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\begin{document}

\title{Linking human impacts to community processes in terrestrial and
freshwater ecosystems}



\author[1,2]{Ian R. McFadden}%
\author[1]{Agnieszka Sendek}%
\author[3]{Morgane Brosse}%
\author[3]{Peter M. Bach}%
\author[3]{Marco Baity-Jesi}%
\author[1]{Janine Bolliger}%
\author[1]{Kurt Bollmann}%
\author[1]{Eckehard G. Brockerhoff}%
\author[3]{Giulia Donati}%
\author[1]{Friederike Gebert}%
\author[1]{Shyamolina Ghosh}%
\author[3]{Hsi-Cheng Ho}%
\author[3]{Imran Khaliq}%
\author[1,3]{J. Jelle Lever}%
\author[3]{Ivana Logar}%
\author[1,3]{Helen Moor}%
\author[3]{Daniel Odermatt}%
\author[1,2]{Loïc Pellissier}%
\author[3,4]{Luiz Jardim De Queiroz}%
\author[1]{Christian Rixen}%
\author[3]{Nele Schuwirth}%
\author[3]{J. Ryan Shipley}%
\author[3]{Cornelia W. Twining}%
\author[1]{Yann Vitasse}%
\author[3,5]{Christoph Vorburger}%
\author[1,6]{Mark K. L. Wong}%
\author[1]{Niklaus E. Zimmermann}%
\author[3,4]{Ole Seehausen}%
\author[1,2]{Martin M. Gossner}%
\author[3]{Blake Matthews}%
\author[1]{Catherine H. Graham}%
\author[3,7]{Florian Altermatt}%
\author[3]{Anita Narwani}%
\affil[1]{Swiss Federal Institute for Forest, Snow and Landscape Research (WSL)}%
\affil[2]{Institute of Terrestrial Ecosystems, ETH Zürich}%
\affil[3]{Swiss Federal Institute of Aquatic Science and Technology (Eawag)}%
\affil[4]{Institute of Ecology \& Evolution, University of Bern}%
\affil[5]{Institute of Integrative Biology, Department of Environmental Systems Science, ETH Zürich}%
\affil[6]{School of Biological Sciences, The University of Western Australia}%
\affil[7]{Department of Evolutionary Biology and Environmental Studies, University of Zurich}%


\vspace{-1em}



  
  \date{May 26, 2024}


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\selectlanguage{english}
\begin{abstract}
Human impacts such as habitat loss, climate change and biological
invasions are radically altering biodiversity, with even greater effects
projected into the future. Evidence suggests human impacts may differ
substantially between terrestrial and freshwater ecosystems, but the
reasons for these differences are poorly understood. We propose an
integrative approach to explain these differences by linking impacts to
the fundamental processes that structure communities: dispersal,
speciation, ecological selection and ecological drift. Our goal is to
provide process-based insights into why human impacts, and the responses
to these impacts, may differ across ecosystem types within a
mechanistic, eco-evolutionary comparative framework. To enable these
insights, we review and synthesize i) how the four processes can
influence diversity and dynamics in terrestrial and freshwater
communities, focusing on whether their relative importance may or may
not differ among ecosystems, and ii) how human impacts can alter
terrestrial and freshwater biodiversity in different ways due to
differences in process strength among ecosystems. Finally, we highlight
research gaps and next steps, and discuss how this approach can provide
new insights for conservation. By focusing on the processes that shape
diversity in communities, we aim to mechanistically link human impacts
to ongoing and future changes in ecosystems.%
\end{abstract}%



\sloppy


\textbf{}

\subsubsection{}

{\label{339777}}

\textbf{Contents}

{\ref{874970}}~ Introduction

{\ref{187142}}~ Linking human impacts to community
processes

~~~~~~~~{\ref{915286}}~ Dispersal

~~~~~~~~{\ref{826592}}~ Speciation

~~~~~~~~{\ref{284690}}~ Ecological selection

~~~~~~~~{\ref{380504}}~ Ecological drift

{\ref{443870}}~ Synthesis and future directions~

{\ref{264515}}~ Acknowledgements

{\ref{730256}}~ References

\par\null

\subsection{Introduction}

{\label{874970}}

It is well-known that humans are fundamentally altering the biodiversity
and functioning of ecosystems through impacts such as habitat loss,
climate change and biological invasions (Ruckelshaus \emph{et al.} 2020;
P\selectlanguage{ngerman}örtner \emph{et al.} 2021). However, recent studies suggest that global
change drivers may cause divergent biodiversity responses in terrestrial
and aquatic ecosystems (Blowes \emph{et al.} 2019; van Klink \emph{et
al.} 2020), making it difficult to forecast future biodiversity changes.
Indeed, that differences exist in human impacts across ecosystems was a
key finding of the recent IPBES Global Assessment (Díaz \emph{et
al.}2018; Ruckelshaus \emph{et al.} 2020), which emphasized the
importance of better understanding these differences. One potential
reason the relative importance of human impacts may vary across
ecosystems, beyond differing strength of impact, is that differences
exist in the relative importance of core processes that govern
biodiversity dynamics in these systems. Identifying differences in the
strength and type of processes most important in a given ecosystem may
allow for the development of comparative frameworks to understand and
mitigate human impacts in both terrestrial and aquatic communities.

