Introduction
One of the features that most universally affects ecological systems is
ecosystem (patch) size. Ever since the foundational work by MacArthur &
Wilson (1967), it has been known that ecosystem size affects diversity,
abundance, and extinction risks. This finding has broad implications
from theoretical ecology (Luo et al., 2022) to conservation biology
(Riva et al., 2024). A second feature that universally affects
ecological systems is their spatial connection through the movement of
non-living materials, creating spatial coupling or even spatial
feedbacks (Gravel et al., 2010; Loreau et al., 2003; Peller et al.,
2024), whose effects can depend on the type of ecosystems connected
(Gounand et al., 2017; Osakpolor et al., 2023; Pichon et al., 2023), as
ecosystem type can determine the type and quality of resources flowing
(Gounand, Harvey, et al., 2018; Gounand, Little, et al., 2018; Pichon et
al., 2023; Sitters et al., 2015). However, despite recognising the
ecological significance of these two features, we do not know how the
size of ecosystems of different types influences the effects of resource
flows on their function.
Worldwide, different and heterogeneous ecosystems are connected through
flows of resources (Gounand, Little, et al., 2018), forming
meta-ecosystems (Gounand, Harvey, et al., 2018; Harvey et al., 2023;
Leroux et al., 2017; Loreau et al., 2003; Osakpolor et al., 2023). For
example, a meta-ecosystem could be composed of a stream and a forest
(Harvey et al., 2023; Leroux et al., 2017; Osakpolor et al., 2023),
where leaf litter falls from the forest into the stream (Cereghetti &
Altermatt, 2023; Naiman & Link, 1997), fish carcasses are brought from
the stream to the forest by bears fishing (Helfield & Naiman, 2002),
and emergent aquatic insects end their life cycle in the forest
(Scharnweber et al., 2014). In a meta-ecosystem, ecosystems can sustain
their consumers mainly through their resources (autotrophic ecosystems)
or through resources coming from other ecosystems (heterotrophic
ecosystems). For example, a forest can be net autotrophic because it has
enough plant production to sustain the growth and reproduction of
terrestrial insects. In contrast, a stream can be net heterotrophic
because it does not produce enough algae to sustain insects–such as
stoneflies–that, therefore, rely on the input of leaves from the
terrestrial ecosystem. The flow of resources between autotrophic and
heterotrophic ecosystems is widespread in nature (Gounand, Little, et
al., 2018). For example, deep benthic systems (heterotrophic) are
coupled to pelagic systems (autotrophic) (Griffiths et al., 2017;
Schindler & Scheuerell, 2002), small headwater streams (heterotrophic)
are coupled to riparian forests (autotrophic) (Carter et al., 2024;
Gounand, Little, et al., 2018), and coral reefs (heterotrophic) are
coupled to pelagic ecosystems (Morais et al., 2021; Morais & Bellwood,
2019).
Resources flowing between autotrophic and heterotrophic ecosystems can
positively or negatively affect the overall meta-ecosystem production
(Allen et al., 2024; Gounand et al., 2017; Pichon et al., 2023).
Resource flows are predicted to increase the production of the two
ecosystems if the production of the two systems is limited by the
compounds that the other provides (Pichon et al., 2023). By contrast,
resource flows would likely decrease production if the ecosystems
provided each other with unnecessary compounds, making limiting
resources less dense and, therefore, less available (Pichon et al.,
2023). For example, if a stream connected to a forest receives leaf
litter rich in carbon, this flow of resources will increase stream
production if the stream is carbon-limited and decrease it if it is
nitrogen-limited (Pichon et al., 2023). Also, if a stream is connected
to a terrestrial ecosystem from which it receives too many resources
such as hippo faeces, these resources cannot be absorbed by algae as
quickly as received, and subsequently provoke hypoxia, reducing fish
function (Dutton et al., 2018). As the connection among autotrophic and
heterotrophic ecosystems via resource flows is widespread in nature,
understanding how their connection influences their production will
deepen our understanding of a ubiquitous ecological process, ultimately
enhancing our ability to manage landscapes effectively.
However, how the size of the autotrophic and heterotrophic ecosystems
shape the effects of this spatial coupling has been overlooked. The size
of an ecosystem can alter its permeability to resource flows and,
therefore, how they affect its production. This interaction between
ecosystem size and resource flows was shown, for example, by field
studies on unidirectional flows of resources to islands, where smaller
islands get proportionally more resources coming from the ocean than
larger ones, increasing the density of spiders and insects in islands in
Baja California (Mexico) (Polis & Hurd, 1995) and birds in islands in
British Columbia (Canada) (Obrist et al., 2020). Furthermore, resources
flowing bidirectionally between ecosystems of different sizes can change
the overall biomass in these ecosystems, as shown by a protist
experiment (Giacomuzzo et al., 2024). In light of the influence of
ecosystem size on meta-ecosystem total biomass and the fact that
ecosystems have different sizes in nature, we expect that including
ecosystem size should be important in understanding how resource flows
between autotrophic and heterotrophic ecosystems change the total
biomass of meta-ecosystems.
Here, we studied how relative ecosystem size mediates the effects of
spatial couplings on meta-ecosystem biomass using a protist experiment.
Such microcosm experiments are a well-known approach to study general
questions in ecology and allow a high level of control and a close
conceptual analogy to mathematical models, yet still contain realistic
ecological dynamics (Altermatt et al., 2015; Benton et al., 2007). We
studied two-patch meta-ecosystems composed of one autotrophic and one
heterotrophic patch with only non-living resources flowing between the
two ecosystems (‘patch’ and ‘ecosystem’ are used as synonyms). We
investigated the effects of resource flows in three meta-ecosystems with
the same total volume but different relative patch sizes: one in which
the autotrophic patch was larger, one in which the heterotrophic patch
was larger, and one in which both patches were of the same size. We
investigated the effects of resource flows by comparing the same three
meta-ecosystems (varying in patch size of heterotrophic and autotrophic
systems) and manipulating if patches were coupled or not (i.e.,
connected or unconnected by resource flows).