem theory and also its empirical
tests to cross-ecosystem studies.
Second, we suggest the explicit consideration of the
nutritional quality of matter flowing between ecosystem
compartments using a stoichiometric perspective on metaecosystem dynamics. This is because nutrient-rich material
in the form of organisms is transferred from aquatic to
February 2015 / Vol. 65 No. 2 • BioScience 179
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more extensively as a potential predictor of the richness
and composition of aquatic communities, because it may
reflect the long-term environmental conditions in aquatic
ecosystems better than the snapshot measures of water
chemistry. Remote-sensing-based metrics may also be helpful in theoretical studies considering regional species pools
and regional productivity instead of focusing solely on local
communities and local productivity. We think that following these approaches would facilitate the shift from purely
aquatic or purely terrestrial research toward a more holistic view on how biological communities and ecosystems
function.
References cited
Figure 3. A comparison of diversity in lakes in
unproductive and productive catchment. The blue circles
are lakes, the arrows represent the passive dispersal
of algae among lakes, and dark green color represents
a productive catchment, whereas the light green color
indicates a less productive catchment. The top row
represents landscapes with a higher number of lakes,
whereas the bottom row represents landscapes with fewer
lakes. In the panels on the right, the lake with the dotted
boundary represents a single productive lake (due to the
point source of nutrients), whereas the solid boundary line
indicates that all the other lakes in the same landscape
are unproductive. Different symbols depict different algal
species. Despite the high local nutrient supply, species
richness may be low in lakes in an unproductive catchment
because of a small regional species pool and associated
low influx of colonists into the lake. In contrast, lakes in
a productive catchment may have overall higher species
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spatial scales, advances in remote sensing have provided
metrics useful as proxies for estimating the extent of crosssystem flows of matter from land to water. We therefore
argue that catchment level productivity (e.g., NDVI values
reflecting terrestrial productivity) should be considered
180 BioScience • February 2015 / Vol. 65 No. 2
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Janne Soininen ([email protected]) and Miska Luoto are affiliated with the Department of Geosciences and Geography at the University
of Helsinki, Finland. Pia Bartels is affiliated with the Department of
Ecology and Environmental Science at Umeå University, in Umeå,
Sweden. Jani Heino is affiliated with the Finnish Environment Institute,
Natural Environment Centre, Biodiversity, in Oulu, Finland. Helmut
Hillebrand is affiliated with the Institute for Chemistry and Biology of
the Marine Environment, at Carl-von-Ossietzky University Oldenburg, in
Wilhemshaven, Germany.
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