While studies have compared differences in processes and properties for
terrestrial and marine habitats (May \emph{et al.} 1994;
Grosberg\emph{et al.} 2012; Webb 2012), comparisons are lacking for
terrestrial and freshwater ecosystems, which are highly threatened
(Carpenter\emph{et al.} 2011; Belletti \emph{et al.} 2020). Here, we aim
to build a more complete understanding of how and why human impacts vary
among terrestrial and freshwater ecosystems by identifying differences
and similarities in the fundamental processes shaping communities
(Figure 1). To do this, we build on theory that distinguishes four
fundamental processes (Box 1) that comprehensively describe how species
are gained and lost from communities (Vellend 2010, 2016): dispersal,
speciation, ecological selection (hereafter selection) and ecological
drift (hereafter drift). These processes capture the mechanisms by which
community attributes such as species richness, species-abundance
relationships and species turnover emerge, and are general enough to
allow for comparisons across many ecosystem types. Importantly, the
relative importance of these processes is likely modulated by
ecosystem-specific physical and spatial attributes, for example the
properties of media (e.g., air vs water), or geometric constraints of
habitat (e.g., open vs dendritic, Table 1). Thus, our goal is to develop
mechanistic bridges between global change drivers and their impacts by
explicitly considering both the fundamental processes and properties
shaping ecosystems (Figure 1, Table 1).

\par\null\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.98\columnwidth]{figures/Figure-1-Framework/Figure-1-Framework}

\end{center}
\end{figure}

\begin{quote}
\textbf{Fig. 1 \textbar{} Impact, process and response-based approach to
understand ecosystem change.} Focusing on community processes can enable
mechanistic inferences into how humans alter ecosystem dynamics in
terrestrial and freshwater systems via effects on community structure
and dynamics. Traditional approaches (i) identify various patterns of
ecosystem responses and attribute them to specific human impacts, but
often do not explicitly consider the community processes which shape
these responses, in particular across ecosystems. By considering these
processes we can better fill the twin knowledge gaps of (ii) how humans
change the relative importance of processes operating in communities,
and (ii) how these processes then create the observed ecosystem
responses.
\end{quote}

\par\null

\begin{quote}
\textbf{Box 1 \textbar{} Glossary and contextualization of community
processes used in the approach ~}

\textbf{Dispersal: ~}The movement of organisms among sites (Stevens
~\emph{et al.} 2014) and the process by which species can be added to a
local site from a regional species pool via immigration, or removed from
a local site via emigration. Along with speciation, dispersal is one of
the two processes that can add species to communities (MacArthur \&
Wilson 1967; Vellend 2016).~ ~ ~ ~ ~ ~ ~

\textbf{Speciation: ~}The process describing species splitting into two
more or less reproductively isolated populations, either due to
geographic barriers (allopatry) or in situ divergence (sympatry, Coyne
\& Orr 2004; Hern\selectlanguage{ngerman}ández-Hernández~\emph{et al.} 2021). Speciation has not
traditionally been a focus in community ecology. However, this process
is now recognized as an important mechanism influencing the size of
regional species pools and the assembly of communities from them
(Ricklefs 1987; Mittelbach \& Schemske 2015). Speciation is one of two
processes that can add species to a local community, along with
immigration via dispersal. Allopatric speciation of geographically
isolated populations in particular may increase the size of the regional
species pool, and thus the number of species arriving at local sites. ~~

\textbf{Ecological selection:} Species differ in their population growth
rates. These differences emerge from the sum of the absolute fitness of
all individuals in a population. Such species-level differences can
cause variation in their relative abundance over time, which defines a
selection process operating at the level of species that shapes
community structure (Vellend 2010, 2016). Ecological selection is the
best-studied of the four fundamental processes (Cottenie 2005), and is
the most diverse with respect to ecological mechanisms it encompasses.
It includes i) the impact of the environment in filtering and sorting
species from the species pool (e.g. ``constant'' selection,
Leibold~\emph{et al.} 2004; Vellend 2010; Soininen 2014), ii) density
and frequency-dependent effects of interactions (e.g. competition,
predation and mutualisms), and iii) impacts of environmental
heterogeneity over space or time (variable selection). ~ ~

\textbf{Ecological drift:~}The change in relative abundances of species
over time due to random variation in births and deaths of individuals
(Hubbell 2001; Vellend 2010; Gilbert \& Levine 2017), leading to
stochasticity in species' abundances over time. Drift can ultimately
only erode local biodiversity due to random losses of species from
communities because it does not generate or introduce new species
(Vellend 2016). Drift is likely the least well-studied of the four
community processes, despite the fact that it can play an important role
in community assembly even when other more deterministic processes are
operating and species are not ecologically equivalent (Gilbert \& Levine
2017; Svensson~\emph{et al.} 2018). One signature of drift is that its
influence is greater when population sizes are small, such as on
islands, isolated lakes, and in small habitat patches (Hubbell 2001;
Melbourne \& Hastings 2008; Orrock \& Watling 2010). ~ ~

\textbf{Note:} Ecological selection and drift refer to community-level
processes shaping diversity in mixed-species assemblages (see Vellend
2016), not changes in allele frequencies or abundances within
populations of single species as in evolutionary biology and population
genetics.~~
\end{quote}

\par\null

\subsection{Linking human impacts to community
processes}

{\label{187142}}

In our approach (Figure 1) we focus on how human impacts are mediated by
the fundamental community processes of dispersal, speciation, selection
and drift. These processes are not always considered in studies of
ecosystem responses to impacts, yet it is these processes that generate
the observed responses of ecosystems. Traditional approaches to studying
human impacts have documented how one or several drivers such as warming
or habitat loss have altered the diversity of ecosystems such as changes
in richness, turnover and abundance. These include studies comparing
terrestrial and marine ecosystems (Blowes \emph{et al.} 2019) or less
often terrestrial and freshwater systems (van Klink \emph{et al.} 2020).
While many of these studies speculate as to the ultimate causes of
ecosystem outcomes, few explicitly consider the full set of processes
shaping diversity in ecosystems. In our approach we fully incorporate
these processes of dispersal, speciation, selection and drift to more
mechanistically link ecosystem responses now and in the future with the
myriad ways humans alter these systems.

To demonstrate the utility of our approach, we provide an example of how
it could be used to interpret and explain a hypothetical scenario in
which freshwater ecosystems are experiencing less warming-related
turnover (also known as thermophilization) than terrestrial ecosystems
(Figure 2). By examining each fundamental community process as a
possible driver of this pattern, we can gain a better understanding of
why differences may emerge across terrestrial and freshwater ecosystems
and use this knowledge to make predictions about other systems and
future impacts. In this hypothetical scenario, freshwater communities
are buffered from some warming effects due to the heat capacity of
water, which slows warming in this system. This buffering effect
decreases the selection pressure on freshwater communities to
thermophilize, and could drive the hypothesized pattern in which
freshwater communities experience weaker warming impacts. However, it
may also be that the dendritic nature of freshwater systems, in
particular streams and rivers, may reduce the capacity for warm-adapted
species to enter the community via dispersal, which would also reduce
thermophilization. Finally, our approach increases the focus on the
under-studied processes of drift and speciation as drivers of changes in
current and future ecosystems. For example, in the longer term,
freshwater communities may recover from warming-related species losses
faster than terrestrial ecosystems through speciation. This is because
the more isolated habitat structure and smaller population sizes of many
freshwater taxa may promote in situ speciation in this ecosystem.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.98\columnwidth]{figures/Figure-2-Example/Figure-2-Example}

\end{center}
\end{figure}

\begin{quote}
\textbf{Fig. 2 \textbar{} Demonstration of the impact, process \&
response approach for a warming scenario.} In this hypothetical scenario
terrestrial communities (red circle) are turning over towards more
warm-adapted species (aka thermophilizing) faster than freshwater
communities (blue circle). The four fundamental community processes are
then used within the context of our approach (Figure 1) to explain this
pattern as a function of potential differences in each process between
ecosystem types (see main text and Table 1).
\end{quote}

\par\null

To implement the proposed approach, we synthesize the literature to find
key differences and similarities in how the four major processes operate
within each system. We then use the differences and similarities we
identify to infer how terrestrial and freshwater biodiversity may
respond to, and recover from, these impacts. For each process, we
provide non-exhaustive but representative examples of some of the major
ways humans impact terrestrial and freshwater ecosystems, paying
particular attention to how the process~might influence the magnitude of
human impacts. We also identify the physical, habitat, chemical and
community properties that likely underlie differences in the strength of
processes among ecosystems (Table 1). Although we focus primarily on the
understudied comparison between terrestrial and freshwater ecosystems,
we also reference marine systems when information on oceanic
environments and taxa can provide insights for comparisons.

\par\null

\subsubsection{\texorpdfstring{\emph{Dispersal}}{Dispersal}}

{\label{915286}}

Dispersal of organisms (Box 1) may be limited either by a lack of
intrinsic adaptations for efficient movement or extrinsic barriers
isolating species from suitable habitats. Here we first compare
differences between terrestrial and freshwater systems in species
intrinsic factors, i.e., attributes such as differences in physiology,
behavior and life history (Stevens \emph{et al.} 2014; Comte \& Olden
2018). We then compare ecosystems in terms of extrinsic factors causing
dispersal limitation, such as differences in habitat barriers,
configuration and connectivity (Campbell Grant \emph{et al.} 2007;
Baguette \emph{et al.} 2013). Overall, findings from the literature
suggest that despite the often-strong intrinsic dispersal abilities of
freshwater taxa, the limitations of extrinsic habitat structure
ultimately lead to lower overall dispersal of taxa compared with
terrestrial ecosystems.

Studies comparing dispersal distances for the same taxa, such as plants
or invertebrates, in terrestrial and freshwater habitats suggest
freshwater organisms have larger intrinsic dispersal abilities
(Boedeltje \emph{et al.} 2003; Kappes \emph{et al.} 2014). A key factor
that may lead to differences in dispersal abilities is the medium within
which species move. Water is \textasciitilde{}800x denser than air, and
the high buoyancy this creates selects for passively-dispersed life
stages and promotes long-distance dispersal by flows and currents
(Bonte\emph{et al.} 2012; Cornell \& Harrison 2013; Srivastava \&
Kratina 2013). These differences may be associated with life history and
morphological traits in both aquatic and terrestrial organisms (but see
Green \emph{et al.} 2022 for a caution on using traits as proxies for
dispersal distance). For example, small-bodied organisms generally
require fewer adaptations and lower energetic costs to disperse longer
distances in water, whereas traits designed to increase drag are
required to overcome gravity in air (Dawson \& Hamner 2008).

Despite the intrinsic potential for long-distance dispersal of many
freshwater organisms, freshwater habitats may be effectively the most
dispersal-limited of all major ecosystems. This is most likely due to
the high degree of spatial isolation inherent in the structure of lakes,
streams and rivers (Comte \& Olden 2018) and the steep environmental
gradients within them (e.g., of light, Stomp \emph{et al.} 2007). Lakes
are in many ways similar to oceanic islands, in that resident species
are isolated by an uninhabitable terrestrial matrix (Kappes \emph{et
al.} 2014), though systems of ponds and lakes can be interconnected by a
network of aquatic corridors (Baguette \emph{et al.} 2013). Rivers
experience isolation at two levels: first, because there is little
exchange of organisms across drainage basins (Leuven \emph{et al.} 2009)
and second, because dispersal is constrained by the dendritic structure
and directional flow of river networks (H\selectlanguage{ngerman}änfling \& Weetman 2006;
Campbell Grant \emph{et al.} 2007; Carrara \emph{et al.} 2012;
Wubs\emph{et al.} 2016). Therefore, many riverine organisms have
relatively small ranges and high levels of differentiation across river
branches. Due to the many spatial limitations just described, freshwater
communities are often not saturated (Shurin \emph{et al.} 2000;
Irz\emph{et al.} 2004), suggesting species are limited in their ability
to reach new sites via dispersal (Shurin \& Smith 2006).

Human impacts can cause extrinsic limitations to dispersal by creating
barriers, increasing heterogeneity or by reducing habitat connectivity
(Figure 3). In highly fragmented landscapes, distances that must be
traveled in order to grow and reproduce often increase, which may lower
the fitness of dispersing organisms (Stamps \emph{et al.} 2005).
Although effects of habitat fragmentation are most often studied in
terrestrial environments, they are thought to be more severe in
freshwater systems (Fuller \emph{et al.} 2015). Human-made obstacles
such as river-crossings and dams can heavily alter species dispersal
patterns, including aquatic invertebrates (Brooks \emph{et al.} 2013;
Sondermann \emph{et al.} 2015), fish (Barbarossa \emph{et al.} 2020;
Duarte \emph{et al.} 2021) and plants (Merritt \& Wohl 2006). In
addition, in comparison with terrestrial habitats, fragmentation in
dendritic river networks creates habitat patches that are smaller and
more varied in size (Fagan 2002; Fuller \emph{et al.} 2015). Finally, as
river networks influence dispersal between lakes, their fragmentation
may also affect lake-dwelling species (Yi \emph{et al.} 2010).

\par\null\par\null\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.98\columnwidth]{figures/Figure-3-Links/Figure-3-Links}

\end{center}
\end{figure}

\begin{quote}
\textbf{Fig. 3 \textbar{} Links between impacts, processes and responses
across ecosystems discussed here.} An integrative approach to infer how
human impacts may alter biodiversity in terrestrial and freshwater
systems via their effects on the community processes of dispersal,
speciation, selection and drift. Brown lines represent the effects of
human impacts on community processes (note only a subset in bold are
considered here), and green and blue lines the impacts of processes of
varying intensity on biodiversity outcomes in ecosystems. Numbered boxes
provide evidence for each highlighted link via the references listed on
the right.
\end{quote}

\par\null

Habitat fragmentation and landscape modification are also severe threats
to terrestrial biodiversity (Fischer \& Lindenmayer 2007) and the
linkages between terrestrial and freshwater ecosystems. For example,
artificial constructions such as roads or fences have been shown to
obstruct long-distance dispersal in mammals (Seidler \emph{et al.} 2015;
Tucker \emph{et al.} 2018; Barto\selectlanguage{polish}ń \selectlanguage{english}\emph{et al.} 2019), plants
(Dener\emph{et al.} 2021) and even microbes (Le Provost \emph{et al.}
2021), which were historically considered to be
near-universally~distributed. Interestingly, some freshwater species
also utilize the terrestrial matrix for dispersal, which can help these
taxa overcome effects of habitat fragmentation (Zuluaga \emph{et al.}
2022). For example, freshwater invertebrates with adults dispersing
actively over land through the air better track environmental variation
and are less affected by barriers such as dams (Gr\selectlanguage{ngerman}önroos \emph{et al.}
2013; Tonkin\emph{et al.} 2018). Similarly, the ability of freshwater
organisms to use other organisms (e.g., birds) as dispersal vectors may
both mitigate effects of habitat fragmentation and facilitate the spread
of invasive species (Incagnone \emph{et al.} 2015; Coughlan \emph{et
al.} 2017).

\par\null

\subsubsection{\texorpdfstring{\emph{Speciation}}{Speciation}}

{\label{826592}}

Speciation (Box 1) is the engine generating new biodiversity, and not
only many species but the process of speciation itself is under threat
from ongoing human activities (Barnosky \emph{et al.} 2011). Studies
have found that human impacts can both hinder and promote speciation,
which has important implications for the long-term recovery of Earth's
biodiversity (Rosenzweig 2001). Both variation in speciation rate among
lineages and the amount of time and area available for speciation to
occur can influence the size of regional species pools (Rabosky 2020;
Miller \& Román\selectlanguage{english}-Palacios 2021) and community diversity assembled from
it. This process can also add species to communities directly via in
situ speciation (e.g., Gillespie 2004). However, despite an increasing
focus on speciation as a driver of community diversity, less attention
has been paid to how local and regional dynamics of speciation differ
among terrestrial and freshwater habitats, and how these dynamics are
altered by human impacts (Figure 3, Table 1). It should be noted that
while the products of speciation can be quickly eradicated, recovering
new species via this process occurs over longer timescales and is
outside the scope of many traditional conservation approaches.

Though few direct comparisons of speciation rates have been made between
terrestrial and freshwater habitats, the highest observed
diversification rates per clade or lineage per unit time occur in
freshwater ecosystems (Miller \emph{et al.} 2018; Rabosky 2020; Miller
2021). This may be due in part to the greater isolation of freshwater
habitats compared to terrestrial environments (Wiens 2015). However, a
recent study found that on average, terrestrial taxa have higher
diversification rates than freshwater taxa, though this could be due in
part to older colonization events in freshwater (see below,
Roman\selectlanguage{english}-Palacios \emph{et al.} 2022). Future studies should compare
speciation rates among terrestrial and freshwater ecosystems using
matched pairs of clades with similar life histories and a range of
dispersal abilities. Such studies would increase our mechanistic
understanding of both the relative importance of speciation among
terrestrial and freshwater ecosystems and how this process shapes the
response of these systems to human impacts.

In addition to variation in speciation rates, the amount of time and
area available for speciation to occur can also influence the size of
regional species pools, the amount of in-situ speciation, and thus local
community diversity. Terrestrial habitats tend to be older, larger and
more stable over geologic timescales than freshwater habitats such as
lakes and streams (Miller 2021). However, they also tend to be better
connected than freshwater habitats, such that new species more quickly
expand their ranges beyond their site of origin. Thus, it may be
expected that freshwater systems will tend to have smaller regional
species pools, but that in situ speciation plays a larger role than in
terrestrial communities, in particular for organisms that cannot
disperse through air or over land (Gillespie 2004; Miller \emph{et
al.}2018). Indeed, the best-known cases of recent rapid in situ species
radiations occur in lake-inhabiting fishes such as cichlids
(McGee\emph{et al.} 2020), salmoniformes (Hudson \emph{et al.} 2011) and
pupfish (Rabosky 2020; Miller 2021; Richards \emph{et al.} 2021). These
differences between ecosystems in area and time for speciation to occur
have important implications for the recovery of diversity and ecological
function after human impacts such as habitat and species loss. For
example, immigration from the larger and more connected terrestrial
matrix may allow terrestrial communities to recover more quickly over
shorter timescales via dispersal. Whereas in freshwater communities due
to isolation, the much slower process of speciation may be one of the
primary ways these systems recover from species loss.

Perhaps the primary way human impacts can hinder or decrease speciation
rates is through the destruction and fragmentation of habitat within
which new species are formed (Figure 2, Rosenzweig 2001;
Barnosky\emph{et al.} 2011). This occurs primarily through deforestation
(or any land-use conversion or intensification) on land, and damming,
draining and eutrophication in freshwater systems (Butchart \emph{et
al.} 2010; Horvath \emph{et al.} 2019; Ruckelshaus \emph{et al.} 2020).
Due to the relatively small existing area of freshwater habitats
compared to terrestrial regions, future speciation in freshwaters may be
more impacted by habitat loss as the small areas available for
speciation to occur become smaller still. For example, pollution in
freshwater systems can cause eutrophication and hypoxic conditions,
which greatly reduce the available amount and diversity of habitats, and
often create conditions where endemic species lose all habitat at once
(Vonlanthen\emph{et al.} 2012; Frei \emph{et al.} 2022). Over time,
pollution in freshwater systems can reduce and homogenize ecological
niche space previously partitioned by species, especially those formed
via adaptive radiation, and lead to the sudden meltdown of species
richness in a group, such as in whitefish (Vonlanthen \emph{et al.}
2012). Overall, human impacts on local diversity may be greater in
freshwater ecosystems, both in the short term due to habitat loss, over
medium timescales as the recovery of diversity via immigration is
limited by dispersal barriers, and over longer timescales as the
opportunity for speciation becomes diminished due to anthropogenic
activities. Thus, by incorporating speciation into our approach, we can
make inference about future changes in richness and turnover in these
systems (see Figure 1).

In addition to hindering speciation, human impacts may also promote
speciation in terrestrial and freshwater habitats in at least two ways.
First, it has been hypothesized that anthropogenic warming could cause
once connected populations to move up in elevation to separate uplifted
areas such as within a mountain range, isolating populations and
potentially leading to allopatric speciation (Hua \& Wiens 2013).
However, there is no evidence that this process also occurs in
freshwaters, as upward shifts in these habitats are also associated with
strong changes in environmental conditions (e.g., water velocity) that
cannot be easily adapted to (Timoner \emph{et al.} 2020). Second,
disturbances caused by habitat homogenization and human-mediated
dispersal can bring new species into contact and thus promote hybrid
speciation. For example, hybrid speciation at ecological time scales in
response to human-mediated dispersal has been demonstrated in land
plants (Abbott 1992) and freshwater fish (Marques \emph{et al.} 2019).
Once hybrids have formed, altered ecosystems created by humans may
further facilitate their survival and spread (e.g., Hoban \emph{et
al.}2012), but whether this process adds new species or removes them
will depend on whether it is associated with the gain or loss of habitat
and ecological opportunity. Overall, humans have and will continue to
impact the diversity and dynamics of communities by reducing speciation
potential, but may also in some cases promote speciation.

\textsuperscript{}

\subsubsection{\texorpdfstring{\emph{Ecological
selection}}{Ecological selection}}

{\label{284690}}

Understanding biodiversity responses to human impacts will first require
an understanding of how the strength of ecological selection, defined as
deterministic differences in fitness among individuals of different
species (Box 1, Vellend 2016), differs between ecosystems (Figure 3,
Table 1). This is because human impacts such as land use, warming and
invasive species alter the selection regime by modifying either abiotic
gradients and heterogeneity (e.g., climate change velocity,
Loarie\emph{et al.} 2009) or biotic interactions such as competition,
predation and mutualism (e.g., novel species interactions, Alberti 2015;
Alexander \emph{et al.} 2015). Below we compare abiotic and biotic
aspects of selection across terrestrial and freshwater ecosystems, and
how humans alter the selection regime through various global change
drivers.

Selection caused by abiotic factors may vary across ecosystems due to
variation in habitat properties (Table 1) and the strength of
environmental gradients. For example, abiotic gradients can vary between
different types of media (e.g., air vs water) due to differences in
density, viscosity and heat capacities (Table 1). In addition, it has
been suggested there is overall greater environmental structure on land
than in freshwater systems (Herfindal \emph{et al.} 2022). However, some
gradients may be stronger in water than on land, such as for light,
which can promote niche differentiation in freshwater habitats
(Stomp\emph{et al.} 2007). Possibly as a result of these differences,
there is evidence that species sorting--when species tend to be found in
local communities matching their environmental preferences (Heino
\emph{et al.} 2015; Govaert \emph{et al.} 2021)--tends to be stronger on
land. For example, species sorting appears to be weakest in lakes
compared to riverine and terrestrial habitats (Soininen 2014), possibly
due to their isolation (Heino \emph{et al.} 2015). Finally, differences
between ecosystems in environmental structure also have major
implications for the evolution of thermal niches (Steele \emph{et al.}
2018; Sunday\emph{et al.} 2019) and suggest the ability of species to
track thermal optima in response to climate change may differ across
realms (Burrows\emph{et al.} 2011).

In addition to abiotic effects, selection caused by biotic interactions
may also vary between terrestrial and freshwater ecosystems
(Gothe\emph{et al.} 2013; Pringle \emph{et al.} 2016; Garcia-Giron
\emph{et al.} 2020). Specifically, consumptive pressure via predation is
thought to be stronger in aquatic systems (Cyr \& Face 1993; Cebrian \&
Lartigue 2004; Alofs \& Jackson 2014). This may be because freshwater
systems have a higher prevalence of generalist consumers, which cause
strong top-down control (Cyr \& Face 1993; Shurin \& Smith 2006; Alofs
\& Jackson 2014). Alternatively, the generally higher nutritional
quality of freshwater organisms (Twining \emph{et al.} 2019; Shipley
\emph{et al.} 2022) may support larger consumer populations and thus
increase predation pressure. However, the high consumptive pressure in
freshwaters could also be due to greater heterogeneity in density of
predators caused by dispersal limitation, which would lead to a higher
naivete of prey populations compared to terrestrial habitats (Cox \&
Lima 2006; Anton \emph{et al.} 2016, 2020). Finally, many studies
highlight differences in selective pressures within freshwaters between
lakes and streams, and suggest that biotic resistance is weaker in
streams (Mitchell \& Knouft 2009; Alofs \& Jackson 2014).

As outlined above, human impacts are known to have multiple and severe
effects on the selection process, to the point that they can overshadow
effects of natural processes (Leprieur \emph{et al.} 2008). For example,
there is a large body of evidence demonstrating that climatic warming
has different impacts in terrestrial vs marine habitats, however few
studies have compared terrestrial and freshwater ecosystems. Freshwater
habitats, being embedded in a terrestrial matrix, have thermal regimes
that are closely tied to air and thus their temperatures tend to be
similar to surrounding land areas (Grant \emph{et al.} 2021). Among all
major ecosystems, evidence suggests freshwater and terrestrial
communities are less affected by warming than marine systems
(Burrows\emph{et al.} 2011). However, this apparent resilience may lead
to accumulation of greater extinction debts compared to marine systems,
which are experiencing the highest community turnover (Blowes \emph{et
al.} 2019).

A final way humans impact selection regimes is by facilitating
biological invasions. For example, it has been argued that because
freshwater habitats represent a more complex matrix of interacting
abiotic and biotic components, invading freshwater species can more
easily affect the properties and functions of their ecosystems than
terrestrial taxa (Moorhouse \& Macdonald 2015). Invasive species can
also alter selection regimes in communities in a less direct way, by
transmitting diseases or by altering abiotic conditions (via poisoning,
bio-fouling or changing other ecosystem properties, Blackburn \emph{et
al.} 2014). These processes not only affect selection regimes, but can
also facilitate further invasions (Ricciardi 2001; Green \emph{et
al.}2011). Invasive species effects may also be transmitted between
ecosystem in s through various linkages, such as when invasive plants
alter nutrient flows between terrestrial and freshwater habitats
(Stewart \emph{et al.} 2019).

\par\null

\subsubsection{\texorpdfstring{\emph{Ecological
drift}}{Ecological drift}}

{\label{380504}}

Ecological drift (Box 1) is generally the most understudied of the four
community processes, and has primarily been considered in microbes and
terrestrial plant communities (e.g., Hubbell 2001). However, it should
be noted that the more general process of stochasticity is increasingly
studied by ecologists (Shoemaker \emph{et al.} 2020). For example,
rather than being studied directly, the strength of drift has been
inferred from random variation in species abundance distributions (Chase
2010) or quantified as unexplained variation in community dynamics
models (Vellend \emph{et al.} 2014). Because few studies directly
quantify drift, in particular across several ecosystem types, we know
very little about how humans impact this process in terrestrial and
freshwater communities (Figure 3).

There are however a small number of studies quantifying drift in plant
(e.g., Hubbell 2001; Gilbert \& Lechowicz 2004; Gilbert \& Levine 2017),
bacterial (Vanwonterghem \emph{et al.} 2014; Aguilar \& Sommaruga 2020),
and other microorganismal communities (Devercelli \emph{et al.} 2016;
Logares \emph{et al.} 2018; Wu \emph{et al.} 2018; Vass \emph{et
al.}2020). And perhaps the best-documented studies of how drift shapes
community structure come from damselfly (Odonata) communities, where
species appear to closely approach ecological equivalence
(Svensson\emph{et al.} 2018; McPeek \& Siepielski 2019). Despite this
limited evidence, drift may be expected to be stronger in freshwater
habitats than on land due to the much smaller total area these habitats
occupy compared to their terrestrial counterparts (Wiens 2015),
especially as these populations are reduced via ongoing impacts such as
damming.

Despite the paucity of studies quantifying drift, it has been
demonstrated that human impacts can increase the overall importance of
stochastic processes, which include drift, in both freshwater and
terrestrial systems. For example, species losses, nutrient addition, and
warming all increased the relative importance of stochastic processes in
soil microbial communities (Zhang \emph{et al.} 2016). Similarly,
warming and nutrient addition increased the relative contribution of
stochasticity--mainly caused by drift--among lake bacterioplankton
communities (Ren \emph{et al.} 2017). More generally and as mentioned
above, we can expect that drift and drift-related extinctions will be
greater in more isolated and smaller communities (Hubbell 2001; Vellend
2016). Thus, any anthropogenic changes which reduce community size and
increase isolation (e.g., habitat fragmentation or land-use change)
should tend to increase the contribution of drift to community dynamics
(Melbourne \& Hastings 2008). For example, biological invasions can
cause decreased population sizes and thereby increase species
vulnerability to stochastic extinctions (Gilbert \& Levine 2013).
Looking forward, a greater focus on the contribution of drift to
community dynamics is warranted, as it is highly understudied but likely
very important for shaping human impacts in both ecosystems.

\par\null

\subsection{Synthesis and future
directions}

{\label{443870}}

Here we propose an integrative approach for comparing effects of human
impacts on freshwater and terrestrial ecosystems using fundamental
processes (Figures 1-3). We find several key differences in the strength
and operation of these processes that suggest differing biodiversity
responses in terrestrial and freshwater ecosystems (Figure 3, Table S1).
For example, we find evidence suggesting that i) ecological selection
due to abiotic gradients is stronger in terrestrial ecosystems than in
freshwater, ii) that dispersal limitation may be greater in freshwater
communities but by contrast that freshwaters have the highest potential
for recovery via speciation, and iii) that the biggest data gap for
cross-ecosystem comparisons is the relative influence of ecological
drift among the two ecosystems. Overall, we found that quantitative
comparisons across ecosystems are generally lacking, though data
enabling such comparisons may be available for many organisms for the
processes of dispersal and selection. In contrast, cross-ecosystem
studies of speciation rates tend to focus on a few well-characterized
organisms and such studies are largely missing for drift. Filling these
gaps will be essential to fully link the processes of dispersal,
speciation, selection and drift to the future dynamics and recovery of
Earth's biodiversity.

In addition to filling data gaps, the mechanistic realism of our
approach could be increased by considering interactions between
community processes and between human impacts, as well as increased
consideration of linkages and flows between ecosystems. For example,
interactions between community processes can modify the effect of global
change drivers, as has been found in experiments and models
incorporating both competition and drift (Chesson 2000; Orrock \&
Watling 2010; Gilbert \& Levine 2017), as well as experiments on the
relative role of dispersal and selection (Ron \emph{et al.} 2018). In
addition, we focus primarily on the effects of single human impacts,
though we recognize that these drivers interact in nature (Settele \&
Wiemers 2015). For example, invasive species are often positively
affected by global change, such as increases in temperature or land use
intensification (Occhipinti-Ambrogi 2007; Hellmann \emph{et al.} 2008;
Eisenhauer \emph{et al.} 2012). Finally, many of the ecosystem processes
depend to some extent on linkages between the systems, such as flows of
nutrients and organisms between land, rivers and lakes (Soininen\emph{et
al.} 2015). For example, dispersal of many freshwater insects depends on
adult forms that fly (Bilton \emph{et al.} 2003). Furthermore, transport
of nutrients and pollutants between freshwater and terrestrial systems
is known to affect populations in both systems (Kraus 2019; Kraus
\emph{et al.} 2021). As linkages between systems may be affected by
global change drivers (Kraus 2019; Johnson \emph{et al.}2021; Kraus
\emph{et al.} 2021), their role should be more fully addressed in future
developments of the proposed approach.

Realizing the full predictive potential of our approach which links
human impacts to community processes will require increased research
efforts and dialog between aquatic and terrestrial ecologists,
evolutionary biologists, conservationists and policymakers. Future
research aimed at better forecasting human impacts across ecosystems
should include targeted, quantitative studies of the strength and
function of single or multiple processes in both terrestrial and
freshwater ecosystems, especially for the understudied processes of
speciation and drift. Furthermore, our approach could be used to
parametrize a mechanistic model of impacts, processes and biodiversity
outcomes. Such a model would allow researchers to make more detailed,
mechanistic predictions about how diversity may change in a given
ecosystem. This work would build upon recent processes-based simulation
studies used to infer the mechanisms shaping diversity through deep
time, and across communities and spatial scales (Thompson \emph{et
al.}2020; Hagen \emph{et al.} 2021a, b). Mechanistic models, when
combined with data, can provide more detailed estimates of the relative
importance of processes shaping diversity across ecosystems and increase
our understanding of how bio-physical properties influence the balance
and strength of processes (Table 1).

In conclusion, to tackle the ongoing and accelerating impacts of humans
on biodiversity, we must provide a mechanistic understanding of how
these impacts cause changes in terrestrial and freshwater ecosystems.
Our process-based approach developed here may be useful for mitigating
many impacts of global change for several key reasons. First, focusing
on the real-world processes which create the diversity and structure of
communities creates a mechanistic bridge between a given human impact,
such as climate warming, and the outcome of this process on assemblages,
such as increased turnover or decreased diversity. Second, it provides a
foundation for further research, especially quantitative comparisons and
mechanistic models. Thus, in the same way that these processes are meant
to open the 'black box' of community ecology to understand community
patterns (Vellend 2010), our approach has the potential to do the same
for understanding the mechanistic pathways by which humans impact
Earth's biodiversity.

\par\null

\subsection{Acknowledgements}

{\label{264515}}

The authors thank the ETH Board for funding through the Blue-Green
Biodiversity Initiative BGB2020. Icons and landscapes in Fig. 2 and
human impact icons in Fig. 3 are modified from reshot.com.

\par\null

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\par\null

\textbf{Table 1 \textbar{}} Key differences between freshwater and
terrestrial ecosystems in physical, chemical, habitat and community
properties and their hypothesized effect on community processes

\par\null

\textbf{Table S1 \textbar{}~}Overview of studies comparing how human
impacts may, or may not, differ in type or severity among terrestrial vs
freshwater ecosystems, organized by the process which humans impact

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